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Hydrides VERSUS competing options for storing hydrogen in energy systems
301
Journal of the Lea-Common Metals, 74 (1980) 301 - 320 0 Elsevier Sequoia &A., Lausanne - Printed in the Netherlands
HYDRIDES VERSUS COMPETING OPTIONS FOR STORING GEN IN ENERGY SYSTEMS*
HYDRO-
JAMES H. SWISHER
Division of Thermal and Mechanical Energy Stomge, U.S. Department Washington, D. C. 20585 (U.S.A.)
of Energy,
EDWARD D. JOHNSON
OAO Corporation, 2101 L Street, N. W., W~hi~ton,
D. C. 20037 (U.S.A.)
Summary Metal hydrides present many possibilities for energy storage because of their ability to store hydrogen safely and at modest pressures. We compared hydrogen storage in metal hydrides with other forms of hydrogen storage. Comparisons were made using technical performance, cost and safety as the criteria. Hydride applications and alternatives for vehicular applications, chemical heat pumps, engines, peak shaving and others are discussed.
1. Introduction Hydrides are certainly interesting materials for scientific research, as evidenced by the number and variety of papers at this symposium. It should also be realized that hydrides are potentially useful materials for energy storage, gas separation processes and special applications. There is of course competition between hydride devices and other means of providing an engineering function in all the areas where hydrides may be used. The obvious example is in storage applications because methods are available for storing hydrogen in either solid, liquid or gaseous form. From the point of view of an engineer or an energy program administrator, there is a desire to provide stimulation for the research community to focus its research on materials and devices which could lead to commercial applications. With this in mind, an attempt is made here to show where and why hydrides look attractive in comparison with competing options for engineering applications. The criteria used are technical performance, cost and safety characteristics. Whilst no attempt is made to analyze all possible applications for hydrides, a number of the more likely possibilities are discussed.
*Paper presented at the International Symp~ium on the properties and Application of Metal Hydrides, Colorado Springs, Colorado, U.S.A., April 7 - 11,198O.
302
2. Vehicular applications
2.1. Overview One of the larger efforts involving metal hydrides for energy storage is the hydrogen vehicle. The requirements for the fuel storage system of the hydrogen vehicle are (1) a minimum delivery rate of 1.25 kg h-l to meet the suburban driving cycle defined by the Society of Automotive Engineers [ 11, (2) a storage volume which can be accommodated in a car, (3) sufficient energy density (energy per mass) to provide acceptable performance and range, (4) a high degree of safety during use and refilling and (5) a reasonable refill tune. In addition the system should be as self-contained as possible, e.g. no external energy sources should be required for start-up, and the energy requirements for hydrogen retrieval should be as small as possible (or altematively use waste heat from the engine). Three systems in contention for a hydrogen vehicle fuel storage system are metal hydrides, glass microspheres and liquid hydrogen. We will start with the newest idea first, the glass microsphere. Glass microspheres are small glass spheres 6 - 60 E.trnin diameter made from fly ash (Fig. 1). The microspheres are structurally strong and after an initial screening are expected to undergo numerous refilling operations with minimal breakage. As the filling and subsequent containment of hydrogen in the microspheres is a physical process the system is not affected by low levels of impurities. This is an advantage over hydrides where small levels of impurities cannot be tolerated. During filling, the glass spheres are heated in a high pressure hydrogen atmosphere. The hydrogen permeates the glass, the system is then cooled and the residual hydrogen is pumped off. Since the hydrogen is trapped within the spheres, the spheres can be removed from the charging vessel. The decrease in permeability of the glass as the temperature decreases allows the glass spheres to maintain a high internal hydrogen pressure (25 MPa). To extract the hydrogen the glass spheres are warmed to an operating temperature of 200 “c and the hydrogen diffuses out of the spheres. When the engine is shut off, the microsphere bed slowly releases hydrogen until the bed has completely cooled down. The hydrogen released during the cool-down period is absorbed into a smaller metal hydride bed.
ONE lxkAN DIAMETER
Fig. 1. A photograph of small glass microspheres that can be used for hydrogen storage.
303
The metal hydride then provides the hydrogen required for vehicle start-up. This hybrid system, developed by Teitel et al. [2], is called the microcavity hydrogen storage system. The refueling scenario for the microspheres would involve the complete exchange of the microspheres in the fuel tank (or an exchange of fuel tanks). The microspheres would then be transported and recharged with hydrogen at a central processing plant. Turning now to liquid hydrogen storage, several liquid hydrogen vehicle prototypes have been built. All the vehicles have been experimental and little work has been done to optimize fuel tank design. A liquid hydrogen vehicle has a large metal Dewar flask for a fuel tank. The Dewar flask is necessary to provide the insulation required to maintain the low temperature of liquid hydrogen. A semiautomated delivery system has been built for the Institute for Technical Physics of the Deutsche Forschungs- und Versuchsanstalt fur Luft- und Raumfahrt e.V. (DFVLR) [3]. This automated delivery system will fill a 120 1 tank in 5 min. The semiautomated system has several checks to prevent exposure of personnel to the cryogenic liquid. All liquid hydrogen fuel systems deliver the fuel into the combustion chamber in the gaseous state. Some prototype vehicles use electrical resistance heaters to vaporize the liquid hydrogen in the tank, whilst others use liquid withdrawal of the hydrogen followed by vaporization with waste heat from the engine. A third approach is to store hydrogen in metal hydrides. The heat required to liberate the hydrogen from the hydride is provided by waste heat from the engine. Refueling is achieved by admitt~g pressurized hydrogen and removing the heat generated. Current refueling techniques can deliver an 85% charge in 10 min. The unique feature of the hydride system is the equilibrium existing between the hydrogen and the metal On cooling of the system, hydrogen is reabsorbed into the metal, in contrast with the two other systems in which hydrogen, once liberated, remains in the gas phase. 2.2. Energy considerations A recurring energy cost for each fuel system is the energy used in filling and emptying the fuel tank. As is common practice, we shall assume that the hydrogen is gaseous at a pressure of 690 kPa (i.e. hydrogen generated by electrolysis). For liquid hydrogen the overriding energy cost is the energy used in liquefaction. For a refrigeration cycle which is 33% efficient the energy for liquefaction is estimated as 10 kW h kg-l, i.e. 25% of the heat available from combustion of the hydrogen [4] . The recurring energy cost for microspheres is at the refilling stage where the gas must be compressed and the entire system heated to 300 “C:The energy required for heating and compression is estimated to be 3% of the energy available during combustion, (This may be a low estimate because extensive conservation measures were assumed to be employed at the refilling plant.)
304
The work required to pressurize hydrogen for convenient hydriding is the main recurring energy cost for metal hydrides. An estimate of 1.4% for the recurring energy cost has been made by Walters [ 51. The overall energy efficiency of the three fuel storage systems must include the energy cost of storage, of retrieval and of transportation of the fuel. In our calculation of vehicle efficiency we ignore the energy cost of transporting the hydrogen to the user but include the cost of transporting the fuel within the vehicle. Thus the energy cost of storage and retrieval in a microsphere or metal hydride system is negligible whereas for liquid hydrogen the cost of liquefaction is at least 25% of the heat of combustion of the hydrogen. McAlevy f6] has developed a ma~ematic~ model relating various automobile parameters to fuel efficiency for alternative fuels. We used this model to calculate the efficiencies for equivalent hydrogen-fueled automobiles. (Equivalent automobiles have similar performance, integrity, range etc.) The efficiency of the vehicle can be expressed by the parameter WPL/C (km kg W-” h-l), which is the distance a payload is moved per unit of energy. The overall efficiency is greatly dependent on the mass of the vehicle. For vehicles with metal hydride or microsphere fuel storage an increase in the designed range of the system will cause a large increase in the mass and thus a sharp decrease in efficiency. For a liquid hydrogen fuel tank, which has a high energy density, there is relatively little loss in performance as the range of the vehicle is increased. A comparison of efficiency and range is presented in Fig. 2. The energy cost of the liquefaction of hydrogen is included in Fig. 2 assuming an efficiency of 33% [4] in the refrigeration cycle. This correlates to 25% of the heating value of hydrogen. It is apparent from Fig. 2 that for vehicles designed for short-range use the metal hydrides are more energy efficient than liquid hydrogen. The microsphere system described by Teitel and coworkers [ 1,2] has approximately the same weight percentage of hydrogen (3.0%) as does Mg,NiH* (3.16%) and will have the same efficiency according to this model 141. Figure 2 does not include the ~por~tion energy cost. For both liquid hydrogen and microspheres the hydrogen is transported to the distribution center by surface ~~spor~tion. Liquid hydrogen will be more efficient because its gravimetric energy density (141 kJ g-l) is higher than that of the microspheres (8 kJ g-l) [‘7]. (These values do not include the mass of either the Dewar flask or the vessel for containing the microspheres. The additional mass of the container will be dependent on the scale of the delivery vehicle.) The actual effect of transportation costs on overall energy efficiency will also be dependent on the location of production plants relative to consumers. No attempt was made to include these costs. It is clear from the figure that liquid hydrogen vehicles have an energy advantage in long-range vehicles. However, hydride or microsphere storage is more efficient in short-range vehicles. We next address the dollar cost of the fuel tank itself. For liquid hydrogen it is the cost of a Dewar flask. For an automobile system the cost has
305
100
zoo
300
400
500
Range. km
Fig. 2. Plots of vehicle efficiency (km kg W -’ h-l) as a function of the range of the vehicle, after equations due to McAlevy [ 61.
been estimated [8] to be $1250 (1978$) for a production run of 10’ vessels. The automobile Dewar flask designed for DFVLR is a double-walled aluminum vessel with a capacity of 150 1 and weighing 43 kg. The evacuated space between the walls is filled with a multilayer aluminum foil and fiberglass superinsulation. Teitel et al. [l] have estimated costs for an Mg,Nil&-FeTiHa system at $1984 and for the microsphere system at $837. These estimates are for total systems and include metal hydride, tank, supporting structure etc. 2.3. Vehicle operation All of the hydrogen-powers vehicles show superior specific power to battery systems and have equal or greater specific energy than projected battery systems (Fig. 3). However, power and energy are only a few of the criteria on which acceptability is based. The liquid hydrogen fuel tank is the only system that is currently capable of all-weather general purpose operation without supplemental energy for start-up. Since the microsphere system uses a hydride to provide fuel for start-up, the microsphere vehicle and hydride vehicle will have the same initial operating requirements. The hydride FeTiHa exhibits a double plateau in its pressure-composition isotherm [9] which complicates the application of the hydride to a fuel storage system. An internal combustion engine requires the hydrogen to be delivered at a pressure greater than atmospheric. At -8 “C! the pressure of hydrogen over Fe-Ti alloy exceeds atmospheric for hy~ogen-to-Mets
306
10,000
Li FeTiH.
n
MQH.
snd microsphere
I
10
loo Specific
looo
Energy- Wh/&o
Fig. 3. A comparison of the projected specific power and the specific energy for various storage devices.
ratios (H/M) greater than 0.1; when the temperature drops below -8 “C the pressure is below atmospheric for H/M values of less than 0.5. Therefore if ambient temperatures are below -8 “C the vehicle will not self-start when the tank is less than half-full of hydrogen. The lower temperature limit for a full tank of Fe-Ti hydride is -18 “CLAs manganese replaces iron in the alloy the lower temperature limits are raised. Several options have been suggested to overcome the cold weather starting problem. Supplement heating from a battery has been tried, but the battery performance also decreases with temperature and the system does not appear to be practical. A second option under consideration is to use a less stable hydride to provide hydrogen at low ambient temperatures. This low temperature hydride would be recharged from the main tank. Recharging requires energy input because hydrogen will be transferred from a more stable hydride to a less stable hydride. Recharging of the low temperature hydride will be required after a few starts to avoid excessive weight being added to the systern via the less stable hydride and its tankage. Any hydride capable of releasing hydrogen at a low ambient temperature may cause problems at high ambient temperatures. For such an unstable hydride either a heavy tbickwalled pressure vessel is needed to contain the free hydrogen at high ambient temperatures (up to 50 “C) or the hydrogen must be piped to the main hydride tank where the hydrogen will be absorbed. When the second alternative is used the system will suffer decreased cold weather starting capacity after each warm spell of the winter if the vehicle is not used in the interim.
307
A unique limitation of the microsphere system is that a driving time of 25 mm [l] is required to heat the microspheres to operating temperature. Considering the long warm-up time, the microsphere concept may not be viable for vehicles where the primary operating mode consists of intermittent short trips. In Table 1 we present results from a survey of automobile usage. This survey indicates that a large portion of American driving consists of trips of shorter duration than the time required for the microspheres to reach operating temperature. When the microspheres are below their operating temperature the hydrogen supply will come from the hydride bed; however, the hydride bed is designed primarily for receiving hydrogen during microsphere cool-down and not to act as the major hydrogen supply source. A number of short intermittent trips could totally deplete the hydride supply while still leaving a sizable fraction of hydrogen in the micr~pheres. To avoid being a vehicle of very limited application, the microsphere vehicle must demonstrate the capability ta meet the diverse short-range driving needs. Refueling procedures for liquid hydrogen and metal hydride storage systems are comparable. For liquid hydrogen, Peschka and Carpetis [3] have reported a semiautoma~d service station design that fills a tank in 5 min with no exposure of the cryogenic liquid to air. At Brookhaven National Laboratories a recharging system for the Fe-Ti hydride tank has been developed that provides an 85% recharge in 10 min [ 121. The refueling of the microsphere tank involves replacement of the microspheres and then recharging (if necessary) of the hydride bed. Estimates of the time required for the refueling process have not yet been made. 2.4. Capital costs In addition to the cost of the vehicle there are capital costs for building the facilities required to process the hydrogen for storage and for establishing an adequate fuel distribution network. Liquid hydrogen would need an estimated capital ~vestment of $48 X 10’ (1974$) to build sufficient plants to provide liquid hydrogen for all the vehicles in the United States
TABLE 1 Mileage of personal automobile travel
Commutes
All trips
(min)
(S)
(miles)
(%)
(miles)
(%)
<15 15-29 30-44 >45
38.4 29.8 15.5 16.3
30
13.4 23.1 20.5 30.2 12.8
<5 6 - 10 11-20 >21
62.4 16.8 12.5 8.3
Data taken from refs. 10 and 11.
308
[ 131, The cost of a delivery system for liquid hydrogen is estimated as $60 X 10’ for 300 000 service stations plus another $2 X 10’ for trucks to transport liquid hydrogen to the service stations from the liquefaction plants. A similar analysis has not been performed on microspheres or hydrides. Filling stations would either be constructed and/or modified to dispense hydrogen; however, this should not cost nearly as much as the $105 (1974$) required for a liquid hydrogen Dewar flask at each service station. The major capital cost for a hydride system would be a pipeline for delivery to the service stations. The capital cost for the microsphere system would be for plants to process and fill the spheres and for the acquisition of sufficient surface ~~spo~tion vehicles to supply all of the service stations with microspheres. 2.5. Safety The metal hydrides and the microspheres provide significant safety advantages over liquid hydrogen systems. All systems have had safety aspects taken into consideration during the design process and the automotive systems are claimed to have minimal risk to the automobile driver and passengers. The main safety threat is the release and burning of hydrogen following the rupture of a tank. For an automobile fuel tank there would be very small amounts of hydrogen released from a microsphere or metal hydride system. In both of these cases the normal operating pressure of the tank is low so there is little free hydrogen present. In an accident few microspheres would be expected to rupture [l] so only the void hydrogen would be liberated immediately. However, if the fuel tank is at operating temperature (200 “C+the hot spheres could escape and cause severe burns to accident victims who come in contact with them, In addition hydrogen would slowly permeate the spheres providing a supply of hydrogen until the spheres cooled. In a hydride system the metal hydride would liberate free hydrogen after a tank rupture at a rate dependent on the hydride bed temperature, the ambient temperature and the rate of heat transfer to or from the hydride bed. In a safety test of a hydride system, incendiary shells were fired into a hydride tank with only a brief flame ensuing [ 141. Pulsed hydride bed heating has been suggested by Buchner [15] at Daimler-Benz to maintain a nearly constant moderate temperature and pressure in the fuel tank. This approach provides an added safety feature by keeping the hydrogen pressure near 200 kPa. Much work has been done to study the safety of liquid hydrogen. Tests and models have shown that the small amount of liquid hydrogen in an automotive fuel tank (100 l), if released, would dissipate rapidly and present a hazard for only a few minutes. The safety of the automotive application of liquid hydrogen will be greatly dependent on the crashworthiness of the automotive Dewar flask. Peschka and Carpetis [3] have demonstrated with the DFVLR II Dewar flask that loss of the vacuum seal in the Dewar flask does not lead to a dangerous pressure build-up, There are no test results in the literature on the c~hwo~h~ess of liquid hydrogen fuel tanks. There is, however, a report of a trailered vehicle overturning. In this accident there was very little damage and no fire was reported.
309
A large safety problem presented by the liquid hydrogen system is in the transportation of the fuel to the distribution centers. This would involve quantities in the range of 50 000 1 for a tank truck to 946 000 1 for a barge [ 161. Edeskuty and Sheheen [ 171 in a study on liquid hydrogen spills concluded that the time required for a spill to evaporate completely is dependent on the depth of the pool formed and only indirectly on the amount of liquid hydrogen spilled. For an average pool depth of 150 mm, the time calculated until total boil-off had occurred was 875 s. However, it should be noted that the depth of a liquid hydrogen pool will be dependent on the terrain and the amount spilled. Modeling and testing is currently under way to determine how a very large liquid hydrogen spill will behave [ 181. In particular the researchers are concerned with modeling a possible hydrogen fireball that could be formed following a large spill of liquid hydrogen. 2.5. Ou Hook None of the three alternatives can provide all of the desirable features of today’s family car without further breakthroughs in technology. For cars used in urban driving, hydrogen stored in microspheres and/or hydrides are favored over liquid hydrogen for both vehicle efficiency and safety reasons. For fleets of trucks and air or rail applications, liquid hydrogen is favored because the energy stored per unit weight is higher. Also safety could be assured more effectively because the vehicles would be operated and maintained by commercial firms. It is clear that widespread use of hydrogen in vehicles must await more cost-effective hydrogen production technology and a new infrastructure for distributing the hydrogen from production plants.
3. Catalytic ignition system At the Institute of Gas Technology (IGT) a metal hydride storage device with a catalyst (prophets platinum fo~ulation) has been developed to provide an ignition source for gas burners [ 191. The hydride (Fe-T& 1 kg) is stored in a cannister and the hydrogen is released to the catalyst when ignition is desired. The ignition system has a lifetime of 80 000 ignitions (per hydrogen charge) and the catalyst has a lifetime of 1 X lo6 ignitions. A schematic diagram of this system is shown in Fig. 4. The ignition system is capable of auto-ignition in less than 1 s and consumes 0.35 standard cm3 of hydrogen per ignition. The principle operating drawback of this system is a tendency for the catalyst to deactivate in a few days when left dormant. Deactivation is worse in the presence of high humidity. The IGT invention is facing stiff competition from electronic ignition devices which are less expensive. However, only the hydride device will work during a power outage, which suggests its possible use as an ignition device for emergency power systems.
310
Gas
--_)
Calalyot
(Pi)
Hydrogen
COlIclUll
--______---------________
Fig. 4. A schematic diagram of a stationary catalytic ignition system developed by the IGT.
4. Peak shaving Storing of electrical energy produced during off-peak hours for use during peak demand hours is called peak shaving and hydrogen is one possible means of achieving this. An engineering model of a peak-shaving hydride storage vessel was constructed in 1976 [20]. The vessel would be used in a system that generates hydrogen electrolytically using off-peak power and then compresses the hydrogen from 0.5 to 3.5 MPa and stores it in an Fe--% bed. During hours of peak electrical cons~ption the hydrogen would be released and passed through a fuel cell to generate electricity. The system, which contains 400 kg of Fe-Ti alloy, is capable of storing 6.4 kg of hydrogen and discharging it at a maximum sustained rate of 1.3 (kg H,) h-l. One utility company has had a continuous interest in installing a megawatt-size system of this type. An artist’s conception of the system is shown in Fig. 5. Large-scale alternatives to storing hydrogen in metal hydrides for peak shaving are to store it as a compressed gas in constructed tanks and underground caverns. Hydrogen can be stored underground in aquifers, depleted petroleum reservoirs or solution-mined salt caverns, as is currently being done with natural gas. No large technical problems are anticipated in switching from natural gas to hydrogen aside from those posed by the hydrogen emb~t~ement of socks steels.
311
Fig. 5. An artist’s conception
of a peak-shaving system utilizing metal hydrides.
One major capital expense for underground storage will be the increase in the amount of cushion gas that must be left in the storage facility. For a facility with a working capacity of 8.4 X 1016 J a cushion of hydrogen equivalent to 10 X 1015 J would be required [ 51 to maintain minimum withdrawal rates. Walters [ 51 has estimated the capital cost for underground storage at $3 - $6 per 10’ J and operating costs of $1 - $3 per 10’ J. These values assume operating pressures of ‘7 - 21 MPa. Foh et al. [21] in a more recent economic assessment of und~~ound hydrogen storage have arrived at slightly higher estimates. Their estimates for the cost of service for hydrogen storage range from $2.4 to $10 per 10’ J depending on the site chosen (see Fig. 6). In their analysis it was concluded that storing hydrogen underground was as economic as storing natural gas underground. Unfortunately most electric utilities are not currently in favor of hydrogen storage systems for peak shaving. The only utilities showing interest are those combined electric and gas utilities that would be able to utilize hydrogen as a supplement to natural gas. The general prognosis is that hydrogen storage for utility peak-shaving applications may not be competitive with compressed air, pumped hydro and battery storage. Furthermore various options for hydrogen gas storage look more attractive than storage as a hydride.
312 :OST iERVl i/10’
-
1 --
SALT CAVERN
__-_ .e6
5.66
EXCAVATED CAVERN
-
AQUIFER
I -
4.40
DEPLETED PETROLEUM FIELD
Fig. 6. The mean cost f- - -) and range of cost of service for underground storage, in dollars per 10’ joules [ 2 J .
hydrogen
5. Hydride engines, compressors and pumps The first hydride compressor known to us was built in 1971 for laboratory use and is still in working order [ 22] . The origimd system used VHs with a source at 50 “C and a sink at 18 “C. Subsequently van MaI [23] has bui.It a compressor using LaNisH, as the working hydride. (Almost any hydride may be used provided that the soume and sink temperatures can be selected approp~a~ly.) The La&H, compressor is capable of compressing hydrogen from 0.2 to 15 MPa using a soume at 197 “c and a sink at 25 “C. If the use of hydrogen storage becomes widespread there wiII be a great need for compressed hydrogen for ah forms of storage. The metal hydride compressor could fiII this need and could utilize low grade heat, waste heat or solar heat for the energy input. Operating on similar principles to the compressors are hydride engines; instead of releasing the hydrogen at a higher pressure from the warm hydride, the evolved hydrogen is used to do work. Proposals for hydride engines have used the expanding hydrogen to compress a bladder containing water, forcing the water out and thus functioning as a water pump. When the hydride cooIs, the hydrogen is reabsorbed and the water returns to the bIadder. A schematic diagram of this hydride pump appears in Fig. 7. It is estimated that such a device could pump water to a head of 92 m. The amount of water moved per cycle wiII also be dependent on the amount of hydride used in the pump. For large capacity pumps the prime requisite is an inexpensive hydride.
313
Fig. 7. A schematic diagram of a hydride pump (see text for description). An experimental pump [ 24 ] , using LaN&,H,, was capable of pumping water to a head of 9 m with a 3 mm cycle. The cycle time increased as the operating pressure increased. A hydride engine built using an LaNis _ XAl, hydride had a theoretical efficiency of 5.3% and an actual efficiency that varied between 1.6% and 2.4% depending on the maximum pressure of the hydrogen [ 24 1. Efficiency differences from ideality were assessed, and it was projected by Heckes et al. [24] that such an engine could compare favorably with a Rankine cycle engine when both engines used a low temperature source (80 “C) (Table 2). A source temperature of 80 “C could easily be achieved with a solar thermal collector. A design for a power generator using metal hydrides has been proposed by Terry and Schoeppel [ 25). They suggested using the desorbed hydrogen to turn a turbine linked to an electrical generator. The same idea of using the expanding hydrogen to drive a turbine has been proposed on a much smaller scale by Gruen et af. [ 261. Terry and Schoeppel[25] have suggested utilizing spent steam from a nuclear reactor as the energy source. The expanding hydrogen, liberated by the heat, is forced through a turbine connected to a generator and then into a second hydride bed. Heating of the second hydride returns the hydrogen to the original bed. Four beds operate out of phase to maintain continuous power generation.
6. Neutron generators One app~cation of metal hydrides not involving energy storage in the usual sense is the neutron generator. Neutron generators are used in weapons
314 TABLE 2 Comparison of engine efficiencies for operation between 80 and 20 “C Cycle
Theoretical efficiency
Projecfed actual eficiciency
Carnot Rankine, complete expansion Hydride, complete expansion
17 15 14.1
8 10.5
Data taken from ref. 25.
systems and are under development for cancer therapy [ 271. A schematic diagram for a neutron generator designed for cancer therapy is shown in Fig. 8. Two methods for producing neutrons are from the reaction between tritium and deuterium (T(d,n)He reaction) and from the reaction between deuterium and deuterium (D(d,n)He reaction). The basic scheme is to bombard a target with a beam of either deuterium or tritium. The beam consists of atomic and molecular monocations (e.g. I>+, D2+, Ds+) which react with the deuterium or tritium in the target to produce neutrons. The problem is to find a stable compact source of tritium or deuterium. One type of target material that has been successfully used is metal deuterides or tritides. The target materials must be stable under the conditions of the generator. When under ion bombardment the target reaches temperatures of up to 450 “C fn uacuo. These conditions are quite different from those found in storage systems.
Fig. 8. A schematic diagram of a neutron generator proposed for use in cancer therapy.
315
Two metals that have formed successful target hydrides are scandium and erbium. The target is assembled by vapor depositing thin films (10 50 I;lrnthick) of metal (scandium or erbium) onto a copper plate fitted with cooling tubes. The target metal is then hydrided with the desired hydrogen isotopes. To maintain a steady rate of reaction in a T(d,n)He reaction both the beam and the target will contain equal amounts of deuterium and tritium. As the target isotopes react, beam materisl will react with the target material to maintain a constant hydrogen isotope concentration in the target. Currently, target lifetimes are limited by the sputtering of the thin target film and by the d~placement of deuterium and tritium in the target by impurities in the ion beam. A neutron production of 6 X lOlo neutrons s-l has been achieved in a system using the D(d,n)He reaction and a scandium target 10 Mm thick. With a T(d,n)He reaction this would scale up to I X 1013 neutrons s- l, which is suitable for cancer therapy. By increasing the target thickness to 50 pm a target lifetime of 75 h is expected [27]. 7. Metal hydride heat pumps Chemical heat pumps use a thermochemical process to effect the transfer of heat across a thermal gradient. Two separate chemical reactions involving one common reactant, the movement of which accomplishes the heat transfer, are the basis of a chemical heat pump. A chemical heat pump which uses pairs of dissimilar metal hydrides is under development at Argonne National Laboratory [ 26 J . In the heat pump operating cycle there is a low temperature hydride which absorbs heat from a low temperature heat source, e.g. the air. This heat absorption causes the low temperature hydride to dissociate. The hydrogen then flows to a metal that forms a more stable hydride, giving off heat in the process. The high temperature hydride is then heated and the hydrogen flows back to the original metal, releasing heat. A schematic diagram is shown in Fig. 9. The coefficient of performance COPn for heating for the hydride heat pump does not explicitly contain the operating ~rn~~~s of the system; it is determined by the enthalpies of the reactions in the heat pump and is independent of the ambient temperature. Gruen et al. [28] have shown that the temperatures at which a metal hydride heat pump can operate are determined by the enthalpy and entropy of the pair of hydrides chosen. For mechanical heat pumps utilizing a Camot cycle, the COPn is given by the well-known equation COPn = Z’n/(Tn - 7’~) where TH is the source temperature and TL is the sink temperature. The COP, is totally a function of temperatures and decreases as Tn - TL increases. A typical mechanical heat pump efficiency (actual) is shown in Fig. 10 together with theoretical COPns for a metal hydride heat pump and for a chemical heat pump using sulfuric acid and water. The efficiency for a mechanical heat pump can be seen to drop off quickly below -2 “C.;
316
Fig. 9. A schematic diagram of a hydride heat pump, shown in the cooling cycle.
L -40
-30
-20
-10
0
IO
20
Sink Tetnpsntum ‘c.
Fig. 10. The operating efficiencies of three types of heat pumps.
Not all chemical heat pumps display a decline in performance with decreasing temperature, at least theoretically; therefore chemical heat pumps can compete favorably with mechanical heat pumps in the lower temperature region, although they will be less efficient in the higher temperature region. The operating temperatures of a metal hydride heat pump are determined by the enthalpies AH, of the pair of hydrides chosen for the system. One of the great assets of the hydride chemical heat pump is the large range of enthalpies for different metal hydrides. The large number of hydrides
317
allows the system to be selected to meet the heating and cooling needs of the user. Furthermore, the enthalpies of ternary alloys can be varied over a small range by varying the composition of the alloy, allowing for a fine tuning of the heat pump. This is the approach taken by Gruen et al. [ 281 in their hydride heat pump system. Currently CaNiB and LaNis _ XAl, alloys are being considered. By slight changes in the aluminum-to-nickel ratio the enthalpy is varied to provide a system customized for a given climate. The theoretical COPn is about 1.5, (The actual COPn depends on the exact hydride pair chosen.) The operating COPn of the metal hydride heat pump is currently being investigated. For a mechanical heat pump the COPn is less than the theoretical limit because the operating COPn must be calculated from refrigerant temperatures and not source-sink temperatures. This constraint serves to reduce the COPn from the theoretical limit by nearly a factor of 2. The same increase in AT is involved with chemical heat pumps; however, instead of affecting the efficiency (which is temperature independent) the lower ambient temperature limit for a given ‘hydride pair is raised. The COPn s of chemical heat pumps under investigation are shown in Table 3. Although non-hydride chemical heat pumps have similar efficiencies, they may be less costly to m~ufact~e. Also they have larger internal thermal storage capacities than hydride heat pumps. For all the chemical heat pumps a solar thermal collector can be used as the high temperature energy source. The non-hydride systems, because of their large-scale storage capabilities, can be run on a diurnal cycle in which they can almost exclusively utilize a solar source. The metal hydride heat pump, which has been designed to function more as a heat pump than as a storage system, operates on a very short cycle to minimize the amount of hydride necessary. As it runs intermittently, the heat pump cannot utilize exclusively solar thermal energy without an additional storage facility but it benefits from a large COPn when using non-solar or stored solar power. The current l~i~tions on the operation of a hydride heat pump are the uncertainty in the long-term stability of metal alloys in the presence of hydrogen, the uncertainty in the actual coefficient of performance for the TABLE 3 Chemical heat pumps and their COPHS
Heat pump
COPH
Metal hydride
Cl.8 1.5 - 1.6
CaC12 - CHsOH HzS04 - Hz0
1.2 * 1.7
313
system on heating (cooling) and the effect heat exchangers have in altering the operating temperatures of a metal hydride heat pump. All calculations in the literature have been carried out ignoring the temperature gradient across the heat exchanger. If the heat exchange temperature gradient is significant, the lower ambient air temperature limit will be raised by at least the amount of the gradient. Current designs for a chemical heat pump utilizing metal hydrides have not been oriented towards exploiting one of the advantages of the metal hydrides, i.e. the constant COPn at low temperature. Development of an efficient low temperature (-23 oC,5-10 OF)heat pump would provide considerable energy savings. One possible low temperature chemical heat pump is the sulfuric acid-water heat pump. The HzS04-Ha0 system has a theoretical COPn of 1.2 at -40 “C &hen used with a source at 210 “C 1291. Thus once again the commercial use of hydrides in heat pumps faces stiff competition from other chemical and mechanical heat pumps. 8. Concluding remarks
From the preceding sections it is possible to make some judgments on the competition between hydrides and other methods for energy storage and conversion. It appears that relatively unstable hydrides provide an attractive way to provide star&up energy for hydrogen-fueled vehicles. However, glass microspheres and liquid hydrogen look more attractive for the main storage tank. Neutron generators appear to provide a unique oppo~i~ for compounds of hydrogen isotopes, with erbium and scandium deuterides and tritides showing the most potential. The use of hydrides in hydrogen compressors and engines is promising; however, critical comparisons with more conventional approaches have not been made. The same prognosis applies to hydride heat pumps; they offer more versatility than present heat pumps, and possibly other advantages, but additional research and development is needed to show whether their advantages outweigh disadvantages that have been identified. Finally, hydrides for storing large quantities of off-peak electrical energy and hydrides for catalytic ignition of natural gas burners do not seem to be as attractive as the competing ideas. Acknowledgments
The authors wish to thank the following individuals and organizations who provided information for possible use in this paper: Jon Pangbom, Institute of Gas Technology; James Reilly and Gerald Second, Brookbaven National Laboratories; Dieter Gruen, Argonne National Laboratory; Frank Bacon and Clyde Northrup, Sandia Laboratories; Gary D. Sandrock, Intemational Nickel Company; F. J. Edeskuty, Los Alamos Scientific Laboratory; Charles E. Lundin, Denver Research Institute.
319
References 1 R. J. Teitel, T. M. Henderson and J. E. Luderer, Microcavity systems for automotive applications. In Proc. DOE Chemical/Hydrogen Energy Systems Contractor Rev., CONF 781142, November 27 - 30, 1978, U. S. Department of Energy, Washington, D.C., 1979, p. 289. 2 R. J. Teitel, T. M. Henderson, J. E. Luderer and J. Powers, Microcavity systems for automotive applications, Brookhaven National Laboratory Final Prog Rep., Novem-
ber 1978. 3 W. Peschka and C. Carpetis, Proc. 14th Intersociety Energy Conversion Engineering Conf., Boston, M~ach~etts, 1979, American Chemical Society, Washington, D.C., 1979, p. 763. 4 J. J. Thibault, The cryogenic storage of hydrogen. In T. N. Veziroglu and W. Seifritz (eds.), Hydrogen Energy Systems, Proc. 2nd World Hydrogen Energy Conf, Zurich, August 21 - 24.1978, Pergamon, Oxford, 1978, pp. 25 - 54. 5 A. B. Walters, Technical and environmental aspects of underground hydrogen storage, unpublished work, 1975. 6 R. F. McAlevy III, Prediction of vehicle performance and its sensitivity to component improvements: application to electric and hydrogen fueled vehicles. In Proc. 14th
Intersociety 7
8
9 10
Energy Conversion Engineering Conf, Boston, Massachusetts,
1979,
American Chemical Society, Washington, D.C., 1979, p. 337. R. J. Teitel, Development status of microcavity hydrogen storage systems for automotive applications. In Proc. DOE Chemical Energy Storage and Hydrogen Energy Systems Contracts Rev., November 1979, Draft, p_ 39. W. J. D. Escher and E. E. Ecklund, Survey of liquid hydrogen container technology for highway vehicle fuel system applications. In Froc. 14th Znte~ocie~ Energy Conversion Engineering Conf., Boston, Massachusetts, 1979, American Chemical Society, Washington, D.C., 1979, p. 790. J. J. Reilly and R. H. Wiswall, Jr.,Znorg. Chem., 13 (1974) 218. R. H. Asin, Nationwide personal transportation study - purposes of automobile trips and travel, U.S. Department of Transportation, Fedeml Highway Administration, Rep. 10, Washington, D. C., May 1974,~. 16.
11 Annual Housing Survey 1974, United States and Regions, Part A - General Housing Characteristics, U.S. Department of Commerce and US. Department of Housing and Urban Development, Washington, D.C., August 1976, p. 6. 12 E. Behrin, C. J. Anderson, H. Bomelberg, M. Farahat, H. C. Forsberg, C. L. Hudson, B. C. Kullman, L. G. O’Connell, G. Strickland and W. J. Walsh, Energy storage systems for automobile propulsion: 1978 study, UCRL-52553, Lawrence Livermore Laboratory, Livermore, California, December 15,1978. 13 W. F. Stewart and F. J. Edeskuty, Logistics, economics and safety of a liquid hydrogen system for automotive transportation, presented at intersociety Conf on Tmnsport&ion, Denver, Colomdo, September 23 - 27. 1973, ASME 73.XT-78 American Society of Mechanical Engineers, New York, 1973, p. 78. 14 R. L. Woolley, Performance of a hydrogen powered transit vehicle. In Proc. 11th Zntersociety Energy Conversion Engineering Conf, Stateline, Nevada, American Institute of Chemical Engineers, New York, 1976, p. 992. 15 H. Buchner, Metal hydride energy concept. In Energy Storage and Transportation: Prospects for New Technologies, Ispra, October 22 - 26, 1979, Commission of the European Communities Joint Research Centre, Ispra. 16 F. J. Edeskuty (ed.), Critical review and assessment of problems in hydrogen energy delivery systems, Los AZamos Scientific Labomtory, LA-7505-PR, 1977, pp. 31 - 37. 17 F. J. Edeskuty and N. N. Sheheen (eds.), Critical review and assessment of environmental and safety problems in hydrogen energy systems, Los AZamos Scientific Laboratory, LA-7820-PR, 1979, pp. 35 - 36.
320 18 H. C. Hardee and D. W. Larson, Thermal hazard from hydrogen fireballs. In Critical 19 20 21
22
23 24
review and assessment of environmental and safety problems in hydrogen energy systems, Los Alamos Scientific Laboratory, LA-7820-PR, 1979, Appendix 3-A. U.S. Patent 4,021,183 (May 3,1977), to J. C. Gillis and N. Holcombe. J. J. Reilly, in K. E. Cox and K. D. Williams, Jr. (eds.), Hydrogen: Its Technology and Implications, Vol. 2, Chemical Rubber Co., Cleveland, 1977, p. 13. S. M. Foh, M. Novil, P. L. Randolph and E. M. Rockar, Underground storage of hydrogen, Proc. DOE Chemical Energy Storage and Hydrogen Energy Systems Contracts Rev., November 13 and 14, 1979, Draft, p, 43. J. J. Reilly, A. Holtz and R. H. Wiswall, Jr., Rev. Sci. Instrum., 42 (1971) 1485. tf. H. van Mal, Chem. Zng. Tech., 45 (1973) 80. A. A. Heckes, T. E. Hinkebein and C. J. Northrup, Hydride engines. In Proc. 14th
Znte~ociety 25 26
27 28
29
Energy Con~rsion
engineering Conf., Boston, ~~~chusetts,
1979,
American Chemical Society, Washington, D.C., 1979, p. 743. U.S. Patent 3,943,719 (March 16,1976), to L. E. Terry and R. J. Sehoeppel. D. M. Gruen, R. L. McBeth, M. Mendelsohn, J. M. Nixon, F. Schreiner and I. Sheft, HYCSOS: a solar heating cooling and energy conversion system based on metal hydrides. In Proc. 11 th intersociety Energy Conversion Engineering Conf., September 1976, American Institute of Chemical Engineers, New York, p. 681. F. M. Bacon and A. A. Reidel, ZEEE Trans. Nucl. Sci., 26 (1979) 1505. D. M. Gruen, I. Sheft, G. Lamich and M. Mendelsohn, HYCSOS: a chemical heat pump and energy conversion system based on metal hydrides, Argonne National Labomtories, ANL-79-8, April 1979. E. C. Clark, Sulfuric acid and water chemical heat pump/chemical energy storage program, Rocket Research Corp., RRC-78-R-595, 1978, pp. 3 - 21.
Journal of the Lea-Common Metals, 74 (1980) 301 - 320 0 Elsevier Sequoia &A., Lausanne - Printed in the Netherlands
HYDRIDES VERSUS COMPETING OPTIONS FOR STORING GEN IN ENERGY SYSTEMS*
HYDRO-
JAMES H. SWISHER
Division of Thermal and Mechanical Energy Stomge, U.S. Department Washington, D. C. 20585 (U.S.A.)
of Energy,
EDWARD D. JOHNSON
OAO Corporation, 2101 L Street, N. W., W~hi~ton,
D. C. 20037 (U.S.A.)
Summary Metal hydrides present many possibilities for energy storage because of their ability to store hydrogen safely and at modest pressures. We compared hydrogen storage in metal hydrides with other forms of hydrogen storage. Comparisons were made using technical performance, cost and safety as the criteria. Hydride applications and alternatives for vehicular applications, chemical heat pumps, engines, peak shaving and others are discussed.
1. Introduction Hydrides are certainly interesting materials for scientific research, as evidenced by the number and variety of papers at this symposium. It should also be realized that hydrides are potentially useful materials for energy storage, gas separation processes and special applications. There is of course competition between hydride devices and other means of providing an engineering function in all the areas where hydrides may be used. The obvious example is in storage applications because methods are available for storing hydrogen in either solid, liquid or gaseous form. From the point of view of an engineer or an energy program administrator, there is a desire to provide stimulation for the research community to focus its research on materials and devices which could lead to commercial applications. With this in mind, an attempt is made here to show where and why hydrides look attractive in comparison with competing options for engineering applications. The criteria used are technical performance, cost and safety characteristics. Whilst no attempt is made to analyze all possible applications for hydrides, a number of the more likely possibilities are discussed.
*Paper presented at the International Symp~ium on the properties and Application of Metal Hydrides, Colorado Springs, Colorado, U.S.A., April 7 - 11,198O.
302
2. Vehicular applications
2.1. Overview One of the larger efforts involving metal hydrides for energy storage is the hydrogen vehicle. The requirements for the fuel storage system of the hydrogen vehicle are (1) a minimum delivery rate of 1.25 kg h-l to meet the suburban driving cycle defined by the Society of Automotive Engineers [ 11, (2) a storage volume which can be accommodated in a car, (3) sufficient energy density (energy per mass) to provide acceptable performance and range, (4) a high degree of safety during use and refilling and (5) a reasonable refill tune. In addition the system should be as self-contained as possible, e.g. no external energy sources should be required for start-up, and the energy requirements for hydrogen retrieval should be as small as possible (or altematively use waste heat from the engine). Three systems in contention for a hydrogen vehicle fuel storage system are metal hydrides, glass microspheres and liquid hydrogen. We will start with the newest idea first, the glass microsphere. Glass microspheres are small glass spheres 6 - 60 E.trnin diameter made from fly ash (Fig. 1). The microspheres are structurally strong and after an initial screening are expected to undergo numerous refilling operations with minimal breakage. As the filling and subsequent containment of hydrogen in the microspheres is a physical process the system is not affected by low levels of impurities. This is an advantage over hydrides where small levels of impurities cannot be tolerated. During filling, the glass spheres are heated in a high pressure hydrogen atmosphere. The hydrogen permeates the glass, the system is then cooled and the residual hydrogen is pumped off. Since the hydrogen is trapped within the spheres, the spheres can be removed from the charging vessel. The decrease in permeability of the glass as the temperature decreases allows the glass spheres to maintain a high internal hydrogen pressure (25 MPa). To extract the hydrogen the glass spheres are warmed to an operating temperature of 200 “c and the hydrogen diffuses out of the spheres. When the engine is shut off, the microsphere bed slowly releases hydrogen until the bed has completely cooled down. The hydrogen released during the cool-down period is absorbed into a smaller metal hydride bed.
ONE lxkAN DIAMETER
Fig. 1. A photograph of small glass microspheres that can be used for hydrogen storage.
303
The metal hydride then provides the hydrogen required for vehicle start-up. This hybrid system, developed by Teitel et al. [2], is called the microcavity hydrogen storage system. The refueling scenario for the microspheres would involve the complete exchange of the microspheres in the fuel tank (or an exchange of fuel tanks). The microspheres would then be transported and recharged with hydrogen at a central processing plant. Turning now to liquid hydrogen storage, several liquid hydrogen vehicle prototypes have been built. All the vehicles have been experimental and little work has been done to optimize fuel tank design. A liquid hydrogen vehicle has a large metal Dewar flask for a fuel tank. The Dewar flask is necessary to provide the insulation required to maintain the low temperature of liquid hydrogen. A semiautomated delivery system has been built for the Institute for Technical Physics of the Deutsche Forschungs- und Versuchsanstalt fur Luft- und Raumfahrt e.V. (DFVLR) [3]. This automated delivery system will fill a 120 1 tank in 5 min. The semiautomated system has several checks to prevent exposure of personnel to the cryogenic liquid. All liquid hydrogen fuel systems deliver the fuel into the combustion chamber in the gaseous state. Some prototype vehicles use electrical resistance heaters to vaporize the liquid hydrogen in the tank, whilst others use liquid withdrawal of the hydrogen followed by vaporization with waste heat from the engine. A third approach is to store hydrogen in metal hydrides. The heat required to liberate the hydrogen from the hydride is provided by waste heat from the engine. Refueling is achieved by admitt~g pressurized hydrogen and removing the heat generated. Current refueling techniques can deliver an 85% charge in 10 min. The unique feature of the hydride system is the equilibrium existing between the hydrogen and the metal On cooling of the system, hydrogen is reabsorbed into the metal, in contrast with the two other systems in which hydrogen, once liberated, remains in the gas phase. 2.2. Energy considerations A recurring energy cost for each fuel system is the energy used in filling and emptying the fuel tank. As is common practice, we shall assume that the hydrogen is gaseous at a pressure of 690 kPa (i.e. hydrogen generated by electrolysis). For liquid hydrogen the overriding energy cost is the energy used in liquefaction. For a refrigeration cycle which is 33% efficient the energy for liquefaction is estimated as 10 kW h kg-l, i.e. 25% of the heat available from combustion of the hydrogen [4] . The recurring energy cost for microspheres is at the refilling stage where the gas must be compressed and the entire system heated to 300 “C:The energy required for heating and compression is estimated to be 3% of the energy available during combustion, (This may be a low estimate because extensive conservation measures were assumed to be employed at the refilling plant.)
304
The work required to pressurize hydrogen for convenient hydriding is the main recurring energy cost for metal hydrides. An estimate of 1.4% for the recurring energy cost has been made by Walters [ 51. The overall energy efficiency of the three fuel storage systems must include the energy cost of storage, of retrieval and of transportation of the fuel. In our calculation of vehicle efficiency we ignore the energy cost of transporting the hydrogen to the user but include the cost of transporting the fuel within the vehicle. Thus the energy cost of storage and retrieval in a microsphere or metal hydride system is negligible whereas for liquid hydrogen the cost of liquefaction is at least 25% of the heat of combustion of the hydrogen. McAlevy f6] has developed a ma~ematic~ model relating various automobile parameters to fuel efficiency for alternative fuels. We used this model to calculate the efficiencies for equivalent hydrogen-fueled automobiles. (Equivalent automobiles have similar performance, integrity, range etc.) The efficiency of the vehicle can be expressed by the parameter WPL/C (km kg W-” h-l), which is the distance a payload is moved per unit of energy. The overall efficiency is greatly dependent on the mass of the vehicle. For vehicles with metal hydride or microsphere fuel storage an increase in the designed range of the system will cause a large increase in the mass and thus a sharp decrease in efficiency. For a liquid hydrogen fuel tank, which has a high energy density, there is relatively little loss in performance as the range of the vehicle is increased. A comparison of efficiency and range is presented in Fig. 2. The energy cost of the liquefaction of hydrogen is included in Fig. 2 assuming an efficiency of 33% [4] in the refrigeration cycle. This correlates to 25% of the heating value of hydrogen. It is apparent from Fig. 2 that for vehicles designed for short-range use the metal hydrides are more energy efficient than liquid hydrogen. The microsphere system described by Teitel and coworkers [ 1,2] has approximately the same weight percentage of hydrogen (3.0%) as does Mg,NiH* (3.16%) and will have the same efficiency according to this model 141. Figure 2 does not include the ~por~tion energy cost. For both liquid hydrogen and microspheres the hydrogen is transported to the distribution center by surface ~~spor~tion. Liquid hydrogen will be more efficient because its gravimetric energy density (141 kJ g-l) is higher than that of the microspheres (8 kJ g-l) [‘7]. (These values do not include the mass of either the Dewar flask or the vessel for containing the microspheres. The additional mass of the container will be dependent on the scale of the delivery vehicle.) The actual effect of transportation costs on overall energy efficiency will also be dependent on the location of production plants relative to consumers. No attempt was made to include these costs. It is clear from the figure that liquid hydrogen vehicles have an energy advantage in long-range vehicles. However, hydride or microsphere storage is more efficient in short-range vehicles. We next address the dollar cost of the fuel tank itself. For liquid hydrogen it is the cost of a Dewar flask. For an automobile system the cost has
305
100
zoo
300
400
500
Range. km
Fig. 2. Plots of vehicle efficiency (km kg W -’ h-l) as a function of the range of the vehicle, after equations due to McAlevy [ 61.
been estimated [8] to be $1250 (1978$) for a production run of 10’ vessels. The automobile Dewar flask designed for DFVLR is a double-walled aluminum vessel with a capacity of 150 1 and weighing 43 kg. The evacuated space between the walls is filled with a multilayer aluminum foil and fiberglass superinsulation. Teitel et al. [l] have estimated costs for an Mg,Nil&-FeTiHa system at $1984 and for the microsphere system at $837. These estimates are for total systems and include metal hydride, tank, supporting structure etc. 2.3. Vehicle operation All of the hydrogen-powers vehicles show superior specific power to battery systems and have equal or greater specific energy than projected battery systems (Fig. 3). However, power and energy are only a few of the criteria on which acceptability is based. The liquid hydrogen fuel tank is the only system that is currently capable of all-weather general purpose operation without supplemental energy for start-up. Since the microsphere system uses a hydride to provide fuel for start-up, the microsphere vehicle and hydride vehicle will have the same initial operating requirements. The hydride FeTiHa exhibits a double plateau in its pressure-composition isotherm [9] which complicates the application of the hydride to a fuel storage system. An internal combustion engine requires the hydrogen to be delivered at a pressure greater than atmospheric. At -8 “C! the pressure of hydrogen over Fe-Ti alloy exceeds atmospheric for hy~ogen-to-Mets
306
10,000
Li FeTiH.
n
MQH.
snd microsphere
I
10
loo Specific
looo
Energy- Wh/&o
Fig. 3. A comparison of the projected specific power and the specific energy for various storage devices.
ratios (H/M) greater than 0.1; when the temperature drops below -8 “C the pressure is below atmospheric for H/M values of less than 0.5. Therefore if ambient temperatures are below -8 “C the vehicle will not self-start when the tank is less than half-full of hydrogen. The lower temperature limit for a full tank of Fe-Ti hydride is -18 “CLAs manganese replaces iron in the alloy the lower temperature limits are raised. Several options have been suggested to overcome the cold weather starting problem. Supplement heating from a battery has been tried, but the battery performance also decreases with temperature and the system does not appear to be practical. A second option under consideration is to use a less stable hydride to provide hydrogen at low ambient temperatures. This low temperature hydride would be recharged from the main tank. Recharging requires energy input because hydrogen will be transferred from a more stable hydride to a less stable hydride. Recharging of the low temperature hydride will be required after a few starts to avoid excessive weight being added to the systern via the less stable hydride and its tankage. Any hydride capable of releasing hydrogen at a low ambient temperature may cause problems at high ambient temperatures. For such an unstable hydride either a heavy tbickwalled pressure vessel is needed to contain the free hydrogen at high ambient temperatures (up to 50 “C) or the hydrogen must be piped to the main hydride tank where the hydrogen will be absorbed. When the second alternative is used the system will suffer decreased cold weather starting capacity after each warm spell of the winter if the vehicle is not used in the interim.
307
A unique limitation of the microsphere system is that a driving time of 25 mm [l] is required to heat the microspheres to operating temperature. Considering the long warm-up time, the microsphere concept may not be viable for vehicles where the primary operating mode consists of intermittent short trips. In Table 1 we present results from a survey of automobile usage. This survey indicates that a large portion of American driving consists of trips of shorter duration than the time required for the microspheres to reach operating temperature. When the microspheres are below their operating temperature the hydrogen supply will come from the hydride bed; however, the hydride bed is designed primarily for receiving hydrogen during microsphere cool-down and not to act as the major hydrogen supply source. A number of short intermittent trips could totally deplete the hydride supply while still leaving a sizable fraction of hydrogen in the micr~pheres. To avoid being a vehicle of very limited application, the microsphere vehicle must demonstrate the capability ta meet the diverse short-range driving needs. Refueling procedures for liquid hydrogen and metal hydride storage systems are comparable. For liquid hydrogen, Peschka and Carpetis [3] have reported a semiautoma~d service station design that fills a tank in 5 min with no exposure of the cryogenic liquid to air. At Brookhaven National Laboratories a recharging system for the Fe-Ti hydride tank has been developed that provides an 85% recharge in 10 min [ 121. The refueling of the microsphere tank involves replacement of the microspheres and then recharging (if necessary) of the hydride bed. Estimates of the time required for the refueling process have not yet been made. 2.4. Capital costs In addition to the cost of the vehicle there are capital costs for building the facilities required to process the hydrogen for storage and for establishing an adequate fuel distribution network. Liquid hydrogen would need an estimated capital ~vestment of $48 X 10’ (1974$) to build sufficient plants to provide liquid hydrogen for all the vehicles in the United States
TABLE 1 Mileage of personal automobile travel
Commutes
All trips
(min)
(S)
(miles)
(%)
(miles)
(%)
<15 15-29 30-44 >45
38.4 29.8 15.5 16.3
13.4 23.1 20.5 30.2 12.8
<5 6 - 10 11-20 >21
62.4 16.8 12.5 8.3
Data taken from refs. 10 and 11.
308
[ 131, The cost of a delivery system for liquid hydrogen is estimated as $60 X 10’ for 300 000 service stations plus another $2 X 10’ for trucks to transport liquid hydrogen to the service stations from the liquefaction plants. A similar analysis has not been performed on microspheres or hydrides. Filling stations would either be constructed and/or modified to dispense hydrogen; however, this should not cost nearly as much as the $105 (1974$) required for a liquid hydrogen Dewar flask at each service station. The major capital cost for a hydride system would be a pipeline for delivery to the service stations. The capital cost for the microsphere system would be for plants to process and fill the spheres and for the acquisition of sufficient surface ~~spo~tion vehicles to supply all of the service stations with microspheres. 2.5. Safety The metal hydrides and the microspheres provide significant safety advantages over liquid hydrogen systems. All systems have had safety aspects taken into consideration during the design process and the automotive systems are claimed to have minimal risk to the automobile driver and passengers. The main safety threat is the release and burning of hydrogen following the rupture of a tank. For an automobile fuel tank there would be very small amounts of hydrogen released from a microsphere or metal hydride system. In both of these cases the normal operating pressure of the tank is low so there is little free hydrogen present. In an accident few microspheres would be expected to rupture [l] so only the void hydrogen would be liberated immediately. However, if the fuel tank is at operating temperature (200 “C+the hot spheres could escape and cause severe burns to accident victims who come in contact with them, In addition hydrogen would slowly permeate the spheres providing a supply of hydrogen until the spheres cooled. In a hydride system the metal hydride would liberate free hydrogen after a tank rupture at a rate dependent on the hydride bed temperature, the ambient temperature and the rate of heat transfer to or from the hydride bed. In a safety test of a hydride system, incendiary shells were fired into a hydride tank with only a brief flame ensuing [ 141. Pulsed hydride bed heating has been suggested by Buchner [15] at Daimler-Benz to maintain a nearly constant moderate temperature and pressure in the fuel tank. This approach provides an added safety feature by keeping the hydrogen pressure near 200 kPa. Much work has been done to study the safety of liquid hydrogen. Tests and models have shown that the small amount of liquid hydrogen in an automotive fuel tank (100 l), if released, would dissipate rapidly and present a hazard for only a few minutes. The safety of the automotive application of liquid hydrogen will be greatly dependent on the crashworthiness of the automotive Dewar flask. Peschka and Carpetis [3] have demonstrated with the DFVLR II Dewar flask that loss of the vacuum seal in the Dewar flask does not lead to a dangerous pressure build-up, There are no test results in the literature on the c~hwo~h~ess of liquid hydrogen fuel tanks. There is, however, a report of a trailered vehicle overturning. In this accident there was very little damage and no fire was reported.
309
A large safety problem presented by the liquid hydrogen system is in the transportation of the fuel to the distribution centers. This would involve quantities in the range of 50 000 1 for a tank truck to 946 000 1 for a barge [ 161. Edeskuty and Sheheen [ 171 in a study on liquid hydrogen spills concluded that the time required for a spill to evaporate completely is dependent on the depth of the pool formed and only indirectly on the amount of liquid hydrogen spilled. For an average pool depth of 150 mm, the time calculated until total boil-off had occurred was 875 s. However, it should be noted that the depth of a liquid hydrogen pool will be dependent on the terrain and the amount spilled. Modeling and testing is currently under way to determine how a very large liquid hydrogen spill will behave [ 181. In particular the researchers are concerned with modeling a possible hydrogen fireball that could be formed following a large spill of liquid hydrogen. 2.5. Ou Hook None of the three alternatives can provide all of the desirable features of today’s family car without further breakthroughs in technology. For cars used in urban driving, hydrogen stored in microspheres and/or hydrides are favored over liquid hydrogen for both vehicle efficiency and safety reasons. For fleets of trucks and air or rail applications, liquid hydrogen is favored because the energy stored per unit weight is higher. Also safety could be assured more effectively because the vehicles would be operated and maintained by commercial firms. It is clear that widespread use of hydrogen in vehicles must await more cost-effective hydrogen production technology and a new infrastructure for distributing the hydrogen from production plants.
3. Catalytic ignition system At the Institute of Gas Technology (IGT) a metal hydride storage device with a catalyst (prophets platinum fo~ulation) has been developed to provide an ignition source for gas burners [ 191. The hydride (Fe-T& 1 kg) is stored in a cannister and the hydrogen is released to the catalyst when ignition is desired. The ignition system has a lifetime of 80 000 ignitions (per hydrogen charge) and the catalyst has a lifetime of 1 X lo6 ignitions. A schematic diagram of this system is shown in Fig. 4. The ignition system is capable of auto-ignition in less than 1 s and consumes 0.35 standard cm3 of hydrogen per ignition. The principle operating drawback of this system is a tendency for the catalyst to deactivate in a few days when left dormant. Deactivation is worse in the presence of high humidity. The IGT invention is facing stiff competition from electronic ignition devices which are less expensive. However, only the hydride device will work during a power outage, which suggests its possible use as an ignition device for emergency power systems.
310
Gas
--_)
Calalyot
(Pi)
Hydrogen
COlIclUll
--______---------________
Fig. 4. A schematic diagram of a stationary catalytic ignition system developed by the IGT.
4. Peak shaving Storing of electrical energy produced during off-peak hours for use during peak demand hours is called peak shaving and hydrogen is one possible means of achieving this. An engineering model of a peak-shaving hydride storage vessel was constructed in 1976 [20]. The vessel would be used in a system that generates hydrogen electrolytically using off-peak power and then compresses the hydrogen from 0.5 to 3.5 MPa and stores it in an Fe--% bed. During hours of peak electrical cons~ption the hydrogen would be released and passed through a fuel cell to generate electricity. The system, which contains 400 kg of Fe-Ti alloy, is capable of storing 6.4 kg of hydrogen and discharging it at a maximum sustained rate of 1.3 (kg H,) h-l. One utility company has had a continuous interest in installing a megawatt-size system of this type. An artist’s conception of the system is shown in Fig. 5. Large-scale alternatives to storing hydrogen in metal hydrides for peak shaving are to store it as a compressed gas in constructed tanks and underground caverns. Hydrogen can be stored underground in aquifers, depleted petroleum reservoirs or solution-mined salt caverns, as is currently being done with natural gas. No large technical problems are anticipated in switching from natural gas to hydrogen aside from those posed by the hydrogen emb~t~ement of socks steels.
311
Fig. 5. An artist’s conception
of a peak-shaving system utilizing metal hydrides.
One major capital expense for underground storage will be the increase in the amount of cushion gas that must be left in the storage facility. For a facility with a working capacity of 8.4 X 1016 J a cushion of hydrogen equivalent to 10 X 1015 J would be required [ 51 to maintain minimum withdrawal rates. Walters [ 51 has estimated the capital cost for underground storage at $3 - $6 per 10’ J and operating costs of $1 - $3 per 10’ J. These values assume operating pressures of ‘7 - 21 MPa. Foh et al. [21] in a more recent economic assessment of und~~ound hydrogen storage have arrived at slightly higher estimates. Their estimates for the cost of service for hydrogen storage range from $2.4 to $10 per 10’ J depending on the site chosen (see Fig. 6). In their analysis it was concluded that storing hydrogen underground was as economic as storing natural gas underground. Unfortunately most electric utilities are not currently in favor of hydrogen storage systems for peak shaving. The only utilities showing interest are those combined electric and gas utilities that would be able to utilize hydrogen as a supplement to natural gas. The general prognosis is that hydrogen storage for utility peak-shaving applications may not be competitive with compressed air, pumped hydro and battery storage. Furthermore various options for hydrogen gas storage look more attractive than storage as a hydride.
312 :OST iERVl i/10’
-
1 --
SALT CAVERN
__-_ .e6
5.66
EXCAVATED CAVERN
-
AQUIFER
I -
4.40
DEPLETED PETROLEUM FIELD
Fig. 6. The mean cost f- - -) and range of cost of service for underground storage, in dollars per 10’ joules [ 2 J .
hydrogen
5. Hydride engines, compressors and pumps The first hydride compressor known to us was built in 1971 for laboratory use and is still in working order [ 22] . The origimd system used VHs with a source at 50 “C and a sink at 18 “C. Subsequently van MaI [23] has bui.It a compressor using LaNisH, as the working hydride. (Almost any hydride may be used provided that the soume and sink temperatures can be selected approp~a~ly.) The La&H, compressor is capable of compressing hydrogen from 0.2 to 15 MPa using a soume at 197 “c and a sink at 25 “C. If the use of hydrogen storage becomes widespread there wiII be a great need for compressed hydrogen for ah forms of storage. The metal hydride compressor could fiII this need and could utilize low grade heat, waste heat or solar heat for the energy input. Operating on similar principles to the compressors are hydride engines; instead of releasing the hydrogen at a higher pressure from the warm hydride, the evolved hydrogen is used to do work. Proposals for hydride engines have used the expanding hydrogen to compress a bladder containing water, forcing the water out and thus functioning as a water pump. When the hydride cooIs, the hydrogen is reabsorbed and the water returns to the bIadder. A schematic diagram of this hydride pump appears in Fig. 7. It is estimated that such a device could pump water to a head of 92 m. The amount of water moved per cycle wiII also be dependent on the amount of hydride used in the pump. For large capacity pumps the prime requisite is an inexpensive hydride.
313
Fig. 7. A schematic diagram of a hydride pump (see text for description). An experimental pump [ 24 ] , using LaN&,H,, was capable of pumping water to a head of 9 m with a 3 mm cycle. The cycle time increased as the operating pressure increased. A hydride engine built using an LaNis _ XAl, hydride had a theoretical efficiency of 5.3% and an actual efficiency that varied between 1.6% and 2.4% depending on the maximum pressure of the hydrogen [ 24 1. Efficiency differences from ideality were assessed, and it was projected by Heckes et al. [24] that such an engine could compare favorably with a Rankine cycle engine when both engines used a low temperature source (80 “C) (Table 2). A source temperature of 80 “C could easily be achieved with a solar thermal collector. A design for a power generator using metal hydrides has been proposed by Terry and Schoeppel [ 25). They suggested using the desorbed hydrogen to turn a turbine linked to an electrical generator. The same idea of using the expanding hydrogen to drive a turbine has been proposed on a much smaller scale by Gruen et af. [ 261. Terry and Schoeppel[25] have suggested utilizing spent steam from a nuclear reactor as the energy source. The expanding hydrogen, liberated by the heat, is forced through a turbine connected to a generator and then into a second hydride bed. Heating of the second hydride returns the hydrogen to the original bed. Four beds operate out of phase to maintain continuous power generation.
6. Neutron generators One app~cation of metal hydrides not involving energy storage in the usual sense is the neutron generator. Neutron generators are used in weapons
314 TABLE 2 Comparison of engine efficiencies for operation between 80 and 20 “C Cycle
Theoretical efficiency
Projecfed actual eficiciency
Carnot Rankine, complete expansion Hydride, complete expansion
17 15 14.1
8 10.5
Data taken from ref. 25.
systems and are under development for cancer therapy [ 271. A schematic diagram for a neutron generator designed for cancer therapy is shown in Fig. 8. Two methods for producing neutrons are from the reaction between tritium and deuterium (T(d,n)He reaction) and from the reaction between deuterium and deuterium (D(d,n)He reaction). The basic scheme is to bombard a target with a beam of either deuterium or tritium. The beam consists of atomic and molecular monocations (e.g. I>+, D2+, Ds+) which react with the deuterium or tritium in the target to produce neutrons. The problem is to find a stable compact source of tritium or deuterium. One type of target material that has been successfully used is metal deuterides or tritides. The target materials must be stable under the conditions of the generator. When under ion bombardment the target reaches temperatures of up to 450 “C fn uacuo. These conditions are quite different from those found in storage systems.
Fig. 8. A schematic diagram of a neutron generator proposed for use in cancer therapy.
315
Two metals that have formed successful target hydrides are scandium and erbium. The target is assembled by vapor depositing thin films (10 50 I;lrnthick) of metal (scandium or erbium) onto a copper plate fitted with cooling tubes. The target metal is then hydrided with the desired hydrogen isotopes. To maintain a steady rate of reaction in a T(d,n)He reaction both the beam and the target will contain equal amounts of deuterium and tritium. As the target isotopes react, beam materisl will react with the target material to maintain a constant hydrogen isotope concentration in the target. Currently, target lifetimes are limited by the sputtering of the thin target film and by the d~placement of deuterium and tritium in the target by impurities in the ion beam. A neutron production of 6 X lOlo neutrons s-l has been achieved in a system using the D(d,n)He reaction and a scandium target 10 Mm thick. With a T(d,n)He reaction this would scale up to I X 1013 neutrons s- l, which is suitable for cancer therapy. By increasing the target thickness to 50 pm a target lifetime of 75 h is expected [27]. 7. Metal hydride heat pumps Chemical heat pumps use a thermochemical process to effect the transfer of heat across a thermal gradient. Two separate chemical reactions involving one common reactant, the movement of which accomplishes the heat transfer, are the basis of a chemical heat pump. A chemical heat pump which uses pairs of dissimilar metal hydrides is under development at Argonne National Laboratory [ 26 J . In the heat pump operating cycle there is a low temperature hydride which absorbs heat from a low temperature heat source, e.g. the air. This heat absorption causes the low temperature hydride to dissociate. The hydrogen then flows to a metal that forms a more stable hydride, giving off heat in the process. The high temperature hydride is then heated and the hydrogen flows back to the original metal, releasing heat. A schematic diagram is shown in Fig. 9. The coefficient of performance COPn for heating for the hydride heat pump does not explicitly contain the operating ~rn~~~s of the system; it is determined by the enthalpies of the reactions in the heat pump and is independent of the ambient temperature. Gruen et al. [28] have shown that the temperatures at which a metal hydride heat pump can operate are determined by the enthalpy and entropy of the pair of hydrides chosen. For mechanical heat pumps utilizing a Camot cycle, the COPn is given by the well-known equation COPn = Z’n/(Tn - 7’~) where TH is the source temperature and TL is the sink temperature. The COP, is totally a function of temperatures and decreases as Tn - TL increases. A typical mechanical heat pump efficiency (actual) is shown in Fig. 10 together with theoretical COPns for a metal hydride heat pump and for a chemical heat pump using sulfuric acid and water. The efficiency for a mechanical heat pump can be seen to drop off quickly below -2 “C.;
316
Fig. 9. A schematic diagram of a hydride heat pump, shown in the cooling cycle.
L -40
-30
-20
-10
0
IO
20
Sink Tetnpsntum ‘c.
Fig. 10. The operating efficiencies of three types of heat pumps.
Not all chemical heat pumps display a decline in performance with decreasing temperature, at least theoretically; therefore chemical heat pumps can compete favorably with mechanical heat pumps in the lower temperature region, although they will be less efficient in the higher temperature region. The operating temperatures of a metal hydride heat pump are determined by the enthalpies AH, of the pair of hydrides chosen for the system. One of the great assets of the hydride chemical heat pump is the large range of enthalpies for different metal hydrides. The large number of hydrides
317
allows the system to be selected to meet the heating and cooling needs of the user. Furthermore, the enthalpies of ternary alloys can be varied over a small range by varying the composition of the alloy, allowing for a fine tuning of the heat pump. This is the approach taken by Gruen et al. [ 281 in their hydride heat pump system. Currently CaNiB and LaNis _ XAl, alloys are being considered. By slight changes in the aluminum-to-nickel ratio the enthalpy is varied to provide a system customized for a given climate. The theoretical COPn is about 1.5, (The actual COPn depends on the exact hydride pair chosen.) The operating COPn of the metal hydride heat pump is currently being investigated. For a mechanical heat pump the COPn is less than the theoretical limit because the operating COPn must be calculated from refrigerant temperatures and not source-sink temperatures. This constraint serves to reduce the COPn from the theoretical limit by nearly a factor of 2. The same increase in AT is involved with chemical heat pumps; however, instead of affecting the efficiency (which is temperature independent) the lower ambient temperature limit for a given ‘hydride pair is raised. The COPn s of chemical heat pumps under investigation are shown in Table 3. Although non-hydride chemical heat pumps have similar efficiencies, they may be less costly to m~ufact~e. Also they have larger internal thermal storage capacities than hydride heat pumps. For all the chemical heat pumps a solar thermal collector can be used as the high temperature energy source. The non-hydride systems, because of their large-scale storage capabilities, can be run on a diurnal cycle in which they can almost exclusively utilize a solar source. The metal hydride heat pump, which has been designed to function more as a heat pump than as a storage system, operates on a very short cycle to minimize the amount of hydride necessary. As it runs intermittently, the heat pump cannot utilize exclusively solar thermal energy without an additional storage facility but it benefits from a large COPn when using non-solar or stored solar power. The current l~i~tions on the operation of a hydride heat pump are the uncertainty in the long-term stability of metal alloys in the presence of hydrogen, the uncertainty in the actual coefficient of performance for the TABLE 3 Chemical heat pumps and their COPHS
Heat pump
COPH
Metal hydride
Cl.8 1.5 - 1.6
CaC12 - CHsOH HzS04 - Hz0
1.2 * 1.7
313
system on heating (cooling) and the effect heat exchangers have in altering the operating temperatures of a metal hydride heat pump. All calculations in the literature have been carried out ignoring the temperature gradient across the heat exchanger. If the heat exchange temperature gradient is significant, the lower ambient air temperature limit will be raised by at least the amount of the gradient. Current designs for a chemical heat pump utilizing metal hydrides have not been oriented towards exploiting one of the advantages of the metal hydrides, i.e. the constant COPn at low temperature. Development of an efficient low temperature (-23 oC,5-10 OF)heat pump would provide considerable energy savings. One possible low temperature chemical heat pump is the sulfuric acid-water heat pump. The HzS04-Ha0 system has a theoretical COPn of 1.2 at -40 “C &hen used with a source at 210 “C 1291. Thus once again the commercial use of hydrides in heat pumps faces stiff competition from other chemical and mechanical heat pumps. 8. Concluding remarks
From the preceding sections it is possible to make some judgments on the competition between hydrides and other methods for energy storage and conversion. It appears that relatively unstable hydrides provide an attractive way to provide star&up energy for hydrogen-fueled vehicles. However, glass microspheres and liquid hydrogen look more attractive for the main storage tank. Neutron generators appear to provide a unique oppo~i~ for compounds of hydrogen isotopes, with erbium and scandium deuterides and tritides showing the most potential. The use of hydrides in hydrogen compressors and engines is promising; however, critical comparisons with more conventional approaches have not been made. The same prognosis applies to hydride heat pumps; they offer more versatility than present heat pumps, and possibly other advantages, but additional research and development is needed to show whether their advantages outweigh disadvantages that have been identified. Finally, hydrides for storing large quantities of off-peak electrical energy and hydrides for catalytic ignition of natural gas burners do not seem to be as attractive as the competing ideas. Acknowledgments
The authors wish to thank the following individuals and organizations who provided information for possible use in this paper: Jon Pangbom, Institute of Gas Technology; James Reilly and Gerald Second, Brookbaven National Laboratories; Dieter Gruen, Argonne National Laboratory; Frank Bacon and Clyde Northrup, Sandia Laboratories; Gary D. Sandrock, Intemational Nickel Company; F. J. Edeskuty, Los Alamos Scientific Laboratory; Charles E. Lundin, Denver Research Institute.
319
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