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TABLE 6. ESTIMATED FLYWHEEL AND ASSOCIATED TRANSMISSION
CHARACTERISTICS FOR FLYWHEEL-ENGINE HYBRID
VEHICLE PROPULSION SYSTEMS
Vehicle Application
Pa rain etc r
Assumed Speed, mph
Assumed Effective Mass, Ib
Vehicle Kinetic Energy, hp(s)-hr
Maximum Delivered power, hp(s)
Assumed Transmission and Drive Efficiency, percent
Flywheel Energy, hp(s)-hr
Transmission Power, hp(s)
Flywheel Specific Weight, lb/hp(s)-hr
Flywheel Weight, Ib
Transmission Specific Weight, lb/hp(s)
Transmission Weight, Ib
Flywheel and Transmission Weight, Ib
Flywheel Specific Volume, ft3/hp(s)-hr
Flywheel Volume, ft
Transmission Specific Volume, ft /hp(s)
Transmission Volume, ft
Flywheel and Transmission Volume, ft
Flywheel Specific Cost, $/hp(s)-hr
flywheel Cost, $
Transmission Specific Cost, $/hp(s)
Transmission Cost $
Flywheel and Transmission Cost, $
Family Car
60
4,400
0. 27
94
80
0. 34
1 17
100
34
2. 0
230
260
0. 25
0. 085
0. 02
2.3
2.40
100
34
3. 0
350
380
City Bus
30
33, 000
0. 50
180
80
0.62
225
100
62
3. 0
680
740
0. 25
0. 16
0. 03
6.75
6.9
100
62
14. 0
3. 200
3. 300
BATTEULE MEMORIAL INSTITUTE - COLUMBUS L A I3OD ATORI ES
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duty, long lived, and maintenance free and because it is a lower production-quantity
item. For these reasons, the specific weights and volumes of the Rankine and
Stirling engines that were estimated for the bus application are twice as large as those
listed in Table 2 for automobile-type applications. Since gas turbines and direct con-
verters are inherently more durable, maintenance-free devices, their specific weights
and volumes were increased by only 33 percent for the bus application. It is believed
that the cost for any of the heat engines plus required transmissions for the bus appli-
cation would be similar to or somewhat higher than the cost of commercial-vehicle
diesel engines plus hydrokinetic transmissions.
The other five applications of concern currently use, and it is believed will con-
tinue to use, automobile-type propulsion systems. Thus, it was not necessary to alter
the characteristic values listed in Table 2 for any of these applications.
Average engine efficiency over the various duty cycles was assumed to be approxi-
mately 70 percent of projected peak engine efficiency. Since the direct converters will
operate under more nearly study-state conditions and nearer to their design point, their
average efficiency was assumed to be approximately 90 percent of projected peak
efficiency.
The heat-engine propulsion-system weights and volumes listed in Table 3 include
those of the engine, transmission, and fuel and fuel-storage provisions only. The listed
initial costs are those of the engine and transmission only. These represent the major
portions of the propulsion system, and the addition of exhaust and driveline, other than
transmission, components would neither significantly increase estimated system weights,
volumes, and costs nor, more importantly, alter the conclusions resulting from
these estimates.
As stated above, because of the basic operating nature of the direct converters,
they would probably be used as part of a hybrid system along with suitable batteries,
motors, and controls. The weights and volumes listed for the converters in Table 3
include those of the converters and fuel and fuel-storage provisions only. Thus, the
weights and volumes of associated batteries, motors, and controls must be added to
those listed for the direct converters before a direct comparison can be made between
the weights and volumes of the direct-converter systems and those of the engine and
flywheel systems.
. The values listed in Table 4 represent an optimistic estimate of the best physical
characteristics that could be expected to be achieved for each of the alternative systems
10 to 15 years from now, assuming that major research and development efforts were
expended on them. Gas-turbine characteristics are not listed for the utility-car applica-
tion because it is not believed that a competitive small gas turbine [ i. e. , less than
30 hp(s)] could be developed. The estimated heat-engine characteristics calculated for
the low-energy and -power, commuter-car and utility-car applications are believed to
be somewhat overly optimistic, but are adequate for the purposes of this study.
In both Tables 3 and 4, fuel-storage characteristics were calculated assuming
that one of the conventional liquid fuels plus a suitable sheet-metal tank would be used
for energy storage. To determine the effect that the use of alternative chemical-fuel
and thermal-energy storage means would have on propulsion-system weight and volume.
the variation in Rankine (steam)-engine propulsion-system characteristics was
BATTELLE MEMORIAL INSTITUTE - COLUMBUS LAEORATORIES
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calculated for a selection of chemical-fuel and thermal-energy storage means, including
suitable container provisions. The results of these calculations are given in Table 5.
The assumed effective masses listed in Table 6 were arrived at by increasing
the loaded weights of the family car and city bus 10 percent to account for the effective
inertia of rotating parts. The transmission characteristics listed apply to hydrostatic
transmissions which, it is believed, would be most appropriate for use with a flywheel-
engine hybrid system.
The maximum cruising velocities listed in Table 1 are greater than those to which
the acceleration specifications are set for the family-car and city-bus applications.
Thus, if it is assumed that the engines in the hybrid systems are rated to supply maxi-
mum cruising power, the engines would be capable of supplying the energy required to
overcome all rolling and aerodynamic vehicle loadings during the acceleration period
and would provide some of the energy required for accelerating the vehicles. By making
the energy-storage capacity of the flywheels equivalent to the kinetic energy of the
vehicles at the speeds to which the acceleration specifications apply, a suitable reserve
of energy is provided. This energy reserve could be used to accelerate the vehicles to
speeds somewhat higher than that to which the acceleration specifications apply.
SYSTEM EVALUATION
As previously discussed, the alternative propulsion systems and components were
evaluated as to their suitability for urban-vehicle application by comparing their
projected "1980" characteristics with the hypothesized threshold specifications for
various urban-vehicle missions listed in Table 1. In this manner, the obviously unac-
ceptable propulsion systems were identified and the relative attractiveness of the sys-
tems "above threshold" was determined.
Through such comparisons, magnetohydrodynamics and all nuclear devices were
found to be unsuitable for urban-vehicle application early in the evaluation process and,
thus, were not subjected to more detailed analysis. The other alternative systems in-
cluded in this study, however, were evaluated in more detial and, as discussed in the
preceding section, their estimated weights, volumes, and initial costs are listed in
Tables 3 through 6.
All of the characteristic values listed in Tables 4 through 6 that do not meet the
threshold specifications listed in Table 1 are designated by (a). Systems which are not
expected to meet the threshold specification with respect to one or more characteristics
for any given urban-vehicle application are concluded to be unsuitable for the particular
application in question.
Study of Table 4 shows that of the alternative systems and devices studied, only
the three heat engines are expected to exceed all of the threshold specifications. Magne-
luhydrodynamic and nuclear devices were excluded earlier. Flywheel energy storage by
itself is unacceptable from a weight and cost standpoint for all six urban-vehicle applica-
tions, and its estimated volume meets the threshold specifications for only three of the
six applications. Finally, the projected specific cost [i.e., $100/hp(e)] for the direct
converters results in converter costs higher than the threshold specification for all six
applications.
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The system characteristics estimated for gas turbines and Rankine (steam)
engines are well above threshold performance, and it is concluded that these systems
are very attractive alternative propulsion systems for general urban-vehicle applica-
tion. This is particularly true in light of their low-pollutant-emission characteristics
which are discussed in the following section on the state-of-the-art of these engines.
While the estimated system characteristics for Stirling engines also exceed thres-
hold specifications in all cases, they are less attractive than those estimated for gas
turbines and Rankine engines. Thus, it is concluded that Stirling engines are not as
attractive for general urban-vehicle applications as Rankine engines and gas turbines.
The principal exception to this may be the bus application, where the superior efficiency
of the Stirling engine and its potential for high durability may make it a preferred
alternative.
While the costs projected for the direct converters could not meet the threshold
specifications, projected costs are subject to the greatest variation, as noted in Table 2,
so it is prudent to also consider the relative attractiveness of the direct converters and
the heat engines on a strictly weight, volume, and efficiency basis. Again referring to
Table 4, it can be seen that even without adding the additional weight required for
batteries, motors, and controls, the weight listed for the direct converters is much
higher than that for the gas-turbine and Rankine-engine systems for the family-car,
delivery-van, and taxi applications. The weight difference is not so great for the other
applications, but in no case is the estimated weight of the direct-converter systems, less
batteries, motors and controls, lower than that estimated for the gas-turbine and
Rankine-engine systems.
The other major consideration, of course, is efficiency. In Table 3 and the tables
in Appendix B, estimated average efficiencies for the direct converters are one-third to
two-thirds of those for the heat engines. This comparison is even less favorable for the
direct converters if estimated peak efficiences are compared. The direct converters'
low efficiency would result in increased fuel consumption which would in turn result in
increased operating cost and heat release. Exhaust emissions may or may not be in-
creased by this higher fuel consumption, as the direct converters' use in hybrid systems
would mean that their burners would operate at more nearly steady-state conditions and
thus probably emit less pollutant per pound of fuel burned.
Because of their very high projected cost and particularly because of their low
projected efficiency and high weight, direct thermal-to-electric energy converters are
not considered to be attractive alternatives for urban-vehicle propulsion systems.
The data listed in Table 5 show that the use of thermal-energy storage with the
Rankine engine would result in a system that would not meet threshold specifications for
any except the lowest energy vehicle applications. The lithium fluoride system is heavier
and larger than specified for the family-car application, heavier than specified for the
taxi and bus applications, and just under the maximum allowable weight for the delivery-
van application. While somewhat smaller and lighter thermal-energy storage systems
could be used with Stirling engines, they would still be unattractive for these high-energy
applications. Thus, thermal-energy storage is suitable only for the very low energy
applications, and for such applications, it would encounter very strong competition from
battery systems.
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Assuming a 25 percent average engine efficiency over the applicable driving
cycle, which is the average efficiency estimated for the Stirling engine in the commuter-
car application, a lithium fluoride-energy storage device might have a specific weight
of 13 lb/hp(s)-hr [ 3. 30 lb/hp(t)-hr -:- 0. 25] . This specific weight is higher than that
presently being projected for some advanced electrochemical batteries [i. e. , 6 to 8 lb/
hp(e)-hr for sodium-sulfur batteries). Thus, it is questionable that thermal-energy
storage would be found to be the best alternative even for low-energy, urban-vehicle
applications.
Of the alternative chemical-fuel-energy storage means listed in Table 5, only the
cryogenically stored hydrogen system could not meet threshold specifications for all of
the vehicle applications. Its volume was found to be excessive for both the family-car
and city-taxi applications. While the use of alternative fuels generally resulted in sys-
tems that could meet threshold specifications, study of Table 5 shows that they are con-
siderably less attractive from a weight and volume standpoint than the more conventional
fuels (i.e. , gasoline, JP-4, kerosine, and diesel fuel). Only the liquid petroleum gases
(LPG) would result in energy-storage weights and volumes relatively close to those of
the conventional fuels. The alcohols would have a size and weight disadvantage similar
to that of LPG; however, they are more costly and are not known to have advantageous
emission characteristics as has been cited for LPG in piston engines.
The energy specification for the commuter car and utility car are small so the
effect of heavier and larger energy-storage provisions is considerably less pronounced
on their total-system characteristics than for the other applications. Their short range
(50 miles), however, was set primarily out of consideration of a battery system which
could be recharged every night. If a chemical-fuel-energy storage means requiring a
stop at a service station for refueling were used, however, it would probably be desirable
to increase this range,which would in turn increase the weight and size disadvantage of
using an alternative fuel.
Because of the above-discussed weight and size requirements of the fuel plus
appropriate container, it is concluded that the unconventional chemical fuels investitated,
with the possible exception of the LPG fuels, are not attractive for general use in urban
vehicles. The principal exception to this would be the very low energy vehicle applica-
tions. To overcome their weight and size handicap, unconventional fuels such as
ammonia, methane, and hydrogen, would have to possess emission characteristics con-
siderably better than those of conventional fuels before they could be considered as
legitimate contenders for urban-vehicle application. When considering external com-
bustor systems, it is believed unlikely that these latter unconventional fuels will possess
sufficiently improved emission characteristics, particularly with respect to NOX, to
compensate for their weight and size handicap.
While the flywheel has poor energy-storage characteristics [i.e., 100 lb/hp(s)-
hr] , it is limited with respect to how fast stored energy can be removed, or to power,
only by the size of the associated shafting and transmission. Thus, in a flywheel-engine
hybrid system, a chemical-fueled engine would supply the gross amount of energy
required for a given vehicle application, and only that energy required for vehicle
acceleration need be stored in the flywheel. The flywheel could then be "recharged"
between vehicle accelerations.
Study of Table 6 shows that surprisingly small flywheels would be required for
use with flywheel-engine hybrid systems. The biggest change from current systems would
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be with respect to the transmission requirements, particularly for the family-car appli-
cation. Even though a lower power engine could be used, the addition of the flywheel and
particularly the need for a larger and more costly transmission make questionable the
overall desirability of a flywheel-engine hybrid system for the family car application.
For the bus application, where frequent stopping and starting is encountered and where
heavy-duty and costly engines and transmissions are already used, however, a flywheel-
engine hybrid system may be desirable.
The flywheel-engine hybrid system would be a competitor to a battery-engine hybrid
system; thus, the two systems must be studied and compared in detail before it can be
determined which is the more attractive of the two.
BATTELLE MEMORIAL INSTITUTE - COLUMBUS LABORATORIES
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PRESENT AND PROJECTED STATE OF THE ART
FOR ALTERNATIVE SYSTEMS
CHEMICAL FUELS AND EXTERNAL COMBUSTORS
CHEMICAL FUELS
EXTERNAL COMBUSTORS
HEAT ENGINES
BRAYTON CYCLE (GAS TURBINE) ENGINES
RANKINE CYCLE (STEAM) ENGINES
STIRLING CYCLE (HOT AIR) ENGINES
DIRECT THERMAL-TO-ELECTRIC ENERGY CONVERTERS
» MAGNETOHYDRODYNAMICS
THERMOELECTRIC CONVERTERS
THERMIONIC CONVERTERS
THERMOPHOTOVOLTAIC CONVERTERS
OTHER ENERGY STORAGE AND CONVERSION DEVICES
THERMAL ENERGY STORAGE
MECHANICAL ENERGY STORAGE
VEHICLE TRANSMISSIONS
NUCLEAR DEVICES
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PRESENT AND PROJECTED STATE OF THE
ART FOR ALTERNATIVE SYSTEMS
This section of the report contains detailed discussions of the present and projected
physical, performance, and cost characteristics for all of the alternative systems and
devices investigated during the program. The projected characteristics are for a period
10 to 15 years in the future (i. e. , "1980" characteristics). The discussions of the in-
dividual devices and systems contained in this section are organized as illustrated on the
preceding divider sheet.
CHEMICAL FUELS AND EXTERNAL COMBUSTORS
CHEMICAL FUELS
Fuels Investigated
For the purposes of this study, twelve specific chemical fuels were selected as the
most likely candidates for urban-vehicle application and were studied to determine their
physical and cost characteristics. The fuels selected fall into two general categories:
those that are liquids at ambient temperatures and pressures, and those that are gases.
The liquid fuels, i. e. , gasoline, JP-4 (JP-5 and CITE also), kerosine, No. 2 diesel oil,
methanol, and ethanol, represent a class of petroleum products which have been found to
be particularly appropriate as engine or prime mover fuels. The gaseous fuels include
propane and butane, commonly referred to as the liquefied petroleum gases (LPG),
methane and ethane, the principle constituents of most natural gases, and hydrogen and
ammonia.
Current Availability and Application of Fuels
Gasoline, JP-4, kerosine, and diesel oil are readily available in either large,
medium, or small quantities, through bulk distributors and retail outlets such as service
stations. All are used as prime mover fuels, and kerosine and diesel oil are also used
extensively in small-to-medium size industrial, commercial, and residential heating and
hot-water systems.
Ethanol and methanol are currently available in 55-gallon drums and also in smal-
ler quantities. Their chief uses are as industrial solvents, in the preparation of cooling-
system antifreeze solutions, and in laboratory experiments.
Ammonia is becoming more and more readily available in bulk quantities for agri-
cultural use as a fertilizer. The military is also doing some experimenting with am-
monia as an engine fuel.
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Butane and propane are available from bulk distributors in both large and small
quantities, generally for residential and mobile (trailer) heating and cooling. A small
amount of propane is also used as a vehicle engine fuel, principally for inside operating
lift trucks and similar vehicles, and for the tank trucks used to transport the propane.
Methane is available as the major (generally over 90 percent) constituent of natural
gas, which is supplied by pipeline from gas fields in Texas, Oklahoma, and other areas
to consumers throughout most populated sections of the U. S. The main uses of natural
gas are industrial, commercial, and residential heating and air conditioning, manufac-
turing operations such as heat treating, drying, and mineral processing, and cooking.
Methane is also available in high-pressure cylinders for laboratory use. Ethane is not
readily available except as an ingredient in some natural gases.
Hydrogen is available in high-pressure cylinders for laboratory use, in bulk at
atmospheric pressure and temperature for some industrial processes such as in petro-
leum refining, and in liquid form for cryogenic purposes.
Current and Projected State of the Art
Physical and Cost
Characteristics - Fuels
Table 7 contains a listing of some pertinent physical characteristics and current
cost information for the fuels investigated. The physical data were obtained from various
reference books and other sources. (1-5)* The lower heating value represents the total
energy in the fuel that could be converted to useful work in an ideal combustion system.
The stoichiometric air-fuel ratio represents the minimum amount of air (on a weight
basis) required for complete combustion. The flame temperatures are theoretically cal-
culated assuming perfect mixing and stoichiometric conditions. The boiling point at
1 atmosphere indicates the temperature above which the fuel will no longer remain in
liquid form at atmospheric pressure.
The current cost data were obtained from a number of diverse sources. (o-10)
costs of the engine fuels (gasoline, kerosine, and diesel oil) are based on approximate
local (Columbus, Ohio) service station prices, exclusive of road taxes; JP-4, being
similar to kerosine, is estimated to cost the same; the costs of ethanol and methanol are
based on fairly large quantity commercial prices; the cost of ammonia is based on the
selling price of anhydrous ammonia for fertilizer use in quantities normally associated
with farm use; the costs of propane and butane are based on approximate local prices for
trailer and home heating use; and the costs of methane and hydrogen are based on rough
estimates of the cost of providing these gases at 3000 psig pressure or at cryogenic con-
ditions. At atmospheric pressure, methane costs about 60 cents per 1000 cu ft, or
1. 4 cents per pound, and hydrogen might cost about 40 cents per 1000 cu ft or 8 cents
per pound. No cost data could be located for ethane.
Because of the great differences in the present production quantities, sources of
supply, types of distribution systems and general end uses for each of the fuels, current
cost data are being listed here only as a limiting case. That is, the listed costs repre-
sent a maximum potential cost for the fuels in question. If produced and distributed in
the quantities and manner that the current engine fuels are, the cost of many of the fuels
"Superscript numbers refer to references listed in Appendix C.
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TABLE 7. PERTINENT PHYSICAL, CHARACTERISTICS AND
CURRENT COSTS OF SELECTED FUELS
Lower
Heating Value,
Fuel Btu/lb
Gasoline
JP-4, JP-5,
CITE
Kerosine
Diesel No. 2
Ethanol
Methanol
Ammonia(a-)
Butane(a)
Propane'a'
E thane (b)
Ethane(d)
Me thane (b)
Methane(d)
Hydrogen(b)
Hydrogen(d)
19, 100
18,800
19, 200
19, 000
11, 700
8,700
8,000
19,700
20,000
20,600
20,600
21,600
21,600
51,620
51,620
Flame
Density Temperature,
Ib/cu ft F
46.3
48. 5
51. 0
52. 2
49. 1
49. 7
40. 2
35.6
36.2
16. 1
34. 0
8. 5
25.9
1. 1
4. 47
4190
-(0
--(c)
3850
-(c)
-(c)
3092
3583
3573
3540
3540
3484
3484
4010
4010
Boiling
Stoichiometric Point
Air-Fuel at 1 Atm,
Ratio F
14. 8
14. 5
15. 0
15. 0
9. 0
6.4
6. 1
12. 2
5. 5
12. 4
12.4
12. 6
12.6
34. 3
34. 3
145/340
150/550
370/500
400/590
172
149
-27
31
-44
-128
-128
-259
-259
-423
-423
Current
Cost,
3. 5
3. 0
3. 0
2. 7
13.9
9. 5
4. 2
6.0
4. 0
--(c)
-(c)
2. 1
2.4
14. 0
20. 0
(a) In liquid state at 80 F.
(b) In gaseous state at 80 F and 3000 psig.
(c) No data.
(d) In liquid state at atmospheric pressure and cryogenic temperature.
would be less than the current values listed. This lack of sound, comparable cost in-
formation also applies to the various containers for these fuels. Appropriate containers
can range from nonpressurized tanks to low-or high-pressure, pressure vessels requir-
ing suitable safety provisions for automotive application, to cryogenic storage systems
with or without an on board cooling means. Current costs for such containers are not
directly comparable because of the great differences in utilization and production
quantities.
Figure 2 shows the relationship between vapor pressure and temperature for the
six gaseous fuels. The curve for each fuel essentially represents the line of differentia-
tion between the gas and the liquid state; if the existing temperature and pressure define
a point to the left of the line the fuel will be liquid, and if they define a point to the right
of the line it will be a gas. It will be noted that all of these fuels are gases at
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-400 -300
-200 -100 0
Temperature, F
100 200
A-57470
FIGURE 2. VAPOR PRESSURE VERSUS TEMPERATURE FOR
LOW-BOILING-POINT FUELS<3)
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atmospheric pressure and at an ambient temperature of 80 F. On the other hand, all of
these fuels are sometimes shipped, stored, and used in the liquid form, and in order to
do this, either the pressure must be raised, the temperature lowered, or a combination
of both.
Referring to the curves of Figure 2, it is evident that raising the pressure to 10 to
15 atmospheres would be sufficient to maintain butane, propane, and ammonia in the
liquid state; and this is actually standard procedure in the handling of these fuels. The
normal pressure rating on LPG tanks is about 200 psi gage. The great advantage is, of
course, reduced bulk for a given amount of fuel being handled. Ethane requires a pres-
sure of about 43 atmospheres to remain in the liquid state at 80 F. Methane and hydro-
gen cannot be maintained in the liquid state at 80 F under any conditions. The general
practice with these latter two fuels, when it is desired to handle them as liquids, is to
reduce their temperature at atmospheric pressure. For instance, methane will remain
liquid below -260 F at atmospheric pressure and hydrogen will remain liquid below
-423 F at atmospheric pressure.
Physical Characteristics
Fuel Plus Container
As discussed above, appropriate containers for the various fuels can vary widely
as to their weight, size, and complexity. Therefore, when comparing alternative fuels
for vehicular application, it is not sufficient to consider the characteristics of the fuels
alone. Rather, it is necessary to compare the characteristics of the fuels plus appropri-
ate container provisions.
Table 8 contains specific weight, volume, and current cost data for the twelve fuels
of interest expressed in units that are more suited to the purposes of this study. These
data were derived from the physical and cost data listed in Table 7. However, Table 8
also contains data on the specific weight and volume of the fuels plus compatible container
provisions.
The fuel plus container weights were estimated as follows. First, a container
volume of 2 cu ft, or 15 gallons, was assumed. Then, for the normally liquid fuels, a
simple rectangular steel tank made of 24-gauge material was assumed. For the liquefied
petroleum gases and ammonia, a conventional commercially available LPG tank weight
was used, with some allowance being made for adapting the tank for vehicular application.
For the natural gases and hydrogen, catalog data were used for both the high-pressure
container systems and the cryogenic container systems. A spherical fiber-glass tank
rated at 3000 psi was used for the pressurized fuel system, and a spherical vacuum-
insulated cryogenic tank with an estimated boil-off rate of 2 to 5 percent per day was
used for the cryogenic fuel system. The weights determined for these 2-cu ft containers
were then added to the weights of the contained fuel to determine the specific weights on
a fuel-plus-container basis.
The fuel-system volumes were also estimated on the basis of a 2-cu ft container
capacity. For the normally liquid fuels it was assumed that the tanks could be readily
shaped to fit any reasonable space on the vehicle; hence the total fuel system volume was
estimated to be only a small percentage (approximately 8 percent) greater than the fuel
volume alone to allow for the container material and rounded corners. For the liquefied
petroleum gases and ammonia fuels, actual volumes of commercial LPG tanks were
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TABLE 8. SPECIFIC WEIGHT, VOLUME, AND CURRENT COST OF
SELECTED FUELS AND FUEL PLUS CONTAINER
Fuel
Gasoline
JP-4, JP-5,
CITE
Kerosine
Diesel No. 2
Ethanol
Methanol
Ammonia(a)
Butane(a)
Propaneva/
Ethane(b)
Ethane(d)
Me thane (b>
Methane(d)
Hydrogen(b)
Hydrogen(d)
Specific
Weight,
lb/hp(t)-hr
0. 133
0. 136
0. 133
0. 134
0. 218
0. 293
0. 319
0. 129
0. 127
0. 124
0. 124
0. 118
0. 118
0. 049
0. 049
Fuel Alone
Specific
Volume,
ft3/hp(t)-hr
0. 00290
0. 00280
0. 00261
0. 00256
0. 00444
0. 00590
0. 00805
0. 00366
0. 00351
0. 00770
0. 00365
0. 01390
0. 00456
0. 0455
0. 0114
Specific
Current
Cost,
$/hp(t)-hr
0. 0047
0. 0041
0. 0040
0. 0036
0. 0300
0. 0280
0. 0130
0. 0078
0. 0051
~(c)
--(c)
0. 0025
0. 0028
0. 0070
0. 0100
Fuel and
Specific
Weight,
lb/hp(t)-hr
0. 15
0. 15
0. 14
0. 15
0. 24
0. 32
0. 51
0. 21
0. 22
0. 37
0. 25
0. 56
0. 27
1. 49
0.44
Container
Specific
Volume,
ft3/hp(t)-hr
0. 0031
0. 0030
0. 0028
0. 0028
0. 0048
0. 0064
0. 0150
0. 0064
0. 0062
0. 0170
0. 0140
0. 0310
0. 0180
0. 1000
0. 0440
(a) In liquid state at 80 F.
(b) In gaseous state at 80 F and 3000 psig.
(c) No data.
(d) In liquid state at atmospheric pressure and cryogenic temperature.
used, again with some allowance for adaptation to vehicular use. For the natural gases
and hydrogen, although the tanks were assumed to be spherical to minimize weight, the
space required on the vehicle was assumed to be only slightly less than the volume of
the cube with sides equal to the diameter of the sphere.
Because of practical considerations and insulation requirements, selection of
container volumes greater than 2 cu ft would likely result in somewhat lower specific
weights and volumes, and volumes lower than 2 cu ft in somewhat higher specific weights
and volumes. Because of the nature of this study and the cursory nature of the container
information available, however, such variation is not considered significant to the con-
clusion of the study.
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The data in Table 8 illustrate the superiority of the conventional fuels (gasoline,
JP-4, kerosine, and diesel oil) on a weight and volume basis. It should be recognized
that the low cost of these four fuels derive to some extent from the exceedingly great
demand for their use in vehicular, aircraft, and stationary powerplants. However, this
demand is in itself predicated on their eminent suitability for the purpose.
In connection with the cryogenic storage of natural gases and hydrogen, there is
the problem of boil-off rate which can vary from 2 to 20 percent per day, depending on
the quality of insulation and construction of the tank. A 20 percent boil-off rate would
probably be unacceptable because of the poor fuel mileage that would result. A 2 per-
cent boil-off rate might be extremely expensive to achieve in practice.
The selection of 3, 000 psi for the pressure at which the natural gases and hydro-
gen would be stored in the gaseous state is arbitrary. A higher pressure would natur-
ally result in greater energy storage per unit volume, both for the fuel alone and for
the fuel plus tank and auxiliaries. It would however, result in a similar energy storage
per unit weight of fuel plus tank.
There appears to be another alternative in the means of supplying and storing
hydrogen for vehicular use. This alternative is the use of magnesium hydride as an
adsorber-desorber for hydrogen gas at atmospheric pressure, a technique under study
at the Brookhaven National Laboratory. (H) Briefly, a system based on this technique
would operate on a cycle involving the generation of hydrogen gas for use, by heating
the magnesium hydride and regenerating the magnesium hydride by passing hydrogen
gas through it while extracting heat.
The performance capabilities of a magnesium hydride fuel system sized for a
45-pound hydrogen capacity (approximately 920 hp(t)-hr) were estimated using the fol-
lowing assumptions: hydrogen available for regeneration at 40 cents per 1000 cu ft, in-
cluding the costs of the regeneration process; 547 pounds of magnesium-nickel alloy
(5 percent nickel) at 42 cents per pound, including the cost of pulverizing to an average
particle size of 150 microns; and a metal container and heat exchanger (for generation
of the gas) weighing a total of 100 pounds and costing $1. 00 per pound. The resulting
system performance was as follows: 0. 74 lb/hp(t)-hr for fuel and system, 0. 012
ft^/hp(t)-hr for fuel and system, and 0. 35 $/hp(t)-hr for the fuel system alone. At
40 cents per 1000 cu ft, the cost of the fuel only is 8 cents per pound or 0. 004 $/hp(t)-hr.
Comparing the above data with that listed in Table 8 indicates that the magnesium
hydride storage method would result in a significantly lower specific volume but a some-
what higher specific weight than with the cryogenic method. However, consideration of
a fuel system of the magnesium hydride type must take into account the fact that the de-
velopment is still in an embryonic stage and that several problems remain to be solved
before a more complete assessment of its potential can be made.
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EXTERNAL, COMBUSTORS
General Description
For this study an external combustor can be defined as a device for burning a
fuel and air (or oxidizer) mixture for the purpose of producing heat which can be con-
verted to useful work. The combustion takes place outside of (or external to) the de-
vice which converts the heat to work. External combustors are used in connection with
Rankine-cycle (steam) engines, Stirling-cycle engines, and direct thermal-to-electric
energy converters. Although in the true theoretical sense, gas-turbine combustors are
not "external" combustors, the fact that they operate under steady-state conditions and
are a separate, distinguishable component results in their having characteristics very
similar to those of external combustors.
In addition to their use with heat engines and direct converters, there are many
examples of external combustors which produce heat for space heating, for chemical
processes, for water heating, and for cooking. In many cases these same combustors
with modifications could be adapted to produce heat for conversion to shaft or electrical
power. However, for the most part, combustors for stationary or fixed installations
are too bulky, too heavy, and too limited in control range and response to be adaptable
to a vehicular application.
For maximum compactness and efficiency, an external combustor must be de-
signed integrally with the energy-conversion device which produces the useful work
output. However, all combustors will have certain basic configuration features in com-
mon, including: a fuel system, an ignition system, an air supply system, and a com-
bustion system. The fuel system may consist of an injector or other device (e. g. ,
ultrasonic) for atomizing the fuel and introducing it into the combustion-air stream, a
pump for transporting the fuel from the tank to the injector, and a fuel-rate control.
The ignition system might consist of a spark plug or a glow plug with an appropriate
power supply and control switch. Either ignition source could be continuously ener-
gized or energized only during the starting sequence. The air supply system might
include a blower, flow control dampers, and a means for directing and distributing the
air to the proper chambers and passages within the combustor. The combustion system
might consist of a mixing zone where the fuel and air are mixed before ignition, a pri-
mary .combustion zone where most of the combustion takes place, a secondary combus-
tion zone where combustion is completed with added combustion air, and a dilution zone
where more air is added to the combustion gases to produce the final desired outlet gas
temperature.
Current and Projected State of the Art
As mentioned earlier, an external combustor should be designed integrally with
the energy-conversion prime mover to insure maximum compactness and efficiency and
minimum cost. Consequently, it is impractical to compare on an absolute basis the
many different external combustor configurations that would be needed by the different
power sources and fuel types being considered in this study. However, an appreciation
of the approximate relationship between combustor size and energy output can be ob-
tained by considering a "general case" combustor.
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Combustor Performance
A commonly used measure of the performance for combustors in the heat-release
rate per cubic foot of combustion-chamber volume per atmosphere of pressure. Heat-
release rates of combustors are a function of the rate at which air can be passed
through them at a satisfactory pressure drop. In high-pressure-drop combustors, such
as aircraft gas turbines, heat-release rates as high as 15 x 10" Btu/hr-ft^-atm are
used. For industrial process heaters, on the other hand, heat-release rates as low as
0. 5 x 1()6 Btu/hr-ft3-atm are used.
A combustor for an external combustion vehicular powerplant would be designed
for lower air-flow capacity and a lower overall pressure drop than a gas-turbine com-
bustor, to reduce noise and to minimize blower requirements. It would also be de-
signed to operate at atmospheric pressure. Therefore, a heat-release rate of 1 x 10^
Btu/hr-ft3 (390 hp(t)/ft can be assumed as a reasonable combustion-chamber volume
for comparison pruposes. Assuming that the volume required for the air blower and
ducting and the other auxiliaries is equivalent to the combustion-chamber volume re-
sults in a likely specific-heat-release rate of approximately 200 hp(t)/ft3 for an appli-
cable combustor.
A combustor will produce about the same amount of power regardless of the type
of fuel used. The reason for this is that, as mentioned, combustors are limited not by
the amount of fuel that can be pumped through the system but by the amount of air, and
the potential heat released per pound of air at stoichiometric combustion conditions is
very nearly a constant for the fuels of interest (about 1300 Btu/lb air). There are two
exceptions to this, hydrogen and ammonia. With hydrogen the potential heat release is
about 1500 Btu/lb air, thus, the same-size external combustor might be expected to
produce about 15 percent more power when burning hydrogen. Ammonia, though right
in line with the other fuels on heat released per pound of air, would produce less power
than the other fuels in the same size combustor because of ignition and burning-rate dif-
ficulties peculiar to the ammonia fuel.
Performance of Hydrogen and Ammonia
in Internal-Combustion Engines
While the scope of this program excludes internal-combustion (1C) engines, it is
desirable to comment on the potential performance of two fuels, hydrogen and ammonia,
in 1C engines because they are not normally considered as fuels for that type of engine.
Assuming that the two fuels would be carbureted or injected into the engine in the
vaporized liquid form and the operating air-fuel ratio would be in the same relationship
to the stoichiometric ratio as for gasoline, ammonia fuel should theoretically produce
nearly the same power from the same-size, naturally aspirated engine as gasoline fuel,
and hydrogen should produce about 15 percent more power (because of its higher poten-
tial heat release per pound of air). If, on the other hand, the ammonia and hydrogen
fuels were carbureted into the engine in the gaseous form, which might be a logical ap-
proach to promote good mixing of the fuel and air and to minimize fuel system cost and
complexity, theory indicates that the ammonia fuel would produce about 22 percent less
power than gasoline fuel in the same naturally aspirated engine and hydrogen fuel would
produce about 19 percent less power. In the gaseous form the fuel displaces some of the
air that would normally be inducted into the engine, thereby reducing the total weight of
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fuel and air in the engine per cycle. As a comparison, the same engine using natural
gas as the fuel (carbureted) would theoretically produce about 1Z percent less power
than gasoline fuel.
In actual experimental operation of internal-combustion engines on ammonia
2-14)^ about 70 percent of the rated power of the engine on gasoline fuel was pro-
duced when gaseous ammonia was carbureted into the engine, and about 90 percent of
the rated power was achieved when liquid ammonia was injected into the engine induc-
tion air. It was surmised that the full theoretical performance of ammonia as an en-
gine fuel was not achieved because of ignition difficulties and slow burning of the
ammonia-air mixture. These problems could probably be somewhat lessened with
development.
Emission Characteristics of
External Combustors
The air pollutants resulting from the combustion of fuel oil and natural gas in
various types of external combustors has received a good deal of attention. The
emphasis has been primarily on the control of smoke and oxides of sulfur, which are
the principal problems associated with stationary combustion sources. It is notable
that similar combustion sources using the same fuel type can produce significantly dif-
ferent contaminant-emission levels. This fact points up the importance of burner
design, adjustment, and maintenance in controlling exhaust emissions.
Table 9 is a compilation of emission data for natural-gas- and oil-fired combus-
tion equipment of various types in residential, commercial, and industrial applications.
In general, these data show that natural-gas combustion results in lower exhaust-
pollutant-emission levels than fuel oil and that pollutant emissions from stationary
external-combustion sources are very low. Table 10 is a compilation of emission data
for external combustors associated with Stirling and Rankine (steam) engines. Con-
siderable variation exists in these data also.
As a comparison to the emission data in Tables 9 and 10, a standard gasoline
engine without exhaust controls might produce the following emission per 1 million Btu
of fuel consumed: 2. 7 Ib unburned hydrocarbons (HC), 0. 79 lb oxides of nitrogen (NOX),
0. 04 lb oxides of sulfur (SOX), 0. 012 lb smoke, and 0. 09 lb aldehydes(22) Another
reference(25) gives the emissions from a typical conventional gasoline engine without
exhaust controlls as: 900 ppm HC, 3. 5 percent CO, and 1500 ppm NOX.
Study of the data in question indicates that on a mass-emission-rate basis
(mass of emissions per unit energy of fuel burned), external combustors generally emit
less than 4 percent of the CO and HC that uncontrolled gasoline engines do and vary
widely in their emission of NOX. The NOX emissions indicated in Tables 9 and 10 vary
from roughly 5 percent to equivalent amounts of that emitted by uncontrolled gasoline
engines.
Studies have shown that the emission characteristics of most external combustors
are highly dependent on air/fuel ratio and inlet air temperature. General observation
of many combustors indicates that increasing the air/fuel ratio (increasing excess air)
beyond the stoichiometric ratio will increase Nox and SOX and will decrease HC, CO,
and smoke. These affects could be expected on the basis that more air means more
oxygen, both to participate in the formation of NOX and SOX and to promote more
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TABLE 9. EMISSION DATA FOR NATURAL-GAS- AND OIL-FIRED
COMBUSTION EQUIPMENT
Fuel
Emission Levels, Ib per 1, OOP, OOP Btu
Type of Equipment
Reference
CO
HC
NOX
SOx Smoke Aldehydes
Gas Industrial and commercial burners, 15
100-4000 ft3/hr (avg of 12)
Oil Industrial and commercial burners, 15
5-15 gph (avg of 6)
.10
.15
.42
.02
.10
.01
.02
Oil
Oil
Gas
Oil
Gas
Gas
Oil
Oil
Gas
Oil
Gas
Oil
Oil
Test furnace, 1 gph, 75% excess air
General stationary combustion
sources
General stationary combustion
sources
Power-plant boilers
Power-plant boilers
Domestic appliances
Furnaces, boilers, and power plants
Furnaces and boilers
Furnaces and boilers
Stationary combustion sources
Stationary combustion sources
Residential oil burners
Industrial oil burners
16
17
17
18
18
19
20
21
21
22
22
23
23
. 054 . 002
..
..
--
-.
--
. 027 . 004
.104 .018
.618 .031
.038
.075
. 12 . 008
.01
.065
.71
.45
.62
.22
.08
.49
.26
.16
.54
.19
.09
.12
.10
.16
--
1.4
Negl.
--
--
.44
Negl.
.65
.001
.29
2.15
--
.01
--
.16
.02
--
.11
.14
.02
.08
.01
.05
.10
--
--
--
--
--
.01
.01
.0004
.0007
.015
.011
--
--
(a) Data given as percent CO.
TABLE 10. EMISSION DATA FOR EXTERNAL COMBUSTORS ASSOCIATED
WITH STIRLING ENGINES AND STEAM ENGINES
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complete combustion of the fuel. Continued increase in the amount of excess air,
however, can result in lower peak combustion temperatures which lead to less NOX.
Higher air/fuel ratios can also result in less complete combustion and consequent
higher emission of HC and CO owing to reduced peak flame temperature. However
combustors specifically designed for fuel-lean mixtures need not suffer this disadvan-
tage, since a primary combustion zone operating near stoirhiometric can be used.
Figure 3 shows a typical variation of NOX, HC, CO, and SOX with air/fuel ratio.
These data were obtained on an experimental test furnace designed to be typical of
domestic oil-burning equipment. U&) An air/fuel ratio about 2. 3 times the stoichio-
metric ratio yields minimum HC and CO emissions for this unit. Higher air/fuel ratios
result in an increase in HC and CO emissions. The NOX emissions increase almost di-
rectly with increasing air/fuel ratio to a maximum at 3. 6 times the stoichiometric ratio.
Beyond that point, the NOX emissions begin to decrease. SOX emissions increase with
increasing air/fuel ratio until nearly all the sulfur in the fuel has reacted. For most ex-
ternal combustors, the amount of SOX in the exhaust gases can be fairly accurately pre-
dicted from the sulfur content of the fuel.
Figure 4 presents some additional evidence of the relationship between air/fuel
ratio and oxides of nitrogen. These data represent industrial and commercial burners
ranging in heat-release rate from 100, 000 to 4, 000, 000 Btu/hr, using natural gas and
heating oil for fuel. (^) Although there is considerable scatter in the data points, the
curves follow the same pattern as the NOX curve on Figure 3.
Figures 5 and 6 show the variation of NOX, HC, and CO with "Bnlet air temperature
and air/fuel ratio for a 10-hp(s) Stirling engine with a combustion-air preheater running
with No. 2 diesel fuel. (^4) Figure 5 shows that increasing the inlet air temperature has
about the same effect on emissions as increasing the air/fuel ratio does. The higher
inlet air temperature results in higher peak combustion temperatures which promote
more complete combustion as well as higher concentrations of NOX. Higher air/fuel
ratios resulted in lower NOX, HC, and CO emissions as shown in Figure 6. It is seen
that data for this combustor differed from those given for other combustors in regard to
the effect of air/fuel ration on NOX formation.
No emission data were found for external combustors operated on gasoline, the
alcohol fuels, LP gases, hydrogen, and ammonia. The emissions from gasoline and the
alcohol fuels will probably be similar to the emissions from kerosine and JP-4. The
LP gases would probably produce emissions similar to natural gas. The emissions
from hydrogen and ammonia would consist primarily of oxides of nitrogen. Hydrogen
burns with a relatively high flame temperature and therefore might be expected to
produce higher NOX emissions than would other fuels at the same operating conditions.
Ammonia burns with a lower flame temperature and thus would be expected to produce
lower NOX emissions than would the other fuels. Of course, CO and HC emissions
would not exist with these latter two fuels.
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2.0
_20,-
g 15
6"
CO
J3
o>
10
O
I
LL
01.0
O
O
x
O
0.5
0 L_ 01
-I 100
80
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36
0.3
0.2
CD
O
O
O
d
O
O
X
O
O.I
0.0
Natural-gas fired
I
1.0
2.0
3.0 4.0
Stoichiometric Ratio
5.0
6.0
A-57472
FIGURE 4. NITROGEN OXIDES EMISSIONS RELATED TO AIR/FUEL RATIO
FOR NATURAL GAS AND OIL-FIRED IDUSTRIAL AND
COMMERCIAL BURNERS
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600
500
400
E
o.
Q.
O
O
O
300
200
100
200 400 600 800
Burner Inlet Air Temperature, F
1000
1200
A-57473
FIGURE 5. VARIATION OF EMISSIONS WITH INLET AIR TEMPERATURE
FOR GM STIRLING ENGINE
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800
1200 F burner inlet
air temperature
E
a.
a.
o"
X
a>
c
k.
3
-O
C.
C.
o
o
o
25 30
Air/Fuel Ratio
FIGURE 6. VARIATION OF STIRLING-ENGINE COMBUSTOR EMISSIONS
WITH AIR/FUEL RATIO
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HEAT ENGINES
BRAYTON-CYCJLE (GAS-TURBINE) ENGINES
General Description
In its simplest form, the gas-turbine engine consists of a compressor, a combus-
tion chamber, and a turbine. Air is taken in by the compressor at atmospheric pres-
sure, compressed to a higher pressure, and then delivered to the combustion chamber
where fuel is sprayed in and burned. The combustion process takes place at essentially
constant pressure. The resulting high-temperature, high-pressure gas is expanded in
the turbine and then exhausted to the atmosphere. Part of the shaft work developed by
the turbine is used to drive the compressor. The remainder is the output work. A
schematic diagram of the simple turbine engine is shown in Figure 7. The basic ther-
modynamic cycle on which this engine operates is called the Brayton cycle.
Several modifications of the simple cycle have been employed in the development
of gas-turbine engines for vehicular use. In particular, all current engines for on-
highway vehicles have a free power turbine. This is to say that two mechanically inde-
pendent turbine stages are used: one to drive the compressor and the other to drive the
output shaft. This allows a wide range in output-shaft speed which is not possible with
a single-shaft turbine engine. A schematic diagram of a free-turbine engine is shown
in Figure 8.
To improve efficiency, most automotive turbine engines use a regenerative cycle
which incorporates a heat exchanger that delivers heat from the exhaust gas to the
compressed air upstream of the combustor. This cycle is shown schematically in
Figure 9. The heat exchanger is called a recuperator if it has a fixed surface, or a
regenerator if it is a rotary type. Both types are used. The regenerator or recuperator
also decreases the exhaust temperature, but results in a substantial increase in engine
weight and volume.
One recently developed automotive-type gas-turbine engine uses a variable-slip
coupling between the gas-generator section and the power turbine, as shown in Figure
10. This device is used to produce more optimum part-load cycle conditions. In addi-
tion, it provides excellent engine braking.
All of the gas-turbine engine types described above are open-cycle engines; that
is, the working fluid is atmospheric air taken in by the compressor, and the turbine
exhaust is discharged back into the atmosphere. Closed-cycle engines are another
class of gas turbines. In a closed-cycle system, the working fluid is a high-pressure
gas which is continually recycled, as shown in Figure 11. Heat is added to the working
fluid through an additional heat exchanger instead of directly by a combustor. In most
current closed-cycle developments, the intended source of heat energy is a nuclear
reactor, although several fossil-fuel closed-cycle turbines have been built. A precooler
is also required, to dissipate the heat that would ordinarily be rejected in the exhaust
if the engine were open-cycle.
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Fuel in
Air intake
\
X
K'l *
^
Combustor
\
X
\
C.W1UU5T
*
Outpi
Compressor
Turbine
FIGURE 7. SCHEMATIC DIAGRAM OF SIMPLE GAS-TURBINE ENGINE
Fuel in
Air intake
Compressor
r-^k
\ ^
Combustor
Or Co
1
mor
x
esso
r
X
C.XMU
Outpu
turbine Power
turbine
Gas-generator section
A-57475
FIGURE 8. SCHEMATIC DIAGRAM OF FREE-TURBINE ENGINE
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Exhaust
Air
intake
-wv
Combustor
Regenerator
f1
Compressor
Compressor
turbine
Gas-generator section
Output
shaft
Power
turbine
FIGURE 9. SCHEMATIC DIAGRAM OF REGENERATIVE FREE-TURBINE ENGINE
Exhaust
Air
intake
Combustor
Regenerator
Compressor
Compressor
turbine
Power
turbine
Gas-generator section
A-57476
FIGURE 10. SCHEMATIC DIAGRAM OF REGENERATIVE FREE-TURBINE ENGINE
WITH VARIABLE-SLIP COUPLING
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Heat
sink
Precooler
External
heat
source
A/W
Regenerator
Compressor
Compressor
turbine
30utput
shaft
Power
turbine
FIGURE 11. SCHEMATIC DIAGRAM OF CLOSED-CYCLE, REGENERATIVE,
FREE-TURBINE ENGINE
Many other cycle concepts have been proposed and investigated. These include
3-shaft engines with interceding between compressor stages and reheat between turbine
stages, engines with differentially connected gas-generator, power-turbine, and output
shafts, and engines having combinations of these features.
Historical Development
The fundamental principles of the gas-turbine engine were well known by the end of
the eighteenth century; however, turbomachinery operating successfully on the Brayton
cycle is a fairly recent development. The first really successful gas-turbine power-
plant was the aircraft turbojet that developed from intensive work begun in the 1930s.
These units first flew in the early 1940s and were fully operational in military aircraft
by the late 1940s. The early developers had to learn how to design compressors and
turbines of high efficiency and had to find suitable high-temperature materials. Rapid
progress in the development of the aircraft turbine has come as the result of massive
research efforts made in response to the obvious need for high-performance power
plants for military aircraft.
The development of gas-turbine engines for automotive use was begun immediately
after World War II, and has proceeded at a more modest rate of effort. Chrysler in
this country and Rover in Great Britain were among the first to begin developing an auto-
motive turbine. The first Rover turbine-powered automobile ran in 1949. General
Motors began work on the automotive gas turbine around 1948, and Ford around 1950.
All these developers selected the free-turbine configuration early and moved quickly to
the regenerative configuration.
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Parallel development efforts on small turbines for nonautomotive use were
launched by Boeing, Solar, AiResearch Division of the Garrett Corporation, Lycoming,
Continental, and Caterpillar. Most of these engine developments were based on the
single-shaft, nonregenerative turbine.
Principal early automotive-turbine development problems were noise, fuel econ-
omy, durability, and acceleration lag. Acceleration lag proved to be one of the more
difficult problems. For a free-turbine engine to increase power output, it is necessary
for the gas-generator section to speed up. This takes time; hence, there is an accelera-
tion lag as compared with conventional piston engines. Early automotive turbine engines
took about 5 seconds from idle to develop full power, which resulted in unacceptable
throttle response for passenger-car use.
In 1961, the Department of Defense sponsored a competitive design and develop-
ment program for a 600-hp vehicular gas-turbine engine. The fuel-economy target in
this program was a minimum bsfc of 0. 4 Ib/hp-hr, which is equivalent to 34 percent
maximum thermal efficiency. Solar, Ford, and Orenda were the principal participants
in this program. All three received development contracts with the Orenda develop-
ment largely financed by the Canadian Government. While 0. 4 Ib/hp-hr fuel consump-
tion was not achieved, sufficiently low fuel-consumption characteristics were demon-
strated to encourage continued efforts.
In 1963, the Chrysler Corporation undertook a 2-year, 50-car consumer-
evaluation program in which 50 turbine-powered automobiles were loaned for 3-month
periods each to randomly selected families throughout the United States. The fact that
such a program could be conducted bore testimony to tremendous progress. Chrysler
never published in detail on the results of this program, but in press conferences, it
was brought out that the drivers considered the turbine car "completely useable".
Acceleration lag was considered by some to be still a problem. In general, the turbine
car's fuel economy was poorer in the city, better on the open highway, and on the aver-
age, equivalent to that of a modern V-8 gasoline engine.
Current and Projected State of the Art
From the standpoint of physical and performance characteristics, the automotive
gas-turbine engine appears to be an acceptable power plant at present; that is, present
experimental turbine engines could be installed and would perform in conventional pas-
senger cars and trucks, in a manner that would be acceptable to the consumer.
In passenger-car applications, the consumer might find the turbine a little noisier
and a little higher in fuel consumption, but not prohibitively so. Acceleration lag would
probably be perceptible but not overly disturbing. Idle-to-max-power acceleration
times under 1 second are now being obtained. Under some conditions, an exhaust odor
might be noticed. On the other hand, the consumers would gain advantages such as
smoother idling, quicker warmup, freedom from cooling-system performance problems,
and potentially lower maintenance.
The Z-shaft, regenerative gas turbine would have no particular weight and size
advantage over the gasoline engine and at the present would have an initial cost disad-
vantage for passenger-car application. However, the turbine has one advantage
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to the public: turbine exhaust is substantially lower in CO and HC than are current
gasoline engines of equivalent power.
There is considerable interest in the turbine's lower weight in the high-horsepower
(over 300 hp) heavy-duty engine category. One current experimental 375-hp gas-turbine
engine weighs 1700 Ib, compared to at least 2750 Ib for a conventional diesel of the same
horsepower. In addition, the turbine does not require a radiator. As power require-
ments of commercial vehicles continue to increase, the incentive to use gas-turbine en-
gines will also increase.
Durability and maintenance requirements of the automotive turbine have yet to be
evaluated fully; however, the turbine can be considered potentially acceptable and possi-
bly superior to conventional piston engines in regard to these factors. In commercial
aircraft service, maximum time between overhauls for turbojet engines is now as high
as 12, 000 hours, as opposed to 3000 hours for aircraft reciprocating engines. In at
least one instance, no time between overhauls is even specified with measurements of
certain engine performance parameters being relied on to indicate if and when main-
tenance is required. The maintenance labor required for the turbojet is about 1/2 man-
hour per hour of flight time, while that for the reciprocating engine is about 1-1/2 man-
hours per flight hour. There is some reason to doubt that the regenerative automotive
turbine can enjoy this same degree of superiority over the automotive piston engine;
nevertheless, the aircraft experience is an attractive precedent. Chrysler has stated
that no major durability problems resulted during the 50-car program. ^ '
Cost is the principal factor that has yet to be evaluated. Reduction of manufac-
turing cost is probably the area of work receiving the greatest effort at present. There
are two separate camps in regard to cost objectives. In the United States, Chrysler
Corporation has the only development effort aimed at passenger-car applications.
Chrysler is optimistic about eventually being able to produce a turbine engine for this
application for the same cost as and having similar efficiency to a conventional gasoline
engine giving the same performance. On the other hand, General Motors, Ford, and
others are directing their developments toward large commercial-vehicle applications.
In this effort, their avowed target is to meet the manufacturing cost and efficiency of
the diesel engine of equivalent power.
These objectives are not inconsistent. Since the passenger-car power plant
seldom is required to develop full power and has a short life requirement by
commercial-vehicle standards, considerable economies can be made in the design and
materials of the passenger-car engine.
Even though the automotive gas-turbine engine has been under development now
for 20 years, progress in improving the performance of experimental engines is still
advancing rapidly. Many improvements of present engine concepts can be foreseen
but have yet to be made, and many advanced engine concepts have been identified which
have only begun to be evaluated. For example, advances in cooled turbines for air-
craft use, which permit the use of higher turbine inlet temperatures, have not yet been
applied to automotive turbines.
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Current Experimental Automotive Units
Table 11 lists some of the principal features of current experimental automotive
turbines. Also listed for comparison are two experimental turbines for off-highway
and nonautomotive applications. All but the last two are regenerative engines. It is
probable that the performance figures in Table 11 are not all given for the same atmos-
pheric conditions; therefore, the table is not intended to produce a precise comparison,
but rather is intended to give an indication of the general levels of performance being
obtained.
Chrysler Corporation. Chrysler released a detailed description of their fourth-
generation gas-turbine engine in 1964. *'"' This is the engine that was installed in the
50-car-program vehicles. The manufacturers claim this 130-hp(s) unit will provide ve-
hicle activity equivalent-to that of a large V-8 gasoline engine. The fact that automotive
gasoline engines are rated without accessory loads or inlet and exhaust restrictions,
while the turbine rating is presumably "as installed", helps to make this claim credible.
Also, the torque of the turbine engine increases as output speed is decreased; therefore,
the turbine can deliver near-peak power over a broader speed range than the gasoline
engine can.
The Chrysler engine is a free-turbine regenerative unit using twin rotary disk
regenerators, one on each side of the engine, rotating in the vertical plane. This engine
also has variable-angle power-turbine nozzle vanes. At part load, these vanes rotate
so as to reduce the flow area of the nozzle. This has the effect of increasing the
compressor-turbine inlet temperature, and part-load thermal efficiency is thereby
increased. The variable vanes can also be reversed to provide engine braking during
vehicle deceleration.
The gas-generator acceleration time between idle and 97-1/2 percent full speed
was initially 1. 5 seconds; this has been reduced subsequently to less than 1 second.
Chrysler is now in the process of developing a fifth-generation engine. No plans
to place the turbine engine in quantity production have been released by Chrysler.
Rover 2S/140. The Rover and Chrysler engines are the two principal examples of
current engines intended for passenger-car use. Details of the 2S/140 were released
in 1963. (31) The two engines are similar in size and basic arrangement, yet there are
significant differences. Both use a centrifugal compressor, but the Rover unit incor-
porates a variable-inlet-guide-vane feature. The Chrysler engine has a fixed-geometry
axial-flow compressor turbine, while the Rover uses a variable-nozzle-vane radial-
inflow compressor-turbine configuration. The combination of the variable-geometry
compressor and compressor turbine allows the Rover unit to idle at a higher speed,
thereby reducing time required for gas-generator acceleration.
The Rover engine has an axial-flow power turbine, as does the Chrysler; as of
1963, the variable power-turbine-nozzle-vane feature was still under development for
Rover.
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TABLE 11. CURRENT GAS-TURBINE ENGINES
BATTELLE
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Developer
Model No.
Rating,
hp(s)
at rpm
Minimum
bsfc,
lb/hp(s)-hr
Maximum
Thermal Weight,
Efficiency Ib
Length,
in.
Width,
in.
Specific
Height, Weight(a),
in. lb/hp(s)
Specific
Volume
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47
The Rover 2S/140 engine employs twin stationary recuperators to provide regen-
eration. However, a subsequent unit, the 2S/150/R, uses glass-ceramic twin rotary-
disk regenerators, very similar to Chrysler's in shape and location. (32) The 2S/ 150/R
and the 2S/140 presumably use the same basic turbomachinery components.
This basic engine has been used several times in the LeMans 24-hour race, with
good durability records.
General Motors GT-309. The GT-309 engine uses a centrifugal compressor and
axial-flow compressor and power turbines, all fixed geometry. (33) This is the only
engine to date using the variable-slip coupling between the gas-generator and power-
turbine sections, which General Motors calls "Power Transfer". Part-load fuel econ-
omy is enhanced by this feature; in addition, it provides excellent engine-braking
capacity.
The GT-309 regenerator is a drum type, presumably ceramic, rotating in the
horizontal plane and located on the top of the engine.
The General Motors engine is intended for use in commercial vehicles and, at
280 hp(s), is in a power range where it is in direct competition with diesels. The unit
has been installed in a CMC tractor and tested with gross combined weight up to 78, 000
Ib. At 65 mph, the GT-309 delivers the same miles per gallon as a diesel of equivalent
power, but at lower speeds the diesel is more efficient.
The GT-309 is the fifth gas-turbine engine developed by General Motors Research
Laboratories. GM's turbine work has recently branched out to the GM Detroit Diesel
Division, where production versions of the engine are reportedly being designed. (34)
Target dates for production have not been announced. Initial units will reportedly be
intended for industrial rather than vehicular applications.
Ford 707. No performance figures have been released on Ford's most recent
turbine development, except that it is designed to produce 375 hp(s).(3->) This places
the 707 near the top end of the power range now covered by truck diesels. The weight
saving is considerable over a diesel of comparable power.
The engine appears very similar in configuration to the Chrysler engine, with a
centrifugal compressor, axial-flow turbines, variable power-turbine nozzle vanes, and
twin rotary-disk regenerators. Two previous Ford turbine engines used a complicated
3-shaft arrangement that had excellent potential for good part-load economy, but which
has apparently been dropped. (3°)
Ford officials announced last year that they intend to produce gas-turbine engines
for trucks by the "early 1970s". This might be a little optimistic; nevertheless, it
appears that Ford has a substantial turbine program under way.
Cater pillar/Boeing. The Caterpillar Tractor Company recently purchased rights
to the gas-turbine designs and technology that were developed by Boeing. The Boeing
unit is included in Table 11 as an example of the level of efficiency and specific size and
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weight that can be obtained with a free-power-turbine nonregenerative unit. It is
evident that the regenerative feature adds considerably to bulk and weight; however, the
efficiency of nonregenerative units is generally unacceptable for commercial use.
Nevertheless, the Boeing unit is a vehicular engine and has been used in military vehi-
cles and for marine propulsion. The most recent Boeing experimental engines are
reported to be regenerative.
Caterpillar has had a turbine-development program of its own for a number of
years before the Boeing purchase. (38) Caterpillar is reportedly interested in producing
turbine power plants for over-the-highway commercial vehicles.
USAMERDC/AiResearch 331-30. This engine is being developed for the United
States Army Mobility Equipment Research and Development Center (USAMERDC) by
AiResearch and is currently in the final phases of development. It is a single-shaft,
nonregenerative machine and is included in Table 11 as an example of the level of effi-
ciency and excellent specific size and weight that are possible with this kind of machine.
Lycoming ACT-1500. The development of a 1500-hp(s) automotive-type regenera-
tive gas-turbine engine was announced in 1967 by Avco Lycoming Division and the U. S.
Army Tank-Automotive Command. (39) Design targets are 1600 Ib weight and a fuel
consumption of 0. 38 bsfc. This is equivalent to a specific weight of about 1 lb/hp(s) and
a thermal efficiency of 36 percent.
This engine is intended for use in heavy military vehicles, and is mentioned here
as an example of future performance levels that can be expected from very large
engines. Lycoming is a large manufacturer of turbines for helicopter use.
Other Developments. Other organizations are capable at present of producing
automotive-type gas turbines. Both Solar and AiResearch have developed regenerative
engines and both have an incentive to be interested in vehicular engines. Solar has been
a Division of International Harvester for some time, and AiResearch and Mack Trucks
are now both subsidiaries of the Signal Oil organization.
Continental has been both a developer of turbine engines and a producer of vehic-
ular engines for some time, and should be regarded as a potential producer of automo-
tive gas-turbine engines.
Closed-Cycle Gas-Turbine Engines
Closed-cycle gas turbines have not been seriously considered as an automotive
power plant. The principal disadvantage of the closed-cycle system is the large addi-
tional heat exchangers that would be required to transmit the heat to the working fluid
in place of the combustor, and to reject the heat that normally goes out the exhaust.
The required precooler alone would probably be three times the size of a conventional
automobile radiator. A second disadvantage is the internal lubricant sealing problem.
Since conventional lubricants leaking into the working fluid would decompose thermally
and foul the system, it has been necessary to use either gas-lubricated bearings or
elaborate sealing systems in current experimental nuclear-powered units.
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Closed-cycle Brayton engines are of interest principally for the conversion of
nuclear power - both reactor and radioisotope - and solar energy. The Army has had
a 400 hp(e), portable, reactor-powered system under development for several
years. (^0) Thj-S system uses nitrogen gas as the working fluid.
Smaller Brayton-cycle units ranging from 4 hp(e) to 27 hp(e) have been investigated
by NASA and USAF for aerospace applications.'41'^' These units use either argon or
helium-xenon mixtures as the working fluid. Gas-lubricated bearings are used in the
turboalternator units.
At present, closed-cycle gas-turbine engines do not appear to have any potential
for automotive use.
Practical Size Range
From available data on current experimental automotive units, it appears that
gas-turbine engines are certainly practical in the range of 100 hp(s) and above. How-
ever, turbines do not miniaturize very well. As design horsepower is reduced, the
design speed of turbine engines increases and the diameter of the rotating components
gets smaller. Very small rotating components usually suffer from high ratios of tip
clearance to blade height, which tend to degrade component efficiency. In addition, the
cost competitiveness of the turbine suffers as design horsepower is reduced.
The Williams Research Corporation has built a 70-hp(s) regenerative turbine
engine which delivered good efficiency installed in a pleasure boat. This organization
has also built a 30-hp(s) engine that ran at 85, 000 rpm with only slight penalties in
component efficiency. It appears, then, that a 30 hp(s) vehicular gas turbine may be
practical from a performance standpoint, but would probably suffer a cost disadvantage.
It is not believed, however, that a competitive vehicular gas turbine of the low capacity,
16 hp(s), required for the utility-car application could be developed.
Fuel Requirements
Gas-turbine engines will tolerate a broad range of fuels, and operate best on fuels
such as kerosene, JP-4, No. 1 diesel, and No. 2 diesel. Unleaded gasoline can be
used, but it is not as desirable as a turbine fuel. Chrysler Corporation, in the 50-car
program, recommended that unleaded gasoline be mixed with diesel fuel in warm
weather, or used by itself in cold weather. '4^' Turbine engines should not be run for
extended periods on leaded fuels because the combustion products cause fouling and
corrosion.
Air Filtration
Entrained dust particles can cause erosion and rapid deterioration of the rotating
components of unprotected turbine engines; therefore, efficient air filtration must be
provided. Because turbine engines consume large quantities of air and are sensitive
to inlet pressure losses, the required filter units are fairly large - comparable in size
to the radiator of a conventional automobile. The air-filtration problem has been
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investigated in some detail by the U. S. Army Tank-Automotive Command and by
USAMERDC and has been found to be manageable in terms of military operational re-
quirements. '44) Passenger-car requirements are probably less severe.
Emission Characteristics
To date, the data on gas-turbine-engine exhaust emissions are somewhat limited;
however, it is apparent that the gas-turbine exhaust can be substantially cleaner than
exhaust from present controlled-emission gasoline engines in regard to unburned hydro-
carbons and carbon monoxide. Also, no lead compounds are present in the turbine
exhaust, as unleaded fuels are used.
General Motors has recently released data comparing emissions from their
GT-309 gas-turbine engine and an equivalent, controlled-emission gasoline engine on
the California Cycle. ( ' These data indicate that less than 1 ppm unburned hydrocar-
bons and less than 50 ppm (0. 005 percent) carbon monoxide were present in the turbine's
exhaust over a broad range of operating conditions. These figures cannot be compared
directly with existing gasoline-engine standards, however, because the total mass of
gas-turbine exhaust is 10 to 100 times higher than that of a gasoline engine of equivalent
power, depending upon the mode of operation.
On a total-mass-flow basis, the GT-309 emitted only about 16 percent of the
hydrocarbons and 12 percent of the CO that were emitted by the controlled-emission
gasoline engine with which it was being compared. Since the California test emission
rating for that gasoline engine was 222 ppm hydrocarbons and 1. 28 percent CO, the
emissions from the turbine would be equivalent to those of a comparable gasoline engine
having an emission rating of approximately 36 ppm hydrocarbons and 0. 15 percent CO.
This level of emission is only 4 to 5 percent of that which is typical for noncontrolled-
emission gasoline engines.
Also, a comparison of total mass of emissions is not necessarily valid either.
It can be conjectured that when the GT-309 engine is operating in an environment in
which the ambient concentration of hydrocarbons is 1 ppm or above, as is sometimes
the case, and if the turbine engine emits less than 1 ppm in the exhaust, then the turbine
is actually cleaning up the environment. Therefore, it appears that the net contribution
to atmospheric pollution is the meaningful parameter. Further, it is apparent that net
contribution is meaningful only when compared with an acceptable standard ambient
pollution level.
In regard to oxides of nitrogen, the GM data show that the GT-309 emits a greater
total mass of NOX than the comparable piston engine in fact, 1. 75 times as much. The
computed composite emission value for the particular gasoline engine in question is about
1620 ppm. Thus the equivalent value for the GT-309 is about 2800 ppm NOX. Ford and
Chrysler, on the other hand, indicate that emission of NOX from present engines can be
below projected standards of 350 ppm, on a corrected basis. Apparently, the turbine
does not enjoy a clear advantage with respect to NOX, although emission levels can be at
least potentially acceptable for the near future.
In regard to smoke and odor, two manufacturers claim no problem with either,
and a third admits to having problems with both under some operating conditions.
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Anyone who has stood close to a taxiing jet aircraft and watched a jet take off
knows that gas-turbine exhaust can exhibit an objectionable odor and can contain smoke.
Also, it is known that a poorly designed or defective burner of any type can deliver poor
combustion. It is doubtful that gas-turbine researchers have a complete understanding
of how burner-design variables relate to exhaust-emission levels. Therefore, it seems
reasonable to expect some significant differences in exhaust-emission levels between
different engines, and also to expect improvements to be made.
One final consideration with respect to emission characteristics is the fact that
gas turbines have very low oil consumption and the level of oil consumption does not
tend to drastically increase with age as with gasoline engines. This will result in less
deterioration in emission characteristics with age.
"1980" Physical, Performance, and
Cost Characteristics
Specific Weight. It is doubtful that the weight characteristics of gas-turbine
engines for automobiles will be improved substantially in the next few years. While
technological advances and design improvements may make weight reductions possible,
cost-reduction considerations will probably require weight-increasing compromises
such as the use of lower strength or higher weight, but lower cost, materials. A spe-
cific weight of 3. 0 lb/hp(s) including air cleaner is projected for a "1980", 180 hp(s)
engine. While there is no representative 30 hp(s) engine upon which to base a projection
of specific weight, it is believed that the specific weight of a "1980"-, 30 hp(s) engine
would be approximately 4. 0 lb/hp(s).
Specific Volume. The specific volume would be on the order of 0. 09 ft /hp
including air cleaner for a 180-hp(s) gas-turbine engine. Again, while there is no repre-
sentative 30 hp(s) engine upon which to base a projection, it is believed that the specific
volume of a "1980", 30 hp(s) engine would be approximately 0. 12 ft^/hp(s). Of the above
volumes, required air-cleaner volume would be on the order of 0. 01 ft /hp(s), including
ducting.
Efficiency. A maximum brake thermal efficiency of 35 percent should be attain-
able with a 180 hp(s) engine, equivalent to a bsfc just under 0. 4 lb/hp(s)-hr. At 30 hp(s),
some penalty can be expected perhaps as maximum of 30 percent efficiency could be
obtained. Part-load efficiency characteristics are shown in Figure 12. The curves
shown are representative of three types of engines, one with variable-angle power-
turbine nozzle vanes, another with a variable-slip coupling between the gas generator
and power turbines, and the third with variable-geometry compressors and turbines.
These curves are drawn with the assumption that the power turbine is running at peak-
efficiency speed for the given power level. The Rover engine characteristic is consid-
ered typical of what could be achieved in "1980".
Startup Characteristics. The start-up time of a gas-turbine engine is dependent
upon the capacity of the starting system and will probably be less than 15 seconds to
power output under normal ambient conditions. Warm-up time for full power is more
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rapid than for a conventional piston engine. The starting sequence is as follows: the
cranking motor turns the gas-generator section up to ignition speed at which the fuel is
turned on and lit off. The cranking motor continues to assist up to some speed below
idle at which the turbine is self-sustaining. In all probability, starting systems will be
fully automatic, and the sequence will be initiated by the driver simply turning the
"ignition" key.
Power-Output Characteristics. Although peak power-turbine speeds will be in the
range of 20, 000 to 50, 000 rpm, reduction gearing is used to reduce the output-shaft
speed to conventional IC-engine speeds.
Typical speed-torque characteristics are shown in Figure 13. As is evident from
this figure, the torque of the turbine engine rises as the output-shaft, or power turbine,
speed is reduced. Since there is no fixed coupling between the free power turbine and
the gas-generator section, the engine will operate with the output shaft stalled. In the
example shown in Figure 13, the stall torque is over three times the maximum-speed
torque, although the ratio of stall torque to maximum-power torque is about 2. 2. Since
the power-turbine and output shaft can be stalled while the gas-generator section con-
tinues to run, and since the engine has a large torque increase at stall, the use of
a fluid coupling, or torque converter is not required in an automatic transmission de-
signed for use with the gas turbine.
Corresponding typical speed-power characteristics are shown in Figure 14. At a
given gas-generator speed, the power output is dependent upon the output-shaft speed.
The dashed line is drawn through the peaks of the power curves and represents an
optimum-load characteristic for free-turbine engines. Fortuitously, vehicle road-load
speed-power curves are similar in shape, and, therefore, near-peak economy during
cruising can be obtained with a single high-gear ratio.
As mentioned previously, the gas-turbine engine cannot deliver a sizable instan-
taneous step increase in power, since the gas generator must change speed to signifi-
cantly change power in present turbines. However, it has been demonstrated that full-
acceleration response time can be brought below 1 second. Since gasoline engines have
carburetion and induction lags on the order of a few tenths of a second, the turbine
acceleration lag is probably not distressing, if even perceptible. Engines using a.
variable-geometry compressor and compressor turbine could be controlled so as to
change air flow and hence, power, without change in gas-generator speed, thereby
further improving the response characteristics of the turbine engine.
Power-Surge Capability. The gas turbine does not have a power-surge capability
in excess of maximum power.
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100
Chrysler 4th generation
20 40 60 80
Percent of Maximum Power
100
FIGURE 12. PART-LOAD EFFICIENCY CHARACTERISTICS OF REGENERATIVE
GAS-TURBINE ENGINES
350
100 percent gas generator!
speed I
20 40 60 80 100
Percent of Maximum Output-Shaft Speed. A-5747?
FIGURE 13.
SPEED-TORQUE CHARACTERISTICS OF A FREE-TURBINE
AUTOMOTIVE GAS-TURBINE ENGINE
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o>
5
o
O_
X
o
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liquid pump draws liquid at a low pressure from the reservoir and forces it under high
pressure through the heater where the liquid picks up heat from the expander exhaust
vapor. For some working fluids and/or engine applications, this liquid heater is not
used or required. The high-pressure liquid then enters the vapor generator where it is
converted to superheated vapor by further heating. The hot, high-pressure vapor is then
metered into an expander through a flow-control valve. The vapor expands to a low
pressure and temperature and gives up energy to the expander. The vapor leaves the
expander and enters the liquid heater where it gives up more heat energy. The vapor is
then converted back to a liquid in a condenser, which is typically air cooled as shown.
The resulting low-pressure liquid is then returned to the reservoir by a pump to com-
plete the fluid cycle.
Output
shaft
Vapor-flow
control valve
Condenser
^^ Liquid
I /""N condensate
\J pump
» 1
Liquid-reservoir
or storage tank
Exhaust
High-pressure
liquid pump
FIGURE 15. SCHEMATIC OF TYPICAL RANKINE-CYCLE
ENGINE COMPONENTS
Historical Development
The use of Rankine-cycle engines in vehicles dates back to 1827, or before, when
primitive steam engines were used to power coaches. Steam cars were sold commer-
cially in the late 1800s and early 1900s with familiar names such as Stanley Steamer,
Locomobile, White, and Doble. Of the 126 different makes of steam cars produced, the
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Stanley Steamer was the most popular. Most of the early steam automobiles, like the
Stanley, had a boiler containing a large quantity of water, and used a noncondensing,
single-expansion engine. With the large thermal-storage capacity of the boiler, up to
1/2 hour was required to get going from a cold start. Since they were noncondensing,
these engines also used large quantities of water. The Stanley Steamer was produced
from 1899 through 1925, reaching a peak production of 2,500 vehicles in 1910. A con-
denser in the system was introduced in 1915.
The Stanley Steamers were quiet-running, lightweight cars of surprisingly good
performance, even by present standards. Although maximum continuous speeds were
limited to 45 to 60 miles per hour by the low heat-release rate of the boiler, tremendous
bursts of acceleration and high speed were possible because of the large energy-storage
capacity of the boiler.
Perhaps the most advanced vehicle built during the steam era was the Doble Steam
Car, produced by Abner Doble during the twenties. The latest Doble was built in 1930.
The vehicle had a 142-inch wheelbase and weighed 5, 500 pounds. The 4-cylinder steam
engine ran condensing and was rated at 150 hp(s), continuous. The engine had a low-
inertia monotube boiler that was electrically controlled and could get up to pressure
within 30 seconds.
Initially, the gasoline auto gained popularity because it was free from the frequent
water stops and difficult boiler-control problems of the steam car. However, the ulti-
mate demise of the steam car was apparently not due to lack of continuing engineering
development but to lack of adequate service and repair facilities, and to the high costs
resulting from low-volume production, as compared with the more popular gasoline car.
In 1951, the McCulloch Corporation undertook the development of a modern auto-
motive steam power plant for installation in a luxury sports-type car. Although many
advantages were claimed for the engine, no units were ever sold, and the program was
dropped in 1954.
In 1964, the Convair Division of the General Dynamics Corporation published a
design study of a steam-turbine power plant for a military battle tank. From this study,
it was concluded that the steam power plant was well suited to the tank application, as
well as to other off-road vehicular applications. Design-optimization and cost studies
remained to be done at that time, however.
Apparently, the only automotive steam powerplant that has been offered for gen-
eral sale in recent years is made by the Williams Engine Company of Ambler, Pennsyl-
vania. In 1967, the Williams brothers were taking orders for their complete power
plants for $6,450, or a Chevelle automobile with their engine installed for $10,250.
These engines were to be made in lots of ten after accumulation of sufficient orders.
Actual development and/or design studies of automotive steam powerplants are
being conducted by other organizations at present. Thermo Electron Engineering Cor-
poration (TEECO) of Waltham, Massachusetts, has developed a sealed, 0. 13 hp(e)
portable steam-powered generator unit. More recently, TEECO has prepared prelimi-
nary paper studies of automotive units. Controlled Steam Dynamics of Mesa, Arizona,
has reportedly been engaged in the development of both a small unit of a few horsepower
output and an automotive-size steam power plant. Details of this development are
reported to be forthcoming in 1968. Other developments receiving publicity recently
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include: Gibbs & Hosick of Winston-Salem, N. C. , with several swash-plate piston
expanders ranging up to ZOO cu in. displacement; Smith and Petersen of Midland City,
California, with a steam engine for installation in a Volkswagen and a 42 cu in. outboard
engine converted to steam; and Pritchard Steam Power, Ltd. , of Victoria, Australia,
with a 100-hp, V-2 steam engine in a 5-ton truck.
The development of steam power plants for other applications, such as large
electric utility generation plants, marine propulsion units, and railroad locomotives are
well known. One perhaps lesser-known development was a unit built by the Besler Cor-
poration for aircraft propulsion that flew for short periods in 1933.
Recently, Rankine-cycle engines have received some consideration for use in
small, silent, military ground power units and for aerospace energy-conversion applica-
tions. The work on the small ground power units has been sponsored principally by the
United States Army Mobility Experimental Research and Development Center
(USAMERDC) at Fort Belvoir, Virginia. Various experimental units have been built
using steam, mercury, and organic fluids' ' as the working fluid. One such unit is the
4-hp(e) SCAP system, which is a turbine-drive, mercury, Rankine-cycle generator set
developed for USAMERDC by TRW(47).
Units for aerospace application are being developed principally for the conversion
of solar energy, radioisotope power, or reactor power to electrical energy. Developers
include TRW, Philco-Ford, Aerojet-General, and AiResearch. Working fluids for the
systems are primarily liquid metals or organic fluids. \ '
Current and Projected State of the Art
Automotive experience with Rankine-cycle (steam) engines of recent design is very
limited. However, presuming that the application of modern technology to the design of
Rankine-cycle engines can produce durable, trouble-free systems, the Rankine-cycle
engine must be regarded as a potentially quite acceptable passenger-car power plant.
The operating characteristics of positive-displacement Rankine-cycle (steam)
engines are likely to produce strikingly different sensations to the driver or passenger
accustomed to gasoline-engine-driven cars. One of the principal differences is that
the steam engine stops whenever the vehicle stops; there is no need to unclutch and
"idle" the engine. During acceleration, manual changes in the cutoff position will pro-
duce about the same sensation as gear changes in a conventional automatic transmission.
If a continuously variable automatic cutoff control is used, there will be no gear-change
sensations whatever.
The principal noise from the steam engine is that emitted by the combustion
system. Since the expander is relatively quiet, there will probably be little impression
that the power plant is laboring under high-speed or rapid-acceleration conditions.
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Current Experimental Automotive Units
General. All recent Rankine-cycle engines built for automotive use have steam
for the working fluids and have piston-type expanders. The engines are semiclosed-
cycle systems that require some make-up water. Boilers are low-thermal-inertia or
"once-through" types, which give good response and which permit rapid warmup from a
cold start. The pistons are lubricated by conventional hydrocarbon oils, which need to
be separated from the feedwater to avoid fouling problems in the boiler. Freezing is
still a problem in cold weather. It can be assumed that an objective in advanced develop-
ments will be to achieve completely sealed, self-lubricating systems.
Data on several automotive steam engines, either built or studied since 1950, are
given in Table 12.
The Williams Engine. As previously stated, the Williams engine is apparently the
only automotive steam power plant that has been offered for general sale in recent years.
The expander is a 4-cylinder, single-acting, single-expansion crosshead machine of
105-cu in. displacement with a 3. 50-in. bore and a 2. 75-in. stroke. A number of stan-
dard gasoline-engine parts are used in construction of this engine. Each cylinder has
an intake valve, an exhaust valve, and an exhaust port in the cylinder which is uncovered
near the bottom of the piston stroke. The intake-valve motion is controlled by a cam-
shaft having compound lobes which change the valve timing as the camshaft is moved
axially into one of four positions.
With steam at 1000 psi and 1000 F, a power output of 150 hp(s) or 250 hp(s) is
claimed for continuous or intermittent operation, respectively. Stall torque is 1105
Ib-ft, and good fuel mileage is claimed - 20 to 30 mpg.
Steam is exhausted at 3 to 9 psig and 230 to 300 F. The steam passes through a
shell-and-tube feedwater heater and then through a condenser, which is a heavy-duty
conventional radiator. The condensate and any uncondensed steam is pumped by a gear
pump to a water reservoir which is vented to the atmosphere. Water loss through this
vent can amount to 12 gal in 500 miles. Water is pumped into the boiler through the
feedwater heater by a 3-cylinder piston pump. Water flow is controlled by a boiler
pressure control.
The steam generator is a stainless-steel monotube flash boiler heated by an on-off
pressure-atomizing fuel burner which is controlled by a boiler temperature and pressure
control. Normal operation from a cold start can be resumed in 30 seconds.
The pistons are lubricated by oil injected through the cylinder wall.
Turbine Versus Positive-Displacement Expanders
In the past, turbine expanders for automotive steam engines have not been attrac-
tive. There are two principal reasons for this. One is the large number of stages
required to obtain efficient operation with steam turbines. The second is that in auto-
motive applications, turbine expanders require a transmission, while piston-type
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TABLE 12. DATA ON SEVERAL AUTOMOTIVE STEAM ENGINES EITHER BUILT OR STUDIED SINCE 1950
CD
>
H
H
m
r
r
m
Z
m
o
z
(A
H
C
H
m
i
o
o
r
c
z-
co
c
m
o
H
O
5
m
Engine
Developer or Researcher Reference
Williams Engine Co. 49
McCulloch Corp. 50
Gibbs Hosick Trust 51
Richard J. Smith 52
USAMERDC --(d)
Thermo Electron Engineering 53(e)
Corporation
Microtech Research Co. 54(e)
General Dynamics/Convair 55^ '
S. W. Gouse. Jr. 25. 56(c) $/hp(s)
150
at 2400
120
at 1200
60
at 2500
250
at 6000
3
at 3600
100
at 1680
175
500
--
50
1000 5.4 0.14 -- PO<30 3.4 1.67 44
1000
2000 8.0 -- 23 FPO<30 -- 1.25
900
2000 -- -- -- -- 2.3
850
1000 -- -- 28 FPO-14 1.8
700
700 20.0 -- 19 PO-120
850
1200 5.0 -- 28 -- 1.1
1250
1500 17.2 0.67 16 FPO-500
1100
1200 -- 0.16 22 -- -- 1.5-2.0
1000
5-10 -- 25-30 -- -- -- 3 (?)
2500 -- -- 24 FPO<10
670
01
vO
(a) PO time to power output; FPO * time to full power output.
(b) Torque ratio = the ratio of stall torque to rated torque.
(c) Power-surge ratio = the ratio of short-term, "burst" , power to continuous rated power.
(d) Estimated parameters for steam engine currently under test by United States Army Mobility Equipment Research and Development Center.
This engine is not for vehicular application, but is included to illustrate state of the art for small steam engines.
(e) Paper studies only.
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expanders, in general, do not. The variable cutoff capability of the piston expander
enables high stall torque to be generated and eliminates the need for gear changes. In
addition, the rotation of piston expanders can be reversed without transmission gearing.
Nevertheless, a turbine expander would have a favorable torque characteristic,
similar to that of a two-shaft gas turbine, with a stall torque about twice the maximum-
power torque. Therefore, the steam-turbine transmission would be simpler than that
required for a gasoline or diesel engine. Because of the inherent durability advantages
of the turbine, as compared to piston expanders, Rankine-cycle turbine engines are
potentially attractive for use in commercial vehicles such as trucks and buses, where
higher power engines are required and where higher first cost can be justified to obtain
longer life and lower maintenance costs. In addition, the use of higher-molecular-
weight fluids, such as organic fluids, enables the use of fewer stages in the turbine
expander. Should any of these fluids prove acceptable for use in automotive Rankine-
cycle engines, the turbine expander would look more attractive.
Alternative Working Fluids
As discussed earlier, for some nonautomotive applications, liquid metals and
certain organic fluids as well as water are being used as the working fluid in Rankine-
cycle engines. These alternative fluids generally are being used in systems with tur-
bine expanders.
The attractiveness of candidate fluids depends upon their chemical, mechanical,
and thermodynamic properties, including long-term chemical stability at elevated tem-
peratures and lubricating qualities. One problem with organic-fluid systems is that
they are currently limited to relatively low vapor generator temperatures. This is
because current candidate fluids generally are not chemically stable at temperatures
above 600 to 700 F, although data on the long-term thermal stability of these fluids are
far from complete. Some organic fluids, particularly those containing only carbon and
fluorine, are reported to be stable at temperatures higher than these, but their overall
desirability for use in Rankine engines has not yet been determined.
The potential for designing systems in which the working fluid can also act as a
lubricant enhances the desirability of using the organic fluids in positive-displacement
as well as in turbine systems. One additional attractive feature of the organic-fluid
system is its freedom from the freezing problem.
A major problem with liquid-metal systems for automotive application would be
their toxicity hazard.
In conclusion, because of water's good chemical, mechanical, thermodynamic,
and thermal stability properties, particularly with respect to use in lower power,
positive-displacement expander systems, it is currently the most likely candidate for
passenger-car Rankine-cycle engines. If the lubrication and freezing problems encoun-
tered with water are not satisfactorily resolved, however, one of the organic fluids may
prove desirable for this application. For vehicle applications where a turbine expander
is desirable, such as where long life and maintenance-free operation are required, the
use of fluids other than water appears to be desirable. In any event, much investigative
work remains to be done with respect to alternative fluids before optimum fluids can be
determined for various vehicle applications.
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Emission Characteristics
The emission characteristics of Rankine-cycle engines would be consistent with
those discussed earlier for the general class of external-combustion engines. Reported
data for the Williams engine are: 20 ppm HC, 0. 05 percent CO, and 70 ppm NOX. The
HC and CO emissions, while very low compared with those of gasoline engines, are not
outstanding in comparison with those attained with other external combustors.
"1980" Physical, Performance, and Cost Characteristics
Specific Weight. The specific weight of the two existing automotive steam engines
listed in Table 12 is in the range of 5 to 8 lb/hp(s). These specific weights include the
weight of all components shown on the schematic in Figure 15. It is important to note
that for automotive application, a transmission will probably not be required. Since
many of the engines listed in Table 12 are design studies only, are in the earliest stages
of development, or have some existing shortcomings that remain to be resolved, no
reduction of this specific weight, 5 to 8 lb/hp(s), is estimated for fully developed, "1980'
engines.
Specific Volume. The specific volume listed for the Williams steam engine is
approximately 0. 14 ft3/hp(s). Assuming a higher condensing capacity and resolution of
other existing shortcomings, an equivalent specific volume, 0. 12 to 0. 16 ft /hp(s), is
estimated for fully developed "1980" engines.
Efficiency Characteristics. With accessory loads, heat losses, etc. , the maxi-
mum reported brake thermal efficiency of current engines is around 28 percent. Future
development work should increase this to about 30 percent.
Figure 16 shows the variation in brake thermal efficiency at typical road loading
conditions claimed for two engines. These data should not be considered typical of
steam engines in general but do show that efficiency holds up well down to low power
levels. Engines should and will be tailored to give best economy for the applicable
vehicular duty cycle.
Start-Up Characteristics. Current steam engines require under 30 seconds to
develop power output from a cold start and somewhat over 30 seconds to full power. One
design study on steam engines claims a start-up from a frozen condition will be possible
in 10 seconds. The boiler will require more sophisticated controls as boiler size,
thermal inertia, and start-up time are reduced. Such controls could be developed on the
basis of current technology. The danger of damage from water in the cylinders during
a cold start has been eliminated by suitable valving.
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30
c 20
o>
G
UJ
E
O)
jc
o
m
10
Rated speed
O McCulloch engine (ref. 50)
D TEECO engine (ref. 53)
0 10 20 30 40 50 60 70
Percent of Rated Power
80
FIGURE 16. STEAM-ENGINE EFFICIENCY AT TYPICAL
ROAD LOADING CONDITIONS
Power-Output Characteristics. Table 1Z gives data on the rated power, speed,
and torque ratio of pertinent engines. Figure 17 gives the torque and power as a function
of speed in dimensionless terms for two engines. The difference in just these two
designs reflects the great flexibility in output available with steam power. A typical
reciprocating steam engine will have provisions for varying the cutoff point. The cutoff
affects the amount of steam allowed to enter the cylinder and do work on the piston.
Varying the cutoff effectively accomplishes the same thing as a gear shift in a transmis-
sion. Thus, no one dimensionless plot would be typical of steam engines in general.
Power-Surge Capability. Early steam cars had large power-surge capability
because of thermal-energy storage in the boiler. However, as system weight, size, and
start-up time are decreased, the thermal inertia of the boiler is reduced and the power-
surge capability diminishes. One current engine which has about a 30 second start-up
time has a short-duration power-surge capability of 167 percent of rated power. Another
experimental vehicle engine had a power-surge capability of approximately 125 percent
of rated power. Since future engines will have low-thermal-inertia vapor generators to
minimize weight, size, and start-up time, an effective power-surge capability of 125
percent of rated power is assumed for the "1980" engine.
Specific Cost. It is impossible to fix a specific cost figure based on current data
because no figures are available which reflect volume-production quantity. The best
available evidence indicates that the specific cost will be less than that of a comparable
diesel engine and about equal to that of an equivalent V-8 engine with automatic trans-
mission: The additional cost of the transmission will be eliminated with steam power.
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220
Percent of
rated torque
Percent of
rated power
o McCulloch engine(Ref.50)
a TEECOengine(Ref.53)
0 20 40 60 80 100 120 140 160
Percent of Rated Speed A-57478
FIGURE 17. EXAMPLE STEAM-ENGINE TORQUE AND POWER
CHARACTERISTICS
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Assuming that: (1) a steam engine will cost about the same as an equivalent V-8 engine
with automatic transmission; (2) current larger V-8 engines cost about $2 to 3/hp(s)
and that smaller engines might cost $3 to 4/hp(s); (3) automatic transmissions cost about
$1. 0/hp(s); and (4) in equivalent installations the steam engine would be rated at about
20 percent less power than actual available V-8 engine power, an estimated cost of
approximately $4 to 6/hp(s) is calculated for the steam engine.
STIRLING-CYCLE ENGINES
General Description
In its most common form, the Stirling-cycle engine, sometimes called a regenera-
tive hot-air engine, is an external-combustion, closed-cycle, piston-type power plant
that uses a gaseous internal working fluid, usually hydrogen or helium. In one currently
preferred configuration, pistons are arranged in pairs and linked together through a
common drive mechanism. One piston is called the displacer, and the other is called
the power piston, as shown in Figure 18. Heat is added by an external combustor to the
heater tubes connected to the cylinder on one side of the displacer piston. The water-
cooled, cooler tubes are connected to the other side of the cylinder between the displacer
piston and the power piston. The heater tubes and cooler tubes are connected to each
other through the regenerator, which is usually composed of small cylinders filled with
a fine wire matrix. At any instant, the internal working-fluid pressure is essentially
uniform, since the working spaces formed by the two pistons are connected to each other
through the cooler tubes, regenerator, and heater tubes.
In operation, the motion of the power piston lags the motion of the displacer piston
by about one-quarter of a revolution. Referring to Figure 18, when the power piston is
in the downward position, the displacer piston is moving upward. This forces the bulk
of the working fluid around through the heater tubes, regenerator, and cooler tubes to
the "cold" space below the displacer. In so doing, the bulk of the fluid is cooled and its
pressure drops. The power piston then moves up, compressing the fluid and increasing
its pressure. The displacer then moves downward, moving the bulk of the fluid back to
the hot space and further increasing its pressure. The power piston then moves down-
ward, expanding the fluid and completing the cycle. These motions do not occur sepa-
rately, but are effected by the out-of-phase motion of the two pistons.
Because the internal fluid pressure is low when the power piston moves upward
and high when the piston moves downward, a net work output is produced. The effect of
the regenerator is to improve efficiency by storing heat from the hot fluid as it flows
toward the cold space and releasing the heat when the flow reverses.
Fuel is burned in the combustion chamber shown at the top of the engine, and the
combustion gases pass through the array of heater tubes surrounding the combustion
chamber. The gases leave at a high temperature and, therefore, to conserve heat
which would otherwise be wasted, exhaust gases pass through the preheater which cools
the exhaust and warms the incoming combustion air.
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Fuel Nozzle
Cooled Exhaust
Outlet
Preheater Spiral
Passages
Preheater
Assembly
Hot Exhaust
Hot Space
Regenerator
Cylinder
Cooler Tubes
Cold Space
Power Piston
Rhombic Drive
Power Piston
Connecting Rod
Timing Gears
Combustion Chamber
Heater Tubes
Hot Combustion Air
Displacer Piston
Combustion Air
Inlet
Cooling Water
Connections
Seal
Buffer Space
Seal Assembly
Displacer Piston Rod
Power Piston Rod
Power Piston Yoke
Power Piston Yoke
Pin
Displacer Piston
Connecting Rod
Displacer Piston
Yoke
FIGURE 18. CROSS-SECTION DRAWING OF DISPLACER-TYPE
STIRLING ENGINE
Figure reprinted from Reference (58) by permission
of General Motors Corp.
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Historical Development
The Stirling-cycle engine is not a recent development. It was patented in 1816 by
Robert Stirling, a Scottish clergyman. Thousands of engines operating on this principle
were built and used in the nineteenth century, usually without the regenerative feature.
These engines were slow and inefficient, but were desirable because they were substan-
tially safer than the steam engines of that day. However, they were eventually made
obsolete by the development of the more efficient and compact internal-combustion
engines, and by the advances in the art of making explosion-proof steam-engine boilers.
Just prior to World War II, the N. V. Philips Laboratories at Eindhoven, Nether-
lands, undertook the development of a modern version of the Stirling engine as a means
of producing electrical power in remote areas. This new breed of Stirling engines,
characterized by high efficiency and quiet operation, has remained in the experimental
stage through the present time. The principal developments in the U. S. have been
carried out by General Motors Corporation under a cooperative licensing agreement
with Philips. However, other organizations have shown interest and have built and
tested experimental units. In addition, it is estimated that thousands of rudimentary
hot-air engines have been built by hobbyists in basement workshops.
Current and Projected State of the Art
Until the present, applications of the Stirling-cycle engine have been limited
because of some unsolved development problems that resulted in poor durability. How-
ever, as a result of continued development by Philips Laboratories and by various divi-
sions of General Motors Corporation over the past 10 years, Stirling engines are now
being built that will run for long periods of time. A few engines in the U. S. have been
run for over 1000 hours. These engines are much quieter than 1C engines, quite effi-
cient, and are roughly equivalent to the present diesel engines in size and weight.
Excellent throttle response is attainable, and exhaust emission from the external com-
bustor is characteristically clean. Thus, the Stirling engine is considered to be a
potentially acceptable vehicular power plant. In relation to present passenger-car
engines, the primary factors limiting the use of Stirling engines are the relatively high
first cost, weight, and size and the large radiator that is required. While the gasoline
engine rejects most of its waste heat through the exhaust, the Stirling engine rejects
most of its waste heat through the radiator.
Mechanical Configuration
To date, the combined work of Philips and General Motors has produced the only
highly sophisticated Stirling-cycle power plants that have been publicized. The Stratos
Division of the Fairchild-Hiller Corporation is known to have been conducting a Stirling-
cycle development program; however, their developments have not been made public.
The currently most popular mechanical configuration, shown in Figure 18, was devel-
oped by Philips. It is called a "displacer" type, as opposed to an alternative configura-
tion called the Rider type, after a hot-air engine built in 1876. The Rider engine has
two separate cylinders, but operates on essentially the same thermodynamic cycle as
the displacer engine. Stirling's original engine was a displacer type. The unique
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feature of the Philips' configuration is the "rhombic" drive mechanism. Although some-
what complex compared with conventional crankshaft drives, the rhombic drive produces
theoretically perfect balance of all inertial forces, and eliminates side-thrust on the
pistons.
The power output of the Stirling engine can be controlled by raising or lowering
the combustion temperature; however, this method is too slow for most applications
because of thermal lag of the heater tubes. To achieve rapid response to power-demand
changes, the output of the engine is modulated by changing the pressure level of the
hydrogen or helium internal working fluid. To reduce power output, some of the gas is
pumped out by a small diaphragm pump to a reservoir which is kept a'c elevated pres-
sure. Conversely, to increase power, some gas is admitted back in.'co the engine. Maxi-
mum mean pressure level inside the engine is usually about 1500 psia.
To achieve rapid power cutoff, a port between the "cold" space and the buffer
space can be opened, which reduces the power-piston compressi.on. Excellent throttle
response is said to be obtained by this means.
Current Experimental Units and Applications
Experimental units have been produced by General Motors' Allison Division,
Electromotive Division, and Research Laboratories, and by the N. V. Philips Labora-
tories of the Netherlands. Table 13 summarizes data on. these engines. Perhaps the
most widely publicized development has been the 10-hp(s) GPU-3 single-cylinder engine
built by CM Research for evaluation by USAMERDC as a "silent" military ground power
unit. This engine has passed 500-hour Army qualification tests. The GPU-3 is
expected to be essentially "inaudible" at a distance o'i 100 meters and to operate at 27
percent maximum thermal efficiency.
CM Research also has a 10-hp(s) Model 1036R engine which is used for research
purposes and which is similar to the ground power engine.
The Allison Division worked on a Stirling-cycle solar-energy power plant for
aerospace application from 1959 to 1963 under an Air Force contract. The engine was
similar to the GM Research engines, and delivered 7 hp(s) with a 4. 95-cu in. -
displacement single cylinder at 30 percent thermal efficiency. The latest engine in this
development was designated the PD-67.
The Electromotive Division has built 1-, 2-, and 4-cylinder units of the 8015
engine which develops 95 hp(s) per cylinder at 1500 rpm. A 4-cylinder, 360-hp(s)
version in a generator set was tested by the U. S. Navy. Presumably, this engine
could also be used for marine propulsion.
The Philips Company also has a number of Stirling-engine configurations. The
single-cylinder Model 3015, developing 40 hp(s) has been subjected to sound emission
tests in this country by the U. S. Navy. Philips is also developing a 4-cylinder, 236-cc
displacement-per-cylinder marine unit to deliver 120 hp(s) at 3000 rpm.
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TABLE 13. TYPICAL STIRLING-ENGINE DATA
Developer
Model No. of Rating,
No. Cylinders hp(s) at rpm
Max. Brake
Thermal Envelope Specific Specific
Efficiency. Volume^, Volume(a), Weight(a\ Weight(a\
percent
ft3
ft3/hp(s)
Ib
lb/hp(s)
CM Research
CM Research
Philips
CM Electromotive
Philips
CM Allison
GPLI-2
GPU- 3
3015
8015
Marine
PD-67
One
One
One
Four
Four
One
7.5 at 3600
10.0 at 3000
40.0 at 2500
380 at 1500
120 at 3000
1 at 3000
23
27
39
30
40 (calc.)
30
3.04
4.89
6.40
130.00
23.20
3.50
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Emission Characteristics
As discussed in the section External Combustors, very low mass-emission levels
of CO and HC have been reported for Stirling engines (i. e. , less than 1 percent of those
of comparable uncontrolled-emission gasoline engines). While reported NOX emissions
are lower than for comparable uncontrolled-emission gasoline engines, considerable
variation exists between engines, and reduction of NO emissions should be possible
through directed research.
"1980" Physical, Performance, and
Cost Characteristics
Specific Weight. In the range of 180 hp(s) to 16 hp(s), a specific weight for the
reciprocating assembly of 5 to 6 lb/hp(s) should be obtainable by 1980. This includes
combustor and preheater, but excludes the radiator, flywheel, and other accessories.
The Philips' marine engine in Table 13 approaches this weight. Assuming a typical
radiator and fan weight of 40 Ib for a 150-hp(s) conventional automotive gasoline engine.
and assuming that a radiator 4 times as large is required for automotive applications of
the Stirling engine, a figure of about 1 lb/hp(s) should be added for the radiator. In
addition, other accessories are estimated to weigh about 2 to 3 lb/hp(s). This puts the
specific weight of a complete Stirling-engine power plant for vehicle application in the
range of 8 to 10 lb/hp(s). The transmission weight would be about the same or slightly
less than that of conventional gasoline-engine automobiles.
Specific Volume. Assuming advances in compactness, particularly with respect
to component configuration and nesting to reduce maximum envelope dimensions, can be
achieved, the specific volume of the basic engine, combustor, and preheater assembly
may approach 0. 07 to 0. 10 ft /hp(s) over the 180 to 16-hp(s) range. Required radiator
frontal area would be approximately 0. 06 ft^/hp(s), and the volume required for radia-
tor, fan, and other accessories would run an additional 0. 07 to 0. 10 ft^/hp(s). This
results in a projected Stirling-engine specific volume of approximately 0. 15 to 0. 20
ft3/hp(s).
Efficiency. The most recent design studies indicate that larger sized Stirling
engines are capable of operating with maximum brake thermal efficiencies as high as
40 percent. The particular engine for which this study was made, however, did not
have a cooler fan. With design compromises to reduce cost, no increase is expected in
this value by 1980. Smaller engines would have peak efficiencies between 30 and 40
percent. Figure 19 shows the maximum thermal efficiency that might be considered at
part load. In general, the best part-load efficiency would be obtained at moderate
engine speed with moderate-to-high working pressures, rather than at high speed and
low working pressure.
Start-up Characteristics. Start-up time from the cold condition is determined by
design variables. A start-up period of 10 to 15 seconds can be achieved; however,
necessary design compromises may require a period of up to 30 seconds. Restart
should be virtually instantaneous. As with conventional automotive engines, several
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minutes' driving would be required in cold weather to warm up crankcase and power-
train lubricants before full power could be developed.
Power-Output Characteristics. The torque characteristic of Stirling engines
droops, as speed in increased, somewhat more than that of gasoline or diesel engines,
although the specific torque characteristic is a function of design parameters in any of
these engines. However, the Stirling engine torque droops naturally as speed is
increased owing to reduced effective heat-transfer capacity and increased aerodynamic-
friction losses. Figure 20 shows a typical torque characteristic which rises about 20
percent as speed is reduced to 30 to 40 percent of maximum. Torque drops off sharply
at lower speeds owing to leakage past the power piston. With the drooping torque curve,
power drops off less rapidly as output speed is reduced. If desired, greater torque
droop could be obtained artificially by a scheduled reduction in working pressure with
speed. Of course, this would require a derating of a given displacement engine.
The maximum output speed of current Stirling engines tends to be lower than that
of conventional automotive gasoline engines. The maximum continuous speed of a
10-hp(s) power plant would be about 3600 rpm, while a 200-hp(s) unit would run up to
1500 to 2500 rpm, depending upon the number of cylinders. Units designed specifically
for automotive applications would probably have somewhat higher peak speeds. Idle
speeds would probably be about the same or somewhat lower than that of conventional
gasoline engines.
Throttle response would be excellent, perhaps equivalent to that of the diesel.
Power-Surge Capability. It would be possible for a Stirling engine to have a small
power-surge capability in excess of the maximum steady-state power output, provided
the reciprocating unit was built with the additional capacity. This power-surge capa-
bility has already been partially accounted for in that the radiator and fan have been
sized with reference to present automotive radiators rather than with reference to cur-
rent Stirling-engine practice which is based primarily on continuous rated power
operation.
Specific Cost. Actual production cost figures have not been established; however,
General Motors' cost studies indicate potential costs 15 to 20 percent higher than those
for diesel engines. Some of the engine cost could be offset by the use of a simpler
transmission; however, this saving in turn would be offset by the larger radiator
required. Assuming that diesel engines for automobile application would cost twice as
much as gasoline engines for this application* and that automobile gasoline engines cost
$2 to 3/hp(s), the Stirling power plant specific cost is estimated at $5 to 7/hp(s).
"Diesel engines for truck and bus application cost something like $15 to 20/hp(s). However, if they were produced to meet the
less severe automobile duty cycle and in the quantities required for automobile application, they could be expected to cost
substantially less.
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20 40 60 80
Power Output, percent
100
FIGURE 19. TYPICAL STIRLING-ENGINE PART-LOAD EFFICIENCY
CHARACTERISTICS
o>
(T
i.
O
TJ
O
5
O
Q.
O
Q:
c
0>
O
140
120
100
Maximum working pressure
20 40 60 80 100
Percent of Rated Speed A-57479
FIGURE 20. TYPICAL STIRLING-ENGINE TORQUE AND POWER
CHARACTERISTICS
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DIRECT THERMAL-TO-ELECTRIC ENERGY CONVERTERS
MAGNETOHYDRODYNAMICS
General Description
Principles of MHD
Magnetohydrodynamic (MHD) power generation is based on the same principle as
a conventional rotating-armature generator, that is, a conductor moving through a
magnetic field causing an electric current to be induced in itself. The MHD generator,
instead of using a solid metal conductor as the moving element, utilizes an ionized gas
or a liquid metal flowing at high velocity through a magnetic field. A current is produced
in the gas at right angles to the direction of flow, and electrodes placed in contact with
the fluid carry the current to the load. Figure 21 illustrates the basic elements of an
MHD generator.
iElectrodes
Fluid out
*-Magnet
FIGURE 21.
SCHEMATIC DRAWING OF MAGNETOHYDRODYNAMIC
GENERATOR
The power produced by an MHD generator depends on the velocity of the fluid, its
conductivity and the strength of the magnetic field. The absence of moving parts permits
operating temperatures significantly higher than those possible in dynamic heat engines.
thus providing a higher Carnot efficiency potential. However, high Carnot efficiencies
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cannot be attained in practice by simple MHD power system because the heat-sink or
exhaust temperature must be relatively high to maintain the gases in a conducting state.
MHD Cycles and Requirements
There are three basic cycles that can be used with MHD power generation: the
single open cycle, the open cycle with heat recovery, and the closed cycle. The simple
open cycle involves a once-through pass of the working fluid with no recirculation and
no attempt to make use of the heat remaining in the discharged gases. In the open cycle
with heat recovery, the heat energy available in the discharge gases is recovered in a
heat exchanger. This recovered heat may be utilized to heat the air for combustion or
to generate steam for additional power.
Figure 22 is an example of an open-cycle MHD power system with heat recovery.
In this example, the exhaust gases are first passed through a regenerator, where some
of the available heat is transferred to the combustion air, and then through a boiler
where steam is generated as part of a conventional Rankine-cycle, rotary-generator
power system.
Combustor-
Fuel
Seed
pMHD generator
- Generator
Compressor
Steam turbine
Condenser
Combustion
air
-Feed pump
r Regenerator
Boiler
* Exhaust
FIGURE 22.
SCHEMATIC DIAGRAM OF OPEN-CYCLE MAGNETOHYDRODYNAMIC
POWER SYSTEM WITH HEAT RECOVERY
The closed-cycle MHD generator involves complete recirculation of the working
fluid. Figure 23 illustrates a closed-cycle, nuclear-powered, MHD-steam power sys-
tem. The MHD working fluid passes through the nuclear reactor, through the MHD
generator, through a boiler, through a compressor, and back to the nuclear reactor.
lle.it is transferred from the MHD working fluid in the boiler to generate steam for the
Rankine -cycle, rotary-generator power system.
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/WVW1
Nuclear
reactor MHD
generator
Boiler'
Pump
FIGURE 23.
SCHEMATIC DIAGRAM OF CLOSED-CYCLE, NUCLEAR-POWERED,
MAGNETOHYDRODYNAMIC-STEAM POWER SYSTEM
When a gas is used as the working fluid in an MHD generator. It must be ionized
in order to be conductive. lonization of gases can be accomplished in a number of ways,
two of which are thermally and magnetically. Thermal ionization has received the most
attention in MHD development because it is more easily achieved and appears to be a
more practical approach.
For successful thermal ionization, not only must the gases be at a high tempera-
ture, but small amounts of a metallic salt must also be added. The minimum tempera-
ture at which an MHD generator can be expected to produce useful power is in the range
of 3000 to 4000 F. Temperatures as high as 5000 or 6000 F are required to achieve high
conversion efficiencies. Alkali metals such as cesium, rubidium, and potassium have
been found to be the best seed materials. Potassium is most commonly used because of
its availability and relatively low cost. The optimum quantity of seed material is about
1 or 2 percent of the total mass of the working fluid. Greater percentages actually
result in a reduction in the ionization potential of the gas. Most fuels when burned with
the proper amount of air will produce flame temperatures on the order of 3500 F, which
is marginal for an MHD generator. Increasing the percentage or richness of oxygen in
the combustion air or preheating the combustion air may be essential in a practical MHD
system.
Current and Projected State of the Art
Experimental Accomplishments
Serious MHD development work is being pursued by only a relatively few organiza-
tions. Among these are several universities such as Stanford University, the University
of Illinois, and Sheffield University in England. Corporations involved in MHD work
include: AVCO-Everett, Westinghouse, Atomics International, and General Electric.
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As an example, AVCO-Everett has recently tested an MHD generator utilizing the
exhaust from a rocket engine as the working fluid. This system produced 31,600 hp(e)
for a period of about 1 minute. The magnet of this unit was uncooled; consequently, a
longer run time would not be possible without danger of burning up or melting the sys-
tem. During the run the gas flow was about 6900 pounds per minute and the seed, potas-
sium hydroxide, represented about 1-1/2 percent by volume. The fuel for the rocket
engine was ethyl alcohol plus oxygen. The generator portion of the experimental unit
was on the order of 25 feet long and 8 by 8 feet in cross section. The field strength of
the magnet was about 36, 500 gauss, and 134, 000 pounds of copper was used in the mag-
net. The temperature of the working gases entering the magnetic field was about 4950 F.
At Westinghouse an experimental MHD generator using fuel oil plus oxygen, with
potassium as the seed material, developed about 13.4 hp(e) for 10 minutes. Atomics
International is working on liquid-metal alternating-current MHD generators. They have
succeeded in inducing 10 hp(e) output using NaK as the working fluid in a blowdown sys-
tem. At General Electric, experiments are being carried out on a nonequilibrium MHD
generator using seeded inert gases. This type of MHD generator eventually is expected
to be appropriate for use with a gas-cooled nuclear reactor as the heat source because
the MHD fluid temperatures can be much lower for a given level of conductivity. Work
is also going on at a number of universities and at AVCO-Everett on the feasibility of
using powdered coal as the fuel for an open-cycle, regenerative-type MHD system.
Development Problems
The principal problems facing further development of the MHD concepts are: the
high working-fluid temperatures required, recovery of the seed material, erosion of the
electrodes, corrosion by the seed material, recovery of the exhaust heat, and the field
strength of the magnet.
The high temperature requirements pose a fuel and a combustion problem and also
Lead to almost an absolute necessity for making use of the heat energy in the exhaust
gases. The combustion of normal hydrocarbon fuels with air does not lead to high enough
temperatures to result in an efficient MHD generator. Preheating the combustion air
leads to more practical energy outputs, and of course the substitution of oxygen for some
or all of the combustion air will result in even better energy-conversion efficiencies.
As was previously mentioned, the heat still available in the gases discharging from the
MHD generator can be used effectively to preheat the combustion air and/or to generate
steam in a Rankine-cycle power plant.
Because of its cost, recovery of the seed must be considered essential in a practi-
cal system. This poses many problems, particularly when powdered coal or Bunker C
oil is used, since these fuels produce a considerable amount of ash which would inter-
fere with successful and complete recovery of the seed.
Regarding erosion of the electrodes it has been suggested that consumable elec-
trodes be used in a manner similar to the electrodes in an arc furnace. Whatever final
electrode materials and configurations are developed, it is probable that, replacement
of the electrodes will be a maintenance requirement in MHD generators.
The maximum potential magnetic-field strength of conventional magnets today may
be on the order of 35, 000 to 40, 000 gauss. It is hoped that advances in the technology of
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so called superconducting magnets will soon make possible much higher magnetic-field
strengths, say on the order of up to 100,000 gauss, and thus will hasten the development
of practical MHD systems.
Corrosion by the seed material and erosion by the combustion gases, which in the
case of powdered coal would contain coal slag, are serious problems both within the
MHD generator itself and in any heat exchangers used to recover the exhaust heat.
These problems are further complicated by the fact that the walls must be electrically
insulating and must be able to withstand high temperatures.
Emission Characteristics
Unburned hydrocarbons and carbon monoxide in the exhaust gases from an MHD
generator should be negligible. However, the appreciably higher temperature essential
for MHD power generation would probably lead to emission of nitrogen oxides in quanti-
ties considerably greater than those in a normal combustion system. In open-cycle
systems, traces of the seed material might be present.
"1980" Physical, Performance,
and Cost Characteristics
Work on MHD power generation is in such an early state of development that most
of the characteristics of interest to this study are not yet defined, particularly when
considering automotive application. While a number of prototype commercial MHD
generators should be in operation by 1980, a number of significant development prob-
lems, as discussed above, remain to be satisfactorily resolved.
Applicable Size Range. Most researchers in the field have concluded that the MHD
generator is primarily suited to the production of large amounts of power. It has been
estimated that the lower power limit for a useful MHD generator is about 1 megawatt
[ 1340 hp(e)] . The principal reason for this minimum practical-power-level limitation
is that all of the loss mechanisms, such as viscous drag on the walls, heat transfer to
the walls, losses associated with the magnetic field coil, etc. are all items that are
proportional to the surface area of the device, while the power output is proportional to
the volume. Thus, as the size or volume of a given MHD generator is reduced, the
surface-to-volume ratio increases, hence increasing the losses relative to the power
output until a point is reached where the final net power output is not worth the trouble.
Efficiency. The potential thermal efficiency of an MHD system without heat
recovery in the exhaust can probably never exceed 10 to 15 percent. However a com-
bined MHD-steam power plant can probably be 5 to 10 percent more efficient than a steam
power plant alone.
Specific Cost. It has been estimated that the capital cost of MHD power plants may
be somewhere between $40 and $110/hp(e) in 10 or 15 years. This applies to multimega-
watt [ 1000's of hp(e)] sizes and is comparable to costs predicted for central-station steam
power plants in the same time period.
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THERMOELECTRIC CONVERTERS
General Description
The physical basis of thermoelectric generation of power is the Seebeck effect, or
generation of voltage when two dissimilar metals or semiconductors are joined together.
In order for the voltage thus generated to be sensed externally, the junction of the two
materials must be maintained at a temperature higher than the opposite ends of each seg-
ment of material. The output voltage of such a junction is proportional to the tempera-
ture difference between the hot and cold ends of the metals and to the difference between
an intrinsic property known as the Seebeck coefficients of the two metals.
A second phenomenon of importance in thermoelectric-power generation is the
Peltier effect, which is the pumping of heat into or out of a junction when a current
passes through the junction. In a thermoelectric generator, the Peltier effect tends to
lower the hot-junction temperature and raise the cold-junction temperature. The rate of
heat flow into or out of a junction is proportional to the current, the algebraic difference
of the Seebeck coefficients of the two materials, and the absolute temperature of the
junction.
A third thermoelectric effect known as the Thomson effect occurs when the Seebeck
coefficient of a thermocouple leg varies with temperature. Heat is generated in the leg
at a rate dependent on current, temperature, and the rate of change of Seebeck coeffi-
cient with temperature. As a rule, the Thomson effect can be ignored if the mean
Seebeck coefficient of the materials over the applicable temperature range is used.
Historical Development
The Seebeck effect has been extensively used in temperature measurement but has
only been used for power generation since the late 1940s. Although the metals known to
exhibit relatively high Seebeck voltages were low cost, the efficiencies of electrical
power generators using these metals was 1 percent at best. Telkes(72) in a 1947 article
which reviewed the state of thermoelectric-power generation at that time, showed that
materials containing bismuth and antimony, tellurium compounds, and lead compounds
could be used to achieve power-generation efficiencies of about 5 percent. During this
same period, the gas-controls industry was conducting research on thermoelectric
generators which could use the heat from gas burners and pilot lights to generate the
electrical power required to operate blowers and automatic controls. ' '
The advent of semiconductor transistors and diodes with consequently increasing
knowledge of semiconductor technology control of resistivity, control of purity,
methods of material preparation coupled with requirements of the space program, led
to heavy expenditures on research in all phases of thermoelectric power generation.
From 1958 to 1962, the U. S. Navy Bureau of Ships sponsored many research programs
aimed at finding new materials capable of high-temperature operation, and at improving
the thermoelectric efficiency and general utility of known thermoelectric materials.
Since 1962, the emphasis in thermoelectric-power-generation research has shifted from
materials development to hardware development. Improved electrical contacts,
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improved burners, and stronger and more chemically stable thermocouples have served
to improve reliability, reduce weight, and bring actual generator efficiencies closer to
theoretically possible values.
Current and Projected State of the Art
There is no technical or natural limit to the power capacity of a thermoelectric
generator. Power output is directly proportional to the number of thermocouples for
given heat-source and heat-sink temperatures; there is no optimization of power or
efficiency to be achieved by increasing or decreasing size. As a consequence, thermo-
electric generators presently are used in applications where one or more of the following
requirements exist:
(1) Power required is less than 0.4 to 0.7 hp(e).
(2) Silent operation is necessary.
(3) Frequent maintenance, predictable or unpredictable, is undesirable or
impossible.
(4) Reliability and long life are required.
(5) Operation from any heat source is required.
Advanced Development Units
and Applications
Thermoelectric generators have been built and operated satisfactorily in a variety
of situations. They have been operated from many different heat sources, including
butane, gasoline, kerosine, propane, JP-4, and fuel-oil burners; radioisotopes; and
nuclear reactors. With the exception of one generator which produced over 6. 7 hp(e),
all of the generators have been in the subhorsepower range.
A brief description is given of several thermoelectric generators which have been
built recently and which could be classified as advanced development models.
0.4 hp(e)-Marine Engineering Laboratory. This generator described by Neild, *
contains a thermoelectric section, a fuel tank with capacity for 8 hours' operation, a
liquid fuel burner, and a fan for cooling the thermocouple cold junctions. The burner
uses diesel, JP4, or kerosine fuel.
The thermoelectric section consists of 30 modules mounted in a cylinder surround-
ing the flame. Each module contains eight lead telluride thermocouples. The thermo-
couples are mounted in a hermetically sealed enclosure to prevent oxidation. Cold-
junction temperature is maintained at 350 F by forced convection cooling of aluminum
fins that receive waste heat from the thermocouples. The thermoelectric section,
including cooling fins, weighs 13.9 pounds. The hot junctions are kept at 1050 F.
Although service-life data are limited, it appears that generators of this type have
operated successfully in excess of 1000 hours.
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Statistical data on this generator are:
Power Output 0.4hp(e), 26. 8 v, 11. 2 amp
Weight Without Fuel 22. 5 Ib
Dimensions 11 x 18 x 17 in. (1. 95 ft3)
Efficiency 3. 7 percent
Specific Weight 56 lb/hp(e)
Specific Volume 4.9ft3/hp(e)
Start-up Characteristics 8 minutes with wick preheat;
3 minutes with propane preheat
0.4 hp(e) Vehicle Mounted Power Supply, U. S. Army Electronic Command. This
generator(75) is intended to be used to charge batteries for communications equipment.
It is carried on a jeep or other vehicle and uses the vehicle fuel supply. The entire unit
consists of thermoelectric converter, burner system, fuel system, cooling system, and
power conditioning, electrical control, and monitoring circuitry.
The thermoelectric section is similar to the thermoelectric section of the 0.4 hp(e)
MEL unit described above. Lead telluride thermocouples are mounted in a hermetically
sealed cylindrical unit which surrounds the burner. Fins are used on both the hot side
and the cold side to enhance heat transfer to and from the modules. The converter
weighs 12.2 pounds.
The burner uses liquid fuel which is atomized for combustion by an ultrasonic
atomizer. Primary air is supplied by a blower. Fuel is pumped in by means of an
electric fuel pump through an input connection provided. A fuel metering valve is also
provided.
Thermocouple cold-junction temperature is maintained by means of air driven by
an electric fan over aluminum fins.
Power conditioning circuitry regulates the output voltage to 29 volts and an
oscillator is used to drive the ultrasonic atomizer. Controls provide for start-up and
run conditions and protect against circuit overload.
Statistical data are given below:
Power Output 0.4hp(e), 28 v, 10. 7 amp
Dimensions 13-1/4 in. in diameter x 23 in. high (1.83 ft )
Weight 25 Ib
Efficiency Not known
Specific Weight 62 Ib/ hp(e)
Specific Volume 4.5 ft3/ hp(e)
Start-up Time 5 minutes
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0. 75 hp(e) Vehicle Mounted Power Supply, USAMERDC. This unit differs from the
unit described above only in power output, size, and weight. Data are as follows:
Power Output 0.75 hp(e), 28 v, 20 amp
Dimensions 14-5/8 in. in diameter x 25 in. high (2.44 ft )
Weight 35 Ib
Efficiency 3. 5 to 4. 0 percent at rated load point
Specific Weight 47.5 lb/hp(e)
Specific Volume 3.30 ft3/hp(e)
Other Low-Power Generators. In addition to these generators, a number of others
have been described in the literature in recent years. For several years the U. S. Army
Electronics Command has been developing a silicon-germanium-based generator that
uses a multifuel burner and generates 0. 13 hp(e). Net efficiency of this system is
2. 75 percent. Specific weight is 330 Ib/hp.
General Instrument Corporation markets a line of thermoelectric generators which
utilize flameless, catalytic burners.(^o) The generators can use either butane or pro-
pane gas and are available in sizes up to 0.4 hp(e). The thermoelectric material used
is bismuth telluride.
Plevyak*' ') has described a 0. 2 hp(e) propane-fired thermoelectric generator used
to power remote microwave telephone equipment. Two classes of generators were field
tested under adverse conditions. One was a lead telluride-based system which was
heated by a propane flame burner. The other was a bismuth telluride-based system
which was heated by a catalytic propane burner. A cost analysis was performed by
Plevyak, assuming a 30-year life of the microwave equipment, and a useful life of 5 years
for the thermoelectric generator. He compared costs per year for power generated by a
thermoelectric generator with cost per year for power from batteries and from engine-
generator sets. In the range up to 0.4 hp(e) the thermoelectric generator provided the
lowest cost power. Above 0.4 hp(e), the engine-generator sets were cheapest.
40 hp(e) Generator Feasibility Study
All of the generators described thus far have been in the fractional horsepower
range. As has been indicated, situations calling for larger amounts of power have been
better served by engine-generator sets. The principal advantages which thermoelectric
generators can claim over the engine-generator set, silence and maintenance-free
operation, have not been sufficient to outweigh the lower specific weight and lower cost
of the engine-generator set. An exception to this generalization was an undertaking of the
U.S. Army Transportation Research Command' ' to determine the feasibility of a
40 hp(e) thermoelectric generator to be used to power a silent boat driven by an electric
motor. A thermocouple using silicon-germanium alloy for use in such a generator was
designed and life tested. Thermocouples were built which demonstrated an efficiency of
10 percent (thermocouple efficiency only). The couples were run for several hundred
hours in air, with hot-junction temperatures as high as 1920 F, with no degradation in
output power. Thermocouples were also subjected to shock and vibration tests and did
not show signs of cracks or increases in electrical resistance at the conclusion of the
tests.
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Following the successful silicon-germanium thermocouple development, the design
of a 40 hp(e) system was undertaken to determine the size, weight, and general charac-
teristics of such a system.
The final generator design called for a system of 42 self-contained subassemblies.
Each subassembly contained four thermocouple modules of 8 1 thermocouples each sur-
rounding a heat source which radiates energy at 2360 F. The complete generator was
estimated to weigh 1000 pounds, with dimensions of 31 x 31 x 20 inches, or 11 cubic
feet. The calculated overall efficiency was 6.5 percent. These figures imply a
specific weight of 25 lb/hp(e) and a specific volume of 0.3 ft^/hp(e).
The generator described here was not built. Although the contractor (RCA)
considered such a generator feasible, he believed that some difficulty might be encoun-
tered owing to limitations of construction materials at the design-point operating tem-
peratures. Nevertheless, these difficulties could probably be overcome in the course of
a development program.
Probable Areas of Improvement
Future improvement in the efficiency, cost, and specific weight of thermoelectric
generators could be achieved through: discovery of new alloys or compounds with in-
creased figure of merit; improvement of existing materials; or improved construction
techniques. The figure of merit, which is commonly designated as Z, is a measure of
the usefulness of a material for thermoelectric application.
It is impossible to predict the discovery of new materials. A large amount of
searching for new thermoelectric materials was done in the late 1950s and early 1960s.
A great deal of information was gained on many materials. Criteria to serve as guide-
lines in investigating materials were formulated. However, the materials of maximum
figure of merit at the present time, with the exception of germanium-silicon alloy, are
simply modifications of materials which were known over 10 years ago. There is no
known theoretical limit to the figure of merit, but it has been observed that the "ZT"
product, or dimensionless figure of merit, never exceeds a value of 1.0 or 1. 1. Very
little research on thermoelectric materials is now being done. The advent of therm-
ionics for use at temperatures above 2000 F has reduced the impetus for finding thermo-
electric materials to operate in that temperature range. For these reasons, it would be
unwise to hold out the hope for new materials showing significant improvements in the
intermediate future.
Improvements in existing materials are likewise difficult to predict, particularly
as far as the figure of merit is concerned. The technology of producing materials should
steadily improve, and result in materials of more predictable properties and more
stable properties than are now available. One might also expect improvement in the
mechanical properties. The brittleness and low tensile strength of lead telluride are
areas where improvement could be expected. Reductions in cost should follow
increased production.
It appears that the best possibilities for thermoelectric-generator improvement
are in the areas of design and construction techniques. The table below lists the partic-
ular aspects of device construction where improvement is needed, what the manifestation
of this improvement should be, and what the prospects are for the particular
improvement.
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TABLE 14. AREAS WHERE IMPROVEMENT IS NEEDED IN
THERMOELECTRIC-GENERATOR
CONSTRUCTION
Area of Improvement
Effect of Improvement
Prospects
Reduce contact
resistance
Improved thermal
insulation around
thermocouples between
hot and cold sides
Use of cascaded thermo-
couples or segmented
legs
Provide means to allow
for thermal expansion
mismatches at interfaces
Increased efficiency; permitted
shorter legs and higher power-
to-weight ratio; reduced cost;
improved reliability
Increased efficiency; permitted
shorter legs with higher power-
to-weight ratio and lower cost
Increased efficiency
Improved reliability
Probably a slow,
continuous
improvement
Unknown
Very good
Good
Current Programs Aimed at
Technological Improvement
Several current research programs which may provide insight into the future of
thermoelectric generators are discussed in the following paragraphs.
Atomics International, Canoga Park, California, is working to improve efficiency
by developing a cascaded thermoelectric module. A cascaded module is one in which two
thermocouples are in series thermally so that heat rejected by the cold junction of the
first-stage thermocouple is absorbed by the hot junction of the second-stage thermo-
couple. The first-stage thermocouple is made of materials which exhibit their maximum
figure of merit in the temperature range of the first stage. The second-stage thermo-
couples are made of different materials, which exhibit their maximum figure of merit in
the lower temperature range typical of the second stage. A cascaded generator can be
operated effectively over a larger temperature difference than that possible with only one
stage. A principal difficulty in cascading is in providing good heat transfer from the
first stage to the second stage while providing electrical isolation.
The Atomics International work is in the early stages at this time. (^9) Individual
modules have been built and tested, and a design analysis has been carried out. The
design objective is to achieve 10 to 11 percent module-only efficiency and a specific weight
of 18. 5 lb/hp(e) for the converter, exclusive of heat source and sink provisions. The
module uses a silicon-germanium thermocouple stage operating between 1840 F and
1100 F and a lead telluride stage between 1000 F and 400 F. Maximum theoretical module
efficiency for such a unit is approximately 14 percent.
Assuming that the thermoelectric modules constitute one-half the weight of the
entire generator, a system weight of approximately 36 lb/hp(e) is projected for a system
using these cascaded modules.
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Another thermoelectric program(SO) which could have results significant to this
study is the TURPS (T_errestrial Unattended Reactor Power Supply) Program being
carried out for the Air Force by the Martin Company. The heat source in the TURPS
system is to be a nuclear reactor, but the thermoelectric section design and performance
would be applicable to a hydrocarbon-fueled system.
The design goal of the TURPS program is to achieve 134 hp(e) based upon a thermo-
electric converter with power-to-weight ratio of 10 lb/hp(e) for the converter portion
alone. Converter-only efficiency has not been specified, but efficiency of individual
couples has been 9 percent.
The TURPS system will probably use thermocouples having lead telluride n-type
legs and lead-tin telluride p-type legs. The legs are designed with very high cross-
sectional area-to-length ratios. Thermocouples under test have used legs 1.25 inches
in diameter and 0. 10 inch thick. The thermocouple legs are formed by pressing and
sintering lead telluride powder in a very thin metallic jacket. The jacket provides
strength and protection from contamination.
This program is in its beginning stages at this time. Thermocouples using
different jacket materials have been designed and life tested, and the theoretical analysis
of thermocouple performance has been done. The p-type materials showed severe
degradation on life test. The problems of jacket material, pressing procedure, and
overall element design have not yet been resolved.
The approach to thermocouple design used in this program, known as the high-
power-density or HPD approach, is used to achieve higher power-to-weight ratios for
a. given material in a given temperature range. In principle, this is feasible. (81) How-
ever, the approach has several technical limitations. High power density implies a
high thermal flux in the material, with resultant high thermal stresses. Lead telluride
is quite brittle, and cracking due to thermal stresses has always been a problem. Fur-
thermore, as thermocouple legs are made shorter and shorter, the effect of electrical
contact resistance becomes greater, and thermocouple efficiency decreases. It appears
that encapsulation might provide part or all of the answer to the thermal stress problem,
but the problem of contact resistance remains. Presumably, development work will
provide improved bonding techniques and result in lower contact resistances in the
future.
Finally, a program being conducted at Battelle Columbus which may result in
higher efficiency thermoelectric generators is aimed at developing segmented thermo-
couple legs. (82) Segmenting differs from cascading in that there is only one generator
stage in segmenting, as opposed to two in cascading. A segmented thermocouple leg is
made up of sections or segments of different material. Each segment is located at the
point in the leg where the temperature is most favorable for the material of which the
segment is made.
The theoretical efficiency of a segmented silicon germanium-lead telluride thermo-
couple operating between 1800 F and 100 F has been calculated to be 15 percent for the
thermocouple module alone. Such a figure has not been achieved in practice, however.
The principal problems are related to achieving a suitably strong, low-resistance bond
between the lead telluride and germanium-silicon. The problems are complicated by
the thermal-expansion mismatch between the materials. Two approaches are used to
solve this problem. One is to sandwich intermediate materials between the thermo-
electric materials to take up the expansion stresses. However, the choice of
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intermediate materials and the method used to bond them to the thermoelectric materials
is complicated by the sensitivity of the thermoelectric material to high temperatures
and contamination. The second is to use a pressure contact between the lead telluride
and germanium-silicon segments. Such contacts are relatively high in electrical
resistance, however.
"1980" Physical, Performance, and
Cost Characteristics
In estimating the potential performance of thermoelectric systems, the following
are assumed:
(1) No improvement in figure of merit or temperature capability of existing
materials will occur, nor will new materials of higher figure of merit be
found.
(2) The high power density and cascade concepts will be successful and embodied
in one generator.
(3) Burner efficiency will be increased to 80 percent.
(4) Approximately 10 percent of generated power will be used to drive blowers,
pumps, and power conditioning systems.
(5) One-half of the total system weight will comprise the thermocouples, and
one-half will comprise the burner, cooling system, and auxiliary equipment.
Specific Weight and Efficiency. At present, a 0. 75 hp(e), 4 percent efficient sys-
tem with a specific weight of 47. 5 lb/hp(e) has been built. A 40 hp(e), 6. 5 percent
efficient system with specific weight of 25 lb/hp(e) is regarded as several years in the
future. Cascaded modules designed for 11 percent efficiency and a specific weight of
18. 5 lb/hp(e), module only, have been built, but not a complete system using cascaded
modules. The potential efficiency of a cascaded or segmented module alone is 14 to 15
percent. High-power-density modules, 9 percent efficient and weighing 10 lb/hp(e),
have been demonstrated.
According to the previously listed assumptions, a cascaded system, if built at the
present time, would weigh 36 lb/hp(e) and would be 7. 7 percent efficient; the high-power
density system, if built at the present time, would weigh 20 lb/hp(e) and be 6. 3 percent
efficient. The potential efficiency of a cascaded or segmented system would be 0. 7
(burner plus auxiliary losses) x 0. 15, or approximately 10 percent. If such a system
were built with the high-power-density design, the weight might be
20-^M xf 6'3
hp(e) / \ 10.0
These figures represent the highest system efficiency and the lowest specific weight
that can be envisioned at this time. However, the possibility of achieving this value by
1980 is reasonable.
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Specific Cost. Cost of thermoelectric-generator systems in the high-power
ranges is difficult to estimate. Costs of present generator systems are in the neighbor-
hood of $11, 200/hp(e). This cost is expected to decrease to approximately $3, 000/hp(e)
which is in the same ratio as the weight per hp(e). While building units at a rate of
several hundred thousand per year would result in a drastic cost reduction, it is believed
that the cost would still remain extremely high for automotive application.
THERMIONIC CONVERTERS
General Description
Thermionic conversion of heat to electricity is based upon application of the
Edison effect, that is, the evaporation of electrons from hot bodies. As thermal energy
is added to the body (emitter) additional energy is imparted to its electrons until some
of them evantually are able to surmount the potential barrier at the surface. If a second
electrode (collector) is placed near the emitter and the two electrodes are connected
through an external circuit, some of the emitted electrons will possess sufficient energy
to traverse the space between the electrodes, and (providing the potential barrier at the
surface of the collector is smaller than that at the emitter) do useful work in the external
circuit. The number of electrons (current) emitted by a heated body increases expo-
nentially as the temperature of the emitting body increases. Thus, the conversion
efficiency should also increase rapidly as the emitter temperature is raised.
This would be true except for the fact that the electrons in the space surrounding
the emitter eventually begin to exert a strong repulsive force, owing to their similar
electrical charges, on additional electrons which are attempting to leave the emitter.
This layer of electrons which surrounds the emitter is normally called the space-charge
cloud or just the space charge. In order to provide the high current densities necessary
for reasonable efficiencies, some means of space-charge compensation must be
provided.
It is possible to reduce the spacing between the emitter and collector to a small
enough value so that space charge has very little effect upon diode performance. The
spacings required, however, are extremely small, less than 0.001 inch, and the
mechanical problems involved in such a scheme have led to abandonment of efforts to
produce practical, close-space converters.
The alternative method of space-charge compensation that has been almost
universally adopted is the introduction of positive ions into the interelectrode space.
The usual source of ions is cesium vapor, which can be ionized by contact with the hot
emitter.
For a given diode spacing and cathode temperature there is an optimum cesium
vapor pressure. The performance of the converter is significantly degraded if the
operating conditions depart appreciably from optimum.
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Current and Projected State of the Art
Major characteristics of thermionic conversion devices which are usually consid-
ered advantageous are:
(1) There are no moving parts.
(Z) Operation at temperatures up to 4000 F is possible.
(3) Waste heat can be rejected at relatively high temperature, i. e. ,
up to about 1300 F.
(4) The power-to-weight ratio is relatively high.
The characteristics listed above make thermionic conversion highly attractive for
potential long-duration space missions, especially since nuclear energy appears to be
ideally suited to provide the thermal energy and there do not appear to be any major
materials-compatibility problems. For this reason a major portion of the experimental
research in thermionic conversion is directed toward developing nuclear-powered con-
verters for space applications. Some work has been done on solar-powered converters,
but the effort in this area has not been as great. Finally, work is being done to develop
fossil-fuel-fired converters, primarily for military applications where portability or
silent operation is the objective. The status of fossil-fuel-fired converters will be
examined since this is the most appropriate to automotive application.
Current Experimental Units
As one might suspect, the major problem encountered in adapting thermionic
converters to fossil-fuel-fired heating is prevention of oxidation of the hot metallic
parts. Three organizations have been conducting research on this problem in the
United States. Thermo Electron Engineering Corporation(83> 84> 85> (TEECO), Waltham,
Massachusetts, and Consolidated Controls Corporation(86) (CCC), Bethel, Connecticut,
have been developing silicon carbide flame barriers while Radio Corporation of
America'" ' (RCA) at Lancaster, Pennsylvania, has been working on a heat-pipe concept
which uses an alumina tube exposed to the flame.
It is believed that units designed by TEECO are the most pertinent to this study.
Present capabilities and anticipated improvements for the TEECO fossil-fuel-fired
thermionic conversion system are enumerated in the discussion which follows,
A 6.1 hp(e) flame-heated power supply was designed at TEECO about a year ago.
To our knowledge, this large unit has not been built, but individual modular diodes
which would be used in it have been built and tested in 0. 4 hp(e) experimental units. As
designed, the high-temperature burner would heat the diode emitters to 2750 F and would
be capable of converting 50 percent of the available heat of combustion of the fuel to heat
for the emitter. Energy conversion is accomplished in 0. 13 hp(e) modular diodes con-
nected in series. One great disadvantage of thermionic converters is that they are low-
voltage high-current devices with a typical output voltage of 0. 5 to 0. 7 volt. Thus, in the
6. 7 hp(e) generator designed by TEECO, 48 diodes are connected in series and produce
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an output voltage of 24 volts. Provisions have to be made for failure of an individual
diode in the open-circuit mode to prevent complete loss of power. The conversion
efficiency of the diode itself is calculated to be 10 percent.
Data taken from the literature on theTEECO 6. 7 hp(e) unit are:
Specific Weight 20 lb/hp(e) including batteries to power
blowers and fuel pump during start-up, but
not including power conditioning equipment,
, if required
Specific Volume 0. 45 ft3/hp(e)
Cost Estimated by TEECO to be about $30 or less
for each 0. 13 hp(e) modular converter [i.e.,
225 $/hp(e)], if mass produced. This cost is
for the thermionic module only and does not
include the cost of blowers, combustor, heat
exchanger, etc.
Efficiency As designed, about 5 percentof thermal con-
tent of fuel is converted to usable electricity.
Power-Output Characteristics Each diode will provide about 0.4-volt output.
Series parallel connections are required to
provide higher voltages.
Start-Up-Time The best start-up times so far achieved
with smaller units are just under 10
minutes.
Variations in power level can have extremely detrimental, sometimes catastrophic
effects on thermionic converters, so that provisions for handling load fluctuations would
have to be engineered into the system.
"1980" Physical, Performance, and
Cost Characteristics
.Continued research and development with fossil-fuel-fired converters will proba-
bly result in increasing the conversion efficiency of the diode itself to about 15 percent
and in a doubling of system efficiency to about 10 percent by 1980. The heavy currents
from such devices will probably limit the specific power per individual diode to about
its present level in order to minimize lead losses.
In principle, any number of 0. 13 hp(e) therimonic diodes can be stacked to build a
power source of any desired power level. Maximum efficiency of the system requires
that all diodes operate at very near their optimized temperature, however, so appropri-
ate combustor and air preheater designs would have to be developed.
Assuming a doubling of system efficiency by 1980, system specific weight, size,
and cost will be at least halved. Thus in 1980 it will probably be possible to build a sys-
tem weighing about 10 lb/hp(e), having a bulk of about 0. 2 ft^/hp(e), and costing about
$100/hp(e) for the thermionic module alone. It is unlikely that instant start-up can ever
be achieved, but the lag may be reduced to only 3 to 4 minutes.
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THERMOPHOTOVOLTAIC CONVERTERS
Thermophotovoltaic generation of electrical power (TPV) is a relatively new and
undeveloped technique. As far as is known, no truly practical devices of this type have
been built. Several low-power laboratory models have been built(°°>°9) an(j a 4.0 hp(e)
system was completely designed and analyzed, but was not built. Research on photo-
cells for use in TPV converters is being conducted at MIT(91) and Delco. (92)
The problems standing in the way of development of practical systems that are
competitive with other types of energy-conversion systems are known, and approaches
to the solutions of these problems are under consideration. However, until more per-
formance data are available, it is difficult to assess the potential of this type of converter
for powering an urban vehicle.
General Description
The processes by which thermal energy is converted to electrical energy in a TPV
converter are the following. The stored energy of a heat source is used to heat a body
usually a cylindrical rod of silicon carbide, or a mantle, which then emits electro-
magnetic radiation. The emitted radiation impinges on a photovoltaic converter. That
portion of the radiation which falls in a suitable wavelength range, to be described later,
is converted into electricity. The remainder is either reflected back to the emitter
and absorbed, helping to maintain the emitter temperature at a desired level, or is lost
through scattering or absorption by passive components of the system.
A typical configuration is to have the photovoltaic cells mounted on the walls of a
cylinder surrounding the emitter rod. Another configuration places the emitter at the
focus of a parabola, with the photovoltaic cells mounted on a flat screen normal to the
axis of the parabola.
The emitter is either a cylindrical rod of a refractory material such as silicon
carbide, or a radiating screen such as the Auer-Welsbach mantle, which is a mesh
screen of thorium oxide with 0.8 percent cerium oxide added to give strong spectral
emittance in the visible wavelength band. The emitter is usually heated to about 2600 F.
The requirements for the emitter are that it be capable of operating at high temperature
for long periods and that it have a high radiant emissivity in the wavelength range of
interest.
The photovoltaic converters are similar in construction and identical in their
principle of operation to the solar cells used to provide power for spacecraft and satel-
lites. However, because of the spectral characteristics of the energy emitted by a
2600 F source, germanium rather than silicon is best suited for TPV.
All photovoltaic cells are made of semiconductors which are characterized
by an energy gap, Eg, which is defined as the minimum energy required to raise an
electron to a "free" state. Only that radiation of wavelength, X, such that X £
1. 24 micron-electron volts T- i *. n. u «. j ,. i
, where Eg is in electron volts, can be converted to elec-
Eg
trical power in a photovoltaic cell of energy gap Eg. Furthermore, when radiation of
energy greater than Eg is absorbed by the photovoltaic cell, the electrical-energy
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output cannot be more than Eg. Thus, the radiation of energy less than Eg is not con-
verted at all, and only a fraction of the radiant energy greater than Eg is converted to
electrical energy. Finally, part of the radiation striking the cell is reflected rather
than converted. Reflection losses can be reduced by use of antireflection coatings, and
by arranging the cells in such a way that reflected radiation returns to the source.
Current and Projected State of the Art
The photovoltaic cells must be made from semiconductors of very high purity and
crystalline quality. The only materials from which reasonably good photocells have been
made, and their energy gaps, are the following:
X for Energy Gap,
Energy Gap, ev microns
Germanium 0.66 1.88
Silicon 1.09 1. 14
Cadmium telluride 1.44 0.86
Cadmium sulfide 2. 38 0. 52
Gallium arsenide 1.41 0.88
Indium arsenide 0.36 3.45
Of these materials, only germanium, silicon, and indium arsenide could be con-
sidered for converting the emitted radiation from a rod at 2600 F to electrical power.
Germanium has been shown to be the best material for this application in view of its
favorable energy gap and its cost, which is believed to be lowest of the three.
The efficiency of a TPV system can be expressed as:
T) .= T) x T] X T) X ?1 ,
overall cell sp s ace '
where
overall = overall efficiency, ratio of net electrical-energy output to thermal
energy of fuel consummed
T) = cell efficiency, ratio of gross electrical energy to usable radiation
cell
energy
sp = spectral efficiency, ratio of usable to total radiation energy emitted
by the source
rj = source efficiency, ratio of total radiation energy emitted to thermal
energy of fuel consummed, this accounts for all stack losses, and
other system conduction and convection losses.
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r)acc = accessary efficiency, ratio of net electrical energy to gross electrical
energy; this accounts for losses due both to the electrical regulator
required and to auxiliary drives.
The spectral efficiency 7]Sp, of systems using germanium and silicon photovoltaic
cells and a 2600 F emitter was calculated as a function of black-body-emitter tempera-
ture by Kittl. He found that spectral efficiency for a germanium cell converter was
approximately 25 percent if none of the nonusable radiation were returned to the source,
and would increase monotonically to approximately 75 percent if all of the nonusable
radiation were returned to the source. Spectral efficiency for a silicon cell system in-
creased from approximately 5 percent with zero return to approximately 30 percent with
90 percent return, and thence to approximately 85 percent with 100 percent return of
nonusable radiation to the source. This calculation shows the importance of returning
nonusable radiation to the source.
Current Research Efforts
The following three approaches have been used in attempting to increase the re-
turn of unusable radiation to the source:
(1) Use of transparent cells provided with highly reflective coating on the back
surface (nonabsorptive cells)
(2) Addition of chemical additives to the emitter to increase the emission in
the usable energy range (selective emitters)
(3) Use of multilayer filters which, by means of optical interference, reflect
unusable radiation and transmit usable radiation.
Nonabsorptive Cells. The first approach has had limited success for several
.easons. Although highly pure germanium is transparent to radiation at energies lower
than the energy gap, it is also highly electrically resistive. Selected impurities can be
added to the germanium to reduce electrical resistivity and thereby increase cell effi-
ciency, but the transparency decreases. Addition of metallic "fingers" to the front
surface reduced cell resistance, but further reduced transparency. Cell efficiency as
high as 22 percent has been reported, as opposed to 28 percent for absorptive cells.
Haushalter, et al.^0), predicted a maximum product of cell and spectral efficiencies of
15.8 percent for a nonabsorptive system.
Selective Emitters. The second approach, use of a selective emitter, shows
promise of success on the bases of results obtained by Kittl(°9). The intent is "to find
materials which have high spectral emissivity below the absorption edge wavelength in
the region of maximum cell response and low emissivity in the near infrared beyond the
absorption edge wavelengths". Calculations showed that metals such as tungsten,
iridium, and platinum offer little chance of achieving the desired emission. A. second
class of materials investigated for this purpose, rare-earth oxides, was considered
promising. A mantle of 70 percent thorium oxide and 30 percent erbium oxide showed
a sharp emission peak at a wavelength of 1. 5 microns.
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An experimental test of TPV converters was conducted using several different
rare-earth-oxide mantles. The mantles were open-mesh screens heated by a propane
burner. An Tloverall/Haux efficiency of 4. 35 percent was achieved with a mantle of
10 percent erbium oxide and 90 percent theorium oxide. It was believed that higher effi-
ciency could have been achieved with a closer mesh in the mantle.
The use of selective emitters is an interesting concept, and the results achieved
by Kittl for erbium and thorium oxides were very promising. However, the ability of
emitters of these materials to operate with stability at temperatures of 2600 F for long
periods of time has yet to be demonstrated. The performance of these emitters depends
upon the inclusion of trace impurities, which probably do not have vapor pressures the
same as those of the host materials. Thus, the composition-dependent spectral charac-
teristics of these emitters would be likely to change over a period of time. Further
investigation of this point is required.
Absorptive Cells Used With Spectral Filters. A system consisting of an absorptive
cell used in conjunction with filters which reflect nonusable energy to the emitter and
transmit usable energy to the cell appears to be promising for achieving a high efficiency
of conversion of radiant energy to electrical energy. The photovoltaic cells used in
such a system are inherently more efficient than those used in a nonabsorptive system.
Cells used by Kittl, for example, converted 28. 3 percent of usable radiation impinging
on them to electrical power, whereas nonabsorptive cells converted a maximum of
22 percent of the usable radiation to electrical power. There has been difficulty in
obtaining filters of high spectral efficiency, although Haushalter et al. recently reported
a filter with spectral efficiency of 70 percent. Haushalter calculated a maximum ^gn x
r)Sp efficiency of 22. 5 percent for a system of absorptive cells and filters.
4.0 hp(e) System Design Study
Possibly the best indication of the current state of the art is an engineering study
sponsored by USAMERDC, Ft. Belvoir, Virginia. (9°) Six TPV systems were contem-
plated, all to operate in a 125 F ambient with the following power and weight:
0. 4 hp, 35 pounds without fuel
0.4 hp, 35 pounds including 8 hours fuel
0.67 hp, 35 pounds without fuel
0. 67 hp, 35 pounds including 8 hours fuel
4.0 hp, 100 pounds without fuel
4.0 hp, 150 pounds without fuel.
Comprehensive designs were carried out, and each part of each system was optimized
for performance and weight.
It was concluded that the desired systems could not be built to operate with photo-
voltaic cells available today if ambient air at 125 F is to be used as the heat sink. Their
data indicated that a specific weight of approximately 49 lb/hp(e) and an overall efficiency
of 4. 6 percent could be achieved on a 4. 0 hp(e) system without fuel. If water at 80 F
could be used for cooling the cells, specific weight would reduce to 37. 5 lb/hp(e). These
figures assumed cells with characteristics superior to those available now, but which
could be developed in about 1 year.
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The following efficiencies were calculated for the proposed 4.0 hp(e) system:
Heat of Combustion of fuel, hp(t) 87. 0
Gross Electrical Power Generated, hp(e) 5. 04
Net Electrical Power Generated, hp(e) 4.0
Source Efficiency, percent 48. 1
Spectral X Cell Efficiency, percent 12.0
Gross Efficiency, percent 5.8
Net Efficiency, percent 4.6
"1980" Physical and Performance Characteristics
Three approaches to TPV conversion have been considered: use of nonabsorptive
devices with reflective back surface, use of absorptive devices with selective emitters,
and use of absorptive devices with filters. The absorptive cell-filter combination
appears to be the most promising at the present time. This approach has a theoretical
efficiency higher than that of the nonabsorptive-cell system. The selective emitter sys-
tem is relatively untried, and even if it were successful, it seems unlikely to bring about
efficiencies higher than those of the absorptive cell-filter system.
The point of departure for projecting future developments in TPV is the 4.0 hp(e)
system designed by GM The product of spectral efficiency and cell efficiency was
12 percent. However, in the report it was stated that if available light filters could be
successfully applied, this product could be increased to 22. 5 percent. As discussed,
Kittl reported a cell efficiency of 28. 3 percent, and Haushalter reported a filter having
a spectral efficiency of 70 percent. Therefore, at the present time, this product could
possibly be as high as 19. 8 percent.
In speculating on the potential specific weight and overall efficiency of a TPV sys-
tem, it is reasonable to assume the following improvements:
(1) Increase source efficiency from 48 to 65 percent
(2) Increase accessary efficiency from 80 to 85 percent
(3) Increase product of cell efficiency and spectral efficiency from 12 percent
to 22. 5 percent.
With these three improvements, overall system efficiency should increase from
4.6 percent to 12. 5 percent. If specific weight were inversely proportional to efficiency,
the potential specific weight could be speculated to be reduced from 37. 5 lb/hp(e) to
13 lb/hp(e). This specific weight is quite optimistic as the 37.5 lb/hp(e) specific weight
was calculated for the water-cooled system.
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OTHER ENERGY STORAGE AND CONVERSION DEVICES
THERMAL-ENERGY STORAGE
General Description
A thermal-energy storage system is one in which thermal energy is stored in a
material by means of some physical change in the material. Examples of physical
changes in materials which can result in a release of stored thermal energy are: change
from one crystalline form to another, condensation, solidification, and drop in tempera-
ture. All of these physical changes are reversible in the sense that heat supplied to the
material will reverse the change of phase or temperature. Thus a thermal energy stor-
age material can be cycled many times between reasonable upper and lower limits of
thermal energy.
Current and Projected State of the Art
Thermal-energy storage has been and is being used in a number of ways. Hot-
water storage has been used in industries and central power stations for steam generation
during peak load periods. In these applications the water is not only heated but is also
pressurized to increase its heat-storage capability. The interest in solar heating a num-
ber of years ago created a necessity for finding low-cost materials with the necessary
qualifications for storing solar energy as thermal energy during the day and releasing it
during the night. Thermal-energy storage has been investigated for use in the space
program, particularly for earth satellite power systems depending on solar energy. Sub-
marine propulsion systems, which must at times operate independently of atmospheric
air, are another application of thermal-energy storage which has received some attention.
Finally, at least one experimental thermal-energy storage Stirling-engine, vehicle pro-
pulsion system has been built.
Physical and Cost Characteristics
Thermal-Energy-Storage Materials
Materials absorb or release heat while undergoing one or more of the following
physical changes: change from one crystalline form to another, liquefaction (melting) or
solidification (freezing), sublimation, evaporation or condensation, the heat of solution
and absorption, and a change of temperature in a single phase. Of these many possibil-
ities the most important for practical application are the change from the solid to the
liquid state and vice versa, and the change of temperature in a single phase. Both of
these physical changes are completely reversible without affecting the chemical makeup
of the material involved, occur in a useful temperature range for many common materi-
als, and result in a fairly high magnitude of thermal-energy storage.
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Figure 24 is a graphical illustration of the thermal-energy content of a typical sub-
stance versus its absolute temperature, showing three transitions as it changes from one
crystalline form to another, followed by melting, and finally evaporation. The slopes of
the curves and the heights of the vertical lines represent the quantities of heat energy that
that can be stored in or released from a unit weight of the material. The temperature
limits indicated by the vertical dashed lines represent the region in which this particular
material would be useful in a thermal-energy storage system.
A good heat-storage material must have a high specific thermal-energy storage
capacity over the operating temperature range of interest. It should have a low vapor
pressure at the maximum operating temperature to avoid restrictions on the container
configuration and to reduce container weight. High density is essential to reduce the bulk
volume of the system. The material should be chemically stable at all temperatures so
that the cyclic heating and cooling or change from liquid to solid will not result in deter-
ioration of the material or its container. It must be noncorrosive to the container and any
heat transfer surfaces in contact with it. It must be rechargable by some efficient
method. The material should also be noncombustible and nontoxic for obvious reasons.
Finally, it must be low cost and abundant.
Further restrictions are placed on the selection of a heat-storage material and its
associated thermal-energy storage system by the particular requirements and operating
characteristics of the application. These restrictions are dictated not only by the general
application (such as, in this case, urban vehicles) but also and possibly more importantly
by the device which is selected to convert the stored thermal energy to useful motive
power. These requirements and operating characteristics include: temperature range;
the relative importance of size, weight, efficiency, and cost; and the cycling frequency
and rates of discharge and recharge.
The maximum temperature range within which thermal-energy storage systems
could be expected to operate profitably is fairly easy to establish. The lower limit of
this temperature range would depend on the available heat-sink temperature, which in
the case of a vehicle is the temperature of the ambient air, and on the minimum accept-
able energy-conversion efficiency. With ambient air as the heat sink, the lowest prac-
tical temperature limit would be about 500 F. The upper temperature limit is dictated
largely by the problem of containment. If the container were metal, an upper temperature
limit of 1600 or 1700 F would be acceptable. If the container were made of a refractory
material, a much higher upper temperature limit could be allowed. As a compromise in
the search and evaluation of potential heat-storage materials, an upper temperature of
about 2800 F seems appropriate.
A fairly thorough screening of materials for heat capacities in the solid and liquid
states, for heats of fusion, for usable temperature ranges has been carried out in re-
cent years and reported in the literature(93, 94) Qf the hundreds of materials studied,
only a relatively few had high enough heat capacities or heats of fusion to be considered
as competitive with other forms of energy for vehicular use. Even fewer actually met
the other qualifications considered desirable for the application.
Table 15 presents pertinent data for five materials with heats of fusion over
0. 12 hp(t)-hr/lb and with melting points within the usable temperature range. These
compounds would be suitable in thermal-energy storage systems involving solidification
of the material, to release the stored heat. Lithium hydride has the highest heat of
fusion per pound of any substance known. Unfortunately it is also a very expensive
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£
<§
o>
Si
c
UJ
0>
Vaporization
Total
thermal energy
available in
operating-
temperature
range
Melting
-Crystalline phase transition
r Operating - temperature H
limits I
Absolute Temperature
A-57480
FIGURE 24. THERMAL-ENERGY CONTENT-TEMPERATURE DIAGRAM
FOR TYPICAL THERMAL-ENERGY STORAGE MATERIAL
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material and has a low density which would result in a larger volume for a given heat
capacity. Another potential disadvantage of lithium hydride is that it is flammable and
will react violently with moisture in the presence of air.
Sodium fluoride and lithium fluoride appear to be attractive compounds for vehicle
application. Sodium fluoride has the lowest cost in dollars per unit energy storage
capacity, but its melting temperature may be a little high for metal containers. Lithium
fluoride has the highest heat of fusion per cubic foot and is second only to lithium hy-
dride in heat of fusion per pound. It also has a very favorable melting temperature.
These characteristics of lithium fluoride would probably result in its being the best
choice of the molten materials for vehicular application.
Table 16 presents pertinent data for a number of metallic oxides which have high
specific heats in the temperature range of interest in this study and melting points which
are above the maximum potential operating temperature. The metallic oxides, com-
monly called refractory materials, are relatively inexpensive and readily available
(with the exception of beryllium oxide). Beryllium oxide would be an excellant material
to use were it not for its extremely high cost. Lithium oxide has the highest specific
heat of any of the materials listed in Table 16. It is susceptible to chemical change,
however, when exposed to atmospheric moisture at ambient temperature or to air at
elevated temperatures.
In the second group of materials in Table 16 are some of the elements of low
atomic weight and one compound. These latter materials also have sufficiently high
heat capacities in the solid state to be attractive for thermal-energy storage systems.
However, they are not as stable at temperatures above 2000 F as the refractory ma-
terials and therefore would have to be contained in such a way as not to come in contact
with air or other reactive gases. They are also somewhat more expensive than the re-
fractory materials.
Physical Characteristics - Thermal-Energy-
Storage Systems
General System Configuration. Figure 25 is a simplified illustration of a basic
thermal-energy storage system. The components of this system include: the heat-
storage material and its container, referred to in combination as the thermal storage
tank; the intermediate heat-transfer fluid loop with its circulating pump and the heat
exchanger which supplies heat energy to the prime mover; and the recharging system
with a combustion chamber, a fuel input line, and an exhaust system.
The container of the heat-storage material serves the important functions of in-
sulating the material against excessive direct heat losses to the outside, preventing or
minimizing deterioration of the material and any components in contact with it, restrain-
ing the material in whatever shape or form is desired for best performance, and provid-
ing the means for transferring heat to and from the material during operation and re-
charging. So the container is obviously more than just a. box. In performing all these
functions, the container and its associated insulation must be able to withstand high
temperatures, must be lightweight, must be nonreactive, and should be low in cost.
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TABLE 15. PERTINENT PHYSICAL CHARACTERISTICS AND CURRENT COSTS OF MATERIALS WITH HIGH
HEATS OF FUSION AND QUALIFYING AS ATTRACTIVE THERMAL-ENERGY-STORAGE
MATERIAL
Compound
Lithium Hydride (LiH)
Lithium Fluoride (LiF)
Lithium Hydroxide (LiOH)
Magnesium Silicide (Mg2Si)
Sodium Fluoride (NaF)
Heat of
Fusion,
hp(t)-hr/lb
0.432
0.176
0.149
0.142
0.135
Specific
HeatU).
hp(t)-hr/lb F
0.000420(c)
0. 000145(d)
0.000142(c)
--
O.Q00110(e)
Melting
Temperature,
F
1270
1558
884
2016
1854
Densit/b),
Ib/cu ft
48.7
164.3
88.0
--
168.4
Current
Bulk Price.
$/lb
9.50
1.55
0.54
--
0. 14
(a) The listed values will be higher at higher temperatures.
(b) In solid state.
(c) At 122 F.
(d) At 50 F.
(e) At 212 F
TABLE 16. PERTINENT PHYSICAL CHARACTERISTICS AND CURRENT COSTS OF MATERIALS
WITH HIGH THERMAL-ENERGY-STORAGE POTENTIAL IN SOLID STATE
Material
Beryllium Oxide(BeO)
Magnesium Oxide (MgO)
Aluminum Oxide (Al^Otf
Silicon Dioxide (SiO2)
Titanium Monoxide (TiO)
Zirconium Dioxide (ZrO2)
Lithium Oxide (Li2O)
Boron Carbide (B4C)
Beryllium (Be)
Boron ( B)
Carbon (C)
Specific
HeatCO,
hp(t)-hr/lb-F
0.000176
0.000133
0.000114
0.000106
0. 000090
0. 000059
0. 000274
0.000184
0.000262
0. 000204
0.000145
Melting
Temperature,
F
4500/4550
5070
3650/3700 .
3110
3180
4800/4900
2600/2860
--
2341
4172
>6400
Density
Ib/cu ft
188
223
248
145
307
352
125
158
115
153
140
Current Bulk
Price.
$/lb
12.00
0.03
0.05
0.0075
0.25
0.48
--
--
--
--
--
.(a) Average value for temperature range 500 to 2200 F.
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Fuel input
Air
input"
Combustion
chamber
Intermediate heat-
transfer-fluid loop
Heat storage
material
Insulation
Container
-A/VWXA-
Exhaust
Pump
-Prime mover
heat exchanger
Prime mover
A-57481
FIGURE 25. SCHEMATIC DIAGRAM OF BASIC THERMA.T -ENERGY
STORAGE SYSTEM
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Two problems that must be solved when dealing with a molten heat-storage sub-
stance in a container are thermal expansion and corrosion. Most substances when
changing from the liquid to the solid state usually either expand or contract. When a
material expands in a confined space, the danger of rupturing the container wall or of
crushing any heat-exchanger tubes imbedded in the material could be great. Lithium hy-
dride and lithium fluoride contract upon solidyfying, while lithium hydroxide changes
volume only slightly. These volume changes can generally be accommodated with careful
design. A problem which can arise when the heat-storage material contracts is the prob-
lem of spalling or pulling away from the heat-transfer surface. The lithium fluroride
and lithium hydroxide compounds have been shown not to flake off or pull away from the
surface but appear to form a tight bond during the solidification process. Examples of
materials which have been found to be reasonably compatible from a corrosion stand-
point with the lithium compounds include nickel or a nickel-molybdenum alloy for lithium
hydroxide, and Hastelloy N for lithium fluoride.
The design of a container for a refractory heat storage system could be consider-
ably less critical than that for the molten heat storage system. Since there is no change
of state involved in the contained material there would be no problem of expansion or
contraction and chemical deterioration would be minimal or nonexistent. On the other
hand, a refractory heat storage system might operate at a higher initial temperature;
consequently the container might have to be lined on the inside with an insulating refrac-
tory material. Furthermore, the brittleness of refractory materials would impose a
burden on the design and mounting of the system.
If a molten heat storage material is used in a thermal-energy storage system, the
question might arise as to the possibility of pumping this material in the molten state
directly to the prime-mover heat exchanger instead of using an intermediate heat-
transfer fluid. Experience has shown, in sodium-cooled reactors for instance, that there
are enough problems in trying to contain molten materials in one location at high temp-
erature without trying to pump them through pipes to a remote location. Therefore,
pumping the heat-storage material itself appears to be impractical for the passenger-
vehicle application. A liquid metal for the intermediate heat-transfer fluid would be un-
acceptable for the same reasons. Water or any other conventional liquid could not be
used because of the high temperatures and resultant high pressures. The best choice
would appear to be a gas, and the most logical gas to use is air. If the circuit could be
permanently sealed, then helium or even hydrogen would be preferable. However, a
perfectly sealed system is considered impractical for this application.
A molten heat storage material having its entire mass contained in a single tank
might conceivably maintain the same bulk temperature throughout as it cools, because of
convection currents set up in the melt. Under these conditions, the outlet temperature
of the intermediate heat-transfer fluid would not be higher than the average bulk temper-
ature of the tank contents. On the other hand, if the system were designed such that the
same total weight of heat-transfer material was dispersed in a number of individual con-
tainers with the intermediate heat-transfer fluid passing successively through these con-
tainers, then its outlet temperature could reach a value higher than the average tempera-
ture of the material in all of the containers since a temperature gradient would be set up
from one container to the next. The second configuration, though probably considerably
more complex, is the more desirable condition since the prime mover would be supplied
with a higher temperature heat-transfer fluid over a longer period of time, thus permit-
ting a greater overall energy-conversion efficiency.
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The overall size and weight of the intermediate heat-transfer fluid loop, and the
power required for pumping the air, can be significantly reduced by pressurizing the air.
Pressures up to 100-psi guage might be appropriate in a passenger-vehicle system. The
pressurization also provides a means for controlling the output of the thermal-energy
storage system, for instance, reducing the pressure by bleeding air out of the system
will reduce the heat output, and increasing the pressure will increase the heat output.
Recharging of the thermal-energy storage system can be accomplished in a number
of different ways. One possible approach is to use products from the combustion of
fuel, either passing the hot gases directly through the thermal storage tank, passing
them over a heat exchanger mounted externally on the thermal storage tank, or using
them to heat an intermediate heat-transfer fluid which is then pumped directly through
the thermal storage tank. This combustion system could be integral with the thermal-
energy storage system, as indicated in Figure 25, or it could be completely separate.
Recharging could also be accomplished utilizing electrical energy such as in resistance
heaters imbedded in the heat-storage material, or resistance heaters located in a heat
exchanger external to the thermal-energy storage system and heating air which is then
passed through the thermal storage tank.
System Design Studies. A number of design studies have been conducted for
specific applications of thermal-energy storage systems. Table 17 is a compilation of
available data from three such studies. The third study listed was recently completed
by the General Motors Research Laboratories and represents an excellent projection of
the physical characteristics that would be applicable to future vehicular thermal-energy
storage systems.
TABLE 17. ESTIMATED PERFORMANCE OF PROPOSED AND EXPERIMENTAL
THERMAL-ENERGY STORAGE SYSTEMS
Specific Specific Specific
System Size, Weight, Volume, Cost,
Application Reference Material hp(t)-hr lb/hp(c)-hr ft3/hp(t)-hr $/hp(t)-hr
CM Research-Submarine 95 Aluminum 66,700 5.3 0.045 1.0
oxide
CM Allison Div. -Satellite 96
CM Research- Vehicle
Lithium
hydride
Lithium
fluoride
11.6
100
3.9
3.3
0.240
0.032
2.0
GM Research - Submarine. This design study was undertaken on the basis that
even though nuclear propulsion systems were proving to be extremely successful there
would still be a need for submarines of small displacement, more limited submerged
ranges, lower cost, and freedom from radiation hazards. The submarine size selected
for the application of a thermal-energy storage system was slightly under 1500 tons
submerged displacement with a horsepower requirement of 2, 228 hp(s).
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Total power-plant weight for this system was to be 484,000 pounds. The main
components of the proposed power plant included: an aircraft-type gas turbine for sur-
f^.ce or snorkel running and submerged emergency speed, two large Stirling engines
for submerged cruising, one smaller Stirling engine for submerged creeping, and two
even smaller Stirling engines for auxiliary electrical power generation, plus the
necessary speed reducers, clutches, piping and insulation, expansion joints, and valves.
The function of the thermal-energy storage system was to provide the power for
completely submerged operation. A recharging capability was included in the form of
a liquid-fuel combustion system. The total weight of the Stirling engines, gas turbine,
and other equipment mentioned above came to 129,000 pounds, leaving 355,000 pounds
available for the thermal-energy storage-system weight. Aluminum oxide was selected
as the heat-storage material to operate between an upper temperature of 2800 F and a
lower temperature of 1350 F. The thermal storage tank consisted of a metal shell with
a refractory insulation lining on the inside and a fibrous insulation on the outside. The
tank, insulation, and combustion recharging equipment weighed approximately 46,000
pounds, and the heat-storage material weighed 309, 000 pounds.
The thermal-energy storage system was capable of delivering a total stored energy
of 66, 700 hp(t)-hr. The heat loss from the thermal storage tank was estimated to be
less than 0. 05 percent per hour. Using this total available stored energy in the main
propulsion Stirling engines, the submarine would be capable of sustaining a speed of
15 knots for slightly over 22 hours, which would give it a submerged range of about
340 nautical miles. At an emergency speed of 35 knots, and using the thermal-energy
storage system stored energy in the gas turbine, the running time would be approximately
1-1/2 hours.
GM Allison Division - Satellite. This design study was conducted to determine the
requirements for a thermal-energy storage system to be incorporated with a solar power
system on board an earth satellite. This solar power system was designed to produce
the heat energy required for a 4 hp(e) Stirling-engine-powered generator system. The
basic power plant for this satellite consisted of a solar-energy collector and absorber,
an intermediate heat-transfer fluid and pump, a heat reservoir, and a Stirling engine
driving an electric generator. The heat reservoir was necessary because for the
particular earth orbit mission envisioned for this satellite it would be in the earth's
shadow for approximately 40 percent of each orbit, or about 35 minutes out of each
90 minutes.
The thermal-energy storage system proposed for this application consisted of
conically shaped lithium hydride elements encased in electro-formed nickel or stainless
steel jackets and disposed in a configuration allowing free passage of the intermediate
heat-transfer fluid around each element. Minimum insulation was provided around the
entire assembly because of the short charge and discharge cycle and because minimum
weight was of paramount consideration.
A system sized for a 300-mile-high earth orbit was estimated to weigh a total of
45 pounds, including 15 pounds of the heat-storage material, lithium hydride, to occupy
a space of about 2-3/4 cubic feet, and to be capable of producing about 11.6 hp(t)-hr from
one charge. The authors concluded that a thermal-energy storage system based on these
approximate specifications and performance data was technically feasible.
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GM Research - Vehicles. General Motors is currently actively engaged in an
investigation of the application of thermal-energy storage to passenger vehicles. Their
current investigation was a natural extension of their studies for the submarine applica-
tion, with the added impetus of the current search for a nonpolluting vehicle power plant.
As a tangible demonstration of the technical feasibility of a thermal-energy storage
system for vehicular application, a system was installed in a conventional Corvair auto-
mobile. This system consisted of a thermal-energy storage unit mounted in the front,
trunk compartment of the vehicle and a Stirling engine located in the rear, engine com-
partment of the vehicle. Air was used as the intermediate heat-transfer fluid. The
heat-storage material used was aluminum oxide in pellet form about 1 inch long, hexa-
gonal in cross section, with a hole in the center of each and cutouts at the corners of the
hexagon so that when properly stacked in the thermal storage tank there would be a mul-
tiplicity of air passages through the system. This was the same general configuration
that was studied earlier for the submarine application.
The system worked and the vehicle performed reasonably well, amply demonstrat-
ing the technical feasibility of thermal-energy storage for vehicular use. No significant
attempt was made to optimize the system either before or after installation in the ve-
hicle; consequently, actual performance data from this application are not of direct
interest to this study.
Further work in this area at General Motors, including both theoretical and labo-
ratory studies, has resulted in an attractive second-generation concept for a vehicular
thermal-energy storage power system. This concept is the third one listed in Table 17
and involves a unique and compact integration of the thermal-energy storage system and
the prime mover. Using lithium fluoride as the heat-storage material and a Stirling
engine as the prime mover, the key to this concept is having the Stirling-engine heater
tubes buried right in the heat-storage material. Thus, no intermediate heat-transfer
fluid with its associated hardware and losses is required.
The lithium fluoride is cycled within a temperature range from 1600 F maximum
down to a cutoff temperature of about 750 F. In this temperature range both the heat of
fusion and the sensible heat in the molten and solid states is utilized. With the engine
heater tubes buried right in the lithium fluoride, a minimum temperature difference can
be maintained between the heat-storage material and the working fluid of the Stirling
engine.
Electric immersion heaters embedded in the lithium fluoride would supply the heat
for recharging. A complete recharge from full discharge would take about 4 hours.
Control of the engine power output and compensation for the decreasing lithium fluoride
temperature would be accomplished by varying the working-fluid pressure within the
engine. It is possible, by increasing the working-fluid pressure, to maintain a full-load
maximum efficiency capability up to the point where about 85 percent of the stored ther-
mal energy is consumed. During the last 15 percent of energy release, the engine
efficiency and maximum power capability would taper off.
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MECHANICAL ENERGY STORAGE
Three principal means of mechanical-energy storage have been considered:
(1) the storage of kinetic energy in flywheels, (2) the eleastic deformation of solids
(springs), and (3) the compression of a gas, namely in terms of a hydraulic accumulator
system. A discussion of each of these follows.
Flywheels
In the past few years there has been a renewal of interest in flywheels as sources
of stored power. Their use is most appropriate in applications where many charge and
discharge cycles are contemplated, and where the energy does not have to be stored for
a long period of time. They have the desirable feature that all of their stored energy
can be removed rapidly (i. e. , they can be designed to have a high specific power).
These characteristics make them natural candidates for hybrid vehicle propulsion
systems, where they would be used in conjunction with an engine of reduced size, and
more importantly for this study, of reduced pollutant emissions.
General Description
A flywheel for automotive application would most likely be a steel disk thicker at
the hub than at the rim. It would be enclosed in a housing that would be evacuated to a
pressure of 0. 1 to 0. 01 atm by a small vacuum pump. Pressure-lubricated ball bear-
ings would probably be required. The wheel and its auxiliaries present no serious tech-
nical problems.
The difficult problem is how to couple the flywheel to the vehicle wheels in an ef-
fective and inexpensive way. A steplessly variable drive is desired, since the flywheel
must decelerate smoothly as the vehicle accelerates. The candidate systems include
hydrostatic and electrical drives, traction drives, special torque converters, and
planetary gearsets which allow changes in flywheel speed to be compensated by changes
in engine speed. The torque converter is perhaps the lowest cost system, but it re-
quires development to meet the unusual operating conditions. The hydrostatic system
is perhaps the most desirable in regard to performance and size. The planetary gear
system is an interesting compromise whose possibilities have not been completely ex-
plored.
Current and Projected
State of the Art
Existing Units and Applications. Several applications in which flywheels have
been used for moderately long-time energy storage are listed in Table 18. Of these,
the Oerlikon Electrogyro is the only application to vehicle propulsion that was put into
commercial service. It was a transit bus that had no engine. When the bus stopped to
take on passengers, electrical contact was made with an overhead terminal and energy
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was imparted to the flywheel. These buses were used in several European and African
cities, but are no longer being built. Even though the flywheel used was a very large,
low-performance one, the failure of the Electrogyro is attributed tu operating incon-
veniences resulting trom lack of an engine rather than to any technical difficulty with the
flywheel.
TABLE 18. EXISTING UNITS AND APPLICATIONS OF FLYWHEELS FOR F.NERCY STORAGE
System
Reference Application
Wheel Energy Specific
Type and Stored. Weight3), ' ivc
Weight hp(s)-hr lb/hp(s)-hr System
Status
Oerlikon Electrogyro 97
Gyreacta and Hydreacta
Transit bus
Vehicle
transmission
Steel
rim.
3300 Ib
Steel
disk,
230 Ib
12
275 Electric No longer being built
Planetary
Development
suspended?
General Dynamics 98
North American 99
Aviation
Power supply Glass
for aerospace fiber,
use 56 Ib
1.5
38
Electric
Reunite power Steel 1.7
storage lor disk, for
aircraft widely 100-lb
control variable wheel
functions
Apparent
success
59 Hydrostatic Under
development
(a) Wheel only.
The Gyreacta transmission is a more modern, higher performance device that
augments the performance of an engine. The flywheel and planetary gearbox are in-
corporated into an attractively compact unit that is similar in appearance to a passenger-
car automatic transmission. The gearbox is quite complicated, however, and to the
best of our knowledge the developer has not placed this unit on the market.
Physical, Performance, and Cost Characteristics. The effectiveness of a flywheel
in storing a given quantity of energy in H unit of minimum size and weight depends upon
the material of the wheel and its general configuration, i. e. , whether rim, disk, or
specially contoured. The conventional design of a rim with spokes or a relatively light
webb is best only if there are simultaneous limitations on the diameter and the rotative
speed such that the centrifugal stress in the material does not limit the design. When
there is a choice of speed, a disk can be run approximately 50 percent faster than a thin
rim of the same diameter, because it retains its material primarily by radial tension
rather than by hoop tension. Although heavier, the disk will store approximately
20 percent more energy per pound of flywheel material. As the rim is made thicker in
the radial direction, the advantage of the disk increases. The limiting case of a thick
rim is a disk with a very small hole in the center. This will store just half as much
energy per pound as the unpierced disk.
Because of the centrifugal stresses in a disk are greatest at its center, they can
be significantly reduced by the addition of a relatively small amount of material in the
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form of added axial thickness at this point. The wheel can then be run faster, and will
store still more energy per pound. The most effective wheel on an energy-per-pound
basis is thought to be one shaped so as to approximate a condition of uniform centrifugal
stress throughout the disk. Some wheels of this type have been made with rim thick-
nesses only 5 to 10 percent of the thickness near the hub. As a practical device, how-
ever, the constant-stress wheel is a victim of diminishing returns. The housing re-
quired to enclose its large diameter, thin rim weighs far more than that portion of the
wheel, and the large area at high speed increases the aerodynamic drag loss. The most
practical configuration for use in a vehicle is thought to be a disk with the hub thickened
by 50 to 100 percent to reduce the stress.
Material. If energy stored per pound of flywheel is taken to be the criterion of ex-
cellence, and if simultaneous limitations oh speed and diameter do not prevent working
the material to its allowable stress, then the effectiveness of a flywheel material can be
shown to be directly proportional to its strength-to-weight ratio. When choosing among
the metallics, therefore, there is little motivation for use of anything but steel forgings.
Very high performance steel wheels for aircraft applications are presently being de-
signed for working stresses approaching 140, 000 psi. Stresses on this order can be
contemplated because of the accuracy with which the loading is known. For vehicle ap-
plications, however, it is anticipated that the safety measures required would make
wheels working much above 100, 000 psi uneconomical.
In the area of the nonmetallics, there are a number of modern materials with
strength-to-weight ratios considerably better than those of steel. The most attractive of
these at present is a composite of glass fibers wound in an epoxy filler. Although its
strength-to-weight ratio is about three times that of steel, this material is not really
that much better because it is nonhomogeneous and must, at the present state of the art,
carry its stresses as a hoop. The weight saved over that of a constant-stress steel
wheel virtually disappears unless the glass-fiber rim is made quite thin. Since the
glass-fiber wheels are also expensive, bulky, and limited in their ability to transmit
high torque to the hub structure, they are not recommended except for very high perfor-
mance aerospace applications, where the ambient vacuum makes it unnecessary to house
them. More advanced materials utilizing fibers of materials such as boron, graphite,
and silicon carbide(^O)
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Specific Weight, Volume, and Cost. Attainable specific-weight figures for a few
appropriate combinations of material and shape are given in Table 19. The weight and
occupied volume of the housing required have also been estimated, but these should be
regarded as very rough preliminary figures. The bearing assembly and vacuum pump
are included, but not the gear reduction. For automotive application, a specific weight
and a specific volume of approximately 100 lb/hp(s)-hr and 0.25 ft^/hp(s)-hr, respec-
tively, appear to be most appropriate. These values would apply to a wheel designed for
a stress level somewhat below 140, 000 psi and having a basic disk configuration with the
hub thickened by 50 to 100 percent. Assuming a minimum O. E. M. or dealer1 s cost of
approximately $l/lb, the estimated specific cost would be $100/hp(s)-hr.
TABLE 19. SPECIFIC WEIGHT AND VOLUME OF FLYWHEELS AND HOUSING
Specific Weight,
Wheel Only,
lb/hp(s)-hr
Constant-thickness 111
steel disk, 100, 000 psi
Constant-thickness 79
steel disk, 140, 000 psi
Constant-stress steel 58
wheel, 140, 000 psi
Glass-fiber rim, 38
90 percent yield stress
Estimated Estimated Occupied
Weight With Volume of
Housing, Housing,
lb/hp(s)-hr cu ft/hp(s)-hr
132 0.32
94 0. 23
78 0.43
76 1.27
Springs
Although springs are the most commonly used energy-storing devices, they are
presently used only where relatively small amounts of energy are to be stored. The
storage of quantities of energy large enough to accelerate a vehicle requires an advance
in the state of the art in that springs having superior specific-weight and specific-volume
characteristics must be developed. This means that steel would be replaced by elas-
tomerics as the spring material. The development would be a difficult one, particularly
in regard to life of the spring elements and efficiency of energy recovery.
General Description
The energy storage unit lor use in a vehicle would no doubt consist of a multiplicity
of small springs rather than one large one. This would make it easier to manufacture,
would make possible fuller development of potential material properties, and would allow
attractive packaging configurations. In addition to the springs themselves, a mechanism
would be required for combining the iorces and motions of the many springs into a single
output, a speed-increasing gear train, and a variable-ratio drive to match the varying
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torque requirements of the vehicle to the varying torque delivery of the springs. The
combining and speed-increasing requirements would be simplified if the springs were
mounted in series, so that their deformations would add.
Current and Projected
State of the Art
Existing Applications. No pertinent applications of springs are known, nor are
there applications of elastomerics with the intent of storing appreciable quantities of
energy for reuse. Energy storage springs presently in use are of steel and are mostly
of the spiral-motor or the Belleville type. If made large enough to provide power for a
road vehicle, they would be far too heavy.
Physical, Performance, and Cost Characteristics. The most important indicator
of performance for a vehicle propulsion spring has been identified as the ratio of the
energy stored to the weight of the spring material storing it. Most common springs
are so heavy that they do not store enough energy to accelerate their own masses to
highway speed. For materials with linear stress-strain curves, the energy stored per
pound is proportional to
_S2
Mp '
where
S = working stress
M = appropriate modulus of elasticity
p = density.
Either shear or tensile effectiveness may be calculated by using the appropriate modulus
and working stress. Elastomers usually have seriously nonlinear stress-strain curves,
making it necessary to estimate the area under the curve to determine the energy stored.
Effective use of any material depends, of course, on stressing all of the material used
to the same extent.
Metallic Springs. Among the metallics, the above index of merit indicates steel
to be the best spring material. Steel in tension or compression is about ZO percent bet-
ter than steel in shear, but is not recommended because of the difficulty in handling the
small deflections and large forces that result from pure tensile loading. Uniform shear
can be conveniently applied by using the material in the form of a thin-walled tubular
torsion bar. Such a tube in torsion can store twice as much energy per pound as a solid
round in torsion (a conventional helical compression or tension spring), and can store
about five times as much energy per pound as a rectangular bar in bending (a conventional
leaf or spiral motor spring). It is also better than a Belleville washer, which has
similar difficulties in working all its material to capacity. Spiral motor springs and
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Belleville washers have been favored in the past because the force delivered remains
relatively constant over a large deflection range. Where really high energy storage is
required in a light package, this is a luxury that cannot be afforded. Changes in force
must be compensated for by the driveline.
Elastomeric Springs. Among the elastomerics, natural rubber and urethane have
the highest energy-storage potentials per pound of material. Neoprene rubber is the
only other material that comes close to these two. Energy storage is virtually the same
in tension and in shear, since the deformation amounts to several hundred percent
causing the material to be basically in tension in either case.
Rubber springs are loaded by bonding opposite faces to rigid metal parts which
move relative to one another. It is very important to avoid high tensile stresses at the
edge of the bond area, since, otherwise, peel failure will prevent full loading of the rub-
ber. For this reason, tubular sections in torsion are recommended over tensile ar-
rangements or shear pads. The shear rate may be applied radially by an internal bar
and an external sleeve that rotate relative to one another, or it may be applied axially
by two end plates with relative rotation. The latter arrangement seems particularly
well suited for large-capacity springs, since many sandwiches could be stacked end to
end.
Urethanes cannot be loaded in the same way that rubbers are, since strong ad-
hesive bonds cannot be made to them. A mechanically applied tensile load would probably
be used. Considerable development would be required here.
Specific Weight and Volume and Bulk-Material Costs. Comparative performance
and cost figures for springs of steel, natural rubber, and urethane in appropriate load-
ing conditions are given in Table 20. These figures cover only the spring itself, and not
the driveline components. The estimates are nonconservative, the steel being worked
to its endurance limit and the elastomers to their breaking points.
It can be seen that the steel spring is more than 100 times heavier than the elas-
tomeric ones for a given quantity of energy stored. It is so heavy, in fact, that the
energy stored will accelerate the mass of the spring to only 56 mph. Consequently, any
vehicle that is driven solely by a steel spring which moves with the vehicle is limited by
theory.to a top speed of less than 56 mph. A vehicle with an engine in addition to the
spring could, of course, go faster, but it appears evident that the amount of energy
storable would not be sufficient to appreciably affect performance or economy. Steel is
not, therefore, a candidate material for a vehicle propulsion spring.
If the elastomers are now compared, it can be seen that the urethane springs are
somewhat smaller and lighter than those of natural rubber. It is believed, however,
that this advantage will be more than offset by the difficulty in loading the urethanes and
by their higher cost. In addition, the urethanes have a potential hysteresis problem that
results in inefficient recovery of the energy stored. The amount of hysteresis can be
changed greatly by adjustment of the formulation o* the urethane. Attempts to increase
the hysteresis for applications where damping is desired has been very successful. It
is not known how much effort has been expended on attempts to decrease the hysteresis.
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MEMORI
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TABLE 20. PROPERTIES OF SPRING MATERIALS
Steel in shear,
90, 000 psi
Natural rubber,
unfilled, in shear
DuPont LD-167 urethane,
Shore A 95, in tension
Acushnet Elastacast
polyurethane, Shore A 85,
in tension
(a) Weight of system would probably be 50 to 100
(b) Volume of system would probably be 50 to 100
(c) Approximate cost of basic material purchased
Weight of Volume of
Spring Material Spring Material Approximate
Only(a), Only(b), Efficiency,
lb/hp(s)-hr cu ft/hr( s) -hr percent
19,100 39.0 99
93 1.36 90
69 1.02 75
58 0.82
percent greater than this.
percent greater than this.
in large quantities. Cost of part fabrication not included.
Current Material
cost(c),
$/lb
$0. 20
1. 00
1. 00
n
o»
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112
On the basis of the above discussion it is concluded that the most likely spring de-
sign would be natural rubber fabricated in tubular sections and loaded in torsion.
Compression of Gases
General Description
The most likely system for utilizing the compression of a gas as a means of storing
energy for automotive application would be one utilizing a hydraulic accumulator. Such a
system might consist of a spherical accumulator with a flexible diaphragm to separate the
gas from the hydraulic fluid. The gas and hydraulic fluid would both be at the same high
pressure. When the compressed gas expanded, or released energy, the hydraulic fluid
would be forced out of the accumulator and through a variable-displacement, hydrostatic
"pump-motor". As the hydraulic fluid passed through the motor it would experience a
drop in pressure and would drive the pump shaft, which would be geared into the drive
train of the vehicle in question. After passing through the motor, the hydraulic fluid
would be collected in a low-pressure reservoir.
To store energy in the gas, the hydraulic "pump-motor" would be driven as a pump
and would pump the low-pressure fluid in the reservoir back into the high-pressure ac-
cumulator. This would result in a compression of the gas and storage of energy in the
accumulator.
Current and Projected
State of the Art
To investigate the characteristics of such a system, a number of optimistic as-
sumptions were made. First it was assumed that the accumulator would be a uniform-
wall-thickness sphere made of steel with a permissable design stress of 100,000 psi.
Maximum accumulator pressure was assumed to be 5000 psi. While this pressure is
higher than that used in the great majority of industrial hydraulic systems, some air-
craft applications and a few industrial applications operate at pressures this high or
higher.
With these assumptions as a starting point, an analysis then was conducted to
determine the optimum gas-expansion ratio from the stand points of both weight and size.
That is, what gas-expansion ratio would result in the maximum number of hp(s)-hr being
stored per pound of accumulator, including the weight of both the steel sphere and hy-
draulic fluid? Likewise, what expansion ratio would result in the largest number of
hp(s)-hr being stored per cubic foot of accumulator?
The results of this analysis are listed in Table 21. The optimum expansion ratios
were determined, assuming both an isothermal and isentropic expansion process. When
optimizing for weight, the optimum expansion ratio calculated on an isentropic basis
was 14 percent less than that calculated on an isothermal basis. Likewise, when opti-
mizing for volume, the ratio calculated on an isentropic basis was 21 percent less than
that calculated on an isothermal basis. Since the amount of (.nergy available during an
isothermal expansion is greater than that for isentropic expansion, the former was as-
sumed to exist, and the specific weight and volumes for the accumulator and fluid were
calculated on this basis.
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TABLE 21. SPECIFIC WEIGHT AND VOLUME OF HYDRAULIC ACCUMULATORS
FOR COMPRESSED-GAS-ENERGY STORAGE(a)
Expansion Ratio
Optimized for Weight
Expansion Ratio
Optimized for Volume
Expansion
Process
Isothermal
Isentropic
Isothermal
Isentropic
Optimum
Expansion
Ratio
2. 06
1. 77
2. 72
2. 15
Specific
Weight,
lb/hp(s)-hr
495
--
515
~
Specific
Volumefc),
ft3/hp(s)-hr
7. 86
--
7. 50
~ "
(a) Hydraulic accumulator assumed to be uniform-wall-thickness sphere made of steel with a permissable design stress
of 100, 000 psi. Maximum internal pressure assumed to be 5000 psi.
(b) Weight of steel sphere and hydraulic fluid only.
(c) Volume of accumulator only.
VEHICLE TRANSMISSIONS
(MECHANICAL, HYDROKINETIC, HYDROSTATIC)
In this discussion, the transmission is defined as a variable, mechanical-shaft-
power converter for the conversion of mechanical-shaft power at one condition of speed
and torque to a variety of conditions of speed and torque. Its primary functions are to
provide a means of (1) uncoupling the engine for starting and idling purposes,
(2) varying the torque and speed ratio betwen the engine and driven wheels as required,
and (3) changing the direction of vehicle motion. For certain transmissions and appli-
cations it is the additional function of the transmissions to provide a vehicle braking
capability by converting mechanical-shaft energy into thermal or some other form of
energy.
General Description
Vehicle transmissions are usually divided into four principal categories:
mechanical, hydrokinetic, hydrostatic, and electric. Each of these principal categories
in turn is composed of a myriad of specific transmission concepts. For example, under
mechanical transmissions would be classified positive engagement, friction drive, and
viscous drive devices. These classifications can in turn be broken down into numerous
specific concepts. For the purposes of this study, however, only the most advanced con-
cept with respect to automotive application is discussed for each category. Also, only
the first three categories fall within the scope of this program and are discussed in the
following paragraphs.
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Mechanical Transmissions
Figure 26 shows the arrangement of components in a typical 3-speed mechanical
transmission clutch and gearbox. Power is transmitted from the engine flywheel to the
main drive gear by means of the dry, single-plate friction disk, and in this particular
unit, normal force for the friction elements is supplied by a diaphragm-type spring.
During gear changes, the clutch fork is actuated in the direction indicated; this causes
the diaphragm-type clutch spring to bow and permits the retracting springs to draw the
friction elements out of contact. With the clutch disengaged, the desired gear ratio is
selected by means of a shifting linkage which slides one or two gears along a splined
shaft to produce a particular combination of mesh. With the exception of the direct-
drive condition, power flow always passes through the main drive gear into the counter-
shaft, and then back to the clutch sleeve through one of the gear combinations and a.
splined connection. Small friction clutches known as synchronizers are commonly inter-
posed between the clutch sleeve and the main drive and second-speed gears to match the
speed of these elements just before they are brought into mesh.
Variations of this basic arrangement include the use of helical clutch springs in
place of the diaphragm type-spring shown, the addition of more gear ratios, and the in-
corporation of overdrive units. In heavy-duty trucks, some two-plate clutch designs are
used, and 10, 12, and 15-speed transmissions are not uncommon. In the case of a
10-speed transmission, the 10 ratios are usually obtained by integrally mounting a
2-speed reduction unit in series with a 5-speed unit.
Hydrokinetic Transmissions
Hydrokinetic transmissions, often referred to as automatics, are installed in
approximately 80 percent of currently built American passenger cars. As shown in
Figure 27, these transmissions consist of two basic sections - a torque converter and a
planetary gear box. The converter section is filled with hydraulic fluid, and power is
transmitted from the engine flywheel to the input shaft of the planetary gear box by means
of the hydraulic coupling action of the converter. Engine torque applied to the converter
pump forces an energy transmitting flow of oil into the converter turbine - the turbine,
in turn, absorbs this kinetic energy and converts it into a torque acting on the planetary
input shaft. Manual and/or automatic actuation of the transmission controls then locks
or engages various elements of the planetary gear set to provide two definite forward
gear ratios, one reverse ratio, and one neutral position.
Common variations of the arrangement shown in Figure 27 include the use of two
planetary gear sets to obtain 3 or 4 forward-speed ratios; the use of converter lock-out
clutches to provide a positive mechanical input for certain gear ranges and/or cruising
operation; and the use of multiple and/or variable converter elements to provide certain
desirable operating characteristics for particular applications. Various combinations of
band and multidisk clutch units are applied for engaging the planetary gear elements, and
some differences also exist in the hydraulic circuits which control the shifting sequence.
The latter usually involve mechanical or hydraulic governor devices which sense engine
and vehicle speed, and another element which utlizes engine manifold vacuum and
throttle position as a measure of operating load. It should be noted that the torque con-
verter is also a torque multiplier, and that individual converter designs are tailored to
provide the desired combination of torque multiplication and responsiveness.
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DRIVEN PLATE
FLYWHEEL
CRANKSHAFT
PRESSURE PLATE
I- DIAPHRAGM
SPRING
PIVOT POINT
CLUTCH FORK
ETRACTING SPRING
MAIN DRIVE GEAR
-CLUTCH SLEEVE
COUNTERSHAFT
FIRST GEAR
RATIO 2.58 TO 1
SECOND GEAR RATIO 1.48 TO I
THIRD GEAR
RATIO I TO 1
REVERSE GEAR RATIO 2.58 TO 1
FIGURE 26. MECHANICAL TRANSMISSION CLUTCH AND GEAR BOX
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r
Planetary
Input Shaft
Engine
Flywheel
-Torque Converter
-Converter Turbine
Converter Pump
FIGURE 27. HYDROKINETIC TRANSMISSION
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Hydrostatic Transmissions
A hydrostatic transmission consists essentially of one or more positive-
displacement hydraulic pumps connected by pressure lines to one or more positive-
displacement hydraulic motors. Displacement of the pump and motor may be either
fixed or variable, and the pump and motor may or may not be of identical construction.
By definition, hydrostatic transmissions differ from hydrokinetic transmissions in that
power is transmitted by means of hydraulic pressure as opposed to fluid velocity.
Hydrostatic transmissions have no typical configuration, but their principle of
operation may be visualized by the inspection of Figure 28. In this particular arrange-
ment, a variable-displacement axial-piston pump is used to drive two fixed-displacement
axial-piston motors. A neutral or no-drive condition is obtained by setting the variable-
displacement pump for zero flow. Gradually tilting the movable swash plate in one
direction then causes a gradual increase in forward vehicle motion, and tilting in the
opposite direction produces a gradual increase in reverse motion.
Variations of this basic arrangement are limitless. All of the positive-
displacement pump types (axial-piston, radial-piston, vane, and gear) may be used in
various combinations; the motor and pump units may be "end-to-end" (as shown in
Figure 28) or remotely located and conncected by tubing or hoses; and combinations of
variable displacement units, motor controls, and/or integral gear elements can be ar-
ranged to provide a wide variation in torque and speed chracteristics to match particular
applications.
Current and Projected State of the Art
Properties for a sampling of current vehicle transmissions are shown in Table 22.
As indicated by the column headings, many of these values are estimates. Data sources
included product literature; manufacturers and/or their representatives; local users;
and technical reports, texts, and articles. Data which were not available are indicated
by the notation (n. a. ). General conclusions which may be drawn from these data are as
follows:
(1) For a given class of vehicles, a mechanical transmission offers the
highest peak efficiency, the lowest unit cost, and the smallest envelope
volume.
(2) For passenger cars, incorporation of a hydrokinetic transmission
in place of a mechanical transmission results in a relatively small
weight and efficiency penalty and a relatively large cost penalty.
(3) Hydrostatic transmissions are used principally in off-highway equip-
ment where their lower efficiency and high initial cost are offset by
their unique installation and operating characteristics.
(4) Cost, weight, and size figures for industrial units are 5 to 10 times
higher than those for corresponding passenger-car units. This difference
occurs as a function of the number of units produced, the higher number of
speed ratios or ratio spread provided, and the construction requirements
for different duty cycles.
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Prime Mover
Variable Displacement Pump
Fixed Displacement
Motor
Brake pedal
Movable manifold
t
Reversing cylinders
FIGURE 28. HYDROSTATIC TRANSMISSION
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All three of these mechanisms are highly developed, with hydrostatics being the
least so in this respect. While improvements will continue to be made in these trans-
missions, they will not be of a nature to permit estimates of "1980" specific parameters
to deviate significantly from those listed in Table 22. This is particularly so when
considering the fact that the transmissions of interest to this program are generally
of lower power than those listed in Table 22. The principal exception to the preceding
is the case of high power, high pressure, ultrahigh speed, hydrostatic systems.
NUCLEAR DEVICES
With respect to nuclear devices, the two major sources of nuclear energy, fission
reactors and radioisotopes, and two direct nuclear-to-electric energy-conversion de-
vices, alpha- and betavoltaic cells, were studied to determine whether they would be
applicable for use in urban-vehicle propulsion systems in the next 10 to 15 years. The
alpha- and betavoltaic cells would most likely be used with a radioisotope energy source;
therefore, in the following paragraphs, radioisotopes are discussed in connection with
both thermal and direct electrical power generators.
One vehicle-propulsion-system consideration that is unique to nuclear devices is
the fact that to have radioactive material in a highly populated area would require ade-
quate shielding and many safety precautions. The allowed radiation dose for the general
public is 0. 5 roentgen equivalent man (rem) per year or approximately 1.4 mrem/day.
To protect passengers, pedestrians, and the general environment from a harmful radia-
tion dose, the shielding would have to be "4 TT", that is, completely surrounding the
radioactive source. Also, the shield would have to withstand any conceivable major
accident.
Reactor Systems
Three reactor systems typical of portable or semiportable systems were investi-
gated. These are the ML-1 military reactor and the SNAP-8 and SNAP-50/SPUR space
reactor systems. The ML-1 is gas-cooled and the SNAP-8 and the SNAP-50/SPUR are
liquid-metal cooled. Water-cooled reactor systems were not investigated because of
their inherently large size.
ML-1 System
General Description. The ML-1 is a mobile power plant developed for the United
States Atomic Energy Commission and the United States Army Corps of Engineers by
Aerojet-General Corporation. It consists of a nuclear reactor which supplies heat
energy to a closed Brayton-cycle (gas turbine) engine, which in turn drives an electric
alternator. Oxygenated nitrogen is used as the working fluid in the closed Brayton-cycle
engine. The power plant is rated to produce a net output of 400 hp(e) at an ambient air
temperature of 100 F and 670 hp(e) at -65 F.
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TABLE 22. PROPERTIES OF
Manufacturer
and
Model
General Motors
Corporation
3 Speed Manual
Ford Motor Company
3 Speed Manual
Fuller Transmission Div. ,
R-96, 10 Speed Manual
Fuller Transmission Div.,
RT-910, 10 Speed Manual
General Motors
Corporation,
3 Speed Turbo-
Hydra-Matic
Ford Motor
Company,
3 Speed, C-4
Cruise-O-Matic
Allison Div.. CMC
6 -Speed Transmatic
Allison Div., CMC
6-Speed Transmatic
Dynex
(Hydro- Planetary Hub)
Eaton, Yale & Towne
(Taurodyne 2/60)
Sta-Rite Industries
(C-3 Series Hydrostatic)
Sundstrand
Principal
Applications
Full Size
Passenger Car
Compact
Passenger Car
Heavy Duty Linehaul
Truck
Heavy Duty Linehaul
Truck
Full Size
Passenger Car &
Light Duty
Delivery Vans
Compact
Passenger
Cars and
Econoline Vans
Cross Country Bus
Heavy Duty Trucks
Linehaul Trucks
Heavy Duty-
off Highway
Vehicles
Tractors, Fork- Lift
Trucks, Dump
Trucks, etc.
Machine tools, farm
accessories
Tractors, Etc.
Small Garden
Tractors
Estimated Actual
Power Capacity, hp(s)
270
200
284
340
300
200
300
180
400
60
30
81.4
12
Input Speed
at Max Power, rpm
4500
4300
2100
2100
4700
4300
2100
2000
2500
2500
3000
~3000
~3200
Input
Torque at
Max Power,
ft. Ib
Mechanical
315
244
710
850
Hydrokinetic
335
244
750
472
Hydrostatic
840
126
52.5
142
19.7
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CURRENT VEHICLE TRANSMISSIONS
Ratio
Spread
Weight,
Ib
Estimated Specific
Weight,
lb/hp(s)
Estimated Specific
Envelope Volume,
Volume, ft3 ft^p(s)
Estimated
Dealers
Cost,
$
Estimated
Estimated Peak
Specific Cost, Efficiency,
$/hp(s) percent
Transmissions
2.41:1 112.5
2.99:1 99
9.65:1 820
8.05:1 706
Transmissions
5.06:1
185
0.4
0.5
2.9
2.1
0.6
0.90 0.003
0.76 0.004
7.07 0.025
7.15 0.021
1.30 0.004
69
64
1.800
1,950
210
0.3
°'4 95
6.3
5.7
0.7
4.97:1 145
0.7
0.80
0.004
195
1.0
18.5:1 990 3.3
15.8:1 530 2.9
Transmissions
10.15 0.034
6.31 0.035
3,600 12.0
2,500 13.9
90
18:1 n.a.
8,000
20.0
10:1 194
3.2
2.44
0.04
n.a.
80
n.a. 50
1.7
0.52
0.017
265
8.8
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
676
120
8.3
10.0
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The reactor is a heterogeneous, water-moderated system, fueled with enriched
uranium dioxide. An integral lead radiation shield surrounds the reactor to protect per-
sonnel during reactor shutdown. The core, shielding, and pressure-vessel assembly
are enclosed in a tank of borated water during operation to provide additional neutron
shielding.
Major components of the power-conversion system are a turbine-compressor set,
reduction gear, alternator and starting motor, precooler with fans, recuperator, switch
gear, and connecting piping e,nd valves. The hot gas leaves the reactor at 1,200 F;
expands in the turbine; passes through the low pressure side of a regenerative heat ex-
changer (recuperator) and through an air-cooled precooler where the waste heat is re-
jected to the atmosphere. After being compressed in the compressor, the gas is pre-
heated to about 800 F as it passes through the high-pressure side of the recuperator.
It then flows through the reactor to the turbine inlet, completing the cycle. The turbine
is direct-coupled to the compressor and drives the alternator through a gear box.
Physical, Performance, and Cost Characteristics. The total weight of the ML-1
system is approximately 40 tons(lOl). The reactor alone weighs 30, 000 Ib. The power-
conversion equipment weights 30, 000 Ib, the control c&b 5, 000 Ib, and the auxiliary
equipment 12,000 Ib. The specific-weight requirement for the entire system was set at
150 lb/hp(e).
Because of radiation problems, however, even this high system weight would have
to be increased drastically if the system were to be used for urban-vehicle propulsion.
Twenty-four hours after shutdown the radiation level is 150 mrem/hr at 24 feet, and
during operation, the control cab is kept 500 feet from the reactor. At this distance the
radiation is at a safe level. Thus, if the system is to operate at full power, either the
control cab must be kept 500 feet away, maxing it unusable for motor vehicles, or the
shielding must be increased drastically.
The approximate dimensions of this system are as follows: reactor system,
9 x 8 x 9 ft; power conversion equipment, 14 x 9 x 8 ft; and control cab, 12 x 7 x 7 ft.
As mentioned above, the control cab is located 500 feet from the reactor during opera-
tion. It is possible that a more condensed version of the system could be made, but
this would increase the radiation problem and, consequently, system weight and cost.
The restart time of the ML-1 system as well as any thermal reactor system fol-
lowing a long shutdown period may be as much as 15 to 20 hours. The reason for this
is as follows. During reactor operation, many fission products are formed, two of
which are iodine-135 and xenon-135. Xe-135 is a reactor "poison" in that it has a high
affinity for absorbing neutrons. Xe-135 is formed from the radioactive decay of 1-135.
At reactor power for several days, the 1-135 and Xe-135 are in equilibrium. However,
upon shutdown, the 1-135 ceases to be produced but Xe-135 continues to be produced
from the decay of 1-135 still present. The Xe-135 level rises above its equilibrium level
since there are no neutrons present to "burn it out". The peak of Xe-135 occurs about
10 hours after shutdown and most reactors do not incorporate enough fuel to override
this "poison" effect to any great extent. It this type of power plant were shut down for
a short period, the Xe-135 increase is only slight, so the system could possibly be
restarted in 30 to 60 minutes. It takes 60 hp(e) maximum of auxiliary power for start-
up. The shutdown time is instantaneous in emergencies. For normal shutdown,
however, the auxiliary power system is used and the time required is about 20 to 30
minutes.
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The cost of the entire reactor plant could not be determined by the literature
search. However, the cost of the 49 kilograms of 93 percent enriched U-Z35 (as UO2),
which is needed for the reactor core, is approximately $44,000. This is only one part
of a very intricate system. It is estimated that this type of plant would cost about
$1,500,000.
SNAP-8 System
General Description. The SNAP-8 reactor is being developed by Atomics Inter-
national under contract to the Atomic Energy Commission. It consists of a nuclear re-
actor which supplies heat energy to a closed Rankine-cycle engine, which in turn drives
an electric alternator. The system is designed to produce a net output of 13. 4 to
134 hp(e)(10^) in a space environment.
The SNAP-8 system has a uranium fueled, zirconium hydride-moderated reactor.
Both men and electronic devices must be shielded from the radiation environment of the
reactor. Shielding materials were selected for maximum protection with minimum
weight. The neutron shielding material that best meets these criteria at high temper-
atures is lithium hydride. Gamma shielding materials that are attractive are tungsten,
depleted uranium, lead, and lead compounds. 003)
NASA is developing the power-conversion system for SNAP-8. The system under
development consists of four loops: a NaK, primary-heat-transfer loop through the
reactor and boiler operating at 1100 to 1300 F; a. mercury Rankine-cycle loop through
the boiler, turbine, and condenser operating at 1250 to 680 F; a main NaK heat rejection
loop through the condenser and radiator operating at 665 to 497 F; and an organic cooling
loop for the electrical components and bearings operating at 250 to 210 F.
Physical, Performance, and Cost Characteristics. The specific-weight design
objective for the SNAP-8 system is approximately 130 lb/hp(e) for the 134 hp(e) version.
The specific weight for the lower power versions would be proportionately higher. A
much higher weight would be required for an urban-vehicle installation, however, as the
SNAP-8 system uses only shadow shielding. That is, shielding on one side only, be-
tween the reactor and the power-conversion components and manned module.
While the SNAP-8 has not yet been assembled as a complete flight-rated system,
we can gain some insight into its probable weight range from available information on
the SNAP-10A system which uses a similar reactor. Using a SNAP-lOA-type thermo-
electric power-conversion system, a typical manned nuclear power system, with 4 TT
shielding and a separation distance between reactor system and power station of 30 to
50 feet, would require total system weights in the range of 40, 000 to 70, 000 Ib. If the
station was moved to" within 10 feet of the reactor, the weight would be well over
80,000 Ib.
The shape of the SNAP-8 system with shadow shielding is conical, with a length of
102 inches and a diameter ranging from 40 to 62 inches. The power-conversion system
would be attached to the reactor system. It has a diameter of 62 inches and a length of
120 inches. The size of the manned portion of the system would depend upon the number
of people necessary for a particular mission. It would probably be at least 6 feet in
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diameter and 8 feet in length. If we assume that enough shielding has been supplied, this
portion of the system could be attached to the power-conversion portion. The total
length of the spacecraft would, therefore, be approximately 26 feet and the diameter ap-
proximately 6 feet.
The start-up time for SNAP-8 is 3 hours for manned operations. (104) The shut-
down time is instantaneous in emergencies. Normal shutdowns take up to a half hour.
This system could probably be redesigned to operate normally with a scram-type shut-
down. No appreciable auxiliary power is needed for start-up or shutdown.
The cost of the SNAP-8 type system is very high. NASA estimates the cost of one
SNAP-8 system to be 3 to 5 million dollars. (105) This cost estimate does not consider
the 110 million dollars already spent in perfecting the SNAP-type system. (106) This
price is quite high, but the system is in the experimental stage. With increased produc-
tion and advances in technology, it is possible that the cost could be brought down by one
to two orders of magnitude.
SNAP-50/SPUR System
General Description. The SNAP-50 is being developed jointly by the Atomic
Energy Commission and the Air Force. It also uses a closed Rankine-cycle engine to
drive an electric alternator. The project has as its aim the demonstration of the feasi-
bility of a space nuclear electric power system rated at 400 to 1340 hp(e) at a specific
weight in the neighborhood of 15 lb/hp(e) unshielded.
Although moderators are common in nuclear reactors because they maximize fuel
efficiency, they do limit temperature and, hence, overall thermal-cycle efficiency. In
addition, moderators are bulky, increasing both the size and weight of a reactor.
For these reasons, it was necessary in SNAP-50 to go to an unmoderated, or fast,
design. Absence of a moderator calls for use of fuels with high uranium-235 content.
The fuel for SNAP-50 will probably be the oxide, carbide, or nitride of uranium.
The power comparison system for SNAP-50 consists of three loops: the reactor-
coolant loop, using lithium; the Rankine-engine loop, using potassium; and the radiator
loop, using a sodium-potassium eutectic. In the reactor loop, liquid lithium flows
through the reactor core, extracting the heat produced by nuclear fission. Exiting the
core at around 2000 F, it flows in a single pipe to a boiler, where some of its heat is
transferred to the potassium Rankine-engine loop. In the condenser, the heat lost by
the condensing potassium vapor is transferred to the sodium-potassium of the radiator
loop.
Physical, Performance, and Cost Characteristics. Most of the technical data and
new advances in the SNAP-50/SPUR system are classified. Since this report cannot
give up-to-date data on the progress of the SNAP-50, it will be assumed that the design
objectives can be reached in the near future, and conclusions will be drawn from these
design objectives.
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The weight of the SNAP-50 system, unshielded and with 400 hp(e) output, would be
6000-7000 Ib. (107) For urban-vehicle application, however, a man-rated, 4 TT shield
would be required. As stated previously, the lithium-hydride shield provides the maxi-
mum protection with minimum weight. But even this shield at a power between 2700 to
4000 hp(t) which is required to produce 400 hp(e), with a separation distance of 50 feet,
and a dose rate of 2. 0 mrem/hr, weighs in the neighborhood of 100, 000 Ib. This results
in a specific weight of approximately 250 lb/hp(e). Thus, even though the specific weight
of the reactor, power-conversion system, and the radiators has been cut down by the
SNAP-50-type system, the weight of the shielding still makes it unacceptable for an
urban-vehicle propulsion system.
The SNAP-50 power plant is 10 feet in diameter and about 35 feet long. (!08) This
is assuming enough shielding can be provided so the passenger section can be connected
to the power-conversion section.
The start-up time for the SNAP-50 is approximately 3 hours. This time is re-
quired to develop the operating conditions in the external loops. The shutdown time is
instantaneous. Neither start-up nor shutdown require any extensive auxiliary power
source.
Since the SNAP-50 system is still in the development stage, there are no cost
figures. It is probable, however, that development of this system will cost around
2 billion dollars, and it is improbable that the cost of one system could be reduced to the
$5,000 to $25,000 range.
Summary of Characteristics and
Conclusion as to Feasibility
Three main types of portable reactor systems have been investigated. These in-
clude (1) the ML-1, a water-moderated system with a water and lead shield; (2) SNAP-8,
a uranium-fueled and zirconium hydride-moderated reactor with a lithium hydride shield;
and (3) SNAP-50/SPUR, a fast reactor (no moderator) with a lithium hydride shield.
Water-cooled reactor systems were not investigated because by their nature they are
not portable.
The cost of any of these reactor systems is in the millions of dollars. The specific
weight of the most compact, completely shielded system would be greater than 250
lb/hp(e) or hp(s). The start-up time of all three systems is impractical.
It may be feasible in the future to lower the cost of materials and to devise some
system to overcome the long start-up time. It is unlikely that the shielding weight prob-
lem will be solved, however. The radiation near a power reactor is so intense that it
takes enormous amounts of shielding to provide maximum protection with the minimum
weight. A hundredfold improvement in the protection-per-unit-weight characteristic of
shielding materials is required before nuclear reactors would be suitable for vehicle use.
For these reasons, nuclear reactor systems must be ruled out as potential propulsion
systems for urban vehicles.
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Radioisotope Thermal Power Generators
Since the advent of nuclear-energy research, radioisotopes have become increas-
ingly important in a variety of applications. One of these involves the use of radioiso-
topes as the energy source for power generators. The energy released by radioisotopes
may be converted either into thermal energy, which may then be used as the energy in-
put to a variety of heat engines and direct thermal-to-electric energy converters, or
directly into electrical energy. Radioisotope thermal power generators are discussed
in this section of the report and direct nuclear-to-electric power generators are dis-
cussed in the following section.
The radiation emitted by radioisotopes is mainly of three types, alpha, beta, and
gamma. An alpha particle is identical with the nucleus of helium atom. It has very
little penetrating power. Beta particles, usually negative electrons, are much more
penetrating but less damaging than alpha particles. Gamma rays are highly penetrating
electromagnetic waves similar to X-rays.
When nuclear particles or gamma rays are absorbed in a thermal generator, they
produce highly localized heating effects along their trajectories (thermal spikes). The
effects almost instantaneously average out, and heat is produced on a macroscopic scale.
Specific Power of Generator
An important parameter in radioisotope generator design is the specific power,
Psp, of the fuel in w/g. Once the average amount of energy absorbed for each disinte-
gration within the generator, Eav, is determined, a simple equation for PSp may be
derived. It is:
Psp = 2. 12 x 103 REav/AT1/2
where
Psp = specific power, w/g of fuel compound
Eav = the average energy absorbed in the fuel per disintegration, Mev
T1/2 = half life, yr
A = atomic mass, g/mole
R = mass of the isotope per unit mass of fuel compound.
This equation is valid only for a pure isotope at time (t) = 0. It can be multiplied by
e"^1 to bring in time dependence with X being a characteristic factor for an individual
isotype. It is desirable to have both T\/2 and psp large, but as the equation shows, they
are reciprocally related.
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General Engineering Considerations
Proceeding to the purely engineering matters involved in the construction and the
operation of radioisotope power generators, it i 3 noted that radioisotopes do not require
the formation of a critical assembly as fission reactors do. In principle, radioisotopes
could be used in any quantity, in any configuration, to attain any power level desired.
However, there are technical problems in application.
First, the fuel must be shielded. This leads to consideration of a compact fuel
body. Thus, the designer is faced with the problem of extracting heat from a compact
structure. At high power levels (tens of horsepower thermal power), holes must be
fabricated in the fuel block and the thermal energy transferred by fluid convection since
pure heat conduction and radiation are no longer adequate. This approach leads to
moving parts (pumps) and thus the reliability of the system is reduced.
The second and most serious roadblock to the attainment of high power levels with
radioisotopes is the scarcity of fuel. Table 23 (page 129) shows the quantities of radio-
isotopes that will probably be available over the next decade. The projected power rating
for all of the usefull radioisotopes produced per year (assuming a nominal conversion
efficiency of 10 percent) is only about 2000 hp(e)/yr over the 1970-1980 period. This is
not much power by terrestrial standards. The concensus is for conserving the limited
supply of radioisotopes for use in generators in a low-power range rather than attempt-
ing to construct multihosepower-sized power plants that are better served by fission
reactors. Radioisotopes can perform at these low-power levels where other power
sources are totally unacceptable. But if radioisotopes are to be used in horsepower
size power systems their availability is of major importance. Therefore, the avail-
ability of radioisotopes is critical in their application for motor vehicles.
Another engineering constraint is imposed by a basic property of radioisotopes,
the exponential decay of their energy output with time. As the isotope decays, so does
the flux and the thermal-energy-reservoir temperature. There are three ways to com-
bat this problem: (1) store the excess energy at the beginning of life and use it later to
boost the power to the required level; (2) employ a higher-than-necessary thermal power
at the beginning of life, discarding the excess energy, and terminate the use of the heat
source when the power has dropped to the design level; (3) use a radioisotope whose
daughter also generates power; and (4) employ long-lived fuels. All generators now in
operation have used the last approach in solving the power-flattening problem by using
seed fuels like Pu238 and Sr90.
The inexorable decay of the radioisotope fuels results in a generator that cannot be
turned on or off at will. Thermal power will be produced continuously, although elec-
trical energy may be diverted or controlled if desired. Hence, cooling provisions must
be provided to remove the waste heat from the generator at every step in the installa-
tion and operation. This is a serious restriction of the use of radioisotopes in systems
with intermittent power demand.
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Selection of Radioisotope Fuels
In selecting the best radioisotope for power production, high power density, long
life and low cost are desirable in a potential fuel. Conversely, fuels of high toxicity or
those emitting high energy gamma rays are undesirable. A systematic selection pro-
cedure must be set up to choose the few best among the more than 1, 000 possible
radioisotopes.
The first sifting is done on the basis of physical properties. Using the following
criteria, the many possibilities can be reduced to 36.
(1) The half life should be greater than 100 days and less than 100 years.
The 100-day limitation eliminates the many short-lived isotopes that
would present severe fuel processing and power-flatening problems.
Fuel should have a half life of at least a few months to account for
encapsulation time and to permit some stockpiling. Radioisotopes
with half lives over 100 years generally have unacceptably low
specific powers, as shown by the equation on page 126.
(2) The specific power should be greater than 0. 06 hp(t)/lb. This criteria
is established to eliminate the many nuclei that emit a few weak
particles but still have acceptable half lives. The larger fuel capsules
associated with low specific power fuels lead to higher shield weight
and lower generator efficiencies.
(3) Pure or nearly pure gamma emitters should be eliminated because of
their shielding and handling problems.
The first sieve just described is too coarse for practical purposes, so a second
screening is performed using chemical and engineering properties for guidelines. The
two criteria in this second sieve are:
(1) The fuel should be relatively noncorrosive, compatible with structural
materials, and stable in time.
(2) The fuel should have good engineering properties at moderately high
temperatures. The engineering properties of interest are the prac-
tical power density, melting point, dimensional stability, gas evolution
(e. g. , helium buildup in alpha emitter), thermal conductivity, and
density.
Only 17 potential fuels remain after these criteria are applied. From this group
the Atomic Energy Commission has focused its production efforts on eight radioisotopes.
These fuels, listed in Table 23, have the most appropriate properties for vehicle
application.
The first four isotopes in Table 23 - all beta emitters - are fission products that
are recovered from nuclear-fuel-reprocessing plants. The last four, which emit alpha
particles, must be produced artificially in nuclear reactors. The alpha emitters are
currently more expensive than the fission products, but they are much easier to shield
and have good power densities. For use as an automobile power source, the higher cost
of alpha-emitting fuel is more than offset by the weight reduction because heavy radia-
tion shields are not needed.
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TABLE 23. CHARACTERISTICS OF RADIOISOTOPIC HEAT SOURCES^10*?, n°)
0
H
H
m
r
r
m
n
0
5
r
z
to
H
C
H
m
i
o
o
r
c
DD
C
w
r
o
0
*
o
5
m
*M
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
y _ » T
Half Life, yr
Compound Form
Compound Specific Power,
hp(t)/lb
Power Density,
Compound, hp(t)/ft<3)
Types of Radiation, Major(a)
Shielding Required
Melting Point, F
Annual Availability,
1967-70, hp(t) Annual
Present Price, $/hp(t)
Projected Annual Availability,
1970-802,732
160
123, 000
NA
NA
Cm-244
18. 10
Cm203
1. 70
1, 140
an
Minor
>2, 732
134
324, 000
NA
NA
(a) Legend: a- Alpha, /3-Beta, y-Gamma, n-Neutron, X"-Penetrating bremsstrahlung.
(b) NA-not available.
(c) AEC Division of Isotope Development projections as of July 1965. These have undoubtedly been increased in more recent projections.
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130
Assuming a 13. 4 hp(e) source [about 67 hp(t)] with a separation distance of 3 feet,
and 0. 5 rem/hr radiation dose allowed, the shielding weight for Sr-90, Cs-137, and
Ce-144 would exceed 5000 Ib. Thus, these three radioisotopes are not suitable for urban-
vehicle applications.
The melting point for Po-210 (489 F) is unacceptably low. This results from the
fact that energy converters used in vehicle applications will reject heat to the ambient
air. Allowing suitable temperature differentials for heat transfer on both the hot and
cold sides of the energy converter plus some safety factor with respect to the melting
temperature of Po-210 would probably result in an effective temperature difference of
less than 200 F across the energy converter. This would result in an unacceptably low
efficiency and large energy converter. Had it not been for its low melting temperature
Po-210 would have been the preferred isotope for vehicle application.
Of the remaining four isotopes, Cm-242 would appear to be most appropriate for
automotive application. It currently has the lowest price of the four and its specific
power in compound form is quite high. * Unfortunately, even though a vehicle propulsion
system using Cm-242 as the energy source might be developed to have acceptable physi-
cal and performance characteristics, the ultrahigh cost and unavailability of Cm-242, or
any of the other radioisotopes for that matter, makes the use of radioisotopes unfeasible
for vehicle application in the foreseeable future.
Summary of Characteristics and
Conclusion as to Feasibility
Radioisotope thermal power generators could probably be developed to have weight
and size characteristics that would be acceptable for vehicle application. They have a
number of overriding shortcomings, however, that make them unfeasible now and in the
foreseeable future for urban-vehicle application. The first of these are their very high
cost and unavailability. The isotope identified as most appropriate for vehicle applica-
tion, CM-242, currently costs $123, 000/hp(t) and its current annual availability is only
160 hp(t). While other isotopes could be considerably less costly and are more readily
available than Cm-242, particularly Po-210, they have other drawbacks that make them
undesirable for vehicle application. In any case, the tremendous cost reduction and in-
crease in availability required to make radioisotopes feasible for vehicle application ap-
pear to be far in the future.
Another major problem with radioisotopes is their inexorable decay which results
in a thermal generator that cannot be "turned off". If a radioisotope thermal generator
of "cruise power" size were used, a' large increase in the "thermal pollution" of the city
would result. The use of low power radioisotope-thermal generators combined with
thermal energy storage (discussed earlier in this report) would not be as desirable as
using electric or methane heaters for this usage.
°The specific power of a radioisotope thermal-energy source would be much lower than that of the radioisotope compound alone,
as to remove the heat generated by the radioisotope, a suitable heat exchanger along with auxiliaries such as pumps would be
required. The radioisotope could then be deposited on one side of the surface of this heat exchanger.
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Radioisotope Direct Electric Power Generators
Two methods for converting nuclear energy from a radioisotope source directly
into electric energy were investigated: (1) collection of charged particles in a retarding
field, and (2) generation of ion pairs in the neighborhood of a solid-state p-n junction.
The alpha cell is a typical example of the first, and the betavoltaic cell the second.
The Alpha Cell
General Description. The alpha particle is essentially a helium atom with the two
electrons removed. Certain radioisotopes decay by the emission of an alpha particle,
the half-life of the decay varying widely with the isotope. The alpha particles emitted
are monoenergetic, with energies for isotopes of interest typically in the range of 5 to
6 million electron volts (Mev). At birth each alpha particle has a charge of +2. Thus,
the underlying principle of the alpha cell process makes use of the fact that this alpha
decay process yields charged particles in motion.
If an alpha emitting isotope is distributed in a sufficiently thin layer, rather than a
thick fuel region, so that an appreciable fraction of all the alpha particles produced in
the layer can escape from the surface with much of their initial energy and charge
intact, these particles can be collected on an insulated electrode. The first few alpha
particles reaching the electrode will deposit their charge and dissipate their energy as
heat. However, after a number of alphas have been collected, the insulated electrode,
by virtue of its surplus of positive charge, will attain a high voltage with respect to the
emitter layer. Subsequent alpha particles will do work against this field: they will ar-
rive at the electrode with their initial kinetic energy exhausted but will deposit the
charge.
In effect, the arrangement discussed above is analogous to a capacitor, as shown
in Figure 29, with the alpha particles doing the charging. The charge separation caused
by the energetic alpha particles driving their way to the insulated electrode, the col-
lector, is neutralized by a flow through an external circuit of the electrons which were
left behind. This electron flow through an external circuit is a source of direct elec-
tricity produced without the use of a heat cycle.
The discovery that charged-particle emission can buildup a voltage on a properly
insulated electrode may be traced to work by Mosley in 1913. A critical problem existed
in reducing the concept to practice. Along with the positively charged alpha particles
come secondary electrons which have an opposing charge. Measurements indicate that
approximately 10 secondary electrons are released from the surface with each alpha
particle. (HI) Although these secondary electrons have very low energy (approximately
97 percent have energy less than 100 ev), they are produced in such abundance that their
total negative charge more than offsets the positive charge buildup. Thus, a low-energy
negative charge, rather than the desired positive high-energy charge, is emitted from
the cathode.
A properly designed control grid placed close to the cathode can be used to over-
come this difficulty. A negative potential applied to the grid will repel the secondary
electrons to the cathode. A simple configuration which illustrates this concept is shown
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Emitter
(Cathode)
Collector
(Anode)
Thin layer of
alpha emitting
radioisotope
\Electron flow in
external circuit
VACUUM
-vV/yVVV'vV
1
ELECTRICAL LOAD
FIGURE 29. THE PRINCIPLE OF THE ALPHA CELL'S OPERATION
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in Figure 30. A cylinder electrode, the emitter or cathode, can be a. tube or rod coated
with a layer of alpha emitter several tenths of thousandths of an inch thick. This thick-
ness is required since the range of alpha particles in metals is of the order of
4 x 10-4 inch. Surrounding the cathode in this example is a squirrel-cage grid at nega-
tive voltage with respect to the cathode. The grid is composed of small-diameter wires
and is sufficiently open to permit the alpha particles to reach the collector, or anode,
while at the same time preventing the secondary electrons from escaping. Because the
electrons have very low energy, only a few hundred volts bias on the grid will suppress
them. To hold back the electrons when the anode is at several million volts requires an
increase in voltage to the kilovolt range. The grid can be designed to be almost com-
pletely open, typically 90 percent or more of the total grid area is not blocked by grid
wires.
Physical, Performance, and Cost Characteristics. Experiments were performed
in late 1962 which showed that the principles of operation of the device were sound in the
relatively low voltage range obtained at that time (approximately 50,000 volts). How-
ever, for efficient operation it has been calculated that the device must operate in the
range near 1 megavolt. Because of microdischarging in the cell, significantly higher
voltage cannot be obtained at this time. Because of these microdischarging problems,
present efficiency achieved with alpha cells is less than 5 percent.
The power range for a generator based on the alpha-cell direct-conversion concept
is not yet firmly established. However, it appears at present that individual generator
cells with useful outputs of 0. 013 to 0. 13 hp(e) may be achieved.
Since the radioisotope used in the alpha cell must be spread on a tube or rod in a
layer approximately 4 x 10"^ inch thick and since low energy conversion efficiencies are
achieved, surface areas of thousands of square feet would be required for several tens
of horsepower electrical output.
As with all radioisotope systems, a major problem with an alpha cell system is
the cost and availability of the fuel. The most promising alpha emitters are Pu-238,
Cm-242, and Cm-244. For use in an alpha cell, the least expensive of these, Cm-242,
would cost $123, 000/hp(t).
Betavoltaic Devices
General Description. When beta particles are absorbed by a semiconductor they
dissipate most of their energy by ionizing the atoms of the solid. The carriers gener-
ated in this fashion are in excess of the number permitted by thermodynamic equilibrium
and if they diffuse to the vicinity of a rectifying junction they induce a voltage across the
junction. This phenomenon, which has been termed the electron-voltaic effect is the
basis of the beta voltaic cell.
This effect has been studied as a possible source of electrical power since it
permits the direct conversion of the energy of beta particles emitted by a radioactive
material into electricity. (
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Anode (at positive
potential of several
million volts)
Alpha particles
Free electrons
turned bock to
cathode by grid
Cathode (coated with
alpha emitter)
Grid (at negative bias
of around 10 kibvolts)
FIGURE 30. USE OF A GRID TO REPEL SECONDARY ELECTRONS^112>
(COAXIAL-CYLINDER GEOMETRY)
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The electron-voltaic effect is one of a class of phenomena (the best known is the
photoelectric effect) in which radiation capable of producing ionization in a solid inter-
acts with a rectifying junction. Other forms of electromagnetic radiation (7-rays and
x-rays) as well as other forms of radiation (jS particles, protons) are possible sources
of such ionization, and the analysis of this effect could be applied to these other types of
radiation as well. Beta rays were chosen as the primary source for direct conversion
because their specific ionization is higher than that of "y-rays. They are, therefore,
absorbed in a reasonable thickness of semiconductor and they can be shielded more
easily. They were preferred over a particles because they are emitted by radioisotope
by-products of the fission process and because their smaller weight produces less radi-
ation damage.
When determining which beta emitter to use as the radioactive source, the radia-
tion damage caused by the source must be considered. It was found that the power out-
put of the cells decreased rapidly because of radiation damage caused by the high-energy
(up to 2 Mev) electrons emitted by a Sr-90 radioisotope used as the primary power
source. Subsequent work has shown that the energy of bombarding electrons had to ex-
ceed a few hundred Kev in order to produce the observed degradation in power output.
Thus, a radioisotope must be chosen which emits primarily low energy beta particles.
For this reason Pm-147 is commonly used. Its half life is shorter than that of Sr-90,
but the maximum energy of the emitted /3-particles is only 230 kev.
Physical, Performance, and Cost Characteristics. Each betavoltaic cell consists
of a thin layer of Pm-147 in the form of hydrated Pm2O3, sandwiched between two
n-on-p silicon solar cells and encapsulated in a stainless steel case. It is approximately
1 cu in. in volume. The most economical thickness of the Pm-147 would be about
4 x 10~4 inch. If the thickness is increased beyond this, the power would not increase
significantly.
In a scoping experiment (all factors not optimized), a 2-curie Pm-147 source
generated about 1. 0 x 10~6 hp (beta power). The amount of energy transferred from a
Pm-147 source to one cell is reduced by self-absorption in the source and, to a lesser
extent, back-scattering from the cell. The maximum theoretical cell efficiency is
approximately 4. 4 percent. Self-absorption in the 2-curie sources involved in the pres-
ent batteries probably reduce the power input to the cells by 75 percent, which would
indicate a maximum efficiency of about 1. 1 percent for electrical power generation from
beta, particles entering the cells and a maximum overall efficiency of about 0. 8 percent.
It is rather obvious that significant improvements in efficiency will have to be made be-
fore betavoltaic devices can be used in a large power system.
Assuming that present betavoltaic cells are about 1 cu in. in volume, have an
efficiency of 1. 0 percent, and have a 1. 0 x 10~6 hp (beta power) source, 100 million
cells would be required to produce an output of 1 hp(e). If these were assembled into
one package, it would be about 40 feet on a side. The weight of such a betavoltaic device
including cell connectors and appropriate shielding would be greater than 10,000 Ib.
The cost of a horsepower-size betavoltaic system would be astronomical. The cost
of the fuel, Pm-147, if it could be obtained, would be in the millions of dollars. The
projected cost of Pm-147 for the 1970-1980 period is $70, 000 hp(t). It is not likely that
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advances in technology will enable the cost to be lowered to an acceptable level in the
intermediate future.
Summary of Characteristics and
Conclusions as to Feasibility
The excessive cost, unavailability, and "no-turn-off" problems cited for radioiso-
tope thermal power generators apply to radioisotope direct electric power generators,
as well. In addition, however, the direct electric power generators have some prob-
lems of their own.
The alpha cell requires considerable development to even achieve practicality at
very low power levels. Operation in the horsepower range cannot be expected in the
near future. The problem of microdischarging must be solved before the system can be
further examined for practicality.
The power output of a betavoltaic cell is limited by its low efficiency, currently
0. 8 percent, and because only a very thin layer of radioisotope can be used per cell.
High power levels are unlikely because of the weight and size of the system. Currently,
100 million cells would be required for a 1. 0 hp(e) output. This would require a volume
40 feet on a side and would weight more than 10, 000 Ib.
For the above reasons, radioisotope direct electric power generators are un-
feasible now and in the forseeable future for urban-vehicle application.
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APPENDIXES
APPENDIX A - DEVELOPMENT OF PROPULSION-SYSTEM VOLUME AND COST
SPECIFICATIONS
APPENDIX B - CALCULATION SHEETS FOR THE ESTIMATION OF PROPULSION -
SYSTEM CHARACTERISTICS FOR VARIOUS VEHICLE APPLICATIONS
APPENDIX C - REFERENCES
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APPENDIX A
DEVELOPMENT OF PROPULSION-SYSTEM VOLUME
AND COST SPECIFICATIONS
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A-l
APPENDIX A
DEVELOPMENT OF PROPULSION-SYSTEM
VOLUME AND COST SPECIFICATIONS
Introduction
The open literature contains a number of vehicle-criteria analyses based on per-
missible component weight, but similar detailed information involving the parameters
of volume and cost could not be found. In view of this critical data gap, it became
necessary to conduct a cursory cost and volume evaluation for a range of current
American vehicles. The vehicles selected for this analysis included four passenger
cars, two delivery vans, and one city bus.
The volume data sought were the envelope volumes for the particular existing
systems, and the cost data of interest were the manufacturers' minimum selling prices
(i. e. , O. E. M. or dealer's cost, manufacturer's plant). Since the purpose of this analy-
sis was to establish acceptable limits or threshold values for volume and cost, it was
decided to consider each vehicle as equipped with its largest optional engine and trans-
mission. In each of the passenger cars, therefore, a V-8 engine (nonracing version
equipped with a 4-barrel carburetor and dual exhausts) and a three-speed automatic
transmission were considered. In the case of the delivery vans, both makes offered a
three-speed automatic transmission, one did not offer a V-8 engine option, and neither
had dual exhausts. The city bus evaluated was equipped with a V-6 diesel engine, an
automatic transmission, and a single exhaust system. The procedures used in this
analysis, and the results, follow.
Volume-Estimation Procedure
The determination of individual component volumes consisted essentially of deriv-
ing envelope dimensions from product literature and/or direct measurement on sample
vehicles. In instances where pertinent product literature was not available and the com-
ponent was inaccessible for measurement, estimates were made by scaling from known
similar components.
The term "envelope volume" as used in this discussion refers to the volume of a
rectangular and/or cylindrical container which would hold the component being consid-
ered and provide a slight operating clearance with adjacent chassis or body structures.
The use of envelope volumes and largest optional components was selected as the best
means of developing liberal volume limits for the alternative propulsion systems. The
conservatism of these limits is increased by permitting the lower performance systems
being considered in this program to occupy the maximum allotted volumes for present,
high-performance systems. In this study, the total propulsion system volume is defined
as the sum of five different component volumes - engine, transmission, driveline, ex-
haust system, and fuel tank.
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A-2
Engine Envelope Volume
The engine envelope was taken as that rectangular space enclosed by the longi-
tudinal distance spanning the rear face of the engine cylinder block and the forward edge
of the radiator; the maximum width dimension across the engine exhaust manifolds and/
or accessories; and the vertical distance spanning the top of the air cleaner and the bot-
tom of the crankcase oil pan. While much of this space is actually vacant, the bulk of
the vacancy is required for cooling, isolation, and servicing. The battery and leads are
not normally located within the rectangular space just described, but were assumed to
be so located in this analysis.
Transmission Envelope Volume
The transmission envelope was chosen to consist of one cylindrical disk which
would enclose the torque converter section of the transmission case, and then one or
two cylindrical or rectangular containers which would hold the gear box and rear-
bearing retainer portions. It should be noted that automatic transmissions occupy
slightly more space than do the corresponding manual transmission and clutch
combinations.
Fuel Tank Envelope Volume
Since fuel tanks of the subject vehicles were all fairly regular in shape, tank en-
velope volume was based on the space required to hold the nominal fuel capacity plus
10 percent.
Driveline Envelope Volume
The term "driveline" as used here comprises both the propeller shaft and the
rear axle assembly. The propeller shaft envelope volume was taken as the area required
by the universal joints times the length of the shaft. Volume assigned to the rear axle
assembly consisted of a cylindrical disk which would enclose the differential section of
the axle housing and two tubular spaces which would hold the axle-shaft portions of the
housing. No arbitrary correction was made to these figures to account for maximum
vertical displacement of the driveline with respect to the body.
Exhaust-System Envelope Volume
Exhaust-system envelope volume was based on cross-sectional pipe and muffler
areas slightly larger than normal, scaled muffler length, and an estimated developed
length for the pipe.
Cost-Estimation Procedure
While several consumer buying guides such as the American Car Price Magazine
list "manufacturing costs" for complete vehicles, similar information for vehicle com-
ponents is, of course, not available. All of the cost data presented here, therefore, are
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A-3
estimates based on suggested retail costs for vehicles, components, and optional
equipment; typical dealer and employer discounts; retail prices of rebuilt units; accept-
able overhaul expenses; and the specific cost for similar equipment.
In general, the final estimated cost fell between the suggested retail price (minus
typical OEM discount and shipping charges) and the suggested retail price of a rebuilt
unit (minus typical discount and shipping charges). However, in view of the gross in-
consistencies discovered in pricing policies, no simple estimating procedure was found
suitable and each component had to be evaluated on the basis of all available inputs.
Again, a degree of conservatism is introduced by permitting the low-performance
vehicles being considered in this program to have the same propulsion-system cost as
current high-performance vehicles. This degree of conservatism does not exist in the
case of the bus propulsion system, however, so the permissible bus propulsion-system
cost was increased 30 percent above the current estimated cost.
Results and Conclusions
The principal findings of this analysis are as follows:
(1) For passenger cars, propulsion-system volume and cost are related
to vehicle curb weight as indicated in Figures A-1 and A-2. Choosing
the extreme cases and extrapolating yields the limit specifications
tabulated below.
Family Commuter Utility City
Car Car Car Taxi
Curb Weight, Ib 3500 2100 1400 3500
Propulsion System
Volume, ft3 28 20 16 28
Propulsion System
Cost, $ 820 670 600 820
(2) For delivery vans with a curb weight of 4500 Ib and a GVW rating of
7000 Ib, the average propulsion-system volume and cost were estimated
to be 28 ft3 and $860, respectively.
(3) For a city bus with a curb weight of 20, 000 Ib and a GVW rating of
30,000 Ib, the current propulsion-system volume and cost were
estimated to be 134 ft^ and $6,240, respectively. Since the maximum
power specified for the low-pollutant bus is similar to that of present
conventional buses, permissible volume and cost for a low-pollutant
system will be increased 30 percent over the current estimated values.
(4) Scatter observed in the passenger-car volume data is mainly a
function of radiator and air-cleaner placement relative to the engine.
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A-4
T> 30
^} *J\y
0)
o
o
oT
E
"n ?O
VJ k W
E
^->
CO
1
c
o
'w 10
^M I V
Q.
O
ot
0
s
/
'
*
r Volume limit ^^
\
V'
x^
^^
'
/
'
/"
1000
2000 3000 4000
Vehicle Curb Weight, pounds
5000
FIGURE A-l. PROPULSION SYSTEM VOLUME VS VEHICLE CURB WEIGHT
\\J\J\J
1/7
° 900
0
TJ
(/)
O
O
£ 700
CO
^. 600
0
\ 50°
o
Q.
^^
r Cost
^
^
limit ^**
^^^^ 0
^^"^
*
1000 2000 3000 4000 5000
Vehicle Curb Weiaht. oounds A-57482
FIGURE A-2. PROPULSION SYSTEM COST VS VEHICLE CURB WEIGHT
BATTELLE MEMORIAL INSTITUTE - COLUMBUS LABORATORIES
-------
A-5 and A-6
(5) Limiting propulsion-system volume and cost specifications for
passenger cars and light-duty delivery vans are nearly the same
because many of the components are identical.
(6) While the rated horsepower of the city bus is less than that of the
smallest passenger car considered, its volume and cost specifica-
tions are grossly higher because of its heavy-duty components and
low volume production.
BATTELLE MEMORIAL INSTITUTE - COLUMBUS LABORATORIES
-------
APPENDIX B
CALCULATION SHEETS FOR THE ESTIMATION OF
PROPULSION-SYSTEM CHARACTERISTICS FOR
VARIOUS VEHICLE APPLICATIONS
BATTELLE MEMORIAL INSTITUTE - COLUMBUS LABORATORIES
-------
TABLE B-l. ESTIMATED PROPULSION SYSTEM CHARACTERISTICS FOR COMMUTER CAR APPLICATION
0)
H
H
m
r
r
m
2
ID
0
5
r
2
O)
H
H
C
H
ID
1
O
O
r
c
z
0)
c
(A
OJ
o
H
O
5
m
Parameters
Maximum Delivered Power, hp(s)
Assumed Transmission and Drive Efficiency, %
Maximum Engine Power, hp(s) or hp(e)
Rated Engine Power, hp(s) or hp(e)
Specific Engine Weight, lb/hp(s) or hp(e)
Engine Weight, Ib
Specific Transmission Weight, lb/hp(s)
Transmission Weight, Ib
Energy Delivered to Vehicle, hp(s)-hr
Energy Delivered by Engine, hp(s)-hr or hp(e)-hr
Peak Engine Efficiency, %
Assumed Average Efficiency, %
Required Stored Energy, hp(t)-hr
Specific Weight of Conventional Fuel, lb/hp(t)-hr
Energy Storage Weight, Ib
Engine. Transmission, and Conventional Fuel
Weight, Ib
Specific Engine Volume, ft /hp(s) or hp(e)
o
Engine Volume, ft
Specific Transmission Volume, ft3/hp(s)
Transmission Volume, ft3
Specific Volume of Conventional Fuel,
ft3/hp(t)-hr
Gas
Turbine
30
90
32.4
32.4
4.0
130
0.5
17
13.4
14.9
30
21
71
0.15
11
160
0.12
3.89
0.004
0.13
0. 0029
Rankine
Engine
30
95
31.6
25.2
7.0
180
0.0
0.0
13.4
14.1
26
18
78.4
0.15
12
190
0.15
3.78
0.0
0.0
0.0029
Stirling
Engine
30
85
35.4
35.4
9.4
330
0.7
24
13.4
15.8
36
25
63.2
0.15
10
360
0.18
6.38
0.005
0.18
0. 0029
Flywheel
Energy
Storage
30
80
37.5
37.5
--
--
2.0
75
13.4
--
--
80
16.8
-------
TABLE B-l. (Continued)
0
H
H
m
r
r
m
m
2
O
5
r
z
H
C
H
m
o
0
r
c
3*
CD
c-
r
to
o
5
H
O
5
m
Parameters
0
Energy Storage Volume, ft
Engine, Transmission, and Conventional
Fuel Volume, ft3
Specific Engine Cost, $/hp(s) or hp(e)
Engine Cost, $
Specific Transmission Cost, $/hp(s)
Transmission Cost, $
Specific Energy Container Cost, $/hp(t)-hr
Energy Container Cost, $
Engine, Transmission, and Energy Container
Cost, $
(a) Considered to be part of a hybrid system.
cost of associated batteries, motors, and
(b) Value given in hp(s)-hr.
(c) Value given in lb/hp(s)-hr.
(d) Value given in ft3/hp(s)-hr.
(e) Value given in $/hp(s)-hr.
Gas Rankine Stirling
Turbine Engine Engine
0.21 0.23 0.18
4.2 4.0 6.7
4.5 5.4 6.4
150 140 230
0.4 0.0 1.0
13 0.0 35
Nominal Nominal Nominal
--
160 140 260
Their power rating is based on the cruise
Flywheel
Energy
Storage
4.20
4.9
--
--
3.0
110
100
o;
ST
*%
ET
g.
vt
Oa
0_
n
o
'
5
weight, size, and
to
-------
TABLE B-2. ESTIMATED PROPULSION-SYSTEM CHARACTERISTICS FOR UTILITY CAR APPLICATION
o
H
H
m
r
r
2
m
o
5
r
z
VI
H
C
-T
m
i
o
o
r
c .
2
o
c
in
r
o
o
H
0
5
fn
(A
Parameters
Maximum Delivered Power, hp(s)
Assumed Transmission and Drive Efficiency, "Jo
Maximum Engine Power, hp(s) or hp(e)
Rated Engine Power, hp(s) or hp(e)
Specific Engine Weight, lb/hp(s) or hp(e)
Engine Weight, Ib
Specific Transmission Weight, lb/hp(s)
Transmission Weight, Ib
Energy Delivered to Vehicle, hp(s)-hr
Energy Delivered by Engine, hp(s)-hr or hp(e)-hr
Peak Engine Efficiency, %
Assumed Average Efficiency, %
Required Stored Energy, hp(t)-hr
Specific Weight of Conventional Fuel, lb/hp(t)-hr
Energy Storage Weight. Ib
Engine, Transmission, and Conventional Fuel
Weight. Ib
Specific Engine Volume, ft3/hp(s) or hp(e)
Engine Volume, ft
o
Specific Transmission Volume, ft /hp(s)
Transmission Volume, ft3
Specific Volume of Conventional Fuel,
ft3/hp(t)-hr
Gas
Turbine
rt ""^
2. v*
- 3
£2 O
0> O"
2.
C. re'
re1 6
0 ^
c -**
5! "
n n
Q. 0
S ^
oa
01
c
3-
5'
re
o
--
Rank in e
Engine
16
95
16.9
13.5
8.0
110
0.0
0.0
10.7
11.3
25
17.5
64.6
0.15
10
120
0.16
2.16
0.0
0.0
0.0029
Stirling
Engine
16
85
18.8
18.8
10.0
190
0.7
13
10.7
12.4
35
24.5
50.6
0.15
8
210
0.20
3.76
0.005
0.09
0.0029
Flywheel
Energy
Storage
16
80
20
20
--
--
2.0
40
10.7
--
--
80
13.4
100(c)
1340
1400
--
--
0.02
0.40
0.25
Thermo- Magneto-
Thermo- photo- hydrodynamics
electric Thermionic voltaic and Nuclear
Converter3^ Converter'3' Converter^) Devices
16
80
20
10
13
130
--
Weight
10.7
13.4
10
9
149
0. 15
22
150+
--
--
--
Volume
0. 0029
16
80
20
10
10
100
--
16
80
20
10
13
130
--
of battery, motors, and controls
10.7
13.4
10
9
149
0.15
22
120+
0.2
2.0
--
of batteries,
0.0029
10.7
13.4
13
11.5
116
0.15
18
150+
--
--
--
motors, and controls
0. 0029
0
o
c
a.
o
ST
c
1.
0>
o;
re
if
3-
o
o'
CD
o'
1
l/»
re
re
o
c
£.
to
I
OJ
-------
TABLE B-2. (Continued)
m
J>
H
H
ra
r
r
m
m
2
O
5
*
z
(A
H
H
C
-1
m
i
o
o
r
c
z
OJ
c.
-------
TABLE B-3. ESTIMATED PROPULSION SYSTEM CHARACTERISTICS FOR DELIVERY VAN APPLICATION
o
H
H
m
r
r
m
2
m
o
a
r
1
H
c
H
m
i
o
o
r
c-
z
0
c
r
>
0
o
>
H
0
5
m
VI
Parameters
Maximum Delivered Power, hp(s)
Assumed Transmission and Drive Efficiency, %
Maximum Engine Power, hp(s)or hp(e)
Rated Engine Power, hp(s) or hp(e)
Specific Engine Weight, lb/hp(s) or hp(e)
Engine Weight, Ib
Specific Transmission Weight, lb/hp(s)
Transmission Weight, Ib
Energy Delivered to Vehicle, hp(s)-hr
Energy Delivered by Engine, hp(s)-hr or hp(e)-hr
Peak Engine Efficiency, %
Assumed Average Efficiency, %
Required Stored Energy, hp(t)-hr
Specific Weight of Conventional Fuel, lb/hp(t)-hr
Energy Storage Weight, Ib
engine. Transmission, and Conventional Fuel
Weight, Ib
o
Specific Engine Volume, ft /hp(s) or hp(e)
o
Engine Volume, ft
0
Specific Transmission Volume, ft /hp(s)
o
Transmission Volume, ft
Specific Volume of Conventional Fuel,
ft3/hp(t)-hr
Energy Storage Volume, ft3
Gas
Turbine
96
90
107
107
3.3
350
0.5
54
60
67
33
23
290
0.15
44
450
0.10
10.7
0.004
0.43
0. 0029
0.84
Rankine
Engine
96
95
101
81
5.6
450
0.0
0.0
60
63
29
20
315
0.15
47
500
0.13
10.5
0.0
0.0
0.0029
0.91
Stirling
Engine
96
85
113
113
8.4
950
0.7
79
60
71
39
27.5
257
0.15
39
1100
0.16
18.1
0.005
0.56
0. 0029
0.74
Flywheel
Energy
Storage
96
80
120
120
--
--
2.0
240
60
--
--
80
75(b)
100
-------
TABLE B-3. (Continued)
(D
H
m
r
r
m
2
m
2
0
5
r
z
in
H
H
C
H
m
i
o
o
r
c
0)
c-
r
(D
0
H
O
5
m
M
Parameters
Engine, Transmission, and Conventional Fuel
Volume, ft3
Specific Engine Cost, $/hp(s) or hp(e)
Engine Cost, $
Specific Transmission Cost, $/hp(s)
Transmission Cost, $
Specific Energy Container Cost, $/hp(t)-hr
Energy Container Cost, $
Engine, Transmission, and Energy Container
Cost, $
(a) Considered to be part of a hybrid system.
and cost of associated batteries, motors,
(b) Value given in hp(s)-hr.
(c) Value given in lb/hp(s)-hr.
(d) Value given in ft3/hp(s)-hr.
(e) Value given in $/hp(s)-hr.
Flywheel
Gas Rankine Stirling Energy
Turbine Engine Engine Storage
12 11 19 21
3.4 4.4 5.4
360 360 610
0.4 0.0 1.0 3.0
43 0.0 110 360
Nominal Nominal Nominal 100(e)
7500
400 360 720 7900
Thermo-
Thermo- photo-
electric Thermionic voltaic
Converter^ Converter^3) Converter^3)
Similar to 14+ Similar to
Thermionic Thermionic
Not known but is expected to be $100/hp(e)
or higher
--
--
Cost of batteries, motors, and controls
Nominal Nominal Nominal
..
Very high
Their power rating is based on the cruise power requirement which is one-half the maximum power. The
and controls would have to be added to the values listed
in this table for the converters alone.
Magneto-
hydrodynamics
and Nuclear
Devices
o1
o c
0 3
CL
K" 3
o
c cr
li
e
o;
fT
«-i
5;
tn
S
"H.
o
1 to
weight, size,
-------
TABLE B-4. ESTIMATED PROPULSION SYSTEM CHARACTERISTICS FOR CITY TAXI APPLICATION
0)
H
H
m
r
r
rn
2
m
2
O
X
>
r
z
H
H
C
H
m
t
o
0
r
c
2.
m
c
r
CD
O
H
O
5
m
Parameters
Maximum Delivered Power, hp(s)
Assumed Transmission and Drive Efficiency, %
Maximum Engine Power, hp(s) or hp(e)
Rated Engine Power, hp(s) or hp(e)
Specific Engine Weight. lb/hp(s) or hp(e)
Engine Weight, Ib
Specific Transmission Weight, lb/hp(s)
Transmission Weight, Ib
Energy Delivered to Vehicle, hp(s)-hr
Energy Delivered by Engine, hp(s)-hr or hp(e)-hr
Peak Engine Efficiency, %
Assumed Average Efficiency, %
Required Stored Energy, hp(t)-hr
Specific Weight of Conventional Fuel, lb/hp(t)-hr
Energy Storage Weight, Ib
Engine, Transmission, and Conventional Fuel
Weight. Ib
Specific Engine Volume, ft /hp(s) or hp(e)
Q
Engine Volume, ft
0
Specific Transmission Volume, ft /hp(s)
q
Transmission Volume, ft
Specific Volume of Conventional Fuel,
ft3/hp(t)-hr
Energy Storage Volume, ft3
Gas
Turbine
48
90
53.4
53.4
3.7
200
0.5
27
100
111
31
21.5
516
0.15
77
300
0.11
5.87
0.004
0.21
0. 0029
1.50
Rankine
Engine
48
95
50.5
40.4
6.4
260
0.0
0.0
100
105
27
19
553
0.15
83
340
0.14
5.60
0.0
0.0
0.0029
1.60
Stirling
Engine
48
85
57.5
57.5
9.0
520
0.7
40
100
118
37
26
455
0.15
68
630
0.19
10.90
0.005
0.29
0.0029
1.32
Flywheel
Energy
Storage
48
80
60
60
--
--
2.0
120
100
..
--
80
125(b)
ioo(c)
12,500
13. 000
--
0.02
1.20
0.25
31.0
Thermo-
electric
Converter'3
48
80
60
30
13
390
--
Weight of
100
125
10
9
1390
0.15
210
600+
--
--
Volume of
0. 0029
4.04
Thermionic
Con verted3 '
48
80
60
30
10
300
--
battery, motors.
100
125
10
9
1390
0.15
210
510+
0.2
6.0
--
Thermo-
photo-
voltaic
Con verted3'
48
80
60
30
13
390
--
and controls
100
125
13
11.5
1090
0.15
160
550+
--
--
Magneto-
hydrodynamics
and Nuclear
Devices
o1
c
3
D-
0
o-
O
C
C
a-
ST
^
vt
fu
T3
O
O>
5'
3
\
J?
o.
5'
c
Z
O
3
batteries, motors, and controls
0.0029
4.04
0. 0029
3.16
I
~J
-------
TABLE B-4. (Continued)
o
H
m
r
r
m
Z
m
2
O
3
j,
r
z
(n
H
C
H
m
i
o
0
c
OJ
C'
(A
r
0)
0
H
O
5
m
in
Gas Rankine Stirling
Parameters Turbine Engine Engine
Engine, Transmission, and Conventional Fuel 7.6 7.2 12
Volume, ft3
Specific Engine Cost, $/hp(s) or hp(e) 4.0 5.0 6.0
Engine Cost, $ 210 200 340
Specific Transmission Cost, $/hp(s) 0.4 0.0 1.0
Transmission Cost, $ 21 0.0 58
Specific Energy Container Cost, $/hp(t)-hr Nominal Nominal Nominal
Energy Container Cost, $ ------
Engine, Transmission, and Energy Container Cost, $ 230 200 400
(a) Considered to be part of a hybrid system. Their power rating is based on the cruise
and cost of associated batteries, motors, and controls would have to be added to the
(b) Value given in hp(s)-hr.
(c) Value given in lb/hp(s)-hi.
(d) Value given in ft3/hp(s)-hr.
(e) Value given in $/hp(s)-hr.
Thermo-
Flywheel Thermo- photo -
Energy electric Thermonic voltaic
Storage Converter^ Converter^3) Converter^3)
32 Similar to 10+ Similar to
Thermionic Thermionic
Not known but is expected to be $100/hp(e)
or higher
-~
3.0
180 Cost of batteries, motors, and controls
100^ Nominal Nominal Nominal
12,500
13. 000 Very high
power requirement which is one-half the maximum power. The
values listed in this table for the converters alone.
Magneto -
hydrodynamics
and Nuclear
Devices
TI
o
TO C
O-
to' O
c cr
o ^
§ 5
c
D>
CT*
oT
51
3-
vt
U
T3
"5.
O
(U
5'
3
1
weight, size.
I
CO
-------
TABLE B-5. ESTIMATED PROPULSION SYSTEM CHARACTERISTICS FOR CITY BUS APPLICATION
m
H
H
PI
r
r
m
z
m
6
^
r
z
tn
H
H
C
-t
t
O
O
r
c
z
0)
c
r
CD
0
a
H
O
5
m
Parameters
Maximum Delivered Power, hp(s)
Assumed Transmission and Drive Efficiency, %
Maximum Engine Power, hp(s) or hp(e)
Rated Engine Power, hp(s) or hp(e)
Specific Engine Weight, lb/hp(s) or hp(e)
Engine Weight, Ib
Specific Transmission Weight, lb/hp(s)
Transmission Weight, Ib
Energy Delivered to Vehicle, hp(s)-hr
Energy Delivered by Engine, hp(s)-hr or hp(e)-hr
Peak Engine Efficiency, "]t>
Assumed Average Efficiency, "Jo
Required Stored Energy, hp(t)-hr
Specific Weight of Conventional Fuel, lb/hp(t)-hr
Energy Storage Weight, Ib
Engine, Transmission, and Conventional Fuel
Weight, Ib
Specific Engine Volume, ft /hp(s) or hp(e)
Engine Volume, ft
Specific Transmission Volume, ft3/hp(s)
Transmission Volume, ft3
Specific Volume of Conventional Fuel,
Gas
Turbine
180
90
200
200
4.0
800
2.0
400
400
445
35
24.5
1820
0.15
270
1500
0.12
24.0
0.02
4.0
0.0029
Rankine
Engine
180
90
200
160
10.0
1600.0
2.0
320
400
445
30
21
2120
0.15
320
2200
0.24
36.5
0.02
3.04
0.0029
Stirling
Engine
180
85
212
212
16.0
3390
3.0
640
400
470
40
28
1680
0.15
250
4300
0.30
63.6
0.03
6.36
0. 0029
Flywheel
Energy
Storage
180
80
225
225
--
--
3.0
680
400
--
--
80
500
Thermo-
electric
Converter'3'
180
80
225
112
17
1900
--
Weight of
400
500
10
9
5560
0.15
830
2700+
--
--
--
Volume of
0.0029
Thermionic
Converter^
180
80
225
112
13
1460
--
battery, motors,
400
500
10
9
5560
0.15
830
2300+
0.27
30.2
--
Thermo-
photo-
voltaic
Converter^
180
80
225
112
17
1900
--
and controls
400
500
13
11.5
4340
0.15
650
2600+
--
--
--
Magneto -
hydrodynamics
and Nuclear
Devices
o
c
3
a.
o
cr
re
c
3
£
£
or
J
f
1
B>
O
1
re
re
o.
o
c
batteries, motors, and controls
0.0029
0.0029
vO
ft3/hp(t)-hr
Energy Storage Volume, ft
5.28
6.15
4.86
125
16.1
16.1
12.6
-------
TABLE B-5. (Continued)
m
H
m
r
r
m
2
m
2
0
5
r
z
H
c
H
m
i
o
o
r
c
2
03
C'
r
CD
o
-JO
H
0
5
fn
in
Thermo-
Flywheel Thermo- photo -
Gas Rankine Stirling Energy electric Thermionic voltaic
Parameters Turbine Engine Engine Storage Converter^3) Con verted3) Converter^3)
Engine, Transmission, and Conventional Fuel 33 46 75 130 Similar to 46+ Similar to
Volume, ft3 Thermionic Thermionic
Specific Engine Cost, $/hp(s) or hp(e) -- Not known but is expected to be $100/hp(e)
or higher
Engine Cost, $ ------
Specific Transmission Cost, $/hp(s) 14.0
Transmission Cost, $ 3200 Cost of batteries, motors, and controls
Specific Energy Container Cost, $/hp(t)-hr 100^ Nominal Nominal Nominal
Energy Container Cost, $ 50,000
Engine, Transmission, and Energy Container Should be similar to or some- 53,000 Very high
Cost, $ what higher than cost of
commercial -vehicle diesel
engine and hydrokinetic
transmission [i.e. , $25 to
35/hp(s)]
(a) Considered to be part of a hybrid system . Their power rating is based on the cruise power requirement which is one-half the maximum power. The
and cost of associated batteries, motors, and controls would have to be added to the values listed in this table for the converters alone.
(b) Value given in hp(s)-hr.
(c) Value given in lb/hp(s)-hr.
(d) Value given in ft3/hp(s)-hr.
(e) Value given in $/hp(s)-hr.
Magneto-
hydrodynamics
and Nuclear
Devices
% g"
TO P
Q.
c cr
o" =
c
o;
TO
O
i-i
3-
u
T3
P hi
o' i
i-» _
1"
1
weight, size.
-------
APPENDIX C
REFERENCES
BATTELLE MEMORIAL INSTITUTE - COLUMBUS LABORATORIES
-------
C-l
APPENDIX C
REFERENCES
Chemical Fuels
(1) Gruse, W. A., Motor Fuels, Reinhold Publishing Corp. , New York (1967).
(2) Taylor, C. F. , The Internal Combustion Engine in Theory and Practice, Volume 1,
The Technology Press of the Massachusetts Institute of Technology and John Wiley
and Sons, Inc., New York (I960).
(3) Gas Engineers Handbook, The Industrial Press, New York (1965).
(4) Salisbury, J. K. (editor), Kent's Mechanical Engineers Handbook, Power Volume,
12th Edition, John Wiley and Sons, Inc. , New York (1950).
(5) Obert, E. F. , Internal Combustion Engines, 2nd Edition, International Textbook
Co. , Scranton (1950).
(6) "Quarterly Report on Current Prices", Chemical and Engineering News (January,
1963).
(7) Final Report, "Alternate Sources of Fuel and Power for Army Materiel Use", to
U. S. Army Materiel Command Board, Project AMCB-3-62 (May 1, 1963).
(8) Reid, W. T. , "Kilowatts for Cars - A Comparison of Energy Costs for Electric
Automobiles", AIAA Paper No. 66-978 (November, 1966).
(9) Trayser, D. A. , et al. , "Deterioration of Fuels and Fuel-Using Equipment", Final
Report from Battelle Memorial Institute, Columbus Laboratories, to Office of Civil
Defense, Department of the Army (August, 1967)
(10) Weber, R. J. , and Dugan, J. F. , "Methane-Fueled Propulsion Systems", AIAA
Paper No. 66-685 (June, 1966).
(11) Winsche, W. E. , et al. , "Metal Hydrides as a Source of Fuel for Vehicular Pro-
pulsion", unpublished paper (September, 1967).
External Combustors
(12) Starkman, E. S. , Newhall, H.. K,, Sutton, R. , Maguire, T. , and Farbar, L. ,
"Ammonia as a Spark Ignition Engine Fuel: Theory and Application", SAE Pre-
print No. 660155 (January, 1966).
(13) Pratt, D. T. , and Starkman, E. S. , "Gas Turbine Combustion of Ammonia", SAE
Preprint No. 670938 (November, 1967).
BATTELLE MEMORIAL INSTITUTE - COLUMBUS LABORATORIES
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C-2
(14) Pearsall, T. J. , and Garabediam, C. G. , "Conbustion of Anhydrons Ammonia in
Diesel Engines", SAE Preprint No. 670947 (November, 1967).
(15) Chass, R. L., and George, R. E. , "Contaminant Emmissions from the Combustion
of Fuels", APCA Journal, J_0 (February, I960).
(16) Wasser, J. H. , Hangebranck, R. P., and Schwartz, A. J. , "Effects of Air-Fuel
Stoichiometry on Air Pollutant Emissions From an Oil-Fired Test Furnace",
APCA Paper No. 67-1Z4, U. S. Public Health Service, Department of Health,
Education and Welfare, Cincinnati, Ohio (1962).
(17) "Cleaning and Purification of Air in Buildings", Building Research Institute,
National Academy of Sciences - National Research Council, Publication 797
(I960).
(18) George, R. E. , and Chass, R. L. , "Control of Contaminant Emissions From
Fossil Fuel-Fired Boilers", APCA Journal, JT7 (6) (June, 1967).
(19) Hall, E. L. , "Products of Combustion of Gaseous Fuels", Proceedings of the 2nd
National Air Pollution Symposium (May, 1952).
(20) Smith, W. S. , "Atmospheric Emissions From Fuel Oil Combustion", Public Health
Service Publication No. 999-AP-2, U. S. Department of Health, Education and
Welfare, Public Health Service (November, 1962).
(21) Kweller, et al. , "Measurement of Trace Constituents in Combustion Products of
Operating Gas-Fueled Equipment", Institute of Gas Technology (July, 1967).
(22) Hoven, H. H. , Risman, A. , and Connar, J. F. , "The Development of Air Con-
taminant Emission Tables for Non Process Emissions", APCA Journal, 16 (7)
(July, 1966).
(23) Burroughs, L. C. , "Air Pollution by Oil Burners Measurable but Insignificant",
Fuel Oil and Oil Heat Journal (June, 1963).
(24) Lienesch, J. H. , and Wade, W. R. , "Stirling Engine Progress Report: Smoke,
Odor, Noise, and Exhaust Emissions", SAE Preprint No. 680081 (January, 1968).
(25) "The Automobile and Air Pollution: A Program for Progress", Report of the Panel
on Electrically Powered Vehicles, U. S. Department of Commerce, Part I
(October, 1967), Part II (December, 1967).
Brayton-Cycle (Gas Turbine) Engines
(26) Huebner, George J. , Jr. , "The Chrysler Gas Turbine Story", Proceedings of the
Institution of Mechanical Engineers, 179, 257-279 (1964-65).
(27) Huebner, George J. , Jr. , "The Chrysler Regenerative Turbine-Powered Passen-
ger Car", SAE Paper 777A (January, 1964).
BATTELLE MEMORIAL INSTITUTE - COLUMBUS LABORATORIES
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C-3
(Z8) Chapman, W. I. , "Chrysler's Gas Turbine Car, Powerplant Design Character-
istics", SAE Paper 777B (January, 1964).
(29) Roy, Amedee, et al. , "Chrysler's Gas Turbine Car, Materials Requirements",
SAE Paper 777C (January, 1964).
(30) De Claire, G. , and Bell, A. H. , "Chrysler's Gas Turbine Car, Laboratory Pro-
cedures and Development Methods", SAE Paper 777D (January, 1964).
(31) Penny, Noel, "Rover Case History of Small Gas Turbines", SAE Transactions, 72,
131-177 (1964).
(32) Staff, "Le Mans Rover - B. R. M. ", Automobile Engineer, pp 362-363 (August,
1965).
(33) Turunen, W. A. , and Collman, J. S. , "The General Motors Research GT-309
Gas Turbine Engine", SAE Transactions, Paper No. 650714,^5 (1966).
(34) Kelley, Ken, "New Turbine Approach at GM", Automotive News, p 8
(November 20, 1967).
(35) Staff, "Ford's Prototype 707 Truck Turbine", Gas Turbine, p 15 (November -
December, 1966).
(36) Swatman, I. M. , and Malohn, D. A. , "An Advanced Automotive Gas Turbine
Engine Concept", SAE Transactions, ^9, 219-227 (1961).
(37) Quan, D. , "The Orenda OT-4 600-hp Gas Turbine", SAE Paper 879A (June, 1964).
(38) Johnson, L. E. , and Davis, W. W. , "Evolution of a Turbine Engine for Industrial
Markets", SAE Transactions, 7_5, Paper No. 660035 (1966).
(39) Staff, "Automotive-Type Turbine Announced by Army, AVCO", Automotive News
(March 13, 1967).
(40) Bailey, John A. , et al. , "Status of the Army Closed Brayton-Cycle Gas Turbine
Program", ASME Paper 67-GT-13 (March, 1967).
(41) McCormick, J. E. , and Redding, Tony E. , "3-Kilowatt Recuperated Closed
Brayton-Cycle Electrical Power System", Advances in Energy Conversion
Engineering, pp 1-7 (1967 Intersociety Energy Conversion Engineering Conference,
Miami Beach, August, 1967).
(42) Pietsch, A. , "Reactor-Powered Brayton Cycle for Large Space Stations",
Advances in Energy Conversion Engineering, pp 65-75 (1967 Intersociety Energy
Conversion Engineering Conference, Miami Beach, August, 1967).
(43) Huebner, George J. , Jr. , "Automotive Turbine Engine Developments and Fuel
Requirements", P. D. No. 29, 7th World Petroleum Congress, Mexico City,
April 2-9, 1967.
BATTELLE MEMORIAL INSTITUTE - COLUMBUS LABORATORIES
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C-4
(44) DuRocher, Lionel J. , and Giannotti, Hugo, "Development of an Advanced Air
Cleaner Concept for Army Vehicular Gas Turbines", SAE Paper No. 670733
(September, 1967).
(45) Cornelius, Walter, et al. , "A Combustion System for a Vehicular Regenerative
Gas Turbine Featuring Low Air Pollutant Emissions", SAE Paper No. 670936
(October, 1967).
Rankine-Cycle (Steam) Engines
(46) "The Determination of the Practical Feasibility of Employing a Closed-Cycle
Freon Vapor Engine for a Silent Ground Power Unit", Final Report from Battelle
Memorial Institute, Columbus Laboratories, to USAMERDC, Contract No.
DA-44-009 Eng. 3991 (June 27, 1962).
(47) "Design, Fabrication, and Test of Experimental Model SCAP System", Final
Report from TRW Equipment Laboratories to USAMERDC, AD 473791L
(September, 1965).
(48) "Comparisons of Weights and Performances of Solar Dynamic Energy Conversion
Systems", Final Report by Air Force Aero Propulsion Laboratory, Wright
Patterson Air Force Base, AFAPL-TR-65-44.
(49) "Those Bloomin1 Steamers!", Car Life, pp 30 & 31 (April, 1967).
(50) Dooley, J. L. , and Bell, A. F. , "Description of a Modern Automotive Steam
Power Plant", SAE Paper S338 (1962).
(51) "New Revolver-Like Steam Engine", Popular Science, pp 84-88 (February, 1966).
(52) "Steam-Powered Automobiles May Solve Pollution Problems", Product Engineer-
ing, pp 25, 26 (April 10, 1967).
(53) Harvey, R. J. , "Advanced Steam Engine for Automotive Propulsion", Report to
the Department of Transportation (September, 1967).
(54) Starrell, J. K. , et al. , "Vapor Cycle Power Plant for Low-Power Mobile
Applications", AD 250497, pp 1-165 (1959).
(55) Millman, V. , "Advanced Technology Applied to the Steam Powered Vehicle",
SAE Paper 931A (1964).
(56) Gouse, S. W. , Jr. , "Automotive Vehicle Propulsion Part I: Steam Engine
Part II: Total Energy Ecology Implications", Advances in Energy Conversion
Engineering, pp 917-924 (August, 1967).
(57) "A Steam System for Automobiles", A study of an advanced control system for
low thermal inertia steam engines, Battelle Memorial Institute, Pacific
Northwest Laboratories (November, 1967).
BATTELLE MEMORIAL INSTITUTE - COLUMBUS LABORATORIES
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C-5
Stirling-Cycle Engines
(58) Flynn, Gregory, Jr. , Percival, Worth H. , and Heffner, F. Earl, "GMR Stirling
Thermal Engine", SAE Transactions, 6£, 665-684 (I960).
(59) Welsh, Harvey W. , and Monson, Donald S. , "The Stirling Engine for Space Power,
1962 Progress Report", SAE Paper 549C, presented at National Aerospace
Engineering and Manufacturing Meeting, Los Angeles, California, October 8-12,
1962.
(60) Aeronautical Systems Division, Flight Accessories Laboratory, Wright-Patterson
Air Force Base, Ohio, Final Report on "Potential Capabilities of the Stirling
Engine for Space Power", ASD-TDR-62-1099 (March, 1963).
(61) Heffner, F. E. , "Highlights from 6500 Hours of Stirling Engine Operation", SAE
Transactions, 74, Code 650254, 33-54 (1966).
(62) Meijer, R. J. , "Philips Stirling Engine Activities", SAE Transactions, 74, Code
65004, 18-32 (1966).
(63) Heffner, F. E. , "Stirling Engine Ground Power Unit", Final Report to U. S. Army
Engineer Research and Development Laboratories on Contract No. DA-44-009-
ENG-4968, General Motors Research Laboratories (1963) (AD 427293).
Magnetohydrodynamic s
(64) Rosa, Richard, and Kantrowitz, Arthur, "MHD Power", Science and Technology
(September, 1964).
(65) Brogan, Thomas R. , "Recent Progress in the Development of the Combustion
MHD Generator", AVCO-Everett Research Laboratory Report AMP 189
(February, 1966).
(66) "Electricity From MHD", Proceedings of the 1966 International Symposium on
MHD.
(67) Young, W. E. , et al. , "Recent Studies of Advanced Coal Burning Power Plants",
Westinghouse Research Center Scientific Paper 67-9 D8-MHDCF-P1 (April, 1967).
(68) Weller, A. E. , and Reid, W. T. , "The Economic Position of MHD for Central
Power", ASME Preprint 64-WA/ENER-1 (November, 1964).
(69) Mattsson, A. C. J. , et al. , "Energetics 6: MHD Power", Mechanical Engineering
(November, 1966).
(70) Yeh, Hsuan, "Status of MHD Power Generation for Terrestrial Applications",
AIAA Paper No. 66-1013 (November, 1966).
(71) Young, W. E. , et al. , "Energy Systems: The MHD Combination", Mechanical
Engineering (November, 1967).
BATTELLE MEMORIAL INSTITUTE - COLUMBUS LABORATORIES
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Thermoelectric Converters
(72) Telkes, M. , "The Efficiency of Thermoelectric Generators", J. Appl. Phys. ,
J_8, 1116-1127 (1947).
(73) Fritts, R. , "The Development of Thermoelectric Power Generators", Proc.
IEEE, 51_, 713-721 (1963).
(74) Neild, A. B. , "Thermoelectric Generators for Military Portable Power", Society
of Automotive Engineers, Chicago, Illinois, May 15-19, 1967, Preprint No.
670452.
(75) Nystrom, Thomas L. , "Thermoelectric Power System (300-560 Watts) Used as a
Vehicle Mounted Battery Charger", Proc. 21st Annual Power Sources Conference,
PSC Publications Committee, Red Bank, New Jersey, 1967, pp 121-124.
(76) Energy Conversion Digest, p 3 (August, 1967).
(77) Plevyak, T. J. , "A 160-Watt Experimental Thermoelectric Power Plant for
Telephone Microwave Equipment", Proc. Intersociety Energy Conversion
Engineering Conference, Los Angeles, California, September 26-28, 1966, pp
141-147.
(78) Radio Corporation of America, Electron Tube Division, Harrison, New Jersey,
"Optimization of Silicon-Germanium Thermoelectric Modules for Transportation
Corps Silent Boat Design", Contract No. DA-44-177-TC-857, U. S. Army
Transportation Research Command, Fort Eustis, Virginia (May, 1963),
AD 412341.
(79) Rocklin, S. R. , "Design and Development of a High Efficiency Cascaded and
Segmented Thermoelectric Module", Advances in Energy Conversion Engineering,
papers presented at the 1967 Intersociety Energy Conversion Conference, Miami
Beach, Florida, August 13-17, 1967, pp 207-219.
(80) The Martin Company, Final Report on "Ground Power Thermoelectric Generator
Investigation", Technical Report No. APL-Tr 66115, October, 1966, Contract
No. AF 33 (615)-3520. Performed for Air Force Aero Propulsion Laboratory,
Air Force Systems Command, Wright-Patter son Air Force Base, Ohio.
(81) Battelle Memorial Institute, Final Report on "HPD Thermoelectric Program
Optimum Length Investigation", June 30, 1967, Contract No. DA-44-009-AMC-
1824(X). U. S. Army Engineer Reactors Group, Army Nuclear Power Program,
Fort Belvoir, Virginia.
(82) Freas, D. G. , and Mueller, J. J. , "Silicon-Germanium Lead Telluride Segment-
ing for Improved Thermoelectric Efficiency", Proc. 1966 IEEE/AIAA Thermo-
electric Specialists Conference, pp 12-1 to 12-19.
BATTELLE MEMORIAL INSTITUTE -? COLUMBUS LABORATORIES
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C-7
Thermionic Converters
(83) Lazaridis, L. J. , Pantazelos, P. G. , and Shai, I. , "Design of a Gas-Fired
Thermionic Power Supply for Domestic Furnaces", presented at the Winter
Annual Meeting of the American Society of Mechanical Engineers, 1966.
(84) Lazaridis, L. J. , "Life Tests on Flame Heated Thermionic Coverters", pre-
sented at the 21st Annual Power Sources Conference, Atlantic City, 1967.
(85) Lazaridis, L. J. , and Pantazelos, P. G. , "Design of a 5-Kilowatt Flame-Heated
Thermionic Power Supply", 1966 IEEE Conference Record of The Thermionic
Conversion Specialists Conference, Houston, Texas, November 3 and 4, 1966,
p 126.
(86) Engdahl, Richard E. , "Fossil Fuel Heated Thermionic Diodes", ibid, p 133.
(87) Eastman, G. Y. , Ernst, D. M. , Hall, W. B. , Kessler, S. W. , and Turner,
R. C. , "Review of Fossil-Fuel-Fired Thermionic Energy Converters", ibid,
p 121.
Thermophotovoltaic Converters
(88) Werth, J. , "Thermophotovoltaic Energy Conversion", Proc. 17th Annual Power
Sources Conf. , PSC Publications Committee, Red Bank, New Jersey, 1963,
pp 23-27.
(89) Kittl, E. , "Thermophotovoltaic Energy Conversion", Proc. 20th Annual Power
Sources Conf. , PCS Publications Committee, Red Bank, New Jersey, 1966,
pp 178-182.
(90) General Motors Corporation, Santa Barbara, California, "Final Report on
Engineering Investigation of a Thermophotovoltaic Energy Converter", by Roger
W. Haushalter, Contract DA-44-009-AMC-622(T),. U. S. Army Engineer Research
and Development Laboratories, Fort Belvoir, Virginia, June, 1966, AD 636 484.
(91) Wedlock, B. D. , and Siegel, Robert, "Investigation of P-I-N Germanium Diodes
for TPV Conversion", Proc. 20th Annual Power Sources Conference, PSC
Publications Committee, Red Bank, New Jersey, 1966, pp 182-186.
(92) General Motors Corporation, Delco Radio Division, "Final Report on Study of
Germanium Devices for Use in a Thermophotovoltaic Converter", by D. P.
Crouch and R. W. Beck, Contract DA 28-043-AMC-1420(E), U. S. Army
Electronics Command, Fort Monmouth, New Jersey, July, 1966.
BATTELLE MEMORIAL INSTITUTE - COLUMBUS LABORATORIES
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C-8
Thermal Energy Storage
(93) Wilson, H. W. , Beahm, K. W. , Cooper, W. J. , "Determination and Analysis of
the Potentialities of Thermal Energy Storage Materials", ASD Technical Report
61-187, from Gallery Chemical Company to Flight Accessories Laboratory,
Aeronautical Systems Division, Air Force Systems Command, Wright-Patterson
Air Force Base, Ohio (June, 1961).
(94) Lipmann, David, and Stoltenberg, M. P. , Heat Storage Materials, Lithium
Corporation of America, Incorporated (June 1, 1961).
(95) Flynn, Gregory, Jr. , Percival, W. H. , and Tson, Michael, "Power From
Thermal Energy Storage Systems", SAE Preprint No. 608B (November, 1962).
(96) "Investigation of a 3 KW Stirling Cycle Solar Power System, Volume VI: Energy
Storage System Analysis and Experimental Research", Report No. WADD-TR-
61- 122, from ALlison Division, General Motors Corporation to Flight Acces-
sories Laboratory, Aeronautical Systems Division, Air Force Systems Com-
mand, Wright-Patter son Air Force Base, Ohio (February, 1962).
Flywheels
(97) "The Oerlikon Electrogyro", Automobile Engineer, p 559 (December, 1955).
(98) Roes, John B. , "An Electro-Mechanical Energy Storage System for Space
Application", Energy Conversion for Space Power, Nathan W. Snyder (Ed. ),
Academic Press, New York (1961).
(99) "Energy Storage Substation Concepts for Aircraft Actuation Functions", North
American Avaiation Technical Report AFAPL-TR-66-29 to Air Force Aero
Propulsion Laboratory, Wright Patterson Air Force Base, April, 1966.
(100) West, Philip, "Advanced-Fiber Composits Spark Materials Revolution", Prod.
Engr. , 33(22), 107-116 (Oct. 23, 1967).
Nuclear Devices
(101) "The ML-I Design Report, Army Gas-Cooled Reactor System Program",
IDO-28550 (May, I960).
(102) "Systems for Nuclear Auxiliary Po\ver . . . an Evaluation", TID-20079, 9-11
(January, 1964).
(103) Johnson, C. E. , and Mason, D. G. . "Spacecraft", _3(7), 1099-1105(1966).
(104) Johnson, C. E. , and Geotz, C. A., "SNAP-8 Reactor and Shield", AIAA J. ,
J_ (10), 2355-2361 (i963).
BATTELLE MEMORIAL INSTITUTE - COLUMBUS LABORATORIES
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C-9 and C-10
(105) "Nuclear Space Power Systems", AT-MEMO-75859 REV 2 (Sept. , 1964).
(106) Hanchett, J. , "Whither Now SNAP-8", Nucleonics, 2_5 (5), 54-57(1967).
(107) Osmun, W. G. , "Space Nuclear Power, SNAP- 50/SPUR", SPACE/
AERONAUTICS, 38-45 (Dec. , 1964).
(108) Gruntz, R. D. , and Rackley, R. A., "SNAP-50/SPUR Power Conversion Sys-
tem Objectives, Current Status, and Lunar Applications", SAE Aerospace Fluid
Power Systems and Equipment Conference, pp 210-215 (May, 1965).
(109) Rodden, R. M. , "Radioisotope Energy Sources for Small Manned Antarctic
Stations", Nuclear Applications, _3, 226-232 (April, 1967).
(110) Corliss, W. R. , and Harvey, D. G. , Radioisotopic Power Generation, Prentice-
Hall (1964).
(Ill) Anno, J. N. , "Secondary Electron Production From Alpha Particles Emerging
From Gold", J. Appl. Phys. , 14 (12), 3495-3499 (December, 1963).
(112) "The Alpha Cell Direct-Conversion Generator", NASA CR-54256 (November,
1964).
(113) Rappaport, P., "The Electron-Voltaic Effect P-N Junctions Induced by Beta
Particle Bombardment", J. Appl. Phys., 25, 1422-1429 (November, 1954).
BATTELLE MEMORIAL INSTITUTE - COLUMBUS LABORATORIES
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