ENVIRONMENTAL HEALTH SERIES
Air Pollution
POWER SYSTEMS
FOR ELECTRIC VEHICLES
L-OO
U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
Public Health Service
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AP37
POWER SYSTEMS
FOR
ELECTRIC VEHICLES
A Symposium sponsored by
The U. S. Department of Health,
Education, and Welfare
Columbia University
and
Polytechnic Institute of Brooklyn
April 6-8, 1967
Chairman
H. B. Linford, Columbia University
Co-Chairmen
H. P. Gregor, Polytechnic Institute of Brooklyn
B. J. Steigerwald, Public Health Service
U. S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
Public Health Service
Bureau of Disease Prevention and Environmental Control
National Center for Air Pollution Control
Cincinnati, Ohio
1967
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The ENVIRONMENTAL HEALTH SERIES of reports was estab-
lished to report the results of scientific and engineering studies of
man's environment: the community, whether urban, suburban, or
rural, where he lives, works, and relaxes; the air, water, and earth
he uses and re-uses; and the wastes he produces and must dispose of
in a way that preserves these natural resources. This SERIES of
reports provides for professional users a central source of information
on the intramural research activities of the Centers in the Bureau of
Disease Prevention and Environmental Control, and on their coopera-
tive activities with state and local agencies, research institutions, and
industrial organizations. The general subject area of each report is
indicated by the letters that appear in the publication number; the
indicators are
AP — Air Pollution
RH — Radiological Health
UIH — Urban and Industrial Health
Reports in the SERIES will be distributed to requesters, as sup-
plies permit. Requests should be directed to the Center identified on
the title page.
Public Health Service Publication No. 999-AP-37
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PREFACE
The National Center for Air Pollution Control, the Polytechnic
Institute of Brooklyn, and Columbia University sponsored a sym-
posium on Power Systems for Electric Vehicles at Columbia Uni-
versity April 6, 7, and 8, 1967. The purpose of the symposium was to
provide a coordinated review of current research activities related to
power systems for electric vehicles. The meetings were designed both
to define the present status of our knowledge and to stimulate
research.
As chairmen of the symposium, we want to thank Dr. John H.
Ludwig and Mr. Anthony H. Sweet of the Public Health Service for
their cooperation and assistance; Mrs. Anne Cassel, of Dr. Ludwig's
staff, provided editorial services in preparation of these proceedings.
Finally, the devotion of Miss Donna McManus, Secretary of the Sym-
posium, to the never-ending details is greatly appreciated.
Henry B. Linford
Harry P. Gregor
Bernard J. Steigerwald
New York
Washington, D. C.
1967
This report was prepared for publication at facilities oj the
Public Health Service in Cincinnati, Ohio, under the direction of
Kenneth Cassel, Jr., Publications Officer, National Center for Urban
and Industrial Health.
iii
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CONTENTS
Introduction HENRY B. LINFORD 1
PROBLEMS IN OUR CITIES: polluted air, tangled transit;
why we think about electric vehicles.
Introduction RICHARD s. MORSE 3
An Examination of Alternatives to the Gasoline Engine
AUSTIN N. HELLER 5
Impact of Electric Vehicles on Urban Problems
CLARK HENDERSON 13
Impact of Electric Power Systems on Urban Traffic Flow,
Control, and Facilities - WALTER HELLY 19
TYPES OF POWER SOURCES: reviews of system develop-
ments, including several kinds of hybrids.
Introduction _. WILLIAM T. REID 25
Hybrid Power Systems for Vehicles GEORGE A. HOFFMAN 27
NASA Work on High-Energy-Density Electrochemical Power
Devices ERNST M. COHN 43
Potential Battery Systems in Vehicle Propulsion
ROBERT C. OSTHOFF 51
Battery-Powered Electric Vehicles D. THOMAS FERRELL, JR.
and ALVIN j. SALKIND 61
Power Systems for Electric Vehicles for Military and Com-
mercial Use EDWARD A. GILLIS 71
Fuel Cell — Battery Power Sources for Electric Cars
GALEN R. FRYSINNGER 83
Electric Vehicle Research HOWARD A. WILCOX 91
European Developments of Power Sources for Electric
Vehicles - M. BARAK 105
AUXILIARY SYSTEMS: component developments; other
aspects of the total vehicle system.
Introduction PAUL L. HOWARD 121
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Electronic Circuits for Speed Control and Braking
VICTOR WOUK 123
The Mechanical Design of Electric Automobiles
CARL A. VANSANT 143
Electrochemical Systems ANDRE J. de BETHUNE 155
Separator Systems HARRY P. GREGOR 173
BATTERIES AND FUEL CELLS: status of current research
on specific energy-storage systems; how they perform.
Introduction ARTHUR FLEISCHER 199
Lead-Acid Batteries and Electric Vehicles DAVID L. DOUGLAS 201
High-Energy Non-Aqueous Battery Systems for Electric
Vehicles M. EISENBERG 209
An Electrically Rechargeable Zinc-Air Battery for Motive
Power D. v. RAGONE 225
Zinc-Air Batteries for the Electric Vehicle NIGEL i. PALMER 231
Note on Problems Related to High-Energy Batteries
SAMUEL RUBEN 242
The Atomics International Sodium-Air Cell L. A. HEREDY,
H. L. RECHT, D. E. MC KENZIE 245
Aluminum Fuel Cell for Electric Vehicles ....SOLOMON ZAROMB 255
Performance and Economics of the Silver-Zinc Battery in
Electric Vehicles GEORGE A. DALIN 269
A Sodium-Sulfur Secondary Battery T. w. DEWITT 277
Lithium Nickel-Halide Batteries R. c. SHAIR, A. E. LYALL,
H. N. SIEGER 289
A State-of-the-Art Automotive Fuel Cell PHILIP DANTOWITZ,
LONNY GADDY 297
Direct Hydrocarbon and Methanol-Air Fuel Cells
C. E. HEATH, E. H. OKRENT, M. BELTZER, G. CIPRIOS 307
Hydrogen-Air Fuel Cells for Vehicle Propulsion G. E. EVANS 313
VI
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POWER SYSTEMS
FOR ELECTRIC VEHICLES
We have allowed ourselves to so contaminate the air space above
most of our major cities as to endanger the health of the inhabitants.
Although the total contribution of pollutants emanating from in-
ternal combustion engines is debatable, we do not have to argue about
exact percentages, since there is almost uniform agreement that
automobiles are a major contributor.
The U. S. Department of Health, Education, and Welfare, through
its National Center for Air Pollution Control, asked us to cooperate
with them in arranging to bring together the leaders of the scientific
and engineering world and delineate the state of non-polluting power
systems existent today. We hope to point out where progress can
be made most expeditiously, with the result of reducing the lead
time for the commercialization of the electric vehicle.
Henry B. Linford
Columbia University
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PROBLEMS IN OUR CITIES: polluted
air, tangled transit; why we think about
electric vehicles.
The air pollution problem is more complex than is generally
recognized. It varies in form and intensity throughout the country,
but the gasoline automobile is a principal contributor to the total
pollution. This country now has over 80,000,000 automotive vehicles;
no simple mechanism can be suggested to attack a problem of this
magnitude.
The automotive industry must of course undertake the introduc-
tion of improved automotive engine designs and emission control
devices. Our committee" is receiving excellent cooperation -from the
automobile companies and the oil, chemical, and other industrial
groups. I am confident that the emission characteristics of future
automobiles will be substantially improved.
' Mr. Morse is a member of the Technical Advisory Board of the U. S.
Department of Commerce. Currently he is chairman of the study panel
appointed by the Secretary of Commerce to deal with the automotive
air pollution problem.
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The public must clearly recognize its responsibility and assume
the necessary costs associated with our country's desire to improve
the environment in which we live. The Federal government must
develop early and realistic short- and long-term goals and standards,
both for quality of our air and for the transportation systems them-
selves.
In retrospect it appears that knowledge and technology available
several years ago were adequate to reduce harmful emissions from
the gasoline engine substantially, but we have only recently initiated
appropriate action at the national level. Because of the 80,000,000
automotive vehicles now in use, the introduction of new and ad-
vanced propulsion devices can at best reduce air pollution to a limited
extent in the next decade. It is a sad commentary indeed that the
very industrial society that is supposed to improve standards of living
has in fact made many of our cities uninhabitable relative to the
quality of air that was known 50 years ago.
New types of automotive vehicles must be developed that will
operate without generating harmful emissions. Our panel studies
are specifically directed toward the identification of new types of
motive power that are technically and economically feasible and
to which the scientific and industrial capabilities of this country can
be most profitably employed. The immediate needs of certain areas
such as Los Angeles and New York City and future requirements of
our growing urban society dictate that government and industry
implement a more effective national program than has been evident
in the past decade.
Richard S. Morse
Massachusetts Institute
of Technology
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AN EXAMINATION OF
ALTERNATIVES TO THE GASOLINE AUTOMOBILE
Austin N. Heller
Commissioner, Department of Air Pollution Control
New York, N. Y.
Right here in New York City, because of the convenience and
transportation flexibility of the private automobile, New York auto-
motive vehicles consume a billion gallons of gasoline each 12 months.
Because of the less than perfect efficiency of the internal combustion
engine, about 70 million gallons of this total is discharged into the
atmosphere as waste products — unburned hydrocarbons. This
wasted gasoline costs New York City motorists about $25,000,000
a year at current prices.
A study of carbon monoxide, hydrocarbons, and oxides of nitro-
gen emissions has been completed by the New York City Depart-
ment of Air Pollution Control. I would like to bring to your atten-
tion some pertinent data, that will be included in the report of the
study.
For the City of New York, we estimate that each day, automobile
traffic produces 8,300,000 pounds of carbon monoxide, 1,000,000
pounds of hydrocarbons, and 212,000 pounds of oxides of nitrogen.
This information is being correlated with traffic patterns in the
City, and the initial evaluation indicates a direct relationship be-
tween traffic congestion and emissions. For example, if the average
route speed of an autombile is 25 miles per hour, the exhaust gases
on the average contain 3 percent carbon monoxide and 1,275 parts
per million hydrocarbons.
On another basis of calculation, each car emits 0.17 pound of
carbon monoxide each mile it travels at 25 mph. At 10 mph, the
rate of emission rises to 0.35 pound of carbon monoxide per mile
traveled. This fluctuating rate, crucial to those of us charged with
air pollution control in New York City, is due to the increasing
traffic congestion resulting from more and more automobiles travel-
ing over a static amount of road space and crowding into a fixed
area of land space.
New York City traffic Commissioner Henry Barnes is fond of
telling people that the only way to get to the west side of Manhattan
is to be born there. Although things are not that bad, I would like to
emphasize that things are not that bad — yet.
Since the beginning of this year, the Department of Air Pollu-
tion Control has maintained a five-station carbon monoxide moni-
toring program in midtown and lower Manhattan, with the samplers
operating continuously 24 hours a day, 7 days a week. The first of
HELLER
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the stations to go into operation and the one from which the most
data have been obtained, is located at 45th Street and Lexington
Avenue. A summary of data from this site for the first 2 months
showed that during a normal workday the hourly carbon monoxide
concentration average cannot be expected to fall below 15 ppm from
9:00 a.m. to 7:00 p.m. The New York State ambient air quality ob-
jective states that 15 ppm for 8 consecutive hours should not be
exceeded more than 15 percent of the time on an annual basis.
On January 24, at the 45th Street location, 15 ppm as an hourly
average was exceeded for 11V2 consecutive hours, with a high hourly
average of 31 ppm. On January 26, 15 ppm was exceeded at 45th
Street for 9 hours, with the highest hourly average being 30 ppm.
On that same day, and at the same time in the morning, the high
hour at Times Square was 21 ppm; at Canal and Church Streets it
was 35 ppm; and at Herald Square it was 25 ppm.
The influence of traffic upon carbon monoxide values in Man-
hattan cannot be doubted. The early morning low, followed by a
jump in concentrations during the rush hour is easily discernible.
Variations in traffic flow, not only on weekdays but on Sundays as
well, can be read from the variations in carbon monoxide readings.
Nationally, 70 million automobiles consume some 600,000 tons
of gasoline per day and discharge more than 270,000 tons of carbon
monoxide, unburned hydrocarbons, nitrogen oxides, and organic lead.
Where is our automobile population heading? In 1900, this
nation's auto population was nil. Today there are more than 70
million private motor cars in the United States, and this figure is
expected to rise to 120 million by 1980 — approximately a 70 percent
rise. For New York City, this rise will be a minimum of 70 percent
and I remind you that 1980 is only 13 years away.
From the standpoint of air pollution control, I would like to
examine the various types of automobile engines that are either
presently in existence or are in a prototype stage.
GASOLINE ENGINES
The potential sources of air pollutants emitted from gasoline
engines include blowby gases from the crankcase, exhaust gases, and
evaporative losses from the fuel tank and carburetor.
Control of blowby gasses, i.e., the combustion gases that blow
past the reciprocating piston, is being approached. These blowby
gasses are trapped in the crankcase and piped to the intake mani-
fold where they are returned to the combustion chambers and
burned.
Hydrocarbons evaporate from the car's fuel supply system, gas
tank, and carburetor into the atmosphere. Experimental systems
for control of evaporative losses of hydrocarbons have been designed
and appear to be technically feasible.
Alternatives to the Gasoline Engine
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A national control program is scheduled to begin in late 1967
with the introduction of 1968 models. These vehicles will be equipped
to control crankcase emissions completely and to control exhaust
emissions to specified levels. Federal regulations for all 1968 models,
with engines greater than 140 cubic inches in displacement, restrict
carbon monoxide concentrations to 1.5 percent and hydrocarbon con-
centrations to 275 ppm in the exhaust stream.
Since the vehicles produced during 1967 will pollute the air
throughout their 10-year life span, emission controls must relate to
air quality standards and projected requirements for populations
in the year 1977.
The consideration of air pollution control devices is far from a
panacea. A report issued this past December (1966) by the Cali-
fornia State Motor Vehicle Board points up the very high deteriora-
tion rate of the automotive pollution control device that has been
mandatory in that State since 1966.
Tests were conducted on 552 fleet cars with control devices to
determine whether they could meet California's criteria for hydro-
carbon and carbon monoxide emissions. A direct correlation in the
testing indicated that the higher the mileage of the vehicle, the
greater the ineffectiveness of the air pollution control device. Of
227 vehicles driven less than 2,000 miles, 37 percent failed to main-
tain pollution below state limits. Of 15 vehicles driven over 20,000
miles, 13 vehicles, or 87 percent failed. From these data, we can
conclude that an extension of this trend indicates that at 50,000
miles all of the vehicles will fail to meet California standards for
hydrocarbon and carbon monoxide emissions.
Despite this evidence, "first generation" automotive emission
controls are rather impressive accomplishments and are a significant
step in controlling air pollution. However, it is interesting to ex-
amine these accomplishments in light of the substantial rise made in
pollution sources during these past 14 years. Since 1953, the vehicle
population has increased 64 percent and gasoline consumption has
increased 73 percent.
The importance of figures such as these is that they present a
direct challenge to the various professionals to move faster than the
expected rise in population and the concomitant rise in the auto-
mobile population. Looking back, we would have had to reduce
emissions by at least 70 percent since 1953 just to stand still.
The task that faces planners in air pollution control in rela-
tion to the gasoline engine seems to be formidable, but I do believe
that it can be mounted.
The problem of technology keeping up with rising population, be
it human or automotive, may be new to the engineer and scientist,
but it is old hat to to the developmental economist, who long has
had to contend with Malthusian equations. Perhaps we might borrow
HELLER
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from the economic developer one of the most useful tools at his com-
mand — that of developmental capital, or seed money.
If the automotive and gasoline industries who have a stake in
finding viable solutions to the problem of automotive pollution con-
trol would voluntarily pool 5 percent of their advertising budgets,
which would amount to about 12 million dollars based on 1965 data,
toward a joint effort at new and better controls, I believe progress
would indeed be hastened.
GAS TURBINE
Following World War II, the gas turbine engine became the
dominant source of power for the aircraft industry. The gas turbine
engine for land-vehicle use had to be much smaller, however.
Such an engine has now been developed by the automotive in-
dustry, but fuel consumption is extremely high and must be reduced
especially at less than peak load.
The turbine blades must be subjected to the maximum cycle
temperatures, thereby necessitating a great amount of excess air to
cool the hot gas temperatures.
Another deterrent to widespread utilization of the gas turbine
engine in automobiles is the very high cost of needed regenerators
or heat exchangers. If ways can be found to raise the maximum
cycle temperatures and reduce engine cost, there is no reason why
the turbine engine cannot become competitive for automotive use.
The cost of manufacture is likely to remain high for many
years, and should the cost go down, there is still the consideration
of extensive retooling on the part of the automotive industry to meet
any changeover from piston engines.
From a pollution control standpoint, exhaust emissions from gas
turbine engines are very low. Tests indicate that emissions of un-
burned hydrocarbons and carbon monoxide from a turbine-powered
vehicle are approximately one-tenth those from a piston-engine-
powered vehicle, based on emissions per pound of fuel burned. Un-
fortunately, emissions of oxides of nitrogen do not compare favorably
with those from the present gasoline engine.
Considering the stage of development of turbine-powered
vehicles, we should point out that even for heavy trucks, which are
considered the most likely outlet for first application of the gas
turbine, production is well beyond 1970.
Through our efforts, New York City is to become the test site
for the first thorough evaluation of a gas turbine bus. The bus is
expected to be delivered before the end of this year and, through
the cooperation of the Department of Air Pollution Control and
The New York City Transit Authority, will be given its first practical
emission and performance tests.
Alternatives to the Gasoline Engine
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ELECTRIC AUTOMOBILE
Between 1959 and 1961 there was a great flurry of activity when
several newly designed versions of the electric automobile were
placed on the market.
The various batteries considered for the light trucks and pas-
senger vehicles represented a considerable improvement over those
that were tested during the first quarter of this century. Nonethe-
less, advances in technology have not yet provided a satisfactory
battery.
The storage-capacity-to-weight ratios for the relatively inex-
pensive lead-acid type of battery were too low, limiting the speed
and range of the vehicles. The nickel-cadmium and silver-zinc
batteries were too expensive.
The production and sales of battery-powered, material-handling
transport devices for industrial use have gained great acceptance.
Today, more than 100,000 are used inside plants in the United States
where air pollution from internal combustion engines is either
dangerous or undesirable.
Despite this positive start, electric vehicles for street and high-
way utilization have not fared well commercially.
After several years of testing on the part of commercial cor-
porations and governmental agencies a few facts have become ap-
parent. We have found that the energy storage capacities of con-
ventional batteries are too limited or too expensive to provide an
acceptable energy source for electric passenger cars, or any short-
distance electric vehicle.
A substantially superior method of storing electrical energy
that will provide much greater storage capacity per unit weight
and volume will have to be developed to meet the needs. A great
deal of progress needs to be made on electric motors, controllers,
and chargers. Further, we have seen that research and development
must explore the entire electric automobile, not merely a few in-
dividual components.
Significant improvements in the energy density of storage bat-
teries have been made in recent years. Further improvement may
prove to be difficult, and there exists doubt that they can meet the
requirements of electric vehicles. Because of the physics of the
problem, we know that frequent acceleration, gradiant climbing,
and high-speed driving place a heavy burden on batteries, reducing
the range of the vehicle.
Because of the basic limitations of existing battery systems, a
number of companies are conducting research on new systems.
Several of these designs show considerable promise, but as yet it
is not clear whether or not they would be desirable for electric
HELLER
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automobiles. All of these new systems are in the developmental
stage, and will require between 5 and 10 years of additional de-
velopmental work and testing before they can be shaped into an ac-
ceptable energy source for electric vehicles. The most essential com-
ponent of an electrically powered vehicle is the battery, and it is
confronted with serious developmental problems. Consequently,
until a battery with suitable characteristics is developed, there is
little incentive to attempt development of a practical electrical car.
Until now, most of the road testing for electric vehicles has
taken place in converted gasoline-powered cars. These internal com-
bustion cars, stripped of their engines and adapted to operate with
rechargeable batteries, are not an ideal medium for testing electric
car performance. A specially designed "electric car" should eliminate
all components not specifically needed, reduce the weight of other
parts, and allocate a much greater share of the weight to the motor,
the batteries, and other essential electrical equipment.
Certain steps must be taken if electric powered vehicles should
become a reality, and here I am not referring to that old saw about
a new market for long extension cords. Nationally, it would require
retooling by tens of thousands of small service stations to meet the
energy needs of the vehicles and to adapt to the radically different
type of engines.
The market potential of an electric car is probably one-third
of the automotive market, based on utilization of the car as a second
car or one used primarily for short-distance urban driving. I should
like to point out that we in air pollution control have a great interest
in even one-third reduction of automotive pollution.
The use of the electric car as soon as one becomes feasible would
have immediate effects on the carbon monoxide and hydrocarbon
emission problems of our major cities. I am particularly interested
in the adaptation of the electric car for use as passenger taxicabs.
In New York City, taxis account for about 50 percent of all mileage
driven, and most of this driving is done in highly congested areas.
The introduction of this vehicle would therefore result in a 50 per-
cent reduction of automotive pollution.
FUEL CELLS
The development of fuel cells for energy is now new. Like all
inventions, the spirit of the age dictates whether or not the inven-
tion is to be used. For example, the telephone was invented in
Austria in 1867, but the need for it was not seen until it was in-
vented a second time, in 1876 by Bell. Movable type, was invented
by Pi-Cheng in China in 1041, but it remained for Gutenberg to
re-invent and utilize movable type in the mid-Fifteenth century
before it took hold.
The fuel cell was invented in England in 1839, but it waited for
the development of the NASA space program to provide the first
10 Alternatives to the Gasoline Engine
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practical application. Fuel cells have a number of advantages: high
theoretical efficiency, relatively noiseless operation, and most im-
portant from my standpoint, they are relatively free of pollution.
Several major manufacturers have built and tested vehicles
powered by fuel cells and have demonstrated that fuel cell tech-
nology has progressed to a point where a high-output vehicle power
plant is technically feasible. As with most good things, though,
there are problems that must be conquered. At present, fuel cells
have a heavy weight, large volume, and a short lifetime, and they
utilize costly components. They depend on a complicated start-up
and shutdown procedure, and there are safety problems involving
high voltages and electrolytic leaks.
The costs of the fuel cells today are certainly not competitive
with the costs of gasoline-powered engines. However, should it ever
come to pass, the economics of mass production is expected to result
in a sharp reduction in overall cost. The maintenance costs for fuel
cells are completely unknown, and very little indeed is known about
the longevity and reliability of complete fuel cell systems.
The commercial possibilities for fuel cells being utilized in
vehicle propulsion are good. How quickly and how completely they
will be realized depends on the speed and ingenuity with which we
can overcome major problems that now prevent fuel cells from
competing with conventional power plants.
I question whether the public will pay exhorbitant prices for
air pollution attachments or pollution-free cars. If we can develop
a pollution-free car that is also a better car and can be economically
produced so as not to be priced out of its market, this car will be
a prototype that can enter mass production immediately.
I encourage the research and development of such cars, but I
also ask that you give consideration to the marketing factors as well,
for if we cannot sell it to the public, it is all for naught.
It is evident to me, as I imagine it is to you, that it is safe to
assume that full-scale application of alternatives to the gasoline-
powered automobile are between 5 and 10 years away, depending
upon the direction we finally take. What do we in air pollution con-
trol do in this interim? It is necessary to take positive action, because
to do nothing is not merely to stand still, but to double and triple
the problem for the next generation of planners.
I suggest that we concentrate in these next few years on two
key programs: alteration of the urban traffic pattern, and an ac-
celerated program to reduce emissions from certain critically im-
portant segments of the traffic load — such segments as taxicabs
and buses, for example.
To give you just one idea of how altering the traffic pattern
can result in a decrease of pollution, I would like to cite some data
from the New York City Department of Traffic.
HELLER 11
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During peak hours of traffic, we know that there is concomitant
traffic congestion. Increased traffic congestion means increased
carbon monoxide and hydrocarbon emissions.
Several years ago, when Fifth Avenue and Madison Avenue
were two-way thoroughfares, the City Department of Traffic calcu-
lated that during peak hours of traffic, the average automobile
traveled at 3.5 miles an hour. The changeover of these two traffic
arteries to one-way streets resulted in an increase to nearly 18
miles per hour.
An increased emphasis on the development of rapid, convenient,
and attractive mass transit systems, similar to those now being
operated in Japan, may result in a decrease in the anticipated rise
of automobile congestion in the metropolitan area, and therefore a
reduction in the anticipated rise in automotive pollution.
Similarly, serious study should be given to the possibility of
using mini-cabs, because of their lower exhaust volumes; or of using
lead-free gasoline for cabs so that they may be re-equipped with
catalytic mufflers without fear that the catalyst will be ruined by
lead deposits. Catalytic mufflers are presently being tested on 10
new Transit Authority diesel buses.
I take heart from the progress and achievements we report
in the following pages. Knowing the problems and having seen
limited success, I hope that your inborn scientific inquisitiveness
asks you, as do I, for an immediate acceleration into the research and
development of battery systems, fuel cells, and gas turbines.
When John Kennedy was President of the United States, I had
the good fortune to be associated with the Federal Government.
During a meeting that took place at the time, a task force report
was submitted to the President on a vital subject. The Chairman
paused to direct a question to the Chief Executive: "Mr. President,"
the Chairman said, "Do you believe in the power of an idea whose
time has come?" The President rose and answered, "Indeed I do."
I submit that air pollution control is an idea whose time has
come, and the next few years will demonstrate the power of that
idea. We in air pollution control cannot wait for the development
of the pollution-free car; we must reduce pollution and we must
reduce it now.
12 Alternatives to the Gasoline Engine
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IMPACT OF ELECTRIC VEHICLES
ON URBAN PROBLEMS
Clark Henderson
Stanford Research Institute
Menlo Park, Calif.
BACKGROUND
Electric vehicles have already had a major impact on urban
development. Electric rail cars and electric elevators, both intro-
duced in the late 1880s, were the most important factors in urban
transportation for at least 40 years. Their influence on the pattern
of urban development is clear. For example, central business districts,
such as the Chicago Loop, developed rapidly as old six-story build-
ings were torn down and replaced by taller buildings with elevators.
Much of the growth in urban population was accommodated in sub-
urban communities, which developed in radial patterns along the
rail lines extending from the central business districts.
The elevator, of course, has continued its important role. Wilfred
Owen reports that there are 45,000 elevators in New York City and
that the total length of elevator shafts is equal to the total length of
subway tracks. In fact, some people ride greater distances in eleva-
tors each day than they travel on the ground.*
The electric rail car has not fared as well. Streetcar systems have
almost disappeared from U.S. cities. Rail mass transit systems con-
tinue to play an important part in some large cities; and a few new
systems, such as BART in the San Francisco area, are either being
constructed or planned.
Electrically powered trolley buses are attractive because they
are quiet and produce no exhaust, but they have never been exten-
sively used because of their need for an overhead power supply and
because of the high labor costs associated with their operation. Elec-
tric automobiles, which were introduced somewhat later than rail
cars and elevators, had a relatively small impact on urban develop-
ment.
Gasoline-powered automobiles and buses began to dominate
urban development in the late 1920s. The influence of the bus has
declined, but that of the automobile has increased tremendously
through the years. Today the automobile is the most important
mode of transportation in almost all urban areas and is certainly
the dominant influence in urban development.
* See Wilfred Owen, The Metropolitan Transportation Problem, Anchor
Books edition, 1966.
HENDERSON 13
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NEED FOR IMPROVEMENTS
At present, we have many reasons to be discontented with the
transportation services available in urban areas and with the systems
that are used. Public transportation services are declining even
though urban populations are growing, and many people who do not
have access to an automobile suffer inconvenience and hardship.
Emissions from buses, automobiles, and service stations add greatly
to air pollution problems. Automobiles not only create noise and
traffic congestion, but they also require unsightly facilities and are
costly and unsafe. Automobiles are also charged with encouraging
the unattractive pattern of development called "urban sprawl."
A number of different courses can be followed to improve urban
transportation services. Continued development of new rail systems
and new highways will undoubtedly provide some relief, but we can-
not expect these systems, even on a much larger scale, to correct all
present deficiencies while meeting growing needs. It has also been
suggested that much of the problem might be solved by improving
the design of cities: old cities could be reconstructed and new cities
could be built with purpose of reducing the need for automobiles
and emphasizing pedestrian travel and public transportation.
NEW SYSTEMS
A third approach, which interests me greatly, would be to
develop entirely new transportation systems for use in both existing
and future cities. The need for new systems has been recognized in
many quarters, and considerable research is being sponsored by
federal agencies with the hope that new transportation systems can
be developed. An especially noteworthy program, sponsored by the
Department of Housing and Urban Development, seeks to plan re-
search, development, and demonstration programs for breakthroughs
in urban transportation. We at SRI have recently begun one of the
research projects in that program.
We have been impressed with the number and variety of trans-
portation concepts that have been put forward in recent years. We
have encountered several dozen different suggestions and proposals.
However, only a few of the inventors and promoters have actually
constructed and demonstrated their systems or even their key com-
ponents. Apparently, most of the concepts have yet to be fully
worked out on paper.
Considerable work must be done before any new urban trans-
portation systems are completely developed and ready for service.
We have little doubt that a number of new systems are technically
feasible, but it is not yet clear whether they can be made attractive
on economic, social, and political grounds. However, if all goes well
a variety of new systems may be in service within 5 to 15 years
As it happens, most of the new system concepts call for electrical
14 Impact on Urban Problems
-------
power rather than for internal combustion engines. Consequently,
if new systems are developed from current research efforts, we may
enter a second "golden era" of electric vehicles.
Let us consider several of these electric vehicles and some of
their possible effects on urban problems.
LARGE ELECTRIC VEHICLES
First, assume that it is possible to replace conventional auto-
mobiles, trucks, and buses with battery-powered vehicles capable
of providing the same services at the same costs. It seems likely that
the most important result of this change would be the elimination of
air pollution caused by internal combustion engines and fuel evapo-
ration. This gain might be partially offset by the emissions from
electrical power-generating plants, but that problem can be avoided
by locating steam plants outside urban areas or by using nuclear
power.
Electric vehicles may also be somewhat quieter than conven-
tional vehicles because engine noises will be reduced; however, other
disadvantages of the automobile will not necessarily be overcome.
Large electric vehicles will still cause congestion; they will continue
to be involved in accidents; and they will use unsightly roads, bridges,
parking lots, and other facilities.
The function and characteristics of service stations might change.
Batteries would ordinarily be recharged at home, and visits to local
service stations would therefore be less frequent. However, when
batteries had to be charged at a service station, the traveler would
be delayed long enough that he might want to use the time for a
meal, entertainment or shopping. Thus, service stations could become
less numerous, but they might compensate by offering more services
and they might be located according to new criteria.
The use of thousands of large-capacity battery chargers in resi-
dential districts might require additional distribution capacity in
excess of the off-peak capacity that is already available during the
night. If so, these requirements may help to accelerate programs to
replace unsightly overhead distribution lines with underground
facilities.
The use of battery-powered vehicles might tend to encourage
the use of tunnels, underground garages and stations, and other
enclosed spaces where conventional vehicles cause ventilation
problems.
One can also visualize reasonably simple schemes for low-speed
movements of empty vehicles. Thus, the adoption of large electric
vehicles would probably encourage the development of automatic
or remotely controlled parking facilities in central business districts
and at other congested areas, such as airports.
HENDERSON 15
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These advantages — except for reduced air pollution — do not
appear to be compelling. Consequently, if we want car owners to
replace conventional vehicles with battery-powered vehicles volun-
tarily, the new vehicles should be attractive in terms of improved
service to the owner and lower costs. It may be many years before
such vehicles are available.
SMALL ELECTRIC CARS
In the meantime, there are other near-term possibilities for elec-
tric automobiles. One of these is to find new ways to use the low-
performance vehicles that can be produced now. I am thinking of
vehicles with speed and range characteristics more like golf carts
than like automobiles. I am also thinking of their use in low-density
residential areas where public transportation is now almost non-
existent. Vehicles of this kind could be used to carry commuters to
and from their stations and for other services entailing less than,
say, 3 to 5 miles of travel.
The small electric cars could be privately owned. I'm told that
several thousand such vehicles are in use today in Long Beach,
California, primarily by elderly people. It may be more desirable
for a public agency or corporation to furnish vehicles as a public
service to an entire community. If this were done, the agency would
own, maintain, and service the vehicles; collect fares or rentals; and
attend to the storage and redistribution of idle vehicles.
Systems of this kind have not been developed, so comments
about their value are somewhat speculative. It appears, however,
that such systems would offer a number of advantages.
Small electric cars would be hired by users on a variety of terms
—by the month, by the day, or by the ride. The vehicles would be
managed as a common pool rather than on individual assignments,
i.e., a commuter would turn in his vehicle each morning and would
receive a different one at night. Vehicles would be redistributed and
reused to reduce the requirement for cars, to minimize the need for
parking facilities, and to make cars readily accessible to all qualified
users. Thus, many travelers who do not have regular access to a
private automobile could be given prompt and convenient service.
The vehicles would be small and highly standardized, and there-
fore would probably be less expensive than conventional automobiles
to purchase and operate. If a high rate of utilization could be
achieved, the cost per ride should also be low.
The vehicle would have simple controls and would operate at
speeds up to about 25 mph. With additional speed restrictions to, say,
15 mph, the vehicles might also be used safely by children and handi-
capped people who are not qualified to operate conventional auto-
mobiles.
The problems of using small electric cars are currently being
investigated. It appears that storage, redistribution, and manage-
16 Impact on Urban Problems
-------
ment present the greatest difficulties. Vehicle design would not be
a serious obstacle, but special efforts would be needed to minimize
costs.
AUTOMATIC CARS
Now let me turn to another family of electric vehicles that may
also have a major impact on urban problems. I will use the term
"automatic cars" as the name for a group of systems that call for the
operation of relatively small vehicles on exclusive rights-of-way
under automatic controls. Examples of such systems are the Westing-
house Transit Expressway and the Stevens-Adamson Carveyor. At
least a dozen such systems have been proposed within the last decade.
These systems would have fixed routes and would require ex-
clusive rights-of-way similar to those of modern rail transit systems,
but the structures would be much less ponderous. Automatic cars
would be much smaller than rail cars and would not, in most cases,
operate in trains. Automatic car systems of many kinds can be en-
visioned. The systems could vary greatly in features such as speed,
frequency of service, capacity, network configurations, and stations.
One of the simplest automatic car systems was the automated shuttle
train considered some years ago for service between Times Square
and Grand Central Station. Automatic cars in that service would
require low speed, frequent departures, and high capacity. Service
would be provided on a single, short route with a station at each
end. Another group of systems has been proposed to link airports
with central business districts. These systems require high speed,
low frequency service, and low capacity. A trunk line would connect
the two areas, and two or more stations could be provided at each
end of the route. A third group of automatic car systems has been
proposed for circulation in central business districts. Such systems
would have low speed, frequent service, and high capacity. The
network of routes would be complex, with many links and stations.
High standards of safety should be attained, and the noise and
aesthetic problems of rail systems would be partly avoided.
Automatic car systems would be permanent installations but
would provide considerable flexibility in design and planning. Sys-
tems could be installed piecemeal; they could be tailored to match
varying requirements; and they could evolve to satisfy changing
needs.
Such systems have one major limitation — they cannot provide
door-to-door service throughout the entire urban complex. The
stations would have to be widely separated in residential areas
where transportation demands are low and irregular. However,
other schemes, such as the small electric cars described above, could
be used to supply this service.
DUAL-MODE TRANSPORTATION SYSTEMS
Dual-mode transportation systems represent still another ap-
proach to the exploitation of electric vehicles. In recent years, several
HENDERSON 17
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proposals have been advanced for the development of such systems.
The proposals call for small electric vehicles capable of operating
on two different types of roadway. In one mode, they operate under
the driver's control on city streets with battery power (in the same
manner as the small electric cars described above) at about 25 mph.
In the second mode, they operate under automatic control on an ex-
clusive right-of-way at about 60 mph, using power drawn from an
electrical distribution system in the roadway. The cars would change
from one mode to the other at entry and exit ramps in much the
same way as automobiles enter and leave freeways.
Again it must be emphasized that these systems have not been
developed and cannot be evaluated at present. It appears that the
dual-mode system would combine most of the advantages of the
small electric cars and the automatic car systems previously de-
scribed. It would be somewhat less flexible in installation than the
automatic systems but would eliminate the need for some of the
transfers that passengers would have to make with automatic car
systems.
High standards of safety can be provided on the exclusive right-
of-way. The roadways for automatic operation would ordinarily be
elevated or underground. Roadways would require less land than
freeways for equal volumes of traffic. Driverless operation on the
exclusive right-of-way would be used to redistribute empty vehicles.
Automatic parking and the pooling of vehicles might eliminate most
of the parking problems in congested areas.
The dual-mode systems are relatively complex. They will be
more difficult to develop than small electric cars and automatic cars
and may be more costly as well.
SUMMARY
Large electric vehicles would reduce air pollution but would
not necessarily make other contributions to the solution of urban
problems.
Small electric cars suitable for use in low-density traffic areas
could provide one essential stage of an area-wide public transporta-
tion service, and automatic car systems could extend the public serv-
ice throughout the urban area. Technically, these two systems appear
to have the potential for improving transportation services to most
of the urban population within a relatively short time.
Dual-mode systems combine most of the advantages of the small
car systems and automatic car systems but, because of their com-
plexity, may not be available for many years and may be more
costly than the other two alternatives.
Economic, social, and political questions have not yet been re-
solved for any of these advanced systems.
18 Impact on Urban Problems
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IMPACT OF ELECTRIC POWER SYSTEMS
ON URBAN TRAFFIC FLOW, CONTROL,
AND FACILITIES
Walter Helly
The Polytechnic Institute of Brooklyn
Brooklyn, N. Y.
The transition to electric propulsion for automobiles is viewed
primarily as a means to achieve significant reductions in air pollu-
tion and other urban irritants. Though these environmental con-
siderations are paramount, some attention should be given to the
perhaps less dramatic effects on the transportation system itself.
It will be argued here that these are fairly complicated and that
some are not altogether beneficial.
AUTOMOTIVE TRANSPORT TODAY
The vehicles used include trucks for local delivery and for
longer trips, taxis and buses for transit, and private cars for
commuting, shopping, personal travel, and business. All vehicles
have a range of 200 to 300 miles between quick refuelings. However,
they differ significantly in performance. The normal full-size Ameri-
can car can accelerate from 0 to 60 mph in less than 15 seconds,
and from 30 to 50 mph in about 5 seconds. It can climb a 5
percent grade (about the steepest experienced often on streets
designed for heavy traffic) at a speed greater than any present
or contemplated speed limit. In contrast, a Volkswagen acceler-
ates from 0 to 60 mph in about 27 seconds, from 30 to 50 mph in
9 seconds. Its top speed on a 5 percent grade is about 55 mph.
A heavily laden trailer truck may take more than 100 seconds to
accelerate from 0 to 60 mph, and may be unable to go at more
than 10 to 15 mph on a 5 percent grade.
Such a wide disparity in performance is harmful to traffic
flow capacity and to safety. Capacity is lowered by vehicles
that cannot maintain optimum flow speeds. For example, in the
Lincoln and Holland Tunnels under the Hudson River, the peak
flows occur at about 20 miles per hour. Some trucks cannot main-
tain this speed on the upgrades and so effect a serious reduction
in the number of vehicles these tunnels can serve.
Safety is affected adversely when vehicles of widely different
acceleration capabilities are fairly closely spaced at high speed on
expressways. These vehicles with low maximum acceleration are
forced to oscillate through a relatively dangerous wide range of
velocities if they consistently attempt to achieve and maintain
HELLY 19
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"optimally close" headways in a stream where the majority of
vehicles are very powerful performers (Helly, 1959). Further, the
underpowered vehicles face difficulties in merging with high-speed
expressway traffic. It may be argued that safety considerations for
high-speed roads are not important in congested urban centers
where average effective speeds may be below 10 miles per hour.
However, one must remain aware of the fact that today a very
high proportion of urban trips take place partially on sections of
expressways with free flow and speeds of 50 mph or more.
Because of the wide range in size and performance of present
vehicles, our road system is generally designed in a very generous
manner to provide for everyone. Lanes are wide, and where high
speeds are possible and legal, every effort is made to bank curves
for such speeds and to provide long acceleration lanes for merging
by relatively low-powered vehicles. In signalized grids, the timing
of light cycles is geared to the capabilities of all but the most
sluggish participants.
There is one important property of present day automotive
traffic that is present for every type of vehicle and on every road
or street. That is the very high probability of propulsion failure.
A typical vehicle may experience about one unexpected stoppage
per 10,000-mile-year as a result of running out of fuel, mechanical
malfunction, or tire failure. This may not appear to be very large
until one considers that a three-lane flow on a 5-mile-long section
of expressway produces about 25,000 vehicle-miles in a 1-hour
peak period. This would correspond to an expected average of
about 21/2 stoppages. Because a high proportion of stoppages occur
in the main traffic lanes before the affected vehicles can get off,
the effect on flow capacity is disastrous: it is part of our common
experience that the typical commuter inches his way past such a
bottleneck at least once daily!
To provide succor to the stranded and to refuel all vehicles,
the present transport system includes a tremendous network of
service stations. In 1963 the United States contained 211,000 of
these, employing 730,000 persons including proprietors, with gross
annual sales of 17.8 billion dollars. These figures are for service
stations only; they do not include dealers or repair garages.
Because range limitations are a leading obstacle to acceptance
of electric vehicles at today's stage of development, it would be most
desirable to obtain the distribution of daily travel distances for
various classes of vehicles. Unfortunately, there is little data at
hand. One would suppose—hopefully—that the urban vehicle needs
a smaller range than the national average. Unfortunately, the
evidence does not support this premise. A Regional Plan Association
of New York study showed that 72 percent of Manhattan vehicles
travel more than 50 miles per day. This is definitely above the
national average, which must be based on about 10,000 miles per
20 Impact on Urban Traffic
-------
vehicle per year. Manhattan, like other city centers, has an unusually
high proportion of traffic in the form of high-mileage buses, taxis,
and commercial vehicles. Available estimates show that, in New
York, the average taxi goes about 100 miles per nominal 8-hour
shift (a little over 200 miles per day) and the average bus goes
about 100 miles per day.
Direct fuel costs may be very important in the selection of
power plants for commercial vehicles. There are at present about
40,000 electric bread and milk trucks for local deliveries in Great
Britain. These are used, despite the inconvenience of a 30-mile
range and a 20-mph top speed, because they save money in a
country where off-peak electric power is especially cheap and where
gasoline is taxed much more highly than in the United States.
THE TRANSITIONAL PHASE
The English experience with electric delivery vehicles suggests
that the present state of development is marginally sufficient for
such vehicles in urban areas. However, a somewhat higher per-
formance will be required for passenger vehicles. In the transitional
phase, where electric cars must survive in an environment of
high-powered internal combustion competition on expressways as
well as on congested city streets, the minimum acceptable per-
formance specifications might be:
1. Acceleration from 30 to 50 mph in 10 seconds. To achieve this,
a 3,000-pound vehicle requires about 25 horsepower more
than needed to cruise at a steady 50 mph.
2. A top speed of 50 mph on a 5 percent grade. To achieve this,
a 3,000-pound vehicle requires about 20 horsepower above that
needed to cruise at a steady 50 mph on a level road.
3. A range of 50 to 75 miles.
Though these specifications are offered rather arbitrarily, they
represent about the minimum performance for safe operation on
expressways, for minimum degradation of flow, and for a reasonably
significant fraction of the range demand distribution. They would
satisfy many commuters and shoppers. However, unless very rapid
recharges or battery exchanges are developed, taxis would necessarily
continue to use internal combustion engines.
To summarize, early and relatively primitive electric vehicles
would be acceptable for local delivery trucks and to some commuters.
These vehicles must offer performances at the lower end of the
present day spectrum, or better. Because roads and control systems
have generally been designed for present vehicles of all types, such
an introduction of electric cars would not raise new problems in
traffic management.
HELLY 21
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THE FULLY ELECTRIFIED ERA
When reasonably priced batteries or fuel cells are developed
to a stage matching present automobile capabilities, there should
be a fairly rapid and widespread transition to electric propulsion.
The transition may well be accelerated by governmental incentives
or selective taxation, in view of the environmental advantages to
be gained.
From a traffic viewpoint, the advantages of such a transition are:
1. The flow capacity of the street network will be improved. A
small improvement may be expected from greater uniformity in
vehicle responses to driver inputs. The major gain will come
from a significant reduction in unexpected vehicle stoppages
that delay heavy traffic. The simple electric power plant, with
very few moving parts, is very unlikely to expire unexpectedly.
Further, there should be no sudden equivalent to an empty gas
tank. A fuel cell or battery at the end of its range will still
give some power, enough for a vehicle to remove itself from
interference with the traffic stream.
2. Safety will be improved. It will be possible to give all vehicles
flexible and smooth performance characteristics within a fairly
close range, thus minimizing the buildup of irregularities and
shock waves in traffic. Braking, if in part by motor induction,
will be simpler and more reliable. There will be less danger
of fire and no danger of carbon monoxide poisoning from leaky
exhaust systems.
3. External controls, such as automatic roadways, will require less
instrumentation than for internal combustion power plants.
4. The cost of underground roadways will be reduced by up to
50 percent because of smaller ventilation requirements.
Some aspects of the transportation system will be largely
unaffected by the transition because there will still be old vehicles
and because there will still be a wide variation in vehicle size and
in driver performance. These aspects include the geometric design,
construction, surfacing, lighting, signing, and control of the road
network. Parking will be affected only insomuch as there may be
reduced requirements for garage ventilation and for fireproofing.
One facet of the transition is likely to raise serious problems.
It certainly is most attractive to the motorist to experience fewer
mechanical failures and to be able to refuel at night by tapping
any electric outlet. The gasoline station operator is not likely to
be as favorably impressed. One may anticipate that a general
transition to electric power would ultimately force the closing of a
great majority of the country's 211,000 service stations. The direct
economic and social consequences to these businesses and their
employees can be weathered, just as with other major technological
22 Impact on Urban Traffic
-------
dislocations in the past (but one must be prepared for substantial
political difficulties and a certain amount of government spending
in relief of this declining industry). There is another problem,
more important in the long run. The many service stations today
get their primary income from fuel sales and from routine main-
tenance. Yet, from the motorists' viewpoint, the stations' ubiquity
and long open hours are at least as important for emergency assis-
tance, for rest rooms, and for communications. If the great majority
of service stations were to close, the government would have to
assume a major new responsibility for the nation's 3 million miles
of roadway. A precedent for government-organized service exists
on some toll and interstate roads. Experience with these leads one
to suspect that vast improvements would be demanded if such
arrangements become more widespread.
A possible amelioration of this problem is to continue to fuel
vehicles at service stations by using interchangeable fuel cells and
specially designed hoists. There are other advantages to such a
system. The problem of the long-distance motorist would be solved
completely. A very large proportion of urban cars today are stored
on the streets, not in garages. If this were to be the case in the
future, these vehicles could readily be fueled with interchangeable
cells; it would be most awkward for them to require lengthy
overnight curbside access to electric power outlets. Further, if
the fuel cells are very expensive and have a useful life longer
than that of the typical vehicle, the motorists would find it advan-
tageous to rent rather than own them.
REFERENCE
Helly, W., 1959, Dynamics of single lane vehicular traffic flow.
Research Report No. 2. Center for Operations Research. M.I.T.,
Cambridge, Mass. pp. 45-49.
HELLY 23
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TYPES OF POWER SOURCES: reviews
of system developments, including
several kinds of hybrids.
Electric power -for automobiles came long before piston engines.
The first electrically propelled vehicle on record apparently was that
of Davenport, a Vermont blacksmith who constructed an electrically
powered car in 1834 and ran it on a short stretch of track. Davidson,
a Scotsman living in Aberdeen, built a more sophisticated electrically
propelled vehicle in 1837. This was only 44 years after Volta's first
primary battery, and but 7 years after the principles of the first elec-
tric motor had been demonstrated by Negro at the University of
Padua; it is evident that engineers then were as alert as they are
today in picking up new ideas and in doing something about them.
Davidson's vehicle used primary batteries of iron and amalga-
mated zinc plates immersed in sulfuric acid. His motor was crude,
with the rotor a wooden cylinder carrying copper-wire-wound iron
bars between the poles of two electromagnets, with commutating
switches mounted on the axle. Basically, it was the same as traction
motors used today. His system developed 2 horsepower and was able
to pull a 6-ton coach at 4 mph on wooden rails.
In 1851, a similar, but larger, vehicle was built in the United
States by Page. His vehicle developed 16 horsepower and ran at
speeds up to 19 mph, but the severe jolting over track joints in short
order ruined the Grove primary cells he used.
25
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The rapid development of steam during this period diverted at-
tention from battery-powered vehicles, and not until 1888 did the
first real electric "automobile" make its appearance, almost simul-
taneously in England and in the United States. By 1903, the electric
"Torpedo" built by Walter Baker had achieved a speed of 120 mph.
The trend though was largely to luxury cars for wealthy old ladies,
each automobile complete with velvet upholstery and a cut-glass
vase to hold fresh flowers. Over the next 25 years, many versions of
electric automobiles were built. But with the great developments in
piston engines, including invention of the self starter, the electric car
was gradually replaced and few were built after about 1930.
Now, the pendulum swings back. Once more electric automobiles
are in the public eye, thanks mainly to the fact that they do not emit
air pollutants. Electric automobiles offer other advantages for urban
transportation: they can be small for crowded city streets, nearly
noiseless for residential areas, and extraordinarily convenient for
stop-and-go driving. It is unlikely that electric automobiles will
ever compete performance-wise with piston-engined cars, and their
range is always likely to be limited to 100 miles or so before recharg-
ing or refueling is required. Hence, in the future, electric automobiles
seem most likely to move us about within cities, with high-powered,
high-speed, massive piston-engined cars still being used between
cities and for general-purpose driving.
William T. Reid
Battelle Memorial Institute
26
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HYBRID POWER SYSTEMS FOR VEHICLES
George A. Hoffman
University of California
Los Angeles, Calif.
INTRODUCTION
Air pollution has reached catastrophic proportions in many
metropolitan areas of the United States. Much of the contamination
in the urban air originates from our essential travel by automobile,
bus, and truck. Engine exhausts account for over half of the be-
fouling of the atmosphere in New York City and for over three-
quarters of the smog plaguing Los Angeles.
It is becoming alarmingly clear that present attempts at con-
trolling and abating the noxious exhausts of the piston engine are
too feeble and inadequate. In his message to Congress Jan. 30, 1967,
President Johnson said:
The sheer number of motor vehicles may, within a
decade or two, defy the best pollution control methods we
can develop. If this proves true, surely we cannot con-
tinue to use the type of internal combustion engine now in
service. New types of internal combustion engines — or
indeed new propulsion systems — may be required.
I recommend an increase of 50 percent in funds to
expand our research efforts.
A steep escalation of the war on air pollution must be under-
taken soon if urban America is to avoid inundation and suffocation
by the effluvium from our transportation. Significant reduction of
smog, possibly leading to its elimination, can be envisioned through
two distinct approaches to automotive design.
The first approach, illustrated in Figure 1, hypothetically con-
sists of an elaborate array of chemical reactants and scrubbing equip-
ment, occupying most of the space in the trunk compartment of a
family car and estimated to cost upward of $500 per vehicle. Pure
and breathable air could flow from the final exhaust pipe of this
imaginary apparatus. But the accumulated tankage of absorbed
products of combustion would require emptying and replenishing
every time one pulls into the corner gas station to fill up the empty
tank, and probably at a cost comparable to that of the fuel pur-
chased. The logistics and diseconomies of this design approach for
pollution abatement by total exhaust suppression appear enormous.
The costs and intricacies of full exhaust absorption will undoubtedly
cause the American public to resist strongly the mass adoption of
this scheme for air purification.
HOFFMAN 27
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A: UNIT TO CONVERT CO AND HYDROCARBONS TO C02
AND H,O
B: AQUEOUS SCRUBBER AND NOX, C0.2 ABSORBER
E: EMPTYING OUTLET
F: FILLING INLET
Figure 1. Total exhaust controls for piston-engine cars.
The other widely discussed alternative for eliminating auto-
motive air pollution on a national scale is the early and compre-
hensive substitution of electric-battery-operated automobiles for
conventional cars (Hoffman, 1966). But this candidate for cleaning
up the urban air also must surmount some formidable obstacles. The
concept of phasing out, scrapping, or outlawing a hundred million
vehicles powered by internal combustion engines in the United States,
and making the transition to a comparable number of electric vehicles
in a dozen years or two could easily dismay the hardiest proponent.
The appalling obstacles yet to be surmounted in the design of a viable
all-electric vehicle are also not to be dismissed too lightly: to this
day we lack even small fleets of demonstration prototypes, let alone
a single vehicle (as in Figure 2) at reasonable cost, range or per-
formance. Batteries exceeding the critical level of 50 watt-hours
per pound are still in the laboratory stage.
C: CONTROLS M: MOTORS
Figure 2. All-electric battery-operated automobile.
28
Hybrid Power Systems
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Therefore comes our pessimism concerning near-term solutions
for enhancing the quality of urban air through radical automotive re-
design. In spite of the stated willingness of many to switch im-
mediately to battery-operated cars, or of the certainty that com-
plete exhaust chemoabsorption is technically feasible, neither of these
re-engineered vehicles could arrive cheaply enough upon the auto-
motive scene by 1980 to offer a modicum of relief in air pollution
levels.
A third possibility for significantly reducing smog is reviewed
in this paper: hybrid power systems for passenger cars, buses, and
trucks. A hybrid is defined as the enhanced offspring of the union
and cross-breeding of the mature form of a male plant species (in
this case the internal combustion engine) with the equally mature
form of the female of another species (the electric motor generator).
The power package proposed for consideration here is a transitional
intermediate between the all-electric and the all-combustive power
supply. as shown in Figure 3. The hybrid system consists of an
internal combustion engine of rating much reduced from the con-
ventional motor that it supplants driving at its optimal speed a gen-
erator. The generated electric power goes to the traction motors
(integral with all the wheels) and to a sizeable secondary battery,
also connected with the four electric motors. (The suggestion of a
traction motor in each wheel of the vehicle is only illustrative. Early
designers of hybrids might find it expeditious and simplest to use a
single motor driving two wheels, rear and front, through a differ-
ential and axle. This would obviate the problems of greatly increased
unsprung weight that will eventually arise from installing motors
within the wheel rims.) Solid-state controls divert power from both
the generator and the battery to the electric motors when the driver's
power demand exceeds the capability of the constant-speed engine.
When the accelerator pedal demands less power than the internal
combustion engine delivers continuously, the controls divert some
power to the wheels and some to recharge the battery. If the battery
is topped-off, a progressively lesser amount of fuel is metered out to
the internal combustion engine.
Replacement of the piston-engine under the hood of today's
vehicle by hybrid power sources is the topic of this paper, and its
objective is to show the advantages of this substitution in terms of
performance, economy, volumetrics. planning, and environment.
These advantages have led most Detroit manufacturers to become
actively interested in the hybrid concept.
The idea of hybrids occurred to automotive designers a genera-
tion ago, though the earliest literature dates back only to the mid-
1950's. More recently. Prof. J. L. Shapiro of the California Institute
of Technology and Dr. A. L. Stanly of TRW Systems, Inc.. studied
this concept and brought it vividly to the author's attention in 1966.
Prof. Shapiro dubbed it the "gasoline-electric" automobile, whereas
Dr. Stanly called it the "quasi-electric" economy car, certainly as
appropriate an appellative as our "hybrid" terminology. Dr. G. R.
HOFFMAN 29
-------
B: BATTERY C: CONTROLS
E: INTERNAL COMBUSTION ENGINE M: TRACTION MOTOR(S)
G: GENERATOR
Figure 3. Schematic of hybrid power systems for
cars, trucks, and buses.
Frysinger of the U. S. Army has also aptly used the noun "hybrid"
in connection with a power system analogous to the one in this study,
but with the internal combustion engine replaced by a non-regenera-
tive hydrocarbon fuel cell (Frysinger, 1966). Not to be outdone by
all this activity in the United States, A. P. Zobov in the U.S.S.R. has
also engineered a hybrid system by modifying a Moskvich sedan. In
England, Jack Brabham, of Grand Prix fame, proposed a battery-
powered passenger car with a gasoline engine for charging and
occasional peak power.
WHY HYBRID POWER SYSTEMS?
The negative or detractive reasons urging us toward the design
of hybrids stem from two sources. First, vehicles operated entirely
by battery will not exist in large numbers for dozen of years to
come, and even then another decade or two must pass before a dent
can be made in the soaring pollution rate. Things look pretty grim
for all-electrics until the end of this century. On the other hand,
tacking thorough exhaust purifiers onto present-day engines and
vehicles is probably an insufferably expensive complexity that might
be shunned by most of the motoring public.
The contributive or positive reasons for developing hybrids are
threefold: first, they present an early capability for reducing exhaust
pollution substantially by use of existing automotive technology;
second, hybrids promise a perceptible lowering of operating and
fuel costs; third, many beneficial attributes emerge when planning
the transition to an all-electric 21st-century America.
The hybrid's exhaust would be proportionately less polluting
than its conventional counterpart, for these reasons:
1. In the first-generation hybrid the power output would be about
one-half that of the engine it replaces. (It could be even less
30
Hybrid Power Systems
-------
in later designs.) Expelled contaminants therefore should also be
half of those found in conventional exhaust, if contaminant rate
may be assumed proportional to energy-rate as a first approxi-
mation.
2. The engine-generator set could be run at fairly constant speed,
near the rpm at which minimum specific fuel consumption is
achieved and with fixed-setting carburetion. With the patterns
of fuel consumption, engine rpm, and time at rpm stipulated in
Figure 4, a further pollutant reduction of one-third can be calcu-
lated from just running engines at constant optimum speed with
non-variable carburetion. For example, Figure 4 shows that an
engine dwells only 40 percent of the operating time at its level
of least specific consumption, which occurs at 0.3 to 0.6 of maxi-
mum rpm.
3. Fixed-speed internal combustion engines allow most easily the in-
corporation of regenerative heating of inlet air by heat ex-
changers and recirculation of exhausted gases. The chilled ex-
haust from the exchanger swept turbulently over large catalytic
surfaces, and its partial readmission into the inlet, should lead to
an additional reduction of the unburned hydrocarbons and nitric
oxide produced earlier in the combustion chamber. This reduction
of smog agents, with a well-designed regenerative exchanger,
could amount to another one-half.
A well-engineered hybrid power pack could be one-fourth to
one-eighth as emissive of smog products as its equivalent piston en-
gine if the three effects just cited are properly utilized and com-
bined. Moreover the heat-exchanger surfaces offer a fine oppor-
tunity and base for chemical activation (perhaps in the form of
expanded nickel or graphite coatings) and might also be an excellent
suppressor of most contaminants except CO2.*
The second class of beneficial contributions from hybrid power
systems is lowered operating costs. Without changing the driver's
'•"It is a matter of conjecture how long mankind will consider the massive
and rapidly rising effluvium of COj as innocuous. In the first century after
the birth of industrial civilization, the CO.. content of the atmosphere
rose imperceptibly by a few percent. Since the turn of this century, it
has risen about another 10 percent. It is now increasing at the rate of
about 0.5 to 1 percent per annum. At this rate of accumulation, some
meteorologists claim the re-radiated heat from earth is decreased notice-
ably, and by the end of the 21st century the average surface temperature
of this planet will register a dozen or so degrees higher than it does now.
The polar icecaps will then partially melt, inundating most coastal terri-
tories less than 100 feet above sea level.
I do not necessarily hold to such cataclysmic predictions: methods of power
generation alternative to combustion will be more widely used: and vast
expansion of reforestation, landscaping, and agricultural activities will
absorb much of the added CO.. Fixation of carbon dioxide from exhausts
into disposable carbonates or by manipulated photosynthesis will un-
doubtedly be investigated in the next century, and a workable solution
to the suppression of CO- emissions might be adopted.
HOFFMAN 31
-------
2.0
2 1.5
CL
S
Z>
CO
•z.
O
fj
1.0
CL
CO
3 0.5
UJ
cc
r
~i
i 1
i 1
j r
"RELATIVE SFC =-
"RELATIVE
0.5
UJ
5
0.4 H
_i
_i
UJ
0.3 I
UJ
^
0.2 §
UJ
UJ
0.1 £
0.5 1.0
RELATIVE ENGINE SPEED"
SFC AT rpm AND MAXIMUM TORQUE
MINIMUM SFC
_ rpm AT SPEED
~ rpm EQUIVALENT OF 120-mph ROAD SPEED
c FRACTION OF ENGINE TIME AT rpm IN AN URBAN TRIP
COMPENSATED FOR IDLING TIME AND BRAKING BY
ENGINE DRAG
Figure 4. Fuel consumption, engine speed, and time
at speed for a 3-year-old engine.
power demand for performance, speed, acceleration, and grade
climbing, fuel consumption would be slashed one-third just by the
constant-speed operation of the engine (note the patterns in Figure
4). Cheaper and less-refined fuels than those presently required
for our variable-speed engines can be fed into fixed-speed engines.
32
Hybrid Power Systems
-------
In sum, the fuel costs of hybrids might well be one-half to two-
thirds of those of gasoline for conventional engines for equal auto-
motive missions.
Maintenance costs of hybrids also may be perceptibly lower.
The on-board diagnostics of constant-speed engines can be readily
visualized as a welcome simplification of today's complexities of
dashboard gages, warning lights, and other puzzlers. Compressor,
gasifier, and power vanes are all on the same shaft as the generator's
rotor and at the same rpm. The single rotating part implies much
savings in upkeep. Finally the hard-mounted engine-generator unit
and the nonmechanical power transmission to the wheels allow for
easy powerplant plug-in and replacement, less destructive vibration,
and freer interchange of parts.
The last category of advantages for hybrids is less technical
and more an ease-of-planning benefit. Hybridizing the power systems
of the automobile, the truck, and the bus buys a few decades of
breathing time for the more orderly research and development of
individual travel means. These few decades of respite could permit
a more unhurried evolution of advanced batteries for the all-electric
car and a more gradual introduction of the driving populace to
electric propulsion. The industrial complex would be less dislocated
by the smooth transition of automotive manufacturers from produc-
ing all-combustive power components to the mass-production of
battery-motor systems. Public utilities would have a more gradual
time-table for national rewiring and up-rating of electric distribution
to home garages, parking lots, and service stations and for the electri-
fication of our networks of federal highways and limited-access ex-
pressways.
The future motorist driving a hybrid vehicle might also avoid
for a few more decades the logistics of battery recharging or re-
placement, and might cling sentimentally for a while to the habit
of "filling-her-up-with-gas" every 300 or 400 miles, aside from the
cleaner air and the smaller burden on the pocketbook. And the
quiet hum of the turbine-generator-motor might yet prove a less
offensive sound than the muffled roar of the V8.
Essentially, the hybrids might prove the least traumatic sequen-
tial conversion of the internal combustion engine species of vehicles
dominating the 20th century into the unavoidably electric mainstays
of the 21st. A transition scheme is given in Figure 5.
COMPOSITION OF FIRST-GENERATION HYBRID
POWER SYSTEMS FOR AUTOMOBILES
This section examines a specific family of ground-transporta-
tion vehicles, the family passenger car, as an example of analogous
studies regarding buses and trucks.
I believe that the future electrically propelled vehicle should
be a look-alike, act-alike, simple counterpart of the piston-engined
HOFFMAN 33
-------
YEAR
1960-1970 1970-1980 1980-1990 1990-2000 2000
2050
N
H
ON-ELECTRIC
ENGINE _ i
POWER
BATTERY = Q
POWER
O
^_ LL. LJ
0 H |
,• -\ rn
ELECTRIC
REPLENI:
OUTLETS
z
K£ = 0
o w
LJ W
0 °
cc o:
< LJ
CD c/5
8^J
HYBRIDS
H
V,
Vs
0
»
V4
*
Ve
Vz
%
ALL-ELECTRC
0
1
1
tffv
*
jfl POWER INDUCER
.^Mbw IMBEDDED IN FREEWAY
777W'^|l^v'/AV/'AV PAVEMENT
> . LEGEND
1 I AUTOMOBILE, TRUCK,
ID 9 OR BUS
cm BATTERY
0 ENGINE/GENERATOR
W PISTON ENGINE
Figure 5. Scheme for a transition from non-electric to all-electric vehicles via hybrids.
-------
vehicle with comparable dimensions, performance, and speeds (Hoff-
man, 1967). Apart from changes under the hood, which tend to be
of least concern to most outside observers or to the passenger, I am
firmly convinced that hybrid vehicles must conform to the major
characteristics of conventional cars of today as regards configuration,
power, and spectrum of sizes offered to the public (from small or
compact cars up to large luxury automobiles). This premise defines
the basic composition and design criteria of the first generation of
hybrid-powered automobiles.
The •weight compostion of family passenger cars of the late
1960's, ranging in curb weight from 1,500 pounds (a small car) to
over 5,000 pounds (a large, luxurious one), is uniform in the pro-
portions of weight of components and in the linearity of subsystem
weight with curb weight (Hoffman, 1962). Table 1 lists the ratios
of subgrouping weights to overall weight in this decade's automobiles;
these ratios are the starting point of this synthesis of hybrid power
vehicles.
The next step is elimination or transformation of the car's build-
ing blocks that are altered by the substitution of the hybrid power-
plant. From the parametric analysis in Table 1 one can forecast that
future hybrid vehicles that are identical twins to conventional ones
in shape, size, and performance would still call for two-thirds of the
vehicle weight to be conventional subsystems, and one-third in-
novated power systems. In this section I will discuss this innovative
third of the vehicle system weight, and its subdivision into engine
(denoted in Table 1 by A), generator (B), motors (also amounting
to B as a first approximation) and X, the battery weight.
Engine
Having fulfilled the "look-alike" requirement, one must next
choose engines to satisfy the "act-alike" requirement. Providing a
constant level of power comparable to that available in conventional
cars traveling at 50 mph gives three results (see Figure 5, Hoffman,
1967):
1. Accelerative performance from 0 to 30 mph superior to that
given by piston-engine cars, even with constant torque provision
(rather than power) up to 15 mph.
2. Equivalent performance from standstill to 60 mph, (that is, equal
time to accelerate from 0 to 60).
3. Somewhat inferior performance beyond 60 mph, but boostable
in hybrids by injection of battery-supplied spurts of power.
The actual power available to the driving wheels of modern
passenger cars at 50 mph is shown in Figure 6 for standard and inter-
mediate-performance engine options. An apparent satisfaction with
0.025 to 0.035 horsepower per pound seems to emerge as the urban
public's demand level for automotive power, the lower value being
for "standard" and the upper for the next-in-line ("intermediate")
consumer option of engines.
HOFFMAN 35
-------
Table 1. WEIGHT APPORTIONMENT IN CONVENTIONAL
AND HYBRID VEHICLES
«£
o OD
Ic'SJ
gs
Component _~
"j s
5 c
S. o
*- 0.
Se
> o
C 0
o
O
.c
QO
'£
* Redesign possibilities from substitution
•S of a hybrid powerplant
o
Body 0.33 Elimination of transmission hump; fewer vi-
Trlm 0.14
Engine 0.15
bratlon mounts; redistribution of weights
Simplification of dashboard furnishings, controls
See text
Starter 0.005 Becomes traction motors in the wheels
o"£ 2
^ M no
2'S '3
„* *
5 = e
J= OJ 3
o> c u
» 2
Q.
"° E
Is
0.32
0.14
A
B
Transmission 0.040 Eliminated
Suspensions 0.060 Better balanced front-rear springing; low
center of gravity
Glass 0.032 No alterations
Wheels 0.025 Incorporation of traction motors
Tires 0.032 No significant change
Brakes 0.038 Decelerating function partially taken over by motors
Steering unit 0.016 More equitable weight distribution
0.056
0.032
0.030
0.034
0.020
0.016
Rear axle, drive line 0.043 Not needed
Exhaust, smog system 0.014 Heat recuperator at engine controls exhaust
Battery 0.012 Size increased to meet high-power peaks
X
Radiator, full 0.014 Eliminated
Fuel tank, full 0.044 More modestly sized for lowered fuel consumption
Generator, controls 0.005 Growth into full-power generator on turbine shaft
0.022
B
Sum = A+2B+X + 0.67 = 1.00
If we assume that the electrochemical conversion efficiency of the
hybrid's electric motors and generator is 93 percent, the engine
power requirement is then
(0.03 ± 0.005)
hp =
W
(1)
0.932
and the least horsepower required of the engine is about
hpmln,i.c.E. = 0.029 W
where W is the curb weight of the car in pounds.
The maximum speed of hybrid autos on level grade and without
battery augmentation is calculated by equating the actual propulsive
power at the wheels
hpwt,eels = 0.025 W (2)
with the power needed to overcome aerodynamic drag and tire
rolling resistance.
Assume that the tire rolling tractive force (SAE, 1967) of
modern tires is
0.015 W, Ib (3)
and that the aerodynamic drag of a well-streamlined family car is
7 x ID-6 W v2, Ib (4)
36
Hybrid Power Systems
-------
0.05
0.04
Q.
.C
jf
Q.
H-
- 0.03
OL
LU
O
Q.
O
<
0.02
0.01
SMALL
O
COMPACT FULL-SIZE
O O
INTERMEDIATE
O
LARGE
-0
INTERMEDIATE PERFORMANCE OPTION
Q^ O
6
STANDARD
PERFORMANCE OPTION
1
1
2,000
4,000
6,000
CURB WEIGHT, Ib
Figure 6. The actual mean power available at the
driving wheels of modern passenger cars traveling
at 50 mph with standard option and intermediate
performance engines.
The total force resisting automotion, i.e., expressions (3) plus
(4), when multiplied by the speed, v (mph), and the appropriate
conversion factor, is close to the total automotive drag power that is
required of the motors. This tractive power requirement (hp) is then
0.00267 (0.015 — 7 x lQ-« v2) W v (5)
where
0.00267 =
1.467 (ft/sec) /mph
550 (ft-lb/sec)/hp
At the top speed of the hybrid not utilizing batteries for power,
the horsepower in expression (5) balances the motor-delivered power
HOFFMAN
37
-------
in (2). Equating expressions (5) and (2) yields the maximum speed,
v, to be 103.5 mph. This speed is without resort to battery augmenta-
tion, which presumably will be employed mostly for upgrading,
during short periods, the poor passing performance of hybrids beyond
60 mph.
The characteristics of some conventional automotive engines
that could drive the generator in a hybrid are listed in Table 2.
They were derived for a unit of 50- to 100-horsepower output by
updating material presented earlier (Table 2, Hoffman, 1967).
Observe that the relative characteristics of these engines do not differ
by amounts significant enough to indicate a clear preference for
hybrid vehicles. The selection of the gas turbine for this study,
therefore, is not to be construed as definitive or final, but rather is
only illustrative. The piston engine (rotary, diesel, or Otto cycle)
certainly remains a strong candidate for powering the generator in
hybrids.
Table 2. PROJECTED CHARACTERISTICS OF PRESENT-DAY
AUTOMOTIVE POWERPLANTS UNDER 100 HORSEPOWER
Gasoline Diesel Rotary piston Gas turbine
piston engine piston engine engine
Engine weight per horsepower, Ib/hp
Best thermomechanical efficiency,
installed under the hood
Least fuel consumption at optimum
speed, Ib/hp-hr
Relative fuel costs
Lowest number of reciprocating and/or
rotating parts between combustion
chambers and power pick-off
Mass production cost, dollars/hp
3 to 4
0.2 to 0.25
0.4
highest
13
(6 cyl)
2 to 3
4 to 5
0.3 to 0.35
0.4
intermediate
13
(6 cyl)
2 to 3
2 to 3
0.3 to 0.4
0.4
highest
3
(2 rotors)
2 to 3
1.5 to 2.5
0.25
0.35 to 0.45
lowest
1
3 to 4
The properties of the gas turbine in Table 2 vary slightly from
the data on automotive gas turbines usually found in the literature
(Turenen and Collman, 1965). This difference comes from calculating
the entries in Table 2 for a new, simpler, and smaller breed of auto-
motive turbine engine with carburetion fixed for constant power
output, no reduction gears to the output shaft, no power transfer
clutch, and omission of the many parts required for the mechanical
transmission of power to the wheels.
From the gas turbine's mean value of 2 pounds per horsepower
in Table 2, we may conclude that the engine weight ratio in hybrids
will be about
A = (2 Ib of eng/hp) x (0.029 hp 'lb of W)
or about A = 0.058, nearly 6 percent of the vehicle weight.
With typical power densities of 10 to 12 horsepower per cubic
foot for turbine and rotary piston machines, the engine in a full-
sized hybrid car would occupy 8 or 9 cubic feet, half of the volume
now taken by the engine-transmission assembly.
38
Hybrid Power Systems
-------
Generator and Motors
Alternating-current generators in aircraft today operate at top
speeds of 8,000 to 10,000 rpm, half to one-third of the turbine's
shaft speeds of 20,000 to 30,000 rpm. But reduction gears should be
avoided in hybrid systems to achieve quietness and higher efficiency,
and to reduce manufacturing costs, it also would be desirable to
mount the generator's rotor directly on the power shaft. This ar-
rangement would require a sharp upward revision of the ratio of
axial length to diameter of the stator and rotor. The presently favored
configurations with length-to-diameter ratios near unity can prob-
ably be slimmed down to ratios of 1.5 to 2.5 or even higher for the
hybrid turbine-generator sets without incurring a weight penalty
and still avoiding gear reduction.
The rated outputs of aircraft generators and of some advanced-
design oil-cooled motors are plotted in Figure 7 against their weight.
The mean ratio of weight to power of these generators and motors
is about 1 pound per horsepower. Then the fraction of vehicle weight
to be allocated first to the generator, B, and then again to the motors
in the wheels is
B.ng = (1 Ib of gen-mot hp) x (0.029 hp Ib of W) x 2 x
x (0.932 + 0.93)-1 = 0.032
In other words, about 6.5 percent (= 200 x 0.032) of the vehicle
weight will be in generator and in traction motor(s).
Battery and Controls
At the bottom of Table 1 we see that the sum of component
weight apportionments is
1.00 = A + 2B + X + 0.67
from which the weight ratio that can be allocated to batteries and
controls in first-generation hybrids becomes
X = 1 — (0.67 + 0.058 + 2 x 0.032)
or about X = 0.21.
The variable-frequency stepless and continuous controls regu-
lating a 200-horsepower motor in a truck wheel weigh about 80
pounds (SAE, 1966). Assuming the weight of well-designed electric
controls to be proportional to the controlled motor's output, we can
deduce that controls will take up about 0.01 of the curb weight,
leaving one-fifth of the vehicle weight for battery.
At least two distinct design possibilities for charging this sizable
battery pack are available:
1. Using a fraction of the power generated by the turbine-generator.
The constant power level from the generator can be selected so as to
exceed by a carefully preset amount the road-driver demands for
most driving situations. That portion of electrical energy from
HOFFMAN 39
-------
LEGEND: ( O SUNDSTRAND
AIRCRAFT A-C GENERATORS, OIL-COOLED j + BENmx
fj COMMUTER TRAIN TRACTION MOTOR, GARRETT
A ARMY TRUCK IN-THE-WHEEL MOTOR, LEAR-SIEGLER
/S PROTOTYPE ELECTRIC CAR MOTOR, GM
1,000
100
cc
UJ
§
UJ
1
I
10
1
= ' o/:
: / '
; /'•
*% i
I Of1" :
0
— 1 Ib/hp^ :
^ _
i i i i i 1 1 1 1 i i i I I i 1 1 1 I I I I I I II
100
5
^
of
UJ
g
10 p
0.
Q
UJ
1-
<
cr
1
10 100 1,000
MOTOR OR GENERATOR WEIGHT, Ib
Figure 7. Weight and power of selected electric
motors and generators.
the power-plant that is over and above the energy requirement
for the trip can then be metered out at preset rates to the battery.
An accurate allocation of power of this type should be based on
a predictive catalog of vehicle usage, in the form of speed-time
plots for a variety of trips, driver behaviors, locations, grades,
and weather conditions.
40 Hybrid Power Systems
-------
2. Regenerative braking. An urban electric vehicle could recover
about half of the kinetic energy of deceleration, presently dissi-
pated in friction heating of brakes and engine linings (Hoffman,
1963). With a third of the urban trip time spent in deceleration,
perhaps one-sixth of the energy now required for automotive
travel could be recaptured into the battery. Traction motors are
easily and instantly operable as generators, braking the vehicle
down to 5 mph, below which speed conventional brakes and park-
ing locks could take over. A simplified alternative design of first-
generation hybrids might thus consist of the turbine-generator
connected only to the driving wheels, and with regenerative
braking as the sole battery energizer.
To explore the potentialities of battery installation amounting
to one-fifth of the curb weight, consider a conventional lead-acid
battery, capable of storing 10 watt-hours per pound, and delivering
20 watts per pound within a half-hour discharge time.
The short-time power boost derivable from the 0.20 W in bat-
teries is then
0.20 W x (20 w/lb) x (0.001341 hp/w) or 0.0054 W, hp
namely, a 20 percent increase in power pulses.
This power boost can raise the accelerative and passing per-
formance and the grade-climbing ability of hybrid-power-system
vehicles to that of conventional piston-engine cars over all con-
ceivable urban and highway driving conditions (see Figure 8). The
other effect of battery power augmentation is to increase the maxi-
mum speed of this vehicle to 110 mph.
The patterns in Figure 8 indicate that this first system design of
hybrid power for automobiles provides too much driving power at
speeds below 50 mph and not enough beyond 80 mph, even with bat-
tery boost. Follow-on parametric studies should explore power match-
ing at 30 mph as a starting point (rather than the 50-mph base of this
preliminary study), with occasional battery-power augmentation
beyond 30 mph on demand. With this better balanced arrangement,
we forecast that a hybrid-power passenger car will:
1. Pollute our air only one-eighth to one-fourth as much as today's
conventional car.
2. Consume only half as much fuel.
3. Compare well in size, performance, and top speed.
4. Disrupt industry less than any other system in the eventual transi-
tion to all-electric cars.
REFERENCES
Frysinger, G. R., 1966. Fuel cell — energy storage hybrid systems
for vehicles. Paper No. 66-975, 3d Annual Meeting, AIAA,
Boston, Mass. (Dec. 2).
HOFFMAN 41
-------
o
o:
o
<
1.0 r-
0.8
0.6
0-4
0.2
INDUCTION MOTOR
BATTERY BOOST
INTERNAL COMBUSTION '
ENGINE WITH AUTOMATIC
TRANSMISSION
20 40 60 80
VEHICLE SPEED, mph
100
120
Figure 8. Torque-speed characteristics of conventional
cars and hybrid power automobiles.
Hoffman, G. A., 1962. Automobiles today and tomorrow. RM-2922-
FF, the RAND Corp. (Nov.) ($3.00).
Hoffman, G. A., 1963. Electric motor cars. RM-3298-FF, The RAND
Corp. (Mar.) ($3.00).
Hoffman, G. A., 1966. The electric automobile. Sci. American.
215:34-40 (Oct.).
Hoffman, G. A., 1967. Systems design of electric automobiles. J.
Transportation Res. 1(1): 3-19.
Society of Automotive Engineers, 1966. W. Slabiak. An AC individual
wheel drive system for land vehicles. SAE Paper No. 660134.
(Jan.).
Society of Automotive Engineers, 1967. D. M. Tenniswood and H. A.
Graetzel. Minimum road load for electric cars. SAE Paper No.
670177. (Jan. 13).
Turunen, W. A., and J. S. Collman, 1965. The General Motors re-
search GT-309 gas turbine engine. Res. Publ. GMR-495, General
Motors Corp. (Oct.).
42
Hybrid Power Systems
-------
NASA WORK ON HIGH-ENERGY-DENSITY
ELECTROCHEMICAL POWER DEVICES
Ernst M. Cohn
National Aeronautics and Space Administration
Washington, D.C.
To ensure crew safety during manned space flights and to
maximize the probability of success of unmanned space missions,
all spacecraft subsystems must have extraordinarily high reliability.
In addition, each subsystem should be as small and as light as
possible, to make available a maximum of space and weight for
payloads. These are the general goals of the electrochemical research
and development program that aims at providing reliable, high-
energy, and high-power-density subsystems for space. Since the
same type of criteria, together with other considerations, must be
applied to automotive power plants, a number of space-oriented
electrochemical projects are of terrestrial interest as well. The
methods used and results obtained in these studies may find more
or less direct commercial use. And even if a given space-type
device is not practical under everyday conditions, the lessons learned
from its development are often helpful in adapting it to meet other
physical and economic requirements.
BATTERIES
Organic Electrolyte Systems
Among pertinent battery tasks is the search for high-energy-
density electrochemical couples, i.e., fuel-oxidant combinations, with
compatible electrolytes. For 6 or more years, the Army, Navy, Air
Force, and NASA have sponsored in-house, grant, and contract work
on the use of organic solvents with inorganic solutes as battery
electrolytes. All of these efforts have shown how difficult it is to
obtain highly pure cell components; and purity appears to be a
requisite for compatibility. Even if this problem is solved, the fact
remains that all organic electrolytes investigated to date have ionic
conductivities so low that they are unsuitable for high rates of
discharge. Thus, although high energy density may be achieved,
high power density has yet to be shown. We can go even further
and say that an operable, compact, high-power cell would have an
inherent difficulty: Because no electrochemical reaction is 100 per-
cent efficient, the rate at which heat is produced per unit volume
in such a system must be higher than in a conventional one;
hence, special engineering skill is necessary to maintain thermal
control. The negative results of these efforts are valuable lessons
in themselves. In addition, a potentially positive result has come
from one of the contracts in this area.
Since mid-1963, Monsanto Research Corporation has been inves-
tigating their dry-tape battery or fuel cell (US Pat. 3,260,620 and
COHN 43
-------
3,293,080) under various NASA contracts (currently NAS3-7624).
One might have a prolonged semantic discussion as to whether this
is a battery or a fuel cell. Suffice it to say that the device employs
a multiple-reserve construction, i.e., the active components are pack-
aged separately and combined only immediately before use. Because
of the bulk and weight of its inevitable non-active components
(packaging, drive, controls, housing), the dry tape does not appear
to be applicable for 4- or 5-hour missions, the maximum duration
of driving a conventional car before refueling. One of the cathodic
materials studied under this task does, though, have some interesting
properties that make it, as well as related substances, worth further
consideration. Trichlorotriazinetrione (also called trichloroisocyanuric
acid) is a solid, organic bleach that contains 85 percent equivalent
chlorine on a weight basis. In combination with magnesium, for
example, the theoretical energy density of the electrochemical couple
alone is 750 watt-hours per pound. An electrolyte more compatible
than those now available remains to be found; further, the weight
of a battery would obviously be much more than that of the active
materials themselves. But this bleach offers the energetic advantages
of chlorine without the tankage weight penalties and the toxicity
hazards entailed by the use of that gas.
I should like to suggest, therefore, that bleaches be considered
as oxidants in batteries or battery/fuel-cell hybrids. If an acceptable
solid electrolyte can be found, a flow-through cathode might be
combined with it, through which a liquid bleach or a bleach dis-
solved in an inert solvent is pumped. The lightweight storage
tank for the oxidizer might contain a flexible bladder so that the
spent liquid can be stored in the same container before re-processing.
The solid electrolyte must prevent diffusion of materials into the
anode. The flow-through cathode would permit continuous flushing
of the cell, and thus undesirable build-up of products could be
avoided.
Alkaline Batteries
Let me now switch from a futuristic to a more immediate
electrochemical system, the zinc-oxygen cell. A number of projects
aimed at improving the silver oxide-zinc space battery have direct
bearing on that system, particularly on a rechargeable zinc-oxygen
or zinc-air battery. When the zinc anode is recharged, i.e., when
zinc is electroplated from the electrolyte back on the anode, it becomes
covered with uneven zinc deposits. Zinc dendrites or needles tend
to penetrate the cell separator and eventually to short-circuit the
cell by contacting the cathode.
In a contract with the Goddard Space Flight Center
(NAS5-3908), scientists at Leesona Moos Laboratories observed
three distinctive types of zinc dendrites. Mossy, pine-tree-shaped,
or acicular deposits were formed, depending on the overpotential.
The mossy kind, deposited at up to 0.1-volt overpotential, is least
objectionable; but this means that zinc plates have to be recharged
very slowly. Small hydrogen bubbles and local density differences
44 NASA: Electrochemical Power Devices
-------
between zincate-rich and zincate-depleted electrolytes caused local-
ized convection at dendrite tips and thus contributed to undesirable
needle growth. Once started, needles grew more easily than smooth
deposits, because zincate ions diffused slowly; the needles "reached
out" for more zinc. Needle growth was worst at the edges of
electrodes. Masking the edges with non-porous polyethylene or
other plastic was proposed as a solution to edge growth. Unsymmet-
rical pulse charging was suggested to the contractor, who did indeed
find that 30 percent on—60 percent off resulted in lower dendrite
growth than continuous charging. Thus a "half-wave" or other type
of discontinuous charge may overcome some of the rate limitation
set by the low overvoltage requirement for continuous charge.
A parallel effort at Yardney Electric Company (NAS5-3873)
confirmed a number of these findings. In addition, examination of
zinc deposits in membranes established that they penetrate by
deposition within the separator rather than by mechanical puncture.
High overpotential and low zincate concentration in solution are the
chief causes for such harmful growth. Furthermore, the overpoten-
tial at which penetration takes place was found to be a function
of the partition of zincate ions between the adsorbed state (on
separator fibers) and the free state (in solution, held within the
separator). The lower the partition ratio of adsorbed to free zincate,
the more resistant is the membrane to zinc penetration. Although
cellophane soaked in 44 percent KOH gave a ratio of only 0.03, it
is not stable enough to be a good separator.
The best organic separator found thus far, for both stability
and low zinc penetration, was prepared by the Borden Chemical
Company. (NAS5-9107). A 10 percent aqueous solution of methyl
cellulose (700g) and a 10 percent aqueous solution of (polyvinyl
methyl/maleic anhydride) (300g) were mixed at 0°C. The solution
was poured on a glass plate, leveled with a doctor blade, and water
was allowed to evaporate before the film was stripped from the
glass. Even so, the 60 or so cycles obtained with silver oxide-zinc
cells would obviously be far from commercially acceptable. Relief
may come from projects that started for completely different reasons.
An inorganic separator was developed by Astropower Laboratory
to operate silver-zinc cells at much higher temperatures than the
normal range of about 0° to 40°C. It has been incorporated into
test cells (NAS3-7639) and sustained well over 1,000 cycles at 35
percent depth of discharge and 25°C; even at 50 percent depth of
discharge and 100°C, it has sustained more than 250 cycles. Another
inorganic separator, electrodeposited calcium hydroxide, has been
studied for us by General Electric Company (NAS5-9168) and is
now being further evaluated. A separator made by Radiation
Applications, Inc., specifically for sterilizable batteries (JPL con-
tract 951015) still remains to be tested for resistance to zinc pene-
tration. In principle, however, inorganic or organic high-temperature
or sterilizable separators may turn out to be tougher and longer-
lived under ambient conditions than their more traditional precursors.
COHN 45
-------
We have started work on primary and secondary zinc-oxygen
cells for space during the past fiscal year. Of particular interest
are preliminary results obtained at Union Carbide Corporation
(NAS5-10247), where a zinc-oxygen 16-AH secondary cell was dis-
charged about 20 percent in 2 hours and recharged in 6 hours. This
8-hour cycle has been repeated 14 times thus far. Such charge/
discharge times approach those one might expect for everyday
battery uses.
Auxiliaries and Test Procedures
Whereas the cathode of the zinc-oxygen (air) battery is only
a partial product of recent fuel-cell research, the fuel cell as a
whole has been incorporated into batteries, not as a power-producing
but as a gas-consuming device. The gassing of zinc plates, which
evolve hydrogen with almost any electrolyte under almost all
circumstances, would make a sealed battery structure impossible if
the gas could not be consumed again. Miniature fuel cells inserted
in silver-zinc cells have operated satisfactorily for up to 1 year
(NAS5-9594) and are being further developed and evaluated by
Astropower and at NASA's Goddard Center. The same purpose
can be achieved by fuel-cell type auxiliary electrodes that have
been used by General Electric Company (NAS5-3669), among
others, to remove both hydrogen and oxygen from sealed silver-
zinc cells.
The question of charge control is not simply answered; the
correct method depends on the size of the power supply and on
the time available for recharging, according to a General Electric
Company report on contract NAS5-9193. They recommended con-
stant current charge for small (up to 15 watt) nickel-cadmium
power sources and 1-hour charge time; and constant voltage/
coulometer cut-off with auxiliary gas-recombination electrodes for
larger power supplies. A simple electrochemical coulometer made from
commercial cadmium plates was described by E. R. Stroup in NASA
report X-716-66-462. It functioned well with nickel-cadmium bat-
teries at 0° to 25°C for at least 1 year. Although these charge-
control devices and procedures were developed specifically for nickel-
cadmium batteries—the space workhorses—I cite them because they
indicate what can be done and must eventually be adapted to ensure
safe and reliable operation of other battery power plants.
Like all battery manufacturers and many other battery users,
we have our own test programs and procedures. An original con-
tribution to this type of activity arose from a brief contract with
Radio Corporation of America (NASw-1001) to develop statistical
methods for correlating the voluminous test data. It is a non-
destructive procedure for determining the quality of a battery.
Though perhaps not an original discovery of Mr. J. H. Waite, (it
appears to have been used on relays in WW II), it is potentially
applicable to any mass-produced item and looks particularly valu-
able as a deterministic, instead of the usual probabilistic, statistical
approach.
46 NASA: Electrochemical Power Devices
-------
Briefly, the procedure is as follows: A statistically meaningful
sample of a population is tested to destruction under normal operating
conditions, while significant data are being recorded, particularly
frequently during the initial test period. The data are manipulated
to show differences, for the initial portion of the life test only,
among samples that passed and those that failed for a variety of
reasons. One can then take another member of the same population,
obtain initial life data only, and match the results to those previously
obtained, in order to predict whether this specific member will
complete its mission or, if not, what will cause it to fail.
It is not yet known whether a general mathematical basis,
similar to conventional statistics, can be evolved for this approach.
Nor are the physical details for meaningful battery testing fully
explored as yet. Preliminary indications from nickel-cadmium bat-
tery tests are that the method may be made fail-safe; i.e., although
tests showed likely premature failure, some battery packs completed
their mission; but no "sound" batteries, according to prediction,
failed.
FUEL CELLS
For the present, at least, NASA is concentrating on one type
of fuel cell only, the hydrogen-oxygen cell with alkaline electrolyte.
Although the Bacon-type intermediate-temperature (250°C) fuel cell
for the Apollo spacecraft needs no added catalyst (the nickel elec-
trodes are catalytic enough at that temperature), low-temperature
fuel cells, operating at temperatures up to about 150°C, do need
catalysts on both anode and cathode. The greatest problem of
inefficiency consists of the voltage loss at the cathode of low-
temperature systems, where 0.2 to 0.3 volt per cell is lost as soon as
any power is drawn. This is true even for the best platinum or
silver electrodes.
To overcome this loss, I started a program at Tyco Laboratories
in mid-1965 (NASw-1233) on preparation and screening of a series
of alloys that are potential cathodic electrocatalysts. Thus far,
osmium and certain titanium alloys have been identified as promising
materials. They are perhaps of dubious commercial value. On the
other hand, nickel carbide was found to be about 90 percent as
good as platinum. That is not good enough for space, but in view
of its price and availability it may attain commercial importance.
I hasten to add that, thus far, it has been too unstable to be useful
for any length of time. Similar preparations are being made for
us by the Bureau of Mines (Contract W 12,300) and being evaluated
for possible ground applications by six firms. We furnish free
catalyst samples to them, in return for which we obtain the test
results. Since this mutual aid program has just begun, it is too
early to speculate on the results.
Another problem of gas-fed fuel-cell electrodes has been with
us for a long time. In 1839, Sir William (later Lord Justice) Grove
COHN 47
-------
mentioned, in describing the very first hydrogen-oxygen fuel cell,
that the seat of the electrochemical reaction is at the three-phase
interface. He thought that power production occurs where the
gaseous reactant, liquid electrolyte, and solid electrode meet.
Though he was almost right, modern work has shown that the
locus of reaction is just below this boundary, in a thin film of
electrolyte above the visible meniscus. Gas does not react at the
solid surface until after it has been dissolved in the electrolyte.
On a grant to Professor Bockris at the University of Pennsyl-
vania (NsG-325), a graduate student has been working on the
behavior of the pore of a gas-fed fuel-cell electrode, deepening
the insight gained into this problem by Professor Meissner's students
at Massachusetts Institute of Technology, Dr. Will at General Electric
Company, and others. Mr. Cahan observed boiling of the meniscus
in a model pore. Further work showed that about 90 percent of
the current was produced in a meniscus ring about 10~s centimeter
thick. In other words, only about 5 percent of the catalyst now
used in fuel-cell electrodes appears to perform its function; the
rest is wasted. This study points out that extremely thin electrodes
would suffice for the electrochemical reaction.
On the other hand, the thinness of the active layer may simply
be the result of the way we operate fuel cells today. They are
still being built according to Grove's recipe, maximizing the three-
phase interface. If the active layer is so thin only because all
of the sparsely soluble gas is used up in that region, then it may
be possible to improve the power density of a fuel cell by separating
the two operations, gas saturation of electrolyte and electrochemical
reaction, that are now so closely coupled in the electrode. This
decoupling does not make sense for a single cell, but it may result
in advantages for a stack of 30, 60, or even more cells. We now
have three contractors taking a look at the engineering problems
of novel types of fuel cells that would break with the Grove
tradition. Furthermore, new work on optimization of porous elec-
trodes is underway. In general, fuel cell power plants of today
are not much more than assemblies of slightly scaled-up laboratory
cells. A good deal of imaginative chemical engineering remains
to be done.
But, though the core of a fuel-cell plant represents a fairly
loose coalition of individual cells thus far, their combined behavior
is far from that of the same number of single cells. Something
always gets lost in the stacking of them, and it doesn't much matter
which kind of basic building block is being used. Whereas single
cells might perform well for a year or even several years, the very
best stacks have only lasted for hundreds (say 1,800) instead of
thousands of hours so far. The reasons for failure depend on the
type of cell, on the method of operation, and on the control devices
to name only the most obvious. Not enough full-size stacks have
yet been built to know details about the behavior, reasons for and
mechanisms of failure of any fuel-cell system in existence today
48 NASA: Electrochemical Power Devices
-------
The first extensive fuel-cell stack test is being carried out
by Allis-Chalmers Manufacturing Company (NAS8-2696). We are
doing here what has been done with batteries routinely for years.
However, in view of the much higher cost of these stacks, the
program now comprises only eight of them. The stacks are being
built as much alike as possible, though we will take advantage of
improvements that become apparent as we go along. Each stack
will be run under standard conditions for the first 200 hours to
ensure uniformity. After this check-out procedure, each stack will
be tortured in a different manner until failure: repeated starts
and stops, simulated external short circuit, unbalanced gas pressure,
improper water removal, excessive temperature, etc. The results
will permit a rather modest statistical assessment of full-size stacks,
their uniformity, their capability to withstand abuse, their mode of
failure under different stresses, their safety, and their reliability.
Though this is the first such systematic evaluation of fuel-cell stacks,
it will have to be repeated, and on a much larger scale, if commer-
cial use of them is ever to become widespread. I hope that this
exercise will set a helpful precedent and that many of the results
and conclusions will be applicable to other than the specific system
with which they will have been obtained.
Although the Allis-Chalmers Manufacturing Company system
still uses ordinary asbestos to contain the electrolyte, we already
know that a considerably improved material must take its place, if
long-term operation of matrix-type cells is to be achieved at mini-
mum rates of degradation. Several contractors, among them Electro-
Optical Systems (NAS3-2781), are studying composite asbestos
matrices or completely different substances for this purpose. In
addition, Union Carbide Corporation is upgrading its free-electrolyte
fuel cell for the high power and energy densities required in space
(NAS3-9430). It will be interesting and instructive to compare the
performance and longevity of this modified equipment with that
using an electrolyte matrix, since both will be tested under very
similar load conditions.
CONCLUSION
This summary of space-oriented electrochemical research and
development, aimed at producing compact, lightweight reliable power
sources, clearly shows that we have no ready-made, 50- to 100-
kilowatt product directly applicable to consumer uses. Yet, I hope
you will agree that these ideas, approaches, data, and devices can
contribute to our obtaining considerably improved batteries and
fuel cells, be they upgraded conventional ones or radically new
ones. This program should help solve some long-standing problems,
lead to simplified controls, and perhaps result in easier operability
and maintainability of such power plants. We do, of course, hope
that some of the concepts will end up as practical hardware that
can be applied where and when electric vehicles may prove to
be feasible.
COHN 49
-------
POTENTIAL BATTERY SYSTEMS
IN VEHICLE PROPULSION
Robert C. Osthoff
General Electric Company
Sclienectady, N.Y.
In the past most efforts to study the potential opportunities for
electric vehicles have centered on the lead-acid battery as the energy
source. This choice was based primarily upon the low cost and ready
availability of the lead-acid battery. However, the relatively low
energy content of the lead-acid system resulted in vehicles with
very limited ranges; further, the weights of the batteries were ex-
tremely great, compared to the weight of the very high-capability,
gasoline-powered, internal combustion engine. During the period of
1958 to 1962, at least three such converted vehicles powered by lead-
acid batteries were built by the General Electric Company. These
vehicles were plagued by limited range, although acceleration and
top speed were adequate for ordinary city driving. Nonetheless,
their unpredictable range made these test vehicles highly unsatis-
factory. At that time, no other battery candidate systems were avail-
able to consider as alternative power sources. Little or no analytical
work was done to predict vehicle properties in advance of building
such vehicles. As a consequence, interest in battery-powered vehicles
declined at General Electric and also at other organizations that
had shown early interest.
In England, where other economic considerations are involved,
engineering development and manufacture of electric vehicles has
proceeded with considerable success. Delivery trucks powered by
lead-acid batteries have had broad acceptance in Great Britain.
As research and devedopment on fuel cells increased in the
early part of this decade, there came an awareness of the very great
desirability of this power source for vehicle propulsion. The suc-
cessful application of fuel cells to the Gemini space flights increased
the public interest in the fuel cell. Nonetheless, the questions of
cost, life, simplicity, and reliability of fuel cells have precluded their
use in vehicles during the next few years on a large commercial
scale.
Within the past 2 years the development (to the point of feasi-
bility demonstration) of the zinc-air battery, the sodium-sulfur
battery, and the other existing high-energy-content systems has
established a new technological base for a variety of concepts of an
electric-powered vehicle. Additionally, the widespread concern over
pollution of the air by gasoline-powered internal combustion en-
gine systems has considerably increased the interest in electric
vehicles.
OSTHOFF 51
-------
It is natural that analyses of vehicle systems at General Electric
involved the transportation, battery, and systems competence of the
Company. The analyses presented here are based on studies of the
zinc-air battery, because a large amount of background informa-
tion existed at General Electric. The techniques are applicable to
any battery system, however, provided the investigators have suffi-
cient technical information concerning the battery.
For initial consideration, Figure 1 shows the forces required to
accelerate a 2-ton vehicle as a function of its speed. The linear por-
tion just below 1,200 pounds of force is established by an initial ac-
celeration requirement (in this case, 9 mph per second) and the
current limit for the two direct-current traction motors that were
assumed to power the vehicle. Also plotted in Figure 1 is the drag
force. The accelerating force is chosen so that it crosses the drag
force curve at 80 mph. The so-called balancing speed of 80 mph is
chosen so that some acceleration will be available at the design
cruising speed of 50 mph.
1,200
1,000
800
o
600
400
200
CURRENT
LIMIT
-CONSTANT ACCELERATION
\
CRUISE SPEED
40 60
SPEED, mph
80
100
Figure 1. Acceleration force vs. speed for 2-ton vehicle.
In Figure 2, the information of Figure 1 is replotted as power
against speed. About 50 kilowatts of power is required to reach the
balancing speed of 80 mph. To proceed further in this analysis, let
52
Potential Battery Systems
-------
us examine the data of Figure 3. At the top is plotted the accelera-
tion to 50 mph, followed by a period of constant (level-terrain)
cruising at 50 mph, and then by braking at a constant rate. Braking
deceleration at 5 mph per second was assumed, but the actual value
is not important to the analysis. The middle section of Figure 3
represents the motor current divided into both armature and field
currents, assumed to be controlled separately. In this scheme the
armature current remains constant throughout the acceleration and
then drops to that current level required to maintain the constant
speed. On the other hand, the field current is constant through the
constant initial acceleration period, then drops until the constant-
speed portion of the cycle is reached, and then increases slightly for
the constant-speed part of the mission. In the analysis no regenera-
tive braking was assumed, and hence the current in the two motors
in the vehicle will drop to zero when braking commences. In the
lower section of Figure 3, the power delivered by each motor is
shown through the cycle. The top power level does not necessarily
represent motor sizes. Traction motors can withstand prolonged
periods of overload. The sizing of the motors requires sophisticated
engineering judgments that need not be discussed here.
100
o
CM
ro
CTl
CD
cc
LJ
o
CL
40
20
40 60
SPEED, mph
80
100
Figure 2. Acceleration power vs. speed for 2-ton vehicle.
Mathematical models were established for these types of
vehicles based upon the considerations just discussed, and computer
programs were established. For a typical analysis, we selected a
2-ton vehicle with a 1,000-pound zinc-air battery as its power
OSTHOFF
53
-------
source. Some properties of the battery must be known to complete
the analysis. These data include polarization curves, power-rate
data, and voltage cut-off level.
Figure 3.
O.
E
o.
co
Q.
E
LU
tr
ce
Z)
o
o
D-
ou
40
20
0
60
40
20
0
60 '
40
20
0
/
/
~\
/
50 mph
AF
vl
"53.2 hp
I
1
25
WATUF
FIELD
.8 hp
\
iE
\
\
|
20
40 60
TIME, sec
80 100
Motor current and power requirements for a
specified cycle.
The analytical techniques are illustrated in Figures 4 and 5.
Figure 4 shows an acceleration to 50 mph, followed by immediate
braking; in Figure 5 the acceleration is followed by a period of
constant-speed level driving and then by braking. The constant-
speed range value may be conveniently varied (a geometric progres-
sion is convenient in computer programming). For all of these runs
the initial constant acceleration rate, A0, is 9 mph per second. Table
1 summarizes the typical vehicle properties that were commonly
employed, the input data, and computed results for the vehicles.
Of greater significance for the battery problems are the data in
Table 2. In this table the miles per cycle, that is, the distance as
represented by the ordinate for the cycles of Figures 4 and 5 are
54
Potential Battery Systems
-------
.c
Q.
E
LU
>
VELOCITY, 50 mph
ACCELERATION = 9 mph/sec
BRAKING AT 5 mph/sec
RANGE, miles
Figure 4. Acceleration of a 2-ton vehicle with a 1,000-
Ib zinc-air battery to 50 mph followed by braking.
o
o
CONSTANT SPEED
BRAKING AT
5 mph/sec
ACCELERATION = 9 mph/sec
RANGE, miles
Figure 5. Acceleration of a 2-ton vehicle with a 1,000-
Ib zinc-air battery to 50 mph followed by constant speed
driving and then braking.
OSTHOFF
55
-------
Table 1. TYPICAL VEHICLE PROPERTIES
Program constants
Balancing speed on level load, mph 80
Retardation (braking rate), mph/sec 5
Input Parameters
Gross weight of car, tons 2
Initial acceleration, mph/sec 9
Weight of Zn-air battery, Ib 1000
Cruising speed, mph 50
Propulsion system potential, volts 72
Computed Results
Max. power/motor at wheel rim, hp 26.6
Specific load at full power, w/lb 49.6
Speed to reach full power, mph 11.2
Acceleration on reaching full power, mph/sec 8.98
Current drain at full power, amp 689
Weight of control, Ib 44
Volume of control, in.3 1096
Acceleration on reaching cruise speed, mph/sec 1.4
Power/motor to maintain cruise speed, hp 8.45
Current drain at cruise speed, amp 218.7
Specific load at cruise speed, w/lb 15.7
tabulated, as is the average speed, the range attainable for that
particular cycle, and the computed weight of motors and gears.
Note that as the number of miles per cycle increases the range at-
tainable with the fixed weight of batteries also increases.
Since the power requirements of the motors are accurately
known as the vehicle is accelerated and run at the top speed, from
the computer program results one can calculate the amount of
energy required for any specific mission within the framework de-
scribed. One can also obtain the fraction of total battery energy
(or capacity) required for each accelerate/brake or accelerate/
cruise brake situation. These energy requirements as fractions of
battery capacity for a given distance are easily summed to synthesize
range values for any mission. Obviously, for a given size, the mis-
sions that involve more stops and starts give shorter range values
than do those missions in which most of the total distance traveled
is at the cruising speed. The utility of such a system of calculations
is that one can calculate vehicle properties in terms of range for a
desired mission, rather than for average missions, which are generally
unrealistic.
56 Potential Battery Systems
-------
Table 2. SAMPLE VEHICLE PROPERTIES FROM COMPUTER PROGRAM
Miles
per cycle
0.21
0.25
0.32
0.4
0.5
0.63
0.8
1
1.6
2
2.5
4
5
10
16
20
25
40
50
Avg speed,
mph
29
32
34
37
39
40
42
43
45
46
47
48
48
49
49
49
49
49
49
Range,
miles
67
77
91
105
120
135
151
166
193
205
215
232
239
252
258
260
262
264
265
Weight of gears and
motors, Ib"
436
417
400
383
366
350
335
308
297
287
270
264
249
243
241
239
237
236
° This weight is for continuous duty at the specified duty cycle for
a direct-current, two-motor traction system.
The effect of top speed on range is illustrated in Figure 6. The
ranges for 50-mph and 70-mph cruise speeds are given as a function
of miles per cycle. Other calculations show that the range is further
decreased by increased cruising speed as the vehicle weight is in-
creased. This appears to be the case even if the total weight of
batteries is proportionately increased.
The properties of the zinc-air cell systems of most interest in this
analysis are represented graphically in Figure 7, in which are plotte'd
both the polarization curve for a General Electric zinc-air battery
and the energy content curve for the same battery.
These properties may be combined as illustrated in Figure 8,
in which the energy content in watt-hours per pound is plotted
against the power density expressed as watts per pound. The portions
of the curve of interest for acceleration power and for range energy
are indicated. Since a combination of these properties is clearly re-
quired for vehicle propulsion, a hybrid power-source system is
suggested. Because high energy content and high power density
generally are not compatible in battery systems, a dual battery
system may be required. These properties are illustrated in Figure
9, in which current is plotted as a positive quantity in both directions
OSTHOFF 57
-------
300
.2 200
'E
100
ACCELERATION = 9 mph/sec
I I i I
50-mph CRUISE
i-mph CRUISE
I
0.2 0.5 0.8 1.2 2 5 10
DISTANCE OF RUN, miles
20
50
Figure 6. Variation of range of a 2-ton vehicle with a
1,000-lb zinc-air battery over various run (miles
per cycle) ranges.
125
100
a
oc.
UJ
50 100 150 200
CURRENT, amp/1,000 cm"
250
300
Figure 7. Polarization curve and energy content of
zinc-air cell.
58
Potential Battery Systems
-------
from the voltage axis for a high-energy-content battery and for a
high-power-density battery.
In such a system the high-energy-content power source could
be a battery or a fuel cell, and the other power source could be any
battery (or other system of high power capability) capable of de-
livering high currents to supply acceleration power. The high-energy
power sources can then be used to recharge the high-current (high-
power-density) source at the stop condition if the latter power
source is a battery. This will be done at relatively high efficiency
because of the operation of the high-energy-content system near the
open-circuit voltage.
POWER DENSITY, w/lb
Figure 8. Watt-hours per pound vs. watts per pound
for zinc-air batteries.
In summary, a computer program has been established with
which the properties of electric vehicles can be estimated. Pre-
liminary studies based on zinc-air battery systems have been made
in which the vehicle range for any mission can be calculated. The
utility of hybrid battery systems or battery and fuel cell systems
has been shown.
OSTHOFF 59
-------
VOLTS (E)
I REGION FOR HIGH-POWER-DENSITY
BATTERY RECHARGING
REGION OF ENERGY /
FOR EXTENDED RANGE /
I I I I
CURRENT OF
HIGH-ENERGY-CONTENT
BATTERY, amp
CURRENT OF
HIGH-POWER-DENSITY
BATTERY, amp
Figure 9. Two-battery hybrid system polarization curves.
ACKNOWLEDGEMENT
This paper is based upon the work of Mr. C. E. Kent, Dr. J. L.
Manganaro, and Dr. H. A. Christopher of the General Electric Re-
search and Development Center in Schenectady, N.Y., and on the
work of Mr. J. E. Wallace and Mr. H. G. Moore of General Electric's
Transportation Systems Division in Erie, Pa. The efforts of these
gentlemen are gratefully acknowledged.
60
Potential Battery Systems
-------
BATTERY-POWERED ELECTRIC VEHICLES
D. Thomas Ferrell, Jr.,
and Alvin J. Salkind
The Electric Storage Battery Company
Philadelphia Pa.
The Electric Storage Battery Company (ESB) has gained years
of practical experience with various types of battery-powered vehicles
now operating and now is engaged in research and development on
new power sources for electric vehicles.
We have supplied power systems for three types of electric
vehicles now in operation. In the first category are battery-powered
fork-lift trucks and golf carts. About 100,000 to 200,000 units of each
type are in use in the United States, an indication that quiet, clean
operation combined with low maintenance and low operating costs
makes battery power attractive to many users. Battery-powered
vehicles have proved themselves in many other applications in-
cluding mining equipment and in-plant utility vehicles. The principal
battery used for these types of applications is lead-acid.
ESB increased by 18 percent the energy output per unit volume
of its Exide-Ironclad cell construction with improvements intro-
duced between 1952 and 1954; we further increased the energy output
by another 43 percent with improvements introduced between 1954
and 1966. At present, this long-life, deep-cycle type of lead-acid
battery delivers approximately 1.4 watt-hours per cubic inch at
the 6-hour rate.
The second category of electric vehicles for which ESB can
provide operating data is the delivery truck. Vehicles of this type
have used Exide-Ironclad batteries since 1910. Our most recent
experience is with route trucks manufactured by Battronic Truck
Corp., an ESB affiliate. Some of these trucks have been in use for
more than 3 years. The power system is an 84-volt Exide-Ironclad
battery of either 425 ampere-hours (42TSC11) or 340 ampere-hours
(42TSC9). The smaller battery delivers approximately 11 watt-
hours per pound.
In typical multi-stop delivery service, such as on milk or bread
routes, the present Battronic trucks have a loaded range of about
40 miles and a top speed of about 25 miles per hour (Figure 1). The
maximum gross weight is 9,500 pounds, including the load. Com-
bined load and battery weight is 5,000 pounds; route requirements
determine the size of the battery, which weighs 2,000 to 3,000 pounds.
Data on these vehicles are summarized in Table 1.
FERRELL, SALKIND 61
-------
Battronic
Electric Truck
Figure 1. Truck powered by an 84-volt
Exide-lronclad battery.
Table 1. OPERATING CAPABILITIES OF A DELIVERY TRUCK POWERED
BY LEAD-ACID BATTERY
Load capacity:
Gross maximum weight
Top speed on level road (loaded)
Acceleration
Hill climbing speed (10% grade)
2,000 to 3,000 pounds
(vehicle and load): 9,500
Present
25 mph
1.5 mph/sec
12 mph
Maximum range on one charge of
the battery (loaded, 8 stops per mile) 40 miles
Time required for full charge of the
battery (after complete discharge)
Type of motor
Type of controller
up to 10 hours
series traction
contractor
pounds
Presently
possible
35 to 40 mph
1.5 mph/sec
14 mph
60 miles
up to 10 hours
compound traction
solid state
62
Battery-Powered Vehicles
-------
TRUCK IN USE AT MAPLEHURST FARMS DAIRY, LINCOLN, R.I.
Operating experience has shown over-all costs (depreciation
included) of $4.76 to $6.00 per day for an electric truck, compared
with $7.00 to $9.00 per day for a gasoline truck on similar routes.
Thus, the possible savings are $1.00 to $4.25 per day. Projected im-
provements in the lead-acid batteries, the controller, the motor, and
the chassis are expected to yield speeds up to 35 to 40 mph and a
maximum loaded range of about 60 miles. Many delivery routes and
other functions could be handled by the existing or improved trucks.
Benefits that would be realized are lower operating costs and re-
duced noise and air pollution.
The third type of electric vehicle for which we have operational
data is the small personal car, such as the ESB-Exide Electric Car
(Figure 2). This vehicle operates with twelve 6-volt golf-cart-type
batteries (3LDT-9). which produce a total of 10.1 kilowatt-hours.
Without sophisticated electronic controls, aerodynamic streamlining,
improved motors, or special tires, this vehicle has a range of 35 miles
and maximum speed of 40 mph (Table 2). The economy of operation
is shown in Table 3. Projected improvements are expected to result
in a maximum-load range of 50 to 60 miles and a top speed of
50 mph (Table 4).
FERRELL, SALKIND
63
-------
Figure 2. Electric car powered by twelve 6-volt golf-
cart-type batteries.
Table 2. PERFORMANCE CHARACTERISTICS OF PRESENT ESB-EXIDE
ELECTRIC CAR
Maximum speed: 40 mph on a level road
Range: 25 to 35 miles per battery charge
Acceleration: from stop to 20 mph in 3 seconds
Hill-climbing rate: 10 percent grade at 20 mph
Our total experience with the three types of electric vehicles
so far discussed is summarized in Figure 3. This figure, a plot of
range versus the ratio of available power to total vehicle weight,
shows the effect of weight and acceleration losses on the range of
vehicles at a speed of 30 mph. Other charts at other speeds show that
at a ratio of 4 watt-hours per pound, with a vehicle traveling at 15
mph, the range without stops would be 60 miles; at 30 mph, 50 miles;
and at 45 mph, 42 miles. Then range is reduced as the number of
stops increases. This ratio of 4 watt-hours per pound of total vehicle
weight is approximately the ratio for existing vehicles.
64
Battery-Powered Vehicles
-------
Table 3. ECONOMY OF OPERATION OF PRESENT ESB-EXIDE ELECTRIC CAR
Estimated cost of operation for 5 years driving 5,000 miles per year
Electricity
Battery replacement
Licenses
Greasing
Repairs with labor
Contactor tips
Motor brushes
Total
$375
450
50
12
35
17
$939
Estimated cost per mile: 3.50
*The ESB-Exide Electric Car uses 0.75 kilowatt-hour of alternating
current for each mile of driving. Based on a household rate of 20
per kw-h, the cost of electricity for recharging the battery is 1.5C
per mile.
Table 4. PROJECTED PERFORMANCE CHARACTERISTICS OF SMALL
"URBMOBILE" POWERED BY EXISTING LEAD-ACID BATTERIES
Maximum speed: 50 mph on a level road
Range: 50 to 60 miles per battery charge
Acceleration: from stop to 20 mph in 3 seconds
Hill-climbing rate: 10 percent grade at 30 mph
With present technology, the speed and range of electric vehicles
are limited by the energy available in the power source (as shown
in Figure 3). With the power sources now available on a production
basis, economics pretty well limit the choice to the lead-acid battery
system with the limitations of speed and range indicated.
Although ESB is continuing research and development on the
lead-acid, nickel-iron, nickel-cadmium, silver-zinc, and silver-
cadmium systems, it is likely that improvements in the performance
and economics of these systems will be incremental. It appears that
large increases in range or speed of an electric vehicle will come only
with the introduction of a new power source that can be produced
cheaply enough to provide energy at about the same cost per kilowatt-
hour as the lead-acid battery, i.e., in the range of $50 to $100 per
kilowatt-hour initial cost. A number of other requirements must
be considered, such as power density, convenience of use, start up,
and life.
ESB is investigating, in research and development stages, a
number of systems that show some promise for application to electric
vehicles. These include fuel cells, metal-air cells, non-aqueous
systems, and cells with organic active materials. At present, all of
these systems show some limitations for vehicle applications.
FERRELL, SALKIND 65
-------
80
70
GO
(/)
1 50
uj
z 40
<
30
20
10
o
STOPS PER MILE
0
I
I
I
I
0123 456
RATIO OF ENERGY TO TOTAL WEIGHT OF LOADED VEHICLE, w-hr/lb
Figure 3. Effect of vehicle weight and acceleration loss
on range (30-mph cruising speed).
Fuel cells are currently unattractive in terms of cost, life, con-
venience, and power density. Since fuel cells have received con-
siderable attention generally, we will briefly, describe the work of
ESB on other systems, with comments about their future possibilities.
ESB has been investigating metal-air cells for more than 10 years
by several approaches involving both static and dynamic systems.
Most of our efforts have been concentrated on the zinc-air system,
which seems to offer the best combination of characteristics.
Our most recent zinc-air projects have been in static systems in-
corporating very thin air electrodes with a third electrode for re-
charging the zinc electrode. A typical seven-cell secondary battery
of this type is shown in Figure 4. The output performance, power
density as a function of current density, is shown in Figure 5, which
indicates an average peak power for the secondary battery of 88
milliwatts per square centimeter. This figure also gives the data
for a primary zinc-air cell of similiar design and shows that even
greater output is possible with the present secondary design. As an
example of the progress that has been made, the lowest curve indi-
cates the performance of an earlier cell.
The effect of temperature on performance is shown in Figure 6.
These data were obtained at close to isothermal conditions, i.e., the
battery was not permitted to heat up as a result of chemical ineffi-
66
Battery-fowl-roil Voliii-Ics
-------
ciencies. Even under those very severe conditions, useable power was
obtained at low temperatures.
Figure 4.
ESB research metal-air cell.
Figure . shows the cycle life performance of zinc-air cells of the
present design, indicating good performance for over 50 cycles. We
believe that the loss of performance is associated with the zinc elec-
trode. In half-cell tests, the air electrode has given continuous per-
formance equivalent to over 120.000 miles at 30 mph. Indicated
energy density for the zinc-air system is in the range of 50 watt-
hours per pound. Although recent discoveries show promise for im-
proving the cycle life of the zinc electrode, many more improvements
in separators and zinc electrode reehargeabihty will be required to
make this system practical.
FERRELL. SALKIXD
67
-------
120
1100
o
^
> 80
05
Z
UJ
60
40
20
PRIMARY BATTERY
l-ZAP-94
SECONDARY BATTERY
7-ZAS-266
EARLY DESIGN BATTERY
l-ZAS-292
50 100 150 200 250
CURRENT DENSITY, ma/cm2
Figure 5. Power Output of Zinc-Air Cells.
Non-aqueous systems that use lithium or other alkali metals are
intriguing because of the possibility of energy densities of over 100
watt-hours per pound. ESB Research has built cells with lithium
anodes that delivered more than 100 watt-hours per pound at low
discharge rates. Although these systems are capable of high energy
densities, the low conductivity of the electrolytes makes the power
densities rather low. Considerable improvements in rate capability
will be required to make these systems attractive for vehicle applica-
tions.
Systems that use organic compounds as active materials have in-
trigued battery investigators for a number of years. Our recent in-
vestigation of certain organic compounds indicates good recharge-
ability and an energy density of over 50 watt-hours per pound of
cathode in small laboratory cells. These systems show promise for
high-rate rechargeable batteries, but will require more development.
At present, the economics are uncertain.
Let me summarize our experiences. We believe it is feasible to
construct delivery and small personal electric vehicles that will elimi-
nate air pollution and satisfy a need for transportation over a range
of approximately 50 miles. This can be done with existing commer-
cial batteries. For much greater ranges, newer power systems will
be required.
Battery-Powered Vehicles
-------
CELL OUTPUT, amp-hr
Figure 6. Effect of temperature on performance of zinc-
air cell (l-ZAP-97, 4-hour rate).
FINAL CYCLE
(APPROXIMATELY 100)
CELL OUTPUT, amp-hr
Figure 7. Typical cycling performance of zinc-air
secondary cells (4-hour rate).
FERRELL, SALKIND
69
-------
POWER SYSTEMS FOR ELECTRIC VEHICLES
FOR MILITARY AND COMMERCIAL USE
Edward A. Gillis
U. S. Army Engineer Research and Development Laboratories
Fort Belvoir, Va.
INTRODUCTION
Many power sources and many combinations of power sources
have been proposed as the power plant for electric vehicular pro-
pulsion. In a recent report to the CTAB Panel of the U.S. Depart-
ment of Commerce on Electrically Propelled Vehicles, the U. S. Army
Engineering Research and Development Laboratories (USAERDL)
reported for the U. S. Department of Defense (DOD) on the DOD
program in electrical propulsion. It was pointed out that the types
and severity of problems and the areas of emphasis for military
vehicles were quite divergent from those for private vehicles, but
that the basic technology being developed would, in most instances,
be applicable to all electric propulsion programs.
The following are some of the conclusions presented in that
report1
1. An all-battery power plant is not feasible for military vehicles
because the charging facilities cannot be provisioned in a military
environment.
2. The most promising power plants were gas turbines, fuel cells,
and hybrid systems. State of the art and improvement potentials
were considered when determining the applicability of these
power plants.
3. The prime mover portion of a hybrid power plant where batteries
are a part of the hybrid system should have the peak power
capability. For operational characteristics required by military
vehicles, it appears that dependence upon energy storage for
peak power is not practical whereas it may be for commercial
or private vehicles.
4. The three primary hybrid systems that can be considered are
(1) gas turbine and fuel cell, (2) gas turbine and battery, and
(3) fuel cell and battery.
Battery and fuel cell hybrids have been proposed and prototypes
constructed for continuous duty power plants to meet the following
objectives: (1) relatively low weight, (2) fuel economy, (3) silence,
and (4) low pollution.
In the analysis of this hybrid, however, the most important
considerations are weight and fuel consumption. For instance, if
weight alone were the only consideration, turboalternators in the
capacity of prime movers or peaking devices can be installed at
GILLIS 71
-------
2.5 to 4.0 pounds per kilowatt. Batteries or fuel cells either alone
or hybrided cannot approach this weight, but in noise or pollution
or part load fuel consumption, the turboalternator may fail to meet
requirements. If fuel consumption alone is important, a fuel eel]
is the most efficient energy conversion device available, but heavy.
Because of the interest in air pollution reduction, the gas turbine
is deleted from this discussion, although for military purposes the
gas turbine or gas turbine hybrids look extremely attractive. This
discussion, therefore, will be concerned with the two remaining
power sources: (1) fuel cells and (2) fuel cell and battery hybrids.
FUEL CELL SYSTEM ANALYSIS
There are nearly as many different types of fuel cell systems
as batteries. In fact, a fuel cell system itself may be a hybrid with
more than one fuel conditioner or with hydrogen storage capabilities
for peak power demands. To meet military requirements the fuel
cell systems must use a logistically available liquid hydrocarbon
fuel such as Compression Ignition—Turbine Engine (CITE) fuel
or JP-4. This fuel is converted to a hydrogen rich stream for use
in the fuel cell. Ambient air is the oxident in all cases.
Fuel cell systems operating on logistic fuels have been researched
at USAERDL for more than a decade, and continue to be the most
attractive energy conversion device for future general purpose sta-
tionary power plant use. A fuel cell system that is attractive for
ground power may not, however, be attractive for vehicular use,
since it would have been optimized for silence, fuel economy, and
ruggedness, whereas for vehicular use it must be optimized for
weight as well. As an analytical tool to aid in the optimization
of fuel cell and hybrid systems, a Figure of Merit equal to:
_, ,, x Power plant weight (Ib)
Power (kw) — ;—-
' 100 Ib/kw
Fuel consumption (kw/hr)
was used as measure of efficiency to indicate the power available
to move the vehicle and payload, minus the power required to
move the power plant. The 100-pounds-per-kilowatt constant is
representative of the power-to-weight ratio of military vehicles
now being designed.
The type of hydrocarbon-air fuel cell system with the highest
Figure of Merit at the 40- to 100-kilowatt power level required
by most military vehicles was an acid electrolyte, hydrogen-air
fuel cell. Three types of fuel converters to change the liquid fuel
to hydrogen were analyzed:
1. A steam reformer with the unpurified reformate passed directly
to the fuel cell.
2. A partial oxidation unit with product gas passed directly to
the cell.
72 Power for Military and Commercial Vehicles
-------
3. A steam reformer—partial oxidizer combination with the partial
oxidizer used for peak loads and transient operation. Schematics
for each of these systems are shown in Figure 1. The weights
for 40-kilowatt output, with modest state of the art improvements
are shown in Figure 2 as a function of the fuel cell current
density. (No frame or structural weight was charged to the
power plant, however, since it was assumed the fuel cell would
be built into the vehicle either using or contributing to its
structural frame). Also shown in Figure 2 is the specific fuel
consumption for the power plants with a spent fuel exhaust
containing 10 percent hydrogen by volume.
FUEL-
AIR-
PARTIA
OXIDIZE
-
^
L
R
1
i CELL
' STACK
1
r
i
i
i
L
iii i i i i i
— AIR
00
REFORMER |
FUEL
/
,'
\
00
r. ._ ._
"i r
1 CELL j
_j STACK |
L
| I I^| |I I |
C^XZ>
1
1
WATER
CONDENSER
AIR
AIR
I- — AIR
Figure 1. Fuel cell system schematics
GILLIS
73
-------
I
C3
O
Q.
2,500
2,000
1,500
1,000
500
O REFORMER SYSTEM
D PARTIAL OXIDIZER SYSTEM
O REFORMER PLUS PARTIAL OXIDIZER
1.0
0.8 3
0.6
o~
u_
0.4 <"
0.2
50 100
CURRENT DENSITY, amp/ft2
150
Figure 2. Forty-kilowatt indirect fuel cell power plant
weights and specific fuel consumptions.
It can be see in Figure 2 that the reformer system is nearly
twice the weight of the partial oxidation system, but uses only
three-quarters of the fuel. When the Figure of Merit is applied to
the systems as shown in Figure 3, the reformer and partial oxidizer
systems are nearly equal, 0.2. By comparison, the Figure of Merit
for the 94-horsepower spark ignition engine of an M37 truck is
0.095 at maximum power. For most commercial or private vehicles,
100 pounds per kilowatt will not give adequate performance. If
the weight-to-power ratio of modern compact automobile (25 to
40 Ib/kw) is substituted, the partial oxidizer and fuel cell combi-
nation is far superior to the other two systems.
In Figure 3, the systems all show a maximum at 85 amperes
per square foot, but the fuel cell at this current density is far
from its peak power, and additional ancillary devices can be installed
74
Power for Military and Commercial Vehicles
-------
to allow the cell to operate continuously at peak power. Figure 4
shows the Figure of Merit over the full range of power, with average
or most economical power still at 40 kilowatts, but peak power
between 64 and 70 kilowatts. For the partial oxidation system,
the maximum specific power density of 66 watts per pound occurs
close to the peak power point.
0.4
_Q
1—
X
o
UJ
1-
z
<
_J
CL
cr
1 | |
g
O
Q.
J£
s
—
0
o
T— t
"?
cc
UJ
g
o
CL
s
z
o
1 —
Q.
D
z
o
o
1
LJ
^>
LL.
0.3
0.2
0.1
© REFORMER SYSTEM
D PARTIAL OXIDIZER SYSTEM
O REFORMER PLUS PARTIAL OXIDIZER
50 100
CURRENT DENSITY, amp/ft2
150
Figure 3.
Forty-kilowatt fuel cell power plant figure of
merit versus current density.
POWER SYSTEM ANALYSIS
Two battery systems were analyzed for use in the hybrid
system. The first is an idealized 100-watt-hour-per-pound battery
with unlimited charge or discharge rates and a 75 percent wattage
efficiency over one cycle. This represents the goals for vehicular
batteries needed for the hybrid system calculations. The second
battery analyzed is an idealized metal-air type, limited on charge
and discharge rates by air electrode performance only and limited
on total charge by a maximum active metal weight. This battery
discharges between 1.27 and 1.0 volt and charges between 1.9 and
GILLIS
75
-------
2.23 volts, depending on ampere rate. Other goals for vehicular
batteries that are important but do not enter into the calculations are
(1) charge rates equal to discharge rates, (2) cycle life of over 10,000
cycles, (3) minimum of 50 percent of energy delivered at 80 percent
of open-circuit voltage or higher, (4) self-discharge of less than 5
percent per day, and (5) 18-month activated life.
o:
LJ
o
Q_
0.3
0.2
0.1
© REFORMER SYSTEM
D PARTIAL OXIDATON SYSTEM
O REFORMER PLUS PARTIAL OXIDIZER
40
80
% POWER
120
160
16 32 48 64
POWER, kw
Figure 4. Fuel cell power plant figure of merit
for 10 to 170 percent of rated power.
HYBRID SYSTEM ANALYSIS
A fixed-weight power plant was used for the hybrid analysis
with the battery comprising 0 to 100 percent of the weight. Figure
5 shows the peak power per pound of power plant capability as a
function of time for various fuel cell and battery combinations
with the ideal 100-watt-hour-per-pound battery. Also plotted is
76
Power for Military and Commercial Vehicles
-------
the recharge time of the battery when the power plant is providing
less than peak power. For example, 200 watts per pound can be
provided for 4 minutes by a 10 percent battery hybrid system. The
recharge time with no net power out (all of the fuel cell power
recharging the battery) is about 14 minutes. But if 40 watts con-
tinuous power is used by the vehicle (i.e. cruise power), it requires
40 minutes to recharge the battery, since there is little excess fuel
cell power for charging. If the battery proportion increases to 50
percent, 200 watts per pound is available for a longer time, about
18 minutes, but since the fuel cell is much smaller, 33 watts
capability, the recharge time at idle is over 2 hours and the system
cannot sustain 40 watts per pound continuously.
;
of
O
Q.
<
LLJ
Q.
RECHARGE TIME WITH
CONTINUOUS POWER OUTPUT OF
Q NONE
<3> 20 watts
°0 A 40 watts
10"
TIME, seconds
10*
Figure 5. Ideal battery and fuel cell hybrid system
peak power, discharge time, and recharge time.
Figure 6 shows much the same peak-power-versus-time capa-
bility for the metal-air battery, but anode and cathode limitations
are apparent. Figure 7 shows the ratio of charge to discharge
times as a function of continuous power required in watts per
pound.
The minimum-weight hybrid system was calculated for two
types of duty cycles. All of the duty cycles assume a peak power
period, a continuous power period, and an idle period, v> ith the
battery fully recharged at the end of the cycle. Duty cycles 1 and
2 are short-term "milk-route" types of cycles, characterized by
short peak power times. Duty cycle number 3 is a military battle-
GILLIS
77
-------
field power profile recently reported, and duty cycle number 4 is
a USAERDL estimate of an 8-hour military mission based on proving
ground tests on the fuel-cell-powered M37 truck. Table 1 shows
the characteristics of the duty cycles and the operating characteristics
of a hybrid system containing the idealized 100-watt-hour-per-pound
battery. The hybrids listed under each duty cycle have the maximum
performance possible under the peak-to-average-power ratios and
time ratios dictated by the duty cycles. As would be expected, the
"milk-route" duty cycles, 1 and 2, have superior power plant char-
acteristics, both in peak power density and in continuous power capa-
bilities. The minimum-weight hybrid for the military vehicle duty
cycles has much poorer performance, especially in continuous power
capability, which may be considered as the reserve or emergency
power. Figure 8 shows the horsepower requirements for three classes
of military vehicles, with the peak power requirement occurring over
a wide range of conditions. It is not inconceivable that the peak power
100% FUEL CELL
10"
103
TIME, seconds
104
Figure 6. Metal-air fuel cell hybrid system peak
power versus peak power time.
78
Power for Military and Commercial Vehicles
-------
may be demanded for more than 1 hour continuously, especially
since these are rough terrain vehicles with up to a 60 percent
grade requirement. If the power plant is designed for duty cycle
number 3, once the peak power demand has exceeded 1.2 hours,
or if a series of short peak power bursts total this time without a
substantial idle time, the available power drops to about 20 percent
of peak. This is not sufficient power to maintain convoy speeds
on level ground, much less on 3 percent grades.
10
20 30
CONTINUOUS POWER, w/lb
40
Figure 7. Metal-air battery and fuel cell hybrid
charge-to-discharge-time ratio versus continuous power
GILLIS
79
-------
Table 1. DUTY CYCLES AND OPERATING CHARACTERISTICS OF HYBRID
SYSTEM CONTAINING IDEALIZED BATTERY (battery, 100 w-hr/lb)
Hybrid power plants Duty cycle number
characteristics 1 2 3 -b
Duty cycle characteristics
Peak power (Pt)
Peak power time (tp)
Average power (Pa)
Average power time (ta)
Idle time (ti)
Hybrid operating characteristics
Peak power density, w/lb
Continuous power, as % Pt
Continuous power, w/lb
Figure of Merit
1
Vz min
0.5 Pt
3 min
3 min
187
35
65
0.178
1
Vz min.
0.25 Pt
3 min
3 min
300
22
65
0.185
1
1.2 hr
0.2 Pt
15.6 hr
7.2 hr
80
19
15.5
0.176
1
1 hr
0.4 Pt
4 hr
3 hr
85
35
30
0.179
Table 2 shows the same duty cycles and power plant perform-
ance as above. However, it augments the first in that the hybrid
characteristics have been calculated for both types of batteries, and
a comparison of an all-fuel-cell power plant with a hybrid power
plant has been added. The specific fuel consumptions and Figures of
Merit shown are better for the fuel cell power plants than the
hybrids in all cases. The higher fuel consumption for hybrid sys-
tems and its consequent effect on the Figure of Merit due to the
inefficiency of the battery in that from 35 to 100 percent more
energy has to be put into a battery than can be taken out. Also, in
a hybrid system a relatively small fuel cell works at full power
continuously, either helping power the vehicle or charging the
battery. Little advantage can be taken of the fuel cell's ability
to operate with higher efficiency at part load.
For duty cycles 1 and 2, substantial savings of power plant
weight are possible with the hybrid systems. The all-fuel-cell
power plant weighs from 2 to 4.5 times the hybrid systems weight.
For military duty cycles this weight savings is not substantial. An
increase of less than 25 percent in power plant weight for either
duty cycle 3 or 4 and all fuel cell power, the vehicle will not be
restricted by peak power times. If the weight of fuel required
for the mission is added, the weight differential drops to under
12 percent. The continuous peak power capability seems well worth
the extra weight.
SUMMARY
Battery and fuel cell hybrid power plants do not offer appreciable
savings in weight over fuel cell power plants for use in military
vehicles. All-fuel-cell power plants offer the advantages of much
80
Power for Military and Commercial Vehicles
-------
lower fuel consumption, and more important, continuous peak power
capability. Conversely, fuel cell and battery hybrid systems do offer
the potential of a lightweight power plant for electrically powered
commercial vehicles. For the military vehicle that may require
extended periods of operation at high power levels and where
noise or air pollution are not handicaps, hybrid systems utilizing
engines as well as electrochemical power plants may be more suitable.
90
80
70
60
OL
LLJ
50
o
a.
UJ
co
o:
2 40
30
20
10
GROSS VEHICLE WEIGHT
16,000 Ib
8,000 Ib
4,000 Ib
UJ
Q
Q:
o
I
I
I
A
T
I
UJ
Q
I
10 20 30 40
VEHICLE SPEED, mph
50
60
Figure 8. Vehicle horsepower requirements as a
function of speed and slope.
GILLIS
81
-------
Table 2. COMPARISON OF ALL-FUEL-CELL POWER PLANT AND HYBRID
POWER PLANT
Hybrid power plants
characteristics
Ideal battery (100 w-hr/lb)
Duty Cycle Number
1234
Advanced metal-air battery
(107 w-hr/lb at 1-hr rate)
Duty Cycle Number
1234
Duty cycle characteristics
Peak power, (Pt)
Peak power time, (tp)
Average Power, (Pa)
Average power time, (ta)
Idle time, (ti)
1111
1/2 mln 1/2 min 1.2 hr 1 hr
0.5 Pt 0.25 Pt 0.2 Pt 0.4 Pt
3 min 3 min 15.6 hr 4 hr
3 min 3 min 7.2 hr 3 hr
1111
1/2 min 1/2 min 1.2 hr 1 hr
0.5 Pt 0.25 Pt 0.2 Pt 0.4 Pt
3 min 3 min 15.6 hr 4 hr
3 min 3 min 7.2 hr 3 hr
Hybrid operating characteristics
Peak power density, w/lb 187 300. 80 85 133 162 83 80
Continuous power, as % Pt 35 22 19 35 36 24.5 22 43
Continuous power, w/lb 65 65 15.5 30 48 40 18 34.5
Battery only time at Pa 1.5 min 1 min 4.8 hr 1.6 hr 1.2 min 1.8 min 4.7 hr 1.7 hr
Specific fuel consumption, Ib/kw-h 0.874 0.860 0.820 0.812 0.914 0.982 0.813 0.840
Figue of Merit 0.178 0.185 0.176 0.179 0.166 0.157 0.178 0.171
All-fuel-cell power plant
Relative wt, fuel cell/hybrid
Specific fuel consumption, Ib/kw-h
Figure of merit
2.76 4.5 1.19 1.27 1.99
0.625 0.622 0.534 0.640 0.625
0.224 0.225 0.262 0.218 0.224
2.43 1.24 1.19
0.622 0.534 0.640
0.225 0.262 0.218
82
Power for Military and Commercial Vehicles
-------
FUEL CELL —BATTERY POWER
SOURCES FOR ELECTRIC CARS
Galen R. Frysinger
U. S. Army Electronic Components Laboratory
Fort Monmouth, N. J.
In recent approaches to the electric automobile, attempts have
been made to redesign vehicles to make them more appropriate to
electric-drive concepts; to rethink the way in which people use
vehicles so that an electric car may fit into a new type of transpor-
tation mode. All of the thinking is dominated by the fact that the
electrical automobile cannot have all the versatility, range, speed,
and other characteristics associated with present internal-combustion-
driven vehicles. These considerations come back to basic limitations
of the battery, which must contain the stored electrical energy
required for the complete duty period.
BATTERY PROBLEM
The basic limitations a power source places on a vehicle are
(1) the limited range or the number of miles the vehicle can travel
without refueling, (2) the acceleration characteristics of the vehicle,
and (3) the percentage of time the vehicle is available for its
prescribed duty. For the conventional internal combustion engine
the range depends upon the amount of gasoline carried, which is
commonly adequate for about 300 miles, and the acceleration is
dependent upon the design horsepower of the engine, which in
most American cars is adequate to allow high-speed passing. The
off-duty time for a conventional petroleum refueled vehicle is the
5- to 10-minute period required every 250 to 300 miles to refuel
the gasoline tank.
For a vehicle battery, the range is limited by the energy
density (kw-h/lb) of the battery; the acceleration is limited by
the power density (maximum kw) of the battery; and the off-duty
time is, of course, the time required to recharge the battery for
the next discharge period. Conventional batteries that have been
used, such as the lead-acid, nickel-iron, or nickel-cadmium, and
in some experimental cases silver-zinc, all have distinct limitations
in these three areas and also have the additional limitation of
higher cost than those presently associated with the power source
in a conventional vehicle.
Several new battery systems under investigation would sig-
nificantly increase the energy density of future vehicle batteries.
Based on the recent development of highly advanced fuel cell air
cathodes, new zinc-air systems have become possible with conven-
tional alkaline electrolytes. They are, however, limited to about
80 watt-hours per pound. There are some difficulties in achieving
FRYSINGER 83
-------
very high rates without drastically affecting the energy density,
and long life recharge capability is still highly uncertain. Investi-
gators have explored two types of nonaqueous electrolytes. The
first consists of salts dissolved in organic electrolytes used with
alkali metal anodes and halogen cathode materials. These have
been extensively investigated by National Aeronautics and Space
Administration for satellite communication batteries, but do not
appear attractive for vehicle propulsion because of the very low
rates that are obtained and the many problems involved with the
dissipation of heat generated by the inefficiency of the reactions.
The second major category of non-aqueous systems includes the
molten electrode or molten electrolyte types. The sodium-sulfur
battery announced to the public by Ford Motor Company only last
month is an example of this type of cell. The lithium-chlorine
batteries being developed by General Motors use a molten lithium
anode, lithium chloride electrolyte, and a porous carbon cathode
in which chlorine gas is either evolved during recharge or consumed
during the discharge. These systems based on sodium or lithium
anodes and sulfur-, chlorine-, or possible fluorine-based cathode
materials have theoretical energy densities sufficiently high that
practical devices in the range of 150 to 200 watt-hours per pound
are within the realm of feasibility. Together with Ford and General
Motors, several other investigators are trying to tackle the problems
associated with the containment of these reactive materials in con-
figurations that can make them attractive as high-power-density,
high-energy-density batteries. The concept of using a molten elec-
trolyte battery operating at elevated temperatures introduces new
types of operational patterns for vehicle propulsion. The whole
question of start up and how one maintains the system in molten
condition ready for operation over extended periods must be
considered.
If one requires equivalent performance from any new electrically
propelled vehicle, as is now obtained from conventional vehicles using
internal combustion engines, the present performance parameters
must be determined. The conventional power plant weight associated
with typical vehicles is shown in column 2 of Table 1.
This includes just the engine weight. The weight of the transmission
and the power train for this calculation were assumed to be
equivalent to that required in the future for the electrical drive
train system. Column 3 shows the fuel weight normally carried
in these vehicles immediately after refueling. The total weight of
the power plant, the fuel, and the tankage is shown in column 4.
The amount of energy that can be generated from this fuel in the
internal combustion engine converter is, of course, a function of
the conversion and utilization efficiency. Based on detailed calcu-
lations, the specific fuel consumption for these vehicles during their
normal duty cycle, which includes periods of idle, traveling across
country, on secondary roads, on highways, and at peak power, has
been calculated in terms of pounds of petroleum fuel per kilowatt-
84
Fuel Cell —Battery Power Sources
-------
Table 1. WEIGHT AND FUEL CONSUMPTION SUMMARY FOR
CONVENTIONAL INTERNAL COMBUSTION POWER PLANTS
Vehicle
Type
% ton
% ton
2V2 ton
5 ton
10 ton
Tank
Conven-
tional
power-
plant
weight,
Ib
418
590
1,550
1,675
2,200
4,870
Fuel
weight,
Ib
108
146
305
476
1,036
2,741
Total wt
(PP, fuel,
and tank),
Ib
555
773
1,932
2,270
3,447
8,068
Duty cycle
SFC,»
Ib/kw-h
2.10(11%)
1.53(12%)
1.02(18%)
0.97(19%)
1.02 (18%)
1.41(13%)
kw-h
Equiv
51
95
299
490
1,015
1,940
w-hr/lb
97
129
161
229
314
254
a Specific fuel consumption.
hour and is shown in column 5. From these data the kilowatt-hour
equivalent of the fuel, and the watt-hours per pound of total engine
and fuel can be readily calculated. This represents the energy
density of the power source in conventional vehicles. The watt-
hours per pound range from approximately 100 for the one-quarter-
ton truck to over 300 for the 10-ton cargo carrier. From this com-
parison it is obvious that no aqueous electrolyte battery, not even
the 80 watt-hours per pound estimated for the zinc-air system, can
directly replace the engine and fuel tank in conventional vehicles
and give equivalent performance. If the molten battery systems
could be developed to store 150 watt-hours per pound, they could
be used as direct replacement for conventional engine power sources
in all but the very heavy and long-range vehicles. Improved versions
with energy densities of up to 300 watt-hours per pound would
be required for the larger trucks and combat tanks.
Table 2 shows the power requirements, in kilowatts provided
to the drive train, required during idle, cross country, secondary
roads, highways, and at peak power.
Table 2. POWER REQUIREMENTS TO DRIVE TRAIN
OF CONVENTIONAL VEHICLES
(in kilowatts)
Idle
Vehicle (30%)
1/1 tnn
% ton
O1A tnn
5 ton
10 ton
Tan Ir
Cross
country
(20%)
2.4
5.6
11.5
17.4
61.3
153
Secondary
(20%)
4.5
9.4
19.9
29.4
97.5
261
Highway
(25%)
6.3
12.2
24.0
33.4
104.0
Peak
(5%)
37
55
108
130
216
400
FRYSINGER
85
-------
Internal combustion engines have been designed on the basis of
providing the peak power required and in many cases almost
idle during normal operating conditions for highway transit. To
give equivalent performance the battery must of course be capable
of providing the same output. These discharge rates are no problem
for the molten electrolyte batteries because of the very high con-
ductivity of the electrolytes and because of the fast electrode
reactions at these elevated temperatures. Most of these peak demands
require discharge at about the 1-hour rate. Molten batteries are
capable of high capacities and efficiencies even at 15- to 20-minute
rates. The molten electrolyte systems, therefore, have sufficient
power density for these vehicle applications.
The third major limitation of battery systems is the high
percentage of nonduty time during which the battery must be
recharged. Various proponents of the family-type electric car have
suggested the overnight recharge to replace the electrical power
consumed during the day's use. This immediately limits the type
and extent of use of the electric automobile because it must stay
within its 1-day range, often mentioned as a maximum of 150 miles,
and can only be recharged at home or where other suitable charging
equipment may be available. An alternative would be supplying
energy to the electrical automobile by a method analogous to that
now employed for refueling conventional hydrocarbon-burning
vehicles. This requires a fast charge capability for the battery so
that its energy content can be completely renewed in a period of
approximately 10 to 15 minutes, which is only somewhat longer
than now required for obtaining a full tank of gas at the gasoline
service station. The kinetics of the electrode reactions of the molten
electrolyte cells are sufficiently rapid so that charging of a total
system within 15 minutes is a possible feasible solution. Special
designs to minimize resistance in the electrodes and electrolytes
must, of course, be taken and some loss of efficiency in recharge
is inevitable. If high-power charging stations were available across
the country, such as gasoline service stations are now, the total
energy content for another 200- to 300-mile trip could be transferred
to the molten electric battery in this short period of time. One of
the principal problems is, however, the large amount of current
that must be conducted through the cables that link the charging
station to the battery. This represents a very significant bus bar
weight on the vehicle. This extra weight in terms of cell terminals
and conductor hardware is so large that it drastically reduces the
effective watt-hours per pound of the battery. It is, therefore,
highly unlikely that a 15-minute recharge battery can be achieved
with the watt-hours per pound required for the vehicles listed in
Table 1. At first glance the alternative appears to be either to
reduce the range of the vehicle and to allow fast recharge so that
an additional increment of power can be quickly taken on as
required or to remain with the concept of overnight charging and
increase the range of the vehicle by developing batteries with
significantly higher watt-hours per pound.
86
Fuel Cell —Battery Power Sources
-------
FUEL CELLS
Fuel cells have also been proposed for vehicle propulsion.
Except for the cases where very special fuels and oxidants such
as tank hydrogen and tank oxygen have been utilized, very little
progress has been made. The economics of electric vehicle propul-
sion does not lend any attractiveness to the utilization of special
fuels such as hydrogen or hydrazine in fuel cells for vehicle propul-
sion. The cost of hydrocarbon fuels for utilization in hydrocarbon-
air fuel cells for vehicle propulsion is, of course, extremely attractive,
but the present day complexity and weight of hydrocarbon air systems
make them infeasible for vehicle mounting. One principal problem
is that a fuel cell is much like an internal combustion engine in
that it must be sized to deliver the peak power demands. The peak
requirements for the military vehicles shown in Table 2 would have
to be used as the design point for the rated performance of the
hydrocarbon-air fuel cells. At the present power density of the
fuel cell (50 to 100 Ib/kw), it is evident that the hydrocarbon-air
fuel cell weight would be in excess of the total vehicle weight.
Even though a fuel cell would have a high energy conversion
efficiency (40 to 50%) at its rated power, which is the peak power
requirement, as listed in Table 2, its energy conversion efficiency
at normal cross country power requirements could be less than 10
percent and not much better than internal combustion engine
efficiencies.
FUEL CELL —ENERGY STORAGE HYBRID SYSTEM
Molten electrolyte batteries have the advantage of high power
density, easily providing the peak currents required for fast accel-
eration and for heavy service, but have limitations in the total
amount of energy that can be stored in a given amount of weight
They also have the disadvantage of often requiring longer periods
for recharge or for short charge periods having significantly lower
energy storage capacities. Fuel cells on the other hand have the
advantage that they can convert a hydrocarbon fuel, which has an
extremely high watt-hours per pound of theoretical energy, very
efficiently into electrical energy, but have the difficulty of having
very low overload capabilities, and therefore, have to be designed
for the peak power requirements that are only utilized 5 percent
of the total time during a day's operation. In the design of a future
power source that is to be strictly limited to the weight of the
existing engine (transmission and drive train excluded) and its
normal amount of fuel, and that is to be optimized for lowest specific
fuel consumption over its average daily duty cycle (optimum high
efficiency) it is logical to choose a fuel cell and battery hybrid
system. A hybrid power source system would consist of approxi-
mately an equal weight of hydrocarbon fuel, fuel to electrical
converter, and battery. The one-third of the weight that is battery
would store electrical power for use during peak requirements or
FRYSINGER 87
-------
where the power required is above the average. The one-third of
the weight in the fuel to electrical converter could be a highly
simplified liquid hydrocarbon fuel cell used as a battery charger
running at an average power level for the entire daily duty cycle,
with power being stored during idle or below-average demand
periods for use during periods of above average drain.
The exact proportion of weight in the fuel cell battery charger
and that in the battery must, of course, be dependent on the particu-
lar duty cycle of the vehicle, with the peak-to-average power ratio
being the most important design parameter. Table 3 indicates the
rated kilowatt capacity of the fuel cell and the kilowatt-hours of
battery storage for the vehicles previously described.
Table 3. FUEL CELL CAPACITY AND BATTERY STORAGE
REQUIRED FOR VARIOUS CONVENTIONAL VEHICLES
Vehicle
% ton
%ton
2Vz ton
5 ton
10 ton
Tank
Rating, kw
2.4/4.5/6.3/37
5.6/9.4/12.2/55
11.5/19.9/24.0/108
17.4/29.4/33.4/130
61.3/97.5/104.0/216
153.0/261.0/ /400
Rating of
fuel cell," kw
4.5
9
20
30
100
175
Max. power
from fuel
cell, kw
6.75
13.5
30
45
150
263
Energy
storage, kw-h
37
50
94
102
78
164
a Based on 0.8-volt single cell.
The fuel cell battery charger is given a nominal power rating
corresponding to 0.8 volt per single cell and is capable of overrated
operation up to the maximum kilowatts listed in the fourth column
of Table 3. In each case, the secondary road and highway steady
power demands can be satisfied by the fuel cell, with the battery
being used only during peak power demands.
A typical system may use the molten electrolyte lithium-chlorine
battery, which operates at 450° to 600°C together with a reformer
and a molten carbonate matrix electrolyte fuel cell operating at the
same temperature. Since excellent electrical transient response is
provided by the storage battery energy, the lag time of the reformer
and fuel cell can be rather lengthy, thus allowing very simple,
largely self-regulating controls.
88
Fuel Cell —Battery Power Sources
-------
FUEL CELL AND BATTERY VEHICLE POWER
SOURCE PERFORMANCE
In Table 4 the distribution of weight between fuel cell compo-
nent, battery component, fuel, and fuel tankage is given for the
vehicles.
Table 4. WEIGHT DISTRIBUTION OF FUEL CELL AND BATTERY VEHICLE
POWER SOURCE
JD
ll
'3 ~v
52
155(4.5kw)
297(9kw)
630(20kw)
900(30kw)
l,785(70kw)
3,675(175kw)
*"" "^
> U
ii
!5 §1
/ \ ^ c: ^ J-i
^ (ii .— o a) ^3 — +^
'o —
£ f
'5 *j
§ -O
246
334
626
680
855
1,094
.£?— -
5.2
139
128
609
621
1,051
2,969
"5^
-C ~_
'5 c
S ™
15
14
67
69
117
330
JD
^
"o 'S
K 5
555
773
1,932
2,270
3,808
8,068
o a>
^ o
•tj 3
Q S
52
52
54
54
51
48
^£ !=J
'~ C S F"
0^ O Q. — i
36
36
38
38
36
34
' co K
0.51
0.51
0.49
0.49
0.51
0.54
(2,000
w-hr/lb)
c
"ra
CT 4
LU ^
278
256
1,218
1,242
2,102
5,938
CO
ro
Q
o
4
C\J
2.4
1.2
2.9
2.1
1.8
2.0
JD
-=
5
500
332
630
550
560
735
Each hybrid system meets the duty cycle power requirements
shown in Table 2 and has the equivalent weight of the conventional
engine as given in Table 1. This means that the hybrid fuel cell and
battery system postulated for each of these vehicles will be equiva-
lent in every respect to the conventional internal combustion engine
in performing the specific duty cycle. The calculations were made
on the basis of sizing the fuel cell and battery according to Table 3
requirements and using all excess allowable weight for fuel. The
power source efficiency was calculated during performance of the
actual duty cycle and is shown in column 7 of Table 4. Columns
8 and 9 indicate the allover utilization efficiency and the specific
fuel consumption based on an assumed 70 percent efficient drive
train. Based on these efficiency calculations, 2,000 watt-hours
are obtained from every pound of fuel consumed. This gives a
hybrid fuel cell and battery system for these vehicles an energy
density of from 500 to 700 watt-hours per pound.
Table 5 summarizes the performances of the conventional in-
ternal combustion engine and the battery and fuel cell system.
FRYSINGER 89
-------
Table 5. PERFORMANCE COMPARISON OF THE CONVENTIONAL INTERNAL
COMBUSTION ENGINE AND THE BATTERY AND FUEL CELL HYBRID
SYSTEM
Conventional
Fi>el cell — battery
Vehicle
Type
% ton
% ton
2Vz ton
5 ton
Combat tank
Efficiency,
11
12
18
19
13
Days
before
refueling
0.44
0.45
0.71
0.84
0.64
W-hr/lb
97
129
161
229
254
Efficiency,
36
36
38
38
34
Days
before
refueling
2.4
1.2
2.9
2.1
2.0
W-hr/lb
500
332
630
550
735
Efficiencies of the battery and fuel cell system can be improved by
a factor of 2 to 3, giving energy densities that would allow these
vehicles to go three to five times as far before refuelings.
CONCLUSIONS
Through the use of a fuel cell and battery hybrid power source,
all of the so-called vehicle "battery" problems can be overcome.
This power source allows an electric vehicle to have:
1. Full-range capability.
2. Excellent acceleration characteristics.
3. Very fast energy refuel.
Achieving the performance outlined for the vehicle requires
the successful development of a 150-watt-hour-per-pound molten
electrolyte battery and a 20- to 35-pound-per kilowatt hydrocarbon
fuel cell. Research progress indicates that these goals should be
achieved in operational hardware within the next 5 to 10 years.
90
Fuel Cell —Battery Power Sources
-------
ELECTRIC VEHICLE RESEARCH
Howard A. Wilcox
General Motors Corporation
Warren, Michigan
Among the several types of automobiles presently being con-
sidered for electrification are small urban cars of limited range and
acceleration performance, larger cars intended to be competitive
with today's general purpose passenger vehicles, and special auto-
motive vehicles such as taxi-cabs.* Hence considerable research
and development effort is going into electrochemical energy systems.
Our effort in this area is focussed on two principal objectives:
(1) to conduct exploratory research into a -wide variety of promising
electrochemical couples such as lithium-sulfur, lithium-air, sodium-
chlorine, and sodium-sulfur and (2) to carry out research and
development work on high-performance energy conversion systems
based on the lithium-chlorine couple.
This latter objective is pinpointed here because our work to
date under the first objective indicates that the lithium-chlorine
couple holds the best promise for satisfying most of the applications
of greatest interest to us. Table 1 explains one of the reasons why
this is true. It gives the energy storage density of various reactant
pairs in watt-hours per pound of reactant—that is, per pound of
reactants that have to be carried in the automobile either before or
after the battery has been discharged.
Table 1. COMPARISON OF THEORETICALLY AVAILABLE ENERGY FROM A
VARIETY OF REACTANT PAIRS
Reactant pairs Li/CI Li/S Na/CI Na/S Zn/Air Li/Air H202
' ll050 U3° 60° 30°-650 50° 2'000 i'600
From the energy storage density standpoint, the lithium-chlorine
couple looks good; the lithium-sulfur couple looks slightly better,
the sodium-chlorine couple looks pretty good; and the lithium-air
couple looks best of all. However, the presently achievable electro-
chemical reaction rates at the air electrode are not good even with
the most expensive catalysts, and the direct lithium-air couple still
suffers from that obstacle, so it remains at present an item of purely
potential interest to us. In fact, our study of electrode reaction rates
Other types of vehicles that are already electrically powered or are
being considered for electrification include light, medium, and heavy
trucks; locomotives; and off-road vehicles such as dump trucks and
scrapers.
WILCOX 91
-------
for these couples has caused us to look most favorably, as of now, on
the lithium-chlorine couple. Figure 1 depicts a research-type electro-
chemical cell we have used to explore the performance potential
and operating features of this couple.
CHLORINE INLET!
GAS OUTLET
J CHLORINE
I LITHIUM
LITHIUM CHLORIDE
CHLORINE SEPARATOR
SCREEN
v^^i\ A^mm-^ts i
LITHIUM CHLORIDE-
LEVEL MINI
" ' ---— • LITHIUM ELECTRODE
CHLORINE ELECTRODE
Figure 1. Vertical experimental lithium-chlorine cell.
The chlorine electrode, which is the positive pole of the cell,
utilizes a porous carbon frit to separate the molten lithium chloride
electrolyte on the one side from the hot chlorine gas on the other.
The molten lithium chloride does not wet the carbon frit. The
gaseous chlorine molecules are forced through the pores of the frit
to the interface between the carbon and the lithium chloride. There
they go into solution, at which time most of them dissociate, gain an
electron from the carbon, and become negative ions in the melt. We
have devoted considerable attention to the basic physical mechanisms
underlying the operation of this electrode, and believe we have
achieved an excellent understanding of its electrochemical behavior.
Moreover, we have rather extensively studied its operating features
from an engineering point of view, and believe that it poses no insur-
mountable problem to the future development of a very high-
92
Electric Vehicle Research
-------
performance battery. Our tests have shown the feasibility of achiev-
ing chlorine electrode current densities up to 4,000 amperes per
square foot without significant polarization losses, and still higher
current densities are possible.
Similar studies have been accomplished on the lithium electrode,
which is the negative pole of the cell. Here we use a porous,
stainless steel structure to bring molten lithium from the one side
to the molten lithium chloride on the other. The stainless steel is
completely wetted by at least a thin film of molten lithium, so
lithium chloride is excluded from touching it as long as some excess
of lithium remains. Thus, the electrochemical reaction occurs on
the surface of the thin film of molten lithium facing the lithium
chloride. Our tests show that current densities achieved at this
electrode can exceed 40,000 amperes per square foot without notice-
able polarization losses. Hence, we believe that the lithium-chlorine
electrochemical cell is potentially capable of very high-power-
handling capacity, and, further, we suspect that this power-handling
capacity is ultimately going to be limited more by the chlorine
electrode than by the lithium electrode.
Most of our current research problems in connection with this
system involve trying to find materials capable of long life while
handling the highly corrosive hot chlorine gas, the molten lithium
chloride, the molten lithium, and the corrosive impurities that may
creep into the system. Our research is still in an early stage, and
potential life data for a lithium-chlorine electrochemical power
system cannot yet be presented.
On the basis of our work to date, we believe that development
of a rechargeable electrochemical power system is technically feasible.
In the 100 hp class with an energy storage capacity of about 100
hp-hours, we believe we can probably achieve a lithium-chlorine
electrochemical power system with the characteristics listed
in Table 2.
Table 2. CHARACTERISTICS OF LITHIUM-CHLORINE POWER SYSTEM
Open circuit voltage 3.5 volts per cell
Average operating voltage 3.2 volts per cell
Power capacity 0.2 hp per pound of
electrochemical power system
Energy storage capacity 0.13 hp-hour per pound of
electrochemical power system
As yet, however, we are far from being assured that we can solve
our problems of long operating life, operating hazards, and a
favorable first-cost picture.
Recently General Motors described and demonstrated two full-
scale electric vehicles and showed that there are currently no
WILCOX 93
-------
absolute technological barriers to such developments (Rishavy, Bond,
and Zechin, 1967; Marks, Rishavy, and Wyczalek, 1967; Agarwal,
1967; Wyczalek, Frank, and Smith, 1967; and Winters and Morgan,
1967). Let me tell you briefly about these research vehicles.
One (Figure 2) was a battery-powered car we called Electrovair
II. This car used a 530-volt silver-zinc battery as its basic energy
storage unit. The silver-zinc battery has about five times the energy
storage capacity, and also about five times the power-handling
capacity, as the same weight of typical lead-acid batteries with
which we are all rather familiar. Since they contain a lot of silver,
they are fearfully expensive—much too expensive ever to be practical
for use in mass-produced automobiles. However, they represent the
most advanced type of battery system available today for vehicle
propulsion. Even so, they fall far short of the energy storage and
power capabilities of an engine fueled with gasoline. The Electro-
vair II therefore weighed 3,400 pounds, which is 800 pounds (or
about 30 rr) more than its counterpart., the standard 1966 Corvair.
It also had an operating range of only 40 to 80 miles (depending on
how it was driven), as compared to the 250- to 300-mile range of
the standard 1966 Corvair with a full tank of gasoline. A weight
and performance comparison is given in Table 3.
Figure 2. Electrovair II.
94
Electric Vehicle Research
-------
Table 3.
WEIGHT AND PERFORMANCE COMPARISON OF ELECTROVAIR II AND CORVAIR
Electrovair II Corvair
Weight, Ib 3400
Acceleration (0 to 60 mph) sec 16
Top speed, mph 80
Range, miles 40-80
Power train weight, Ib 1,230
2,600
16
86
250-300
610
The silver-zinc battery pack for the Electrovair II was composed
of 286 cells, and it did not represent a serious development problem.
It was quite otherwise, however, with the electric motor system (see
Figure 3) that took the power from the batteries and converted it
to torque at the wheels of the Electrovair II. In fact, this car was
intended to be a test-bed to evaluate the electric motor drive
system. This propulsion unit was newly developed for this applica-
tion, as well as for others, and it used a set of high-power electronic
switches called thyristors (or silicon controlled rectifiers—SCR's)
to control its torque output. The more conventional direct-current
electric motor utilizes a commutator instead of electronic switches,
the commutator being in effect a set of mechanical switches for
accomplishing much the same function. However, a mechanical
commutator is limited to moderately low switching rates by mechan-
ical and inductive sparking problems; on the contrary, however, the
electronic switches are able to function at relatively high rates.
Consequently, it is possible, using the high-power electronic switches
in what we call the "modulating inverter assembly" (Figure 4) to
design the motor to operate at greatly increased rotational speeds—
about 13,000 rpm—the conventional direct-current electric motor
operates at about 3,500 to 4,000 rpm in this horsepower range.
These high speeds (at the same torque levels) imply, in turn, a
correspondingly increased power-handling capacity for the same
weight of motor. Table 4 gives a weight analysis for this motor
system.
Table 4. WEIGHT ANALYSIS OF ELECTROVAIR II MOTOR
AND RELATED CONTROL SYSTEM
(in pounds)
Motor
Gear box
Modulating inverter assembly
Control electronics
Cooling system
Cables, etc.
TOTAL
130
25
235
24
80
56
550
WILCOX 95
-------
Figure 3. Electrovair II.
96
Figure 4. Modulating inverter.
Electric Vehicle Research
-------
Using such a high-speed motor system, we found it possible to
deliver approximately 100 peak horsepower to the Electrovair II
from a motor weighing only 130 pounds. The conventional type of
direct-current motor in this same application would have required an
estimated 500 to 800 pounds to yield the same number of horsepower.
This extra motor weight would then have required further structural
material and weight in the body of the vehicle, and all this extra
weight would have produced correspondingly increased power and
acceleration losses. By using the lightweight high-power motor
controlled by electronic switches, it was possible to achieve accel-
eration performance in the Electrovair II comparable to the accel-
eration performance of the standard 1966 Corvair, despite the greater
weight of the Electrovair II.
The electric car was simple to operate, but complicated in its
control circuitry. The driver was required to manipulate only the
steering wheel, brake, an on-off switch, a forward-neutral-reverse
lever, and an accelerator pedal that provided smooth, "step-free"
acceleration and speed control over the whole operating range.
Figure 5 shows (schematically) the car's control circuitry. The low-
power logic circuits were required to shut off the main battery
power if anything went seriously wrong, to prevent over-speeding
or over-accelerating, to prevent sudden reversals of output torque
under any conceivable malfunction condition, and so forth. It is
very important, of course, that adequate safety features be built
into such a car, because the hazards are otherwise significant; at
the same time and for the same reason, it is very important that
the operating procedures required of the driver be extremely simple,
smooth, and similar in character and effect to what he is already
accustomed to.
DRIVER POWER__
VROLS' DEMAND
LOW-
POWER
LOGIC
CIRCUITS
SPEED SIGNAL
1 CONTROL
fsiGNAL
LOW-
POWER
TRIGGER
CIRCUITS
j TRIGGER
D-C POWER YPULSESA-C POWER
SILVER-ZINC
BATTERY PACK
SOLID-STAT
INVERTER A
POWER CONTF
E
\|p _ ._
ROL
3-PHASE
A-C
' INDUCTION
MOTOR
SHAFT
•*-
POWER
Figure 5. Control circuitry for Electrovair II.
WILCOX
97
-------
Our second experimental vehicle powered by electricity (Figure
6) was called the Electrovan; it was produced mainly for study,
on an experimental basis, of the problems of powering a passenger
vehicle with an electric fuel battery. The fuel cell modules were
developed by Union Carbide, and we developed the means of putting
them together into a successful fuel battery.
Figure 6. Electrovan.
Figure 7 shows a phantom view of the Electrovan with its
electric drive train displayed rather completely. In this van you
will note that we used a battery composed of 32 fuel cell modules
connected in series to produce an open circuit voltage of 520 volts.
This battery had a power capacity of 32 kilowatts continuous and
160 kilowatts for short time periods. Almost the same drive motor
system was used in the van as was used in the Electrovair, but
in the van it was able to produce 125 horsepower, instead of the
90 or 100 that it developed for the Electrovair, because the fuel
battery voltage in the van did not sag as much under load as did
the voltage of the silver-zinc battery in Electrovair.
The van's fuel battery uses pure hydrogen as the fuel and
pure oxygen as the oxidizer; these can be stored either in high-
pressure containers or in liquid form at very low temperatures. A
little study—summarized in Table 5—shows that cryogenic storage
is best.
98
Electric Vehicle Research
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WATER
CONDENSER
LIQUID HYDROGEN TANK
LIQUID OXYGEN TANK
ELECTROLYTE
RADIATOR
A-C
INDUCTION MOTOR
GEARBOX
ELECTROLYTE
RESERVOIR "
32 FUEL CELL
MODULES
Figure 7. Phantom view of Electrovan.
Table 5. COMPARISON OF WEIGHT, VOLUME, AND RANGE CHARACTERISTICS
OF CRYOGENIC AND COMPRESSED GAS FUEL SYSTEMS
Item
Total volume, ft3
Total weight,
(incl. vaporizers), Ib
Total weight of product,
Ib of H2 and 02
Total specific volume
ft" per Ib H, and 02
Total specific weight,
Ib/lb H2 and 02
Range, miles
Cryogenic
13
389
108
0.12
3.6
100 to 140
Compressed
gas
at 5000 psi
6
358
47
0.13
7.6
45 to 65
Ratio
2.2
1.1
2.3
0.9
0.5
2.3
I want to point out most emphatically, however, that stored hydro-
gen and oxygen gases, whether at high pressure or at low temperature,
would not be satisfactory or practical as an energy source for pas-
senger vehicle transportation.
WILCOX
99
-------
Figure 8 shows the four loops of plumbing used in our Electro-
van. These loops contained hydrogen, oxygen, liquid potassium
hydroxide in aqueous solution, and cooling air. Keeping leaks out
of all this plumbing was of course a major headache during the
debugging of the van.
VENT
£>-^- SOLENOID
VACUUM LINE
RESERVOIR
BURST
LIQUID H2
I
L-LJ
REGULATOR
AND
JET PUMP
CONDENSER/
EXHAUST
WATER
COOLER KM
AIR
Figure 8. Schematic of Electrovan fluid system.
Figure 9 shows the electrical diagram of the fuel battery system
of the van. Since the potassium hydroxide electrolyte had to be
circulated to cool the modules, there was an electrical shunt across
the fuel battery through this electrolyte circulation loop. Our plumb-
ing design was aimed at minimizing this electrical leakage, but
even so it amounted to 2.4 kilowatts.
Table 6 shows the weight analysis and performance levels
achieved with the Electrovan compared with those of a standard
production van.
At this point I should mention that we are interested not only
in the purely technical problems involved, but also in all aspects of
potential markets for our electrochemical power systems and their
100
Electric Vehicle Research
-------
associated electric drives. Some possible applications under study
are listed below:
Small cars
Full-size cars
Taxis and busses
Light, medium, and heavy trucks
Locomotives
Off-road trucks and scrapers
Industrial variable-speed drives
Load levelers for utilities
FRONT OF VEHICLE
ROOF MODULE
FANS DUCT FANS
«i—OO
Jt
GROUND
OCURRENT
MONITOR
OIL
PUMP
KOH
PUMPS
+ DC RIGHT SIDE
.GAS SOL
BLOWER OIL FAN
CONDENSER-
COOLER FAN
Figure 9. Schematic of Electrovan fuel cell electrical
system.
Table 6. COMPARISON OF PERFORMANCE AND WEIGHTS OF ELECTROVAN
AND CMC PRODUCTION VAN
Total vehicle weight, Ib
Fuel cell powerplant weight, Ib
Electric drive weight, Ib
Powertrain total weight, Ib
Acceleration (0 to 60 mph), sec
Top speed, mph
Range, miles
Electrovan
7,100
3,380
550
3,930
30
70
100 to 150
CMC van
3,250
870
23
71
200 to 250
WILCOX
101
-------
Many of these markets are highly dynamic and are influenced not
only by economic, but also by political and psychological, pressures.
In summary:
1. Potential advantages of electric vehicles include quiet operation,
efficiency not limited by heat engine cycle, reduction of emis-
sions, and use of low-cost electricity generated by nuclear reactors
when fossil fuels become more scarce.
2. Research on electric vehicles offers potential for commercial
payoff in many other fields.
3. Any power system, considered in the whole, presents possible
pollution problems.
4. It is still too early to state definitely that any kind of electric
automobile will soon be able to win an economically significant
and viable place for itself in the total transportation picture.
Returning to comment briefly on the first of these points, the
single most important reason for the current strong resurgence of
interest in electric motor cars is the growing problem of air pollu-
tion; however, we are also interested in electric vehicles because
the development of efficient and constantly improved systems of
energy conversion is vital to the automotive transportation industry.
For this reason, we •will continue to pursue aggressively any method
of energy conversion that offers potential for improving our vehicles.
I should like to mention at this point that if one is willing to
accept the relatively low performance levels currently promised
by the all-electric automobile, then we should also take a careful
look at how much emissions can be reduced from a oar powered
by a small internal combustion engine that features the same per-
formance level as the all-electric. Emission level comparisons can
only be validly made on the basis of comparable performance levels.
Concerning the second point, greatly improved batteries and
also improved electric motors are going to be needed if millions
of electric cars on the highways are ever to become a practical
reality. As far as energy storage and power-handling capacity is
concerned, passenger cars with internal combustion engines perform
very well indeed. Consequently, much improvement in batteries
and electric motors is needed if they are to be able to perform
approximately as well as the internal combustion engine. Even if
we are not able, in the near future, to achieve the high levels of
performance required for a truly practical electric passenger vehicle,
improved batteries and motors will undoubtedly produce commercial
payoffs in other applications. We are interested in improving the
electric power train of diesel locomotives, for example. We are
interested in improving our starter batteries. So we believe we
have justification for pushing ahead with our research and develop-
ment in all these fields.
102 Electric Vehicle Research
-------
Turning to the third point, the fact is often overlooked that
any power system creates pollution problems. Suppose, for example,
one has a completely sealed battery that draws and stores energy
from an electric wall plug and delivers it on demand to the wheels
of the car. This would appear to be a power system wholly free of
pollution problems. However, power drawn from the wall plug has
to be generated somehow, and presently it is generated by a central
power-generating plant that uses oil or coal as fuel. This plant
has an exhaust stack, of course, and at all times emits oxides of
sulfur, nitrogen, and carbon, because these are combustion products
when coal or fuel oil is burned in air. Oxides of sulfur and nitrogen
are, of course, some of the major offenders in air pollution. Carbon
monoxide is another undesirable pollutant if present. However, it
usually is not found in significant amounts in power plant stack gases.
Of course, many technologists believe that most of our energy in
the future will be generated by large nuclear reactors. However,
these reactors have pollution problems too, since radioactive waste
products are generated and have to be disposed of appropriately.
Consequently, we see that any power system, considered in the
whole, generates some sort of pollution problems.
With respect to the fourth point, it is believed that electric
vehicle propulsion is technically feasible. However, our research has
not indicated when it would be economically feasible or acceptable
to customers in terms of safety, utility, and convenience.
In some applications, such as diesel-powered locomotives, electric
drives have thus far demonstrated their capacity to do the job
better than mechanical power trains. However, the personal pas-
senger vehicle imposes its own very stringent and special require-
ments on its power train. Many American passenger cars are
required to have a heated cabin when it is cold outside, an air-
conditioned cabin when it is not cold outside, several powerful
headlights, electric window lifts, a radio, a record player, and other
convenience items. All these take power—in some cases as much
power as is required to propel the car along the road at moderate
speed. The modern American passenger car requires peak power
levels five to seven times larger than it uses at urban speeds
Moreover, it is frequently driven hundreds of miles with only a
few brief stops for "re-energizing." Hence, the major obstacle to
the early mass use of an all-electric car is the present day lack
of an adequately high-power plus high-energy-capacity storage or
fuel battery at reasonable cost. Some batteries have good power
but poor energy storage capacity, some have the reverse, but none
is adequate in both areas at once.
We know that any vehicular power plant must be developed
first for technical feasibility, next for producibility and cost, then
for durability under engineering test, and finally for safety, relia-
bility, and convenience in the hands of the public.
WILCOX 103
-------
At this time not all the answers to attaining these goals with
electric power are known. Consequently, we believe that our ex-
tensive research and development work must be continued if we
are to continue to make necessary progress in energy conversion.
REFERENCES
Society of Automotive Engineers, 1967 (Jan. 13).
E. A. Rishavy, W. D. Bond, and T. A. Zechin. Electrovair—a
battery electric car. SAE Paper No. 670175;
C. Marks, E. A. Rishavy, and F. A. Wyczalek. Electrovan—a
fuel cell powered vehicle. SAE Paper No. 670176;
P. D. Agarwal and T. M. Levy. A high performance AC electric
drive system. SAE Paper No. 670178;
F. A. Wyczalek, D. L. Frank, and G. E. Smith. A vehicle fuel
cell system. SAE Paper No. 670181;
C. E. Winters and W. L. Morgan. The hydrogen-oxygen thin fuel
cell module. SAE Paper No. 670182.
104 Electric Vehicle Research
-------
EUROPEAN DEVELOPMENTS OF POWER SOURCES
FOR ELECTRIC VEHICLES
M. Barak
Chloride Electrical Storage Company Ltd.
London, England
INTRODUCTION
The immediate availability of conventional storage batteries has
stimulated in Europe the building of a number of small electric
vehicles for testing. At the same time many companies have initiated
programs on the research and development of novel devices such as
fuel cells. The progress of this work on passenger-carrying vehicles
and power systems is reported here.
The use of electric power to drive automobiles is by no means
new. In 1899 M. Jenatzy, a Belgian racing driver, momentarily won
the world speed record at 65.8 mph with his electric car La Jamais
Contente when he defeated a fellow racing driver, the Comte de
Chasseloup-Laubat, also driving an electric car (Electricity Council,
1966). Unfortunately, shortly afterwards, Jenatzy forsook his first
love and turned to the gasoline internal-combustion-engine vehicles,
which at the turn of the century, because of their greater range and
speed, were rapidly ousting the electrics from popular favor in many
cities in the United States as well as in Europe. With the prolifera-
tion of the internal-combustion engine we now face the menace of
chronic traffic congestion in the cities, as well as the acute discom-
fort and hazards to health caused by noise and noxious fumes. On
the latter point, it has been estimated that about 5 million tons of
carbon monoxide belch forth from the exhaust pipes of the world's
internal-combustion-engine vehicles each year. This is equivalent to
a daily output of this lethal gas of about 400 million cubic feet; in
addition the vehicles emit quantities of partly oxidized hydro-
carbons, some of which are believed to be carcinogenic.
It is worth noting that the internal-combustion engine is con-
suming its fuel and pouring out noxious fumes even when the vehicle
is stationary and idling in traffic. This does not apply to an electric
vehicle.
The wheel has now turned full circle. Interest in electric auto-
mobiles has been re-kindled in many of the highly developed coun-
tries of the world. What factors led to the demise of these vehicles,
and how can they be resuscitated?
THE POWER UNIT
The obvious deficiency lay in the power unit. Lead-acid and
nickel-iron alkaline storage batteries have to date provided the only
BARAK 105
-------
source of power sufficiently reliable and durable to meet industrial
and commercial requirements. But these systems have relatively low
power densities, in the range of 10 to 15 watt-hours per pound for a
running time of 2 to 5 hours, so that a large weight of battery has
to be carried to provide horsepower equivalent to that of even a
small automobile. For 10 horsepower, for example, this amounts to
between 1,000 and 1,500 pounds.
A few years ago the silver oxide-zinc couple made a meteoric
appearance on the battery scene, offering a four- to five-fold increase
in the energy density of the conventional storage batteries. Un-
fortunately, it proved to have poor reversibility and a low cycle life
in multicell systems and this, together with the high material cost,
has restricted its commercial use.
More recently still, fuel cells have received much publicity.
Theoretically and in fact on the laboratory scale, these systems offer
a considerable reduction in weight, although not in overall volume,
and they offer the added advantage that the fuel can be replenished
in about the same time as would be taken to refill the gasoline tank
of a conventional automobile. Fuel cells are therefore attractive for
this purpose, but they are complex systems and rather expensive to
make, and at present, the most effective fuels are expensive or diffi-
cult to handle. Hybrid systems sometimes called semi-fuel cells,
having a metal anode, such as zinc — as in storage batteries — and
a fuel-cell-type cathode consuming oxygen or air such systems are
under development both in primary and reversible forms and show
some promise. Reversible semi-fuel cells, like storage batteries, how-
ever, have the disadvantage that several hours are required for re-
charging.
Within the past few months disclosures have been made of new
electrochemical couples, in particular sodium-sulphur and lithium-
chlorine. These show, on paper at least, the highest energy densities
attainable so far, but for various reasons, they have to be worked
in the temperature ranges 300° to 500°C and 500° to 650°C, respec-
tively. A preheating source is therefore necessary, and these systems
would probably have to carry a considerable amount of insulation for
intermittent use. Also, the active materials would present dangerous
hazards in the event of an accident.
ENERGY DENSITIES OF DIFFERENT SYSTEMS
Figure 1 shows the energy densities in watt-hours per pound
for different durations of discharge for various systems mentioned
(Barak, 1965). The values given for the Bacon hydrox cell, consum-
ing hydrogen and oxygen and working at temperatures around 200°C
and pressures of 600 psig, include the estimated weight of all the
ancillary equipment and are still the most favorable for fuel cell
systems.
106 European Developments
-------
150
100
80
60
50
' 40
: 30
- 20
15
10
8
6
5
4
3
2
1.5
1
'X LITHIUM-CHLORIDE
X SODIUM-SULFUR
X ZINC-AIR
LEAD-ACID
NICKEL-CADMIUM (IRON)
J L
_LJ I L
_L
30
60
346
10
hr
20
40 60 100
LOG TIME
Figure 1. Energy densities at different rates of
discharge for various battery systems.
These curves indicate that for continuous durations of discharge
of less than about 1 hour, storage batteries have a higher density than
fuel cells. This becomes more and more pronounced as the current
approaches starter battery currents. It is unlikely, therefore, that
any of these other systems will displace conventional storage batteries
for this service in internal-combustion-engine vehicles. On the other
hand, for periods of discharge of about 2 hours and longer, common
to traction service, fuel cells show a continuing superiority over stor-
age batteries. The position of the semi-fuel cells and the new high-
temperature storage batteries should be noted.
DEVELOPMENTS IN ELECTRIC TRACTION
Work on electric passenger vehicles is being actively carried
out in Great Britain. The climate is particularly favorable because
electric vehicles for industrial and commercial uses have been popular
for a longtime. Hender (1965) has shown (Figure 2) how the num-
ber of battery-electric vehicles registered for road use increased be-
tween 1953 and 1964. 'nhese now number over 40,000, and with the
70,000 battery-operatt electric industrial trucks, make a total popu-
lation of about 110,000.
BARAK
107
-------
- 150
CO
o
_
-2
> in
P CTl
< d
cc
o
o
1952
1956
1960
1964
100
YEAR
Figure 2. Increase in the number of battery-electric
vehicles registered for road use (excluding p.c.v.s.)
with comparative fuel costs of oil and electricity.
Experience over many years has proved that, in Great Britain
at least, the fuel and maintenance costs are less for an electric than
for an internal-combustion-engine truck; the life expectancy is ap-
preciably greater, and there are other benefits, such as freedom from
noise and fume. The favorable cost of electricity compared with fuel
oil is indicated by curves a and b in Figure 2. Some comprehensive
figures for economies in depreciation, maintenance, and running costs
achieved by the use of electric fork-lift trucks are given in The Elec-
tric Industrial Truck, published by the Australian Lead Development
Association, 95 Collins Street, Melbourne. The annual figures for
depreciation and maintenance of an electric vehicle are estimated to
be only 51 percent of those for a diesel truck and the economy in
fuel, 67 percent. Within recent years also, considerable improve-
ments have been made in the energy density of storage batteries of
both the tubular type and the flat plate-glass retainer types. For
example, at the 5-hour rate, the gains in watt-hours per pound and
watt-hours per cubic inch have been about 35 percent, and at the
30-minute rate, about 40 percent (Barak, 1965).
The advent of new materials, e.g., lighter and stronger plastics
suitable for battery cases, and of improved designs to reduce the
proportion of inert metal in the battery will undoubtedly allow still
further gains. Unfortunately, though significant, these gains will be
marginal and it is unlikely that spectacular improvements will be
achieved again.
108
European Developments
-------
Several electric passenger vehicles with storage batteries as the
power unit have been produced during the past few years. Four of
these were recently demonstrated by the Electricity Council and de-
scribed by K. W. C. Jeremy (1966). Three were conversions of in-
ternal-combustion-engine cars, but the fourth, the Scamp, made by
Scottish Aviation Ltd., was specially designed for the purpose. The
objective was a "town-about" car, small enough to avoid traffic
congestion, easy to maneuver and park, quiet and fumeless, and
therefore a noncontributor to the smog problem, with a fairly lively
performance, and a range of about 20 to 30 miles.
The characteristics of one of the converted Mini-travellers made
by the British Motor Corporation and the Scamp are given in Table 1.
Table 1. CHARACTERISTICS OF THE MINI-TRAVELLER
AND THE SCAMP
Mini-traveller Scamp
Curb weight, Ib 2,499 1,000
Motor V67 type made by AEI, Ltd., Twin, made by CAV, Ltd.,
with chain drive to original driving onto each rear wheel
differential.
Rating 96 v, 110 amp 24 v, 150 amp (each)
equiv to 12 hp equiv to 2.7 hp
Weight, Ib 178
Battery 96 v, 66 amp-hr (1 hr) 45 v, 105 amp-hr (5 hr)
Power, kw 6.3 1
Energy, kw-h 6.3 (1 hr) 5 (5 hr)
Weight, Ib 830 400
Controls Thermistor pulse unit with
regenerative braking
Weight, Ib
Maximum speed, mph
Range, miles
110
41
30
35
15-20
The range of these cars is generally regarded as being too low.
Range should be nearer 100 miles on one charge, and this calls for
a system with higher energy density.
Table 2 lists some of the salient features of the storage battery on
the Mini-traveller and also the minimum specification for an energy
source to meet this requirement.
I will mention various other custom-built cars under construc-
tion and present photos of some of these.
BARAK 109
-------
The Carter Coaster is made by Carter Engineering Co., of Tarn-
worth, Staffordshire. This vehicle is expected to have a top speed
of about 30 mph and a range of 40 to 50 miles.
The Winn car is made by the Telearchics Ltd. of Lechlade,
Gloucestershire, who are also developing battery-propelled motor-
cycles.
•
110
European Developments
-------
-------
Table 2. STORAGE BATTERY CHARACTERISTICS AND SPECIFICATIONS
Power
Weight, Ib
Watt-hr/lb
Volume, ft3
Watt-hr/ft3
Range at 30 mph,
miles
Life, cycles
years
Costs, capital
£/kw
recharging
or refueling
Storage battery
96 v, 66 amp-hr (1 hr)
6.3 kw-h or kw
830
7.6
5
1,300
30
1,500 to 2,000
5 to 6
£150 ($420)
£23 ($64)
0.8d/kw-h (0.930
0.33d/mile (0.380
Minimum specifications
25 kw-h (2 hr)
< 1,000
25 to 30
6
2,000
50 to 100
> 1,000
4
< £250 ($700)
£20 ($56)
Note: The cost of petrol (gasoline) in Great Britain is about 5s 3d (73 cents)
per gallon; for a medium-sized car consuming about 1 gallon each 30 miles
this is equivalent to a cost of about 2 d (2.3 cents) per mile.
Tube Investments Ltd., has built a small-two seater city car with
an all-in curb weight of about 1,000 pounds. It is powered by lead-
acid storage batteries of 48 volts and 350 pounds weight. This vehicle
is fitted with a semiconductor-controlled rectifier (SCR) pulse-
controller system of novel design, developed by Lansing-Bagnall Ltd.
Tests have been made over several hundred miles and the range in
open country at a speed of about 25 mph is 50 to 60 miles. On tests
in commuter traffic this car has traveled 27 miles with 117 stops on
one charge.
Ford Company of Great Britain is building two models ulti-
mately to be fitted with the sodium-sulfur batteries under develop-
ment at the Research and Development center in Detroit. It is under-
stood that the initial trials will be made with conventional storage
batteries.
An interesting development in France is a battery-powered bus
made by Sovel for operation in the airport of Marseilles. In Italy the
Fiat Company is understood to be experimenting with electric cars.
The Urbanina, made by Bargagli Cristiani of Santa Groce, a small
car for town use, of very novel design, was recently demonstrated at
the Italian Motor Show.
112
European Developments
-------
All of these vehicles are powered by the well-proved lead-acid
storage batteries. Recognizing the limitation imposed by the weight
of these batteries, engineers are examining all aspects of the vehicle
to obtain the best performance possible, e.g., the use of SCR pulse-
control systems to ensure the maximum efficiency of use of the
battery, design of chassis and body to keep the weight to a minimum
and provision of regenerative braking, which is estimated to increase
economy of power by 15 to 30 percent.
FUEL CELLS AND METAL-AIR CELLS
Three industrial companies in Great Britain are actively en-
gaged in developing fuel cells, some of which could be used to power
electric vehicles. Shell Research Ltd., who has demonstrated units
of about 1-kilowatt capacity with an alkaline electrolyte consuming
hydrazine and hydrogen and a 5-kilowatt hydrogen-oxygen battery
linked to a methanol reforming unit, has lately turned attention to
cells with acid electrolyte, breathing air. Figure 3 shows a 300-
watt battery of this type with a sulfuric acid electrolyte (private
communication with K. R. Williams, Shell Research Ltd.). Two
marked advances have been made recently. The first concerns the
use of porous plastic substrates with higher temperature stability than
materials used earlier; the second concerns the hydrogen and oxygen
electrodes, which are now capable of working at current densities as
high as 500 milliamperes per square centimeter in sulfuric acid
at a cell voltage of 0.6 volt.
Energy Conversion Ltd., who is associated with Pratt and Whit-
ney Co. and is working on a variety of systems, has recently de-
veloped a methanol-air battery with internal reforming and hydro-
gen diffusion membranes of palladium-silver alloy. A 4-"bi-cell" is
shown in Figure 4 (private communication with J. C. Hart, Energy
Conversion Ltd.). This works at 200° to 250°C and has an output
of about 170 watts at 2.7 volts overall. By virtue of their agreement
with Leesona Moos, Energy Conversion Ltd. is also engaged in work
on the zinc-air system.
Electric Power Storage Ltd., a member of the Chloride Group
of Companies, has been working primarily with hydrogen-oxygen or
air and hydrazine-oxygen or air cells (Barak, 1963). Many 60-watt
cell modules of each type have been built and have given satisfactory
lives up to 10,000 hours on test (Gillibrand and Gray, 1966). Typical
cells are shown in Figures 5 and 6. Batteries of 1 to 2 kilowatts have
also been built. Figure 7 shows a 2-kilowatt hydrogen-oxygen battery
of 63 cells, having an overall voltage on load of about 36 volts
mounted on a small industrial truck.
Joseph Lucas and Co. has recently announced association with
General Dynamics in the development of semi-fuel cells of the zinc-
air type; experimental work on these systems is also being done by
Energy Conversion Ltd., and by Electric Power Storage Ltd.
BARAK n3
-------
Figure 3. 300-watt methanol-air battery with sulfuric
acid electrolyte (Shell Research, Ltd.).
OTHER DEVELOPMENTS IN EUROPE
In Germany, the Varta Company has been engaged in fuel cell
development for some years and recently demonstrated a fork-lift
truck powered by a hydrogen-oxygen fuel battery of about 2 kilo-
watts with an overload factor of about 3. This battery has electrodes
of the sintered nickel, DSK type developed by Justi (1962), giving
energy densities of about 10 watts per pound and about 1 kilowatt
per cubic foot (Justi and Winsel, 1962).
114
European Developments
-------
Figure 4. 120-watt methanol-air battery operating
at 200°C (Energy Conversion, Ltd.).
BARAK
115
-------
Figure 5. 60-watt, low-temperature, H2-02 cell
(Electric Power Storage, Ltd.).
Siemens-Schuckertwerke has also experimented with hydrogen-
oxygen fuel batteries as a means of propulsion. Using electrodes of
the Justi type, Siemens has built a 24-volt assembly with an output
of about 400 watts to power a small boat. (Sturm, Nischik and Weid-
lick, 1966). Siemens has stated however, that "predictions of fuel
cells entering into the fields of automobiles and large-scale power
generation are purely speculative and must be considered unrealistic
in the light of present knowldge" (Sturm, 1966).
In France, the Office National Industrial de 1'Azote (O.N.I.A.)
has experimented with cells of the Bacon type working at about
200°C and at both high and atmospheric pressures (Laroche, 1965).
Units up to 2 kilowatts have been constructed, and a battery of 20
kilowatts is under development. To avoid the problem of carrying
heavy bottles of gas, O.N.I.A. is developing batteries with built-in
units such as ammonia-crackers to produce on-site hydrogen.
Compagnie Generale d'Electricite (C.G.E.) is engaged in work
on low-temperature hydrogen-oxygen cells and has produced units
with outputs up to 1 kilowatt (24 volts) (C.G.E., 1966).
In Sweden, Allmana Svenska Elektricka Aktiebolaget (A.S.E.A.)
is carrying out a considerable program of work in the development of
116
European Developments
-------
low-temperature hydrogen-oxygen fuel batteries (Lindstrom, 1966).
Using sintered nickel electrodes of their "quadrus" type with nickel
boride catalyst, A.S.E.A. has made a number of 5-kilowatt units,
some of which have been tested on fork-lift trucks and also on other
large industrial trucks. A.S.E.A. is also building a 200-kilowatt unit
that consumes hydrogen from an ammonia cracker to power a coastal
submarine.
Figure 6. 60-watt, low-temperature, N2H4-02
oxygen cell (Electric Power Storage, Ltd.).
In Switzerland, Brown, Boveri Co., Ltd. is experimenting with
low- and high-temperature systems that consume a variety of fuels,
particularly methanol and hydrogen (Plust, 1966). Though primarily
looking at low-output applications such as harbour buoys, TV re-
peater stations, and so on, the company has a high-performance
H2~O2 unit of several hundred watts output. In Italy, the Fiat Com-
pany is also known to be experimenting with low-wattage units
that consume hydrogen and to be looking at possible developments
of electric automobiles.
SUMMARY
Several European Companies are exploring possible uses of fuel
batteries for electric traction with the system generally chosen
being the low-temperature hydrogen-oxygen (or air) cell. Although
none of these developments has yet reached the stage where it could
BARAK 117
-------
be adopted for passenger vehicles, they should supply data for proper
feasibility studies in the near future.
Some of these systems could meet many of the requirements
in the specifications in Table 2, but none could be made at a cost
anywhere near the limit of £20 ($56) per kilowatt. Much of the
capital cost of fuel cells is absorbed in the noble metal catalysts.
Some progress is being made in reducing the amount necessary, but
this is slow and the search for cheap alternatives that could be used
with cheap low-grade fuels has not so far achieved a breakthrough.
The materials of construction and other components also appear to
make the cost prohibitive. Whether these problems can be solved
will depend on the progress made during the next few years.
Figure 7. 2-kilowatt, hydrogen-oxygen low-temperature
battery on an industrial truck (Electric Power
Storage, Ltd.).
REFERENCES
Barak, M., 1963. Some developments in fuel cells. In: Fuel cells.
Chemical Engineering Progress Technical Manual. American
Institute of Chemical Engineers, New York, N. Y. p. 74.
Barak, M., 1965. Developments in electrochemical energy-conversion
devices, batteries and fuel cells. Proc. Inst. Elec. Engrs 112-1439
(July).
118
European Developments
-------
Compagnie, Generale d'Electricite, 1966. Les piles a combustibles on
centre de recherches de la Compagnie Generale d'Electricite.
Marcoussis, France (Oct.).
Electricity Council, 1966. British electric passenger cars. An interim
report. Electricity Council, London SW1, England. (Mar.).
Gillibrand, M. I., and J. Gray, 1966. A Ikw hydrogen fuel battery.
Presented at 5th International Power Sources Symposium, Sept.
1966, Brighton, England. Publication pending in: Power sources
1966. Collins, ed. Pergamon Press, Ltd., London, England.
Hender, B. S., 1965. Recent developments in battery electric vehicles.
Proc. Inst. Elec. Engrs. 112:2297-308 (Dec.).
Jeremy, K. W. C., 1966. The electric car. Report by the Electricity
Council's Appliance and Method Research Panel. Reproduced
from the March/April edition of Electricity, published by the
Electricity Council, London SW1, England.
Justi, E., and A. Winsel, 1962. Kalte Verbrennung. Franz Steiner
Verlag G. m. b. H., Wiesbaden, Germany.
Laroche, J., 1965. Les piles a combustible a moyenne temperature.
Revue Generale de 1'Energie. 74: 63-66.
Lindstrom, O., 1966. Fuel cells for traction purposes. Electrical Rev.
pp. 243-46 (Aug. 12).
Plust, H. G., 1966. Aussichten der Brennstoffzelle als Automobilantrieb.
Automobil Rev. No. 25. Special publication by A. G. Brown,
Bovin and Cie., Baden, Switzerland.
Sturm, F. v., 1966. Electrodes in fuel cells. Siemens Rev. 33:118-23
(Mar.).
Sturm, F. v., H. Nischik, and E. Weidlich, 1966. Fortschrifte in der
Brennstoffzellen-Entwicklung. Ingenieur Dig. Vol. 2, issue 5.
BARAK
-------
AUXILIARY SYSTEMS: component
development; other aspects of the total
vehicle system.
Having worked with energy conversion and batteries since 1934,
I am gratified to see the progress made in new and promising systems,
either in production or in advanced stages of development. Before
World War 17 the only secondary batteries were the lead-acid and
nickel-iron in this country and lead-acid, nickel-cadmium, and
nickel-iron in Europe.
At a time when a new urgent requirement for high-energy
systems arises, many candidates are available thanks to the foresight
of DOD and NASA in promoting very effective and far-reaching
investigations of energy conversion systems from the beginning of
World War II to the present time.
If similar efforts had been made on the rest of the components
needed for the complete electric vehicle design, we would be much
nearer to a matched power source and electric vehicle system.
We will go through a period of overdesign (larger power require-
ments) for the power source until a more efficient motor and control
system are available. Thus, I strongly recommend that concentrated
efforts be made to stimulate work in these other areas to bring them
abreast of the power source effort.
121
-------
I do not subscribe to the continuous complaint that the electric
vehicle is impractical because of the lack of a suitable power source.
This is true only because of the lack of development work on the
rest of the components and systems required. A practical electric
vehicle must be developed from the ground up as a system and not
as individual components. We should not blame the power source
for inefficiencies of the remainder of the system.
Although the power sources under development are not yet a
final product, toe can feel proud of the advances to date and the
number of candidates available for design into the proper end
product. With a little down-to-earth planning there is no reason
why a successful system cannot be developed in the next 5 to 10 years.
Along with this development another major problem arises —
educating the public to accept a practical electric vehicle regardless
of appearance and limited performance. This effort may require as
much time as developing the vehicle.
Paul L. Howard
Consultant
Centreville, Md.
122
-------
ELECTRONIC CIRCUITS FOR SPEED
CONTROL AND BRAKING
Victor Wouk
Gulton Industries, Inc.
Neio York., N.Y.
SUMMARY
Modern high-power semiconductors in comparatively simple cir-
cuits, controlling power from batteries that are capable of being re-
charged rapidly, make a battery-powered bus technologically practical
for some urban applications. Regenerative braking can substantially
extend the range between charges, in stop-and-go traffic, besides
being a necessity for safety.
Some basic concepts of electronic circuitry incorporate "off-the-
shelf" high-power semiconductors and 100-ampere-hour cells that
can be charged at rates up to 1,000 amperes, for an operational bus.
INTRODUCTION
Experimental electric vehicles, operated with both batteries and
fuel cells have been developed utilizing modern high-power semi-
conductors and power sources (SAE, 1967). David Ash (1967), a
car-performance analyst, stated that the performance of some experi-
mental cars compares favorably with that of conventional automo-
biles. An electric bus with electronic controls would give similar ac-
ceptable performance. High speed and long range between charges
are not mandatory for a bus in urban traffic. A battery-operated
delivery van would also be satisfactory in urban applications, because
a van does not need high speed and long range. Hender (1965)
reported that in Great Britain approximately 40,000 vehicles are
battery-operated. The high cost of gasoline in Great Britain makes
this particular factor of great importance, in addition to air pollution
reduction. Since at present no major economic drive in the United
States would encourage the use of battery-powered delivery vans, an
initial political solution is probably most acceptable. For the intro-
duction of battery-powered vehicles for reduction of air pollution,
I suggest that we start with the battery-powered bus for the following
reasons:
1. The bus is a much more obvious offender than a delivery van as
contributor to air pollution because it is much larger and creates
such obvious clouds of exhaust.
2. There are fewer buses than delivery vehicles.
3. The bus is generally public-owned, so the problem of initially
higher cost should not be a major deterring factor.
WOUK 123
-------
4. The sense of community participation would be much greater
with a bus, since so many people ride buses. Large numbers of
citizens could experience pollution-free transportation, thus en-
couraging further expansion of battery-powered vehicles as better
power sources are developed.
SPEED CONTROLS
The major objectives of electronic circuits for speed control and
braking, in comparison with "conventional controls", are these:
1. Maximum utilization of stored energy for driving the vehicle,
i.e., minimum power dissipation in the speed controls (for maxi-
mum range between battery charges).
2. Stepless speed control to simulate as closely as possible operation
of the accelerator of a conventional vehicle (for acceptable vehicle
performance).
3. Regenerative braking controls that will return energy to the
batteries during the slowing-down of the vehicle (for further
extension of vehicle range).
The objectives can be met by means of modern solid-state elec-
tronic circuitry. The degrees to which the objectives can be met are
essentially governed by economics. Although the developmental costs
of some automatic controls may be high, with advances in integrated
and etched circuits techniques, the actual electronic logic circuitry
can be cheap.
CONVENTIONAL CONTROLS
Figure 1 is a typical example of conventional controls, as used in
the Henney Kilowatt. The .batteries are switched from parallel to
series-parallel to series arrangement by means of heavy-duty con-
tractors. Also, the field winding of the motor is switched with
paralleled and series resistance.
The electric motor in the "first generation" of electric vehicles
can be a direct-current motor, "series" connected. The direct-current
motor requires considerably fewer components and much simpler con-
trols than an alternating current (SAE, 1967), and since this paper is
confined to vehicles for urban use, which means short hauls and
stop-and-go traffic, the increase in flexibility of performance possible
with alternating current is not discussed. Further, the series direct-
current motor is recognized as ideal for vehicles, because at low speeds
its high torque can be used for rapid acceleration.
The control method of Figure 1 is a "lossless" speed control,
(except for the lowest speed), because essentially all of the power
coming from the battery goes into the motor. No power is dissipated
in loss elements to control the speed. This system is not completely
satisfactory, however. Besides the operational disadvantage of contac-
124 Speed Control and Braking Circuits
-------
tors switching heavy currents (currents above 500 amperes have been
measured in performance of this car), which can result in rapid
deterioration of the contacts in heavy stop-and-go traffic, the per-
formance is jerky. Each time the system switches, the motor power
changes rapidly, producing a discontinuous change of acceleration,
or a jerk.
When being interrupted by a contactor, the currents develop
microscopic amounts of ozone. The quantity is so small, that even if
all internal combustion vehicles were changed overnight to electric
vehicles with contactor controls, the increase in ozone concentration
in urban areas would be scarcely detectable. Further, the use of
electronic controls in any practical electric vehicle (see next section)
would eliminate all ozone.
ELECTRONIC CONTROL
Figure 2 shows the basic circuit that can provide stepless power
control. This circuit is known variously as ''variable width pulse,"
"pulse width modulation", or "time ratio control." It is referred to
herein as variable width pulse, or VWP
In Figure 2, the source of power, assumed to be a battery, is
permanently connected for maximum voltage output. A switch (S)
is opened and closed rapidly to produce pulses of controlled width
at the terminals (T) of the motor. Across the terminals of the motor
is a diode (CR1), connected so that it does not conduct when the
switch is closed. The series motor field is (F), the armature (A).
It can be shown (Wouk, 1962) that if the switching frequency
of the switch (S) is high enough, then the motor will operate as
though a steady direct-current voltage were applied to the terminals,
the direct-current value being the average value of the voltage ap-
plied. This average is given by the equation:
where
tli2 = time on (See Figure 3)
T = time between start of successive pulses.
With the switch closed for a short percentage of the total period,
tj (Figure 3a), the equivalent voltage Eollt, applied to the motor is
low and ihe resulting speed is low. The current that is drawn depends
upon the load on the motor. If the switch is closed for a longer portion
of the period, t2 (Figure 3b), the effective voltage applied to the
motor terminals," is higher, and hence a greater speed is called for.
The ratio of t/T is the "duty cycle."
If the switch (S) of Figure 2 is an electronic solid-state device,
such as a thyristor (the generic term for an SCR or semiconductor-
controlled rectifier), then it can be turned on and off with great
WOUK
125
-------
POWER CIRCUIT
50 mv
KEY SWTCH
FORWARD-
REVERSE -1-
12V. "SWITCH-DASH
NOTE:
D DENOTES CONTACTOR
AUXILIARY SWITCH.
CONTROL CIRCUIT SWITCHES
MARKED RS, FS, HI, FR, LO
ARE CAM-ACTUATED.
CONTROL CIRCUIT
KEY SWITCH OFF
Figure 1. Diagram of conventional controls.
126
Speed Control and Braking Circuits
-------
rapidity, in the order of microseconds. (With a solid-state control, the
current is interrupted and started with no arcing whatsover, and ac-
cordingly no ozone is generated. Thus the ozone problem discussed
earlier is completely academic.)
u
i-;
CRI
MOTOR
Figure 2. Basic variable-width pulse circuit for stepless,
"lossless" direct-current motor control.
The function of the diode CRI of Figure 2 is to bypass the motor
current from the battery during the "switch off" interval. Without
this diode, often called a free-wheeling diode, if the switch were to
attempt to interrupt the current in the inductive circuit of the motor,
high voltages would be generated that would vitiate the simplifying
assumptions.
The circuit shown in Figure 2 can effect motor voltage control
with no inherent power loss (except for unavoidable dissipation in
the bypass diode CRI, and in the switch itself). Also, the following
additional performance characteristics are readily obtainable with
simple feedback controls.
Smooth Acceleration
Even if the accelerator were to be "floorboarded," i.e., depressed
so as to demand virtually continuous conduction through the switch
(S), the pulse width could be increased comparatively slowly to
ensure gradual increase of voltage application to the motor.
This "gradual" could be over a period as short or as long as is
desired, depending upon acceptable acceleration characteristics. It
WOUK
127
-------
could be adjustable from the dashboard to give the vehicle a "jack-
rabbit" getaway characteristic, or a "little old lady" slow-acceleration
characteristic (presently associated with electric-vehicle perform-
ance).
(a)
F
'-
n
\
F
F ft m
—
L
j
LOW
SPEED
(b)
Eln (t2/T)
HIGH
SPEED
Figure 3. Waveforms of voltage applied to motor,
low and high speeds.
Battery Current Limitation
Circuitry can be incorporated to limit the battery current, both
for automobile performance control and for battery protection.
Thus, with a stalled motor (starting against locked wheels, etc.)
the VWP could be maintained at a low value by circuits that would
override the position of the accelerator and thus prevent damage to
the control elements, the rectifiers, the motor brushes, or possibly
the battery.
128
Speed Control and Braking Circuits
-------
"Wild-Eyed Automation"
In principle, such an electronic control can be readily adopted
to the "highway of the future" dream of completely automated and
guided vehicles. Where starting or stopping a car or controlling a
speed is a simple matter of electronic circuitry (not further linked
to mechanical systems in the control), then a true over-all electronic
control system can be effected with all its attendant possibilities of
accurate response, automatic control, and other advantages.
Figure 4 shows the relative simplicity of the over-all controls.
The speed-control circuits develop signals determined by the position
of the tap on the potentiometer (Rl), which represents the position
of the accelerator pedal. The signal developed from this block deter-
mines when the power SCR will be turned off in the cycle shown
in Figure 3.
CR2
1
ACCELERATOR PEDAL
CONTROL
DASHBOARD PICKUP CONTROL
TO REGENERATIVE BRAKING
CIRCUITS CONTROL
Figure 4. Block diagram of control circuitry for
full performance.
If the current becomes too great the current detection could over-
ride the speed-control circuit to limit the current. Further, if the
pulse width has been reduced to minimum and the current is still
excessive, then these circuits could decrease the pulse repetition rate.
POWER CIRCUITS
A feasible electronic power circuit is discussed here to illustrate
basic cost considerations.
Data recorded on the Henney Kilowatt indicate that for accept-
able performance under maximum load, such as going up a long steep
WOUK
129
-------
hill, 500 amperes might be drawn from the battery (Private com-
munication with C. Gold, Yardney Electric Corp, New York, N.Y.).
Hence, the electronic controls would have to be designed for this
"worst case." The 500 amperes are not delivered at full battery
voltage, however, but at a lower voltage, 25 volts, corresponding to
lower speed. Therefore, the duty cycle would be, let us say, 25
percent. Figure 5a shows the required motor current of 500 amperes.
Figure 5b shows the applied motor voltage, assuming 100 volts from
the battery, with 25 percent duty cycle. The average of this is 25
volts, which we are assuming corresponds to the voltage required
to provide the needed current and hence power for the particular
mode of operation.
(a)
T
500 amp
MOTOR CURRENT
(b)
n
T
100 volts
1
T
25-volt average
T
MOTOR
VOLTAGE
(c)
0
~T
500 amp
1
T
125-amp average
T
BATTERY
CURRENT
Figure 5. Voltage and current waveforms to
illustrate 25 percent duty cycle.
Figure 5c shows the current through the power control switch.
Although the average current flowing is 125 amperes, unfortunately,
from an economic point of view, the heating and current ratings in
these semiconductors depend to a large degree on root-mean-square
(rms) values of current. In a pulse of this nature
J-rms = Ipeak V
So, the rms of this waveform is 500/\/ 4, or 250 amperes. For reliable
design this is too close to the maximum rating available at present
from a single inverter type of SCR. Hence, two SCR's would have to
be paralleled.
If it were possible simply to parallel two SCR's, then each would
handle approximately half the peak current, or 250 amperes, with
each handling the rms current of 125 amperes, a safe value. Parallel-
ing SCR's simply and reliably usually is done by dissipative methods
130
Speed Control and Braking Circuits
-------
(Gutzwiller et al., 1964). A method of avoiding dissipation is to in-
corporate several SCR's, but operate them sequentially, as indicated
in Figure 6, which shows a possible complete power control circuit.
The rms currents would be 500 '\/ 8, or 178 amperes, a safe value.
The explanation of the mode of operation is shown in Table 1. This
is a "dissipationless'' SCR turn-off method in that the energy in the
capacitor is supplied to the load. Any losses are incidental, i.e., not
inherent in the mode of operation. As components are improved,
efficiencies will be improved.
Figure 7 illustrates some of the more important waveforms of
voltage and current in such a circuit. The relationships are complex.
E
-k
CR2
CR3
^j
1
CR4 CR5
r—
E -
\I
CR6
M
^J^
be
CR7
fcl
*k
CR1 '
M
0
T
0
F
\
Figure 6. Circuit with sequential conduction of power
SCR's for proper current rating.
Table 1. SCR's FIRING SEQUENCE and POWER CONTROL FUNCTIONS
Time
to
tn
t3
t6
SCR(s) Firing
CR2
CR5.6
NONE
CR3
CR4,7
NONE
CR2
FUNCTION
Start application of power to motor.
Apply reverse voltage E to CR2. This blocks
CR2, and commutates motor current to ca-
pacitor C.
C voltage reversed to E. All SCRs blocking.
Motor current flowing through CR1.
Per CR2.
Per t,, except CR3 blocked.
Per t,,.
Repeat cycle.
WOUK
131
-------
All waveforms are idealized. The motor current increases during
the power pulse and decreases during a portion of the turn-off pulse
and during the free-wheeling interval. Also, substantial switching
transients of voltage and current usually occur. These are neglected
for ease of understanding of the basic concepts.
All designations in Figure 7 are in accordance with IEEE stand-
ards. Lower-case letters denote instantaneous values. Upper-case
letters denote average values, or rms values, as would be read by a
meter. We will examine Figure 7 in detail, part by part.
(a) The voltage -applied to the motor. During the horizontal portion
parallel to the zero axis, voltage is applied through CR2 or 3. This
persists for the interval t,, which is adjustable for speed control. At
the end of the interval, when the appropriate turn-off thyristors are
fired (in this case CR5 and 6), initially the capacitor voltage adds to
the battery voltage, so that the motor voltage is doubled. Since it is
assumed that the motor current remains essentially constant during
the entire interval, the voltage across the capacitor reverses at a con-
stant rate, so that the voltage applied to the motor drops to zero
linearly. It is this triangular position of the waveform that represents
the "dissipationless" turn-off, in that the energy in the capacitor is
supplied to the load.
(b) The motor current. The motor current is assumed constant.
(c) The battery current. Note that this current flows while either
power thyristor is conducting and also during the interval that the
turn-off-capacitor, voltage-applying SCR's are still conducting.
(d) The current through one of the switching thyristors, in this case
CR2. Note that the current that flows is equal to the motor current,
and it is assumed that the current turns off instantaneously when the
turn-off thyristors are fired. Further note that the pulses occur Vz
as frequently in (d) as they do in (c), because of the sequential firing
previously discussed.
(e) The current through the second power thyristor, CR3 in this
case. Note that the current is displaced one period of motor pulse
frequency, T, but is otherwise identical.
(f) The current through the turn-off thyristors CR6 and 5. This
current flows only during the interval following the turn-off of CR2.
Its magnitude is equal to the motor current.
(g) The current through the turn-off SCR's, CR4 and 7. This cur-
rent flows only after the turn-off of CR 3. Its magnitude is equal to
the motor current, and it flows only during the capacitor voltage
reversal interval. It is identical to (f), displaced by T.
(h) The bypass diode current. The current in CR1 flows only when
all of the thyristors are extinguished and the voltage applied to the
motor is essentially zero. The magnitude of current is equal to the
motor current. The shorter the interval tj, the greater will be the
interval that CR1 conducts. It is further evident that the interval
132 Speed Control and Braking Circuits
-------
(k)
Figure 7. Basic waveform for circuit in Figure 6.
WOUK
133
-------
depends on the interval that the capacitor is conducting. These re-
lationships are examined at length in Wouk (1962).
(i) The voltage across the turn-off capacitor C. Note the trapezoidal
waveform, with the voltage essentially constant, either positive or
negative, during the power interval and the free-wheeling interval.
It changes linearly from one polarity to the other during the power
thyristor turn-off interval.
(j) The power thyristors CR2 and 3. During conduction the voltage
across both thyristors is essentially zero, even though only one of
them is conducting. At turn-off, the voltage goes negative, because
of the voltage across the capacitor, which initially is equal to the
battery voltage. The voltage rises linearly from —E to +E, as the
polarity of the capacitor is reversed. The interval between the time
that the voltage initially goes negative and the time that it rises
linearly again to zero, is known as the turn-off interval (t0). The
capacitance must be large enough to ensure that this interval is greater
than some specified minimum (currently as low as 20 microseconds
for inverter-type high-power thyristors). This interval of 20 micro-
seconds, in conjunction with the maximum current that ever has to
be turned off, determines the size of capacitor required by the re-
lationship:
C — — x t
u - Y °
This implies that with a light load, the "tail-off" will be longer. Since
tj is usually correspondingly reduced, for an application such as this,
it probably would never be necessary to modify the pulse repetition
rate (T). This is analyzed at great length in Wouk (1962). During
the free-wheeling interval, the voltage across the power thyristors
is constant, equal to the battery voltage. Note that there are no
rapid rises, or high rates of de/dt, in this circuit, which simplifies one
of the design problems of employing thyristors, since no thyristor is
subject to a rapid forward increase of voltage that might cause it to
turn on spontaneously.
(k) The current through the turn-off capacitor. This current is
alternating, with the rectagular peaks equal to the motor current
and lasting during the turn-off interval of the power thyristors.
We emphasize that this circuit is only typical of many that may
be developed; undoubtedly properly applied ingenuity will reduce
the number of costly components without introducing other energy-
dissipating elements. None of the engineering subtleties of circuitry,
such as "softening" circuits or saturating reactors, are included in the
circuit to reduce the di/dt through semiconductors.
At present such semiconductors are expensive. Using General
Electric's type C185C for the power, type C55C for the commutating
SCR's, and type A295B for the bypass diode would cost about $400
in large quantities.
134 Speed Control and Braking Circuits
-------
REGENERATIVE BRAKING
In an urban vehicle, subject to frequent start-and-stop operation,
with the slowing down time equalling an appreciable part of the
accelerating time, a significant amount of power can be regained by
the battery if the kinetic energy of the moving vehicle is returned
by some reasonably efficient system into the batteries. Calculations
of the watt-hours that could be returned to the battery in a bus
slowing down from 30 mph to a halt are given at the end of this paper.
One of the reasons for the disfavor of regenerative braking in
the past, as applied to battery-powered vehicles, undoubtedly has
been the enormous regenerative braking currents. Batteries that
normally could have accepted such rates of recharge would have
required substantial overdesign for normal operations. With the
availability of high-speed recharge nickel-cadmium cells, regenera-
tive braking now becomes practical for a battery-operated bus.
The calculations are completely theoretical and assume no losses
whatsover. Even if the losses in the regenerative braking process
reduce the actual range of extension, regenerative braking offers im-
portant advantages above and beyond range extension.
One of the advantages is safety of operation. Existing battery-
powered vehicles give the driver free-wheeling on straightaways,
which may not be undesirable, except for the uneasy feeling it may
give the driver that he has no control over the car. Down steep
grades, however, the problem of brake failure is a serious one, and
the braking action of regenerative braking adds an important dimen-
sion to driving safety. It acts as a safety device in slowing down the
car, and in addition to the incidental advantage of recharging the
batteries, it also lessens wear and tear on the brakes.
Figure 8 shows the basic connections required for regenerative
braking. An additional component is required, in this case shown
as CR2, to conduct the power from the armature of the motor back
to the battery.
As indicated briefly before, the circuit of Figure 6 is indeed in
free-wheeling through CR1, when all SCR's are open, corresponding
to the foot off the accelerator. The current that may have existed
when the foot was removed from the accelerator would decay to
zero, because of circuit losses in the circuit that consists of the three
basic elements — the armature, the field coil, and the diode. A very
slight braking action is not even possible, because without the field
being reversed, the motor could not act as a generator. To be a gener-
ator, the current would have to flow out of the terminals as shown,
rather than into them, and the current could not flow in this direction
through the diode.
Figure 8 shows how motor braking can be introduced, under the
assumption that to minimize maintenance problems the design ob-
jective prohibits the use of contactors. For the motor to act as a
WOUK 135
-------
generator, the series field would have to be reversed. Since this is
assumed undesirable, an additional field coil, which could be a shunt
field, would be employed. Current could then flow as shown from
the armature through CR2 into the battery, recharging the battery.
CR2
SERIES
FIELD
CR1
CR3
SEPARATE
CONTROL
FIELD
Figure 8. Basic connections for regenerative braking.
The amount of current flowing back into the battery is given by
Eg — ECR2 — Eb (2)
I =.
R
where
Eg = the voltage generated by the motor, now acting as a generator.
ECR2 = the voltage across CR2.
Eh = the battery voltage.
R = the sum of all circuit resistance, including internal resistance of
the battery, the generator, and the wiring.
(3)
The generated voltage is shown by
Eg = k$w
where
k = a constant.
4> = the flux density in the control field.
w = angular velocity of the armature, which is directly proportional
to car speed. Therefore equation (3) can be rewritten
= k$v
(4)
136
Speed Control and Braking Circuits
-------
As the vehicle slows down, if all other parameters are held
constant, the generated voltage will drop and the feedback power
will drop, causing the braking effect to be reduced. This action is
contrary to present operating experience. Because mechanical brakes
are usually Coulomb braking, i.e., constant braking force (within
limits), the field strength (4>) must be increased as the speed de-
creases, until it is impractical to increase the field strength further
to compensate for the lower velocity.
The foregoing implies indirectly, and it should be stated here
directly, that regenerative braking cannot bring the vehicle to rest.
The function of regenerative braking is to return as much energy
as possible to the battery, not to stop the car. The brake pedal will
therefore be somewhat more complicated in overall function than at
present. For the start of brake travel, the regenerative braking will
take place, but as the car reaches some minimum speed the braking
force will be eliminated by the motor and the operator will have to
depress the pedal farther to put mechanical braking into effect. Since
the kinetic energy is proportional to the square of the velocity, re-
ducing the speed to Vi initial value will absorb 90 percent of the
initial energy.
Here again the electronics will lend themselves readily to auto-
matic control. Depending upon pedal pressure, the field strength
could be controlled automatically to maintain constant current into
the battery independent of speed, with mechanical feedback to
simulate present operation of the foot pedal.
Figure 9 shows a possible implementation of such a circuit.
CR1 and CR2 are the power control SCR and the bypass diode, re-
spectively, for normal-drive operation; these are assumed inoperative
when the driver's foot is off the accelerator. If the brake pedal is
depressed, the rheostat shown will be activated by hydraulic pressure
and will turn on the transistor through the transistor control circuits.
These in turn will apply current to the shunt field, which generates a
voltage at armature terminals 1 and 2. Simultaneously CR3 is
turned on as the voltage is generated at the armature terminals, de-
veloping a voltage higher than the battery voltage. When the SCR
fires, current flows into the battery at a rate determined by
equation (2).
The current can be detected by the same or a different current
detecting circuit mentioned with respect to Figure 4. In all events
this current can be maintained constant by automatic circuits, which
will control the current through Ql and hence the current into the
shunt field coil, which in turn determines the battery current that
will flow. As the motor slows down, the transistor can be driven
further into conduction.
As a final example of the value of regenerative braking and
the magnitudes of current involved, Figure 10 shows some of the
parameters of a 3,000-pound vehicle being held to a 10-mph speed
on a 10 percent grade, with 100 volts of batteries being recharged
WOUK 137
-------
A A A
I vVV
N CR3
p '
j /^r^~\
CRl^
CR2'
SERIES +
C
1
°vl
BP
cor
2 J
t
AKING
^JTROLS- —
1
'1 R1 1
LWW
— " ^
BRAKE
PEDAL
RHEOSTAT |
to MOTOR
CURRENT
DETECTION
CIRCUITS
Figure 9. Circuit diagram for controlled
regenerative braking.
POWER = MGV(SIN$)
M = 3,000 Ib
V= 10 mph
$ = 10% GRADE
P = 6 kilowatts
I = amperes at 100 volts
Figure 10. Regenerative braking requirements of
automobile on downgrade.
138
Speed Control and Braking Circuits
-------
through regenerative braking. The recharging current would be
approximately 60 amperes.
For a 30,000-pound bus under similar circumstances, with 300
volts worth of battery, the recharging current would be 200 amperes.
These figures indicate why, for practical applications in a battery-
powered vehicle, "dynamic braking," where the power is sent into a
resistor (which would allow braking action down to much lower
speeds), is impractical. On a :l-mile grade, for example, where the
resistors would be absorbing power for 6 minutes, the resistor bank
would have-to be disproportionately large to absorb and dissipate
the energy properly.
CONCLUSION
In a battery-powered vehicle, electronic circuits can perform the
functions of electric motor control for maximum utilization of battery
power and can provide performance similar to that of conventional
fuel engines. In addition, electronic controls can allow regenerative
braking, which will provide performance similar in most respects to
conventional braking on conventional automobiles. The circuits that
will obviously work at present, even in quantities of thousands, will
be substantially more expensive than the simple mechanical controls
for a conventional automobile. However, if we apply the lower prices
of low-power semiconductors, the outlook is much more attractive.
In an application such as a bus, where the complete elimination of
the contribution to air pollution is invaluable to the community, the
added initial cost of the battery-powered bus is negligible.
The possibility of polluting the air with ozone, discussed earlier,
is purely academic when semiconductors are used. Some people have
also shown concern about the strain on the electric-power-generating
capacity of the United States if battery-powered vehicles were used
widely. This concern is completely unnecessary, because if all in-
ternal combustion engines were converted to battery-powered vehicles
overnight, the load requirements on the electric utilities would be
approximately doubled. Calculations of power-generation are also
given at the end of this paper.
All facets of the air pollution problem point to the great de-
sirability of putting into operation at once, with today's technology,
a battery-powered bus. When tomorrow's technology eliminates
the problems of range, speed, and initial cost of battery-powered
buses, we will all be able to enjoy the benefits of this pollution-free
type of transportation.
CALCULATIONS OF POSSIBLE EXTENSION OF RANGE
BETWEEN CHARGES WITH REGENERATIVE BRAKING
Data recorded on the Henney Kilowatt indicate that a car with
four passengers running on a level at 30 mph draws approximately
WOUK 139
-------
60 amperes at 40 volts (Private communication with C. Gold, Yardney
Electric Corp, New York, N. Y.). (100 volts of 100-ampere-hour
cells are used.) This represents a drain of 2.5 kilowatts, with a
load of 2,600 pounds, car and passengers.
For a bus with 10 times the number of passengers, assume 10
times the total weight, and 10 times the "road load", or 25 kilowatts;
to keep the ''steady" current below 100 amperes, use 300-volt
batteries.
Then
Mass of bus = m = : ~— = 11.8 x 103 kg
2.205 Ib/kg
At 30 mph,
v = 30 x 1.4667 x 0.3048 = 13.4 mps
(mph) (ft/sec/mph) (meters/ft)
From National Safety Council charts, the safe stopping distance
at 30 mph is 90 feet.
90 feet = 90 x 0.3048 = 27.43 meters
Assuming constant braking effect, so speed reduces linearly to
zero,
13 4
average velocity = —— = 6.7 mps
27 43
time to come to stop T = — = 4.1 seconds
6.7
Initial bus kinetic energy = 0.5 mv2
= 0.5 x (11.8 x 103) x (13.4)2 = 1.05 x 10fi joules
If all of this energy were fed back to the battery by bringing
bus to a halt, then
1.05 x 106 = EIT
where
E = battery voltage, assumed 300 volts for a large bus
T = 4.1 seconds
1.05 x 10"
1 = 4.1 x 300 ^ 85° ampereS
Thus, the required value of current for successful regenerative
braking is well within the range that some nickel-cadmium batteries
can absorb.
The energy absorbed during stopping, approximately 106 joules,
will drive the bus, at the assumed 25-kilowatt drain, for a time
106
140 Speed Control and Braking Circuits
-------
a distance of
40 1X
~X ^ = ^ mile- at 30
The assumed battery capacity of 300 volts, 100 ampere-hours,
will run the bus continuously for approximately
300 x 100
In an urban area such as congested midtown Manhattan, this
distance (30 miles) is more than adequate. Regenerative braking,
which may extend the range by 25 to 50 percent, renders the battery
bus concept even more practical.
CALCULATIONS OF ELECTRICAL-POWER-GEJVERATIWG
CAPACITY REQUIRED FOR INSTANT CONVERSION OF ALL
CONVENTIONAL VEHICLES TO ELECTRIC DRIVE
The following figures are based on approximate parameters in
a region such as New York State and are rounded off for ease of
calculations.
(1) Assume number of vehicles = 10,000,000 = 107
This value is the correct order of magnitude for New York State.
(2) Assume horsepower per vehicle = 250.
A reasonable assumption; horsepower of buses and trucks, is
higher, and of many small cars, lower.
(3) From (2), kilowatts per vehicle = 250 x 0.746 = 200 kilowatts
(4) Assume present electrical-power-generating capacity in New
York State = 20,000,000 = 20 x 10" kilowatts.
This value is fairly close to the actual figure.
At first glance, it might appear as though the additional power
required is indeed formidable. From (1) and (2), the additional
capacity needed =
(5) 200 (kilowatts per vehicle) x 107 (vehicles) = 200 x 107 kilowatts
From (4) and (5) the additional power required is
(6) (5)/(4) = 200 x 107/ 20 x 10fi = 100
i.e., the generating capacity would have to be increased 100-fold.
This figure looks formidable and has been quoted in newspapers;
however, this reasoning incorporates two major fallacies.
FALLACY 1:
The 200-kilowatt power figure of equation (3) is peak power,
used only infrequently, when the vehicle accelerates rapidly or moves
up steep inclines. The average power required is substantially less.
WOUK
-------
Taking into account the lower average power requirement, plus
the acknowledged greater efficiency of a battery-electric motor
system, one might estimate the average power requirement at 20
kilowatts. So, from (6):
(7) 100 x - = 10, or a tenfold increase.
' 200 kw
The tenfold increase is still substantial.
FALLACY 2:
Vehicles are NOT used 24 hours a day. Considering values from
24 hours a day for some taxis to 2 hours a week for some suburban
cars, we can reasonably estimate an average use of 2.4 hours a day.
Using (6) and (7) and 2.4 hours, we get
20 kw 2.4 hr
i.e., the order of magnitude of increased electrical power re-
quired is a doubling, not a 100-fold, multiplication.
The recharging process for batteries is not distributed uniformly
during the day. If we assume the worst, that all vehicles are plugged
in at the same time and recharged in 8 hours, then the value in
equation (8) would be tripled.
Nobody expects electric vehicles to replace internal combustion
engines overnight. More probably, where battery-powered vehicles
are brought into use, the changeover will take place in 10 to 20 years.
The electric utilities can readily absorb, with their anticipated normal
expansion, the additional capacity needed for battery-powered
vehicles.
REFERENCES
Ash, D., 1967. On a clear day you will see the electric car. New York
Times, Magazine Section. (Jan. 29).
Gutzwiller, F. W., (editor), 1964. Silicon controlled rectifier manual.
3d ed. Rectifier Components Dept., General Electric Co.,
Auburn, N. Y.
Render, B. S., 1965. Recent developments in battery electric vehicles.
Proc. Inst. Elec. Engrs. (London), 112:2297-308 (Dec.).
Society of Automotive Engineers, 1967 (Jan. 13).
E. A. Rishay, W. D. Bond and T. A. Zechin.
Electrovair — a battery electric car. SAE Paper No. 670175.
A. Kusko and I. T. Magnuson. Vehicle electric drive systems.
SAE Paper No. 660761.
Wouk, V., 1962. VWP: a new approach to small, light, efficient high
power regulated power supplies. 1962 Institute of Radio En-
gineers International Convention Record, Part 6.
142 Speed Control and Braking Circuits
-------
THE MECHANICAL DESIGN OF
ELECTRIC AUTOMOBILES
Carl A. Vansant
Operations Research Incorporated
Silver Spring, Md.
It is not necessary that the mechanical design of electric cars
differ from that of conventional automobiles. Indeed, many of the
electric cars that have been built have substituted an electric prime
mover for the gasoline power plant in an otherwise conventional
vehicle. There is the feeling, however, that totally new designs might
provide better electric vehicles than can modifications of standard
vehicular designs.
One difficulty of the electric vehicle is that batteries plus elec-
tric motors are costly and heavy in comparison to the gasoline en-
gines that can do similar jobs. There has been no competition. In its
technical performance as a torque producer, the gasoline engine is
far ahead of any prospective challengers. If, however, a valid reason
exists for considering electric motive sources, we have no good reason
not to exploit any mechanical design possibilities inherent in the
new power source.
For vehicular elements other than the power source, there is no
need for complete redesigning. An electric automobile would em-
body the same components, except for the propulsive elements, found
in conventional cars — but they may look different, be made from
different materials, and such. We are fortunate that automotive
technology endows us with a rich art from which to draw know-how
appropriate to our purpose.
Somewhat aside from the internal engineering aspects are
mechanical design factors relating to the marketing and use of elec-
tric cars. At the present stage it is well to take these factors into
account, since they have a quite significant part in the decision
whether or not to design and build electric cars at all. Part of our
purpose here is to construct images of clearly desirable and useful
automotive transportation systems that are consistent with tech-
nological capabilities and economic realities.
ELECTRIC VERSUS CONVENTIONAL CARS
Electric vehicles can be superb machines. The persistent prob-
lem that must be reconciled in all electromotive designs, however —
that of storing sufficient energy in a less-than-enormous package and
at a cost that can be afforded — is the battery problem. It dominates
even the mechanical design of electric vehicles. Except for this
problem, wholly satisfactory electric vehicles could be made by
VANSANT 143
-------
merely replacing the engines of conventional cars with electric motors
and batteries.
The conventional gasoline-powered car is the standard against
which electric vehicles will inevitably be compared. Modern auto-
mobiles typically have weight-to-power ratios on the order of 20
pounds per road horsepower. Reid (1966) shows, on the basis of
published figures, that contemporary cars have weight-to-power
ratios of about 10.9 pounds per horsepower. This figure is degraded
to around 20 when "road" horsepower is considered.
These numbers are significant in assessing the broad potential
of electric cars. For example, 20 pounds per horsepower is a typical
weight for electric motors of standard design and construction. Thus,
it is practically obvious that, barring radically improved motors, an
electric car even without batteries could not match the performance
of a conventional automobile. The energy-to-weight ratio of modern
cars, 80 watt-hours per pound — and that is for the entire car — is
better than the energy density of even the superior silver-zinc bat-
teries. If, then, by some miracle one could effect propulsion from
a battery alone, sans car, the machine still would not be attractive
on the basis of energy density. Because of the low energy density,
the range of electric vehicles will necessarily be limited.
AUTOMOTIVE ENERGIES
Power must be provided in an automobile for both propulsion
and accessories. In general, propulsion power is applied to raise the
kinetic and potential energies of the car and to overcome frictional
and drag forces. The stored energies are potentially recoverable,
but energy applied to overcome frictional and drag forces or to
operate accessories is not. The distribution of automotive energies is
considered in the following paragraphs.
Modern low-profile, low-pressure tires allow a comfortable ride
and good handling, but they consume a significant amount of energy.
Tire rolling resistance is a function of a number of parameters, in-
cluding specific tire design, materials of construction, tire diameter,
car weight, and vehicle speed.
Table 1. ENERGY LOSS IN TIRES"
WEIGHT, Ib
kw-h/mile
0.008
0.010
0.012
0.014
0.016
0.016
0.020
0.024
0.028
0.032
0.032
0.040
0.048
0.056
0.064
0.048
0.060
0.072
0.084
0.092
0.064
0.080
0.096
0.112
0.128
"From Campbell and Hunsberger, 1962.
144 Mechanical Design
-------
Table 1 shows the approximate energy requirement, in kilowatt-
hours per mile, to overcome tire rolling resistance for variations in
car weight and in the tire rolling resistance coefficient. The typical
modern car requires nearly 0.1 kilowatt-hour per mile to overcome
tire rolling resistance.
Table 2. ENERGY LOSS DUE TO AERODYNAMIC DRAG"
(at constant 35 mph)
Air resistance
coefficient,
Ib-hr2/
(ft-mile)2
0.0006
0.0008
0.0010
0.0012
0.0014
15
0.022
0.029
0.037
0.044
0.051
FRONTAL AREA, ft2
20 25
kw-h/mile
0.029 0.035
0.039 0.047
0.049 0.059
0.059 0.071
0.069 0.082
30
0.046
0.059
0.074
0.088
0.103
"From Campbell and Hunsberger, 1962.
Energy is also consumed by aerodynamic resistance. Table 2
shows these energy losses for variations in drag coefficient and in the
frontal area of the car. Of course, aerodynamic drag is strongly de-
pendent on speed. Table 3 shows the effect of speed on the aero-
dynamic losses of the typical modern car. For the typical car, aero-
dynamic losses predominate over other losses at speeds of about 45
mph and greater.
Table 3. ENERGY LOSS DUE TO AERODYNAMIC DRAG*
Speed, mph Energy Loss, kw-h/mileb
10
20
30
40
50
60
70
80
0.005
0.019
0.048
0.077
0.120
0.173
0.235
0.307
aFrom Campbell and Hunsberger, 1962.
''Aerodynamic drag coefficient of 0.0012 and frontal area
of 20 ft2 are assumed.
VANSANT 145
-------
The energy required for acceleration increases the kinetic energy
of the vehicle so that, in principle at least, some fraction of that
energy could be recovered. Table 4 shows the energy amounts re-
quired to accelerate vehicles of various weights to certain speeds.
Table 4. ENERGY REQUIRED FOR ACCELERATON
VEHICLE WEIGHT, Ib
Vehicle
speed, mph
1,000
2,000 3,000
4,000
kw-h/mile
20
40
60
80
0.010
0.040
0.090
0.160
0.020
0.080
0.180
0.320
0.030
0.120
0.270
0.480
0.040
0.160
0.360
0.640
Energy is also required for hill-climbing. Although in the long
run the car will descend as many hills as it climbs, the potential
energy gained during an ascent may be partly lost through braking
when descending. Table 5 illustrates the effect of grade on energy
expenditure.
Table 5. ENERGY REQUIREMENTS DUE TO GRADE
VEHICLE WEIGHT, Ib
Grade, %
1,000
2,000
3,000
4,000
kw-h
—2
0
+ 2
+4
+6
—0.040
+0.040
+0.080
+0.012
—0.080
+0.080
+0.160
+0.240
—0.120
+0.120
+0.240
+0.360
—0.160
+0.160
+0.320
+0.480
Requirements for accessory power can vary widely, from a few
watts to several kilowatts. On the basis of current practice, heating
and cooling are the major items in the energy budget for accessories.
Accessories such as lighting or convenience features normally require
only small amounts of energy.
It is not unusual for automobile heaters to have outputs of 20,000
or 30,000 Btu per hour, the equivalent of about 6 or 8 kilowatts of
electrical resistance heating. Automotive air conditioners often can
pump about 20,000 Btu per hour. In line with electrical air condi-
146
Mechanical Design
-------
tioning practice this output would require about 3 kilowatts of power.
It is easy to see that even a low-powered electric air conditioner can
consume considerable energy.
The requirements for heating would perhaps best be met by
using a non-battery-powered source, e.g., a phase-change system that
could be "recharged" by resistance heating when the batteries are
being charged. One of the best phase-change systems, although some-
what exotic, is the lithium hydride system. Lithium hydride releases
about 1000 Btu per pound upon freezing. A practical lithium hydride
"heat cell" might easily produce more than 350 Btu per pound, or
approximately 100 thermal watt-hours per pound. Chemically fueled
heaters could provide even more heat per unit weight, although the
air pollution problem would have to be reckoned with. Catalytic
heaters that "burn" fuel at low temperatures are, however, reputedly
free of noxious effluents.
TOTAL ENERGY REQUIREMENTS
Total energy requirements for various driving modes are ex-
amined in Table 6. Three types of driving are considered.
Urban — average speed 15 mph; 5 stops per mile
Suburban — average speed 30 mph; 1 stop per mile
Turnpike — average speed 60 mph; 0.01 stop per mile.
Table 6. ENERGY REQUIREMENTS OF A TYPICAL MEDIUM-SIZE
AMERICAN CAR FOR VARIOUS TYPES OF DRIVING
(kw-h/mile)
Energy use Urban Suburban Turnpike
Tires
Aerodynamic losses
Acceleration
0.092
0.011
0.086
0.092
0.048
0.053
0.092
0.173
0.003
Total 0.189 0.193 0.268
The totals in Table 6 are approximations of the energy require-
ments for propulsion. The requirements may be somewhat understated
in that they exclude allowances for system inefficiences and for power-
ing accessories. A "worst-case" accessory load might include an air
conditioner operating at 10,000 Btu per hour and requiring about
1,700 watts; with other small loads, the total accessory load could be
2 kilowatts. In the case considered, then, the energy requirements
would be as shown in Table 7. Reid (1966) determined that modern
cars expend energy at a rate of about 0.3 kilowatt-hour per ton-mile.
For a 3,000-pound car, 0.45 kilowatt-hour per mile would be the
corresponding expenditure rate. This figure seems to agree well with
the numbers derived here for idealized situations.
VANSANT 14V
-------
Table 7 TOTAL ENERGY REQUIREMENTS OF ELECTRIC VEHICLE
DURING VARIOUS DRIVING MODES
(kw-h/mile)
Load totals Urban Suburban Turnpike
Propulsion
Accessories
Propulsion plus
accessories
0.189
0.133
0.322
0.193
0.067
0.260
0.268
0.033
0.301
What were the energy requirements of early electric cars? J. E.
Homan (1911) wrote: "The current consumption of electric vehicles
operated by storage batteries varies more or less with different makes,
but some idea of the same may be obtained from the results of a test
run of 62 miles over dirty and slippery roads recently made in France.
In this run a number of electric vehicles each carrying four passen-
gers and weighing, complete, over 2 tons, covered the entire distance
at an average speed of 15 miles per hour with an energy consumption
of about 160 watt-hours per ton mile. The best performance was that
of a Vedrine, which required 155 watt-hours per ton mile. Under
ordinary conditions, this vehicle consumes 110 to 120 watt-hours per
ton mile."
Although many changes have taken place since that early day,
it appears that the 0.180 kilowatt-hour per mile that would have been
required by a 3,000-pound vehicle is slightly less than would be
required by a present-day vehicle operating under similar conditions.
Consideration of data in Tables 6 and 7 in light of current bat-
tery art tells us plainly that practical battery-powered turnpike
vehicles are not in the immediate offing. For urban-suburban driving,
where long range is not required, strong technical promise for prac-
tical electric vehicles is implied — even for off-the-shelf components.
However, since the power system in a vehicle built with available
elements falls far short of the standard to which we are accustomed,
promising directions for further development should be examined.
INCENTIVES FOR IMPROVED DESIGN
The power source is the only part of the conventional automo-
bile that is inherently different from an electric car. The motor and,
in particular, the ponderous batteries of electric vehicles have led
many to expound the need for lighter weight, more powerful motors
and batteries. Better batteries and motors are, of course, much to
be desired. However, other directions for development that lie within
the area of mechanical design could be at least as rewarding and per-
haps more immediately productive.
Other approaches can be illustrated by analogy in the aphorism,
"One can either make more than he spends or spend less than he
148 Mechanical Design
-------
makes." That is, improving the efficiency of propulsion may be as
fruitful a course to pursue as that of developing batteries that have
higher energy density.
For example, we have considerable incentive for seeking tires
with lower rolling resistance coefficients. Tire rolling resistance is a
major energy loss during propulsion. For the automotive racing
world, tire manufacturers have recently performed some minor
miracles in designing and constructing high-performance racing tires.
Perhaps, if given the challenge, tire manufacturers could develop a
tire with very low rolling resistance for electric vehicle service. Also,
light weight is desirable to reduce tire rolling drag losses and energy
requirements for acceleration and hill-climbing. Although there is
a large incentive for a lightweight vehicle, we are cautioned by
Homan, who wrote, 56 years ago: "There may be some advantages
in light constructions, formerly supposed to be essential, but present-
day practice recognizes the evident fact that strength and durability
are more important considerations."
Nevertheless, light weight does afford a technically more efficient
machine. Eliminating a pound of structure produces far greater
benefit in an electric than in a gasoline-powered car because of the
greater cost and weight of the power system. The best examples of
lightweight vehicular structures are found in racing machines. Al-
though racing cars are highly specialized, they embody techniques
that may be useful in the design of lightweight electric vehicles
(Costin and Phipps, 1962).
Fiberglass has proved to be a practical body material for limited-
production cars. Since this material provides for a lighter weight
shell than steel, it is probably a good choice for electric cars. (One
likely candidate for a near-term electric car is the Meyers Manx,
produced by B. F. Meyers and Company and sold in kit form only.
This handsome car is essentially a fiberglass bathtub on a shortened
Volkswagen chassis. The total weight, without prime mover, is less
than 1,000 pounds.)
Particularly in urban use we have incentive for developing a re-
generative braking system. There has been much talk about this
possibility, but little activity of practical merit.
Vehicles for urban-suburban use have little need for aerodynam-
ically clean design, since they are seldom driven at high speeds.
If we assume no further technical development of components,
about the only things that can be done for an electric car designed
for urban use are to choose efficient tires, to design for lightweight
structure, and to design a regenerative braking system. Because of
large heating-cooling loads imposed by accessories, it is reasonable
to consider even insulated walls and double glass windows. Some
efficiency in design over conventional practice may be gained by
appropriately packaging the components in ways that are impossible
with a gasoline engine.
VANSANT 149
-------
These, then, are the general directions in which engineering
attention can be expected to produce advances in electric vehicle
design. Drawing our attention to just one of the potentially reward-
ing areas, let us consider the effect on cost of reducing the weight of
a 3,000-pound electric car, of which 1,000 pounds is lead-acid battery,
to 2,000 pounds. If an energy density of 15 watt-hours per pound is
assumed, the battery will contain 15 kilowatt-hours. The cost of the
battery will be about $300. (Price estimates for batteries vary con-
siderably, but the U.S. Government procures lead-acid SLI batteries
at prices of about $20 per kilowatt-hour (private communication from
J. Reinman, U. S. Army Tank Center, Warren, Mich.). Reputable
retail establishments sometimes sell batteries for $15 per kilowatt-
hour, based on nameplate rating.) If the weight of the car is only
2,000 pounds, the energy requirement for the same range will drop
by about one-fourth (see Table 8) for urban-suburban driving. The
battery goes from 33 percent to 37 percent of vehicle weight, and
propulsion power requirements decrease by approximately one-
third; this means that if a 15-horsepower motor had been used for
the 3,000-pound machine, a 10-horsepower motor would suffice for
the 2,000-pound vehicle. At 20 pounds per horsepower and $15 per
horsepower this represents a reduction of $75. The motor accounts
for about 10 percent of the vehicle weight in either case.
Exclusive of the power plant, then, a 2,000-pound machine can
cost as much as $150 more than a 3,000-pound machine before the
heavier vehicle becomes more attractive in terms of first cost.
Table 8. EFFECT ON ENERGY REQUIREMENTS OF REDUCING
VEHICLE WEIGHT FROM 3,000 TO 2,000 POUNDS
Energy requirements of
Driving mode 3,000-lb vehicle, 2,000-lb vehicle, Change, %
kw-h/mile kw-h/mile
Propulsion only
Urban
Suburban
Turnpike
Propulsion plus
accessories
Urban
Suburban
Turnpike
0.189
0.193
0.268
0.322
0.260
0.301
0.135
0.130
0.239
0.268
0.197
0.272
—29
-33
—11
—17
—24
—10
Many similar comparisons could be made; the important point,
however, is that the careful weighing of alternatives is a worthy pre-
requisite to actual design.
150 Mechanical Design
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So far, the energy requirements of electric automobiles have
been a principal subject. The whole of automotive mechanical design
obviously involves much more. We are the beneficiaries of long years
of significant progress in automotive technology. To appreciate our
technological inheritance, we need list but a few of the dramatic
early engineering accomplishments:
The differential, which allowed the rear wheels to
negotiate corners without scrubbing; the pneumatic tire,
which made motoring less wearing for both vehicle and pas-
senger; and steering mechanisms able to accommodate the
different turning radii of front wheels.
Now that we are in a technically sophisticated and less romantic
era of transportation technology than our forebears, it may be ap-
propriate to view a vehicle as a box on four wheels. We need no
longer view as a problem the development of, e.g., a satisfactory
braking system. Today, by contrast, the "problems" affecting even
mechanical design appear to spring from economic as much as from
engineering considerations. For example, who would have foreseen,
even 20 years ago, the impact of loan credit on the whole automotive
industry, including the mechanical design of vehicles.
Technically, it appears that a completely satisfactory electric
car can be built today. This is no news; it has been true for at least a
decade. Even from a cost standpoint, satisfactory electric vehicles
that are not prohibitively expensive can be built. From a solely
economic viewpoint, however, the competition of the conventional
car is overwhelming.
THE MODERN ELECTRIC CAR
The various prognosticates of electric automobile developments
are correct in suggesting the small "runabout," particularly the com-
muter vehicle, as the first reincarnation of the electric car. Such a
machine can most comfortably exploit the values placed on light
weight, low speed, and small size. The technology is certainly at hand
for the production of electric vehicles satisfactory for commuter serv-
ice, mainly because great range is unnecessary.
More important, since the air pollution problem is most serious
in the city core, attention should first be directed to urban-suburban
commuters; they are, collectively, perhaps most responsible for and
affected by airborne contaminants.
Although the air pollution problem provides the most recent
mandate for an electric car, I believe that a number of automobile-
associated urban problems could be alleviated by the electric auto-
mobile. Some of these crucial problems are air pollution, parking,
and road space. Electric cars have the potential, through design, of
providing relief in each of these problem areas. Electric vehicles, of
themselves, can meet only the air pollution problem; however, more
useful solutions may be found by a composite problem-solution ap-
proach than by attacks on individual problems.
VANSANT 151
-------
Perhaps as great as the engineering problems associated with
electric automobiles is the acceptance problem. Individuals have
little natural incentive to possess electric vehicles, since gasoline-
powered cars can, now and in the future, outperform electric cars
on almost every score except that of holding down air pollution.
Even on this point, it is not clear that the electric power plant
is the best of the alternative propulsion systems. Steam engines,
turbines, and even improved spark-ignition engines have consider-
able potential for very low pollutant emission under the impress of
an air pollution criterion. Nor are all electric systems necessarily free
of emissions.
In addition, it is practically certain that electric cars will cost
considerably more than equivalent gasoline-powered ones. People
will not, then, gladly abandon their gasoline cars to buy electric cars.
The incentives will have to be created. The banishment of gasoline-
powered vehicles by legislation does not appear to me to be a practical
possibility.
One incentive would strongly encourage the use of electric
vehicles and, at the same time, take advantage of an inherent char-
acteristic of the electric car. That incentive? A place to park. Public
parking structures limited to electric vehicles would surely be in-
centive enough in most urban business districts. Such a scheme is
favored, again, by the zero-emission characteristic of electric cars.
Parking structures could be built underground or anywhere, with
relatively little concern for ventilation. It seems reasonable to insist
also that cars meet such requirements as maximum dimensions or
certain construction features, so that they are compatible with the
parking facilities.
Another proposal is to put commuter vehicles on rails. The steel
wheel on the steel rail is, in a practical sense, unsurpassed for low
rolling resistance. In many cities a number of railroad lines run
from surrounding residential areas into the core areas. Perhaps ar-
rangements could be made to use these tracks as rush-hour commuter
lanes for electric vehicles capable of both on-track (steel wheel)
and off-track (pneumatic tire) service. A speculative design of a
vehicle for such dual service is shown in Figure 1.
There is room for a great deal of variety in design of electric
cars; however, it may be in the public interest to place limits on the
designs of electric commuter vehicles in consideration of the prob-
lems of air pollution, parking space, and road space and to establish
special incentives for the use of these commuter vehicles.
TOWARD A TOTAL DESIGN
If one wishes to build an electric vehicle today, it can be done
by assembling off-the-shelf components. Companies as well as indi-
viduals have done exactly that. Naturally — particularly in view of
152 Mechanical Design
-------
the markedly superior performance of gasoline-powered machines —
it would be desirable to produce a better car than can be assembled
from presently available components.
Figure 1. Two-passenger electric commuter vehicle for
both road and rail service has rubber tires and steel wheels.
The single traction motor is in the rear compartment; bat-
teries are in the fenders. For entrance and exit, the wind-
shield slides back into the roof.
We recognize that financial resources for progress toward the
better electric vehicle will not be unlimited. It is therefore, worth-
while to inquire where research and development payoffs should be
sought. Some possibilities are mentioned in the following paragraphs.
Most proponents of electric cars recognize the need for less-
expensive, lighter weight batteries and motors. These technical areas
probably call for some straightforward inventing — or at least for
rediscovery of some dormant concepts.
The tires appear to have potential for considerable improvement
in relation to the large amount of power they require on contemporary
vehicles. Light weight is desirable, particularly in urban use. A good
regenerative braking system would likewise improve propulsive effi-
ciency for urban driving. For urban-suburban vehicles, we probably
have little incentive for aerodynamically "clean" design.
VANSANT
153
-------
The power requirement total for accessories, it was shown earlier,
can be nearly as great as the requirement for propulsion. I believe we
should expect commuters to demand the level of comfort afforded
by year-round air conditioning. For the electric vehicle, this require-
ment will introduce a new direction for design, i.e., thermal design.
Previously, we have been able to overpower the weather with heaters
and air conditioners that have been merely added to the car. In
electric automobiles, for climate control as well as for other accessory
functions, the cost of providing extra power is incentive for the use
of relatively efficient systems.
Electric cars may, at last, provide the long-lived automobile.
Commercial vehicles often have service lives of 20 years. What if
the life of private electric cars were 20 years rather than the 7 or 8
years now typical for automobiles? From the viewpoint of mechanical
design, 20-year vehicle life is a practical possibility.
Although a number of engineering alternatives for electric vehi-
cle design are well known and are found throughout the spectrum of
modern technology, I conclude that electric cars are really favored
only by entirely new designs — designs that are inherently suited to
electric propulsion. In addition, we should view the electric car as
an evolving vehicle. It has been proposed as a solution to air pollution
problems, but we should not expect it to appear, miraculously, as a
panacea.
Especially for the near future, the electric automobile holds
somewhat limited promise. It will not be able to match, in either per-
formance or range, the fire-breathing machines that are familiar to
us. This is not to say that the promise it does hold is not worthy of
aggressive pursuit — for if the electric car is viewed in the context
of a responsibility toward improving our transportation systems
rather than as merely another automotive alternative, we can hope
to develop imaginative approaches that are genuine improvements
over conventional schemes.
REFERENCES
Campbell, R. W., and J. R. Hunsberger, 1962. Energy recovery in-
centive for regenerative braking. SAE Paper No. 498A. Tables
1, 2, and 3 are derived from the GM road load horsepower equa-
tion in this reference. The equation is attributed to Administra-
tive Engineering Department, Research Laboratories, General
Motors Corporation, Warren, Michigan, General Motors auto-
motive engine test code. 5th ed.
Costin, M., and D. Phipps, 1962. Racing and sports car chassis design.
Robert Bentley, Inc., Cambridge, Mass.
Homan, J. E., 1911. Self-propelled vehicles. Bobbs-Merrill Co., Inc.,
New York, N. Y. pp. 562-63.
Reid, W. T., 1966. Kilowatts for cars — a comparison of energy costs
for electric automobiles. AIAA Third Annual Meeting, Boston,
Mass. Paper No. 66-978.
154 Mechanical Design
-------
ELECTROCHEMICAL SYSTEMS
Andre J. de Bethune*
Boston College
Chestnut Hill, Mass.
The search for more powerful and lighter weight electrochemical
systems for electric vehicles can be facilitated by applying to the
whole chemical gamut of the periodic chart certain basic principles
of thermodynamics and kinetics. The fundamental thermodynamic
relations governing an electrochemical cell or battery were first
derived by Gibbs (1878) and may be expressed in the forms
En. < E = -AG/nF (1)
dE/dT = AS/nF (2)
AH = Q - We (3a)
= heat absorbed — electrical energy delivered (3b)
= TdWe dT- W,, (3c)
= TAS- (-AG) (3d)
= nFT(dE/dT) - nFE (3e)
Heat developed = (-AH-nFEw (4a)
= enthalpy drop — Electrical energy delivered (4b)
where
E,T = working voltage under load
E = theoretical maximum reversible, or open circuit, voltage
T = temperature (Kelvin scale)
F = the Faraday, 96,487.3 ampere-seconds, or 26.802 ampere-
hours
n = 1,2,3 ... as the case may be
Q = heat absorbed by the battery
Wp = electrical energy delivered by the battery
H = enthalpy of the battery (its total energy plus the pres-
sure-volume product)
S = entropy of the battery (measured by J dQrtn-/T)
G = Gibbs function ("free enthalpy") of the battery (its
enthalpy minus the temperature entropy product).
"Theoretical electrochemistry editor, Journal of the Electrochemical Society.
de BETHUNE 155
-------
The maximum voltage is given by the Gibbs function drop ("free
enthalpy," "Gibbs free energy" drop) of the battery per unit
charge delivered. The temperature coefficient of the maximum vol-
tage is given by the entropy gain of the battery per unit charge
delivered. The enthalpy gain of the battery is equal to the heat
absorbed from the surroundings less the electrical energy delivered
(nFEw). Equations (3a) and (3b) express the first law of thermo-
dynamics in its electrochemical form. Equations (3c), (3d), and
(3e) are forms of the Gibbs-Helmholtz equation. The transition
from (3a) to (3c) involves Carnot's theorem in the form dWe =
QdT/T under reversible conditions. The heat developed by the
battery is given by the enthalpy drop less the electrical energy
delivered. Therefore, maximization of the electrical energy output
(e.g., low internal resistance) has the beneficial effect of lowering
the concurrent development of heat.
Thermodynamic information permits a calculation of the maxi-
mum or reversible voltage E of an electrochemical combination.
For example, take the classic zinc-copper cell, a battery that was
popular a century ago, before electricity became commercially avail-
able, when researchers had to assemble their own batteries out of
inexpensive and readily available materials. The schematic diagram
of the zinc-copper cell is
(-)Zn/Zn+ + , SO,=,aq," SO1==, Cu+ + , aq..'Cu( + ) (A)
V° - 0.7628 +0.337 volts
(dV° dT)lso + 0.091 +0.008 mv/°C
The standard electrode potential V°, and its isothermal temperature
coefficient (dV°/dT)(BO (both referred to the standard hydrogen
electrode SHE as zero at all temperatures) are taken from the
compilation of de Bethune and Loud (1964). The Gibbs-Stockholm
sign convention is followed, i.e., the electrode potential is positive
if the electrode is the ( + ) terminal of a cell whose second electrode
is the SHE. The familiar Latimer "oxidation potential" (Latimer,
1952) has the opposite sign and is not used here. The cell voltage
E, following the Lewis-Stockholm sign convention (Christiansen
and Pourbaix, 1954; Christiansen, 1960; Lewis and Randall, 1961;
and Licht and de Bethune, 1957), is computed from
E(Cell) = V (Right) — V(Left) (5)
Note that the "Zellspannung" or "Cell Tension" recommended by
C.I.T.C.E. is V(Left) - V(Right) (de Bethune, 1964), and is not
used here.
For the zinc-copper cell (A), there results
E°(25°C) = (+0.337) - ( -0.763) = +1.100 volts
dE°/dT = ( + 0.008) - ( + 0.091) = -0.083 mv/°C
E°(100°C)= +1.094 volts
156 Electrochemical Systems
-------
The cell reaction
Zn -f- Cu++ + S04- = Cu + Zn++ + SO,- (B)
yields 2 faradays or 192,974.6 ampere-seconds of electricity at a
maximum voltage of 1.100 volts. The maximum energy yielded as
electrical work at 25°C is
2.200 volt-faradays = 212,272 watt-seconds
= 58.964 watt-hours (6)
The volt-faraday is a convenient energy unit in electrochemical
work. Its conversion factors are
1 volt-faraday = 6.023 x 10-3 electron volts (7a)
= 23.061 kilocalories (7b)
= 91.51 Btu (7c)
= 71,165 foot-pounds (7d)
= 96,487.3 watt-seconds (7e)
= 26.802 watt-hours (7f)
At its 23-kilocalories equivalent, the volt-faraday represents, for
example, 92 percent of the heat of combustion of a 25 (large) calorie
"Metrecal" cookie.
In the zinc-copper cell, it takes a minimum of 112.50 grams
of chemicals (if sulfate is used as the counter ion required by the
law of electroneutrality) to yield 1 faraday of electricity. This
comes out to 102.27 grams per volt-faraday. Division of this number
into the conversion factor 26.802 watt-hours per volt-faraday yields
the theoretical maximum energy density as 0.2621 kilowatt-hour
per kilogram at 25°C. At 100°C, the corresponding figures would
be 102.92 grams per volt-Faraday or 0.2604 kilowatt-hour per
kilogram.
In Table 1, similar values are assembled for a number of
well-known batteries. The numbers are based on the data of
chemical thermodynamics (de Bethune and Loud, 1964; Latimer,
1952; Pourbaix, 1966; and Rossini et al., 1951) for the substances
in their standard states and may differ from actual engineering
performance data. For the lead storage battery, the theoretical
maximum energy density is about 0.17 kilowatt-hour per kilogram.
For the zinc-silver, zinc-copper, and cadmium-nickel batteries, it
is about 50 percent higher. For the hydrogen-oxygen fuel cell,
the theoretical maximum energy density jumps spectacularly to 3.5
kilowatt-hours per kilogram, i.e., to 20-fold that of the lead storage
cell. This figure gives the order of magnitude of the optimum that
can be reached in energy densities by going to light-weight active
materials.
de BETHUNE 157
-------
Table 1. STANDARD VOLTAGES AND MAXIMUM ENERGY DENSITIES
FOR A NUMBER OF COMMON BATTERIES*
Maximum energy density,
E°, volts Weight/charge, kw-h/kg
at 25°C at 100°C g/F at 25°C at 100°C
(— )Pb/PbS04, H2S04, PbS04/Pb02(+)
2.0408 2.1528 321.296
(— )Zn/ZnS04,aq.//CuS04,aq./Cu(+)
1.100 1.094 112.50
(-)Zn/Zn(OH)2,KOH,Ag20/Ag(+)
1.590 1.563 157.578
(— )Cd/Cd(OH)2,KOH, Ni(OH)2/Ni02(+)
1.299 1.284 119.566
(-)H2/H+,H20/02(+)
1.229 1.167 9.008
0.1702
0.2651
0.2704
0.2912
3.656
0.1796
0.2604
0.2658
0.2878
3.472
a Based on standard data of chemical thermodynamics (de Bethune and Loud,
1964; Latimer, 1952; National Bureau of Standards, 1952).
Theoretical maximum energy densities may be expected to
exceed operational energy densities by a factor ranging from 3:1
to 10:1 or greater. The theoretical maximum energy density does
not take into account three operational factors that drastically cut
down the ratio of energy to weight: (1) voltage losses due to
polarization and/or internal resistance, (2) the necessary weight
of electrolyte, separator, box, and connectors, (3) the limited depth
of discharge (50 to 75%) per cycle permissible if cycle life is not
to be sacrificed.
In the lead storage cell, for example, a good quality 12- volt
automobile battery, sold by Sears-Roebuck, is rated at 70 ampere-
hours (20 hours rate) and weighs 46 pounds (21 kilograms). On
slow discharge, this battery yields 840 watt-hours, or 18.2 watt-
hours per pound, which is 40.2 watt-hours per kilogram, or 1:4.2
of the theoretical maximum energy density of 0.17 kilowatt-hour
per kilogram. On fast discharges, of the type needed in automobile
propulsion, this battery might yield only 8 to 12 watt-hours per
pound, or 18 to 26 watt-hours per kilogram, i.e., between 1:10 and
1:6.5 of the theoretical maximum.
Dalin, in another part of this book, quotes the following opera-
tional energy densities, in watt-hours per pound. They are here
converted to watt-hours per kilogram and compared with theoretical
figures from Table 1.
158 Electrochemical Systems
-------
Operational Theor. Ratio
w-hr/lb w-hr/kg w-hr/kg Oper:theor
Zinc-silver
Cadmium-nickel
40 to 50
18 to 20
88 to 110
40 to 44
270
290
1:3 to
1:7
1:2.5
The present theoretical study of novel anode and cathode mate-
rials is based extensively on the de Bethune-Loud (1964) compila-
tion of electrode potentials and isothermal temperature coefficients.
These are all referred to the SHE as zero. However, the standard
hydrogen electrode is an inconvenient reference point for the study
of cathodes and anodes of potential usefulness to electric vehicles.
Therefore, other reference electrodes have been chosen as a basis
for comparison. They include two reference anodes: lithium and
zinc-zinc hydroxide, and two reference cathodes: fluorine and sulfur.
These reference electrodes are much closer to the types of half-
cells being developed in actual engineering studies. Two of them,
the lithium anode and the fluorine cathode, represent the extremes
attainable in electrode potential and lightness of weight. The
characteristics of these four reference electrodes are given in Table 2.
Table 2. CHARACTERISTICS OF THE FOUR REFERENCE
ELECTRODES USED IN THIS STUDY"
(referred to the SHE as zero at all temperatures)
Reference anodes
Zn/Zn (OH).,,OH-
V° = —3.045v v°=— 1.245v
(dV°/dT) .so = _0.534mv/°C (dV°/dT)iso= —1.002 mv/°C
6.94 g/F {d2V°/dT2).go= -5.978 ^v/(°C)
49.698 g/F
Reference cathodes
F-/F2 S=/S
V" = +2.87 v V° = —0.447 v
(dV°/dT)i80= _1.83 mv/°C (dV°/dT)lgo= -0.93 mv/°C
19.00 g/F 16.033 g/F
a Data from de Bethune and Loud, 1964.
de BETHUNE 159
-------
In Table 3, chemical thermodynamic data have been assembled
for 28 possible anodes of 15 chemical elements. In the table, the
anodes are listed in alphabetical order of chemical symbols of the
major element. In the periodic chart, the elements considered are
arrayed in the groupings
Anode elements
H
Li Be
Na Mg
K Ca Sc ° ° ° ° Fe
o o \T ooooo
0 (15 °) °
For each anode, the tables lists the following information:
The weight-to-charge ratio (grams per faraday) of the elec-
trode reaction, including the weight of any necessary counter
ions indicated as (Li+) or (H+).
The electrode potential V° at 25°C, and the isothermal tempera-
ture coefficient (dV°/dT)iso at 25°C, both referred to SHE
as zero at all temperatures.
The anode potential E° versus the fluorine cathode and or the
sulfur cathode, calculated at 25° and 100°C. The anode
potentials are all given the negative sign, to correspond to
their observed (—) polarity in the cells; the cell voltages
are given by the same number without the negative sign.
The maximum energy density, in kilowatt-hours per kilogram,
for the given anode when coupled with the fluorine cathode
and/or the sulfur cathode, at 25° and 100°C.
The optimum thermodynamic performance is that of the lithium-
fluorine cell with a maximum voltage of 5.9 volts and an energy
density of 6.1 kilowatt-hours per kilogram. The aluminum hydroxide,
aluminum sulfide, calcium, magnesium, sodium, potassium, yttrium,
hydride, and hydrazoic acid anodes are close seconds, when coupled
with a fluorine cathode, with voltages ranging between 5.1 and 6.2
volts and energy densities between 5.1 and 2.7 kilowatt-hours per
kilogram. The beryllium, acid hydrogen, alkaline hydrogen, hydra-
zine, and scandium anodes give voltages between 2.9 and 4.9 volts,
versus fluorine, and energy densities between 2.2 and 5.4 kilowatt-
hours per kilogram. The aluminum fluoride, cadmium, iron, am-
monium hydroxide, and zinc anodes give voltages between 3.3 and
4.9 volts, versus fluorine, and energy densities between 1.2 and 1.9
kilowatt-hours per kilogram.
160 Electrochemical Systems
-------
Table 3. CHARACTERISTICS OF ANODES"
(V° and (dV°/dT)lso referred to SHE as zero at all temperatures)
(—) Anodes, g/F versus F~/F.n(+) versus S=/S(+)
V°(25°),v E°(v) and kw-h/kgat 25°C E°(v) and kw-h/kgat 25°C
(dV0/dT)lso,mv/°C E°(v) and kw-h/kgat 100°C E°(v)and kw-h/kgat 100°C
AI/AI+++; 899
-1.662 —4.532 4.337
+0.504 —4.357 4.174
AI/AI(OH)3,OH-(Li+); 32.938
—2.30 —5.17 2.668 —1.853 1.0142
—0.93 —5.10 2.632 —1.853 1.0142
AIF6-s,F-(Li+); 60.873
-2.069 —4.939 1.657 —1.622 0.5653
—0.20 —4.816 1.616 —1.577 0.5496
AI/AI2S3, S=(Li+); 31.966
—2.595 —5.465 2.874 —2.148 1.1966
—1.66 —5.452 2.867 —2.203 1.2273
Be/Be++; 451
-1.847 —4.72 5.38
+0.565 —4.54 5.17 —
Ca/Ca++; 20.04
-2.866 —5.736 3.938 -2.419 1.797
—0.175 —5.612 3.853 -2.363 1.756
Ca/Ca(OH)2,OH-(Li+); 43.99
—3.02 —2.57 1.148
—0.965 —2.57 1.148
Cd/Cd++; 56.20
—0.4029 —3.27 1.243
—0.093 —3.14 1.194
Cd/Cd(OH)2, OH-(Li+); 80.153
—0.809 —3.68 0.9947 —0.362 0.1009
—1.014 —3.62 0.9785 —0.368 0.1025
Fe/Fe++; 27.93
—0.4402 —3.31 1.890
+0.052 —3.17 1.810
de BETHUNE 161
-------
Table 3. (continued)
(—) Anodes, g/F versus F-/F2(+) versus S=/S(+)
V(25°),v E°(v)andkw-h/kgat 25°C E°(v) and kw-h/kgat 25°C
(dV°/dT).so,mv/°C E°(v) and kw-h/kgat 100°C E°(v) and kw-h/kgat 100°C
Fe/Fe(OH)2, OH-(Li+); 51.873
—0.877 —3.75 1.418
—1.06 —3.69 1.395
H2/H+, H20; 1.008
0.0000 —2.87 3.845
0.000 —2.73 3.657
H2/H20,OH-(Li+); 24.956
—0.82806 —3.698 2.255—
—0.8342 —3.623 2.209
H2/H-(Li+); 7.948
—2.25 —5.12 5.092
—1.57 —5.10 5.072
K/K+; 39.100
—2.925 —5.795 2.673
—1.080 —5.739 2.647
Li/Li+; 6.94
—3.045 —5.915 6.111
—0.534 —5.818 6.011
Mg/Mg++; 12.16
—2.363 —5.23 4.50
+0.103 —5.09 4.38
Mg/Mg(OH)2,OH-(Li+); 36.11
—2.69
—0.945
Na/Na+; 22.997
—2.714 —5.584 3.563
—0.772 —5.505 3.513
NH4OH/N2,OH-(Li+); 36.630
—0.736 —3.61 1.74
-0.430 0.1697
—0.440 0.1737
-0.381 0.2491
—0.373 0.2439
-2.478 1.205
—2.591 1.260
-2.598 3.031
—2.568 2.996
-1.916 1.821
—1.839 1.748
-2.24 1.151
—2.24 1.151
-2.267 1.557
—2.255 1.548
—0.289
0.147
N2H5+/N2,H+(F-); 13.0135
-0.23 -3.10 2.595
—0.84 —3.03 2.537
162
Electrochemical Systems
-------
Table 3. (continued)
(—) Anodes, g/F versus F~/F.,(+) versus S=/S(+)
V°(25°),v E°(v)and kw-h/kgat 25°C E°(v) and kw-h/kgat 25°C
(dV°/dT)Kolmv/°C E°(v) and kw-h/kg at 100°C E°{v) and kw-h/kgat 100°C
HN,(g)/N2,H+; 43.032
—3.40
—1.193
Pb/PbS04,S04=(H +
—0.3588
—1.015
Sc/Sc+++; 14.97
-2.077
+0.25
Y/Y+ ++; 29.64
2 37?
£- .O / £.
+0.18
Zn/Zn ->"»-; 32.69
-0.7628
+0.091
Zn/Zn(OH)2,OKHLi +
—1.245
—1.002
Zn/ZnS,S=(Li+); 55
1 405
—0.85
—6.27
—6.22
); 152.646
3 23
—3.17
—4.947
4.791
R ?4?
-J .£-'+£.
—5.091
—3.63
—3.49
); 56.638
—4.115
—4.053
.663
2.71
2.69
0 5671
0.5566
3.903
3 780
"} RRff
iC.OOO
2.805
1.882
1.810
1.458 —0.798
1.436 —0.803
0 958
0 952
0.2943
0.2962
0.4613
0.4584
"Data from de Bethune and Loud, 1964.
Table 4 gives similar information for 21 possible cathodes of 13
chemical elements, listed in alphabetical order of chemical symbols
of the major element. In the periodic chart, the elements considered
are arrayed in the groupings
Cathode elements
Cr Mn Fe
Ni
Ag
(15°)
N
Pb
O F
S Cl
Br
Xe
de BETHUNE
163
-------
Table 4. CHARACTERISTICS OF CATHODES"
(V and (dV°/dT)leo referred to SHE as zero at all temperatures)
Cathodes (+), g/F versus (—)Li/Li+ versus (—)Zn/Zn(OH),
V°(25°),v E°(v)andkw-h/kgat 25°C E°(v) and kw-h/kg at 25°~C
(dV°/dT)i,(l,mv/ °C E°(v) and kw-h/kg at 100°C E°(v) and kw-h/kg at 100°C
OH-,Ag20/Ag; 124.881
+0.345
—1.337
OH-,Ag203/Ag; 52.97
+0.564
Br-/Br2; 79.916
+ 1.0652
—0.629
OH-,Br-/Br03-; 30.327
+0.61
—1.287
CI-/CI,; 35.457
+ 1.3595
—1.260
Cr+++/Cr207=; 29.69
+ 1.33
—1.263
Cr(OH)3/Cr04-; 62.69
—0.13
—1.675
F-/F2; 19.00
+ 2.87
—1.83
F-/F20; 14.004
+2.15
—1.184
(Li+)OH—,Fe(OH)3/Fe04=;
+0.72
—1.62
+3.390 0.6839 +1.590 0.2704
+3.330 0.6771 +1.563 0.2658
+3.609 1.615 +1.809 0.4723
+4.110 1.268 +1.310 0.2709
+4.103 1.266 +1.338 0.2767
+3.655 2.628 +1.855 0.6213
+3.599 2.588 +1.834 0.6142
+4.404 2.784 +2.604 0.7580
+4.350 2.750 +2.585 0.7523
+4.375 3.201
+4.320 3.161
+2.915 1.122 +1.115 0.2659
+2.837 1.092 +1.065 0.2540
+ 5.915 6.111 +4.115 1.713
+ 5.818 6.011 +4.053 1.687
+ 5.195 6.648
+ 5.146 6.585
66.285
+3.765 1.378 +1.965 0.4284
+3.684 1.348 +1.918 0.4182
164
Electrochemical Systems
-------
Table 4. (continued)
Cathodes (+), g/F versus (—)Li/Li+ versus (—)Zn/Zn(OH).,
V°(25°),v E°(v)andkw-h/kgat 25°C E°(\i) and kw-h/kgat 25°C
(dV°/dT\0,mv/°C E°(v) and kw-h/kg at 100°C £°(v) and kw-h/kgat 100°C
Mn++/Mn04-; 32.341
+ 1.51
_nfifi
OH-
NH4
(F-),
OH-
,Mn02/Mn04-; 51.654
+0.588
—1.778
+, N2/HN,; 23.028
+ 1.96
-0.14
+4.555
+4.545
+3.633
+3.540
+ 5.005
+ 5.035
H+, N,/H2N,0,; 51.023
+2.65 +5.695
-0.09 +5.662
Ni(OH)2/Ni02; 63.371
+0.490
-1.19(?)
H+, H20/02; 9.008
+ 1.229
_n RAF,
OH-,
S04=
S=/S;
H20/0.,; 17.008
+0.401
-1.680
+3.535
+3.485
+4.274
+4.251
+3.446
+ 3.360
=, PbS04/Pb02; 169.666
+ 1.682 +4.727
+0.326
; 16.033
—0.447
—0.93
+4.792
+2.598
+2.568
Q 77R
O. / / vJ
3 767
1.662 +1.833
1.619 +1.775
4.476
4.503
9 fi^
^_.uoo
2.618
1.348 +1.735
1.328 +1.721
6.379
fi ^AR
O.OHO
3.857 +1.646
3.760 +1.595
0.7174
0.7272
3.031 +0.798
2.996 +0.803
0.4847
0.4694
0.4113
0.4079
0.7058
0.6839
0.3254
0.3274
S04=/S208- 48.033
+2.01
—1.26
+ 5.055
+4.999
2.188 +3.265
2.164 +3.245
0.8954
0.8899
H+,H20,Xe/Xe03; 30.891
+ 1.8 +4.8
3 4
a Data from de Bethune and Loud, 1964.
de BETHUNE
165
-------
In Table 4, the weight-to-charge ratio in grams per faraday gen-
erally does not include the weight of any counter ions required by the
law of electroneutrality. The maximum energy density in kilowatt-
hours per kilogram then represents a ceiling that one would not ex-
pect to attain even under the most idealized conditions. For example,
the maximum energy density of the fluorine cathode is listed in Table
4 at 1.713 kilowatt-hours per kilogram versus the Zn/Zn(OH)2 anode
at 25°C. The inclusion of one mole of lithium ion Li+ as a necessary
counter ion would drop this maximum figure to 1.458 kilowatt-hours
per kilogram, before any further losses are taken into account. Simi-
larly, the maximum energy density of the sulfur cathode, listed as
0.3254 kilowatt-hour per kilogram versus the Zn/Zn(OH)2 anode,
is dropped to a maximum of 0.2934 kilowatt-hour per kilogram by
inclusion of one mole of Li+ as a counter ion. Where the counter
ion is explicitly mentioned, as (Li+) or (F~), its weight is included
in the calculations.
The cathode potentials E° are given versus the lithium anode
and/or zinc-zinc hydroxide anode, calculated at 25° and 100°C.
They are listed with the positive sign, to correspond to their observed
(-]-) polarity in the cells. These potentials equal, as given, the
standard voltages of the cells: (—)anode/cathode( + ).
The optimum thermodynamic performance is that of the fluorine,
oxygen fluoride, and acid oxygen cathodes when coupled with a
lithium anode, with maximum energy densities of over 6 kilowatt-
hours per kilogram. Other cathodes whose maximum energy den-
sities exceed 2 kilowatt-hours per kilogram, when coupled with
lithium, include the bromate-bromide, chlorine, acid dichromate, acid
permanganate, hydrazoic acid, hyponitrous acid, alkaline oxygen,
sulfur, peroxydisulfate, and xenic acid cathodes. Cathodes of silver-
Ill oxide, bromine, alkaline chromate, alkaline ferrate, alkaline per-
manganate, and nickel-IV oxide give energy densities between 1
and 2 kilowatt-hours per kilogram when coupled with a lithium
anode.
Under actual conditions, the reversible, or open circuit, voltage
E would differ from the standard voltage E° by a small term, usually
of the order of ± 100 to 200 millivolts, calculable from the Nernst
Equation, which has also been called the Electrochemical Law of
Mass Action. As applied to a single electrode potential, where the
electrode reaction can be written in the generalized form:
Ox + ne- = Red (8a)
(written in either direction)
the Nernst equation (with the Gibbs-Stockholm sign convention)
takes the form
+ (RT/nF) In (Ox)/(Red) (8b)
+ (0.05916 volt/n) log (Ox)/Red at 25°C (8c)
where (Ox) and (Red) denote the activities, or activity products,
of the electromotively active chemical substances, in their oxidized
166 Electrochemical Systems
-------
and reduced forms, that appear in the electrochemically balanced
expression for the half-cell reaction.
KINETIC FACTORS —
INTERNAL AND POLARIZATION
RESISTANCES
The kinetics of the electrode reactions become of paramount
importance in dealing with an operating battery. The inequality
contained in equation (1) was originally formulated by Gibbs
(1878) in the form
(V"-V) ^ -d£ /de (9)
where V and V" are, in Gibbs' words, "the electrical potentials in
pieces of the same kind of metal connected with the anode and
cathode, respectively," and d£ and de become AG and nF in modern
notation. Gibbs' inequality (equation 9) is more useful than its
modern counterpart (equation 1) since it applies equally to the dis-
charge cycle (V">V) and to the charge cycle (V>V"). It is well
known that the anode on discharge becomes the cathode on recharge,
and vice-versa.
The inequality in equations (1) and (9) can also be expressed,
for a discharging battery, by
V" —V' = Ew = iRx = E —iRj (10)
where
i = total current in amperes
Rx = external or load resistance
R; = internal resistance, including electrode
polarization "resistances," if any.
The maximum current yielded by a cell is its short-circuit current
(Rx=0) equal to E/Rj. The power output is zero at short circuit.
A battery yields its maximum power, equal to E2/4R)7 at a cur-
rent output equal to one-half the short-circuit current. In general,
the power output is given by the parabolic function
P = Ewi = Ei — i2Rj (11)
This result shows the manifest importance of minimizing Rt to obtain
an operational battery.
The internal resistance can be broken up into the ohmic resist-
ance Rs of the electrolyte, equal to pL/A, where
p = resistivity
L = electrode spacing,
A = projected (geometrical) electrode area,
167
-------
plus the polarization "resistances" RA and Rc of anode and cathode.
The polarization resistances are not constants; they vary with the
current density I. For activation polarization (overvoltage), the
resistance has the limiting value
RAct(ohms) = b/2-2.303 I0A (12a)
at zero current density and decreases with increasing current ac-
cording to
RAct(ohms) = (b/IA) log I/I0 (12b)
where b and I0 are the Tafel parameters of the activation over-
potential (Vetter, 1961). For concentration or transport polariza-
tion, the resistance has the limiting value
Rconc(ohms) =RT/nFIdA (13a)
at zero current density and increases with increasing current ac-
cording to
Rconc(ohms) = (RT/nFIA) lnld/(ld-l) (13b)
going to infinity as I approaches the limiting diffusion (transport)
current Id.
In an operational battery, the electrolyte resistivity should not
exceed 10 ohm-centimeters (e.g., the resistivity of a 1 molar sodium
chloride brine). Consider an electrode spacing of 1 millimeter
(0.03937 inch). The unit area resistance A-RS is then equal to
1 ohm-cm2 (or 0.00108 ohm-ft2)
This value probably represents a reasonable norm, and at the same
time, an operational upper limit, for the internal resistance of a
battery. At an A-R value of 1 ohm-square centimeter, an output
current of 600 milliamperes per square centimeter (557 amperes per
square foot) means a voltage loss of 600 millivolts, e.g., about half
of the open-circuit voltage of the hydrogen-oxygen fuel cell.
To the electrolyte resistance must be added the polarization
"resistances" arising from the kinetics of the electrode reactions, as
given by equations (12) and (13). Since activation and transport
polarizations act in series, and since RAct decreases while Rcnnc in-
creases with increasing current, their combined effects can often be
approximated by a linear polarization up to about 99 percent of the
limiting transport current (de Bethune, 1960). Numerical calcula-
tions, based on equations (12) and (13), lead to Table 5, in which
are given the combined activation and polarization unit area resist-
ances of electrodes having the following kinetic parameters.
Tafel constants: b = 0.120 volt
I0 = 10, 1, 0.1, and 0.01 a/cm2
Transport constants: n = 1, Id = 10, 1 a/cm2
168 Electrochemical Systems
-------
Table 5. COMBINED ACTIVATION AND CONCENTRATION
(TRANSPORT) POLARIZATION RESISTANCES OF ELECTRODES
AS A FUNCTION OF CURRENT DENSITY"
Tafel constants: b=0.120 volt, l(1=10, 1, 0.1, and 0.01 a/cm2
Transport constants: n=l, ld = 10, 1 a/cm2
Current
density
(a/cm2)
0
0.1
0.2
0.5
0.8
0.9
0.99
1.00
ld=10 a/cm2
I0=10 a/cm2
A'R
(ohm-cm2)
0.0052
0.0052
0.0052
0.0052
0.0052
0.0052
0.0052
0.0052
ld=10 a/cm2
I0=l a/cm2
A.R
(ohm-cm2)
0.028
0.028
0.028
0.028
0.028
0.028
0.028
0.028
ld=10 a/cm2
I0=0.1 a/cm2
A'R
(ohm-cm2)
0.26
0.25
0.18
0.17
0.14
0.13
0.12
0.12
Id=10a/cm2
I0=0.01 a/cm2
A'R
(ohm-cm2)
2.6
1.2
0.78
0.41
0.29
0.26
0.24
0.24
Current
density
(a/cm2)
0
0.1
0.2
0.5
0.8
0.9
0.99
1.00
ld=l a/cm2
I0=10 a/cm2
A'R
(ohm-cm2)
0.028
0.030
0.033
0.040
0.045
0.069
0.123
QO
ld=l a/cm2
I0=l a/cm2
A.R
(ohm-cm2)
0.052
0.053
0.055
0.063
0.078
0.092
0.146
oo
ld=l a/cm2
l,,=0.1 a/cm2
A'R
(ohm-cm2)
0.28
0.28
0.26
0.21
0.18
0.19
0.24
OO
ld=l a/cm2
L=0.01 a/cm2
A.R
(ohm-cm2)
2.6
1.2
0.81
0.45
0.33
0.33
0.36
OO
lThe body of the Table gives the unit area resistance A-R of the electrode
in ohm-cm2 (1 ohm-cm2 = 0.155 ohm-in.2 = 0.001076 ohm-ft2).
de BETHUNE
169
-------
The degree of constancy of the A-R values in Table 5 measures the
reliability of the approximation of linear polarization as a substitute
for the combined nonlinear activation and transport polarizations.
For the kinetic parameters chosen, the polarization resistance of
the electrode is almost everywhere a small fraction of 1 ohm-square
centimeter (up to 99% of the limiting transport current). Beyond
99 percent of the limiting current, the polarization voltage goes
rapidly to infinity, and the current output remains at its transport
limited value.
Kinetic parameters I0 and Id significantly lower than those con-
sidered in Table 5 would tend to make the battery non-operational.
For example, I0 and Id values of 10~4 amperes per square centimeter
each would correspond to a combined unit area resistance of approxi-
mately 1,000 ohm-square centimeter up to 99 percent of Id.
CORROSION —CELL UNIFORMITY
Batteries made up of active, free energy rich chemicals can be
expected to suffer from corrosion problems in which these active
chemicals react through other reaction paths than those anticipated
or desired. This can be illustrated by the role of the cathodic oxygen
reaction
O, + 4H+ -f 4e~ = 2H,O (14)
in corrosion and in a fuel cell. For corrosion control, reaction (14)
must be inhibited, e.g., by paint films or chemical inhibitors, or it
must be satisfied by a superimposed cathodic protection current to
supply the needed electrons from another source than the metal
under attack. In a fuel cell, reaction (14) needs to be actively
promoted on the fuel cell's catalytic cathodic surfaces. It must also
be vigorously controlled and inhibited elsewhere in the fuel cell to
prevent destructive corrosion of the cell.
Cell uniformity, not only mechanical and geometrical, but also
chemical, electrochemical, and thermal uniformity, becomes of para-
mount importance when cells are stacked together in series to form
a battery (e.g., the voltaic pile). Experience has shown that the
weakest cell in a pile can actually reverse its voltage by polarization
when it is driven by the other cells of the pile generating a current
in excess of its own intrinsic short-circuit current. In such a situa-
tion, the weak cell is rapidly destroyed by the continued operation
of the pile and greatly reduces the effectiveness of the whole battery.
Stringent control of cell uniformity becomes of paramount importance
in ensuring good battery operating characateristics and good cycle
life. A battery, like a chain, is no better than its weakest cell.
REFERENCES
de Bethune, A. J., 1960. Fourth report to the Bureau of Ships.
Boston College (Jan. 1).
170 Electrochemical Systems
-------
de Bethune, A. J., 1964. Tension, cell and electrode. In: Encyclopedia
of electrochemistry, C. A. Hampel, ed. Reinhold, New York, N. Y.
pp. 1102-03.
de Bethune, A. J., and N. A. Swendeman Loud, 1964. Standard
aqueous electrode potentials and temperature coefficients at 25"C.
Published by Clifford A. Hampel, 8501 Harding Ave., Skokie,
111. 60076. 20pp. Also available in: Encyclopedia of electro-
chemistry, C. A. Hampel, ed. Reinhold, New York, N. Y. pp.
414-26 (1964).
Christiansen, J. A., and M. Pourbaix, 1954. Compt. rend. conf. union
intern, chim. pure et appl. (I.U.P.A.C.), 17th Conference, Stock-
holm, 1953. Maison de la Chimie, Paris, pp. 82-84.
Christiansen, J. A., 1960. Manual of physico-chemical symbols and
terminology. J. Am. Chem. Soc., 82(21): 5517-22 (Nov. 9).
Gibbs, J. W., 1878. Trans. Conn. Acad. Sci. 1875-78. Available in:
Collected works, W. R. Longley and R. G. Van Name, eds. Vol.
1, Thermodynamics. Longmans, Green, New York, N.Y. pp. 332-
49, 429. 1928.
Latimer, W. M., 1952. Oxidation potentials. 2nd ed. Prentice Hall,
New York, N. Y.
Lewis, G. N., and M. Randall, 1961. Thermodynamics, 2d ed. Revised
by K. S. Pitzer and L. Brewer. McGraw-Hill, New York, N. Y.
Licht, T. S., and A. J. de Bethune, 1957. Recent developments con-
cerning the signs of electrode potentials. J. Chem. Educ., 34:433-
40 (Sept.).
Pourbaix, M., 1966. Atlas of electrochemical equilibrium in aqueous
solutions. Pergamon, London, Eng; Cebelcor, Brussels, Belg.
Rossini, F. D., et al., 1952. Selected values of chemical thermodynamic
properties. National Bureau of Standards Circular 500.
Vetter, K. J., 1961. Elektrochemische kinetik. Springer Verlag,
Berlin, Ger.
de BETHUNE 171
-------
SEPARATOR SYSTEMS
Harry P. Gregor
Polytechnic Institute of Brooklyn
Brooklyn, N. Y
INTRODUCTION
The science and technology of synthetic membrane systems has
been developed, particularly in the past decade, to the point where
new materials with a wide variety of useful properties are available
for use as battery separators. Some of these new materials are
beginning to find their way into battery separators, and with the
current development of new battery systems, particularly those of
the type required for the electric vehicle, many new developments
may ensue.
For many of the new battery systems now under development,
the availability of new separator systems is an absolute requirement.
Even in older systems such as the lead-acid batteries, the availability
of new separator materials has led to improved performance and
reliability. The failure of a large fraction of all batteries may be
traced to the inadequacy or malfunction of the separator.
In this contribution I will describe the recent advances in the
physics, chemistry, and technology of battery systems that may be
applied to advantage in the fabrication of new and superior battery
separators. I will not describe all of the different materials and
systems that may be employed, but rather will indicate some of
each class.*
Different battery systems require rather different kinds of
separator systems. The separator must, of course, allow for the
passage of the ions that carry the current and, in some cases, they
also must permit the ready flow of solvent. In other systems, it is
desirable to inhibit or even prevent solvent flow. In some systems
certain ionic or molecular species should be prevented from moving
across the separator to the opposite electrode. In most batteries the
separators must prevent the movement of particulate matter away
from the plates and retard, if not inhibit, dendritic growth, particu-
larly from one plate toward the other.
* Dr. Gregor's presentation gives an extensive background of material
from the literature; these references are cited directly as he provided
them. Since he describes various materials and systems as examples of
their class, Dr. Gregor does not regard the literature citations as an ex-
haustive bibliography on separator systems.
GREGOR 173
-------
CLASSIFICATION OF SEPARATORS
Separator systems are best classified on the basis of pore di-
ameters and the nature of those chemical groups fixed to the pore
walls. Although we tend to think of films having cylindrical pores
extending through the membrane, each at right angles to each face,
real systems are not at all like this with the possible exception of
some new films wherein holes are ''punched through" the membrane
by radiation (i;. Separators usually have pores of varying diameter
that extend in a highly branched configuration in all directions from
one membrane face to the other. Indeed, with many membranes
one should think more of a three-dimensional polymeric gel-like
structure; certain membranes are best viewed simply as solid solu-
tions, with no pores, as such. Table 1 lists pore diameters expressed
both in microns (/j.) and Angstrom (A) units; the range of pore
diameters extends over six orders of magnitude for membranes now
available.
Table 1. PORE AND SOLUTE DIAMETERS
Microns (,j)
100
1 mil —
f 10
Millipore -< 1
I o.i
Angstroms
10,000 A
1,000 \
100 /
10
(A)
Colloids
Proteins
Na+. Cl-
1
The selective transport of an ionic and/or molecular specie
across a membrane is controllable by relatively few parameters:
size; charge (whether negative or positive); chemical reactivity with
a fixed (charge) group; solubility.
MACROPOROUS MEMBRANES (d > 1/x)
Highly porous membranes, having pore diameters from 1 to 10
microns, are conveniently prepared by mixing a molten polymer with
fine particles of uniform diameter, the particulate matter occupying
approximately three-fourths the volume of the suspension. One then
extrudes a sheet from this suspension and, after the polymer has
solidified, dissolves the particulate matter, leaving the porous matrix.
Finely divided salt particles have been used for this surface, as have
starch granules, the latter because of their rather uniform size. Such
films are available commercially from several sources, including the
Electric Storage Battery Company(2). The polymers used for this
174 Separator Systems
-------
purpose include polyethylene, polyvinylchloride, and certain of the
fluoro-polymers. A somewhat analogous procedure starts with solid
particles of the polymer, which are sintered together either by heat
or by the use of an appropriate solvent to cement the par tides (3).
The lower limit of the pore diameters for these films is about 1
micron, because this is approximately the size of the smallest particles
available for providing the voids or for cementation.
MICROPOROUS (1 M >d>100A) AND MOLECULAR PORE
(100A >d>lA) MEMBRANES
Polymeric membranes of graded porosity are usually prepared by
dissolving the polymer in one solvent, casting a film and allowing
the solvent to evaporate partially, then coagulating the film by im-
mersion in a non-solvent. Other kinds of polymeric films, particularly
the familiar cellophane film, are produced by extruding a solution
of a cellulosic derivitive (viscose) into a bath that partially coagu-
lates the film and simultaneously converts it back to the original
cellulose (4).
Control of the porosity of films of this type has been the subject
of many investigations; the general principles and procedures des-
cribed by Elford are still commonly employed(5, 6). The higher the
solvent content of the film at the time of coagulation, the greater its
porosity, as a general rule. Elford described films that were re-
markably homoporous, prepared by the use of mixed solvents both
for dissolution of the polymer and for coagulation. These general
procedures apparently have been followed in the preparation of
Millipore membranes, which are usually composed of mixed deriva-
tives of cellulose; similar membranes have been prepared from mix-
tures of cellulose acetate and cellulose nitrate, as an example. Table
1 lists the stated pore diameters of Millipore membranes.
Control of porosity of nitrocellulose membranes has been de-
scribed by Sollner and Gregor, among others, who showed that by
drying a coagulated nitrocellulose film in atmospheres of progres-
sively lowered relative humidity one obtains membranes of graded
porosity(7, 8). This principle has been extended by CarrfS), who
re-swelled nitrocellulose membranes in mixtures of swelling sol-
vents, in this case alcohol-water mixtures, then coagulated the film in
water and obtained films that showed a remarkable degree of dis-
crimination between molecules of low molecular weight, with esti-
mated pore diameters varying from about 5 to 50 Angstrom units.
Similarly, Craig (10) has varied the porosity of cellulosic mem-
branes by using swelling solvents and by stretching membranes uni-
axially to convert the pores from a spherical to an elliptical shape.
With certain kinds of coagulated, porous membranes, drying
of the film may reduce its porosity. This can be avoided by use of
a mixture of two polymers, one being strongly hydrophilic and the
GREGOR 175
-------
other being an insoluble, hydrophobic matrix material. Porosity can
be maintained in a swelling solvent with such films. An example
of membranes of this kind has been described by MindickflJJ, who
cast interpolymer membranes from a mixture of a copolymer of vinyl-
chloride and acrylonitrile (Dynel, Union Carbide Company) and
polyvinylpyrrolidone (PVP), both dissolved in an appropriate com-
mon solvent. After casting the film and allowing the solvent to evap-
orate, one obtains a continuous phase of the water-insoluble polymer
(Dynel) and the macroscopic pores contain the hydrophilic polymer
(PVP). This general interpolymer technique, first described by
Gregor(12), is useful because it is very flexible, it allows one to use
many different polymer mixtures (although selecting an appropriate
solvent may be rather difficult), and it permits one to produce films
with high-speed equipment at relatively low cost. Additional sta-
bility can be obtained by the use of appropriate cross-linking agents,
although their use makes the formulation somewhat more complex.
DETERMINATION OF PORE DIAMETERS
The determination of pore diameters is relatively straight-
forward for highly porous materials; it becomes quite difficult with
pore diameters less than 1,000 Angstrom units. With materials of
larger pore diameter (d > 0.1 /.i), the mercury intrusion technique
gives both the pore spectrum and the total pore volume. Un-
fortunately, mercury intrusion cannot be applied to finer pore sys-
tems, particularly those composed of polymers that are pressure-
deformable or those in which the porosity depends on the presence
of a swelling solvent such as water. Here, the most reliable method
is to measure the permeation across the membrane of solute mole-
cules of different size, as first described by Collander(13). Unfortu-
nately, nature has not provided us with an adequate series of
spherical molecules of different sizes, ones that do not absorb or
otherwise fix themselves to pore walls. Further, as the diameter of
the solute molecule approaches that of the pore, the classical hydro-
dynamic equation of Stokes, as applied to spheres moving in homo-
geneous media through spherical or rectangular pores by Fa.xen(14),
may be subject to considerable error. The same applies to evalua-
tion of the measurement of air or water permeability, because each
pore contributes to the permeability proportional to the 4th power
of its radius; a highly distorted average pore diameter may be thus
calculated.
Staining of molecular pore membranes by the formation of pre-
cipitates, presumably within the pores, has been tried with indifferent
results. The staining procedure usually alters the pore structure,
but by this technique one may observe the occurrence of large voids,
which are difficult to avoid in the formation of many polymeric films
There is considerable evidence that one cannot form precipitates in
molecular pores (of the order of 5 to 50 A); these kinds of mem-
branes may, when used as a part of separators, prevent the growth
of dendrites through them.
176 Separator Systems
-------
ION-EXCHANGE MEMBRANES
Ion-exchange membranes have been used in several new battery
systems. These are films of insolubilized polyelectrolytes, which can
contain the sulfonic, carboxylic, phosphinic, phosphonic, sulfate and
phosphate acids, amino, and quaternary amine fixed-charge
groupsfJSJ. Polymers containing other fixed-charge groups, includ-
ing phosphonium, arsonium, and iodonium, have also been described.
The commercial ion-exchange membranes are of the sulfonic acid
type for the cation-permeable membranes, and of the quarternary
ammonium type for the anion-permeable membranes, because these
groups are strongly ionized, are of reasonable chemical stability, and
do not form ion-pairs or chelate complexes with metallic ions(16).
The most common polymeric matrix for membranes of this kind
is polystyrene; Figure 1 shows diagramatically a typical cross-linked
polystyrene sulfonic acid material. The drawing, roughly to scale,
indicates that the pore diameters are approximately 10 Angstrom
units C17,18).
' ©JTWI 0 ^- ©^S
^e®! ® ^^s® rv
303
Figure 1. A typical cross-linked polystyrene sulfonic
acid material.
GREGOR
177
-------
Techniques for the preparation of ion-exchange membranes in-
clude direct polymerization in sheet form; casting from interpolymer
mixtures; and the preparation of heterogenous membranes by sus-
pending ion-exchange resin particles in a matrix polymer and form-
ing a film by extruding or by casting from a solvent. Bieber de-
scribes several of the latter procedures(19).
Whereas the stability of cation-permeable membranes prepared
from polystyrene sulfonic acid has been suitable (these membranes
can be used at temperatures up to 100°C for long periods without
severe degradation), the same is not true of the quaternary am-
monium anion-permeable membranes, which usually degrade ap-
preciably in weeks or months even at 60°C. The recent synthesis of
a substituted pyridinium compound (2, 6 di-t-butyl pyridinium
iodide) of unusual stability was reported by Okamoto and Shim-
agawa(20). These authors treated 2, 6 di-t-butyl pyridine with
methyl iodide under high pressure. The compound could be sub-
limed at 250°C under reduced pressure without decomposition. Its
unusual stability might be applied to the preparation of anion-
exchange membranes of stability comparable to that of cation-
exchange films.
Ion-exchange membranes are selectively permeable either to
cations or to anions by virtue of the fixed charges contained therein.
The Donnan potential gives rise to an equilibrium that may be ex-
pressed in simplified form as follows for a system of fixed sulfonic
acid groups (ReSO^) in equilibrium with a solution of potassium
chloride:
[K+] [CT]y±= [K+] [Cl-]y±; [K+] = [ReSOg] + [CT],
where [ ] is the concentration of the fixed charges expressed in
molality or in terms of the imbibed water within the pore, y^ is the
mean activity coefficient, and the superscript bar indicates" values
pertaining to the membrane phase. Recent studies have shown that
this simple equation predicts the behavior of many membrane systems
rather accurately if one sets y_ = 0.45(21).
The Donnan equation predicts that membrane selectivity will
fall with increasing concentration of the ambient electrolytic solu-
tion, and this is observed. For maximum selectivity, the concentra-
tion or molality of fixed-charge groups within the film must be as
high as possible; commercial films having molalities (m) in the range
of 5 to 8 exhibit a commensurate ionic selectivity. With highly con-
centrated electrolytes, however, even these membranes will not be
very selective. For example, in 5m potassium hydroxide and a fixed-
charge membrane of concentration 5m, the transport number (frac-
tion of current carried by each ionic species) for the permeable ion
is but 0.73.
Fixed-charge membranes do not show concentration polarization
within the membrane phase, but do exhibit this phenomenon strongly
in the adjacent solution phase. For these reasons, fixed-charge
178 Separator Systems
-------
membranes are favored for use in fuel cells. When relatively dilute
solutions are interposed between the fixed-charge membranes and
the electrodes, however, a marked concentration polarization en-
sus, with an increase in the ohmic resistance of the depicted electro-
lyte layer adjacent to the membrane phase; other undesirable side
effects also occur(22, 23).
When a current is passed across a fixed-charge membrane, the
selective transport of one species over another takes place because
the concentration of ions of the charge opposite the fixed-charge
(counter-ions) is very much greater than that of ions of the same
charge (co-ions). In these narrow-pore systems considerable hydro-
dynamic drag is imposed upon all ions, particularly those whose di-
ameters approach that of the pore. Along with the ionic flux there is
a flow of solvent, termed electro-osmosis. The amount of water trans-
ported by electro-osmosis increases with the size of the counter-ion
and the pore diameter for the "tight" membranes of high selectivity.
This solvent transport can be appreciable; for "tight" membranes it
is about 100 milliliters per Faraday; in "loose" ones it can be of the
order of liters per Faraday. Recent studies by Bagner(24) have
shown that when the diameters of counter-ions are approximately
80 to 90 percent of the pore diameter, "plug flow" results, and
essentially all of the water content of the membrane moves with the
counter-ions.
Although this electro-osmotic flow can be used to advantage in
some systems, it usually presents a problem to be overcome in bat-
teries separators because the solvent flows with the ionic current.
The pressures generated by electro-osmosis are considerable. Electro-
osmotic flow is particularly troublesome in fuel cell membranes, in
which it acts to dry out one electrode and flood the others. Wicking
systems are used to recirculate water under these circumstances.
As mentioned earlier, ions can be made to show marked differ-
ences in permeability in certain films, on the basis of differences in
their hydrated sizes. A most interesting film has been developed by
Yawataya, Oda, and Nishihara(25), who coated the surface of con-
ventional ion-exchange membranes with a very thin (5 /_/,) layer of a
co-condensate of phenol, phenol-sulfonic acid, and formaldehyde, and
achieved a transport number of 0.01 for Ca++ in a sodium-calcium
solution; the latter ion is about twice as large as the former.
The common inorganic ions, particularly the alkali metal cations,
demonstrate conductivity in aqueous solution, an indication that they
are hydrated to an extent that decreases with the ionic size. Values
for non-hydrated and approximate hydrated diameters are given in
Table 2. Because the hydration energies are considerable, extensive
dehydration of an ion on entering a pore system is not readily
achieved. It was observed recently that valinomycin, a cyclic
dodecadepsipeptide, has such a very fine and rigidly controlled pore
diameter that it can produce a structure more permeable to potassium
than to sodium ions by a large factor; this observation shows at least
GREGOR 179
-------
that such systems exist, and they may be the source of the high
level of ionic discrimination demonstrated by biological mem-
branes C26, 27). The usual sequence of hydrated volumes is observed
in most membrane systems, with the permeability series P°tas-
sium> sodium > lithium. The markedly higher hydration of the alkali
metal divalent ions such as calcium and magnesium has allowed
the membranes having very fine pores to show a high selective
permeability to univalent over divalent ions(25).
Table 2. IONIC PROPERTIES
H
Li
Na
K
Me4N
Et4N
Pr4N
Bu4N
OH
Cl
Br
1
SCN
pTS
103
NQ3
X° r in A
350
39
50
74 2.1*
45 3.47
33 4.00
23 4.52
20 4.94
195
76 2.3*
76
77
72
34
40 3.7"
71
Crystal
radii, A
0.68
0.98
1.33
1.81
1.96
2.19
a From distance of approach in solution x° is equivalent ionic
conductance at infinite dilution.
MEMBRANE THICKNESS
The thickness of polymeric membranes can be controlled within
rather wide ranges. Cast films of a number of common matrix poly-
mers can be as thin as 5 microns and still demonstrate reasonable
mechanical strength and the absence of pinholes; the conventional
packaging films vary in thickness from approximately 10 to 50
microns. Membranes as thin as 1 micron have been prepared and
studied; these are very fragile and require support. One means of
support is to cast the film directly onto a thin, fine fabric, as is done
180 Separator Systems
-------
with certain commercial ion-exchange membranes. Very thin films
having a thickness of approximately 0.1 to 1 micron can be cast
directly onto a fine porous matrix such as a treated paper without
the introduction of holes. Also, one can apply the techniques first
described by Loeb and Sourirajanf28/), who cast a film, allowed a
thin skin to form as a result of the rapid evaporation of the solvent
from the surface layer, and then coagulated the film in a non-
solvent. By the choice of appropriate materials, solvents, and con-
ditions, one can produce membranes having a highly porous and
open substructure with a very thin film of the same polymer on one
face. For example, films having an effective thickness of approxi-
mately 1 to 2 microns on a substructure of some 50 to 100 microns
have been prepared.
SOLUTE-BINDING MEMBRANES
Membranes that are impermeable to certain ionic or molecular
species can be made by incorporating on their pore walls substituents
that react reversibly or irreversibly with the mobile species. A
familiar example is the use of cellophane membranes in the silver-
zinc battery, in which under alkaline conditions the cellulose reacts
with silver ions (or colloidal silver) to form an insoluble, immobil-
ized material. The same type of irreversible reaction may result
when rubber membranes vulcanized with sulfur and presumably
containing reactive sulfur compounds are used in lead-acid batteries;
sulfur-antimony complexes apparently form, and the metallic ions,
no longer able to diffuse to the electrode, are "trapped."
The interactions of ionic species with polymers containing specific
binding agents have been studied extensively; the polymers have
been in solution, in resin, and in membrane form. The original stimu-
lation for this work came from Skogseid("29), who showed that the
classical chemical reactions involving aromatic structures can be
carried out with linear or cross-linked polystyrene. With systems of
these kinds, byproducts that are part of the polymeric structure re-
main so and cannot be separated; by judicious use of chemical re-
actions Skogseid prepared a number of materials of rather complex
nature. Figure 2 shows a few of the structures that he prepared and
characterized. Familiar chelate structures are included.
Gregor, Taifer, Citarel, and Becker(30) prepared a number of
chelating ion-exchange resins based upon familiar condensation re-
actions; extensive studies of polymers containing groups capable of
specific interactions with solutes, particularly the transition metal
ions, have been reported (31, 32). The general theory of such inter-
actions has been investigated by Wall (33), Gregor(34), Mora-
wetz(35), and others. The rather common carboxylic polymers
form fairly strong complexes with the alkaline earth and transition
metals; amine polymers form similar complexes with the transition
metals. Because of the high local concentration of ligands or com-
plex-forming groupings in polymeric systems, one usually encounters
stronger binding with polymeric ligands than with monomeric ones.
GREGOR 181
-------
CH2 —CH-
S03H
OH
N02
Figure 2. Some of the complex structures prepared
by Skogseid (reference 29).
Of particular interest to those concerned with battery tech-
nology is the availability of polymers containing the SH grouping,
one capable of rather specific interactions, particularly with heavy
metals(36, 37). BayerfSS) has described an interesting example of
a polymeric chelate demonstrating high specificity (Figure 3), with
which the extraction of gold from sea water was claimed. Preparing
chelate resins or membranes need not entail involved chemical pro-
cedures when the monomeric ligand is available. As an example,
Gregor and Wa\lace(39) prepared thiourea and phenylthiourea
polymers by formation of the condensate (with formaldehyde) as a
cage about a hydrophilic polymer to maintain porosity. The final
product showed a high capacity of this thiol grouping, one that forms
particularly strong complexes with heavy metallic ions, particularly
those of silver, mercury, etc. Many other systems are susceptible to
this kind of cage polymer formulation.
The formation of metal-chelate complex in membranes acts to
bind metallic ions and inhibit their diffusion across the membrane,
and although such retardation can result in a considerably lower
flux, it is still finite because the reaction is reversible. The work of
Wetstone and Gregor(40), in which they studied the relative trans-
port of copper and sodium in a carboxylic membrane, is indicative
182
Separator Systems
-------
I
CH
II
N
I
CH
II
N
N^ ^N^-^N^CH
HC - CH
GLYOXAL BIS-2-HYDROXYANIL
or
Figure 3. A polymeric chelate of high specificity
described by Bayer (reference 38).
of these phenomena. For true "trapping" of the undesirable ionic
or molecular species in a membrane, one must use an irreversible
reaction, of which several are available. Several oxidation-reduction
polymers have been reported by Cassidyf41/), Manecke(42),
GregorfSS), and Overberger(37). Strong polymeric reducing agents
are readily prepared; polymeric oxidizing agents of reasonable sta-
bility and oxidizing power are not known at the present time. A
particularly convenient and simple procedure for preparing strongly
reducing resins (or membranes) was described by Gregor and
Beltzer(43), who prepared pyrogallol-formaldahyde co-condensates
of a high degree of capacity and porosity.
Electrically regenerable ion-exchange systems have long in-
trigued investigators. These systems are electronic conductors and
contain groups that can be rendered ionic or non-ionic, or at least
have a markedly different degree of ionization, as the result of the
imposition of an applied electric potential. Weissf44J reported some
interesting developments. In principle, such systems would attract
anions or cations when polarized (positive or negative), and would
discharge these ions when the polarity was removed or reversed.
MurphyC45) applied this principle for purposes of desalination. Films
that were conductive to electrons were prepared by laying down a
film of activated carbon onto a metallic grid; the film was then
treated with either cationic or anionic polyelectrolytes. Imposition of
the potential onto the grid caused the accumulation of either cations
or anions; these ions were released by reversal of the potential. The
exact chemical reactions that occur are not known; apparently they
do not involve a true oxidation-reduction, charge-transfer process
but rather a charging process at the surface of the charcoal. Alter-
ation of the ionic charge of weakly acidic groups has been reported
GREGOR
183
-------
by Stillman and Gregor(46), who prepared polymers of the poly-
acene-radical type with carboxylic groups as part of the conjugated
ring system and showed that the acidity of the carboxylic acid group
could be increased or decreased by the imposition of a potential from
a battery. Although we do not know whether any of these electron-
conducting systems can be used in battery separator systems, the
possibilities are interesting.
SOLVENT MEMBRANES
We now consider membranes that do not possess pores in the
usual sense. These are commonly called solvent-type membranes.
A typical film of this type is a familiar cellulose acetate reverse-
osmosis membrane first described by Ried(47,) and then improved by
Loeb and Sourirajan(27) for use in desalination. These membranes
selectively dissolve water and not ions by virtue of the unusual
hydrogen-bonded structure of water contained within cellulose
acetate. They can have low resistance by virtue of possessing a
coherent "skin" of extreme thinness, with a porous substructure for
support.
The electrical conductivity of solvent-type membranes, in this
case reverse-osmosis membranes of cellulose acetate, has been meas-
ured by Saltonstall("48), who has also used the time-dependency of
conductivity when the membrane is immersed in an electrolyte solu-
tion to calculate diffusion coefficients in the membrane. Results indi-
cated that the equivalent conductance of electrolyte in the membrane
did not depend on its concentration in the film. Diffusion co-
efficents of sodium chloride in typical membranes were about 6x10~10
square centimeters per second; distribution coefficients (in m) were
about 0.1. The membrane resistance increased markedly with heat
treatment; for example, resistances of membranes heat-treated at
70°C were 3 orders of magnitude lower than those of membranes
heat-treated at 95°C. Resistance of membrane 41 microns thick in
equilibrium with 1 M sodium chloride was approximately 6,000 ohms
for an exposed area of 1.42 square centimeters. Although this specific
conductivity is very low, it is not an unreasonable value for a mem-
brane having an effective thickness of 1 micron. Further, since the
concentrations of acids and bases in membranes of these kinds are
very much greater than those of salts, these films might prove useful
as battery separators under special conditions. The dense packing
and the strongly hydrogen-bonded water structure in these systems
may allow for much more rapid transport of hydrogen and hydroxide
ions than would be encountered in ordinary solution.
The conductivity of acid and base have more recently been
measured in cellulose acetate membranes by Saltonstall(48). Under
these conditions, the equivalent conductivity in the membrane phase
is concentration dependent, but not to a very marked extent. When
comparisons at the same concentration of all solutes in the solution
phase (on a normality basis) were made, it was observed that the
Separator Systems
-------
specific conductivity with potassium hydroxide was twice that for
sodium chloride, and for sulfuric acid was 20 times that for sodium
chloride. These results, particularly with acid electrolytes, appear
very encouraging.
MISCELLANEOUS MEMBRANES
Thus far I have discussed organic polymeric membranes. Many
inorganic membranes do exist, however, and they can provide un-
usual advantages in that they can be made of materials that allow
for a precise and controllable pore structure. Certain ion-exchange
minerals show high discrimination for ions on the basis of size, as
demonstrated by Hechter(49) for sodium-potassium exchange. In-
organic clay membranes were investigated by MarshallfSO,); mem-
branes of beta alumma(51) provide the basis for the operation of the
sodium-sulfur battery described by DeWitt in this volume. Syn-
thetic inorganic membranes of a glass-like nature have also been
described (5 2). Membranes made from Vycor glass have been known
for some time(53). Inorganic exchanges employed as cracking cata-
lysts may also show a uniform pore structure; the work of Barrerf 54)
and of the group that developed the molecular sieves is pertinent
here. Most investigators have been unable to prepare membranes
that demonstrate the highly selective properties of these inorganic
exchange materials, largely because of the problem of incorporating
these into a structure that allows permeation between properly
aligned particles.
KrausfSS) has described inorganic ion-exchange resins; these
and similar systems have been made in membrane form. An example
is a cation-permeable film incorporating zirconium phosphate in
particulate form; hydrous thorium oxide has served in the anion-
permeable membranes(56). Transference numbers of these mem-
branes were 0.90 to 0.96, whereas under the same conditions trans-
ference numbers of organic materials were greater than 0.99. Al-
though not highly selective, these membranes operated at high
temperature and adverse polarizing conditions for rather long periods
without showing some of the defects of the organic membranes.
Certain of these inorganic membranes have an intrinsic solubility,
which, although very low, can be appreciable under conditions of
constant current passage.
Another interesting development has been the availability of
separator materials made from polyethylene and capsulated fibers
and particlesC57j. Cellulose fibers have been enveloped with sheaths
of polyethylene by the use of transition-metal catalysts, resulting in
materials that can be made in membrane form by paper-making tech-
niques. This process is of considerable interest for battery separator
applications because it is simple, because particles that can be con-
verted into membrane form are available in large amounts at a low
price, and particularly because the porosity of the final sheet can
be altered by a further pressing technique.
GREGOR
185
-------
The special requirements of fuel cells call for development of
several different kinds of new separator systems. The ion-exchange
membrane electrolyte provides several advantages, among which are
ease of fabrication of cells containing a solid electrolyte in the form
of thin sheets, resistance to concentration polarization, and the trans-
port of but a single, ionic species. However, organic membranes are
susceptible to attack at the electrodes, their electro-osmotic water
transport dries out one electrode-membrane interface and floods the
other, and the commercial membranes do not possess semi-perme-
ability for different solutes (methanol versus water, as an example).
The use of fluorinated polymers, even as the matrix polymer, imparts
greatly increased chemical stability to the membrane. Further, inter-
polymer systems now available consist of largely fluorinated materi-
als; these possess even greater chemical stability than the fluorinated
polymers. The problem of electro-osmotic water transport is cur-
rently handled by various wicking systems; GregorfSSj has shown
that by the use of a mosaic membrane one can re-transport water
electro-osmotically to the dry membrane side.
Finally, there are some new and interesting possibilities for the
preparation of membranes that will impede one solvent and not an-
other. For example, more polar films not containing aromatic rings,
such as ones prepared from polyvinylsulfonic acid, may be expected
to show much more sorption of water than less polar solvents such
as the alcohols. The recent announcement that certain helical anti-
biotic molecules(26, 27) can apparently form a channel through
which only dehydrated ions can pass suggests that if nature can
produce such a highly selective membrane system similar systems
possibly can be created in a synthetic structure. Finally, the use of
new solvent-type membranes may prove yet another means for con-
trolling the transport of solvents.
HIGH-TEMPERATURE POLYMERS
A recent advance in high polymer chemistry of particular interest
to those concerned with battery separator problems is the preparation
of many new high-temperature polymers. For more information a
collection of papers presented at a recent symposiumfSS) should be
consulted, particularly the papers by Atlas and MarkffiOj and
Mark("67, 62). A more recent review by Marvel(63) describes later
studies.
A few of the new high-temperature polymers deserve mention.
An interesting family of linear, rigid polyether-methylenesulfonic
materials (PEMS) have been reported by Johnson et al.(64),
Karacz(65), Price(66), and Reich(67). These materials are repre-
sented by recurring aromatic rings connected in the para-position by
sequences of —SO2—, —O—, —CH2, —S—, or —CCCH.,), —bonds.
These polymers, particularly the aromatic poly-ether-methylene-
sulfones have specific gravities of 1.20 to 1.26, a tensile modulus of
about 400,000 psi, a tensile strength of about 12,000 psi, and break
186 Separator Systems
-------
elongation ranging from 30 to 120 percent. Some of these polymers
melt or soften reversibly at about 440°C without degradation if the
exposure is short. Viscosities are remarkably constant, even when
the polymer is stored at approximately 400°C. Polymers of this type
are soluble at high temperatures in certain polar solvents such as
dimethylformamide (DMF) and dimethylsulfoxide (DMSO), so they
can be readily prepared in film and fiber form.
Another interesting class of high-temperature polymers are the
poly-benzimidazoles (FBI) prepared from pre-polymers following
the reaction shown in Figure 4, as reported by Gillhamf 68). Proper-
ties of these polymers show very little change at approximately
380°C. Special thermal cures have produced polymers that maintain
noticeable rigidity and strength even at 500°C for some time. Me-
chanical properties of these polymers are unusual; flexural modulus
and strength at room temperature are about 4 x 10" and 0.9 x 10r'
psi, respectively.
2H20
2NH,
Figure 4. Polybenzimidazoles (PBI) as reported
by Gillham (reference 68).
Polyamides containing piperazine are known to be very stable;
one member of this family has been studied in some detail, poly
(isophthaloyl trans-2, 5-dimethylpiperazine), ITDMP(69). This
polymer has a softening range near 400°C; it is soluble in chloroform
and formic acid, in which it has high intrinsic viscosities, lending it-
self to film and fiber formation.
Another class of high-temperature polymers, the polybenzimida-
zobenzo-phenanthrolines (BBB) (see Figure 5), have been prepared
by van Deusen(70) and Marvel(71). Most of these polymers are
soluble in sulfuric acid, benzene sulfonic acid, and concentrated
alkalis. When tetra-aminobenzene is used as the basic reactant, the
BBL ladder polymers formed are less soluble than the BBB poly-
mers. These polymers do not show weight losses of 5 to 10 percent
until heated in air at 550°C
Other high-temperature polymers have been obtained by the
oxidative coupling of aromatic diamines. These show the structure
GREGOR
187
-------
[N = N — Ar — N = N —Ar — O — Ar — N =N]
where Ar is the benzene ring joined in the p-position. Many of these
aromatic azopolymers are stable at high temperature; they exhibit no
weight loss up to 325°C and only 10 percent weight loss upon heating
to 425°C. All of these materials are darkly colored, amorphous, and
soluble only in concentrated sulfuric acid at room temperature. The
work of Kovacic(72), Preston(73), and Bach(74) is relevant.
Finally, two commercially available, high-temperature fibers
deserve mention; the first is a poly — 1, 3 — 4 — oxadiazole, and its
thiol-analogue, shown in Figure 6(75). This polymer is stable at
250°C and is unaffected by reflux for 48 hours in 10 percent of either
sulfuric acid or sodium hydroxide. The second is a poly (phenylene
triazole) (see Figure 7); this is stable at 300°C and can be spun from
formic acid solutions(76/).
HOOC-0}
HOOC-(O
NHr,
Figure 5. Polybenzimidazobenzophenanthrolines
(BBB) prepared by van Deusen and Marvel
(references 70, 71).
N —N
Figure 6. A poly-1, 3-4- oxadiazole and its
thiol-analogue (reference 75).
188
Separator Systems
-------
Many new polymers of exceptional thermal and chemical sta-
bility will find their way into separator applications, particularly in
high-discharge batteries in which the local temperature is high and
in fuel cells operated at high temperatures and under highly adverse
conditions that ordinarily lead to chemical decomposition.
N — N
Figure 7. A poly (phenylene triazole) (reference 76).
FIBRIDS
The use of non-woven fibers as battery separators has found
some application. A new and particularly interesting development in
high polymer technology is the development of "fibrids," a new form
of fibers prepared from a number of commercially available poly-
meric materials. Fibrids can be made from many different polymers
in different waysf 77-79 j; in one procedure the polymer is dissolved
under high pressure and temperature in a liquid that is a solvent for
the polymer under these conditions but is a non-solvent at lower
temperatures and pressures. This solution of the polymer is extruded
through the spinnerets, upon which the solvent literally boils away,
leaving a strand that consists of a very large number of very fine
filaments having appreciable mechanical strength but an extremely
fine diameter and a large surface area. For example, fibers having a
nominal diameter of 25 microns consist of filaments having diameters
of 2 to 4 microns, but these are ribbons having smaller thicknesses.
Their surface area is of the order of 100 square meters per gram
compared to 5 square meters per gram for cotton. Fibrids can be
prepared directly from the polymer at a cost of about 50 cents per
pound, and one can fabricate them into screens and sheets having
a wide range of porosities, a range not ordinarly attainable with
fibrous materials. Many applications of these new polymer forms to
the battery industry should be forthcoming.
CONCLUSIONS
Battery scientists and engineers have been much too depend-
ent for separator materials on commercial films prepared for
other purposes, such as packaging and water treatment. The
amount of material needed for membranes usually is so small that
GREGOR
189
-------
manufacturers and fabricators of plastics have not been encouraged
toward research and development.
Further, research has tended to concentrate separately upon
separator materials or upon electrodes and their interaction with
electrolytic solutions, but not upon the combined system. A funda-
mental investigation of electrode reactions in the presence of well-
characterized separator materials should lead to systems of far-
reaching practical importance.
We may anticipate the development of several new and vastly
improved separator systems based on the new materials and tech-
nology available today. The complexity of battery systems, particu-
larly for electric vehicles, requires an interdisciplinary approach,
with close collaboration on the part of those who synthesize polymers,
prepare them in membrane form, study their structure, and finally
incorporate them into practical battery systems.
LITERATURE CITED
1. News Announcement, General Electric Co.
2. ESB, Incorporated, sold under trade names, Synpor, Porothene.
3. Pall Co., Glen Cove, N. Y.
4. Ott, E., Cellulose, Interscience Pub. Co., N.Y. (1954).
5. Elford, W. J, J. Path Bact, 34, 505 (1931).
6. Ferry, J. D., Chem. Rev., 18, 373 (1936).
7. Gregor, H. P. and Sollner, K, J. Phys. Chem, 50, 53 (1946).
8. Gregor, H. P., Dissertation, May 1945.
9. Carr, C. W, Anderson, D. and Miller, I, Science, 125,1245 (1957).
10. Craig, L. C. and Konigsberg, V, J. Phys. Chem., 65, 166 (1961).
11. Mindick, M. and Oda, R, U. S. Patent. 3,069,340 (Dec. 18, 1962).
12. Gregor, H. P., Jacobson, H, Wetstone, D. M. and Shair, R. C,
J. Phys. Chem, 61, 141 (1957).
Gregor, H. P. and Wetstone, D. M, J. Phys. Chem, 61,147 (1957).
Gregor, H. P. and Wetstone, D. M, J. Phys. Chem, 61,151 (1957).
13. Collander, R, Soc. Sci. Fennica, Comment. Biol, 2, 6 (1926).
14. Faxen, H, Ann. Physik, IV, 68, 89 (1922).
Faxen, H, Arkin Math, Astr. and Fys, 17, No. 27, (1922).
15. Helfferich, F, "Ion Exchange", Mc-Graw-Hill Book Co, Inc.,
New York, N.Y. (1962).
Lakshminarayanaiah, N, Chemical Reviews, 65, 491 (1965).
16. Bastian, R, Dissertation, Polytechnic Institute of Brooklyn, N. Y,
June 1956.
17. Kawabe, H, Jacobson, H, Miller, I.F. and Gregor, H.P, J.
Colloid and Interface Sci, 21, 79 (1966).
190 Separator Systems
-------
18. Gregor, H. P and Kawabe, H., to be published.
19. Bieber, H. H., Dissertation, Polytechnic Institute of Brooklyn,
N.Y., June 1956.
Bieber, H. H., Bruins, P F. and Gregor, H. P., Ind. Eng. Chem.,
50, 1273 (1958).
Imoto, R., Kosaka, Y., and Shimizu, A., Japanese Patent 10, 123.
20. Okamoto, Y. and Shimagawa, Y., Tetrahedron Letters, 3, 317
(1966).
21. Leszko. M. and Gregor, H. P., Roczniki Chemii, 40, (7/8)
1281 (1966).
22. Gregor, H. P and Peterson, M. A., J. Phys. Chem., 68, 2201
(1964).
23. Gregor, H. P and Miller, I. F., J. Am. Chem. Soc., 86, 5689 (1964).
24. Bagner, C., M.Ch.E. Thesis, Polytechnic Institute of Brooklyn,
N.Y., June 1966.
25. Yawataya, T., Hani, H., Oda, Y. and Nishihara, A., Dechema
Monograph, 74, 501 (1962).
Oda, Y., Nishihara, A., Hani, H., and Yawataya, T., Ind. Eng.
Chem. Prod. Res. Develop, 3, 244 (1964).
Asahi Glass Co., Ltd., Japanese Patent 13,009.
26. Bangham, A. D., Standish, M. M. and Watkins, J. C., J. Mol. Biol.,
13, 238 (1965).
Lev, A. A. and Buzhinsky, E. P., Cytology (U.S.S.R.), 9, 102
(1967).
Muller, P. and Rudin, D. O., Biochem. Biophys. Res. Comm., 26,
398 (1967).
27. Tosteson, D. C., personal communication.
28. Loeb, S., and Sourirajan, S., Advan. Chem. Ser., 38, 117 (1963).
Loeb, S., and Sourirajan, S., U.S. Patent 3,133,132.
Loeb, S., Sourirajan, S., and Weaver, D. E., U. S. Patent 3,133,137.
29. Skogseid, A., Dissertation, Oslo, (1948).
30. Gregor, H. P., Taifer, M., Citarel, L. and Becker, E. I., Ind. Eng.
Chem., 44, 2834 (1952).
31. Jenckel, E. and v. Lillin, H., Kolloid z, 146, 159 (1956).
32. Blasius, E. and Olbrich, G, Z. Anal. Chem., 151, 81 (1956).
33. Wall, F. T. and Gill, S. J., J. Phys. Chem., 58, 1128 (1954).
34. Luttinger, L., Dissertation, Polytechnic Institute of Brooklyn,
N.Y., June 1954.
Gregor, H. P., Luttinger, L. and Loebl, E. M., J. Phys. Chem.,
59, 34 (1955).
Gregor, H. P., Luttinger, L. B. and Loebel, E. M., J. Phys. Chem.,
59, 990 (1955).
Andelman, J. B., Hoeschele, G. K., and Gregor, H. P., J. Phys.
Chem., 63, 206 (1959).
GREGOR 191
-------
35. Morawetz, H., Kotliar, A. M. and Mark, H., J. Phys. Chem., 58,
19, (1954).
36. Gregor, H. P., Dolar, D. and Hoeschele, G. K., J. Am. Chem.
Soc., 11, 3675 (1955).
37. Overberger, C. G. and Lebovitz, A., J. Am. Chem. Soc., 77, 3675
(1955).
38. Bayer, E., Angew. Chem., Internal. Ed., 3, 325 (1964).
39. Wallace, R. A., M.Ch.E. Thesis, Polytechnic Institute of Brook-
lyn, N. Y., June 1959.
40. Gregor, H. P., Wetstone, D. M., Discussions Faraday Soc., No.
21, 162 (1956).
41. Cassidy, H. G. et al., J. Am. Chem Soc., 75, 1615 (1953).
42. Manecke, G., Z. Elektrochem. Ber. Bunsenges. physik. Chem.
58, 369 (1954).
43. Beltzer, M., Dissertation, Polytechnic Institute of Brooklyn, N. Y.,
June 1960.
Gregor, H. P. and Beltzer, M., J. Polymer Sci., 53, 125 (1961).
44. Garten, V. A. and Weiss, D. E., Australian J. Chem., 8, 68 (1955).
45. Arnold, B. B. and Murphy, G. W., J. Phys. Chem., 65, 135 (1961).
46. Stillman, I. M. and Gregor, H. P., to be published.
47. Reid, C. E. and Breton, E. J., J. Appl. Polymer Sci., 1, 133 (1959).
Reid, C. E. and Kuppers, J. R., J. Appl. Polymer Sci., 2, 264
(1959).
Reid, C. E. and Spencer, H. G., J. Phys. Chem., 64, 1587 (1960).
Reid, C. E. and Spencer, H. G., J. Appl. Polymer Sci., 4, 354
(1960).
48. Saltonstall, C. W. et. al., O.S.W. Report, Contract 14-01-0001-338,
Oct. 1966.
49. Hechter, O., Polleri, A., Lester, G. and Gregor, H. P., J. Am.
Chem. Soc., 81, 3798 (1959).
50. Marshall, C. E., J. Phys. Chem., 43, 1155 (1939).
Marshall, C. E. and Bergman, W. E., J. Phys. Chem., 46, 52
(1942).
51. DeWitt, T. W., personal communication.
52. Harrington, R. V., personal communication.
53. "Vycor", Corning Glass Works.
54. Barrer, R. M., Proc. Sco. Chem., 99 (1958).
55. Kraus, K. A. and Phillips, H. O., J. Am. Chem. Soc., 78, 249
(1956).
56. Rajan, K. S., Boies, D. B., Casolo, A. J. and Bregman, J. I,
Desalination, 1, 231 (1966).
1 Q2
Separator Systems
-------
57. Orsino, J. A., Herman, D. F. and Brancato, J. J., U.S. Patent
3,121,698 (Feb. 18, 1964).
Herman, D. F. and Dunlap, I. R., TAPPI, 4S, 418 (1965).
Herman, D. F., Kruse, U. and Braneoto, J. J., J. Polymer Sci.,
Part C, No. 11., 75 (1965).
58. Gregor, H. P., U. S. Army Electronics Res. and Dev. Lab. Re-
port, Contract DA-36-039-SC 89197.
59. Segal, C. L., Ed., "High Temperature Polymers", M. Dekker,
Inc., N. Y., (1967).
60. Atlas. S. M. and Mark, H. F., Angew. Chem., 72, 249 (1960).
61. Mark. H. F., J. Polymer Sci., 9C, 1 (1965).
62. Mark, H. F.. paper at IUPAC Symposium, Tokyo, Sept. 1966.
63. Marvel, C. S., paper at IUPAC Symposium, Brussels, June 1967.
64. Johnson, R. H. et. al, paper at A.C.S. meeting, Sept. 1966.
65. Karacz, F. E., Polymer Letters 3, 561 (1965).
66. Price, C. C., J. Polymer Sci., 2A, 1511 (1964).
67. Reich, L., Polymer Letters, 3, 231 (1965).
68. Gillham, J. K., Appl. Polymer Symp., 2, 45 (1966).
69. Bruck, S. D., Polymer, 5, 435 (1964): 6, 49, 319, 483 (1965).
70. van Deusen, R. L., J. Polymer Sci., B4, 216 (1966).
71. Marvel, C. S., J. Polymer Sci., Al, 1531 (1963); A3, 3549 (1965).
72. Kovacic, P., et. al., J. Am. Chem. Soc., 85, 454 (1963).
73. Preston, J., J. Polymer Sci., A4, 529 (1966).
74. Bach, H. C., paper at A.C.S. Meeting, Sept. 1966.
75. E. I. du Pont.
76. Chemstrand Co.
77. Morgan, P. W., U. S. Patent 2,999,788 (Sept. 12, 1961).
78. Blades, H. and White, J. R., U. S. Patent 3,081,519 (March 19,
1963).
79. Blades, H. and White, J. R., U. S. Patent 3,227,664 (Jan. 4, 1966).
193
-------
NOTES ON A CARBON DIOXIDE
SCRUBBING SYSTEM
Harry P. Gregor
Fuel cells that employ alkaline electrolytes and air as one of the
fuels all suffer from one common problem — the carbon dioxide in
the air converts the hydroxide to carbonate with a loss in power
output. The usual remedy is to pass the air through a solution of
base, with periodic replacement. This adds to the volume of the power
unit, and increases its overall chemical requirements. I would like,
therefore, to mention some recent studies on a carbon dioxide scrub-
bing system that may provide a practical answer to this problem.
Traditionally, acidic gases such as carbon dioxide have been re-
moved by chemical scrubbers; those that use monoethanolamine are
typical. Here, an acid base reaction removes the acidic gas, and the
absorbant is regenerated by heating because the equilibrium is shifted
towards the uncombined reactants at elevated temperatures. The
technology of liquid scrubbers is well advanced; these systems have
found large-scale applicability, as in the removal of carbon dioxide
from gas streams of the ammonia process.
The MEA (monoethanolamine) and analogous scrubber systems
have certain advantages and some rather serious disadvantages, par-
ticularly when applied to the electric vehicle. The amine is inex-
pensive, but it is subject to thermal decomposition. Further, amine
odors are noxious and they can be somewhat toxic. Ethanolamine
scrubbers have substantial heat requirements for thermal regenera-
tion because for every pound of carbon dioxide removed a substantial
number of pounds of water must be evaporated. Although this water
can be recovered, the heat loss involved is large and the water re-
covery system must have an appreciable volume.
The use of polymeric amines for removal of carbon dioxide has
been investigated. The reactions of polymeric amines with carbon
dioxide have also been investigated (Gregor and Robins, 1959;
Robins, 1959); the applicability of several polymeric amines for the
removal of carbon dioxide from the the air of submarines and space
vehicles has been investigated (Gregor and Robins, 1959; Robins,
1959; Weber, 1966; Weber, Miller, and Gregor, to be published).
Polymeric amines possess certain important advantages over MEA
and its analogues, and certain relative disadvantages. First poly-
meric amines are much more stable than their monomeric ana-
logues, particularly the ethanolic-substituted amines that are em-
ployed because of their reduced volatility compared to the aliphatic
amines. Polymeric amines also show a higher thermal stability and
are more resistant to air oxidation (Gregor and Robins, 1959; Robins,
1959). The stability of polymeric amines varies; the tertiary alkyl
GREGOR 195
-------
amines are relatively stable; for example, poly-N-methylethylenei-
mine is considerably more stable than polyethyleneimine (PEI) or
polyvinylimidazole. The recently reported polymeric amines of
Gianinni (1967) promise to be even more stable.
Polymeric amines cannot be employed in aqueous solution as is
MEA because of the considerably greater viscosity that obtains. How-
ever, there is no advantage in employing liquid scrubbers in the
electric car; it is, rather, advantageous to use these polymeric
materials in the solid form. Under these circumstances, the amines
absorb relatively small amounts of water, with the amount varying
with the relative humidity of the ambient air. On regeneration, only
a relatively small amount of water must be evaporated; thus, heat
consumption is drastically reduced. Further, polymeric amines have
no odor, and thus their employment in limited space would not re-
quire an after-scrubber of acid, as is used to remove MEA vapors
under these circumstances.
The principal disadvantage of polymeric amines in the solid state
is their low rate of reaction, which is controlled by the diffusion of
carbon dioxide in the solid polymer to the fixed amino groups. The
reaction has been measured (Gregor, Weber, and Miller, to be pub-
lished) and found to be approximately 10~s square centimeters per
second for films in equilibrium with air at 100 percent relative
humidity. At lower relative humidities the process is somewhat
slower and equilibrium not so favorable; both the rate of absorption
and the capacity are approximately halved at 50 percent relative
humidity.
Another of the advantages of polymeric amines is the fact that
their basicity can be controlled, within appreciable ranges, by the
incorporation of salts within the solid phase. Table 1 lists data on
the capacity of a PEI-epichlorohydrin condensate for carbon dioxide
as a function of the gas content in air, its relative humidity, and its
temperature, taken from the dissertation of Weber (1966).
Since the rate of absorption of acid gases by solid-state polymeric
amme scrubbers is controlled by diffusion in the solid, thin film or
fiber absorbers having a maximum area-to-volume ratio are desirable.
The capacity of units designed with different ribbon diameters, based
upon the work of Gregor, Miller, and Wolfe (to be published) is
given in Table 2 and emphasizes this point. For particularly rapid
rates of absorption, the use of solid-state polyamines in the form of
fibrids (Morgan, 1961; Blades and White, 1963 and 1966) would offer
special advantages.
In operation, a polymeric solid-state scrubber would consist of
two filter units operating alternately to produce a continuous stream
of air free of carbon dioxide. The ambient air would pass through
the filter bed and be fed to the fuel cell, with its composition un-
altered except for acidic gases. If appreciable levels of sulfur dioxide
and other acidic gases were present, these would also be removed.
196 A Scrubber System
-------
Other acidic gases would interfere with the fuel cell reaction, and
their removal would be in direct proportion to their acidity. When
the filter was exhausted, it would be sealed by the closure of a pres-
sure valve and heat would be applied to the unit (probably employ-
ing the excess heat generated by the cell); the acid-base reaction is
reversed and almost pure carbon dioxide would be released to the
atmosphere, with an air condenser retarding the evaporation of
water.
This is not to say that polymeric scrubber systems are fully de-
veloped. The capacities for removal of carbon dioxide listed earlier
Table 1. REACTION OF PEI RESIN WTH CARBON DIOXIDE
Temp,
°C
0
20
20
20
20
20
40
70
C02 in
aim, %
1.0
0.30
1.0
0.65
0.30
1.0
1.0
0.30
0.30
Relative
humidity,
%
100
100
100
100
100
70
55
100
100
Capacity,
m-moles/g
4.1
1.8
2.9
2.2
1.3
1.8
1.0
0.45
1.1
Table 2. ABSORPTION OF CARBON DIOXIDE BY POLYAMINE FIBERS
AND FILMS
Fiber or film
thickness, M
150
150
75
38
38
% Theoretical
capacity
50
25
25
50
13
Space between
films or fibers,
A
50
25
25
25
25
Bed
vol,
ft3
16
9
5
5
5
Cycle
time,
min.
60
20
10
15
5
% Removed on
one pass
20
30
30
20
40
Conditions: Air at 25°C, 50 percent relative humidity, 0.5 percent Co,,; for
C02 removal, 15 pounds C02 per hour.
GREGOR 197
-------
are for levels of 0.5 percent, and at 0.05 percent (the usual air level)
the capacities would be lower by a factor of about 10. This capacity
could be increased, however, by using more basic polymers. It is
obvious that several problems must be met, but the availability of a
heat-regenerable scrubber system would make alkaline fuel cells
more practical for electric vehicles, particularly for military vehicles.
For the Union Carbide (private communication with G. E.
Evans) alkaline electrolyte fuel cell, air at a nominal carbon dioxide
content of 0.03 percent is scrubbed to an 80 to 90 percent removal
level by a caustic solution; without such scrubbing, the cell electrolyte
must be replenished 5 to 10 times more often, but this really trans-
fers the problem from the cell solution to the scrubber solution. A
100-watt cell employs from about 500 (4 times the theoretical) to
1,250 liters of air per hour. For a military vehicle, one needs 1 to
10 kilowatts and even higher level power sources. Therefore, a 1-
kilowatt fuel cell would, on the average, use 5,000 liters of air per
hour or require the removal of about 0.1 pound of carbon dioxide
per hour. From Table 2, at a bed volume of 5 cubic feet at a cycle
time of 15 minutes for 15 pounds of gas removed per hour at the
0.5 percent level, this amounts to a bed volume of about 0.3 cubic
foot for the removal of 0.1 pound of gas at a concentration level of
0.05 percent. This figure may have to be increased somewhat to
reduce exit carbon dioxide levels to 10 percent of inlet values.
REFERENCES
Blades, H., and J. R. White (1963). U. S. Patent 3,081,519 (Mar. 19).
Blades, H., and J. R. White (1966). U. S. Patent 3,277,664 (Jan. 4).
Giannini, U. (1967). Presented at I.U.P.A.C. Macromolecular Sym-
posium, Brussels, June 1967.
Gregor, H. P., and J. Robins (1959). Final Report, Contract No.
nr-839(20). U. S. Navy.
Morgan, P. W. (1961). U. S. Patent 2,999,788 (Sept. 12).
Robins, J. (1959). Dissertation. Polytechnic Institute of Brooklyn
(June).
Weber, O. W. (1966). Dessertation. Polytechnic Institute of Brooklyn
(June). See also: NASA Report, Contract No. NAS 9-2792
(Apr. 1966).
198 A Scrubber System
-------
BATTERIES AND FUEL CELLS: status
of current research on specific energy-
storage systems; how they perform.
I have been thinking about the oft-recurring circumstances under
which the battery industry has responded to the needs for batteries.
This industry has been at the threshold of the new in science since
1800, when Volta's pile was announced in England, an event that in-
pired and inaugurated a golden era of science. Within 30 years came
many discoveries about electric current: Oersted on magnetic effects,
Ampere on electrodynamics, the Seebeck effect, Faraday on con-
version of electric current and his discovery of induction.
On the electrochemical side, Davy had isolated the alkali metals
by 1806. Nicholson and Carlisle demonstrated the phenomenon of
electrolysis in 1800, and by 1905 Izarn applied it as a coulometer.
Believe it or not, Grotthus had formulated his theory by 1806. Elec-
trodeposition of metals was demonstrated and an electrochemical
telegraph was in the process of development. In all of these events,
the laboratory needed batteries, the only known source of current
at that time. It was soon found that the Volta pile was most easily
built in fairly large size by using the inetals, zinc and copper. Practi-
cal problems were encountered in scaling up, and especially in the
creation of convenient and easily handled installations.
199
-------
Within 1 year the people who worked with these larger batteries
had already found that -atmospheric oxygen improved the perform-
ance of this particular cell. This observation has been discovered
and rediscovered any number of times in the last 170 years. This
period may be said to have culminated in Faraday's announcement
of the laws of electrolysis. In all of this, electrochemistry and science
and batteries were and are intertwined. Battery technologists find
that their problems in making advances to meet demands are always
at the verge of the unknown and incompletely understood. For ex-
ample, they are pestered by lack of understanding of the structure
of their active materials, especially with regard to the amorphous
materials. They constantly watch for each new material that may
extend the range of application for practical batteries.
The same philosophy applies now to the people who enter and
become part of the battery industry. We see a widening expanse
of the technologists' training, for the battery problems encompass
practically every branch of scientific and engineering effort. I be-
lieve that the battery industry welcomes this increasing horizon,
this broadening of its views, and the edge that it brings in competi-
tive spirit.
Arthur Fleischer
Consultant
Orange, N.J.
200
-------
LEAD-ACID BATTERIES AND ELECTRIC VEHICLES
David L. Douglas
Gould-National Batteries, Inc.
Minneapolis, Minnesota
INTRODUCTION
For the past 50 or 60 years the lead-acid battery has been pre-
dominant in the field of battery electric motive power. Today the
span of battery propulsion covers conventional submarines, small
submersible vessels, industrial trucks, railway cars, mine locomotives,
delivery vans, and golf carts. In the past the list would also have
included passenger cars. Today we are focusing on electric street
vehicles, which include the no longer practical passenger vehicle and
the near marginal delivery van.
There are literally dozens of different types and sizes of lead-
acid batteries on the market — each designed for a specific applica-
tion. Many of these are being used for electric propulsion. Four
of the most important are flat plate motive power; tubular motive
power; golf-cart; and starting, lighting and ignition (SLI) batteries.
Flat Plate Motive Power Cells
For many years flat plate motive power cells have been the
standard of the industry for rugged service in industrial truck and
mine locomotive applications. Figure 1 shows the important elec-
trical characteristics of a typical cell. A battery of six such cells
in a steel tray delivers 5.0 kilowatt-hours and weighs 576 pounds
(8.7 watt-hours per pound). In average service this battery lasts
for 6 to 7 years, giving about 2,500 cycles before the capacity drops
to 80 percent of its rated value.
Tubular Plate Motive Potoer Cells
For certain applications where volume is at a premium, e.g.,
narrow-aisle fork-lift truck, cells with tubular positive plates are
widely used. Figure 2 shows the electrical characteristics of one of
the most popular sizes. A battery of 18 such cells in a steel tray
delivers 20.8 kilowatt-hours and weiphs 1,944 pounds (10.7 w-hr/lb).
The service life may he somewhat shorter than that of the flat plate
battery, but still amounts to 5 to 6 years and about 1,800 cycles.
SLI Batteries
These are familiar to all as the electrical energy storage device
in the electrical system of all automobiles. Being relatively low in
cost and readily available, SLI batteries are frequently used for ex-
perimental electrical vehicles. Important electrical characteristics of
DOUGLAS 201
-------
a common size are shown in Figure 3. The ampere-hour rating of
SLI batteries is specified at the 20-hour rate — quite unrealistic for
motive power applications. Further, these batteries are designed to
deliver large currents for engine starting and to withstand constant
overcharge whenever the engine is running. These factors combine
to make the SLI battery unsuitable for the deep discharges associated
with motive power service. The life of a first-quality SLI battery
used to power an electric vehicle might be less than a year (100 to
200 cycles).
- 1,300
o
- 1,000 o
50
100
200 300 400
DISCHARGE RATE, amperes
- 700
400
500
I- u
Z o
LJ J=
8
cc.
UJ
Figure 1. Discharge characteristics of flat plate
industrial battery (GNB type 72X-13, 1.280 SP. GR.,
77°F, 81 Ib per cell).
Golf-Cart Batteries
Golf-cart batteries would seem to be the closest approximation
to an electric street vehicle lead-acid battery. Two versions of such
batteries are on the market. One is based on industrial battery
technology and has tubular positive plates. Presumably it will give
service similar to that quoted for tubular plate cells. By far the
more common version of the golf-cart battery is based on SLI battery
designs. The principal differences are larger and thicker plates and
more effective plate separation. Figure 4 gives the electrical char-
acteristics of this battery. At the 5-hour rate the energy density is
14 watt-hours per pound and 1.8 kilowatt-hours per cubic foot. A
202
Lead-Acid Batteries
-------
750-pound (curb weight) golf cart with 180 pounds of battery has
a maximum speed of 20 mph and a range of approximately 30 miles.
Properly serviced batteries last 1 to 2 years.
2.00 -
1.70
10
o
X
o
—
Q
1,000
800
o
u_
o
600 LU
400
200
^ 03
O 3
O
a:
UJ
50
150 250
DISCHARGE RATE, amperes
350
Figure 2 Discharge characteristics of Tubular-plate
industrial battery (GNB type 85T-15, 1.280 SP. GR.,
77:F, 85 Ib per cell).
ELECTRIC STREET VEHICLES TODAY
The relatively modest energy-storage capability of the lead-
acid battery and the resultant "poor" performance of electric street
vehicles incorporating them has been the subject of much discussion
in the technical and public press in recent months. It is instructive
to document this as point of reference. The two examples that I
will discuss both incorporate motive power lead-acid batteries.
The first is electric street delivery trucks manufactured and sold
by Gould-National Batteries, inc., 10 to 12 years ago. Altogether
some 50 were made and sold and 10 or more are still giving satis-
factory service in New York City. Table 1 gives some pertinent
characteristics and performance figures.
DOUGLAS
-------
o
2.00
1.75
1.50
16
14
12
P 10
x
o
100
90
80
UJ
O
70 ° «
60 £
cc
LJ
50 5
40
30
20 40
DISCHARGE RATE, amperes
60
Figure 3. Discharge characteristics of SLI battery
(1.280 SP GR.T 77°F, 42 Ib per battery).
For the second example I have chosen one of the many small
passenger cars that have been converted to battery electric pro-
pulsion. This is the "Mini" Traveller, converted to electric drive
by the Traction Division of A.E.I., Ltd., in the United Kingdom
(Jeremy, 1966). Table 2 describes the characteristics and perform-
ance of this vehicle.
204
Lead-Acid Batteries
-------
g 2.00
K 1.80
i 1.60
16
14
12
I
o
CO
Q
10
400 uj
o
300
UJ
200 g
o
100 g
20
60 100 140
DISCHARGE RATE, amperes
180
Figure 4. Discharge characteristics of golf-cart
battery (1.280 SP. GR., 77°F, 62 Ib per battery).
Table 1. CHARACTERISTICS OF THE GOULD "ELECTRUK"
(1.5-ton capacity)
Curb weight, Ib
Lead-acid battery weight, Ib
Payload, Ib
Maximum speed, mph
Range (multiple stops), miles
Battery capacity (5-hr rate), kw-h
9,500
2,450
3,000
18
20
23
Controls
Series — parallel switching
DOUGLAS
205
-------
Table 2. CHARACTERISTICS OF THE ELECTRIC DRIVE "MINI" TRAVELLER
Curb weight, Ib 2,500
Lead-acid battery weight, Ib 829
Payload, Ib 500
Maximum speed, mph 41
Range (constant 30 mph), miles 37
Range (4 stops per mile), miles 16
Battery capacity (6-hr rate), kw-h 9.6
Battery capacity (1-hr rate), kw-h 6.4
Controls SCR
These examples, and many more that could have been chosen,
lead to the not-surprising conclusion that electric street vehicles
powered by industrial lead-acid batteries have performance char-
acteristics much below those of gasoline-engine-powered vehicles.
This leads to the question of what improvements in vehicle perform-
ance might be expected in the near future from projected advances
in the design of lead-acid batteries.
IMPROVED LEAD-ACID BATTERIES
FOR ELECTRIC VEHICLES
The first consideration here is simply that the vehicles just
described, and all others of which I am aware, have "made do" with
available batteries. With the possible exception of the golf-cart
battery, no battery has been designed for this service in terms of
electrical characteristics, size and shape of cells, and life. When the
battery is designed as part of the total vehicle system, appreciable
improvements in performance for the same weight may be expected.
Further, recent advances in plastic technology have made possible
reductions in weight of containers and connectors amounting to 10
percent of the total weight of a golf-cart battery, for example. By
proper plate design, making use of the recent development in
ancillary technology and designing the battery for the vehicle system,
it is anticipated that the energy density (kw-h-lb) may be increased
by 30 to 40 percent over that in industrial motive-power batteries.
Some sacrifice in life will be entailed.
Gould-National Batteries is undertaking the design and proto-
type manufacture of a lead-acid battery optimized for the electric-
street-vehicle application as we understand it. The first attempt at
this has given us a battery on paper that will deliver somewhat over
18 watt-hours per pound at the 5-hour rate. Applied to the Gould
206 Lead-Acid Batteries
-------
"Electruk" the total capacity of the battery would be over 50 percent
greater. Similarly, the "Mini" Traveller would benefit from a 25
percent increase in battery capacity. The life of this battery is de-
signed to be between that of the rugged industrial motive-power
battery and the golf-cart battery, i.e., 3 to 4 years.
None of this is particularly spectacular when compared with
battery systems described elsewhere in this volume; however, this
optimized electric vehicle battery is characterized by being within
reach of present-day technology — no breakthroughs are required.
One other consideration deserves emphasis, the matter of cost.
CONSIDERATIONS OF BATTERY COST
I will take it as established that operating electric street
vehicles will cost substantially less than operating gasoline-engine-
driven vehicles (Jeremy, 1966; Hoffman, 1963; Render, 1965). Here
I want to treat the costs of battery energy sources in a very general-
ized way. The price of the most popular automobiles today falls
between 50 and 75 cents a pound (Hoffman, 1963). SLI batteries,
which are manufactured in quantities of 40 million or more per year,
are priced to the consumer at about 50 cents per pound. Industrial
lead-acid batteries, which are manufactured on a job-shop basis, are
priced at an average of about 70 cents per pound. On this rough
basis the price of lead-acid batteries is at least consistent with that
of automobiles. Translated into dollars of initial cost per unit of
energy, SLI and industrial motive-power batteries sell for approxi-
mately S35 and $70 per kilowatt-hour, respectively. Using Reid's
(1966) estimate of 20 kilowatt-hours as being required for minimum
acceptable performance we can reasonably project that in mass pro-
duction a lead-acid battery for an electric vehicle will sell for between
S700 and 51,400. Other types of batteries, some made on a large scale
today, are priced at from 2 to 20 times the higher of the above values.
One must conclude that the price structure of the lead-acid battery
only will in the near future lend itself to an attractively priced
electric passenger vehicle.
CONCLUSION
My conclusion, which does not differ radically from that of many
others who have discussed this subject, is that for the next 5 to 10
years only the lead-acid battery will be used in significant numbers
of electric street vehicles. Passenger vehicles will perform better
than the current "research" vehicles but their performance will be
marginal compared to that of gasoline-engine-powered automobiles.
Delivery trucks may fare better if urbanization trends continue.
I am sure that I speak for the entire lead-acid battery industry in
saying that every effort will be made to improve and optimize this
most useful of battery systems for the electric vehicle.
DOUGLAS 207
-------
REFERENCES
Render, B. S., 1965. Recent developments in battery electric vehicles.
Proceedings IEE, Volume 112 (Dec.).
Hoffman, G. A., 1963. Electric motor cars. Report No. RM-3298-FF,
Rand Corporation, Santa Monica, Calif. (Mar.).
Jeremy, K. W. C., 1966. The electric car. Report No. 4062, Australian
Lead Development Association, 95 Collins Street, Melbourne,
Australia.
Reid, W. T., 1966. Kilowatts for cars — a comparison of energy costs
for electric automobiles. Paper No. 66-978, AIAA Third Annual
Meeting, Boston, Mass. (Dec.).
208 Lead-Acid Batteries
-------
HIGH-ENERGY NON-AQUEOUS BATTERY SYSTEMS
FOR ELECTRIC VEHICLES
M. Eisenberg
Electrochimica Corporation
Menlo Park, Calif.
INTRODUCTION
In the past decade the desirability of electric vehicle propulsion
has become increasingly recognized. Although the concept is far from
being new and was somewhat popular after the turn of the century,
it was essentially completely eclipsed in the lata 1920's and 1930's by
the rapid advances of the gasoline-powered automotive engine. The
renewed interest in electrically driven vehicles can be attributed to
the following motivations:
1. The advances in electrochemical power sources, e.g. high-energy
batteries, fuel cells, metal-air cells, etc., have progressed to a
point where energy densities required for reasonable vehicle
speed and distances appear to be in sight.
2. The large increase in air pollution in urban areas throughout the
world has placed the long-range acceptability of a hydrocarbon
engine in question. This is particularly aggravated by the antici-
pated growth in the demand for passenger vehicle transportation.
3. The consumer prices for gasoline rising proportionately faster than
the cost of electricity puts greater weight on the exploration of
fuel cell power supplies for vehicles for better fuel utilization
efficiency, and on the desirablity of employing storage battery
operated vehicles, recharged overnight with cheap off-peak power.
4. The progress in the design and efficiency of electric motors in re-
cent years has resulted in a considerable reduction of their weight.
5. Advances in circuitry and semi-conductor devices offer the oppor-
tunity for better utilization of the power on changing loads with-
out appreciable waste of energy.
In order to appreciate the importance of the lithium high-
energy non-aqueous battery system and to gain a better perspective
of its function and the spectrum of possibilities for vehicular propul-
sion, it is of interest to examine briefly all other electro-chemical
power sources that can at least potentially be considered for this
purpose and to analyze their relative merits and disadvantages. An
examination of possible power sources rapidly reduces the possibilities
to the following six:
EISENBERG 209
-------
1. Fuel cells operating on a storable fuel and using air as a cathodic
depolarizer (oxidizer).
2. Electrically regenerative fuel cells, i.e., devices fundamentally
analogous to a storage battery.
3. Metal-air hybrid cells.
4. Rechargeable battery systems of the aqueous type (room tem-
perature).
5. Rechargeable battery systems of the fused salt type (high tem-
perature) .
6. High-energy battery systems of the non-aqueous type (room
temperature).
Before examining the potential applicability of any devices
within the above six classes to the problem of electric vehicle pro-
pulsion, it might be useful to review briefly the technical and eco-
nomic requirements that these electrochemical sources would have
to meet.
The most important question, aside of the problem of the torque
characteristic of a direct-current motor, which will not be discussed
at present, is the general disparity between maximum advertised
horsepower and the brake horsepower of today's automobiles. Only
60 percent of the advertised horsepower (Hoffman, 1962 and 1966) is
left available as brake horsepower to overcome the aerodynamic
drag and the tire drag, and to provide for acceleration. Considerations
of this type lead to the conclusion that a compact-type passenger
vehicle can be conceived with a direct-current motor in the range
of 20 to 30 brake horsepower. If we assume an average electrical load
of 20 kilowatts, an average vehicle speed of 40 mph, and a reasonable
distance range goal of 200 miles between refueling or recharging, it is
obvious that an energy storage capability of 100 kilowatt-hours is
necessary if the electric vehicle is to be something more than a curi-
osity item or a convenience cart.
There are considerable savings in weight when one converts from
internal combustion engines to electric propulsion. It has been esti-
mated in one case that as much as 49 percent of the electric auto-
mobile's weight could be in batteries (Hoffman, 1966). Since this
figure may be somewhat optimistic, we shall assume a more con-
servative figure of 30 percent for a 2,400-pound vehicle. This would
provide for a battery or fuel cell system allowance of 720 pounds. For
a storage of 100 kilowatt-hours of energy, this means an energy dens-
ity of at least 140 watt-hours per pound per cycle. The emphasis is on
the words "per cycle" since it is obvious that for a rechargeable
system this energy delivery must be obtained to whatever percentage
depth of discharge is necessary in order to provide for a reasonably
good, i.e., economical, cycle life. A range of 200 miles is considered
a reasonable minimum to meet the performance requirements to
210 Non-Aqueous Battery Systems
-------
which modern man has become accustomed. The vehicle also must
have enough power level, as discussed previously, in order to reach
top speeds if the vehicle is to qualify for admission to parkways and
freeways.
Thus, in summary, in considering electrochemical power sources
as potential candidates for electric vehicle propulsion, we shall look
toward those capable of meeting an energy density of about 140 watt-
hours per pound, or preferably more. There are other criteria that
must of course be employed in the analysis of the suitability of the
many electrochemical devices that can be proposed. These include
the cost of fuel per mile or per kilowatt-hour, the cost of equipment
amortization over its useful life, the associated safety problems, the
startup and shutdown procedure requirements, maintenance consid-
erations, and availability of recharge or refueling facilities. It is diffi-
cult within the scope of this paper to deal with all of these aspects.
It is for this reason a limitation will be made to the considerations
of the following three criteria: energy density, cost per mile, safety,
and public acceptance.
FUEL CELLS
There are many different types of fuel cells under development,
some using hydrogen or other gaseous reactants, others using stor-
able fuels. Although these systems have different characteristics, one
fundamental aspect common to all fuel cells, unfortunately and sur-
prisingly enough, is not being recognized. The fundamental problem
is that a fuel cell must be regarded as an electrochemical or galvanic
engine, that unfortunately has a high ratio of weight to power, com-
pared to the internal combustion engine. The superiority of a fuel
cell comes through in a shining manner when small amounts of power
are required for a long period of time. Then its inherent efficiency
resulting from the galvanic energy conversion (free of any Carnot
cycle limitations) results in the achievements of high-energy-density
ratios in terms of watt-hours per pound.
It really matters little what the anodic fuel in a fuel cell is, as far
as the weight of the fuel cell plant itself is concerned. It may there-
fore be convenient to examine the data shown in Table 1, which con-
siders a 20-kilowatt hydrogen-air fuel cell operating at an over-all
efficiency of 60 percent. On the basis of near term technology, a rather
optimistic weight of 1,850 pounds was assumed for the fuel cell itself,
including controls but excluding tankage of fuel. In this system there
is no \veight penalty for the use of oxidant since air would be used
and only an air compressor has to be considered. This weight of 1,850
pounds is based on anticipated technology resulting from current
work on the Gemini and Apollo fuel cell programs and compares
rather favorably with the 3,380-pound weight of a 32-kilowatt fuel
cell employed in the recently demonstrated General Motors Electro-
van.
EISENBERG 211
-------
Table 1. 20-KILOWATT HYDROGEN-AIR FUEL CELL POWER PLANT
FOR VEHICULAR PROPULSION"
Fuel cell and controls 1,850 Ib 1,410 Ib
weight (Near-term technology) (Long-term technology)
Mission duration,
hours 5 10 24 100 200 5 10 24 100 200
Vehicle range,
miles 200 400 960 4,000 8,000 200 400 960 4,000 8,000
Total energy
storage, kw-h 100 200 480 2,000 4,000 100 200 480 2,000 4,000
Total system
weight, Ib 1,898 1,939 2,024 2,782 3,624 1,448 1,489 1,584 2,292 3,174
Overall mission
energy density,
w-hr/lb 52.6 103 237 719 1,100 69.1 134 303 872 1,262
aBasis: 0:115 Ib H0/kw-h (60% overall efficiency). Average speed: 40 mph.
If we define, as a mission duration, the total integral time of use
of the vehicle between refuelings and assume a mixed road average
speed of 40 mph, then a 200-mile-range vehicle corresponds to a
mission duration of 5 hours calling for a total energy storage of 100
kilowatt-hours, and the resulting total system weight leads to an
overall energy density for this particular 5-hour mission of only 52.6
watt-hours per pound. When we consider future long-term tech-
nology and a possible cell weight of only 1,410 pounds, only an energy
density of 69.1 pounds results. Extending the mission to 10 hours
over a vehicle mileage range of 400 miles gives us corresponding
energy density ratios of 103 and 134 watt-hours per pound, respec-
tively. The fundamental problem can be even better seen by the
presentation shown in Figure 1, which shows that the realization of
high energy densities by a fuel cell system does not occur until mis-
sion durations well in excess of 10 hours occur, and that, even with
future technology, inadequate energy densities are obtained in the
primary region of interest. Thus, the fundamental difficulty with all
fuel cell systems is that in the case of an electric vehicle in the least
favorable range of mission duration it compares unfavorably even
with conventional batteries.
There are a number of other comments one can make about fuel
cells. For instance, a hydrogen fuel cell would raise, no doubt, serious
objections from a safety point of view. Furthermore, the cryogenic
storage problems or high-pressure storage problems associated with
212 Non-Aqueous Battery Systems
-------
hydrogen would represent serious technical and distribution difficul-
ties. The hydrazine fuel cell, however attractive because of the
storage nature of the anodic reactant, is not considered feasible for
economic reasons. The hydrocarbon fuel with its inherent stability
difficulties and requirements of costly catalysts also appears far from
a practical realization. However, as pointed out previously, the funda-
mental difficulties discussed above — namely, that in the region of
interest for an electric vehicle regardless of its fuel type, low energy
densities result — represent a problem common to all fuel cell
systems.
CO
>-
o
o:
1,000
500
300
100
50
30
CELL WEIGHT ALONE: 1,410 Ib
PRIMARY REGION
OF
CELL WEIGHT ALONE: 1,850 Ib
400
200f 960
I-
5 10 24 40
DISTANCE RANGE, miles
4,000
1 I 1
—I 1-
60 80 100 120 140
MISSION DURATION, hr
+
7,200
1—
160 180
Figure 1. Energy density yields for 20-kw H2 —
air fuel cell as a function of electric vehicle
mission duration.
With reference to Table 1 and Figure 1, it should be noted that
even the optimistic long-term fuel cell weight of 1,410 pounds rep-
resents almost twice the weight allowance that can be made for a
compact family car. In other words, even if one would wish to settle
for a low-energy-density yield, this would result in the removal of
the payload capability of such a vehicle.
ELECTRICALLY REGENERATIVE FUEL CELLS
The fundamental point made above applies also to fuel cells of
this type. Furthermore, an additional increase of weight would result
from the necessity of providing for a separate loop for handling
oxygen in a closed system if this were, for instance, a hydrogen-
oxygen rechargeable fuel cell; whereas, in a primary (non-recharge-
EISENBERG
213
-------
able) fuel cell, one would simply use air without weight penalty in-
volved. Also, from an electrochemists's point of view, the polariza-
tion and consequent energy penalties involved in an electrolysis to a
gaseous form are too excessive to make such a system attractive.
METAL-AIR CELLS
These represent a hybrid concept between a fuel cell and a con-
ventional battery. Actually, the concept is old and cells of this type
known as "air depolarized cells" have been in use for a long time for
railroad signalling. As a result, thin, lightweight, and efficient cath-
odes for the reduction of air have been developed and it has been
possible to design and develop efficient high-energy-density systems.
Of all the systems proposed, the alkaline zinc-oxygen, or air system,
so far appears to be the most promising. Primary batteries in such
a system have been built; they exceed the performance of silver-zinc
batteries. However, rechargeable batteries suffer from the same
fundamental difficulty that the zinc anode suffers in a silver-zinc
battery, namely, short cycle life due to dendritic electrocrystallization
of the zinc upon recharging.
A variant of the approach to the metal-air system calls for the
use of a primary cell with consumable, replaceable zinc anodes. Under
such a concept, solid zinc would be used as a fuel. The engineering
complexities of such an approach remain, however, to be worked on;
regardless of the approach to the zinc-air system, it must be recog-
nized that there is a considerable weight penalty involved because of
the need of providing air cathodes. As a result, despite a high theo-
retical energy density, when one considers the weight of zinc only, the
system is not expected in a rechargeable form to provide energy
densities in excess of 80 watt-hours per pound (see discussion below).
AQUEOUS BATTERIES
The well-known limitation of aqueous batteries, as far as energy
density is concerned, allows us to reduce the discussion of this class
of devices to the silver-zinc battery. With an open circuit voltage of
1.86 volts, this system already represents borderline stability of any
galvanic couple in an electrolyte employing water as a solvent. The
decomposition potential of water (in these batteries) is somewhere
between 1.8 and 2.2 volts. Indeed, a silver-zinc battery cannot be
sealed for this very reason, since a spontaneous, however small,
generation of hydrogen gas occurs on open-circuit stand. This battery
then represents the most energetic system in an aqueous medium with
a theoretical energy density of 208 watt-hours per pound, when we
consider the weight of active materials only. The highest energy
densities achieved for primary cells in this system range between
90 and 105 watt-hours per pound. For secondary cells, energies as
high as 55 watt-hours per pound have been achieved. Thus, a re-
chargeable silver-zinc battery can hardly be expected to provide the
214 Non-Aqueous Battery Systems
-------
energy density required for a vehicle with a practical distance range,
and indeed, all designs for vehicles with such batteries point toward
a distance range of the order of 40 to 60 miles.
FUSED SALT BATTERY SYSTEMS
The achievement of higher energy densities can, of course, be
considered in electrolyte media that are water-free. Such would then
permit the resorting to alkali metals in which the combination of
high electromotive force forms and low equivalent weight is advan-
tageous. Table 2 shows an interesting tabulation of the individual
weight contributions (i.e., a form of a negative merit figure) for
certain metals as anode materials. As can be seen, lithium appears
the most attractive, and surprisingly, some metals like calcium are
more advantageous than sodium. Lithium is approximately 17 times
more advantageous than zinc. On the basis of such elementary con-
siderations, the attractiveness of a lithium battery needs hardly any
emphasis.
Table 2. INDIVIDUAL WEIGHT-TO-ENERGY AND VOLUME-TO-ENERGY
MERIT FIGURES FOR SELECTED ANODE MATERIALS
Material
Lithium
Beryllium
Magnesium
Aluminum
Calcium
Sodium
Strontium
Zinc
Cadmium
g/cm'
0.53
1.82
1.7
2.7
1.5
1.0
2.6
7.14
8.65
Density/
amp-hr
5.70x 10-'
3.70x10-'
10.0 xlO-i
7.40 x 10-'
16.4 x 10-'
19.0 x 10-'
36.0 xlO-'
26.8 xlO-'
46.2 xlO-'
in/ ;amp-hr
0.0298
0.0056
0.0163
0.0076
0.0303
0.0526
0.0384
0.0104
0.0148
Half-
cell"
poten-
tial
(E'),v
3.02
1.70
2.34
1.67
2.87
2.71
2.89
0.76
0.402
Wa'"
Ib/w-hr
1.89x 10-i
2.175x10-*
4.27x10-'
4.43 x 10-'
5.71 x 10-i
7.01 x 10->
12.4xlO-i
35. 3 x 10-J
111.5x10-*
V
in;l/w-hr
9.86x 10-'
3.29 xlO-'
6.96x 10-'
4.55x ID-'
10.5 x 10-'
19.4 x 10-'
13.3 x 10-
13.7 x 10-'
36.8 x 10-'
" For simplicity of comparison, the standard potential values in aqueous
systems, in reference to the standard hydrogen electrode, are taken.
'•As an individual electrode contribution, Wu is defined as:
Ib
W
amp-hr x E°
l' As an individual electrode contribution, Sn is defined as-.
amp-hr x E°
EISENBERC,
215
-------
Fused salt systems can be used for alkali materials, and such
systems as lithium-chlorine and sodium-sulphur at temperatures in
excess of 1,000°F are being worked on. Although these efforts are
essentially in a preliminary laboratory stage, some judgment can
be made as to the ultimate value. Molten sodium reacts violently
with molten sulphur. Since no ceramic diaphragm that would be
used between the two can be expected to be completely failure-proof,
a very serious hazard of a violent explosive reaction would exist. This
would raise serious objection toward incorporation of such a system
in a vehicle. In addition, all molten salt systems are handicapped by
very serious materials of corrosion problems. Last, but not least, is
the problem of startup time that would be required for a vehicle of
this type. A startup time in excess of 5 minutes would hardly be
acceptable to the general public. At the same time, it is difficult to
foresee how a fused salt system could ever be expected to offer a
startup time to full power in less than 30 minutes. Thus, the possible
high-energy-density advantages of these systems would be seriously
overshadowed by the problems discussed above.
THE NON-AQUEOUS ORGANIC ELECTROLYTE
BATTERY SYSTEM
If it is recognized that lithium is a most desirable anode material
in a battery and if one wants to build a battery at room temperature,
the use of a non-aqueous solvent with a broad liquidity range is indi-
cated. Before proceeding to discuss energy densities of batteries, it is
important to realize that they are a function of rate of discharge and
of the cell size expressed in terms of ampere-hour capacity. These
relationships are pictured in Figure 2 for both the silver-zinc battery
and the non-aqueous battery. It will be noted that the slope is
steeper for the non-aqueous battery. Because of the generally lower
conductivity of non-aqueous electrolytes, compared with that of the
aqueous ones, the very high rates of discharge tend to result in a rela-
tively larger impedance losses for the non-aqueous battery. This ex-
plains the steeper slope for the plot against the rate of discharge. At
the same time, it takes the packaging efficiency of larger cells for
the weight advantages of lithium as an anode material to come to
the fore. This again explains the steeper slope in Figure 2. It is there-
fore important in the discussion of any type of a battery to define the
conditions at which the watt-hour-per-pound figures are given. This
explains why rather broad ranges are given for the operational,
practical energy densities in Table 3 for a series of aqueous and non-
aqueous batteries. It will be noted, however, that the non-aqueous
batteries based on lithium offer very substantially larger theoretical
energy densities.
Of the non-aqueous systems, from which so far reasonably suc-
cessful results have been obtained for a secondary battery, the
lithium-silver chloride, the lithium-copper chloride, and the lithium-
nickel fluoride systems should be mentioned. The rechargeability
216 Non-Aqueous Battery Systems
-------
mm
hours
DISCHARGE RATE
.c
5
\-_
C/5
LJ
Q
ce
LJ
z
LU
FIXED DISCHARGE
RATE
NON-AQUEOUS
10
CELL CAPACITY, amp-hr
100
Figure 2. Relationship between (top) energy density
and discharge rate and between (below) energy
density and cell capacity.
EISENBERG
217
-------
characteristics of these systems have been investigated; indications of
favorable performance have already been obtained, even at consider-
able depths of discharge. Figure 3 shows a non-aqueous cell, type
LN-6, cycled at a charge rate of 15 hours (typical of an overnight
charge) and discharged at a mild rate of 30 hours. As can be seen,
most charging occurs under 4.0 volts and typical discharge voltages
are of the order of 2.5 volts. The performance of another non-aqueous
cell, Type LN-7, at a rather high discharge rate of 1 hour is shown in
Figure 4. Here, the discharge occurs at an average plateau voltage of
the order of 2.1 volts. At a typical average rate of discharge corre-
sponding to 5 hours, an average cell voltage of 2.4 volts with fluctua-
tions of the order of ±0.4 volt, can be assumed on the basis of these
initial results. As far as a temperature-range capability is concerned,
the broad liquidity range of the non-aqueous electrolyte provides the
basis for both low- and high-temperature operation. Indeed, one
series of cells has been found to operate satisfactorily over the entire
range of —40° to -4-165°F (Kuppinger and Eisenberg, 1966).
CYCLE 1 | CYCLE 2 CYCLE 3
INTERRUPTIONS I
D C | D C I D
CYCLE 4
C I D
50 100 150
TOTAL OPERATING TIME, hours
200
Figure 3. Case history of discharge (d) — charge (c)
cycling a type LN-6 cell.
It appears, thus, that non-aqueous battery systems can indeed be
constructed, and performance exceeding that of the silver-zinc
battery, the highest-energy-density aqueous type, can be obtained.
This is illustrated in Table 4, where a general comparison is made
between aqueous batteries, metal-air cells, and non-aqueous systems.
A distinction is made in the table between primary and rechargeable
secondary systems, which generally yield lower energy density ratios
218
Non-Aqueous Battery Systems
-------
SENBERG
Table 3. SECONDARY BATTERIES
Aqueous batteries
Types Lead-acid Nickel- Nickel- Silver- Silver-
iron cadmium zinc cadmium
Non-aqueous batteries
Li /copper
chloride
Li /copper
fluoride
Li /silver
chloride
Energy density:
Theoretical on active
material, w-hr/lb
Operational, w-hr/lb
Operational, w-hr/in.-
115 138 107 208 120 503 746 230
5 to 15 7 to 10 12 to 15 25 to 50 16 to 25 (25 to 175) (25 to 200) (25 to 90)
0.4 to 1.0 0.7 to 1.0 0.7 to 1.0 1.7 to 3.5 1.4 to 2.9 -- — —
Operating voltage per cell
(mid-point) 1.7 to 1.95 1.2 1.1 to 1.23 1.3 to 1.5 1.0 to 1.10 2.3 to 2.8 2.3 to 2.8 2.3 to 2.8
Cycles
200 to 500 2,000+ 300 to 3,000 10 to 200 300 to 1,500
50 to 200
(Exper.)
Operating temperature
range, °F
—40 to
-^»0to+140—10to+115—40to+140 0 to +140 —10to+140 +165±
-------
CELL
0 1351
A 1352
20
80 100
min
mm
Figure 4. High-rate operation of cell type LN-7 —
discharge rate, 1 hr; charge rate, 12 hr.
Table 4. ENERGY DENSITIES OF ELECTROCHEMICAL BATTERY SYSTEMS"
(in w-hr/lb)
System
Pb-acid (sec)
Nickel-Cd (sec)
Silver-Cd (sec)
Silver-zinc (prim)
Silver-zinc (sec)
Metal-air cells
Zinc-air (prim)
Zinc-air (sec)
Non-aqueous high energy
CuCI2-Li (prim)
CuCI,-Li (sec)
CuF2-Li
Theoretical
115
107
120
208
208
671'
671
503
503
750
Practical rangeb
7 to 12
8 to 17
12 to 45
20 to 100
15 to 55
30 to (140)
30 to (80)
28 to (230)
20 to (160)
?
"Specific comparison: 10-amp-hr cells; 10-hr rate of discharge
Silver-Zn: 38 to43 w-hr/lb 2.4 to 2.7 w-hr/in.3
Non-aqueous (ELCA Type LN-3): 75 to 82 w-hr/lb 4.3 to 4.8 w-hr/in."
78
90
% Increase of energy densities:
t> Practical energy yield is a function of rate, size, and construction type.
c Considering the weight of zinc only.
220
Non-Aqueous Battery Systems
-------
because of electrochemical design and mechanical design considera-
tions. Thus, a non-aqueous cupric chloride-lithium rechargeable
battery, with a cell capacity in excess of 500 ampere-hours, is
estimated to yield 160 watt-hours per pound per cycle compared to
55 for the silver-zinc battery.
The lower portion of Table 4 shows a specific comparison between
primary cells, comparing a non-aqueous cell, Type LN-3, to a silver-
zinc cell of the same capacity, discharged at the same rate. As can
be seen, an average improvement of 90 percent in energy density per
unit weight and 78 percent in energy density per unit volume, has
been achieved.
The importance of the effect of cell size (capacity) upon both
gravimetric and volumetric energy densities can be seen from the
experimental data given in Figure 5. Indeed, these results provide the
first corroboration of the qualitative figures shown in Figure 2. Going
from a 1-ampere-hour to a 10-ampere-hour cell improves the watt-
hour per pound ratio for silver-zinc batteries about 25 percent. The
improvement is well over 100 percent for the non-aqueous cupric
chloride-lithium battery.
Similarly, as shown in the data of Figure 6, a change in the rate
of discharge has a much more pronounced effect on the energy
density of a non-aqueous cell. In the operation of an electric vehicle
the 5- to 10-hour rate of discharge would be more typical than a
20-hour rate of discharge.
Engineering computations for the non-aqueous cupric chloride-
lithium rechargeable battery indicate that a 700-pound battery could
be built with a base power level of 20 kilowatts and overload capa-
bilities (for acceleration) of 100 kilowatts, i.e. a rate of 5. When
the overload is applied, the 1-hour discharge ratio would correspond
to the average 5-hour discharge rate. The battery would then have
an average power density of 28 watts per pound and an overload
power density of 142 watts per pound. If required, still higher power
densities are possible as a result of recent improvements in current
density capabilities of the system. With a storage capability of 100
kilowatt-hours, the corresponding energy density yield would be 142
watt-hours per pound per cycle. This would correspond to a 75
percent depth of discharge. On this basis, the lithium battery at
least offers the possibilities of achieving the design performance dis-
cussed earlier in this paper.
The new lithium high-energy battery is beginning to emerge
from the pure research area into the initial hardware development
phase. Methods are already being developed for construction of cells
and battery hardware that once sealed can be handled in ordinary
atmospheric environments and that can have the general appearance
of conventional batteries.
Relative to the economic outlook, estimates indicate an eventual
lithium metal price of the order of $2-$3 per pound (Private com-
munication with American Potash Co.). The costs of other com-
EISENBERG 221
-------
100
90
£ 8°
170
o 60
UJ
— 50
H
in
5 40
Q
I 30
LU
5 20
10
0 ELCA LN-3 NON-AQUEOUS
A Ag-Zn CELLS
c
SI
3
>-
K
C/)
"Z.
LLJ
2 °
CD
(X
UJ
468
CELL CAPACITY, amp-hr
10
Figure 5. Effect of cell capacity on energy densities
— non-aqueous lithium battery and silver-zinc battery.
ponents are essentially in line with present-day conventional batter-
ies. With proper production tooling, the lithium battery is not ex-
pected to be unusually costly.
In summary, on the basis of fundamental considerations of energy
density, if we eliminate the fused salt, high-temperature system, only
the lithium-anode-base organic electrolyte batteries appear to offer an
approach toward an energy density of at least 140 watt-hours per
pound per cycle, which is considered essential for a practical vehicle
with a practical distance range.
222
Non-Aqueous Battery Systems
-------
110
100
I 80
P
5 70
UJ
§
r eo
50
I 40
30
20
10
0 ELCA LN-3 NON-AQUEOUS
A
-tY— '
_ — — A~~ '
10
DISCHARGE RATE, hr
20
Figure 6. Effect of rate of discharge on energy
densities —non-aqueous lithium battery and silver-zinc
battery (cell capacity, 10 amp-hr).
REFERENCES
Hoffman, G. A., 1966 the electric automobile. Sci. American. 215:34-
40 (Oct.).
Hoffman, G. A., 1962. Memo. RM 2922 FF. The Rand Corp. (Nov.).
Kuppinger, R. E., and M. Eisenberg, 1966. Non-aqueous organic
electrolyte batteries for a wide temperature range operation.
ECS Meeting. (Oct.).
O
EISENBERG
223
-------
AN ELECTRICALLY RECHARGEABLE ZINC-AIR
BATTERY FOR MOTIVE POWER
D. V. Ragone,
General Atomic Division / General Dynamics
The major obstacle in the path of the development of a significant
electrically driven vehicle is the excessive weight of presently avail-
able electrical energy storage systems. General Atomic, since 1960,
has been engaged in a program to develop a secondary battery system
with a high energy density for use in vehicles. During the process of
selection of an appropriate electrochemical couple, emphasis was
placed on the use of low-cost materials, relatively low-temperature
operation, and freedom from potential operational hazards for the
user. These considerations led to the selection of the zinc-oxygen
(air) couple, operating with an aqueous electrolyte at temperatures
below the boiling point of water.
Some of the important features of the General Atomic zinc-air
battery are as follows:
1. A solid zinc "front surface" electrode is used with the zinc plated
on an inert substrate so that substantially all of it can be used
during discharge.
2. The electrolyte, an aqueous solution of potassium hydroxide, cir-
culates. This allows the reaction product to be swept from the
cell and stored externally.
3. Atmospheric oxygen is introduced as a reactant through a porous
nickel "bubbling" electrode.
OPERATION OF BATTERY
During discharge, air from a pressurized air plenum seeps
through the porous nickel cathode and bubbles into the electrolyte.
A large fraction of the oxygen in the air (assuming that the air supply
is compatible with the cell current demand) combines electro-
chemically with the zinc to form zinc oxide. After some discharge
operation, zinc oxide is formed in the electrolyte as a finely divided
solid. The waste air with its unused nitrogen is continuously swept
from the cell by the circulating electrolyte, as is the excess zinc oxide.
The excess zinc oxide is separated from the electrolyte and stored
outside the cell.
During charge, essentially a reverse reaction takes place. A
potential is applied across the cell while the electrolyte circulates.
Zinc from the zinc oxide is replated on the backing sheet, and pure
oxygen is evolved at the surface of the porous nickel electrode. The
RAGONE 225
-------
electrolyte continuously dissolves zinc oxide from the storage point
outside the cell.
The cells and cell stacks, containing no stored reactants other
than the metallic zinc, can be quite compact and relatively light for
any given active cell area. The system requires several pieces of
equipment in addition to the cell stack, as shown by the schematic
diagram in Figure 1. The electrolyte circuit contains a pump to
WATER CONDENSER
SPENT AIR
OUT
AIR IN
ELECTROLYTE
RESERVOIR
AIR
COMPRESSOR
INSULATION
Figure 1. Schematic of zinc-air battery system.
circulate the electrolyte reservoir and an air separator to remove
spent air entrained in the cell outlet electrolyte. These items are all
arranged close to the cell stack, and the whole assembly is insulated
for temperature control. Two other equipment items, which may be
located some distance from the cell stack, are the air compressor
and the zinc oxide separator and storage container. The zinc oxide
separator removes most of the precipitated zinc oxide from the
electrolyte and stores it as a relatively dry sludge. Some zinc oxide
remains in electrolyte suspension during discharge. Figure 1 also
shows a condenser, which cools the air that has been through the
cells in order to remove most of the water it has picked up from the
electrolyte. The waste heat from this condenser can be used to warm
the passenger compartment of the vehicle.
During discharge, voltage varies with the power demand in a
manner similar to most battery systems. The voltage variation during
226
Rechargeable Zinc-Air Battery
-------
discharge time is relatively flat, until nearly full discharge is reached
(Figure 2), because of the front-surface operation of the zinc elec-
trode. The zinc that participates in the reaction is taken off the elec-
trode and leaves the cell in the circulating electrolyte. The reaction
products, thus, do not impede the reaction. The zinc remaining on
the electrode provides the electrical connection to the terminals. The
discharge may continue until essentially all of the zinc is removed
from the backing sheet.
2.0
1.5
o
1.0
o
0.5
CHARGE
MAXIMUM
CONTINUOUS
POWER
-PEAK
POWER
% POWER
DISCHARGE
10
Figure 2.
TIME, hours
Cell voltage characteristics.
Charging is accomplished at a voltage of about 2 volts per cell.
The speed of charging is fast as compared to average discharge times.
A cell that would give 8 hours of use in an average lift-truck cycle
could be charged in about 2 hours.
HISTORY OF THE PROGRAM
Early experiments (1960-1963) on the air electrodes were per-
formed on small (2 cm2) electrodes. These experiments were mainly
BAGONE
227
-------
concerned with the performance (polarization) of the electrode as a
function of electrode parameters such as porosity, pore size distribu-
tion, and thickness. When satisfactory performance of the air elec-
trode had been achieved and the experiments on the plating of zinc
from a circulating alkaline solution had demonstrated the feasibility
of producing relatively flat zinc electrodes, then a 1-kilowatt-hour
"breadboard" battery was assembled (1964). This was followed by
the construction of 7- and 14-kilowatt-hour prototypes in 1965 and
1966.
Figure 3 shows present estimates of the achievable energy density
plotted against battery size. Because the size and weight of the
auxiliaries do not scale linearly with battery capacity, the larger
batteries show higher energy densities. The system allows flexibility
of design on the question of power versus energy. For example, to
obtain higher power levels a given quantity of zinc may be distributed
over a greater cell area. This increased cell weight lowers the energy
density but raises the power density. Delivery vans, which require
higher power densities than transit buses, are shown in Figure 3 at
the lower portion of the energy-density band.
100
100 200
300 400 500
NUMBER OF CELLS
600 700
Figure 3. Energy density of zinc-air battery
as a function of size.
The amount of space occupied by batteries is also important in
vehicular applications. Figure 4 illustrates one possible configura-
tion of the zinc-air battery in a delivery van. The battery system
pictured has a capacity of about 100 kilowatt-hours and a maximum
228
Rechargeable Zinc-Air Battery
-------
rated output of 60 kilowatts. Gross weight of the van is about
5,000 pounds.
The General Atomic program during the next 2 years will be
directed at the design, building, and testing of a battery for a delivery
van. Capacity will be between 50 and 100 kilowatt-hours. The proto-
type is scheduled to be ready for bench testing in early 1968 and for
preliminary vehicle tests toward the end of 1968.
CONTROLS
ZINC OXIDE STORAGE
.WATER CONDENSER
ELECTROLYTE RESERVOIR (2)
CELL STACK MODULE (6)
'AIR COMPRESSOR
Figure 4. Zinc-air battery system in a delivery van.
RAGONE
229
-------
ZINC-AIR BATTERIES FOR THE
ELECTRIC VEHICLE
Nigel I. Palmer
Leesona Moos Laboratories
Great Neck, N. Y.
INTRODUCTION
The current renewal of interest in the electric automobile can
be attributed largely to two significant developments. The first is
the increasing public concern with air pollution and possible means
of reducing it. The second development is the recent progress that
has been made in energy-storage technology. The prospect of new
power sources with higher ratios of energy to weight has permitted
the electric car designer to project performance levels approximating
those expected by the average driver. The two primary performance
requirements for power sources of the electric car are (1) high
discharge rate capabilities to satisfy acceleration specifications and
(2) high energy capacities to meet operating range requirements.
Although energy conversion devices such as fuel cells, in "which the
source of energy is stored externally, are generally attractive as
regards capacity, they are at present excessively bulky and costly.
Most available storage batteries, although able to sustain reasonably
high drains, have insufficient capacity to give acceptable operating
ranges. The purpose of this paper is to review a new type of power
source — the zinc-air battery, which promises to meet both the
rate and capacity requirements of the electric vehicle.
The zinc-air battery can be regarded as a hybrid fuel cell and
storage battery in that half of the over-all electrochemical couple, the
zinc, is stored within the battery, while the other half, oxygen, is
taken from the surrounding air and electrocatalytically reduced at
a non-consumable electrode.
The essential electrode reactions occurring in potassium hy-
droxide electrolyte are
Zinc
Oxy;
Anode:
ien Cathode:
Zn
l/2
+
o..
2(OH)--
+ H3 0 +
ZnO -
. 2e^
f H2 O +
2(OH) ~
2e
(1)
(2)
Over-all Reaction: Zn -f 1/2 O2 — ZnO E =1.62 volts (3)
Zinc-air and zinc-oxygen primary batteries have been described
(Chodosh, Katsoulis, and Rosansky, 1966" and 1966'1). Briefly, the
basic cell consists of a porous zinc anode, impregnated with aqueous
potassium hydroxide electrolyte, surrounded by a porous separator,
and contained within a structure comprising two hydrophobic air
cathodes electrically connected in parallel (Figure 1). A zinc-air
PALMER 231
-------
cell operating at theoretical voltage (1.62 volts), would have an
energy density of 600 watt-hours per pound, based on the weight of
zinc only. In practice, this figure is reduced by the weight of non-
zinc components, electrochemical irreversibility, etc., to about 150
watt-hours per pound for a primary battery.
CATHODE-
TERMINAL
V V
V
./- ELECTRODE
// SEPARATOR
r
ANODE TERMINAL
-ANODE COVER SEAL
CATHODES
ZINC ANODE
ANODE CONDUCTOR
Figure 1. LML zinc-air single cell cross section.
SECONDARY ZINC-AIR BATTERIES
It is axiomatic that a battery intended for use in electric vehicles
must have long life and full recharge capability. The approach being
followed at Leesona Moos Laboratories in the development of such
batteries is to adhere as closely as possible to the basic configuration
of the primary zinc-air batteries, incorporating modifications as
necessary to achieve the additional capabilities. The purpose of the
following brief analysis is to review the major problem areas of the
over-all system, their present status, and prospects of solution.
232
Zinc-Air Batteries
-------
Cycle Life
The principal single requirement that must be added to the
capabilities of the primary battery is cycle life. Some of the factors
that affect cycle life can be accommodated in a design tradeoff; others
require absolute solution. Depth of discharge is probably the most
important factor in determining cycle life. Battery research has
established that the lower the depth of discharge, the greater the
number of cycles obtained. This could be expected, since the ability
to store charge depends on the conversion of the zinc oxide produced
during discharge back completely to zinc and, more importantly, in
a morphological form suitable for the next discharge. Clearly, the
greater the proportion of zinc converted to oxide during the previous
discharge, the less remains to exert an influence on the morphology
of the recharged zinc. At present, 50 percent depth of discharge ap-
pears to be about the maximum for good cycle life. Note that this
factor of electrode morphology refers only to the consumable elec-
trode. The structure of the air electrode remains unchanged during
the cycling.
Discharge Rate
There is evidence that sustained, high discharge rates reduce
the capabilities of the zinc-air cell for repeated cycling. As analysis
of the urban driving cycle show, however, the sizing of the battery
for peak loads on acceleration results in average discharge currents
of 25 to 35 milliamperes per square centimeter. This rate is con-
sidered relatively low and favors a high cycle life.
Charging Process
Work to date has shown that the charging rate has a profound
effect on the cell cycle life. A critical process to be controlled during
the charge stage is the production of zinc dendrites. The formation
of these crystalline zinc deposits can lead to shorting between the
cell plates and result in sudden failure of the battery. One im-
portant factor in reducing this hazard is selection of the optimum
separator material. New, improved separators, both ceramic and
polymeric, offer considerable promise. Charging rate, method of
charge, anode thickness, and the quantity of electrolyte are other
factors affecting both the growth of zinc dendrites and the life of the
zinc anodes. In general, the lower the charging rate, the more com-
plete the recharging process. The critical step in the recharging
process is reduction of zincate dissolved in the electrolyte to zinc.
Zn(OH)4-~+ 2e-< Zn + 4(OH)- (4)
Work at LML has shown that the crystalline nature of the zinc
deposited on charge will vary depending on whether the process is
activation or diffusion controlled (Oxley, Fleischmann, and Oswin,
1966). Under activation control (low charge rates), the zinc is
deposited in a dense, adherent 'mossy' form, whereas diffusion con-
trol (high charge rates) leads to more-crystalline, less-dense zinc
deposits giving dendrites. This process in turn depends on the
PALMER 233
-------
concentration of zincate, and hence on the quantity of electrolyte. In
this connection a significant difference between silver-zinc and
zinc-air cells may be noted. The latter, having a low-volume air
cathode as the positive electrode, contain proportionately less electro-
lyte. This results in a more activation-controlled process favoring
the preferred 'mossy' type of zinc crystalline growth.
In addition to the technical aspects of the charging process, it is
important to consider vehicle utilization, which ultimately determines
the preferred charging rate;. Obviously the- ideal battery would be
rechargeable in a few minutes, i.e., in the time it takes to refill the
gasoline tank of a car. For many military and space applications,
such rapid charges are highly desirable; this has dictated the need
for charging rates of 10 minutes to 1 hour1 for some' of the studies
funded by the government. Although such rates are partially feasible,
they do incur a penalty of reduced cycle life. For the electric car
application it is clearly reasonable to consider overnight charging
(i.e., an 8-hour rate) as the normal situation. This assumption is
particularly justified, if the; capacity of the; battery is sufficient Coi-
n-lost daily driving requirements. As the subsequent analysis will
show, this appears to be the case of the zinc-air battery.
System Factors
In addition to the battery, a number of system factors must be
considered: air flow, heat removal, water control, and contamination
by COL,. Clearly, each of these is affected by the others, so that
optimization of the over-all power source requires considerable
systems engineering. The thermoneutral voltage corresponding to
the enthalpy change for the over-all zinc-air process is approxi-
mately 1.80 volts. Operation at voltages lower than this results in
evolution of heat. At low rates this heat is advantageous, producing
the natural convective flow of air necessary to provide.1 oxygen to
the cell. At high rates, however, heat evolution becomes significant
and at the same time oxygen demands cannot be satisfied on a
continuous basis by natural connection. Under these conditions,
forced convection by a blower becomes desirable. A system of this
type is shown in Figure 2, a proposed design for a secondary zinc-air
battery. The blower imposes small weight and parasitic power penal-
ties on the battery.
During operation, water vapor is transferred into the air stream.
Since, unlike operation with fuel cells, no net water is formed by
the process, water losses must be made up or prevented. liumidifica-
tion of the air stream is one option that has been studied. At the'
highest rates, however, it may be necessary to supplement convec-
tive cooling with additional cooling by evaporation. Designs are
therefore being considered in which additional water make-up would
be provided directly through the anode top. Finally, the problem of
carbon dioxide take-up from the air by the alkaline1 electrolyte- must
be considered. In practice, primary zinc-air colls have been operated
for 400 hours with no deterioration attributable to COL, uptake
234 Zinc-Air Batteries
-------
•AIR FAN
GAS VENT
WATER MAKE-UP
HEADER
INTERCELL
SEPARATOR
CELL BRACKET
END PLATE
CELL FRAME
Figure 2. A 120-volt zinc-air battery—10-kw-h capacity.
Nevertheless, for truly long-term operation in an electric car, some
form of CO;, removal may be required. Soda-lime scrubbers of the
type used in fuel cells are being studied for this function. Periodic
electrolyte replacement is an alternative solution that will be
considered.
DESIGN OF BATTERIES FOR THE ELECTRIC CAR
In sizing a battery for an electric car, one method could be to
analyse the maximum power and energy requirements of existing
passenger vehicles and translate these directly to battery dimensions.
Such an approach is considered unrealistic, however, not only in
terms of available battery performance but also in terms of the
use patterns for which the electric car is specifically designed. Since
the pressures generated within the urban areas have created the
requirement for the electric car, it is clearly appropriate to base
design of the car and the power plant on urban driving patterns.
As regards the car itself, recent studies have shown how reductions
in over-all vehicle weight, improved vehicle design, and optimum
PALMER
235
-------
tire selection can significantly reduce the energy requirements for
road and drag losses (SAE, 1967). The present design study en-
visions a small commuter-type vehicle with a gross weight including
passengers of 2,000 pounds.
In determining power and energy requirements, the California
City-Driving Cycle was used as the basis for design. This typical
urban cycle, widely applied in air pollution studies, defines the per-
centage of driving spent in the various regimes of acceleration,
cruising, deceleration, and idling. A car operating on this cycle
would cover approximately 22 miles in 1 hour of driving.
The energy required by an electric vehicle will be that required
to overcome road and aerodynamic drag losses during cruising, plus
energy expended during acceleration. These represent the energy
expended on the wheels. The total energy required from the battery
would be higher to account for inefficiencies in the motor, controls,
and transmission. An overall efficiency of 78 percent was assumed
for these factors. Estimated energy requirements are 2.9 kilowatt-
hours per hour of urban driving, increasing to 8.4 kilowatt-hours
per hour at 50 mph steady cruising. Maximum power requirements
for the urban car will occur during acceleration from 0 to 30 mph.
Based on a moderate acceleration performance of 0 to 30 mph in
10 seconds (4.4 ft/sec2), peak power requirement for the 2,000-
pound vehicle is estimated at 20 kilowatts. This requirement would
increase to 33 kilowatts if acceleration from 0 to 30 mph in 6 seconds
were demanded.
BATTERY SPECIFICATIONS
For the purpose of this analysis a nominal voltage of 100 volts
has been assumed. For a point of reference, the requirements are
first analysed in terms of existing primary zinc-air batteries. These,
in effect, represent the ultimate limit of performance of the equiva-
lent secondary batteries. For convenience, a standard Leesona bat-
tery, the 12-volt, 100-ampere-hour model was selected as the basic
design unit. This battery consists of 10 bicells, each with an area
of 278 square centimeters. In specifying the battery, the rate limita-
tion will reflect the current-voltage curve of the cell, whereas
capacity will be controlled, for a given size cell, by the anode thick-
ness and the utilization of zinc. A voltage-current discharge char-
acteristic of the cell is shown in Figure 3. Although the voltages at
the lower current densities are stable under natural convective flow
conditions, forced air circulation is required for stable sustained
operation at current densities over approximately 70 milliamperes
per square centimeter. This is shown in Figure 4, which shows
transient load response for current densities up to 250 milliamperes
per square centimeter and periods up to 30 seconds. Note that for
periods of 1 to 5 seconds voltages may actually be better with
natural convection.
236 Zinc-Air Batteries
-------
1.6
1.4
1.2
1.0
9 0.8
UJ
o
0.6
0.4
0.2
i , i
100 200
CURRENT DENSITY, ma/cm2
300
Figure 3.
Discharge characteristic of zinc-air battery
with blower.
Selecting a current density of 200 milliamperes per square
centimeter to correspond to the peak load indicates a requirement
of 50 batteries, connected 5 in parallel and 10 in series. These
would yield a total capacity of 58 kilowatt-hours, equivalent to
about 11 hours of operation on the urban cycle, with an energy
density of 150 watt-hours per pound.
In estimating the probable specifications of rechargeable secon-
dary zinc-air batteries, one must consider the factors mentioned
earlier. Thus, to obtain satisfactory cycle life, depth of discharge
must be reduced and zinc anode porosity increased, both tending
to reduce the energy-to-weight ratio. In addition, the systems
associated with control of water level, heat removal, and possible
CO., removal will also contribute to weight and volume. Table 1
shows estimated specifications of batteries for the present electric
vehicle application. The first column is based on the performance
PALMER
237
-------
currently available with Leesona standard primary zinc-air batteries.
The second column presents estimates for secondary zinc-air batter-
ies based on current laboratory results. Specifically, the estimates
are based on 200-mil anodes, with 80 percent porosity and operation
at 50 percent depth of discharge (corresponding to 40 percent zinc
utilization). The third column presents some projections based
on assumed future technical progress, such as higher zinc-utilization
factors, use of thicker anodes, and lighter weight construction for
the non-zinc components.
1.4 r
1.2
1.0
UJ
e>
o
UJ
o
0.8
0.6
0.4
•OCV
50 ma/crrf
100 ma/cm2
160 ma/cm2
200 ma/cm2
250 ma/cm2
KEY
• WITH BLOWER
WITHOUT BLQWER
10
20
TIME, sec
30
Figure 4. Zinc-air cell transient load performance.
238
Zinc-Air Batteries
-------
Table 1. BATTERY SPECIFICATIONS FOR 2,000-POUND ELECTRIC VEHICLE
Weight, Ib
Volume, ft3
Capacity, kw-h
Energy density,
w-hr/lb
kw-h /ft3
Operation in urban cycle, hr
Range (at 50 mph), miles
Present
primary
batteries
370
3.6
54
148
15
19
320
Estimated for
rechargeable
batteries
under
development
360
4.5
20
55
4.4
7
120
Longer-term
projections for
rechargeable
batteries
350
4.0
25 to 35
75 to 100
6.0 to 8.0
9 to 13
150 to 220
ALTERNATIVE OPERATING MODES
One of the novel features of the zinc-air battery investigated
at LML is the ability to remove and replace the zinc anode after
discharge. This is made possible by the non-changing form of the
fuel-cell-type air cathode, which in the bicell configuration forms
a case within which the anode is contained. This capability in
effect gives an extra degree of freedom to the battery designer
and offers the possibility of a number of alternative operating modes,
shown m Figure 5. These operating modes may also be considered
in the context of the electric car. Scheme 1 in a standard throw-
away primary cell and is not applicable here.
In Scheme 2, the discharged anode is replaced by a new one
at the end of the discharge, the old one being discarded. Batteries
of this type are already in limited production for a number of
applications. Although such a system would not be economical
for the present application, this mode of operation does offer distinct
advantages for the immediate testing of prototype electric cars
under realistic conditions of battery weight and power.
Scheme 3 is a logical extension of the second. By recharging
the discharged anodes externally under controlled conditions in an
electrolytic bath, one may circumvent many of the difficulties en-
countered in an internally recharged true secondary battery. Because
of the higher depth of discharge capability, competitive economics
might be obtained with this approach. A further operating advan-
tage includes automatic replenishment of water and elimination
of COL, contamination through equilibration in the charging bath.
As with the previous schemes, this system does not seem appropriate
for the private automobile, although it might be feasible for an
industrial application.
PALMER
239
-------
1. PURE PRIMARY
(THROWAWAY)
2. PRIMARY WITH ANODE
REPLACEMENT
3. EXTERNAL RECHARGE
4. SECONDARY WITH
PERODIC ANODE
REPLACEMENT OR
EXTERNAL RECHARGE
[HS( AR[( OR
HI I HARC.E
[JlSCMARut
5. PURE SECONDARY
I
Figure 5. Alternative operating schemes for zinc-air
batteries.
Scheme 4 does appear practical and possibly even optimum for
the electric car application. In this scheme, the replacement and/or
external recharging technique would be used in conjunction with
a true secondary zinc-air capability. The anodes would be replaced
or recharged periodically as well as during long trips. The addi-
tional capability would offer both operational and performance
advantages. Operationally, the replacement could effectively repre-
sent a rapid (almost instant) recharge technique. This feature would
be important during long-distance traveling; the discharged anodes
could be simply replaced at service stations and later recharged for
other customers. As regards performance, by accepting a lower cycle
life (or life between periodic external charges) one could probably
design for higher depths of discharge. Again, as in Scheme 3, the
CCX problem might be bypassed, although in this case an effective
water control system would still be required. Incidentally, the
periodic anode replacement could be regarded as equivalent to
240
Zinc-Air Batteries
-------
the regular car maintenance cycles now standard for existing
automobiles.
Scheme 5 represents a true long-life, high-capacity, secondary
zinc-air battery. Such a battery would only be recharged 'internally'
and would have ultra-high cycle life. It may be that if sufficient
performance and reliability are obtained, this would represent the
ultimate goal for development of a secondary zinc-air battery.
REFERENCES
Chodosh, S. M., E. G. Katsoulis, and M. G. Rosansky, 1966a. Pri-
mary zinc-air battery systems. Proceedings of 20th Annual
Power Sources Conference.
Chodosh, S. M., E. G. Katsoulis, and M. G. Rosansky, 1966b. A
high energy density zinc 'oxygen battery system. Proceedings
of Intersociety Energy Conversion Engineering Conference,
Los Angeles, Calif., September 26-28.
Oxley, J. E., C. W. Fleischmann, and H. G. Oswin, 1966. Improved
zinc electrodes for secondary batteries. Proceedings of 20th
Annual Power Sources Conference.
Society of Automotive Engineers, 1967 (Jan. 13). D. M. Tenniswood
and H. A. Graetzel. Minimum road load for electric cars, Paper
presented at Automotive Engineering Congress (S.A.E.), Detroit,
Mich.
Question by Dr. Samuel Ruben: Would enough zinc be available to
meet the needs of an electric automobile if the zinc-air battery were
to be chosen as the power source?
Answer by Albert R. Cook, Internationl Lead Zinc Research Organi-
zation, Inc.: If a 20-kilowatt motor were needed for a small car, the
battery would require approximately 250 pounds of zinc. The zinc
used would eventually be reclaimed so that the demand for zinc
would be expected to build up gradually with electric vehicle produc-
tion and then level off.
Approximately nine million new automobiles are registered each
year in the United States. In the event that electric automobiles were
produced at the rate of one million per year, it would be necessary
to produce annually an additional 125,000 tons of zinc. I would not
expect this to disturb the present market on account of the number
of new mines coming into full economical production. World mine
output of zinc is expected to increase by over 400,000 tons in 1967,
compared with an increase of 206,000 tons in 1965. The consumption
of zinc in the United States during 1966 was 1,400,000 tons at a stable
price of 14^/2 cents per pound. It would be necessary to watch the
capacity available for the production of 99.99 per cent pure zinc,
since this may be required. Fortunately, such high-purity zinc is
traditionally available commercially, and no difficulty is anticipated.
PALMER 241
-------
Comments have been made concerning the energy density of the
lead-acid battery. It should be appreciated that the starting, lighting,
and ignition battery used in automobiles is most efficiently designed
for its purpose. It can achieve approximately 17 watt-hours per
pound. It is designed for operation under float conditions with oc-
casional heavy current drain and is not, therefore, suitable for deep
cycling. The industrial battery can typically provide up to 12 watt-
hours per pound and successfully withstands deep cycling for long
periods. Given an incentive such as is now provided by the prospect
of a battery- or hybrid-powered automobile, it should be possible to
plan on the near-term achievement of 20 watt-hours per pound for
a lead-acid battery of optimum life. The International Lead Zinc Re-
search Organization, Inc., has just commenced a 6-month study to
define the contribution the lead-acid battery can make to the de-
velopment of the electric vehicle and to identify the characteristics
that should be designed into the optimum power source. The results
of this study will be available to anyone interested.
PROBLEMS RELATED TO HIGH-ENERGY BATTERIES
Samuel Ruben
Reuben Laboratories
When current flows in any electrochemical system, electrode
reactants are consumed and end products are formed. If the products
are soluble, they change the characteristics or the conductivity of
the electrolyte, particularly the temperature coefficients, until satura-
tion is reached, and then accumulation on the electrode surface such
as the anode decreases ionic mobility and ion diffusion. Ions im-
mediately adjacent to the cathode must be discharged in order for
current to flow through the circuit, otherwise an ionic gradient is
established, which is equivalent to electrode polarization drop. This
process is very temperature sensitive, depending upon the depth of
the ionic gradient, the transport number of the ions in solution, and
the concentration of the diffusing ions in mols per liter. To obtain
the minimum electrode polarization drop of potential and the neces-
sary ion diffusion requires large effective anode areas and relatively
close spacing. This would necessitate the use of a porous zinc
amalgam electrode and a residual anode structure that could main-
tain its porosity during recharge cycles.
One important factor that must be considered with zinc alkaline
cells is that as soon as a current is discharged ionization of the zinc
occurs (Zn — 2e —' Zn-+), with the formation of zinc hydroxide,
which dissolves into the alkaline electrolyte, forming zincates. When
the electrolyte is saturated with zincate, the anode reaction product
is then deposited on the anode as a white pasty mass that consists
essentially of zinc oxide (Zn(OH)., ^ ZnO -j- H.,O). This layer of
242
-------
zinc oxide, if thick enough, is gravity sensitive and can, under vibra-
tion, fall off and deposit at the lower end of the electrode. The im-
portant variation is not so much the mechanical effects, but the
change in the temperature coefficients of the system, as it affects the
IR and electrode polarization drop. With a new cell it is possible to
operate at low temperatures without excessive voltage drop or polar-
ization, but as soon as the alkaline electrolyte becomes zincated. the
cell requires higher temperatures and the output drops off very rapidly
with the lowering of ambient temperature. This temperature co-
efficient imposes a problem with an electric car, where atmospheric
changes m temperature can be wide enough to reduce the cell or
battery output to an inadequate value.
One specific problem in zinc alkaline cells is the spacer, which
must allow ionic conduction with a low voltage gradient, particularly
at lower operating temperature, yet prevent any migration of elec-
trode composition or reaction products. Our experience with re-
chargeable cells is that the ma.ior function of the spacer is to main-
tain as uniform a current distribution over the electrode area as pos-
sible. With localized currents of high density, dendritic zinc deposits
produced force their way through the spacer, producing bridging
paths with internal discharge of the cell. In our work with alkaline
cells over the past 25 years, we have found that sintered ceramic
spacers, particulary magnesium oxide, are the only ones with which
we could obtain repetitive results. Once dendritic growth has been
formed on one section of the anode, its mass increases even at low
current densities. Placement of the spacer close to the anode surface
confines solid anodic products and prevents mechanical change. At
the same time, however, it also becomes important to avoid com-
pression of the zinc oxide formed, since a high-density layer rapidly
increases the ionic gradient, causing greater electrode polarization
drop of potential. If the cathode is soluble or if colloids are formed,
the s'oacer must also have a filter action to prevent its metallization
with bridging circuits. Alkaline battery systems utilizing consum-
able anodes such as zinc are inherently not well adapted to series
connection over long cyclic operation. Unless a sustained balance
between the coulombic capacities of anode and cathode is maintained.
progressive changes in unbalanced cells can take place, with detri-
mental effects. The problem of series connection could be a factor:
if a cell is prematurely exhausted, it would set up a reverse potential
This could happen if the cell had excessive local action or initially
insufficient zinc for the proposed current and time of discharge. With
air-zinc cells, the carbonation of the electrolyte by the carbon dioxide
content in the air could materially affect performance
Another problem that mu>t be considered for rechargeable air-
zinc cells, other than the electric power facilities in the average
garage, is the hazard introduced by the generation of oxygen at the
cathode during recharging Oxygen is released at the rate of 0.658
pound per 1.000 ampere-hours. For a 50-kilowatt-hour battery, this
would amount to 35 pounds or more of oxygen per charge
RUBEN
-------
Although other high-energy anode materials look attractive, the
major problem is to cope with their high reduction potentials, which
make necessary the use of non-aqueuous electrolytes. In some in-
stances, inhibitors can make the electrode passive, but as soon as
current is discharged through an aqueous electrolyte, the electrode
area becomes active to the aqueous component of the electrolyte and
hydrogen is displaced, with a loss of anode material.
244 Note
-------
THE ATOMICS INTERNATIONAL
SODIUM-AIR CELL*
L. A. Heredy, H. L. Recht, and D. E. McKenzie
Atomics International
a Division of North American Aviation, Inc.
Canoga Park, Calif.
INTRODUCTION
The aim of Atomics International's sodium-air cell project is the
development of a compact sodium-air cell operating isotherm-
ally at a temperature slightly above the melting point of sodium.
Preliminary engineering studies have shown that this system can be
developed into a battery system with high energy and power density
for application as a power source in automobiles.
The first part of this paper summarizes the general criteria used
in the engineering studies that led to the selection of the sodium-air
system for experimental development. This is followed by a descrip-
tion of the sodium-air cell, a review of the experimental results, and
a discussion of the potential performance of the system.
BATTERY CRITERIA FOR AUTOMOTIVE
POWER APPLICATIONS
The technical and economic feasibility of using secondary bat-
teries as power sources in automobiles requires that the battery
system satisfy a large variety of criteria. The criteria summarized in
Table 1 were used in our study of possible power sources for passenger
automobiles. Although we recognize that possibly no system will
meet all these criteria, the one best filling these requirements would
be most attractive for automotive use. An estimate of energy and
power density requirements in various vehicle applications is shown
in Table 2.
The requirement of high energy density can be best satisfied by
using hydrogen or a light alkali or alkaline earth metal as the anodic
reagent together with a strong oxidizing low-molecular-weight
cathodic reagent. A consideration of other important criteria (e.g.
high power-to-weight ratio, which requires high current density;
long cycle life, which requires uniform deposition of the metal on
charging) shows that the use of molten alkali metals has several
advantages. The rate of electrode reactions at a liquid metal and
*Work performed under company sponsored research program. Patents
pending.
HEREDY, RECHT, McKENZIE 245
-------
electrolyte interface is generally very high and therefore high current
densities can be achieved. Furthermore, the molten state of the metal
assures uniform deposition on charging and thus long cycle life of
the metal electrode. The use of molten metals also permits the con-
struction of very compact cells since most of the metal can be stored
outside of the cell and supplied continuously to the anode compart-
ment at the required rate. In this sense the system can be regarded as
an electrically rechargeable fuel cell as well as a secondary battery.
Table 1. CRITERIA FOR POWER SOURCES IN ELECTRICAL AUTOMOBILES
Performance criteria11
Energy-to-weight ratio15 of cell battery: > 150 w-hr/lb
Power to weight ratiob of cell battery, nominal: > 35 w/lb
Overload capability: greater than 200%
Energy efficiency (% energy out/energy in): — 50%
Other criteria
Long shelf life and cycle life
No severe materials or corrosion problems
Low production cost
Durability and reliablity
Easy startup
"For 4 hours of operation at nominal power level.
This power level permits continuous operation at
a speed of 60 mph on a level road.
b Based on total weight of battery and reactants.
Table 2. ENERGY AND POWER DENSITY REQUIREMENTS
Application
Industrial carts,
small delivery trucks,
other small vehicles
Standard delivery
trucks
Army vehicles
Automobiles
Energy density,2
w-hr/lb
10
40
100
150
Nominal power
density,"''1
w/lb
2.5
10
25
35
"For 4 hours of operation at nominal power level.
bThe battery system should be capable of delivering at least
twice the nominal power output for short periods.
Sodium was chosen from the light alkali metals as the most
promising anode feed material. It has the advantages of a lower
molecular weight than potassium and a lower melting point than
246 Sodium-Air Cell
-------
lithium. The low melting point is a very important factor because
it permits a low operating temperature, and many important require-
ments (materials compatibility, corrosion resistance, safety, ease of
startup) can be best satisfied at low temperatures.
Air was selected as the cathode feed material because it is readily
available from the atmosphere and thus need not be stored and carried
in the vehicle. This advantage outweighs the drawback of relatively
high polarization losses at this electrode.
On the basis of a detailed analysis, the system selected as the
most promising for experimental investigation was a sodium-air cell
operating somewhat above the melting point of sodium. In principle,
the electrochemical reaction of sodium with oxygen from air can be
carried out in one step. However, one-step oxidation requires a highly
conductive electrolyte that is chemically compatible with both molten
sodium and oxygen. Since the development of such an electrolyte
presents a formidable problem, a two-step method was chosen for
development.
THE ATOMICS INTERNATIONAL SODIUM-AIR CELL
The two-step or dual sodium-air cell system can be characterized
as follows:
(—) Na
Molten sodium
salt
NaHgx
Aqueous
NaOH
02 (air) ( + )
Step A Step B
The two cell reactions involved are:
2 Na + 2x Hg = 2 NaHgx (in Step A) (1)
and
2 NaHgx + V2O2 -f H,O = 2 NaOH -f 2x Hg (in Step B). (2)
The amalgam electrode acts as the cathode of cell A and the anode of
of cell B. Sodium is transported across the amalgam electrode by
diffusion and convection.
The net cell reaction and the calculated value of the correspond-
ing reversible cell potential at an operating temperature of 130°C are
Na (liq) + V4 O, (air) -f Vz H,O (50 wt % NaOH)
=NaOH (50"wt % NaOH) ' (3)
E = 2.72 ± 0.03 volt
If the amalgam electrode contains 2 atom percent sodium, the
calculated reversible cell potentials of the two subcells are as follows:
Na/Na-amalgam (2 a/o Na) ; E = 0.83 volt
Na-amalgam (2 a/o Na)/0., (air); E = 1.89 volts
A schematic diagram of the cell is shown in Figure 1. The cell
consists of a sodium compartment, a molten salt electrolyte matrix,
an amalgam electrode, an aqueous NaOH electrolyte compartment,
HEREDY, RECHT, McKENZIE 247
-------
an air electrode, and an air compartment. On discharge sodium is
supplied continuously from an outside container to the sodium com-
partment. Cell reaction (1) takes place in the Na/NaHgx subcell,
and sodium is deposited on the sodium side of the amalgam electrode.
Sodium migrates across the thin, stationary amalgam electrode under
the effect of a concentration gradient and is made available for cell
reaction (2), which takes place in the NaHgx/O2 (air) subcell, where
aqueous NaOH, the product of the overall reaction, is formed. The
aqueous NaOH solution is kept in circulation through the cell and the
electrolyte storage tank. Its concentration is kept constant by the
addition of water at a rate proportional to the rate of NaOH formation.
Na
MOLTEN
SALT
ELECTRO
LYTE
Na-
NaOH (aq)
(50-70 wt%)
(AIR)
AIR
OUT
Figure 1. Schematic diagram of sodium-air cell
On recharge, the aqueous NaOH solution is electrolyzed (reverse
of reaction (2) in cell B); sodium is deposited on the amalgam elec-
trode; and oxygen evolves on the air electrode. The oxygen is re-
leased to the atmosphere. The concentration of the aqueous electrolyte
is kept constant by evaporating water at a rate proportional to the
rate of NaOH electrolysis. Sodium migrates across the amalgam elec-
trode, and in cell A the reverse of reaction (1) takes place. Sodium
is deposited on the sodium electrode and the excess sodium flows
back into the sodium storage tank.
Note that these operating features distinguish the Atomics Inter-
national cell concept from previously proposed designs for a primary
sodium-oxygen cell (Eidensohn, 1962) or a secondary potassium air
cell (Koch and Karas, 1963), in which the two subcells were operated
at widely different temperatures and a circulating amalgam electrode
connected the two subcells.
248
Sodium-Air Cell
-------
EXPERIMENTAL RESULTS
Component Development
Preliminary engineering studies indicated that a key requirement
for developing a high-power-density sodium-air battery is the
achievement of current densities in the range of 100 to 200 milli-
amperes per square centimeter. This current density range — with
other major criteria — denned the basic requirements of the cell
components and set the goals of the component development program.
The component requirements and the results of the first phase of the
experimental program are summarized in this section.
Electrolyte for the Sodium/ Sodium Amalgam Cell
The principal requirements of the electrolyte are
1. It should have a specific conductance greater than 0.1 mho per
centimeter.
2. It should be chemically compatible with sodium and dilute sodium
amalgam.
3. It should contain Na+ as the only cation, and oxidation and reduc-
tion at the electrodes should involve only this ionic species.
A molten sodium salt electrolyte of proprietary composition that
satisfies these requirements was developed. The freezing point of the
electrolyte is 127°C. Its specific conductivity was measured in the
temperature range of 130° to 165°C with a conductivity cell made of
alumina. Conductivity values in this temperature range fit the fol-
lowing equation:
K _ 1,370 exP-L°
where K is the specific conductance in mho per centimeter. The value
of K, calculated from this equation, is 0.28 mho per centimeter at
130°C.
Chemical compatibility of the molten salt electrolyte with the
liquid metal electrodes and various cell structural materials has been
investigated. No significant degradation of the electrolyte could be
detected at temperatures up to 150°C over test periods of several
hundred hours.
Amalgam Electrode
The amalgam electrode must form a thin, continuous layer
anchored firmly in a stable supporting structure. Design studies
have shown that the thickness of the amalgam electrode must be
less than 0.3 millimeter (preferably as small as 0.1 mm) to give high
energy and power density and low materials cost.
Electrodes with amalgam layer thicknesses in the range of 0.35
to 1.0 millimeter have been tested successfully. At operating temper-
atures of 130° to 135°C, current densities up to 70 milliampere per
HEREDY, RECHT, McKENZIE 249
-------
square centimeter (on discharge) and 50 milliampere per square
centimeter (on charge) were obtained with an amalgam layer about
0.4 millimeter thick.
Air Electrode
Good cell performance and economical operation require durable,
low-cost electrodes with polarization (with respect to the theoretical
oxygen reduction potential) less than 0.4 volt at current densities
greater than 100 millamperes per square centimeter.
Most work performed to date has been concerned with the interim
goal of testing commercially available electrodes and adapting them
to the operating conditions of the sodium-air cell. We found that the
required performance could be obtained with electrodes made with
a platinum loading of 10 milligrams per square centimeter, but the
lifetime of these electrodes at 130°C in 50 to 70 weight percent NaOH
was much less than under normal conditions (25° to 70°C in 6N KOH
electrolyte). Flooding of the electrode and solidification of NaOH on
the electrode surface were found to contribute significantly to the
deterioration of performance. Precise control of the humidity and
temperature of the air feed is required to alleviate these difficulties
and ensure stable operation. Typical cathodic polarization data
obtained with a commercially available electrode (American Cyana-
mid AB-6) are shown in Table 3.
Table 3. AIR ELECTRODE POLARIZATION DATA
Air feed Humidified, C02 removed; air pressure: 2.5 psig
Catalyst loading 10 mg Pt/cm2
Electrode surface area 16 cm2
Electrolyte 50 wt % aqueous NaOH
Operating temperature 135°C
Reference electrode Hg/HgO/50 wt % NaOH at 25°C
Measured electrode
Current density, potential,
ma/cm2 mv
0 +100 to+110
(drifting)
20 +35
40 —5
60 —53
80 —125
Cell Experiments
Several complete sodium-air cells were fabricated and tested at
various stages of component development. Figure 2 shows one type
of test cell prior to operation. The sodium feed tank (located to the
250 Sodium-Air Cell
-------
left) is not yet connected to the sodium compartment (at the bottom
of the cell). Connections on top are to aqueous NaOH and air feed
and exit lines.
Figure 2. Sodium-air test cell.
Cells of 0.3- to 2.0-watt output have been operated successfully
at 128 to 136 :C for 50 to 150 hours. Typical data obtained with a
cell of 1.0-watt nominal output (at 50 ma/cm- current density) are
shown in Table 4.
EXPECTED PERFORMANCE OF THE
SODIUM-AIR CELL BATTERY
The expected energy and power density values of a developed
sodium-air cell battery were calculated on the basis of an engineering
design study (Table 5). Design I is based on the assumption that a
current density of 120 milliamperes per square centimeter can be
achieved at an operating voltage of 2.1 volts. Experimental data indi-
cate that this current density could be achieved with moderate im-
provements in the performance of various cell components. Design II
is based on a current density of 200 milliamperes per square centi-
meter at the same operating voltage. Significant improvements in
cell performance are needed to achieve this goal.
HEREDY, RECHT, McKENZIE
251
-------
Table 4. SODIUM-AIR CELL OPERATING DATA
Cell Description
Component
Structural materials
Sodium compartment
Molten salt electrolyte Matrix
NaOH and air compartment
Air electrode
Polypropylene
Porous ceramic impregnated
with molten salt
Teflon
Gold-coated nickel screen with
porous Teflon backing; platinum
loading 10 mg/cm2
Active surface area of electrodes: 8 cm2
Operating mode
Open circuit
Discharge
Charge
Cell
voltage,
V
2.60
2.40
2.30
2.20
3.10
3.20
Current
density,
ma/cm2
20
50
70
20
50
Power
density,
mw/cm2
48
115
154
Table 5. EXPECTED PERFORMANCE OF THE SODIUM-AIR CELL BATTERY
Energy density,
w-hr/lb
Theoretical energy density
of reactants b>c
Power density of cell
battery1
Energy and power densities,
complete battery assembly1"'
Operating period: 4 hr
8 hr
Design la
930
160
240
Design 1
930
215
300
Nominal power
density,
w/lb
1" Design \a Design IIs
60 100
40 55
30 37.5
"Design I based on a current density of 120 ma/cm- at 2.1-v operating voltage.
Design II based on a current density of 200 ma/cm- at 2.1-v operating voltage.
''Based on reversible cell potential.
l'One half of the weight of the stoichiometric oxygen requirement was used in
calculating energy densities, because, on the average, only this amount will
be present as chemically bound constituent in the aqueous electrolyte and
thus contribute its weight to the system.
dExcluding weight of reactants stored outside of cells.
"•Cell battery and reactants; weight of reactants includes weight of water
required to dissolve the produced NaOH to form a 70 wt % aqueous solution,
252
Sodium-Air Cell
-------
Since the cell reactants are stored in tanks outside of the battery,
the energy density of the complete battery assembly is a function of
the operating time or the total amount of reactants carried. Two sets
of data were calculated, for a 4-hour and an 8-hour operating period.
The energy efficiency of the cell on charge-discharge cycle at a
current density of 120 milliampere per square centimeter is estimated
to be about 60 percent.
SUMMARY
A novel compact sodium-sodium amalgam-air dual cell has been
designed and operated. Cell components and complete cells of 0.3-
to 2.0-watt output have been tested successfully at about 130°C
operating temperature. Experimental data and preliminary engi-
neering studies indicate that very high energy and power densities
could be achieved with a fully developed sodium-air battery system.
High expected performance and the advantages of relatively low op-
erating temperature make this system very attractive for develop-
ment as a power source for electric automobiles.
REFERENCES
Eidensohn, S., 1962, Fuel cell systems. U. S. Patent 3,057,946.
Koch, R. L., and H. R. Karas, 1963. Development of a liquid metal-
air secondary battery system. Test data cell concept and program
status, EDR No. 3660, Allison Division, General Motors Corp.
HEREDY, RECHT, McKENZIE 253
-------
ALUMINUM FUEL CELL
FOR ELECTRIC VEHICLES
Solomon Zaromb
Zaromb Research Corporation
Passaic, N. J.
SUMMARY
Although aluminum has long been recognized as a potentially
superior battery anode material, its practical use has been precluded
by excessive passivity in most electrolytes or by excessive corrosion
in non-passivating electrolytes. Now we have succeeded in inhibiting
the corrosion of aluminum in alkaline solutions reproducibly and over
long periods of time while operating an experimental 60- to 80-watt
aluminum-air battery at a current density of 40 to 50 amperes per
square foot and 1.1 to 1.3 volts per cell. This battery has been per-
forming satisfactorily in intermittent tests since April 1966.
Our experiments have generally confirmed the expected high
energy capacity of alkaline aluminum-air batteries. In a highly
conservative design based on present performance data, an over-all
energy density of more than 200 watt-hours per pound was estimated
for a 30-kilowatt, 240-kilowatt-hour power source including all ac-
cessories. Considerably higher densities should be achievable with
moderate development effort.
The chemical rechargeability (by anode and electrolyte replace-
ment) of our aluminum-air battery should constitute an important
advantage for vehicle propulsion in that no overnight recharging
would be required, nor would any megawatt power outlets be needed
for rapid electrical recharging. Other advantages, such as a 1,000-
mile driving range between electrode refuelings, exceptional safety,
functional superiority, and over-all economy should allow aluminum-
driven automobiles not only to compete fully with but to far surpass
present cars.
INTRODUCTION
The fuel cell described here is unique in that it uses solid alumi-
num rather than a hydrogen-containing fluid as its chief anode re-
actant. It also differs from other metal-air batteries in that it is
chemically rather than electrically rechargeable, and also provides
for control of the voltage and power output through adjustment of
the electrolyte level or of the depth of electrode immersion. Of course,
zinc-air batteries with replaceable zinc anodes have been used for
special applications and are even considered for motor vehicle pro-
pulsion. We propose to show however, that for the requirements of
ZAROMB 255
-------
broad-purpose passenger automobiles such as those currently in use,
the aluminum fuel cell appears to provide a practical, economical,
and otherwise adequate primary electrochemical power source, po-
tentially capable of competing with internal combustion power within
the next few years.
Table 1. ELECTROCHEMICAL COMBUSTION ENERGY DENSITIES
Fuel
Liq. H2
Liq. NHs
Gasoline
C
Li
Be
B
Na
Mg
Al
Ca
Zn
Faraday-volts/cm3
Theor. Pract.
0.087
0.18
0.16
0.24-0.3
0.6-1.2
0.32
1.13
1.31
0.166
0.51
0.87
0.31
0.43
0.06
7
0.1
0.09-0.12
?
7
7
0.09
?
0.36
7
0.28
Kw-h/lb fuel
Theor. Pract.
14.9
2.0
2.0
5.0
4.1
7.4
7.8
6.6
2.1
3.5
3.9
2.4
0.74
2.4
0.7
2.0
i
1
i
i
1.1
7
1.6
?
0.5
Remarks
Require prohibitive
catalyst area
Too expensive
Poor conductor
Too bulky
Poor current efficiency
Poorer than Al
Poorer than Al
In Table 1 we speak of "electrochemical fuel" rather than "anode
reactants" to emphasize that neither the weight of oxygen nor that
of water need be considered in preliminary energy-density compari-
sons as long as the latter substances are abundant and readily avail-
able from the surroundings. If we can seriously consider electrically
rechargeable vehicles with a driving range of merely 50 miles for
urban applications, then we should not object to replenishment with
water at 200-mile or even at 100-mile intervals, especially if the
main fuel charge could last for 500 or 1,000 miles. The combustion
energy densities based on the weight of fuel alone may therefore pro-
vide fairly meaningful figures of merit for the best available electro-
chemical fuels.
Table 1 shows that aluminum is one of the most energetic fuels
in terms of both volume and weight. On a volume basis aluminum is
fourth highest, and its combustion energy content is close to two-
thirds of the highest possible value. On a weight basis aluminum is
seventh highest after hydrogen, beryllium, lithium, boron, gasoline,
and carbon. Beryllium and lithium can be immediately excluded as
economically prohibitive for large-scale primary fuel requirements.
Boron is a poor conductor and has otherwise unsatisfactory anode
characteristics, at least in the present state of the art. This brings
aluminum again up to fourth place, after liquid hydrogen, gasoline,
and carbon, with its practical energy content per unit weight again
amounting to two-thirds of the highest possible value.
Our analysis thus far fully covers all the readily oxidizable
elements in the first two rows of the periodic table. As the elements
get heavier, the energy densities become obviously poorer. Never-
theless, to ascertain that the attractive energy density of aluminum
256
Aluminum Fuel Cell
-------
is not somehow outweighed by some of the other potential fuels, let
us review the remaining metals in the third and fourth rows of the
periodic table that might be considered for metal-air batteries. Al-
though sodium may provide a respectable energy density on a weight
basis, it is bulky. In addition, its use in amalgamated form and possi-
bly also at high temperatures renders it far less practical than alumi-
num. Magnesium might come closest to aluminum in terms of theo-
retical energy per unit weight and per unit volume. Yet, in the
strongly alkaline or acidic electrolytes required for air cathodes,
magnesium poses much more severe passivation and corrosion prob-
lems than are encountered with aluminum. The same objection may
apply to calcium. Thus, zinc is left as the only material having
sufficiently attractive anode characteristics to be worth further
consideration.
COMPARISON OF ALUMINUM-AIR AND ZINC-AIR CELLS
Our system resembles the well-known zinc-air batteries in
several respects. We use aluminum anode sheets between two air
cathodes just as in the Leesona-Moos type of primary battery de-
scribed by Palmer or as in the General Dynamics type of secondary
battery described by Ragone, both in this volume. The reactions are
of the same type:
metal -)- oxygen -)- water —* metal hydroxide.
An electrolyte circulating system has also been used with aluminum-
air cells since 1960 (Zaromb and Foust, 1962). There are several
differences, however. No anode grids are required, since the alumi-
num is to be consumed entirely. No separators are used; it is a free
electrolyte system. The energy per reactant weight is at least two-
fold higher for aluminum, as shown in Table 2. Theoretically, alumi-
num would yield a 3 times higher energy density. In practice, how-
ever, the usual operating cell output may be about 1.3 volts with
zinc and around 1.2 volts with aluminum, which still yields the 2:1
ratio in favor of aluminum shown in the last column of Table 2.
Applying Mr. Shair's (in this volume) rule of thumb (approximately
20 percent of the theoretical energy density for the density to be
expected in practice) to Table 2, one reaches an estimate of 160 to 400
watt-hours per pound for a practical aluminum-air battery. On the
basis of the 2: 1 ratio in favor of aluminum, one might double the
energy densities available from present or projected zinc-air bat-
teries, 60 to 100 watt-hours per second for secondary cells and 150
watt-hours per pound for primary cells, and thus again arrive at
an estimate of 160 to 300 watt-hours per pound for practical
aluminum-air batteries. These estimates are generally confirmed by
the experimental data and design analysis presented in the following
sections and summarized in Table 3.
The well-known and quite satisfactory anode performance of
zinc, its relatively slow corrosion rate in alkaline solutions, and its
electrical rechargeability have contributed to the widespread ac-
ZAROMB 257
-------
ceptance of zinc anodes for miscellaneous low-power electrochemical
devices. But, for long-range portable power sources in the range of
10 to 30 kilowatts, we believe that electrical recharging is inferior to
chemical recharging. Moreover, both the anode performance and the
corrosion behavior of aluminum can be controlled.
Table 2. COMPARISONS OF ALUMINUM AND ZINC CELLS
Al
Zn
Total
g/F
26.0
49.7
Without 0.,
g/F
18.0
41.7
cm3/F
12.3
13.5
Kw-h/(lb
Theoretical
1.94
0.58
M H90)
Practical
0.8
0.4
iM(OH)z
= Zn or Al; z = 2 or 3
Table 3. WEIGHT ANALYSIS OF A 30-KILOWATT, 240-KILOWATT-HOUR,
ALUMINUM-AIR BATTERY
Scheme I Scheme II Scheme
Weight, Ib
Anode charge 150 150
Electrolyte charge 650 350
Uncharged battery assembly 200 200
All accessories 200 200
Total weight, Ib 1,200 900
Component life, kw-h
Electrolyte replaced and ppt removed
after 240 120
Anodes replaced after 240 240
Energy density, w-hr/lb, based on
anode replacement intervals 200 267
electrolyte replacement intervals 200 133
Power, kilowatts
Average power 30 30
Peak power without cathode boosting
provision 45 45
Peak power with cathode boosting
provision >90 >90
Power density, w/lb
Average power density 25 33
Peak power density without cathode
boosting 38 50
Peak power density with cathode
boosting >76 >100
Design based on:
(1) 1.2 Faraday-volts/9 g Al or 1600 w-hr/lb Al.
(2) 4.3 Ib initial electrolyte/Ib Al consumed.
300
500
200
200
1,200
120
480
400
100
30
45
>90
25
38
>76
258
Aluminum Fuel Cell
-------
PERFORMANCE OF EXPERIMENTAL
70-WATT ALUMINUM-AIR BATTERY
The experimental battery apparatus is shown in Figure 1. Am-
bient air is pumped first through a concentrated KOH solution, which
removes COL, and humidifies the air, and then to a manifold leading
to 14 parallel porous cathode compartments. The air pressure within
the cathode compartments is usually about 2 to 3 inches of water,
comparable to the maximum height of electrolyte within the cell
compartments during battery operation. The anodes are held by a
plastic cover that fits exactly over the cathode assembly.
Figure 1. Experimental 70-watt aluminum-air battery.
The electrolyte level within the cell compartments is controlled
by a small lab-jack, which raises or lowers a base plate pressing
against a collapsible electrolyte reservoir. The latter also collects
the Al(OH) ; battery reaction product. This reservoir is easily un-
clamped from or clamped onto the base of the battery during pre-
cipitate removal and electrolyte replacement.
When no power is drawn, the electrolyte is allowed to drain
into the reservoir to prevent exposure and corrosion of the aluminum
anodes. For battery operation the reservoir is collapsed partly, as
ZAROMB
259
-------
shown in Figure 1, so that electrolyte is forced into the battery
compartments. The electrolyte is also pumped through a heat ex-
changer to remove the heat dissipated in the battery reaction.
Any hydrogen evolved as a result of anode corrosion is chan-
neled through a single gas-outlet tube connected to the battery cover.
This outlet tube constitutes a simple and rapid corrosion detector;
the nascent hydrogen formed in any corrosion reaction is readily
detectible through its pungent odor, especially when it is all chan-
neled through a single outlet. The occurrence or absence of any
significant corrosion could thus be diagnosed, and the corrosion
could be prevented or kept to reasonable levels through proper
battery operation. We have acquired enough understanding of the
corrosion reaction to prevent or induce corrosion within the same
battery, reversibly and at will.
Our battery discharge curves tend to confirm the absence of a
significant corrosion rate under our experimental conditions. Figure
2 is representative of the kind of output voltage, current, and power
drawn from an immersed electrode area of 1,400 square centimeters
with anodes 0.05 centimeter thick consumed simultaneously from
both sides. At 100 percent current efficiency and an average cur-
rent density of 50 milliamperes per square centimeter, the anodes
would be expected to last for 4 hours, or about 700 coulombs per
square centimeter. Actual integration of the current curve of Figure
2 yields around 500 coulombs per square centimeter. Since about
one-third of the original anode thickness was left at the end of the
run, the figures given here are roughly consistent with nearly 100
percent efficiency.
The data in Figure 2 were obtained in June 1966. Data from
an experiment performed in March 1967 are shown in Figure 3.
The same battery and experimental apparatus were used as in the
earlier runs; however, the anodes were 0.3 centimeter thick and cut
narrower. The immersed electrode area was therefore only 1,250
square centimeters. Nevertheless, the average power output was still
between 60 and 70 watts, at around 40 milliamperes per square
centimeter and 1.2 volts per cell. Similar results were obtained in
even more recent experiments (June 1967). Thus, no observable
deterioration in battery performance occurred during a 12-month
period.
The run of Figure 3 was designed to determine the minimum
weight ratio of electrolyte to aluminum required for satisfactory con-
tinuous battery operation. To obtain this information it was most
expedient to allow 2 kilograms of aluminum to corrode away in an
initial 8-liter volume of 3 M KOH solution. Additional water was
added to make up for evaporation losses. After complete dissolution
of the aluminum, the total volume was again brought to 8 liters. The
final mix therefore contained around 5.8 kilograms A1(OH)S plus
6.7 kilograms 4 M KOH solution. This mix was then re-introduced
into the electrolyte reservoir, and the run was completed.
260 Aluminum Fuel Cell
-------
<
I—
<
cc
o
o:
UJ
Q.
2
<
60
ALUMINUM ETCHED FROM BOTH SIDES
TOTAL APPARENT ELECTRODE AREA -= 1,400 cm-
ALUMINUM THICKNESS ~ 0.02 in. =- 0.05 cm.
70 amp
\
50 ma/cm2
1.1
1.0
O
0.9 >
0.8
0 20 40 60 80 100 120 140 160 180 200
TIME, min.
Figure 2. Typical performance of a 70-watt
aluminum-air battery with thin anodes. Kinks in
curves correspond to load adjustments (June, 1966).
70
60
50
40
30
CONSUMED 2 kg Al -
PER 7 kg of ELECTRODE |
CURRENT (I)
-T
I
J TOTAL ELECTRODE AREA = 1,250 crrv
ALUMINUM THICKNESS = 0.125 in.;
50 amp <-» 40 ma/cm-
_L
1.3
1.2 LU
1.0
0 60 120 180 240 300
TIME, min.
3,000 3,060
Figure 3. Performance of the same battery with
thick anodes of smaller area (March, 1967).
ZAKOMB
261
-------
Figure 3 shows that the battery performance near the end of
the run was not much worse than at the beginning. Hence, the
residual electrolyte weight need not exceed 3.3 pounds per pound of
aluminum consumed, and the corresponding initial electrolyte weight
need not exceed 4.3 pounds per pound of aluminum.
In these experiments we obtained about 1.2 faraday-volts per
gram equivalent (9 grams) of aluminum, or 1,600 watt-hours per
pound of aluminum at current densities of at least 40 milliamperes
per square centimeter. Further, such performance could be obtained
even without replenishment of electrolyte or of water if the initial
charge included 4.3 pounds of electrolyte per pound of aluminum
consumed.
In addition, our single-cell experiments have yielded consider-
ably higher current densities and output voltages with somewhat
more active aluminum alloys. Unfortunately, our present stock of
alloys is not easily adaptable for testing in the multicell battery of
Figure 1. On the basis of our experience to date, however, we expect
no serious difficulty in obtaining at least 1.2 volts per cell at 100
milliamperes per square centimeter and nearly 100 percent current
efficiency with at least one type of a readily available aluminum alloy.
SIGNIFICANCE OF EXPERIMENTAL RESULTS
A design analysis has been performed for a 30-kilowatt alumi-
num-air power source. The block diagram of Figure 4 shows all the
required accessories. The air supply system, including a compressor,
air scrubber, and connecting tubes or manifolds, is essentially similar
to those used in other fuel cells. The electrolyte circulating system,
including a heat exchanger and auxiliary reservoir, is also similar to
those generally used. A multichannel pump, possibly of the peristal-
tic type, is proposed for the purpose of eliminating leakage currents
between series-connected cells. Furthermore, the electrolyte volume
in the reservoirs can be readily altered by application of air pressure
within spaces enclosed by collapsible inner linings and rigid outer
shells. The electrolyte level in the battery assembly is thus pneu-
matically controlled by a simple air-pressure regulator. This also
provides a simple, light, and inexpensive means of controlling the
current, voltage, and power output without recourse to any elec-
tronic circuitry.
The aluminum hydroxide precipitate formed in the battery
reaction is to be collected in one of the electrolyte reservoirs and
preferably returned for credit at a refueling station. Alternatively,
the precipitate may be disposed of whenever fresh water is added
to the system. At least two simple arrangements are being con-
sidered for ejecting the precipitate safely and conveniently.
The cathode boosting means shown in Figure 4 may consist of
a solution of oxidant, such as hydrogen peroxide, which may be in-
jected into the electrolyte entering the battery assembly. The chief
262 Aluminum Fuel Cell
-------
power limitation of the oxygen or air electrode is known to be as-
sociated with the low solubility of oxygen in aqueous alkaline electro-
lyte. Hence, during periods of peak power demand it may be desir-
able to inject an alternative oxidant into the electrolyte. Although
such oxidant would have an adverse effect on hydrogen anodes, we
have found no deleterious effect and even an enhancement of the
activity of aluminum anodes upon introduction of around 1 percent
H.,O., into the electrolyte. This simple method of boosting the cathode
performance should double the peak power output and thus meet all
peak power requirements; no auxiliary batteries or other auxiliary
power sources would be needed.
SCP
AEC
»ROS
EP
J
-
\
x
1
f
1
K ' — I —
AE
AB —AIR BLOWER
AC —AIR COMPRESSOR
AE—AIR ENTRY
AO —AIR OUTLET
AS —AIR SCRUBBER
BA —BATTERY ASSEMBLY
CB —CATHODE BOOSTING MEANS
EP —ELECTROLYTE AND PRECIPITATE
CONTAINER
GC —GASKETING CLAMPS
GM —GASKETING MOLDING
HE-HEAT EXCHANGER
SA —SIDE ARM
AEC —AUXILIARY ELECTROLYTE
CONTAINER
CIL —COLLAPSIBLE INNER LINING
LLR —LIQUID-LEVEL REGULATOR
ROS —RIGID OUTER SHELL
MCP— MULTI- CHANNEL PUMP
Figure 4. Block diagram of a proposed
aluminum-air power source.
ZAROMB
263
-------
Since the required accessories do not otherwise differ appreciably
from those in similar systems, we can formulate a weight analysis of
our proposed power source by referring to the weights quoted by
Dantowitz and Gaddy for an operational hydrazine fuel cell. The re-
quired accessories for 20-kilowatt and 40-kilowatt hydrazine systems
weigh 185 pounds and 235 pounds, respectively. Hence, the acces-
sory weight for a 30-kilowatt system may amount to around 200
pounds. The remaining weight evaluation in Table 3 is derived di-
rectly from the experimental data presented in the preceding section.
The three schemes in Table 3 may again demonstrate the pro-
priety of disregarding the weight of water in the preliminary energy-
density comparisons of Table 1. The initial electrolyte charge will
last for a complete 8-hour operation at average load. Hence, a vehicle
traveling at 60 mph at average power would have a range of 480
miles between refueling stations. By another scheme, however, the
range between stations would be the same, but the driver would have
to stop off midway for a fresh electrolyte charge. The total fuel cell
weight is thus reduced by around 25 percent, and the peak power
density is correspondingly increased to around 50 watts per pound
without recourse to cathode boosting, and possibly to more than 100
watts per pound with our cathode boosting provision.
Where the power density achievable by the first scheme may
be entirely adequate, one can easily increase the range between re-
fueling stations to around 1,000 miles by doubling the aluminum
charge and still keep the initial electrolyte charge down to 500
pounds, of which 150 pounds would be in form of three separate
50-pound dry KOH cartridges providing fresh electrolyte upon addi-
tion of water at successive 200- to 300-mile intervals.
Although the energy and power densities given by Table 3 are
already attractive, they do not reflect the important weight savings
provided by the simple pneumatic power control already mentioned.
The power controller used with the 20- to 40-kilowatt hydrazine
fuel cell weighs 300 pounds, and even if this weight is eventually
reduced by a factor of 2 it would still represent around 15 percent of
the total weight of the power source. Since an air pressure regulator
need not weigh more than 2 pounds, our power control weight is
clearly negligible. Hence, the 15 to 30 percent saving on the weight
of electronic controls becomes equivalent to a corresponding further
increase in effective energy and power densities.
One might argue that electronic inverter circuitry might be re-
quired in any event for use with alternating-current motors, which
weigh only about 1 pound per horsepower, whereas present commer-
cial direct-current motors weigh more than 5 pounds per horsepower.
At least one lightweight direct-current motor has been developed
with an estimated weight of around Vz pound per horsepower (Zeisler,
1964). This motor also requires lower voltages, 2 to 5 volts, which
may further simplify the fuel cell design, e.g., by reducing the re-
quired number of separate electrolyte pumping channels. The heavy
264 Aluminum Fuel Cell
-------
bus bars required with large currents could be kept down to a reason-
able level by placing the battery close to the motor. For example, a
rod of copper 1 square centimeter in cross section and 100 centimeters
long weighs less than 2 pounds. The type of homopolar direct-current
motor reported by Zeisler apparently would provide not only an
adequate but a highly desirable combination with any type of cell.
NEAR-TERM AND LONG-TERM PROSPECTS
OF THE ALUMINUM FUEL CELL
In addition to its important functional advantages, the aluminum
fuel cell appears to be economically attractive both immediately and
long term. At current prices, the cost of aluminum fuel (25 cents/lb)
less the cost of the recoverable A1(OH):1 product (around 5 cents'lb
Al(OH)., or 10 to 15 cents lb Al) may be already comparable to or
lower than the average cost of gasoline consumed in present internal-
combustion engines for the same motive energy yield. In competitive
mass consumption the costs of aluminum should come down, since the
over-all aluminum consumption and recovery cycle would utilize the
cheapest available resources—carbon, and the cheapest available elec-
tricity (usually processed directly near a hydroelectric power plant)
—at a fairly reasonable overall energy efficiency. Modern aluminum
reduction plants can operate at current efficiencies of at least 90
percent and less than 4.1 volts per cell (Oehler, 1963). Hence, the
fraction of electric energy input that may be regained in form of
motive power should amount to at least 90 percent times 1.2 volts
divided by 4.1 volts, or 26 percent. In addition, the salvageable heat-
ing value of the heat generated in the battery may considerably in-
crease the effective efficiency in cold weather.
The production of aluminum near sources of cheap electricity,
i.e., in predominantly rural areas, would also prevent further intro-
duction of smoke-stack pollutants from power plants in urban sur-
roundings. Although aluminum reduction plants do inject pollutants
into the atmosphere, their emissions should be much more readily
amenable to control than motor vehicle exhausts or flues in urban
areas. In addition, the total amount of carbon dioxide that would be
introduced into the earth's atmosphere by each complete aluminum
reduction-consumption cycle can easily be shown to be around half
of what might be produced in a few decades for comparable energy
production by hydrocarbon fuel cells.
This discussion thus far has failed to point out some of the ob-
stacles to an early realization of practical and competitive aluminum-
driven vehicles. The present high cost of air cathodes may constitute
the chief and perhaps only genuine technical-economic barrier. This
high cost cannot be due only to catalyst requirements, since the silver
required for satisfactory cathodes need not cost even $1 per square
foot. We believe that at considerably greater cathode costs ($107ft2)
the aluminum fuel cell may start competing with gasoline engines.
The present costs of $50 to $100 per square foot of cathode area are
ZAROMB 265
-------
thus only 10 times higher than what we regard as the maximum
costs allowable for motor vehicle applications, and these costs may
be attributed chiefly to the small production scale and to initial de-
velopment outlays. Discussions with cathode suppliers lead to the
conclusion that a cost of $5 per square foot on a mass production
scale may be a realistic forecast.
One must also take cognizance of past failures to develop practi-
cal aluminum batteries (Brown, 1893; Sully, 1897; Polcich, 1926;
Vince, 1933; Ruben, 1933, 1951, 1952; Stokes, 1955; Lozier et al., 1959,
1961; Sargent, 1951; Zaromb, 1962). These past attempts were, how-
ever, directed toward small cells for low-power applications where
the use of auxiliary mechanical accessories could not be considered.
It is chiefly for high-power and high-energy-storage applications
that the aluminum fuel cell is attractive.
Other problems have been met since the inception of this work
some 8 years ago (Zaromb and Faust, 1962; Zaromb, 1962; Bockstie,
Trevethan, and Zaromb, 1963). Throughout, we have had to steer a
narrow course between passivation of the anodes and excessive cor-
rosion. We believe we now have enough knowledge of these difficul-
ties and of how to circumvent them to be confident that a practical,
aluminum-driven vehicle will sooner or later make a successful
appearance.
ACKNOWLEDGEMENTS
Acknowledgements are due especially to L. Bockstie, J. O. M,
Bockris, and M. Lasser for help in the early stage of this work (in
1959-1961), and to W. Bojman, R. Edwards, and J. Teale for contri-
butions made in the last 2 years.
REFERENCES
Bockstie, L. D., Trevethan, and S. Zaromb, 1963. Control of Al cor-
rosion in caustic solutions. J. Electrochem. Soc. 110:267-71.
Brown, C. H., 1893. U. S. Patent 503,567.
Lozier, G. S., R. Glicksman, and C. K. Morehouse, 1959. U. S. Patent
2,874,079.
Lozier, G. S., R. Glicksman, and C. K. Morehouse, 1961. U.S. Patent
2,976,342.
Oehler, R. E., 1963. In: Extractive metallurgy of aluminum, G.
Gerard, ed. Interscience, New York, N. Y. Vol. 2, pp. 231-37,
especially p. 232.
Polcich, G., 1926. U. S. Patent 1,771,190.
Ruben, S., 1933. U. S. Patent 1,920,151.
Ruben, S., 1951. U. S. Patent 2,638,489.
Ruben S., 1952. U. S. Patent 2,783,292.
266 Aluminum Fuel Cell
-------
Sargent, D. E., 1951 U. S. Patent 2,554,447.
Stokes, J. J., Jr., 1955. U. S. Patent 2,796,456.
Sully, J., 1897. U. S. Patent 585,855.
Vince, C. H., 1933. British Patent 397,475.
Zaromb, S., 1962. The use and behavior of aluminum anodes in
alkaline primary batteries. J. Electrochem. Soc. 109:1125-30.
Zaromb, S., and R. A. Foust, Jr., 1962. Feasibility of electrolyte re-
generation in aluminum batteries. J. Electrochem. Soc. 109:
1191-92.
Zeisler, F. L., 1964. IEEE National Conference Record on Automotive
Electrical and Electronics Engineering.
ZAROMB 267
-------
PERFORMANCE AND ECONOMICS OF THE
SILVER-ZINC BATTERY IN ELECTRIC VEHICLES
George A. Dalin
Yardney Electric Corporation
New York, NY.
Two principal arguments have been advanced for the electric
car: freedom from pollution of the air, and economy. The pollution
argument is clear enough; each gasoline-powered car emits an esti-
mated ton of pollutants each year. The concentration of pollutants
in New York approached the crisis level last Thanksgiving (1966).
With the increase in the number of cars, the crisis level may be sur-
passed at any time. Although engine manufacturers are working on
reduction of emissions, there is no guarantee that the achievements
in this direction will not be swamped by the increase in the concen-
tration of cars in urban areas. Since the internal combustion engine
produces more than half the total pollution in urban areas, it is
evident that replacement by electric cars could result in a substantial
decrease in pollution.
Evaluation of the economics of the electric car is more complex.
Additional factors such as range, speed, acceleration, life, and even
method of financing must be taken into account. To start with, con-
sider the energy-storage capacity of the batteries that are now used
for various purposes and that might be used to power an electric car.
The energy densities for the various batteries in terms of watt-hours
per pound are (FPC, 1967) lead-acid, 8 to 12; nickel-iron, 11; nickel-
cadmium, 12 to 14; silver-cadmium, 30; and silver-zinc, 40 to 60.
Lead-acid, with the lowest energy density of the lot, has had the
field to itself since 1870, when Sir David Solomons built the first
electric vehicle. The business burgeoned, and between 1900 and 1915
over 100 manufacturers were in operation. By 1912, the business
amounted to 6,000 passenger and 4,000 commercial vehicles yearly.
Range was 20 miles and top speed was 20 mph. It is worth empha-
sizing that traveling at top speed cuts the range, so that top speed
and maximum range cannot be achieved in combination. This is true
of the other batteries and also of internal combustion.
The cost ranged from $2,600 for a Baker Electric to $5,500 for a
Borlund Electric. It would be interesting to calculate the cost per
mile in today's dollars for the Borlund Electric.
After 1914 the manufacture and sale of electrics fell off, and
internal combustion took over the market. By 1933, electrics had
gone off the market (Ference, 1967). Nevertheless, during this period
a number of concepts were proposed that may prove important today.
To eliminate the energy loss at the differential, the rear wheels were
driven by individual motors. Charging at metered outlets and ex-
change of exhausted batteries for fresh batteries at service stations
were both proposed.
DALIN 269
-------
In this .country lead-acid batteries are now used for the propul-
sion of fork-lifts, of which about 100,000 are in use. The battery-
powered fork-lifts are used in enclosed spaces in which internal
combustion fumes cannot be tolerated.
In England there are about 40,000 battery-powered urban
delivery vans, and 60,000 fork-lifts. We see, then, that where the
daily range is sufficiently limited, even lead-acid batteries can meet
the needs.
With reference again to batteries mentioned earlier, it is evident
that nickel-cadmium and nickel-iron do not give a sufficient increase
in energy density over lead-acid to warrant their use in passenger
cars. Nickel-iron batteries are extremely long-lived, however, and
thus are suitable for powering trucks. Some of the batteries originally
put into use around 1910 are believed to be still in operation.
Silver-cadmium, from the standpoint of energy density, is far
superior to the other three batteries; however, it is by far more expen-
sive. It is also substantially more expensive than silver-zinc. The
silver-zinc battery not only tops the list in energy density but is also
a high-rate battery. The General Motors Electrovair II reached a
speed of 80 mph, powered by Yardney silver-zinc batteries.
The first cost of silver-zinc appears high relative to lead-acid,
but a more significant parameter is the cost per mile. To calculate
this, we use the data obtained by installing four Yardney Electric
batteries in a modified Renault Dauphine automobile. The car was
originally modified for use with lead-acid batteries, and was fitted
with a 7.1-horsepower direct-current motor. The Yardney batteries
were designed for aircraft use and therefore were not optimum for
the car. Each consists of fourteen 85-ampere-hour cells, which give
about 120 ampere-hours when new. Figure 1 shows typical discharge
curves for a 100-ampere-hour cell, which is very similar in con-
struction and of only slightly larger capacity. Figure 2 shows voltage
as a function of current density. This curve makes it possible to
predict wattage as a function of current.
The car was run on a flat road; both voltage and current were
observed. Wattage is plotted as a function of speed in Figure 3. Note
that the wattage at each speed varied over a substantial range. Evi-
dently wind is a factor, with low wattages indicating a following wind
and high wattages a head wind. To estimate the energy expended
in traveling 1 mile, a value for 40 mph can be taken from the graph,
a value from the middle of the zone. A speed of 40 mph requires
6,500 watts. With the proper mathematical manipulation we find
that 1 mile requires 160 watt-hours.
At a battery voltage of 75, the number of ampere-hours required
per mile is 160/75 or 2.1. For city driving, an average speed of 25
mph is more likely. At this speed the wattage is about 1,700 and the
voltage about 80. This works out to 68 watt-hours and 1.05 ampere-
hours per mile. On this basis the range would be 85/1.05 or 81 miles.
270 Silver-Zinc Battery
-------
1.8-
1.6-
UJ
|l.4
d
1.2-
1.0-
12 24 36 48 60 72 84 96 108 120
OUTPUT, amp-hr
Figure 1. Typical discharge curves for
100-ampere-hour cell.
2.0-
LlJ
CD
_
o
1.0-
0
0.20
0.05 0.10 0.15
CURRENT DENSITY, amp/in.2
Figure 2. Voltage as a function of current density.
DALIN
271
-------
14 -
SPEED, mph
Figure 3. Wattage as a function of vehicle speed.
In practice the range is about 77 miles. This range gradually de-
creases as the battery ages and is shorter at higher speeds.
The four batteries, including metal cases, weigh 240 pounds. The
weight can be cut by 30 pounds through the use of plastic cases.
The car, without batteries or occupants, weighs 1,335 pounds and is
not streamlined. Maximum speed is 55 mph. The motor is 7.1 horse-
power and is series-wound. With the car on blocks, turning the
wheels at 40 mph requires 2 horsepower.
The speed control in this model is the one that was installed
when it was originally fitted for lead-acid batteries and consequently
is primitive. The control was not changed for the tests with silver-
zinc because the objective was to obtain performance data on the
battery. For the same reason, the low-power motor also was retained.
The control can achieve six speeds by placing the four batteries in
parallel, series-parallel, or series, and through the use of a series
resistor (at bottom speed) and a field shunt.
As stated earlier, the batteries described were not designed for
use in the car — they occupied only about half the available space
in the front and rear trunks. Yet the data can serve as the basis for
a more suitable design and for cost estimates. The number of cells
will be taken as 60 because this number can be charged directly from
110-volt alternating current, with a suitable rectifier, of course. The
space available in the car is adequate for 60 cells of any capacity
up to about 250 ampere-hours; the technology is available for a
battery of almost any size.
272 Silver-Zinc Battery
-------
Let us start with the assumption that a second family car will
be driven about 50 miles per day. Also, assume that a safety factor
of 2 in the range must be provided. This will require that battery
capacity be increased to 124 ampere-hours.
To compare the costs of operating internal combustion and
electric vehicles, we must make some basic assumptions:
Internal combustion vehicle
Retail cost, $3,000.
Life, 6 years at 12,500 miles per year. This figure is based on 50
miles per day, 5 days per week, 50 weeks per year. Gasoline
mileage, 13 miles per gallon at $0.35 per gallon, or 2.7 cents per
mile.
Battery-powered vehicle
Retail cost, $2,500 without battery. Cost is lower than that of the
internal combustion car because cost of engine, transmission, and
auxiliaries is subtracted. Cost of motor, controls, and charger is
included.
Life, 10 years at 12,500 miles per year. Car life is assumed longer
than for internal combustion because the system is much simpler.
Electricity mileage, cost of electricity is $.01 per kilowatt-hour
and charge wattage efficiency is 50 percent. At 160 watt-hours
per mile, cost is 0.33 cents per mile.
Retail cost of the battery, exclusive of silver, is estimated to be
$700 to $900. Our accounting department has suggested that the
lower figure be used for calculations. Life is taken as 2 years when
the vehicle is operated for 12,500 miles per year and as 4 years for
7,500 miles per year.
The silver can be leased or rented. This is now the practice with
batteries for fork-lifts. The silver can be re-used indefinitely with
virtually no loss, and can therefore be regarded as indestructible
collateral for a loan. This places its cost in a different category from
that for the remainder of the battery and the vehicle.*
Battery life, on the basis of laboratory data, can be taken as
500 cycles at 50 percent depth of discharge. The cycle-life is extended
as the depth of discharge is decreased. Moreover, recent develop-
ments in battery construction have demonstrated substantial increases
in cycle life over that quoted here.
With these assumptions, the cost of operating the battery-
powered vehicle is somewhat lower than that of operating the in-
ternal combustion vehicle.
The costs are compared in Table 1 for mileage of 12,500 per year.
The picture changes if the daily mileage is lower. The life of the
battery will be substantially longer at the shallower depth of dis-
charge. We now examine a new set of assumptions. Traveling 30
*Cost estimates based on silver at pegged price of $1.29 per'ounce.
DALIN 273
-------
Table 1. COST COMPARISON BASED ON 12,500 MILES PER YEAR
Fixed costs
Depreciation
Battery
Insurance
License and taxes
Internal
Combustion
$500
0
200
20
720
Battery
Power
$250
350
250
20
720 870
870
Operating costs
Gasoline
Silver rental, 9%
Oil, grease, tires, etc.
Charging
Cost per year
Cost per mile
337
0
140
0
477
Table 2. COST COMPARISON BASED
Fixed costs
Depreciation
Battery
Insurance
License and taxes
Operating costs
Gasoline
Silver rental, 9%
Oil, grease, tires, etc.
Charging
Cost per year
Cost per mile
Internal
Combustion
$500
0
200
20
720
203
0
84
0
287
0
96
75
41
477 212
1,197
9.6C
ON 7,500 MILES
Battery
Powered
$250
175
250
20
720 695
0
96
45
25
287 166
1,007
13.40
212
1,082
8.70
PER YEAR
695
166
861
11. 50
274
Silver-Zinc Battery
-------
miles per day, 5 days per week, 50 weeks per year yields 7,500 miles
per year. Battery life increases to 4 years. Table 2 presents the
cost comparison for 7,500 miles per year. At this mileage, the battery-
powered vehicle provides an even greater cost advantage.
Since the assumptions on which these accountings are based in-
volve some uncertainties, it would be overstating the case to claim
that battery power is substantially cheaper than gasoline. Never-
theless, it appears that silver-zinc battery power is no more expensive
than gasoline power, and if anything is cheaper. Consequently, cost
is no obstacle to the use of silver-zinc for low-mileage applications.
Since batteries emit no pollutants, silver-zinc offers the possibility of
a fast start toward eliminating the pollution blight.
In concluding, I should mention that work is proceeding in a
number of laboratories on systems of still higher energy density.
Specific examples of couples under investigation are zinc-air, lithium-
nickel halide, sodium-sulfur, and lithium-chlorine. Speaking for our
own laboratory, I can say that progress on zinc-air is very satisfactory
and we expect to present results before long.
REFERENCES
Federal Power Commission, Bureau of Power, 1967. Development of
electrically powered vehicles. 50pp. (Feb.).
Ference, M., 1967. Electric vehicles. Statement before Committee on
Commerce and Subcommittee on Air and Water Pollution of the
Committee on Public Works, U. S. Senate. (Mar. 16).
DALIN 275
-------
A SODIUM-SULFUR SECONDARY BATTERY
T. W. DeWitt
Ford Motor Company
Dearborn, Mich.
INTRODUCTION
One of the major barriers to the general use of electric power
in mobile applications is the inadequacy of the power source. This
has been true for over half a century, yet the advantages that could
be gained through the use of electric power are such that technological
advances, as they occur, are continually reviewed for their potential
in overcoming this barrier. A case in point is the widespread
resurgence of interest over the last decade in fuel cells and high-
energy-density rechargeable battery systems.
Research and development in this area has taken many paths
and has had many origins. The result of one approach is the subject
of this paper.
The work that will be described did not originate directly as
a conscious search for a high-energy-density electrical power system,
but is a product of the interaction of fundamental research in solid-
state science and a continuing awareness of the major problems
of the energy storage and conversion field.
SOLID ELECTROLYTES
Common to all secondary cells is an ionically conducting mem-
brane separating reactive materials. The reactive materials are
constrained to combine by ion transport through the membrane and
electron transport through an external circuit where the free energy
change corresponding to the cell reaction is extracted as useful work.
The fundamental idea in this concept of a sodium-sulfur cell is that
of using a solid ionically (but not electronically) conducting mem-
brane between reactants. When such a solid membrane, which takes
the place of the usual liquid electrolyte, is also impermeable and
inert to reactant materials in other than ionic form, the possibility
exists of using energetic reactants in the liquid state. Molten elec-
trodes suffer from no irreversible changes on charge-discharge
cycling and often present fewer problems from polarization at phase
interfaces.
Figure 1 compares the sodium-sulfur cell schematically with
the common lead-acid cell. In the latter, electrons pass through
the external circuit (where they do work) and ions through the
sulfuric acid electrolyte to complete the reaction, which is essentially
the oxidation of lead and the reduction of lead dioxide. The reactants
DEWITT 277
-------
and products are solid, and the electrolyte, liquid. In the sodium-
sulfur system, liquid sodium, which serves as its own electrode,
yields electrons to the external circuit, passes through the separator
as a sodium ion, and reacts with liquid sulfur, which receives
electrons from the external circuit, to form sodium sulfide, which
is also kept liquid by operation at elevated temperature (300°C).
LEAD-ACID STORAGE BATTERY
LEAD
~ SULFURIC-
~ ACID
--^-1
— —
LEAD
DIOXIDE
SOLID REACTANTS,
LIQUID ELECTROLYTE
On discharge, energy is extracted
in external circuit while both
electrodes and electrolyte
undergo chemical change.
FORD SODIUM BATTERY
r .LIQUID SODIUM
;/ CERAMIC
ELECTROLYTE
CONDUCTING
SULFUR
ELECTRODE
LIQUID REACTANTS,
SOLID ELECTROLYTE
Current is carried by sodium atoms
that give up electrons to external
circuit, traverse solid ceramic
electrolyte, and react with sulfur.
Figure 1. Schematic representation of sodium-sulfur cell
and comparison with lead-acid cell.
THE SODIUM-SULFUR SYSTEM
Sodium and sulfur are attractive reactants for several reasons.
Under proper conditions the reaction is electrochemically reversible.
Both are molten at 115°C, and if sodium is added to a fixed amount
of sulfur, a high specific energy can be obtained before the melting
point of the reaction product exceeds 300°C. Both sodium and sulfur
are plentiful and inexpensive and can be contained at 300°C in vessels
made of common materials such as stainless steel and aluminum.
The highest pressure encountered is the vapor pressure of pure
sulfur, about 60 millimeters of mercury at 300°C.
Figure 2 shows the open-circuit potential of the sodium-sulfur
system as a function of composition. The voltage remains constant
at 2.08 until a composition corresponding to sodium penta-sulfide
is reached. It then drops almost linearly to 1.76 at a composition
corresponding to sodium trisulfide. The temperature of operation
is dictated by the melting point of the product, and the constraints
thus imposed are illustrated in Figure 3, which is the phase diagram
for the system Na,S-S (Pearson and Robinson, 1930). At the opera-
278
Sodium-Sulfur Secondary Battery
-------
ting temperature of 300°C, solid will begin to separate out at about
the composition Na.,S3. This composition is the maximum contem-
plated degree of reaction, or the discharged state. On the right-hand
side of the diagram, the sulfur side, the system is two phase at
300°C, liquid sulfur and liquid sodium pentasulfide.
2.0
UJ
13
O
o
1.0
Q.
o
DEPTH OF DISCHARGE, percent
25 50 75
_j I I
100
I
I \ I
0.2 0.4 0.6 0.8
MOLE RATIO (SODIUM/SULFUR)
Figure 2. Open-circuit voltage of sodium-sulfur cell vs.
state of discharge.
CONDUCTING CERAMIC MEMBRANE
The heart of this battery concept is the ionically conducting
solid electrolyte around which most of the research centered. The
original materials examined for use as solid sodium ion conductors
were glasses, principally those in the soda-alumina-silica system.
The requirements for a solid electrolyte material separating sodium
and sulfur are stringent. Good conductivity, chemical corrosion
resistance to the reactants, usable strength, and impermeability are
essential properties.
No one glass composition was found perfectly satislactory, but
in the course of investigation a crystalline material known as beta-
alumina was encountered and recognized as having the sought-after
properties. Beta-alumina is not a form of A1..O.,, but is a sodium
DEWITT
279
-------
1,000
. SULFUR ,
100
40
60 70 80
SULFUR, percent
100
Figure 3. Phase diagram of the system Na2S-S (after
Pearson and Robinson).
aluminate with the generally accepted formula Na,O-HAl,O3. This
is not a new material, but its high ionic conductivity has not been
previously recognized. The structure of beta-alumina (Beavers and
Ross, 1937) is known and is shown in Figure 4. It is a layer structure
in which sodium ions are found in relatively open planes together
with bridging oxygen ions that have the effect of providing a spacing
that results in nearly optimum mobility in this structure for a
monovalent ion the size of sodium. The oxygen ion ''prop" is what
distinguishes beta-alumina from other materials with a layer struc-
ture such as mica. Application of theories and techniques from
solid-state and crystal chemistry resulted in producing proprietary
280
Sodium-Sulfur Secondary Battery
-------
derivatives of beta-alumina that can be made into highly conductive,
stable, impervious shapes by conventional ceramic-forming pro-
cedures. The electrical resistivity of the formed polycrystalline
ceramic is shown in Figure 5. At 300°C the resistivity is about 5
ohm-centimeters, which is comparable to the values for molten salts
and aqueous sulfuric acid and potassium hydroxide.
* $ 9 *
• (i, If # 9 9
* •
Figure 4. The structure of beta-alumina (after Beavers
and Ross). White balls are sodium.
DEWITT 281
-------
300
TEMPERATURE,
200
100
100 —
E
o
E
-§
t 30
Ul
o:
10
1.5
2.0
2.5
3.0
3.5
T(°K)
Figure 5. Electrical resistivity vs. temperature of ion-
conducting ceramic.
LABORATORY SCALE CELL
Ceramic membranes in the form of thin-walled tubes have
been used to construct sealed cells like that shown in Figure 6.
Sodium is contained in a reservoir sealed to the top of the ceramic
tube, the other end of which is closed. Surrounding the ceramic
tube is sulfur contained in a porous graphite electrode that makes
contact with the ceramic and a backing electrode. Typical charge-
discharge curves for a cell of this type with a 3 millimeter thick
layer of sulfur and 0.8-millimeter ceramic wall thickness are shown
in Figure 7. Of note are the high current densities, computed from
surface area of ceramic, that can be achieved at terminal voltages
282
Sodium-Sulfur Secondary Battery
-------
consistent with only pure ohmic resistance of ceramic, sodium
sulfides, and porous electrode. The data contain little indication of
polarization from charge transfer or diffusion-controlled electrode
reactions.
Operating experience has confirmed the expected advantages
of this battery concept over other systems. Being liquid, the reactants
are regenerated to the same physical form during repeated charge-
discharge cycling, obviating a source of limited cycle life as in
cells with solid reactants. The cell reaction is simple; it has no
gaseous reactants or products and no known side reactions or
mechanisms for self discharge. No auxiliary equipment is required
except thermal insulation.
,-y;
SEAL
LIQUID SODIUM
RESERVOIR
BACKING
ELECTRODE
SODIUM-ION-
CONDUCTING
CERAMIC TUBE
SULFUR-FILLED
POROUS ELECTRODE
Figure 6. The construction of a small sodium-sulfur cell.
PROJECTED SCALE-UP
This battery is still in the advanced laboratory stage, and
current emphasis is on obtaining data necessary for proper design
of larger units. No basic barrier to its larger scale development
is known at present, and although problems associated with materials
of construction, configuration, reliability and the like remain to be
faced, it is anticipated that these can be solved.
DEWITT
283
-------
CONSTANT CURRENT CHARGE AND DISCHARGE, amp-hr/lb
o
3.0
2.5
2.0
2.5
2.0
1.5
1.0
0.5
0
1 — 170 ma/cm2 -120 min
2 — 340 ma/cmz- 60 min
3 — 680 ma/cm2 - 30 min
DISCHARGE-
OPEN CIRCUIT
-CHARGE
0.5
0.4
0.3
0.2
0.1
0
0.1 0.2
DISCHARGE ->
0.3 0.4 0.5
MOLE RATIO (SODIUM/SULFUR)
0.6
Figure 7. Terminal voltage vs. state of discharge for steady
discharge and charge currents of 170, 340, and 680 milli-
amperes per square centimeter of ceramic membrane.
When the laboratory data are projected to the design of larger
size units, one result is the proposed cell shown in Figure 8. The
specifications for such a unit are given in Table 1. The rated
energy density, 148 watt-hours per pound, represents about 45
percent of the theoretical 346 watt-hours per pound. These numbers
are compared in Table 2 with those for several conventional batteries.
At the present time it is expected that a battery of the size
shown in Figure 8 will be in operation in a year and one in the
20- to 40-kilowatt range in 2 years. It will certainly be several
years before any appreciable number of these batteries will be
available for the purpose of exploring other applications. Although
the primary interest, and immediate focus, is in the use of this
battery system for motor vehicle propulsion, it is anticipated that
it will ultimately have many other applications, both civilian and
military. Since it operates at elevated temperatures, its usefulness
284
Sodium-Sulfur Secondary Battery
-------
in areas such as miniature batteries and small portable power
sources is limited, except for very special applications. In sizes
of a kilowatt or more, the temperature problem diminishes con-
siderably in magnitude, and in some instances the higher operational
temperature may even be an advantage. A few of the areas in
which it may have applicability include: electric lawnmowers, garden
tractors, farm tractors, submarines, electric railway cars, and mate-
rials handling equipment.
Figure 8. Mock-up of a proposed 2-kilowatt cell, showing
planar array of sodium-filled ceramic tubes.
DEWITT
285
-------
Table 1. SPECIFICATIONS OF PROPOSED CELL
Average power, kw 2
Maximum power, kw 4
Open circuit voltage 2.08
Average discharge voltage 1.75
Ampere-hours 1,850
Weight, Ib 22
Volume, in.3 400
Table 2. COMPARISON OF BATTERY SYSTEMS
Lead-acid
Nickel-cadmium
Silver-cadmium
Silver-zinc
Ford sodium-sulfur
Energy Density, w-hr/lb
(slow discharge, 5 hr)
10
14
24
50
150
Weight/ power ratio,
(rapid discharge, x/4
31
25
15.5
6.5
10
Ib/kw
hr)
APPLICATION IN VEHICLES
It is premature to do anything but speculate on what this
battery could mean for an electric car since there are many problems
to be solved, not only with the battery, but with motors and controls,
before it can become a practical reality. It is nevertheless possible,
and useful, to make a tentative calculation of what might be
achieved. For example, 350 pounds of sodium-sulfur battery in a
1350-pound commuter car should allow it to cruise at 40 miles an
hour for 200-300 miles, at 60 miles an hour for 125 to 200 miles,
and have a top speed of about 70 miles an hour. Such a car's per-
formance should be adequate in an urban-suburban environment.
The chief disadvantage of this battery, even for motive power
use, is the high temperature of operation. The battery is in-operative
at room temperature, so a cold start is impossible, unless auxiliary
heating is used. A large battery has a considerable heat capacity,
and the time required to bring it up to operating temperature with
any practical auxiliary system would probably be unacceptable except
for very infrequent occasions. However, the solution to this problem
is to keep the battery hot all the time. This should present no
problem for an urban-suburban vehicle that will be garaged almost
286 Sodium-Sulfur Secondary Battery
-------
every night for overnight charging anyway. Indeed this has some
advantages since the car would require no warmup period and
would be ready for instant starting at all times. There is no
problem during operation, since internal power dissipation in the
battery is more than sufficient to keep it hot. Indeed, cooling will
undoubtedly be necessary- There will be, of course, occasions when
the car will not have access to charging facilities, perhaps for
several days or even a week or more, and here it should be very
simple to arrange it so the battery uses its own energy to keep itself
hot for such periods of time and still have enough left over for
operation of the car. The minimum charging time for the battery
might be as short as a half hour, depending on the cooling facilities.
Clearly, the question of safety must be faced. There is little
danger arising from an internal breakage in the battery with a
consequent mixing of the reactants. As has been mentioned, there
are no gaseous products involved at all, and under certain circum-
stances, direct reaction between sodium and sulfur is self-attenuating
because the product, sodium sulfide, provides a barrier between
the liquid reactants. The one thing that must be prevented, of
course, is the escape of hot sodium and sulfur from the container
in the event of an accident, and proper packaging is the answer
to this.
REFERENCES
Pearson, T. G., and P L. Robinson, 1930. The polysulfldes of the
alkali metals. I. Sodium. J. Chem. Soc. (London). 1473-97.
Beavers, C. A., and M. A. S. Ross, 1937. The crystal structure of
(beta alumina) Na,OllAl,O,. (In English). Z. Krist. 97:59-66.
DEWITT 287
-------
LITHIUM NICKEL-HALIDE BATTERIES
R. C. Shair, A. E. Lyall,
and H. N. Seiger
Gulton Industries, Inc.
Metuchen, N. J.
INTRODUCTION
The battery industry is partial to a term called the theoretical
energy density. This is calculated by taking stoichiometric amounts
of the anode and cathode materials in their charged state. Then we
look up the Gibbs free energy for the reactions and calculate the
theoretical voltage of the system. Next, we use both the free energies
and the equivalent weights to obtain the theoretical watt-hours per
pound. Unless the electrolyte is required in balancing the chemical
reaction, it will not be used in the weight calculations. To have a
practical working battery, a case is needed, and so are grids to hold
the active materials, tabs to connect plates to terminals, and a
separator that prevents electronic short circuits while holding the
electrolyte that supplies the electrochemical conductivity. The weight
of these items cannot be readily calculated, but practical batteries
have watt-hour per pound ratios that are about 20 percent of the
theoretical values. So let us consider the theoretical value as a
Figure of Merit for comparison purposes, and bear in mind that
20 percent of these numbers are realistic.
The present battery systems have been pushed rather far, and it
is difficult to visualize how the watt-hour per pound ratios can be
increased much more. Significant increases in energy-to-weight ratios
must come from systems that have not previously been reduced to
practice as batteries. The first step is a tabulation of materials that
may serve as anodes and materials that may serve as cathodes.
Lithium is the first metal encountered in the table of electromotive
force, and it is the lightest metal. Between its half-cell potential and
equivalent weight, coupling it with almost any cathode makes an at-
tractive system. Lithium supplies the energy, and the cathode supplies
the weight.
Because lithium is so energetic, it is, as a consequence, extremely
reactive with water, oxygen, hydrogen, and even organic liquids that
contain active hydrogen. As an example of the latter, lithium reacts
with ethyl alcohol to form lithium alcoholate, a reaction that proceeds
with the release of hydrogen gas.
This paper is essentially a progress report on our efforts to de-
velop a battery using lithium as the anode and the nickel halide as
the cathode. The fluoride system has a theoretical energy-to-weight
ratio of 620 watt-hours per pound, which I now feel obliged to divide
SHAIR, LYALL, SEIGER 289
-------
by 5 to obtain 120, and the chloride system has a theoretical value
of 437 watt-hours per pound, a fifth of which is 87.
ELECTROLYTE
Since lithium is so reactive, the first thing one must do is find
a liquid medium with which it is not spontaneously reactive. But
the list of required criteria is quite long. The electrolyte consists of
a solvent and at least one solute. If only one solute is used, then it
must supply the ions that migrate under the electric field, and these
ions must also participate in the charge transfer steps at the elec-
trode and electrolyte interface. If two solutes are used, the one that
does not participate in the charge transfer process at the interface
is called the supporting electrolyte. The supporting electrolyte merely
carries the current in the electric field. The fraction of the current
carried by each kind of ion is its transport number.
In Table 1 are listed some non-aqueous solvents of interest. Five
of these contain a double-bonded oxygen, and the sixth, acetonitrile,
is also an unsaturated compound containing a triple bond-nitrogen.
The dielectric constants for the double-bond-oxygen compounds
range from 38 to 64 and that for acetonitrile is 38.
Solvent
Dimethyl sulfoxide
Nitromethane
o-Butyrolactone
N-methyl-2-pyrrolidone
Acetonitrile
Propylene carbonate
GO
&%
1. 1
.2 •=
:> QJ
0
1.98
0.62
1.75
1.65
0.345
2.53
^V
c
? !E
s <=>
"^ 0.
18.45
—29
-44
—24
—41
^49
c_p
00 o
c
"I =
CD 0
0.
189
101
206
202
82
241
O
-t C
o J2
.32 to
QJ =
.2 S
0
46.7
38
39.1
45.4
38.3
64.4
Specific conductivity,
millimhos/cm
LiCI
9.91
31.8
0.008
0.16
—
0.34
Aid,
0.37
33.3
4.4
—
—
4.5
KPFe
10.2
5.91
—
—
12.0
7.2
NaPF,
10
12.3
13.2
8.2
43.0
7.5
The boiling points and melting points of these solvents are im-
portant because they define the temperature range over which the
battery may operate. This temperature range is widened, however,
by the freezing point depression and the boiling point elevation that
come about upon dissolution of salts.
The solute must dissolve in the solvent, naturally, and it must
ionize. The goal here is to have a high conductivity. There must be
an anion that is the same as the anion in the electroactive species
in the cell. As mentioned before, these requirements may be met by
using two solutes instead of one.
Candidate electrolytes were originally tested for compatibility
with the electrode materials. These tests were run by placing the
electrode and the electrolyte in sealed Kel-F containers for periods
up to 6 months. Two electrolytes have done well in these posts, thus
far — propylene carbonate and dimethyl sulfoxide. The solute was
290 Lithium Nickel-Halide Batteries
-------
potassium hexafluorophosphate for both. The concentrations of solute
taken at the maximum conductivity are:
Propylene carbonate 90g/l KPF(; 7.2 x 10~3 mho/cm
DMSO 190g/l KPF0 11.6xlO-3 mho/cm
Dimethyl sulfoxide is also a solvent for nickel chloride and thus can-
not be used because there will be a self-discharge process at the
lithium electrode. When the propylene carbonate KPFB is used with
this chloride system, LiCl is added to the electrolyte, a specific ex-
ample of the use of two solutes.
LITHIUM ANODE ELECTRODE
When an electrode is fabricated, it must possess electrical con-
ductivity in both the oxidized and reduced states. This statement is
equally true for both anodes and cathodes. Many of the oxidized
materials commonly used are found to be semiconductors. Other
electrodes may have only thin films, or there may be an intimate
mixture; the graphite and flake nickel in Edison storage batteries is
an example of such mixture.
A pasted lithium electrode was developed with the paste con-
sisting of a mixture of powdered lithium and conductive granular
carbon, a binder and a vehicle.
The paste was spread onto the substrate in a dry box. It was
then pressed, after which the solvent was removed by heating in a
vacuum oven and the vehicle leached out.
The electrical capability of an electrode is shown by a polariza-
tion diagram as depicted in Figure 1.
The electrode was discharged against a counter electrode. A ref-
erence electrode, mercury/mercuric chloride, was used to monitor the
voltage of the lithium electrode, while a motorized potentiometer
varied the current through the electrode. The electrodes used had an
area of 10 square centimeters. The charge region is to the right of
zero current, and the discharge behavior is shown on the left of the
origin. The maximum current density shown corresponds to 65
milliamperes per square inch. The attendant polarization was 0.1
volt. In a conventional nickel-cadmium battery, 65 milliamperes per
square inch is equal to about the 1-hour discharge rate.
The lithium anode is far from the limiting factor in this cell,
with regard to performance, and we are not attempting any further
improvements in performance at this time. Our concentration of
effort is on the cathode materials.
NICKEL HALIDE CATHODE ELECTRODE
A survey of cathode materials to be used with a lithium anode
reveals several attractive anion materials. A few selected materials,
SHAIR, LYALL, SEIGER 291
-------
their voltage as a lithium cell, and their total energy content in watt-
hours per pound, are listed in Table 2.
Table 2. THEORETICAL CELL POTENTIAL AND ENERGY CONTENT OF
SELECTED LI — CATHODE COUPLES
Li Cell
athode potential,
actant Product volts
"2 F- 6.05
:i2 ci- 4.02
)uF2 Cu + 2F- 3.55
;uCI2 Cu + 2 CI- 3.06
•JiF2 Ni + 2F- 2.83
-------
At the beginning of the program we were ready to drop NiF2 because
anything more than an electrometer brought the voltage of a cell to
zero rather quickly.
The nickel chloride electrode was more favorable, and a pasted
electrode similiar to the lithium electrode was developed. The initial
current densities achieveable were low, however. Investigations were
directed at doping the nickel halide, improving the conductive di-
luents, and improving the fabricating techniques to get higher current
densities. The techniques developed when applied to the NiF2 elec-
trode made an attractive electrode. A current density of 18 milli-
amperes per square inch, equivalent to the 5-hour rate of discharge,
has been achieved.
CELLS
Cells have been constructed using the pasted lithium and nickel-
fluoride formulations. The active area for 20 ampere-hour cells is
45 square centimeters per electrode side, and eight lithium electrodes
and nine nickel-fluoride electrodes were incorporated into each cell.
The separator was a 60 percent porous sheet of non-woven poly-
propylene. The electrolyte was propylene carbonate with KPF6
added. These cells have the energy content of two nickel-cadmium
cells that weigh 4 pounds compared to the 0.5 pound per lithium cell.
The contribution of electrolyte to the cell's internal resistance was
calculated to be 0.027 ohm. Pulse data such as shown in Figure 2
show an internal resistance of 0.35 ohm. The cell pulsed at 2 amperes
shows a terminal voltage of 2.5 volts. At 4 amperes, the C/5 rate
cell voltage is 2.0 volts.
Several small laboratory cells were built and cycled unsealed
in an argon atmosphere glove box. They were discharged at the
C/10 rate and recharged at 3.6 volts. Figure 3 shows cycle data after
2 and 20 cycles.
DISCUSSION
The feasibility of the lithium-nickel halide battery has been
demonstrated. As a low-rate cell or as a primary cell, its performance
is very attractive. Recent development has been directed at achieving
higher rates of discharge suitable for electric vehicles. At the C/5
rate of discharge, which has been realized, the battery can provide
power for cruising of a vehicle and its energy density of 100 watt-
hours per pound provides an attractive mileage range. Its present
limitation is its power density of 50 pounds per kilowatt, which re-
quires too large a battery to provide acceleration peaks.
Bipolar nickel-cadmium batteries have been developed in our
laboratories. Because of their unique geometry and construction,
these batteries have a weight to power ratio of 3 pounds per kilowatt
for short periods of time — of the order of minutes.
SHAIB, LYALL, SEIGER 293
-------
Q.
E
uJ 3
cc
cc
Z3
o
Q 2
o
-OCV
TIME
Figure 2. Current voltage trace for 20-ampere-hour
lithium cell at 18 milliamperes per square inch.
4
LJ
< 3
—' o
0 2
1
---CYCLE 2
— CYCLE 20
|
I
Figure 3.
TIME, hours
Discharge data for lithium cell
A small bipolar nickel-cadmium booster battery coupled in
parallel with the lithium battery can provide the necessary peak
power. It can be small compared to the lithium battery, since the
duration of acceleration peak loads is short, and during cruising,
the bipolar battery can be recharged in minutes from the main
battery. Bipolar nickel-cadmium battery technology is well-advanced.
Let us look at the application of the lithium battery to electric
vehicle propulsion. A vehicle weighing about 3,000 pounds requires
about 5 horsepower to cruise on level road at 30 mph. For grades
and acceleration peaks, it would require up to 15 horsepower. If we
294
Lithium Nickel-Halide Batteries
-------
assume approximately a 75 percent conversion of electrical power to
mechanical power, then the same number of kilowatts can be used
as indicated above for horsepower. For cruising at 5 kilowatts a
lithium battery with a weight-to-power capability of 50 pounds per
kilowatt would have to weight at least 250 pounds. At 100 watt-hours
per pound, it would be capable of 5 hours of operation, which at
30 mph is 150 miles. To provide the 15-kilowatt peaks would require
about 45 pounds of bipolar nickel-cadmium battery at 3 pounds per
kilowatt. Since 5 kilowatts would still be coming from the lithium
battery, 45 pounds allows some margin of safety and the peak would
be available for several minutes if necessary.
The non-aqueous lithium-nickel halide battery coupled with a
bipolar nickel-cadmium booster has attractive features for vehicular
propulsion. Further work is in progress to optimize the lithium system
and to evaluate its capabilities.
SHAIR, LYALL, SEIGER 295
-------
A STATE-OF-THE-ART
AUTOMOTIVE FUEL CELL
Philip Dantowitz
Monsanto Research Corporation
Everett, Mass.
Lonnie Gaddy
U. S. Army Engineer Research and Development Laboratories
Ft. Belvoir, Va.
INTRODUCTION
Interest in the electric automobile has recently been widely re-
vived. In most of the articles published on the subject, the lead-acid
battery is examined as a source of electrical energy for the vehicle.
But all agree that the lead-acid battery is marginal at best, its basic
shortcoming being the limited range it affords the vehicle at any
reasonable battery weight. Attention has naturally turned to supply-
ing electrical energy by other means, and among these the fuel cell
is almost invariably proposed.
Little factual data exist on large, air-breathing fuel cells suit-
able for automotive use. This paper describes one existing fuel-cell
automotive power plant to allow a comparison with other sources of
energy, such as the lead-acid battery.
THE 5-KILOWATT FUEL-CELL MODULE
Monsanto Research Corporation in cooperation with the U. S.
Army Engineer Research and Development Laboratories has de-
veloped a hydrazine-air fuel cell module that when used in multiples
provides power to drive an M-37 Army truck. The fuel cell module
is designed so that fuel dissolved in the electrolyte can be circulated
through the anode compartments. The circulation rate is high enough
that the electrochemical fuel consumption hardly changes the fuel
concentration, even at high load. This circulating liquid also pro-
vides the means for cooling the fuel cell. The module consists of 140
cells spatially arranged so that anode faces anode (as opposed to a
bi-polar scheme). The anodes are connected electrically in parallel
by a metallic grid; the current is collected and brought to the edge
of the electrode. Similarly, adjacent cathodes face each other, but
in operation they are at different potentials, typically being 0.78 volt
apart. An insulating plastic grid is incorporated in the cathode com-
partment. As with the anodes, the cathode current is collected and
brought to the edge of the electrode. External electrical connections
DANTOWITZ, GADDY 297
-------
are made so that each pair of cells (one cell being one cathode and
one anode) is in series with the next, and appropriate cathodes are
in parallel. Figure 1 shows a schematic diagram of the arrangement
and the external electrical connections. The module consists of two
side-by-side decks of 70 cells each, connected in parallel electrically
and clamped between common end plates. At rated load each cell
is designed to deliver 0.78 volt. Since each pair of cells is wired in
parallel, the rated voltage of the module is:
70
— x (0.78) = 27.3 volts
EXTERNAL PARALLEL
CONNECTIONS FOR
CATHODE
NONCONDUCTING GRID
IN AIR PASSAGE
ANODE
SEPARATOR
CATHODE
Figure 1.
CONDUCTING GRID IN
ELECTROLYTE/ANODE
COMPARTMENT
Hydrazine-air fuel cell module with
external connections.
298
Automotive Fuel Cell
-------
Each electrode has an active area of 62 square inches and is rated
at 44.5 amperes at a rated current density of 103 amperes per square
foot. Two cells in parallel deliver 89 amperes, which is the current
carried by each deck. Twin decks in parallel carry 178 amperes at
28 volts and yield a 5-kilowatt rating. The module is shaped like a
box with overall dimensions of 9 by 9 by 22 V2 inches and a weight
of 95 pounds with electrolyte (Figure 2). At the 5-kilowatt rating,
the power density is 4.38 kilowatts per cubic foot, and the specific
weight is 19 pounds per kilowatt. Under design conditions about
700 watt-hours are generated per pound of hydrazine monhydrate
(about 90^, of ideal). Figure 3 shows the average single-cell polar-
ization curve, based on module tests.
. *
Figure 2. Five-kilowatt hydrazine-air fuel cell module.
At rated power the average single-cell voltage is 0.78 volt, which
is about 48 percent of the thermodynamically reversible voltage for
the hydrazine/oxygen couple. Thermal efficiency at this condition
is about 50 percent, based on the higher heating value of hydrazine.
THE POWER PLANT IN THE M-37 ARMY TRUCK
The power plant consists of three major elements: the fuel cell,
with its controls and accessories; the electric motor; and the motor
DANTOWITZ, GADDY
299
-------
0.90—< - —>
103 amp/ft2
CURRENT DENSITY, amp/ft-'
Figure 3. Average single-cell polarization curve.
40-kw
HYDRAZINE
HYDRATE
FUEL CELL
P.C.M.
CONVERTER
STANDARD D-C
SERIES
MOTOR
VARIABLE
D-C
VOLTAGE
0 to 375
o
X
bj
VARIABLE
TORQUE
§5
< ""
co Q
PROTECTION AND
CONTROL LOGIC
VEHICLE WEIGHT 6,000 Ib
CARGO CAPACITY 1,800 Ib
TOTAL 7,800 Ib
FUEL TANK
Figure 4. The power plant for the M-37 army truck.
300
Automotive Fuel Cell
-------
and vehicle control system. Figure 4 shows a schematic block dia-
gram of the power plant.
At present, the fuel cell is made up of four of the 5-kilowatt
modules described above with a full load rating of 20 kilowatts. The
modules are mounted on a bedpost frame, which in turn is shock-
mounted on the truck chassis under the hood (Figure 5). Although
four modules have been used thus far, space for eight has been pro-
vided and much of the system is designed for eight-module power
of 40 kilowatts. The modules are connected in series, giving a full-
load voltage of over 100 volts for the four-module system and over
200 volts for the eight.
Figure 5. Four 5-kw hydrazine-air fuel cell modules
installed as power plant for M-37 truck.
The installed fuel-cell-plus-accessory weight is 525 pounds for
four modules and 915 pounds for eight. Four modules weigh about
340 pounds dry. The weight of electrolyte required to charge the
system is 100 pounds. The radiator, which is an air-to-electrolyte
heat exchanger, weighs about 45 pounds. The latter is a finned, tube
radiator, very similar to those found on many American cars. To
enhance ram-air cooling, an electric-motor-driven fan is provided.
Fuel is supplied intermittently by a pump to the electrolyte tank.
The "on" versus "off" time of the fuel flow determines the average
DANTOWITZ, GADDY
301
-------
quantity of fuel delivered. This in turn is regulated by the voltage
on the terminals of one of the modules. Furthermore, the control
voltage is a function of current and is highest at no load. This ensures
good transient performance following the sudden application of a
large load despite the inevitable dip in voltage. Parasitic power drain
for the 40-kilowatt system is about 600 watts.
A direct-current traction motor is mounted under the truck
body forward of the differential that it drives. The motor is rated
at a stall torque of 625 foot-pounds; starting current is 525 amperes.
The latter is a short-time rating only. The motor weighs 700 pounds,
and is being used at present only because of its availability. To do
the same job, a motor weighing only 300 pounds has been ordered
and will very shortly be installed in the truck.
The controller for the motor is mounted under the driver's seat.
The basic purpose of the controller is to convert the voltage and
current output of the fuel cell to that required by the motor. The
controller ensures that neither the operator demand nor motor load
shall exceed the power capability of the fuel cell. Figure 6a shows
how motor current, voltage, and power very as a function of
vehicle velocity following a full-throttle start from a standstill.
Figure 6b shows the fuel cell voltage and current on the same
scale. In the span marked A the controller limits the current to
the motor to 525 amperes. Since the short-circuit current of the
fuel cell is about 2,500 amperes, protection is clearly required.
As the vehicle and the motor pick up speed, motor voltage is
increased to maintain the current limit. At the end of span A,
the motor power is equal to 40 kilowatts, which is equal to the
peak fuel cell power (for the eight-module installation). During
the span marked B the vehicle continues to accelerate and the
motor back electromotive force continues to increase, requiring
a higher voltage and a lower current to maintain constant power.
Note that during span B the motor voltage is first below and then
above the fuel cell voltage, requiring a controller with both step-
down and step-up capabilities. At the beginning of span C the
vehicle has reached its maximum speed and no further change in
voltage, current, or power occurs. The present controller weighs
300 pounds, but a redesigned version that weighs 150 pounds will
soon be installed.
The peak acceleration of the vehicle equipped with four
fuel cell modules and a supplementary lead-acid battery pack is
compared with that of a truck equipped with the standard 94-
horsepower gasoline engine in Figure 7. With only 20 kilowatts
of fuel cell capacity the truck readily climbed grades of 20 percent.
The maximum speed on a good level road was 47 mph. Fuel
consumption was between 10 and 15 miles per gallon of hydrazine
menohydrate; that of a standard M-37 vehicle is 7 miles per gallon
of gasoline. The reliability of the fuel cell has been good in the
120 hours it has been used.
302 Automotive Fuel Cell
-------
375 volts
(b)
224 volts
180 amp
VEHICLE VELOCITY
Figure 6. (a) Motor current, voltage, and power as
functions of vehicle velocity, and (b) fuel cell voltage
and current as a function of velocity.
DANTOWITZ, GADDY
303
-------
UJ
Q.
94-hp GASOLINE ENGINE
WITH GEAR SHIFT
TIME
Figure 7. Comparison of acceleration of electric-
and gasoline-powered M-37 truck.
SIGNIFICANCE OF THE PROJECT
As installed in the truck the hydrazine-air fuel cell system
(excluding the motor and controller) gives a very high energy
density, nearly 350 watt-hours per pound for an 18-hour mission.
For an eight-hour mission the energy density drops to 210 watt-
hours per pound, still much higher than is offered by any other
readily available system. The successful operation of the M-37
truck with a system having such a high power density must be
considered an important step forward in the practical application
of fuel cells.
Using the hydrazine-air system, the Army has gained valuable
experience in the operation and control characteristics of an electric
vehicle. The relative simplicity and ease of operation of this large
hydrazine-air fuel cell power plant has been demonstrated by over
100 operating hours in the truck.
FUTURE DEVELOPMENTS
We have seen that a start has been made in applying fuel
cells to automotive use. To make further progress, research and
development efforts must be continued and intensified. These pro-
grams have been split into short-range and long-range efforts for
which successful completion is visualized in 2 and 5 years, respec-
tively.
304
Automotive Fuel Cell
-------
The highest priority short-range program is the minimization
of the use of platinum in the electrodes. With two 5-kilowatt
modules as they presently exist, and reasonable reclamation and
capital costs for the platinum, annual expenditure of well over
$500 is incurred by the use of precious metal. This figure probably
must be reduced to around $100 per year for wide acceptance by
the public. Monsanto has already demonstrated the feasibility of
achieving such a light platinum loading. Much work remains to
be done, however, to optimize the platinum loading and to reduce
the resulting electrode to a practical, reliable, and durable component.
Improvement of reliability and durability of the fuel cell is
second on the priority list. In concept the fuel cell promises to be
highly reliable and very durable, matching and ultimately exceeding
the lead-acid battery in these respects. Although the fuel cell of
today has not lived up to this promise, in a relatively short time
great improvements have been made. Most first-order failure mecha-
nisms have been identified and eliminated so that 1,000 hours of
life is common on a single cell. Since a practical fuel cell battery
contains 30 or more cells in a series, the problem of battery life
is more severe than that of a single cell. Consequently, much work
needs to be done to identify second-order life-limiting phenomena
and to find ways to extend life and reliability. A goal of 1,000
hours of stack life is reasonable for the short term, since it would
give the average motorist 2 to 3 years of operation.
Third priority for the short term is reduction of specific weight.
Engineering studies have shown that a 10-kilowatt fuel cell module
weighing only 75 pounds can be developed in the short term.
Although optimistic, this goal can be reached by the application
of sound engineering principles in a disciplined design effort.
If all three of these goals are met, an attractive first-generation
fuel-cell power plant will result. An annual market of 25,000
units of 10 kilowatts each could very well be captured by such a
machine. A 2,000-pound vehicle so equipped would use about a
pound of hydrazine for every 5 miles. If each vehicle traveled
an average of 15,000 miles per year, the total annual consumption
of hydrazine would be 75 million pounds per year at the end of
the first year and 150 million pounds per year at the end of the
second. Without a doubt, production rates this high would bring
the price of hydrazine down. How far down is a matter of specu-
lation, but it appears possible that with a high enough demand the
price could be reduced to a level roughly competitive with gasoline
prices. The retail hydrazine sales volume would be several mil-
lion dollars a year, sizeable enough to warrant appreciable plant
investment.
Because the early realization of all the goals just described is
unlikely, only an extreme optimist would cite such a probability.
The main point is, nevertheless, that the fuel-cell-powered vehicle,
even for the short range, remains a possibility.
DANTOWITZ, GADDY 305
-------
If the hydrazine industry does not meet the challenge of
achieving a competitive price, a cheaper fuel will have to be
found. Much of the work done on the hydrazine system will be
applicable because the air cathode will still be used. A possible
alternative to hydrazine is hydrogen obtained by on-board con-
version of one of the following: saturated hydrocarbon, methanol,
or ammonia.
In each case the annual cost of fuel for 15,000 miles of travel
would be less than $300 (a 2,000-pound vehicle dissipating 160
watt-hours per mile at the wheels). Production of hydrogen by any
of the three methods offers no scientific obstacle. That is, processes
and materials such as catalysts are well defined and have been
reduced to practice. It remains to engineer the best of them into
a practical, lightweight hydrogen generator for use in an automobile.
Logistic considerations allow only liquid saturated hydrocarbons to
be considered for widespread military use.
In moderate quantities, production of fuel cells with minimized
platinum loading could be easily sustained by present production
of the precious metal. If the fuel cell is to replace the internal
combustion engine to a significant degree, however, then a long-
term goal must be elimination of the use of platinum-group metals
in the fuel cell.
Finally, the ultimate objective of every fuel cell researcher
is the direct oxidation of a low-cost liquid fuel. It has been demon-
strated that under the proper circumstances saturated hydrocarbons
can be electrochemically oxidized at appreciable rates. Thus far,
however, this has been accomplished by using far more platinum
(and other platinum-group metals) than can be tolerated for the
application under discussion. Furthermore, the power density of
the cells produced has been very low, and therefore power plants
utilizing them would be expensive, heavy, and bulky. Research
continues on direct oxidation of liquid saturated hydrocarbons to
reduce the platinum loading and to increase the power density,
and thus decrease the cost, weight, and size of the resulting power
plant. Many are convinced that for nonmilitary use a more timely
consummation of a practical fuel cell can be achieved by using a
fuel that is more active electrochemically than the saturates. The
two most important candidates are methanol and ammonia. These
fuels are reasonably priced for automotive use; both are plentiful
and can be readily stored and distributed.
It has been demonstrated that fuel cells for automotive use
are a possibility. The chances for success depend upon the level
of support the research and development effort gets. A vigorous
and determined program could very well make the fuel cell automo-
bile a reality in the 1970's.
306
Automotive Fuel Cell
-------
DIRECT HYDROCARBON AND METHANOL-AIR
FUEL CELLS
C. E. Heath, E. H. Okrent,
M. Beltzer, and G. Ciprios
Esso Research and Engineering Company
Linden, N. J.
The fuel cell has been proposed as a potential solution to the
problem of automotive exhaust emissions. Such a power plant would
not be subject to the limited range of battery vehicles and would
serve a broader spectrum of transportation needs. The specific re-
quirements for an automotive fuel cell power plant have not been
defined; however, as a first approximation a fuel-cell-powered vehicle
should at least approach the performance capability of the conven-
tional vehicle. For simple control, reliability, ease of start-up, and
adaptability to a variety of driving conditions, system complexity
should be avoided. Thus, direct fuel cells are preferred to indirect
systems. Although high efficiency will reduce operating costs, this
economic advantage would be quickly offset by a fuel cell that con-
tains costly components and materials. A practical fuel cell power
plant should use a readily handled, inexpensive fuel and air as the
reactants. Finally, if the fuel cell is to provide a solution to exhaust
emissions, the fuel cell reactions should yield inoffensive products;
unconverted fuels and intermediates should be contained.
Fuel cell research at Esso Research, although primarily aimed at
the development of military field generators, has identified some of
the problems that must be solved before fuel cells can meet these
requirements. This program includes work on direct liquid hydro-
carbon and methanol-air fuel cells. Both fuels satisfy the require-
ment of a readily handled inexpensive fuel, but the higher cost per
Btu of methanol places a greater premium on high efficiency than
necessary for hydrocarbons.
Substantial progress has been made on the development of hydro-
carbon fuel cells (Figure 1). Since 1959 when direct electrochemical
oxidation of ethane was first demonstrated in our laboratories (Heath
and Worsham, 1963), hydrocarbon fuel cell power density has been
dramatically increased. A modified Bacon cell operating on ethane and
oxygen sustained 1.5 watts per square foot at 400°F and 20
atmospheres. The voltage was 0.2 volt. In 1966, an octane-air fuel
cell, operating at 525°F and atmospheric pressure generated 90 watts
per square foot at 0.5 volt. The early porous electrode required a
complex differential pressure control mechanism to maintain the fuel-
electrolyte-electrode interface. Current electrodes, based on a cata-
lyst-Teflon matrix, are not flooded and can be designed for either
HEATH, OKRENT, BELTZER, CIPRIOS 307
-------
vapor or liquid reactants. Finally, the early work used a 30 percent
KOH electrolyte, which obviously would not remain invariant. The
present cells all use CO2-rejecting acid electrolytes.
l.Or
0.8
0.2
1967
OCTANE-AIR
PYROPHOSPHORIC ACID
268°C, 1 atm
1959
ETHANE-02
KOH
208°C, 20 atm
I
100 200 300
CURRENT DENSITY, ma/cm2
400
500
Figure 1. Progress in hydrocarbon fuel cells.
These performance levels have been achieved under conditions
that ensure rapid electrochemical oxidation and high current densities
— heavy catalyst loadings and temperatures of 500°F. The fuel elec-
trode catalyst was platinum at 50 milligrams per square centimeter.
The catalyst loading can be reduced by a carbon support, but at the
expense of some performance. For example, a butane-air fuel cell
with total catalyst loadings of 3.2 milligrams per square centimeter
for both electrodes yielded 53 watts per square foot. The temperature
was 525°F. Thus, catalyst utilization was increased from 0.7 to 17
milliwatts per milligram. Other supports have been investigated,
but show no major advantage over carbon.
High power densities can be obtained only at temperatures above
300°F. Although phosphoric acid is suitable up to 300°F, new electro-
lytes are needed for higher temperatures. Two new electrolytes now
under development make it possible to reach increased power levels.
One, pyro-phosphoric acid, is similar to the aqueous systems investi-
gated in the past and does not require special electrodes. However,
it is highly corrosive and new materials are needed. The other, mixed
alkali phosphates, is not corrosive, but because of its viscosity will
require development of specific electrodes. The current performances
of these and the conventional electrolytes are summarized in Table 1.
308
Hydrocarbon and Methanol-Air
-------
Table 1. HYDROCARBON FUEL CELL PERFORMANCE
IN DIFFERENT ELECTROLYTES"
Electrode polarization from
theory,b v at 100 ma/cm2
Electrolyte Temperature, °F Butane Oxygen
H,S04
H3P04
Pyrophasphoric acid
Mixed alkali phosphates
205
300
480
480
30 ma/cm2 max.
0.35
0.18
0.27
0.35
0.30
0.08
0.18
aThese data are based on runs of several hours. During prolonged operation
electrode structure can deteriorate through thermal stressing.
bPlatinum catalyst loading: 50 mg/cm2 at each electrode, except 0., in
H,S04 (20 mg/cm2).
Obviously, a fuel cell power plant will consist of more than the
basic module. The hydrogen-oxygen automotive fuel cell power plant
recently demonstrated by General Motors contained 1,485 pounds of
auxiliaries, almost twice the weight of the conventional engine (SAE,
1967). A decane-air battery system was developed and constructed
in our laboratories to determine systems problems that must be faced
in a direct hydrocarbon fuel cell battery (Okrent and Heath, 1967).
This system included auxiliaries for water balance, fuel and air circu-
lation, electrolyte concentration control, and other necessary func-
tions. Although this system was not engineered as a prototype, this
study confirmed that auxiliaries would represent a substantial portion
of a practical power plant.
The study also indicated the problems that would be faced in
using practical hydrocarbon fuels. The electrodes were designed for
use with decane. Although equally effective for vapor and liquid
phase fuel, the electrodes are sensitive to molecular weight. Low-
molecular-weight hydrocarbons will diffuse through the structure into
the electrolyte chamber. When a wide-boiling-range fuel is supplied,
some light ends will cross to the cathode, give rise to a mixed poten-
tial, and reduce cell voltage. This effect was illustrated by experi-
ments in a three-cell hydrocarbon battery in which decane, the re-
search fuel, was replaced with a commercial jet fuel, OTF-90 (Sup-
plied by Humble Oil and Refining Co.). This fuel is a low-sulfur
isoparaffmic fuel with a boiling range of 195° to 290°C. As shown in
Figure 2, performance declined to 30 percent of that of decane. In-
spection of the cells indicated that the fuel had diffused through the
anode.
Based on these studies, preliminary projections of complete
hydrocarbon-air fuel cell power plants can be made. Projected
HEATH, OKRENT, BELTZER, CIPRIOS 309
-------
weights can be compared with the weight of the total fuel cell power
plant used in the General Motors Electro van (Table 2). This power
plant was rated at 21 pounds per kilowatt. The hydrocarbon-air fuel
cell, if built with today's technology, would clearly exceed the weight
and volume of the hydrogen-oxygen battery, which itself was ex-
cessively heavy. The noble metal catalysts alone preclude economical
application of the direct hydrocarbon fuel cell as an automotive power
plant. The search for active non-noble metal substitutes has been
under way for 5 years. Many leads have been followed to extinction;
new leads are being uncovered. The nature of catalyst research makes
it impossible to predict when a suitable material will be found.
Methanol is considerably more active than hydrocarbons and
might be considered for this application. Much progress has been
made in improving this activity (Figure 3). Methanol fuel cells have
yielded power densities as high as 50 watts per square foot at 160°
to 190°F in short laboratory tests. However, it should be emphasized
that deterioration occurs and further research will be required to
maintain this performance. The low temperature activity would make
the methanol cell easier to start than the hydrocarbon cell, but it is
inadequate for an automotive power plant. Attempts to improve the
power density significantly further by increasing cell temperature
have been unsuccessful thus far, although the approach deserves more
consideration.
DECANE
MAXIMUM POWER 3.0 watts
MAXIMUM POWER
1.1 watts
COMMERCIAL JET FUEL
(OTF-90)
1.0 1.5
CURRENT, amperes
Figure 2. Wide-boiling-range fuels reduce
fuel cell performance.
310
Hydrocarbon and Methanol-Air
-------
Table 2. COMPARISON OF WEIGHTS OF COMPLETE HYDROCARBON
FUEL CELL SYSTEMS, ELECTROVAIR POWER PLANT, AND
CONVENTIONAL ENGINES.
System power density,"
Ib/kw
Catalyst quantity,
g/kw
High-performance hydro-
carbon fuel cell systemb 50 to 70
Low catalyst loading
hydrocarbon fuel cell
systemb 65 to 90
GM Electrovan power plant 21
Conventional engine 8 to 12
1,600
60
"Fuel cell power plant densities do not include electric drive. GM system
drive weighed 3.4 Ib/kw.
bBased on fuel cell size. Higher power densities are based on 5-kw design,
lower on 30-kw systems.
T
H2S04 60 to 95°C
NOBLE METAL CATALYSTS
1967 SHORT-TERM
LABORATORY
40 60
CURRENT DENSITY, ma/cm2
100
Figure 3. Methanol — air fuel cell improved
but noble metal catalysts needed.
The question of whether the fuel cell can reduce air pollution
cannot be conclusively answered yet. Direct hydrocarbon cells, oper-
ating at temperatures below 500°F, appear to offer no serious source
of contamination. Studies have shown that hydrocarbons are com-
pletely consumed to carbon dioxide and water in sulfuric acid elec-
trolytes at 90°C. Fuel cell fuels will probably be sulfur and lead-free.
HEATH, OKRENT, BELTZER, CIPRIOS
311
-------
Furthermore, the low temperatures should minimize nitrogen oxide
formation. Hydrocarbon losses from manifolds and tanks may cause
a problem, but using low vapor pressure fuels and proper engineering
should minimize this contribution. No major obstacles to a low-
emission vehicle are known, but clearly much more experience will
be required to define pollution levels in operative systems.
From this discussion it is evident that the present state of hydro-
carbon fuel cell technology cannot meet the requirements of a high-
output, low-cost system suitable for extensive vehicular application.
Much progress has been made during the past 5 years. If this progress
continues and suitable catalysts are developed, a fuel-cell-powered
vehicle could become practical. However, much more research and
development will be needed before this stage is reached.
ACKNOWLEDGEMENT
Much of the work described in this paper was made possible
through the support of the U. S. Army Electronics Command and the
Advanced Research Projects Agency under Contracts DA 36-039
AMC-03743(E) and DA-28-043 AMC-02387(E).
REFERENCES
Heath, C. E., and C. H. Worsham, 1963. Fuel cells (Vol. 2). G. J.
Young, Ed. Reinhold, New York, N.Y.
Okrent, E. H., and C. E. Heath, 1967, A liquid hydrogen fuel cell
battery. Presented at Fuel Cell Symposium, American Chemical
Society National Meeting. Chicago, 111. September 10-15.
Society of Automotive Engineers, 1967 (Jan. 13). C. Marks, E. A.
Riskavy, and F. A. Wyezalek. Electrovan — a fuel cell powered
vehicle. SAE Paper No. 670176.
Hydrocarbon and Methanol-Air
-------
HYDROGEN-AIR FUEL CELLS
FOR VEHICLE PROPULSION
G. E. Evans
Union Carbide Corporation
Parma, Ohio
A very large variety of power sources have been proposed for
the future electric vehicle. In broad terms they can be divided
into two classes—rechargeable batteries and fuel cells. Rechargeable
batteries include conventional types such as lead-acid, nickel-
cadmium, and silver-zinc, and newcomers such as rechargeable
metal-air, sodium-sulfur, and lithium-halogen types. Fuel cells in-
clude the well-known hydrogen-air types, direct or indirect hydro-
carbon-air types, and a large variety of miscellaneous types burning
hydrazine or other even less common fuels.
In general, rechargeable batteries are characterized by high
power density (watts per pound) and low energy density (watt--
hours per pound). The converse is true of most fuel cells: they
offer high energy density, but usually at a sacrifice in power density.
Since Union Carbide Corporation manufactures both rechargeable
batteries and fuel cells, we can be impartial in judgment, and in
fact, what we will propose is a hybrid system: a fuel cell with a
very high energy density to provide long driving range and rapid
refueling, and a rechargeable battery to supply peak loads.
When discussing a proposed power source, it is important to
define and weigh the requirements, so as to avoid the danger of
sacrificing life for low initial cost, or safety for power density.
The following objectives or performance goals for a vehicular power
supply have been used in arriving at the proposed system; not
necessarily in order of importance:
1. High power density, e.g., at least 40 watts per pound;
2. High energy density, e.g., at least 40 watt-hours per pound.
3. Freedom from harmful emissions.
4. Low initial capital cost, rather arbitrarily set at a maximum
of twice the price of an internal combustion engine vehicle.
5. Low fuel cost, comparable to the price of gasoline.
6. Low maintenance costs.
7. Long life—as a yardstick, the power plant should last at
least as long as the body.
8. Safety; last but not least, the electric vehicle should provide
greater safety on the road than the present gasoline vehicle.
EVANS 313
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A package consisting of a hydrogen-air fuel cell, using cryo-
genic hydrogen as fuel, and a rechargeable battery for peak power
appears to be a leading contender for meeting these requirements
in the near future. The operational characteristics of the hydrogen-
air system make it more suitable for industrial or commercial
vehicles—delivery trucks and vans, city buses, taxicabs, etc.—than
for the private automobile. Thus, we visualize the hydrogen-air
fuel cell as a practical power source for industrial and commercial
fleets of vehicles within the near-term future, whereas improved
rechargeable batteries may more readily meet the needs of lightweight
electric vehicles for use as private automobiles under short-range,
urban driving conditions.
In the following discussion, the present status and future poten-
tial of hydrogen-air fuel cells will be discussed in the light of the
above eight objectives. It is important to remember that these eight
objectives must be met simultaneously: there is little merit in a
system that offers high power density at the risk of safety or long
life at the expense of exorbitant cost.
CONCEPTUAL DESIGN
Many of the advantages of the hybrid hydrogen-air fuel cell
can be made more explicit by considering an actual vehicular system.
For this purpose we have started with the General Motors Electro-
van as a baseline. The performance of this vehicle, as well as the
detailed weight and volume breakdown of its components, has been
fully documented in a series of papers presented to the SAE Auto-
motive Engineering Congress this past January. As a short-term,
state-of-art approach, we have simply looked at the problem of
replacing the hydrogen-oxygen fuel cell of the Electrovan with a
hydrogen-air fuel cell plus a rechargeable battery.
Based on Electrovan performance data, a hybrid fuel cell sys-
tem has been sized to provide 32 kilowatts of continuous power
and 160 kilowatts of peak power. A weight breakdown of the hybrid
system is given in Table 1.
In SAE Paper No. 670176, Marks et al. (1967) mention that a
"production model" Electrovan could weigh at least 1000 to 1500
pounds less than the prototype model, by elimination of experi-
mental instrumentation and through normal engineering optimization.
Any such weight savings are in addition to the gains shown in
Table 1, which result from use of a different type power source.
For example, Table 1 shows a weight of 164 pounds for the hydrogen
supply, of which 128 pounds is the weight of the actual fuel tank
aboard the Electrovan. As a conservative estimate, a fully optimized
hydrogen fuel tank for this vehicle will weigh no more than 80
pounds.
314 Hydrogen-Air Fuel Cells
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Table 1. COMPARATIVE WEIGHTS OF ELECTROVAN AND FUEL CELL POWER
PLANT COMPONENTS
(in pounds)
Fuel cell stacks
Peak power battery
Fluid system
Fuel and fuel tankage
Oxygen and oxygen tankage
Electric system
Electric motor drive and controls
Power train total
Electrovan
1,345
—
1,097
164
208
569
550
3,933
Hybrid system—
this study
690
427
704
164
—
380
550
2,915
The weight of the fuel cell stacks is reduced since the fuel
cell is no longer required to handle heavy overloads; a hydrogen-
air system designed to a 32-kilowatt rating without overload require-
ments is only about one half the weight and volume of a 32-kilowatt
hydrogen-oxygen system designed to handle 160-kilowatt peak loads.
Reduced fuel cell volume produces corresponding reductions in the
weight and volume of interconnecting plumbing, tankage, etc. As
a result, the 427 pounds added in a battery to handle peak loads is
more than compensated for by reductions in fuel cell weight. Another
significant weight reduction results from dispensing with a 208-
pound liquid oxygen tank; only about one half of this weight need
be added on in air processing equipment aboard the hydrogen-air
system.
Table 2 lists the performance factors chosen for the Electrovan
system. Simple replacement of the hydrogen-oxygen system of the
Electrovan with the hybrid hydrogen-air system would give above-
target performance, because of the lower power train weight. This
permits design to a lower power level, or alternatively permits an
increase in useful cargo weight.
Table 2. PERFORMANCE
Variable Electrovan
Acceleration (0 to 60 mph), sec 30
Top speed, mph 70
Range, miles 100 to 150
EVANS 315
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Rather than attempting to redesign the Electrovan to a new
set of weight figures, a simpler way is to interpret the data of
Table 1. The Electrovan weighs 7,100 pounds. Table 1 shows that
with a 3,933-pound power train, 55 percent of the total weight is
required to give it the design performance of Table 2. With a
hybrid hydrogen-air system, the same performance can be provided
in a 7,100-pound gross weight vehicle with a power train weight
of 2,915 pounds, or 41 percent of the total weight. In a recent
article Hoffman (1966) indicated that up to 50 percent of the
weight of an electrical vehicle could be assigned to batteries without
endangering utility; by this criterion the Electrovan just misses,
wherein the hybrid system is well within the safe range.
The data in Tables 1 and 2 are not intended to project long-
range future goals, but rather to indicate what can be accomplished
in the near future. Our longer-range projections are far more
optimistic.
Many criteria other than weight and power are important in
evaluating a power system. In the following discussion, the char-
acteristics of the hybrid fuel cell system are weighed against the
eight criteria previously listed.
High Power Density
Weights estimated for the fuel cell of the hybrid system are
based on a peak electrode power density of about 75 watts per square
foot at the 32-kilowatt design point. This represents performance
that has been achieved already in smaller multicell hydrogen-air
systems. This electrode power density represents a reasonable design
goal for a number of nonnoble metal catalysts being investigated by
us and in other laboratories. The trade-off between performance
and cost leads directly to the hybrid system. The many kilowatts of
peak power required for vehicle acceleration are most economically
provided by a rechargeable battery, whereas the fuel cell provides
instant refueling capability and high energy density needed for
an acceptable range in miles.
The feasibility of the hybrid fuel cell and rechargeable battery
combination was demonstrated a few months ago by Kordesch in
a small motorcycle, using a hybrid power plant employing an 800-
watt hydrazine-air fuel cell plus a set of nickel-cadmium 15-ampere-
hour batteries. No electronic charge controls were necessary; a
simple matching of the fuel cell and battery discharge characteristics
provided proper load-sharing.
The rechargeable battery employed in the hydrogen-air design
of this study has been characterized thus far only as weighing
427 pounds, and as being capable of delivering short bursts of power
up to 128 kilowatts. Several of the rechargeable batteries presently
being proposed as the sole power source for electric automobiles
can meet or surpass this requirement; feasibility of the hybrid sys-
tem does not depend upon inventing a new battery system.
316 Hydrogen-Air Fuel Cells
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High Energy Density
The hybrid system contains 120 kilowatt-hours of energy in the
fuel tank. If we assume that storage batteries with an energy
density of 50 watt-hours per pound become available for vehicle
propulsion use, the rechargeable battery of the hybrid system can
store an additional 21 kilowatt-hours for a weight of 427 pounds.
Since the power train weight is 2,195 pounds (including motors and
controls), this gives about 50 watt-hours per pound as the energy
density of the total power supply, including fuel tank and motors.
The energy density of the fuel cell portion of the system is
about 77 watt-hours per pound. If the fuel tank size were doubled,
increasing the range from between 100 and 150 miles to between
200 and 300 miles, the fuel cell energy density would increase to
about 140 watt-hours per pound, with an additional 140 pounds
added to the system weight.
The use of the rechargeable battery for supplying peak power
provides a useful fringe benefit; should the vehicle run out of
hydrogen, the battery will give about 20 miles additional range
to reach the next filling station.
Freedom from Harmful Emissions
The absence of noxious exhaust fumes is perhaps the most
obvious and most easily sold advantage of hydrogen-oxygen or
hydrogen-air fuel cells. The only possible reaction product, even
in theory, is pure water. By-product water from Union Carbide
fuel cells has been checked by the testing laboratory that monitors
Cleveland's water supply and found to meet U.S. Public Health
Service standards for potable water.
It might be argued, since we are proposing hydrogen-air systems
and commercial hydrogen is produced largely by steam reforming
of natural gas, that we are simply moving the source of pollution
from the vehicle back to the hydrogen manufacturing plant. This
is not a valid argument. A major hydrogen production plant can,
and does, cut down emission of atmospheric pollutants to a negligible
level, using techniques that are practical in a plant covering city
blocks in area, but that would be totally impractical to apply to
each of millions of individual vehicles. For example, even in the
long-range future when fossil hydrocarbon fuels become unavailable
or more expensive, the vehicle described in this study could be
operated on hydrogen produced by electrolysis of water by use of
nuclear electricity.
Low Capital Cost
The question of initial capital cost for a fuel cell installation
has probably limited or at least confused the growth of the fuel
cell industry more than any other item. Fuel cell prices today are
quoted anywhere from about $15,000 per kilowatt, to well over
$40,000 per kilowatt, depending upon type. A very large percentage
EVANS 317
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of this cost is for labor. There is scarcely a component or part of
the Union Carbide fuel cell system that can be purchased from a
mass-production supplier; even such simple parts as pipe couplings
may have to be hand-crafted one at a time from bar stock. Under
these conditions, the only realistic gauge available for estimating
potential future prices is the cost of the materials that must be used.
Unfortunately, no studies have been conducted in sufficient detail
to provide quantitative information. The following comments are
intended merely to set some qualitative guidelines.
A fuel cell system may be visualized as a group of electro-
chemical cells plus a group of auxiliary components such as pumps,
fans, and radiators. These auxiliaries can be made of low cost
materials such as a variety of modern plastics or nickel-plated steel
and in general are no more complex in design than the corresponding
pumps, fans, radiators, etc., of the internal combustion engine. For
simplicity, then, let us write off fuel cell auxiliaries as being com-
parable in cost to the corresponding type of equipment required
to operate an internal combustion engine, and focus attention on
the electrochemical cells, which represent the largest unknown in
the cost picture.
The Union Carbide hydrogen-air fuel cell contains nickel, carbon,
and a variety of plastics such as polysulfone, epoxies, polyolefins, and
Teflon. Adding up all of the materials, less catalyst, required for the
32-kilowatt hydrogen-air fuel cell gives a raw materials cost of
about $1,500, or $50 per kilowatt. This estimate is based on our
present purchase prices for raw materials, purchased in lots of a
few hundred pounds (or sometimes a few pounds) per order. There
is reason to believe that a reduction in cost to one half this value
could be achieved under mass-production conditions. This corre-
sponds to a figure of about $1 per pound for a miscellaneous
assortment of nickel, carbon, and plastics, which seems intuitively
reasonable.
In my earlier discussion of power density, a conservative figure
of 75 watts per square foot was selected as the peak power density
for electrodes. This represents our judgment at Union Carbide as
to the best power density achievable at the present state of the art
without using precious metal catalysts, and for an acceptable life-
time. Fabrication of electrode stock is already on a continuous
strip, semiautomatic basis; full conversion to a high-speed auto-
mated production line is simply a matter of the requisite production
engineering and capital investment.
We have not discussed two other problem areas: the controls
that regulate motor speed and the motors themselves. The approach
used by General Motors in the Electrovan supplied a sophisticated,
but rather expensive, solution to the problem. Not being manufac-
turers of traction motors or motor controls, Union Carbide will be
pleased to hear of any practical and economic answers to these
problems from specialists in the field. It is also obvious that any
318 Hydrogen-Air Fuel Cells
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electric vehicle powered by a direct-current source will have to
face and solve the same problems.
From this rather cloudy cost picture, it appears likely that a
hybrid hydrogen-air fuel cell system will remain more expensive
than a corresponding internal combustion engine within the next
decade, perhaps by a factor of 1.5 to 2. A portion of this cost
differential can be written off against operating cost, a portion against
the advantages of silence, a portion against the necessity of air pollu-
tion control, and a portion against the greater safety of a hydrogen-
fueled vehicle as compared with a corresponding gasoline-fueled
vehicle.
Low Fuel Cost
One of the advantages of the liquid hydrogen fuel proposed
for the hybrid system is its low cost. To establish a basis for com-
parison, 1 pound of hydrogen burned in a hydrogen-air fuel cell
gives about the same performance as 1 gallon of gasoline burned
in an internal combustion engine. In recent contracts, government
agencies have purchased liquid hydrogen for prices in the range of
20 cents per pound. Projections indicate that full utilization of plant
capacity with insured sales to fuel distributors could lower the cost
to 10 cents per pound in large quantities. This corresponds to 10
cents per gallon of gasoline, or in other terms, about 1 cent per
mile of travel for a 7,100-pound gross weight vehicle under urban,
start-stop driving conditions. The above figures do not include
road tax. I am sure the Government will find a way to tax hydro-
gen when it is used as a vehicular fuel, but the fuel cost will still
be attractive.
Low Maintenance Cost
Predicting maintenance costs for a system that has not yet
been built invokes more prophecy than science, but a few clearly
defined maintenance requirements can be foreseen. Operation of an
alkaline electrolyte fuel cell on air produces potassium carbonate
from the carbon dioxide in the air. If raw, untreated air is fed
directly into an alkaline fuel cell operating at high power density,
the buildup of carbonate deposits on and in the cathodes can produce
a detectable loss of voltage within a few days. In order to avoid
this problem, we extract a majority of the carbon dioxide from the
incoming air in a "CO2 scrubber," with the bulk electrolyte itself
used as the absorbent. Laboratory experiments with small, 100-watt
hydrogen-air systems have shown that we can get about 500 hours
of operation before it becomes necessary to discard the electrolyte,
flush out the system with water, and refill with clean electrolyte.
Since 500 hours of operation represents 15,000 miles of vehicle
operation at an average of 30 mph, this does not represent an
intolerable maintenance burden. A refill of KOH electrolyte for
the present 32-kilowatt hybrid design would cost about $38 (less
than !/4 cent per mile).
EVANS 319
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Long Life
The life of a fuel cell is highly sensitive to the conditions of
use and the criteria for failure; i.e., how much degradation in per-
formance can be tolerated before the system is declared inoperable?
Under laboratory conditions, we have operated small fuel cell sys-
tems for over 50,000 hours of intermittent duty, or over 14,000 hours
of continuous duty, in the latter case at a time-average power density
comparable to that proposed here for the 32-kilowatt system. Objec-
tively, it must be recognized that these are "best results"—those
isolated results that indicate merely that a goal is attainable, and
not that it has been routinely reached. Consideration of the series-
reliability problem, the fact that one bad cell in a stack can limit
its life, suggests several thousand hours of on-load time as a more
reasonable indication of the state of the art. We realize that a
few thousand hours of life is not enough to provide the foundation
for large-scale use of hydrogen-air cells, and are working hard to
bring the average of the distribution up to the level of those
best results.
The hybrid power source system contains two electrochemical
devices; a fuel cell and a rechargeable battery. Developing a suitable
battery for the hybrid system is a much simpler task than developing
a rechargeable battery for use as the sole power source in an electric
vehicle. In the hybrid system, the battery is subjected to very
shallow discharges, is slowly recharged, and unless the operator
runs out of hydrogen fuel, is never called on for more than a small
fraction of its ampere-hour capacity. Any conventional rechargeable
battery available today shows long life under these use conditions,
and we foresee no major problem in meeting the rechargeable battery
requirements of the hybrid system.
Safety
One of the major advantages of a conversion from gasoline
fuel to cryogenic hydrogen fuel will be a reduced fire hazard in
the event of traffic accidents. Weights assigned to the cryogenic
tankage in the hybrid system are based on the criterion that the
fuel tankage must not rupture unless impact is so severe that there
is no reasonable chance for survival of the human occupants of the
vehicle. Under these circumstances, the major hazard of concern
is the probability that any resulting fire will endanger nearby
humans, vehicles, or structures. Upon rupture and ignition of a
liquid hydrogen tank, the contents vaporize immediately, rising in
a vertical column of nonluminous flame which vanishes within
seconds. In contrast, rupture and ignition of a gasoline tank results
in the flow of heavy gasoline vapor and liquid gasoline across the
ground, acting as a spreading and persisting source of fire. Further-
more, the gasoline flame radiates far more infrared or heat energy
(because of incandescent carbon particles in the flame), leading to
ignition of nearby structures or personnel not directly within the
flame. On the basis of numerous experiments conducted for the
320
Hydrogen-Air Fuel Cells
-------
government, the deliberate ignition of large quantities of liquid
hydrogen unquestionably produces far less fire damage than ignition
of a comparable fuel value of gasoline.
Union Carbide has been engaged in fuel cell research for about
15 years now, proving in and testing hundreds of kilowatts of
hydrogen-fueled fuel cells. In logging over 5 million test hours on
fuel cells, we have had three small fires, each easily extinguished
and without injury or hazard to personnel. More important, in each
of these incidents the causative factor could be traced to the use of
pure oxygen. In a pure oxygen atmosphere almost anything is
flammable (and hydrogen in particular). We do not propose the
use of cryogenic oxygen in the future hybrid fuel cell system for
this reason (but also because the use of oxygen would approximately
double the operating cost per mile).
It is also noteworthy that our 15-year record of handling large
quantities of liquid and gaseous hydrogen without a hazardous
accident has been accomplished without our having to invoke extreme
safety measures that might be impractical to apply in commercial
practice. For example, we do not smoke while transferring liquid
hydrogen. Neither should a filling station attendant when he is
transferring gasoline.
Another problem related to safety concerns the handling of
boil-off from the cryogenic hydrogen tank while the vehicle is
garaged. This problem is easily solved in the hybrid hydrogen-air
fuel cell vehicle, especially for commercial fleet users such as
delivery trucks, buses and taxicabs. Such vehicles see heavy service
5 to 7 days every week. Over night or weekends the boil-off gas
is simply supplied to the fuel cell, providing the fuel energy required
to restore the rechargeable battery to full charge and maintain it
there on trickle charge. This answer is not as directly applicable
to the personal automobile, whose owner may leave it garaged for
a month's vacation.
As a further safety precaution, any garage in which fuel cell
vehicles are stored should be ventilated, to avoid hazard in the
event of fortuitous hydrogen leakage from a defective system.
Adequate ventilation can readily be provided, indeed is often already
available, in large commercial garages of the sort used to store
fleets of buses, delivery service vans, taxis, etc., but is not usually
available in garages for the personal automobile. This again suggests
that the first application of fuel cell vehicles can be to commercial
types.
SUMMARY OF ADVANTAGES
In comparison with the internal-combustion-powered vehicle,
the hybrid hydrogen-air vehicle will produce no air pollution, will
operate silently, will operate at a comparable fuel cost, and will
provide similar'acceleration, speed, and range.
EVANS 321
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In comparison with the rechargeable-battery-powered vehicle,
the hybrid hydrogen-air vehicle will provide greater range (as a
result of higher energy density) and will permit instant "recharge"
by filling the fuel tank. This ability to refuel quickly is a very
important advantage. The vehicle discussed in this paper requires
140 kilowatt-hours of stored energy to achieve the desired range. If,
as it has been suggested, the battery electric vehicle is to be recharged
in 15 to 30 minutes while parked, this would call for a 280- to
560-kilowatt, direct-current charging station; definitely not a job
for an extension cord. Consideration of charging problems again
suggests that the hybrid vehicle may offer competitive advantages
in heavy-duty vehicles with high energy storage requirements, such
as buses and trucks, whereas the battery-powered vehicle may be
more competitive in lightweight personal cars.
In comparison with the vehicle powered by the "pure" fuel
cell the hybrid system offers lower weight, lower cost, an energy
reserve in case the vehicle runs out of fuel, and a means of utilizing
hydrogen boil-off in a safe and effective manner.
SUMMARY OF PROBLEM AREAS
The following areas require investigation and development before
a vehicle powered by the hybrid hydrogen-air fuel cell can become
a reality:
1. Improvement in quality and uniformity of electrodes and fuel
cell stacks to permit the consistent achievement of long life at a
power density of at least 75 watts per square foot, without
increasing cost.
2. Development of a low-cost rechargeable battery offering at least
50 watt-hours per pound and capable of at least short bursts
of power at a 128-kilowatt rate.
3. Development of inexpensive, lightweight motors and motor con-
trols. This problem is a common one for any type of electric
vehicle. Both fuel cells and batteries can provide high voltage
only by coupling a large number of cells in series. Any elec-
trochemical power source can be made lighter, more compact,
and more reliable by reducing the number of series elements
to a minimum. Furthermore, the voltage across the cells of a
battery or fuel cell remains "on," even after the power supply
switch is turned off. At high voltages, this can present a
serious hazard to personnel. Either of the above reasons pro-
vides a strong incentive to design motors and controls to the
very lowest voltage consistent with a tolerable weight and cost.
4. The specific 32-kilowatt hybrid system discussed here has been
presented as a conceptual design, with only a first-order approxi-
mation of weights and other characteristics. An actual engi-
neering design study needs to be conducted in sufficient detail
322 Hydrogen-Air Fuel Cells
-------
to permit an accurate appraisal of system characteristics. In
turn, this information will permit more meaningful intercom-
parisons between vehicles powered with a fuel cell, a battery,
and an optimized hybrid combination.
5. Upon arriving at a design projecting desirable and competitive
characteristics, one must build and test a prototype. No amount
of paper studies can provide the engineering advances achieved
during the building and testing of hardware.
REFERENCES
Hoffman, G. A., 1966. The electric automobile. Sci. American.
215:34-40 (Oct.).
Society of Automotive Engineers, 1967 (Jan. 13).
E. R. Rishavy, W. D. Bond, and T. A. Zechin. Electrovair—
a battery electric car. SAE Paper No. 670175;
C. Marks, E. A. Rishavy, and F. A. Wyczalek. Electrovan—
a fuel cell powered vehicle. SAE Paper No. 670176;
P. D. Agarwal and T. M. Levy. A high performance AC
electric drive system. SAE Paper No. 670178;
F. A. Wyczalek, D. L. Frank, and G. E. Smith. A vehicle fuel
cell system. SAE Paper No. 670181;
C. E. Winters and W. L. Morgan. The hydrogen-oxygen thin
fuel cell module. SAE Paper No. 670182.
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