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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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                               71
                           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.
                                                 i—Electrodes
                                                                 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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                                      93 and 94

      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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                                           95

                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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                                           96

      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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                                      97
 £

 <§

 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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                                            98


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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                                                99
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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                                      100
      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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                                          101

      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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                                          102

      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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                                          103

      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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                                          104

      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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                                          105

                          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
               BATTELLE  MEMORIAL INSTITUTE - COLUMBUS LABORATORIES

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                                             106

 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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