Energy Depot Concept

SP-263

Presented at
International Automotive Congress
January 11-15, 1965

Published by:
SOCIETY OF AUTOMOTIVE ENGINEERS, INC.

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650051

Energy Depot Fuel Production and Utilization

P. G. Grimes
Research Div., Allis-Chalmers Mfg. Co.

ABSTRACT

The Army's fuel logistics problem could be reduced or eliminated by use of nuclear energy in the field. In this concept, nuclear energy is converted to chemical fuels with, locally available raw materials. Hydrogen can be produced by electrolysis of water with electricity from a nuclear reactor system. It can be converted to liquid hydrogen for ease of transportation. Alternately, liquid, ammonia can be produced from the hydrogen and nitrogen extracted from air through liquefaction of air.

These fuels can be used most efficiently in fuel cell systems. The electric powered vehicles in these cases may have distinct military advantages. The fuels can be used to power modified combustion engines.

MODERN FIELD ARMIES today face an increasing fuel logistics problem. The major portion of the supplies brought to a theater of operations is fuel for vehicles, electric power generation, and heating. All studies point to an even greater increase in fuel consumption in the future.

Use of nuclear energy in the field is a potential solution to this problem. However, limitations of size and weight imposed by present nuclear technology prohibit the use of vehicles individually powered with nuclear energy. Therefore, the nuclear energy must be converted to energy forms, that can be used to power individual vehicles.

In this concept, energy output of a mobile nuclear reactor would be processed to storable energy forms readily transportable to the energy consumers. This energy depot would be mobile and could accompany the field army in its operation. The concept would allow an extended operation of field units independent of outside fuel supplies. Field commanders would have greater freedom of operation, thus providing an opportunity to seize and maintain the initiative.

ENERGY DEPOT FEASIBILITY STUDY - A feasibility study of the energy depot concept was undertaken by Allis-Chalmers Manufacturing Co. with Air Products and Chemicals, Inc., as a subcontractor (1).

[Numbers in parentheses designate References at end of paper.]

In this study many conceivable means to power vehicles indirectly with nuclear energy was considered. Numerous criteria or guidelines were used in evaluation and selection of attractive energy depot systems. Such systems have to be mobile, highly efficient, small in size and weight, and capable of operating essentially independent of supply. The energy depots must be road, air, and sea transportable. Thus, all equipment must be contained in modules, not exceeding 30,000 lb in weight and 8.5 x 8.5 x 24ft in dimensions. Source of raw materials for the energy depot is limited to air and water. Earth as a raw material is eliminated because of variable composition. Maintenance materials must be minimal permitting extended operation free of outside supply. Energy forms should, be storable to permit a supply buildup for use during movement of the energy depot. The conversion of nuclear energy to "power at the wheels" requires efficiency to minimize the depot's size and weight.

Using these criteria, analysis of potential energy depot systems leads to the general selection of systems which convert the nuclear energy to chemical fuels. These fuels can be stored, their energy transported in an easily divisible form, and they can be used for heating and to power vehicles.

Two broad chemical approaches can be employed in the energy depot concept; the open cycle and the closed cycle. In the open cycle process, the chemical fuel is synthesized from raw materials (air and water) at the depot site. The fuel is then transported to the user. There fuel is oxidized, energy is extracted, and oxidation products of the fuel are discharged to the atmosphere. In the closed cycle process, the oxidation products are retained at the user, returned to the depot, and reprocessed to fuel.

In the open cycle process, only the elements present in air and water are available to synthesize potential fuels and oxidants.

Consideration of the physical properties, the methods and efficiency of synthesis, the energy content, and the usage of compounds reduced the potential fuels to liquid hydrogen and the hydrogen carrier, liquid ammonia. Potential oxidants were reduced to air and liquid oxygen.

The liquid hydrogen synthesis process involves conversion of nuclear energy to electrical energy, electrolysis of water to hydrogen, followed by liquefaction of hydrogen. Ammonia is prepared by reaction of the hydrogen with nitrogen produced by liquefaction and fractional distillation of air. Both processes are basically techniques of densification of hydrogen for storage and transport.

Radiolytic decomposition .of water and other chemo-nuclear synthesis processes were found to be of low efficiency, and the synthesis product purification process complicated fuel production. Direct thermal decomposition of water requires reaction temperatures too high for an attractive process. Indirect thermal decomposition of water using intermediate reaction steps with thermally regenerable chemicals does not appear to offer a highly efficient process for hydrogen production.

In the closed cycle processes, almost any chemical oxidation reduction process has potential as an energy carrier in the energy depot concept. Considerations of physical properties of compounds, energy content of fuels/oxidants, and efficiencies and methods of synthesis, and state-of-the-art of various powerplants rapidly reduce the list. Of all process considered, only the sodium metal process and the methanol/caustic system survived for further analysis.

In the sodium process, sodium hydroxide solution at the depot is electrochemically converted to metallic sodium, water, and oxygen. Sodium and water are stored and transported to the using vehicle. They are used there to produce electric power for the vehicle drive. The sodium is con-, verted to sodium amalgam in an electrochemical, process producing electrical energy. Amalgam, water, and air are, then supplied to a sodium amalgam/air fuel cell that produces more electrical energy for the vehicle drive. The sodium hydroxide solution product is returned to the depot for reprocessing. Alternately, the vehicle may return to the depot where electrical energy is fed into the electrochemical devices. This reverses the process above and produces sodium metal, oxygen and water from the sodium hydroxide. This process is analogous to the recharging of secondary batteries. The sodium process is potentially very efficient, but in an early state of development (2-4). Operational and, tactical characteristics of the system require further analysis before a system selection.

In the methanol/caustic system, sodium bicarbonate at the depot is reduced with hydrogen to methanol and sodium hydroxide. These are carried to the using vehicle. Methanol is used in a methanol/air fuel cell and to produce electrical energy for the vehicle drive. The oxidation products are returned to the depot as sodium bicarbonate solution. The total weight of the vehicle drive based upon the state-of-the-art of methanol cells eliminated this system in initial studies. Recent advances in methanol utilization efficiency may make this system attractive following further analysis.

Energy-depot fuels need to be used with high efficiency to utilize the nuclear energy most effectively. The feasibility analysis showed that hydrogen and ammonia could be used most efficiently in fuel cells to produce power at the user.

(NOT SHOWN, POOR QUALITY FROM MICROFILM)
Fig. 1 - Artist concept of liquid hydrogen energy depot

(NOT SHOWN, POOR QUALITY FROM MICROFILM)
Fig. 2 - Arrangement for field operation

CONCEPTUAL DESIGN - Allis-Chalmers with Air Products then undertook a conceptual design study of energy depot fuel production plants, and fuel cell powered vehicles using either liquid hydrogen or ammonia (5). The object of this study was to define more clearly the characteristics (weight, volume, processes, and performance) of the depots and vehicles designed for these two fuels. A fuel-cell-powered armored personnel carrier, based on the M113, was selected for the vehicular study.


Fig. 3 - Liquid hydrogen process flow diagram

Other studies have shown the feasibility of mobile nuclear reactor electric power plants. For this analysis, a system capable of producing 3000 kw of electrical energy was assumed.

For purposes of this study, the conceptual designs were projected to the late 1960's state-of-the art representing prototype energy depots and vehicles as designed after completion of an extensive development program. However, the selected conceptual designs are based upon firm engineering principles.

Much of the data used to develop these conceptual designs was made available to the project from the company and government sponsored research in our Research Laboratory. The electrolysis system was developed exclusively from company sponsored research. The fuel cell systems described were developed through our own programs and contracts sponsored by NASA, the Air Force, and the Army.

LIQUID HYDROGEN FUEL PRODUCTION

FACILITY DESCRIPTION - An artist concept of a liquid hydrogen energy depot is shown in Fig. 1. It consists of four modules, exclusive of the reactor system. During operation the electrolysis module -- containing the water purification plant, the water electrolysis plant, and rectifiers -- is located adjacent to the nuclear powerplant. This arrangement allows the water purification plant to use the waste thermal energy from the turbine exhaust (Fig. 2), However, the greater significance of this arrangement is that it requires only a short length of electrical cable to connect the turbine alternator to the electrolysis plant. Since this plant utilizes about 80% of the electrical power, a significant reduction in the weight of electrical cable needed by the energy depot is allowed. This electrolysis module also contains the circuit breakers for the total plant. Raw water is supplied to the purification plant on the electrolysis module by a pump located outside the reactor exclusion radius. A small high pressure hose is used to transport the gaseous hydrogen from the electrolysis plant to the liquefaction plant.

The hydrogen liquefaction\plant is contained on two modules located adjacent to each other outside the exclusion radius. All cold equipment is in two insulated cold boxes on the hydrogen liquéfier cold equipment module. This module also contains the expanders for the hydrogen recycle and nitrogen refrigeration loop. The two major compressors, the hydrogen recycle and nitrogen recycle compressor, are mounted on the hydrogen liquéfier compressor module.

The control module, containing the centralized control panel for the overall energy depot, is adjacent to the two liquefaction plant modules. Space is provided on this module for depot maintenance operations, supplies, and for carrying the cables and hose during transit.


Fig. 4 - Electrolysis module

PROCESS DESCRIPTION - The process system flow diagram for the liquid hydrogen energy depot shown in Fig. 3 can be described in terms of the water purification plant, electrolysis plant, and hydrogen liquefaction plant.

Water Purification - A water purification plant is required to make the raw water suitable for use in the electrolysis plant. Contaminants in the water would remain in the electrolysis cells. The raw water (4-6 gpm) is heated in an atmospheric pressure boiler by the exhaust gas of the reactor gas turbine. One half to one third of the raw water is evaporated, and the remainder continually drained from the boiler to reduce scale formation on the heat-transfer surfaces.

Electrolysis Plant - In this study it was found desirable to operate the electrolysis cells above the pressures required for the feed stream of the hydrogen liquefaction plant. This eliminated the need for feed compressors for that system which resulted in a weight saving for the total fuel production plant. In addition, operation of the electrolysis plant at high pressure eliminates the inefficiencies of the mechanical and cycle losses of the hydrogen feed compressor and increases the product output. The theoretical increase in power, 1.36 kw hr/lb H2, required for operating the electrolysis cells at 1840 psia, over that required for cells operating at atmospheric pressure was used for this design study. It is projected that the high pressure electrolysis process will require 19.86 kw hr/lb produced.

The electrolysis module is illustrated in Fig. 4. Electrolysis cell modules connected electrically in series parallel are assembled in multicell groups fitted into pressure tanks. These modules (Fig. 5) are arranged in four groups of six each. Each group has a rated input of 700 v.

Gas outlets from each electrolysis module are connected to one of the four collection manifolds, which conduct the fluids to the centrally located gas-electrolyte collection and separation equipment, serving all four circuits. Remotely operable valves-provide for isolation of any of the four independent circuits. Pressure throughout the system is regulated by an arrangement of control valves at the outlets of the electrolysis plant.


Fig. 5 - Electrolysis cell module stack


Fig. 6 - Electrolysis cell voltage amperage characteristics (atmospheric pressure)

Individual electrolysis cells are of the series bipolar design. Major components of each cell consist of a hydrogen electrode, an oxygen electrode) matrix and electrolyte and an electrode holder or bipolar plat. Electrolyte is circulated through the cells to provide makeup water, remove gas, and control temperature.

Fig. 6 illustrates electrolysis cell voltage characteristics obtained by Allis-Chalmers with cells utilizing fuel cell bipolar plate construction. Extrapolation of today's state-of-the-art indicates that electrolysis cells can most probably be. developed that operate at 1.535 v per cell and 400 amp/ft2 current density at atmospheric pressure. (This is equivalent to a power requirement of 18.5 kw hr/lb H2 produced.)

Circular bipolar plate geometry permits maximum utilization of a cylindrical pressure vessel (Fig. 7). The proposed design is based on the use of porous sintered metal electrodes. Catalysts are deposited on the electrodes to aid in the decomposition of water by lowering electrode potential.

In order to minimize internal resistance, the cell is designed with a thin KOH filled asbestos membrane. This membrane safely withstands the maximum pressure differentials allowed in the projected pressure control system. Each cell is sealed with O-rings. This seal separates the gas and electrolyte from the pressurizing liquid fill outside the cells. The pressure drop across these seals is negligible, since the liquid fill of the electrolysis module is held at a pressure only slightly less than that of the hydrogen gas. The pressure of the liquid exceeds the internal cell pressure only at shutdown and then only by the head of the liquid in the module.


Fig. 7 - Electrolysis cell module

The electrolysis cells are in pressure tanks (see Fig. 7), designed to take the pressure differential between the cell operating pressure and the atmosphere. Bellows pressurize the inside of the vessel (outside of the electrolysis cells) to the pressure level inside the cells with a nonelectrically conductive liquid. This design eliminates pressure differentials at each of the individual cell junctions. Each electrolysis module contains two units (A and B in Fig. 7) of electrolysis cells operating electrically in parallel. Each unit contains 70 cells in series. The "pressure-seal'' type of closure for the pressure vessel was selected over the more conventional bolted flange to reduce weight and overall diameter.

LIQUEFACTION PLANT - Hydrogen at about 1500 psi is delivered to the hydrogen liquefaction plant. A modified high pressure Claude liquefaction cycle (Fig. 3) was selected for the system because of its efficiency (6).

A hydrogen liquéfier is a specialized combination of compressors, heat exchangers; expansion valves and engines, adsorbers, piping, and other standard types of process equipment. A transportable hydrogen liquefier could be built with present technology, but such a system would not meet the energy depot criteria. Commercial compressors, expanders, and heat exchangers would impose severe limitations on the characteristics of the plant. It is projected that lightweight. non-hydrocarbon lubricated double acting, reciprocating compressors operating at 850-1600 ft/min and 4000 rpm with an efficiency of 75% can be used. Advanced design, non-hydrocarbon lubricated, expanders with efficiencies of 90% would be used. These expanders would be operated at half speed during start up. Advanced concept high surface area heat exchangers will be incorporated in the liquefaction plant design. Anticipated power requirements for liquefying the high pressure stream of hydrogen are 4.6 kw hr/lb liquid H2.

This liquid hydrogen fuel depot is expected to produce 114 lb of liquid hydrogen per hour from an electrical input of 3000 kw plus some thermal energy recovered from the reactor turbine exhaust heat. Characteristics of the liquid hydrogen fuel production plant are given in Table 1.

Table 1 - Characteristics of the Energy Depots

Characteristics

Liquid Hydrogen

Liquid Ammonia

Fuel Production Efficiency %
(Based on higher heating values of fuels.)

68.0

67.1

Production Rate



Fuel, lb/hr

114

710

Equivalent heat. Btu/hr
(Based on higher heating values of fuels.)

6,960,000

6,870,000

Pure water, lb/hr

1050

1136

Hydrogen, lb/hr

117

126

Nitrogen, lb/hr

--

589

Power Requirements



Water purification plant, kwe

7

8

Electrolysis plant:



(a-c), kwe

8

9

(d-c), kwe

2320

2523

Liquefaction plant, kwe

525

--

Nitrogen generator plant, kwe

--

235

Ammonia synthesis plant, kwe

--

85

Transmission and distribution losses, kwe

45

40

Rectification losses, kwe

95

100

Total Electrical Power, kwe

3000

3000

Thermal energy to water purification plant, kwt

390

420

Electrolysis (d-c). kw/hr/lb H2

19.86

19.93

Hydrogen liquefaction, kw/hr/lb H2

4.60

--

Nitrogen generation, kw/hr/lb N2

--

0.40

Ammonia synthesis, -kw/hr/lb NH3

--

0.12

Module Weights



Electrolysis, lb

27,000

29,000

H2 liquefaction, cold equipment, lb

28,000

--

H2 liquefaction, compressors, lb

29.000

--

Control, lb

25,100

25,300

Ammonia

--

30,000

Totals

109,100

84,300

Module Dimensions



Electrolysis, ft

22 x 8.5 x 8.5

24 x 8.5 x 8.5

H2 liquefaction, cold equip., ft

24 x 8.5 x 8.5

--

H2 liquefaction, compressors, ft

24 x 8.5 x 8.5

--

Control, ft

20 x 8.5 x 8.5

20 x 8.5 x 8.5

Ammonia, ft

--

24 x 8.5 x 8.5

LIQUID AMMONIA FUEL PRODUCTION

FACILITY DESCRIPTION - The ammonia production unit is similar to the liquid hydrogen unit except here hydrogen is mixed with nitrogen and converted to ammonia for storage and handling. The ammonia energy depot occupies only three modules exclusive of the nuclear powerplant. Arrangement of the modules is similar to that for the liquid hydrogen depot shown in Fig. 2 with the exception that the two modules of the liquefaction plant are replaced by a single -module adjacent to the control module. Both the nitrogen generator plant and the ammonia synthesis plant are mounted on this single module.

The projections for the size and weight of the electrolysis module in this system are based upon extrapolations of the weights and sizes developed under the liquid hydrogen energy depot design. Production of ammonia from hydrogen and air requires less input energy per pound of hydrogen than that needed for liquefaction of this hydrogen. The production rate for water purification and electrolysis plants are correspondingly about 8% larger than that for the liquid hydrogen plant. Because of the larger production rate, the size and weights of the water purification and electrolysis system are larger than in the liquid hydrogen system.

PROCESS DESCRIPTION - The process system flow diagram for the ammonia system is shown in Fig. 8. The process flow for the production of hydrogen is identical to that discussed before. The electrolysis plant is designed to operate at 2000 psi pressure. This permits hydrogen to be mixed directly with the nitrogen at this pressure in a stoichiometric ratio prior to the final stage of compression required by the ammonia snythesis process. Design of the electrolysis plant for operation at the 5000 psi required by the ammonia synthesis process was discarded as being excessively difficult for the resultant gains in the process. Operation of the electrolysis process at the 2000 psi delivery pressure requires a projected energy input of 19.93 kw hr/lb H2 produced, or 1.43 kw hr/lb H2 above operation requirements of the cells at atmospheric pressure.

NITROGEN GENERATION - Nitrogen for the production of the ammonia is provided by the liquefâction and frac-tional distillation of air (Fig. 8). An advanced design, four, stage, 36,000 rpm, centrifugal air compressor was used in the design. Oversizing by 50% was used for fast start up and operation at extreme altitude conditions. A radial inflow turbine operating at 61,000 rpm and approximately 80% efficiency was used for the expansion engine. Main heat exchangers are envisioned to be of the aluminum plate-and-fin type for maximum heat transfer capacity per unit of volume and weight. The distillation column will utilize either bubble-cap or sieve trays. It is estimated that an energy input of 0.4 kw hr will be needed to produce 99.993% N2. The warm start up time of the plant would be about 12 hr.

AMMONIA SYNTHESIS - Ammonia is synthesized by the reaction of stoichiometric mixture of nitrogen and hydrogen over catalysts

3H2 + N2 = 2-NH3

This equilibrium reaction for the formation of ammonia is favored by high pressure, low temperature, and low concentration of ammonia in the feed stream. The Schematic for the synthesis cycle is shown in the lower portion of Fig. 8. The synthesis reaction is highly exothermic. It is envisioned that the chemical reactor will be cooled by water flowing through coils in the synthesis reactor. The superheated steam so formed is passed .through a conventional -type steam turbine-generator to recover a portion of the heat of reaction as electrical energy.

A pivotal item in any ammonia plant is the synthesis catalyst. The energy, weight, and volume of the plant are largely determined by the ability of the catalyst to effect the synthesis reaction. Available catalysts are relatively crude materials which though studied intensively, have pot been appreciably improved. Cost of the available materials is so low that significant development programs have not been commercially justified. It is projected that an ad-vanced synthesis catalyst can be developed which is more active than the best current materials.

Typical process conditions for commercial low pressure plants are 500 C and 300 atm. Under these conditions, the reactor effluent contains about 20% ammonia and represents a 75% approach to equilibrium. Characteristics of the energy depot plant have been estimated on the basis of a catalyst which produces 35% ammonia in the effluent stream (77% of equilibrium) at 400 C and at 350 atm. The flow rate of gases recycled back to the reactor is approximately equal to the feed gas rate; therefore, the conversion separation circuit operates with a gas flow rate approximately double the feed gas rate. It is anticipated that this combination of conditions will yield a substantial weight and volume savings for the overall plant. Compressors in the ammonia synthesis plant will be of the advanced type discussed before. It is estimated that an energy input of 0.12 kw hr/lb NH3 will be needed for the ammonia synthesis plant.

The ammonia-fuel depot is expected to produce 710 lb of anhydrous ammonia per hour from the 3000 kw output of the nuclear powerplant plus some thermal energy recovered from the reactor turbine exhaust heat. Characteristics of the liquid ammonia fuel production plant are given in Table 1.


Fig. 8 - Liquid ammonia process flow diagram

FUEL PRODUCTION SYSTEM MODIFICATIONS - In the . systems discussed, about 80% of the electric power is consumed in the production of hydrogen. The major inefficiency in the systems is the conversion of nuclear heat to electricity. If thermal energy could be used directly or indirectly in the production of hydrogen, overall fuel production of a depot may be. increased. Recent preliminary studies indicate that the use of thermal energy to operate electrolysis cells at elevated temperatures may reduce the electrolysis cell voltage by about a third. Feasibility studies on the dual pH concept have indicated that the use of thermally regenerated acid and base to maintain a difference in pH at the electrolysis cell electrodes will lower the electrolysis cell voltage by about a half.

Electrochemical techniques of preparation of nitrogen from air may-lower the power requirements for nitrogen production by half.

Improvements in technology and new processes may greatly enhance the fuel production yields over those pro-jected in the conceptual design study. The energy depot concept does not depend upon the development of these techniques but they will greatly aid the program.

LIQUID-HYDROGEN AND AMMONIA STORAGE AND DISTRIBUTION UNITS

The storage and distribution units for liquid hydrogen con sist of a large storage vessel mounted on an 8 ton GOER, and a smaller vessel mounted on the user vehicle.

The GOER is expected to carry about 3200 gal of liquid hydrogen in a vacuum insulated cryogenic storage vessel The vessel receives and discharges liquid hydrogen through an insulated transfer hose through quick disconnect valves During filling, any vaporized hydrogen is returned to the depot for reliquefaction. Discharge of the liquid hydrogen to the user vehicle is effected by vaporizing a small amount of hydrogen in a pressure-raising coil (external radiator) and returning it to the storage vessel. This raises the internal vessel pressure and forces liquid hydrogen through the transfer system.

Because liquid hydrogen can only be stored under cryogenic temperature and because there will be heat leakage to the fuel, there will be some unavoidable loss of fuel dur-ing storage and distribution. During normal conditions, no losses during storage are expected since the amount of hydrogen vaporized by heat leakage will be so small that it will not be vented. However, the transfer of hydrogen from the primary to the secondary vessel requires the cool down of the transfer hose, hose, valves, and piping. This results in vaporization of the first liquid hydrogen contacting the warm surfaces and as this gas must be vented to make room for the incoming hydrogen liquid, there is a loss. Efficiency of the fuel transfer is expected to be about 93%.

The ammonia storage and distribution units consist of a large reinforced plastic ammonia storage vessel mounted on an 8 ton GOER, and a smaller vessel mounted on the user vehicle. Both vessels store the liquid ammonia under pressure at ambient temperature. A small pump is provided to move the. liquid ammonia from the primary vessel to the user vehicle through a flexible hose. The vessels are not vented except during emergency conditions. Because it is unnecessary to vent gas in any of the filling or transfer operations, the efficiency of ammonia transfer is expected to be nearly 100%. The system also has the advantage that the ammonia may be. stored indefinitely in the pressurized tanks without loss.

The characteristics of fuel storage and distribution units are given in. Table 2.

ENERGY DEPOT FUEL UTILIZATION

An integral part of the energy depot concept is the utili-zation of energy depot fuels. The efficiency of fuel utilization will reflect upon the size and number of energy depots needed to support particular units. Ideally, the most efficient powerplants should be used.

Hydrogen and ammonia have been used to power internal combustion engines (7-10). However, present vehicle powerplants would require modification to use these fuels. The fuel utilization efficiency could approach that of gasoline.

Hydrogen and the hydrogen carrier, ammonia, are ideal fuels for fuel cells and high fuel efficiencies can be obtained. Theoretically, electrical power equivalent to the free energy of the fuel oxidation reaction can be produced in the fuel cell. Fuel cells are not carnot cycle limited. Practical factors such as electrode polarization internal resistance and auxiliary power demand lower the net output. The modular nature of fuel cell systems allows the vehicle [designer?] great flexibility. High fuel efficiency, silent operation, and design flexibility are obtainable with fuel cell powerplants and point toward a greater usage of fuel cells in future; vehicles, and are a prime application in the energy depot systems.

Table 2. Characteristics of the Fuel Storage and Distribution Unit

Characteristic

Liquid Hydrogen

Liquid Ammonia

Transfer efficiency, %

93

100

Capacity of 8-ton goer, gal

3200

2900

Depot production hours to fill, hr

17

21

Storage temperature of fluid, F

-423

ambient

Storage pressure (normal) of fluid, psia

14.7

153

Method of delivery from GOER

pr coil

pump

Method of delivery from user vehicle tank

electric heater

pump

Future fuel cell powered military vehicle will require a new design. Present designs will probably not be retrofitted. However, in order to establish the feasibility of the application of fuel cell power to military vehicles, an armored personnel carrier (APC) based upon the M113 was selected for study. Two fuel cell powered vehicle drive systems were investigated for this vehicle: hydrogen/air and dissociated ammonia/air.

[The hydrogen/oxygen system for vehicles was analyzed and found to be very similar to the hydrogen/air system.]

For comparison, the use of hydrogen, ammonia, and gasoline in the APC was also investigated.

FUEL CELL ASSEMBLIES

The fuel, cell assemblies studied were hydrogen/air and dissociated ammonia/air. The hydrogen/air fuel cell assembly was the centerline design. All of the fuel cell systems consume hydrogen as the fuel and oxygen from air as the oxidant. Direct use of ammonia as a fuel in low temperature cells has not presently proved successful, and it must be dissociated into hydrogen and nitrogen for use. In the cases where the fuel and oxidant contain nitrogen, the nitrogen remains inactive and serves only to dilute the consumable gas. As a result, the projected performance of the hydrogen/air and the dissociated ammonia fuel cell are lower than hydrogen/oxygen systems.

Hydrogen consuming fuel cells can be classified as solid electrolyte, liquid electrolyte, or capillary-held electrolyte type. This study deals with the capillary membrane fuel cell long under development at Allis-Chalmers. Tests and analysis have proved that this type of fuel cell is feasible and well suited to use in a military vehicle. This fuel cell is an electrochemical converter that produces electrical energy, water, and heat from a continous supply of hydrogen and oxygen (air). The basic system has been described in detail in other, reports (11-13).

HYDROGEN-AIR FUEL CELL ASSEMBLY

Early in the study it was necessary to designate a power output for the hydrogen/air fuel cell assembly which would approximately satisfy the power requirements for the vehicle drive unit on the APC. This was necessary to determine the weights, sizes and other characteristics of the fuel cell assembly and its components. A gross power output of 160 kw in continuous service was selected as the total power output of the fuel assembly. The auxiliaries for the fuel cell assembly require 12 kw of power. This assembly can produce 180 kw (net) in a 15 minute overload condition.

Hydrogen/air fuel cell assembly designs were projected to be achievable in the late 1960's.on the basis of performance characteristics available in 1963. Actual assemblies developed may differ from those projected, but it is expected that with reasonable research and development, the size, weight, and performance goals are attainable. Fig. 9 shows the predicted performance related to results of tests performed on a fuel cell module built in an Allis-Chalmers development program. The prediction was also guided by results of research and development on hydrogen-oxygen assemblies for aerospace application.

A primary consideration in designing a vehicle power assembly is its weight. In applying fuel cells' to a vehicle . it is possible to project an operating design point so that the fuel cell assembly will be very efficient; that is, operate at low current density (amp/ft2) and at high terminal voltage (see Fig. 9). The total electrode area for the cells is large, and their weight is great. The amount of fuel consumed for a given mission would then be small. If the design point is chosen at a very high current density, then the voltage output of each cell is reduced, and number of cells must be increased to obtain the desired output voltage. The weight of a module for a given power level decreases up to the point where the increased weight, resulting from the number of cells required, overcomes the weight saving because of the reduction in plate area. However, this occurs at a very high amp/ft2 operating point for the projected performance curves. As this operating point increases for a given performance curve, efficiency falls. Consequently, the fuel consumption rate, and the water and heat to be removed all increase—These effects result in a system growth, requiring more fuel and larger capacity auxiliaries. If the performance, curve, required power level, and mission time are known, it is possible to find a point of minimum assembly weight. Thus, the selection of the amp/ft2 design point requires a balancing of the results of mathematical analysis with a knowledge and understanding of the nature of fuel cell development projected to the late 1960's.

With all these factors in mind, the design point of 300 amp/ft2 at 0.825 v was selected for continuous duty of the hydrogen/air fuel cell. A 15 minute overload point of 400 amp/ft2 at 0.758 was used.


Fig. 9 - Projected performance curves for hydrogen/air fuel cells

Process Equipment - Arrangement of the fuel cell process equipment is shown in Fig. 10. The fuel cells are arranged in modules consisting of 91 cells each. Sixteen modules make up the vehicle drive unit and are connected by common manifolds to the cooling circuit, the hydrogen and oxidant supplies. and the moisture removal condenser.

About 70% of the water is removed through the static moisture control system on the hydrogen side of the cell. The remaining 30% of the moisture is removed with the exhaust air. Moisture removal cavities of all cells are connected through a common manifold to the condenser. Pressure within this condenser is automatically maintained by the vacuum fan; thus, the migration of moisture stops at a particular concentration of electrolyte when the correspond -ing vapor pressure matches the pressure maintained in the condenser. Condensed moisture is returned to the air purifier to humidify the incoming air. An air purifier conditions the air entering the cells removing dust, carbon dioxide, and the like, and humidifying of air to a vapor pressure corresponding to minimum desired vapor pressure in the fuel cells.

The fuel cell modules are maintained at a constant temperature of 180 F. Heat is dissipated both through moisture removal from the cells and through a cooling circuit. An electrically nonconductive cooling liquid is circulated through the electrode holders in each fuel cell and the hot liquid is routed through a common header to the coolant radiator. The cooled liquid then goes to the sump tank and a circulating pump forces it through the moisture removal condenser and into the fuel cell modules to make a complete circuit.


Fig. 10 - Hydrogen/air fuel cell vehicle drive unit

The fuel cells are arranged in modules of 91 cells to give a module voltage of 75 v at the continuous load design point. Electrode area was selected to produce 10 kw of power pet module. Current through each module is therefore 133 amp. Under overload conditions each module produces 178 amp at 69 v (12.3 kw).

The modules are arranged in four groups of four modules each. Each group of modules is connected in series to produce 300 v. The groups of modules are arranged so that modules 1-4 are in parallel with modules 5-8; modules 9-12 are parallel with modules 13-16. Switches enable those two parallel groups to provide an output voltage of 600 v and 266 amp when in series, or 300 v and 532 amp when in parallel.

The major design and operating characteristics for the hydrogen/air fuel cell assembly are summarized in Tables 3 and 4.

DISSOCIATED AMMONIA/AIR FUEL CELL ASSEMBLY -A gross power output of 160 kw in continuous service was selected as the total power output of the fuel cells for the dissociated ammonia fuel cell assembly study. This fuel cell assembly has a net power output of 147 kw in continuous service and 179 kw (net) in the 15 minute overload condition. The fuel cell assembly studied is 4% more powerful than required by the APC.

The dissociated ammonia fuel cell assembly differs from the hydrogen/air assembly in two major respects. The hydrogen fuel is diluted with nitrogen and a modification of the moisture removal process is required.

Table 3 - Hydrogen/Air Fuel Cell Assembly, Major Design


Size and Weight

Module weight, lb

101

Module volume, ft3

0.827

Number of modules

16

Assembly weight, lb *

3160,

Assembly volume, ft3 *

45.9

* Includes 16 modules plus auxiliary equipment consisting of radiator, filter, air compressor, condenser, vacuum fan, circulating pump, water pump, plumbing and ducting controls, fluids, and air purifier.

The dissociated ammonia-air cell is not expected to reach the performance of the hydrogen/air cell because of the effect of nitrogen dilution on the hydrogen electrode.

The curve in Fig. 11 shows the performance estimated for this fuel cell projected to late 1960's. This projection assumes considerable development on both the fuel cell and the ammonia dissociator to minimize the amount of and effects of nondissociated ammonia. Rated current density was selected at 225 amp/ft2 at 0.825 v per cell. Overload was selected at 300 amp/ft2 at 0.758 v per cell.

AMMONIA DISSOCIATOR

An ammonia dissociator was conceptually designed to produce up to 20 lb of usable hydrogen per hour for the fuel cell assembly. For this case, the ammonia dissociator produces 26.5 lb/hr of hydrogen in the form of 3:1 hydrogen-nitrogen mixture. This mixture is produced by catalytic thermal dissociation of ammonia at 1700 F and 50 psig pressure. Equipment required was estimated to weigh about 1025 lb and occupy about 12.5 ft3. A schematic representation of the unit appears in Fig. 12.

Table 4 Hydrogen-Air Fuel-Cell Assembly
Operating Characteristics


Continuous Duty

15 min Overload

Gross power, kw

160

196

Auxiliary power, kw*

12

16

Net power, kw

148

180

Assembly voltage (parallel), v

300

276

Assembly voltage (series), v

600

552

Assembly amperage (parallel), amp

532

712

Assembly amperage (series), amp

266

356

Module power, kw

10.0

12.3

Module voltage, v

75

69

Module amperage, amp

133

178

Cell power, kw

0.110

0.135

Cell voltage, v

0.825

0.758

Cell amperage, amp

133

178

Cell current density, amp/ft2

300

400

Operating temperature, F

180

--

Assembly weight/net power ratio lb/3kw

21.4

17.6

Asjembly volume/net power ratio ft3 /kw

0.310

0.255

Fuel consumption, lb H2/net kw/hr

0.108

0.118

Fuel consumption, lb H2/hr

16.0

21.3

Purified air requirements, lb air/hr

790

1050

Air purification chemicals lb/hr

2.7

3.6

* Includes power for compressor, cooling fan. vacuum fan, circulating pump, water pump, and electrical control.


Fig. 1l - Projected performance curves for dissociated ammonia, air fuel cells


Fig. 12 - Ammonia dissociator process flow diagram

Theoretically, 113.3 lb/hr of dissociated ammonia is necessary to supply the fuel cell with 20 lb/hr of hydrogen. To make the process self-sustaining, about 6.5 lb/hr of hydrogen are burned in the reactor to supply the energy for the dissociation process. Included in this figure are possible radiation, diffusion losses, and so on. Therefore, a total of 150.1 lb/hr of ammonia is supplied to the reactor.

A palladium/silver foil hydrogen diffuser used after the dissociator could supply pure hydrogen to the fuel cells. This would allow the projection of hydrogen/air fuel cell system. The palladium/silver diffuser was disallowed because of its volume and because it requires the dissociator to be operated at high pressures.

PROCESS EQUIPMENT - Arrangement of the fuel cell process equipment is shown in Fig. 13. The arrangement is similar to that for the hydrogen/air fuel cell assembly with respect to the temperature control equipment and the supply of purified air to the oxygen electrode. The fuel supply equipment differs in that a hydrogen/nitrogen mixture from dissociated ammonia is fed to the fuel cell hydrogen electrodes. About 75% of the hydrogen in this mixture is used by the fuel cell to produce electrical power. The remaining hydrogen is burned to provide the heat for the dissoci -ator.

Electrical arrangements for the dissociated ammonia/air fuel cells are identical to that for the hydrogen/air cells. This system's auxiliaries consume about 1 kw more power.

The major design and operating characteristics for the dissociated ammonia fuel cell assembly are summarized in Tables 5 and 6.

ELECTRIC DRIVE ASSEMBLY

A detailed analysis was made of the electric drive assembly for the M113 (5). A d-c type motor was selected for the fuel cell powered vehicle. The advantages of the d-c drive assembly are:

1. It is more efficient since fuel cells produce direct current and there will be no losses due to conversion to alternate current.

2. The d-c drive assembly eliminates the need for a-c conversion equipment with its associated control.

3. A simple one-step switching of the fuel cell banks from series to parallel operation will change the output from low speed, high torque to high speed, low torque using full fuel cell output in both ranges.

4. The short time overload capability of the d-c motor is greatly superior to a-c motor.

5. Its ability to weaken its field and deliver constant horsepower with constant voltage and amperage input over a wide speed range (trading torque for speed) closely match normal traction requirements.


Fig. 13 Dissociated ammonia/air fuel cell vehicle drive unit.

Table 5 - Dissociated Ammonia Fuel-Cell Assembly. Major Design


Size and Weight

Number of cells per module

91

Module weight, lb

134

Module volume, ft3

1.09

Number of modules

16

Assemblyweight. lb*

48.00

Assembly volume. ft3 *

64.1

* Includes 16 modules plus auxiliary equipment consisting of radiator, filter, air compressor, condenser, vacuum fan. circulating pump, water pump, plumbing and ducting, controls, fluids, air purifier, and ammonia dissociator.



Table 6 - Dissociated Ammonia Fuel-Cell Assembly, Operating Characteristics


Continuous Duty

15 min Overload

Gross power, kw

160

196

Auxiliary power, kw *

13

17

Net power. kw

147

179

Assembly voltage (parallel), v

300

276

Assembly voltage (series), v

600

552

Assembly amperage (parallel), amp

532

712

Assembly amperage (series), amp

266

356

Module power, kw

10.0

12.3

Module voltage, v

75

69

Module amperage, amp

133

178

Cell power, kw

0.110

0.135

Cell voltage. v

0.825

0.758

Cell amperage, amp

133

178

Cell current density, amp/ft2

225

300

Operating temperature. F

180

--

Assembly weight/net power ratio lb/kw

32.7

26.8

Assembly volume/net power ratio/ft3 /kw

0.436

0 358

Fuel consumption, lb NH3/net kw/hr

0.816

0.894

Fuel consumption, lb NH3/hr

120

160

Purified air requirements, lb air/hr

790

1050

Air purification chemicals lb/hr

2.7

3.6

* Includes power for compressor, cooling fan. vacuum fan. circulating pump, water pump, and electrical control.

Only disadvantages of the d-c drive assembly are that the motor is slightly larger and heavier than its a-c,counterpart, and the d-c motor requires a commutator.

The design analysis indicated that a single shunt wound d-c motor was suitable for the drive of the APC based on M113. At low speeds up to 9 mph, the low voltage high cur-rent parallel circuit output of the fuel cells is applied to the armature of the shunt wound motor. This provides high torque to the motor eliminating the need for mechanical shifting. At higher speeds, the fuel cells are switched to the series circuit placing high voltage, low current source on the armature. Fine control in each range is provided by varying the field current which is excited from a constant voltage control.

VEHICLE ANALYSIS

A mathematical vehicle analysis was developed to provide a means of quickly estimating the weight, size, and power requirements for the APC fuel cell powered vehicle without preparing a detailed drafting layout of each vehicle drive unit. This analysis is basically a weight, volume, power calculation which applies equal averaging to all vehicle components. Detailed analysis would probably arrive at a lower order of change.

The analysis considered:

1. The present horsepower to differential.

2. Size and weight of present powerplant and fuel supply

3. Weight of vehicle per present dimensions.

4. The corresponding values for the new vehicles. This analysis is given in detail in the conceptual design report (5).

FUEL CELL-ELECTRIC MOTOR DRIVE UNITS - Weight-to-horsepower and volume-to-horsepower ratios for the three fuel cell-vehicle drive assemblies are summarized in Table 7. In all cases, horsepower refers to the horsepower deliv-ered to the steering differential of the APC with the vehicle drive units operating at their 15 minute overload rating.

Table 7 - Weight-To-Horsepower and Volume-To-Horsepower Ratios
For the APC Powered By a Fuel-Cell Drive Unit

Component

Hydrogen/Air Fuel-Cell Drive

Dissociated Ammonia/Air Fuel-Cell Drive

lb/hp

ft3/hp

lb/hp

ft3/hp

Fuel cell assembly

16.4

0.259

25.0

0.364

Electric drive assembly

11.2

0.069

11.2

0.069

Vehicle fuel unit

1.5

0.314

4.5

0.163

Totals

29.1

0.642

40.7

0.596

Fuel cell assemblies contain all auxiliaries including the air purifier and ammonia dissociater where applicable. Values for the fuel cell assemblies assume that the electric drive assembly is 88% efficient. A 10% increase in the weight-to-horsepower ratio for mounting the fuel cell components, and a 20% increase in the volume-to-horsepower ratio were included as a packaging factor. The electric drive assembly includes the motor, cooling equipment, and controls.

Fuel rates for the respective fuel cell assemblies were determined by using the 15 minute overload rating adjusted to include the losses to the electric drive assembly. A 5.38 hr operating duration at full power was used to calculate the capacity of the fuel tank. The weights and volumes of the fuel containers were estimated from curves given in APCI-541101 with allowance for ullage, fill, and interconnecting lines, auxiliaries, and a packaging factor (14).

Values given in Table 7 were then used to calculate the characteristics of the respective armored personnel carriers assuming that these values remain constant over the range of power needed. The performance of these vehicles should be equal to the production M113. Since the components are designed at approximately the power required, this is con-sidered a valid assumption. The result of the analysis is given in Table 8.

Table 8 - Characteristics of APC Powered By a Fuel-Cell Drive Assembly

Characteristics

Production
Model M113

Fuel-Cell Powered
Armored Personnel Carriers

Gasoline

Hydrogen/Air

Dissociated ammonia/air

Combat weight to horsepower ratio, lb/hp

146

146

146

Range (full power duration), hr

5.38

5.38

5.38

Horsepower to differential, hp

157

182

203

Vehicle combat weight, lb

22,830

26,600

29.600

Fuel, lb

490

99

828

Fuel, gal

80

16.7

165

Fuel container and supply unit, lb

30

173

83

Fuel container and supply unit, ft3

11

57

30

Powerplant, lb

1810

5020

7350

Powerplant. ft3

50

60

88

Increase in hull weight, lb

--

830

890

Increase in hull volume, ft3

--

56

60

Vehicle height, in.

72

79

80

Vehicle width, in.

106

106

106

Vehjcle length, in.

192

192

192

Fuel rate, full power lb/hr

91.1

18.3

154

Air cleaning chemicals, lb

--

16.4

18.5

INTERNAL COMBUSTION ENGINE DRIVE UNITS - It is apparent that the change in fuel from petroleum to either liquid hydrogen or ammonia will also influence the design (or performance) of the APC when these fuels are consumed in an internal combustion engine. A brief analysis was made on the same basis as for the fuel cell powered vehicle.

Table 9 presents the weight-to-horsepower and volume-to horsepower ratios for the internal combustion engine drive unit fueled by hydrogen or ammonia. In both cases the fuel efficiency of the engine was arbitrarily considered equal to that of the gasoline engine. The engine and its auxiliaries were arbitrarily assumed to be 10% heavier and larger than the gasoline engine. The results of this analysis for equal range vehicles are presented in Table 10. This table shows that the use of either liquid hydrogen or ammonia in the APC will require either a larger and heavier vehicle, or a compromise in the vehicle's range. In particular, the large-liquid hydrogen tanks result in the largest vehicle studied; the roof is raised 11 in. and the hull is lengthened 5 in. In the case of the ammonia fueled internal combustion engine, the roof must be raised 5 in. to provide for the increased volume of the fuel. This vehicle is similar to the three fuel cell powered vehicles in outside appearance.

The fuel rates for the internal combustion engine pow-ered vehicles are considerably greater than for the fuel cell powered vehicles. This is particularly true for the hydrogen fueled vehicles, 33.0 lb/hr versus 18.3 lb/hr. respectively. In the ammonia fueled vehicles, the fuel, consumption rates are 154 lb/hr for the fuel cell powered vehicle versus 206 lb/hr for the internal combustion engine powered vehicle.

Table 9 - Summary of Weight-To-Horsepower and Volume-To-Horsepower Ratios for the APC Powered By an Internal Combustion Engine

Component

Gasoline

Hydrogen

Ammonia

lb/hr

ft3/ hp

lb/hr

ft3/hp

lb/hr

ft3/ hp

Engine and auxiliaries

11.6

0.319

12.8

0.351

12.8

0.351

Fuel Unit

3.3

0.069

2.9

0.607.

7.3

0.264

Total

14.9

0.388

15.7

0.958

20.1

0.615

The efficiency advantage of the fuel cells over internal combustion engines is shown in Table 11. These values were computed from the vehicle analysis of the armored personnel carrier. In all cases they include the auxiliaries. The drive efficiency is defined as the horsepower-hours delivered to the vehicle differential divided by the equivalent energy of the fuel consumed using the higher heating values for each fuel.

The energy depot systems are most easily compared by considering them as complete individual systems. Table 12 gives the number of horsepower-hours delivered to the vehicle transmission for each hour of depot operation, and the system efficiency.

Table 10 - Characteristics of APC Powered By Internal-Combustion Engine

Characteristics

Fuel Consumed

Gasoline
(Production model M113)

Hydrogen

Ammonia

Combat weight-to-horse -power ratio, lb/hp

146

146

146

Range (full-power dura tion), hr

5.38

5.38

5.38

Horsepower to differential, hp

157

170

168

Vehicle combat weight, lb

22,830

24,800

24,500

Fuel, lb

490

178

1,110

Fuel. gal

80

300

220

Fuel container and supply unit. lb

30

314

(Illegible in Original)

Fuel container and supply unit. ft3

11

103

44

Powerplant, lb

1810

2180

2150

Powerplant. ft3

50

60

59

Increase in hull weight, lb

--

1670

620

Increase in hull volume, ft3

--

102

42

Vehicle height, in

72

83

77

Vehicle width, in.

106

106

106

Vehicle length, in.

192

197

192

Fuel rate, full power, lb/hr

91.1

33.0

206



Table 11 - Comparison of Drive Unit Efficiencies

Fuel and Oxidant

Efficiencies, Per Cent

Internal Combustion Engine

Fuel Cell
Continuous Duty

Drive Units
15 Minute Overload

Gasoline/air

21.5

--

--

Hydrogen/air

21.5

45.6

41.7

Ammonia/air

21.5

38.1

34.8



Table 12 - Comparison of Energy Delivered and System Efficiencies


Horsepower-Hours delivered To
Transmission per hour of Depot Operation

System Efficiency %

Energy Delivered to Transmission
Electrical Energy Delivered to Depot

Internal Combustion Engine

Fuel-Cell-Drive Unit

Internal Combustion Engine

Fuel-Cell-Drive Unit

Continuous Duty

15 min. Overload

Continuous Duty

15 min. Overload

Liquid hydrogen

546

1158

1055

13.6

28.7

26.3

Ammonia

579

1026

937

14.4

25.5

23.3

SUMMARY

The fuel logistic problem can potentially be alleviated by using nuclear energy în the field. Feasibility studies indicate the technical possibility of converting nuclear energy, air, and water into the chemical fuels, liquid hydrogen and ammonia in the field. These fuels can be utilized to power internal combustion engines or more efficiently to power fuel cell electric drive, systems. A conceptual design analysis of the fuel production portion of the energy depot system resulted in lightweight compact, mobile units of high efficiency. The study of armored personnel carriers showed the suitability of fuel cell electric motor drive systems for military vehicles. The energy depot system should prove to be an exciting concept for the army of the future.

Continuing studies in the areas of hydrogen and ammonia production and their utilization in internal combustion engines and fuel cells should allow the early realization of the energy depot.

REFERENCES

1. NY010422, "Modile Energy Depot Feasibility Study - Summary Report." Allis-Chalmers Mfg Co., July 12. 1962. Contract Report AT (30-1) 2931.

2. A. B. Rosenthal. "Energy Depot Concept. " "Paper 650050. SAE Transactions. Vol. 74 (1966)

3. APL-TDR-64-41 "Research and Development on an Advanced Laboratory Liquid Metal Regenerative Fuel Cell," Allison Div. of General Motors. Contract Report AF 33(657)-11032.

4. K. Miller. Chem. Eng. Prog.. Vol. 57. (1961), 140.

5. ACNP 64501 "Energy Depot System Study," Allis-Chalmers Mfg. Co.. February 1964. Contract Report AT (30-1)3133.

6. D. B. Chelton, J. W. Dean, and J. Macinko. "Methods of Hydrogen Liquefaction," N. B. S. Report 5520. Oct. 14, 1957.

7. E. Kroch, "Ammonia, A Fuel for Motor Buses," J. Inst. Petroleum, (July 1945). 213.

8. G. Egloff and M. Alexander. "Petroleum Refiner," Vol. 23, No. 6 (1944), 127.

9. W. Cornelius, SAE Meeting. Detroit, 1985.

10. K. O. Lindell. "Introduction to Nuclear Powered Energy Depot Concept," SAE Meeting, Detroit, 1965.

11. J. L. Platner and P.D. Hess, "Static Moisture Removal Concept for Hydrogen-Oxygen Capillary. Fuel Cell," Allis-Chalmers Mfg. Co., Research Div., ASME/AIChe Heat Transfer Conference, Boston, July 1963.

12. J. L. Platner, D.P. Ghere, and R. W. Opperthauser. "Capillary Hydrogen-Oxygen Fuel Cell System for Space Vehicle Application." AIEE Meeting, Washington, August 1963.

13. R. J. Lodzinski. "A 5 KW Hydrocarbon Air Fuel Cell Power Source." AIAA Conference on Aerospace Power Systems, Philadelphia. 1964.

14. ACPI 541301 "Energy Depot Cryogenic Fuel Storage and Distribution Systems Phase I Conceptual Design." Air Products and Chemicals, Inc., Contract Report AT (30-1) 3129.