BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

This invention relates to improvements in hydride heat pumps, and the like. More particularly, this invention relates to improvements in hydride heat pumps for use in refrigera¬ tion, ice making, air conditioning and space heating or cooling applications and methods for using an improved hydride heat pump.

2. RELEVANT BACKGROUND A technical paper on compressor-driven metal hydride heat pumps was presented by Saul Wolf of the US Navy ( Hydrogen Sponge Heat Pump," Intersociety Energy Conversion Engineering Conference, Paper 759196, p. 1348, 1975) and a patent on the concept was granted to Andrew W. McClaine (US Patent 4,039,023) and assigned to the Navy. These publications disclose certain system configurations that include hydride reactors and heat exchangers for a compressor driven hydride heat pump. These are, however, not regarded as ideal for hydride heat pumps. For example. Wolf describes a system with pumped fluid heat transfer through hydride reactors with concentric jacket type heat exchangers. And McClaine describes a system with hydride containers that contain an internal "core" where the hydride is contained. The cores are shell- and-tube heat exchangers for pumped liquid heat transfer. Such shell-and-tube heat exchangers with pumped fluids are not particularly suitable for hydride heat pumps because they have too much mass (from the container, the tubes, and the liquid) . The large mass creates excessive parasitic heat losses associated with heating and cooling this mass. This in turn, reduces the efficiency of the system so much that such systems are impractical for most purposes. Also, parasitic energy is required for pumping the heat transfer liquid, which reduces the system efficiency even further.

In 1993, Alfred E. Ritter (US Patent 4,413,670) described a hydride heat pump that is driven either by a reversible "mechanical pressure varying means" or by "thermally-driven pressure variation." Ritter describes both coil type heat exchangers using pumped liquids and two heat pipe diode heat exchangers to transfer heat into and out of the hydride vessels. Like McClaine (US Patent 4,039,023) the liquid cooled coil type heat exchangers of Ritter have too much mass for most practical hydride heat pumps applications. Also, the use of two or more heat pipes per vessel adds to the mass.

Moshe Ron (US Patent 4,507,263) describes a process for compressing and sintering a mixture of metal hydride particles with aluminum powder in what is called a porous "powder metal hydride" compact (PMH compact) . This aluminum/hydride compact provides improved conduction heat transfer while allowing sufficient porosity for hydrogen flow. While Ron's PMH compact has substantially improved heat transfer, the compact requires a difficult manufacturing process in its construction.

W. E. Wallace (US Patent 4,928,496), described a hydride reactor in which copper or aluminum discs are force fit inside a reactor vessel with hydride particles sandwiched between to improve heat transfer. Such discs are perforated to allow hydrogen gas to pass with low pressure drop. While this approach is better than a plain hydride bed, it still suffers from large temperature drops within the hydride bed.

A process for copper coating metal hydride particles has been developed to improve electrical conductivity in hydride battery electrodes by Park (C. N. Park, Journal of Alloys and Compounds, 182, pp 321-330, 1992) . Park described an electro- less plating process that employs copper sulfate in a sulfuric acid bath to copper plate hydride particles. This process is one method of coating metal hydride particles.

Accordingly, there is a need for an improved heat pump that is not only environmentally clean, but also efficient and economical.

SUMMARY OF THE INVENTION

In light of the above, it is, therefore, an object of the invention to provide an improved hydride heat pump.

It is another object of the invention to provide an hydride heat pump of the type described that can provide cost effective refrigeration, ice making, air conditioning, space heating, water heating, and similar benefits.

It is another object of the invention to provide an heat pump that does not use environmentally damaging refrigerants or other environmentally damaging constituents.

It is another object of the invention to provide an improved heat pump that can be more efficient than conven¬ tional compressor-driven refrigeration process that use CFC, HFC, or HCFC refrigerants. It is another object of the invention to provide an improved heat pump with at least two hydride reactors with an internal structure of high thermal conductivity that enhances internal heat conduction in the hydride bed and prevents migration of hydride particles. It is another object of the invention to provide an improved heat pump that can use an automatic 4-way (or other suitable multiported) conventional valving system for regula¬ tion of hydrogen gas flow.

It is another object of the invention to provide an improved heat pump with an efficient, oil-less hydrogen compressor.

It is another object of the invention to provide an improved heat pump with automatic dampers to direct air flow alternately over the hot and cold reactors for either space cooling or heating.

It is another object of the invention to provide an improved heat pump with internal convection heat transfer by hydrogen gas flowing through a porous metal hydride bed to fixed external cold and hot heat exchangers.

It is another object of the invention to provide an improved heat pump with internal heat transfer provided by a liquid/gas mixture flowing through a porous hydride bed where the liquid is a volatile liquid that evaporates in the one reactor and condenses in the other.

It is another object of the invention to provide an improved heat pump in which internal heat transfer is provided by circulation of a slurry mixture of liquid/gas/solid particles through finned reactor tubes, where the liquid is a volatile liquid refrigerant and its vapor that evaporates in the one reactor and condenses in the other, the gas is hydrogen, and the solid is small hydride particles that desorb hydrogen in the one reactor and absorb hydrogen in the other.

These and other objects, features, and advantages of the invention will be apparent to those skilled in the art from the following detailed description of the invention, when read in conjunction with the accompanying drawings and appended claims.

The invention provides an improved hydride heat pump. The heat pump has an effectively controlled compressor driven system having at least two hydride reactors, each with enhanced internal heat transfer. The heat pump is environmen¬ tally safe and cost-effective, and can meet a worldwide need for refrigeration, ice making, air conditioning, space heating, water heating, and similar benefits.

Thus, according to a broad aspect of the invention, an improved hydride heat pump is provided that has at least two reactors. Each reactor includes a low mass vessel, which contains particles of a metal hydride that are contained in a number of porous structures, which have a high thermal conductivity, such as encapsulating shells, containers, or foams of a material having a high thermal conductivity, such as copper, aluminum, graphite, or the like. A hydrogen gas is contained within the two reactors, and a gas moving mechanism moves the hydrogen automatically back and forth from one of

the reactors to the other. The hydride may be formed into a bed, which may also include an internal heat transfer enhanc¬ ing structure, such as graphite fibers or the like. The hydride particles and graphite fibers may be mixed with a binding metal which is compressed and sintered into a porous powder metal hydride compact. BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated in the accompanying draw¬ ings, wherein: Figure 1 is a schematic cross-sectional view of a packaged hydride heat pump system, in accordance with a preferred embodiment of the invention.

Figures 2a and 2b are schematic plan views of an auto¬ matic damper and fan system in respective operative positions to serve as an air conditioner or heat pump, using the hydride heat exchanging system of Figure 1.

Figure 3a is a schematic cross-sectional view of a hy¬ dride refrigerator, in accordance with another preferred embodiment of the invention. Figures 3b and 3c are van't Hoff diagrams for the respective reactors of the hydride refrigerator of Figure 3a. Figure 4 is a schematic cross-sectional view of a hydride ice maker, in accordance with still another preferred embodi¬ ment of the invention. Figures 5 and 6 are schematic drawings, partly in cross- section, of a split-system hydride air conditioner, heat pump or refrigerator, in accordance with another preferred embodi¬ ment of the invention, showing the paths of hydrogen flow in each direction between the reactors. Figure 7 is a schematic cross-sectional view of a hydride air conditioner, heat pump or refrigerator, in accordance with another preferred embodiment of the invention, in which internal heat transfer is enhanced by the flow of a mixture of hydrogen gas and liquid refrigerant.

Figure 8 is a schematic cross-sectional view of the device of Figure 7 in reverse mode of operation.

Figure 9 is a schematic cross-sectional side elevation view of another embodiment of a heat transferring system according to the invention, in which a slurry of fine hydride particles, hydrogen gas, and a liquid refrigerant are circu¬ lated.

Figure 10 is a schematic side elevation view of another heat transferring system according to the invention in which a slurry of fine hydride particles, hydrogen gas, and a liquid refrigerant are circulated through two finned tube reactors.

Figure 11 is a magnified cross sectional view of hydride particles that have been encapsulated within a thin layer of high thermal conductivity material, compressed, and sintered into a porous powder metal hydride compact, according to a preferred embodiment of the invention.

Figure 12 is a magnified cross-sectional isometric view of a high thermal conductivity porous foam with interconnected pores containing hydride particles, according to a preferred embodiment of the invention.

Figure 13 is a cross-sectional plan view of a cylindrical hydride reactor with internal PMH compact discs and external radial fins, together with an enlarged perspective view of one of the PMH compact disk with a central hole and radial grooves for hydrogen flow.

Figure 14 is a perspective view of a PMH compact disk with a central hole and radial grooves for hydrogen flow, partially cut away to show internal imbedded fibers of a high thermal conductivity material. In the various drawings, like reference numerals are used to denote like or similar parts. Additionally, the drawings are not necessarily drawn to scale.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to a preferred embodiment of the invention, I have discovered that hydride heat pumps can be designed to achieve maximum efficiency when minimum mass hydrides and hydride reactors are used. Moreover, the efficiency may be further improved when the hydride reactors have enhanced heat transfer both within the hydride reactor and between the hydride reactor and the surroundings. Furthermore, the minimization or elimination of parasitic energy for pumping heat transfer fluids is also desirable. The present invention illustrates all of these desirable features.

Certain metal alloys absorb large amounts of hydrogen gas and have been used for hydrogen storage. When hydrogen is absorbed by such a "hydride forming metal alloy", it becomes a hydride, an exothermic process occurs, and heat is liber¬ ated. This heat may be used for space heating, water heating, or other useful heating application. Alternately, when hydrogen gas is desorbed from a hydride, an endothermic process occurs, providing significant cooling. This cooling effect can be used to produce refrigeration for a refrigera¬ tor, ice maker, air conditioner, or heat pump. A heat pump provides space heating in the winter and cooling during the summer. A heat pump can also be configured to heat domestic hot water while providing space cooling. Metal hydrides begin as metal alloys or "hydride forming metals" with good thermal conductivity, but after they have been "activated" by absorbing hydrogen, they become fine particles, eventually a few microns in diameter, with poor thermal conductivity, some for example, having only about 10% of the thermal conductivity of aluminum. The finest particles tend to migrate and plug small orifices and reduce the sealing ability of valves, which may be used in the system. A bed of such fine hydride particles with low thermal conductivity has poor heat transfer and consequently has a low efficiency in a hydride heat pump application.

A hydride heat pump has two or more reactors, each preferably filled with the same metal hydride, and connected by piping to a compressor, or other gas moving mechanism. Each reactor has a vessel to contain the hydride material and hydrogen gas, plus a mechanism such as conductive fins, or the like, to transfer heat to or from the surroundings. Also a mechanism to enhance internal heat transfer within the rector may be provided, such as fibers or metallic parts having a high thermal conductivity. In a two reactor system, initially, one reactor is fully saturated with hydrogen while the other is unsaturated. The first reactor is cooled when hydrogen is drawn from it. As the withdrawn hydrogen is compressed into the second, the second reactor becomes warm. The cold and heat may be used for spare cooling or heating. The process is then reversed. The hydride heat pump does not require environmentally damaging chloro- fluorocarbon (CFC) or HCFC refrigerants and is inherently clean with virtually no adverse environmental impact. The system is also simple, compact, reliable, highly efficient, and economical. Such a cost effective and environmentally clean heat pump system may be used for compressor driven refrigerators, freezers, air conditioners, ice makers, heat pumps, and other similar applications.

With reference now to the drawings, and particularly to Figure 1, a hydride air conditioner or heat pump 10, in accordance with a preferred embodiment of the invention, has two hydride reactors 31 and 32. (Although it will be under¬ stood that the device can be used either as an air conditioner or a heat pump, the device 10 will be referred to herein merely as a "heat pump".) The reactors 31 and 32 may be substantially identical, as shown. The reactors 31 and 32 serve as containers for a metal hydride which undergoes absorption and desorption reactions with hydrogen gas, as a pressure vessel for the hydrogen gas, and as a heat exchanger

to exchange the hydride heat of absorption or desorption with the surroundings.

Accordingly, each of the reactors 31 and 32 has a containment vessel 30 filled with a porous bed of hydride particles 33. The two reactors 31 and 32 are preferably (but not necessarily) filled with the same type of porous hydride, such as LaNi ₅ ,V ₀ . ₉₅ Cr ₀ . ₀₅ , NiZr, Cao. ₇ M _o . ₃ Ni ₅ , or other ones of many suitable hydride materials. For descriptions of suitable hydride materials, reference is made to The Journal of the Less Common Metals, Vol. 174, p. 1092 (1991); Vol. 131, p. 225, p. 385 (1987); Vol. 104, p. 259, p. 307 (1984)), incorpo¬ rated herein by reference. The most suitable hydrides have low mass, high hydrogen absorption, and low cost. The hydride particles in the bed 33 are contained within a porous struc- ture 34 that has a high thermal conductivity to enhance heat conduction. Many other suitable hydrides will be apparent to those skilled in the art.

The porous structure 34 provides enhanced conduction heat transfer within the hydride bed of the reactors. Each bed also has an interior porous artery 35 for hydrogen flow. Each vessel 30 is a container which may have end caps that are welded leak tight, or alternately, at least one of which being attached by an appropriate adhesive to the hydride container, and having external fins 36 for heat transfer to the surround- ings. The fins 36 may be, for example, light weight external radial fins of high thermal conductivity material to exchange heat with the surroundings.

The hydride reactors 31 and 32 are interconnected by pipes 22 and 26 via a 4-way solenoid valve (or other suitable multiported valve) 25 and a hydrogen compressor 20. A suitable 4-way solenoid valve, for example, is described in the catalog entitled "ASCO ^® Solenoid Valves", Automatic Switch Co., Florham Park, NJ, 07932, or in the catalog "Ramco Product Selector Guide," Reversing Valves, Ramco North America, Plain City OH, 43064.

Preferably, the hydrogen compressor 20 is an oil-less compressor, although oil lubricated compressors with oil vapor removal features may be used. Migration of oil vapor that might condense in the hydride bed needs to be avoided, since it may slow the hydrogen absorption/desorption rate. One compressor that may be used is that sold by Thomas Industries Inc., Sheboygan, WS 53082, as the Thomas Industries Single Cylinder, Oil-less WOB-L Piston Compressor, Model 515, or similar device which is designed for closed system operation. In operation, initially one reactor, for example, reactor

31, is fully saturated, or charged, with hydrogen. The other reactor 32 is unsaturated, or discharged. When hydrogen is evacuated from the reactor 31, the heat of desorption of the hydride contained in the reactor 31 is removed and the reactor 31 cools significantly. Therefore, initially, the compressor 20 withdraws low pressure hydrogen gas 27 from the reactor 31 through piping 22 and the 4-way valve 25, and compresses it. The compressed hydrogen gas is then moved through the 4-way valve 25 into reactor 32 via the pipe 26, at a higher pres- sure. As this hydrogen is compressed into reactor 32, the heat of absorption is liberated, and the reactor 32 becomes hot. Thus, the reactor 31 becomes a "cold" reactor and reactor 32 a "hot" reactor.

Preferably, a control circuit may be provided (not shown) to automatically reverse the 4-way valve 25 and the process. Thus, after hydrogen gas has been evacuated from one of the reactors and compressed into the other reactor, the process is automatically reversed to evacuate hydrogen gas from the second reactor and refill the first. During the reverse hydrogen transfer, the reactor 31 becomes hot, and the reactor 32 becomes cold. A fast cycle time is desirable.

As shown in Figures 2a and 2b, a baffle system 39 is provided to control the air flow across the two hydride reactors 31 and 32 of the hydride heat pump 10. The baffle system 39 has inside 42 and outside 42' insulated walls, with

insulated dampers 40 (Figure 2a) and 40' (Figure 2b) to separate air in the indoor space 41 from air in the outdoor space 43. The dampers 40 and insulated walls 42 may be constructed of thin metal sheet surrounding insulating foam, or other suitable material. Examples of such insulated walls 42 and dampers 40 may be found, for instance, in conventional automobile air conditioners. The dampers 40 serve to direct the air flow 46 - 47 from the cold indoor space 41, alter¬ nately over the first reactor 31 when it is cold, as shown in Figure 2a, and then over the second reactor 32 when it is cold, as shown in Figure 2b. The dampers 40 similarly direct the air flow 48 - 49 from the warmer outdoor space 43, alternately over the second reactor 32 when it is hot, as shown in Figure 2a, and then over the first reactor 31 when it is hot, as shown in Figure 2b.

Thus, in operation, the dampers 40 are first moved to the positions shown in Figure 2a to allow indoor air in the indoor space 41 to be blown by the fan 44 over the cold reactor 31 and its heat exchanging fins 36. The indoor air is therefore cooled to provide air conditioning to the indoor space 41. At the same time, ambient outdoor air 48 in the outdoor space 43 is blown by the fan 45 over the hot reactor 32 and its heat exchanging fins 36. Operation in this configuration is continued for a time sufficient to remove the heat delivered from the reactor 32, for example, for a period of several minutes to cool the reactor 32 to near the ambient outdoor temperature.

As the heat transfer processes approaches steady state, the 4-way valve 25 (see Figure 1) is reversed, reversing the hydrogen gas flow from the reactor 32 back to the reactor 31. The reactor 31 now becomes the "hot" reactor, and the reactor 32 becomes the "cold" reactor. At the same time, the dampers 40 are moved to the positions shown by dampers 40' in Figure 2b, in which indoor air 46 from the indoor space 41 is blown by the fan 45 over the cold reactor 32 and its heat exchanging

fins 36. At the same time, ambient outdoor air 48 in the outdoor space 43 is blown by the fan 44 over the hot reactor 31 and its heat exchanging fins 36, until the hot reactor 31 is cooled to near ambient outdoor temperature. This alternat- ing process is then repeated until the temperature of the ambient inside air in the indoor space 41 is cooled to the desired temperature. A multiplicity of reactors 31 and 32 may be used for larger capacity systems.

It will be appreciated that the units shown in Figures 1 and 2 may be alternatively used as an air-to-air heat pump for heating the inside air 41 simply by reversing the dampers. Thus, in the heating season, heat from the cool outside air may be transferred to warm the indoors.

A refrigerator 50 is shown in Figure 3a according to another preferred embodiment of the invention. The refrigera¬ tor 50 has two finned hydride reactors 51 and 52 intercon¬ nected by piping 54 via a reversible compressor 55. Each reactor 51 and 52 is provided with a one-directional heat pipe 56 and 57, respectively. The heat pipes 56 and 57 extend from within the respective reactors 51 and 52 to within a cold space 60 defined by an insulated wall 62. The heat pipes 56 and 57 serve to transfers heat from the cold space 60, providing a high conductance heat transfer with no parasitic pumping power requirements. The reactors and the heat pipes are preferably constructed of thin, light weight material, such as aluminum.

The heat pipes 56 and 57 may be constructed with their inside walls circumferentially grooved with fine grooves, of approximately 30 grooves per inch, to provide a wick to enhance evaporation and distribute the heat transfer liquid, which fills approximately 20% of the internal volume. More¬ over, the heat pipes 56 and 57 are tilted from the horizontal, and, in the embodiment illustrated, are vertically oriented, so that the liquid resides in a pool in the bottom of the heat pipe. When the bottom is warmer than the top, liquid is

evaporated from the pool in the bottom end. The vapor travels upward to the top end of the pipe and condenses, thereby transferring the latent heat of vaporization upwardly within the heat pipe. The grooved wick is designed to only lift the liquid a short distance from the bottom.

Conversely, when the upper end of the pipe is warmer than the bottom end, no heat is transferred by the liquid, since there is no liquid to evaporate from the upper end of the heat pipe. The liquid within the heat pipes may be, for instance acetone, which is chemically compatible with the aluminum pipe wall material and has a reasonable vapor pressure over the temperature range of operation for a refrigerator. Because the latent heat of vaporization is large and the pressure drop is small, the heat pipe has a very high thermal conductance in the upward direction. Design, construction, operation and testing of such one-directional heat pipes is described in the publication "Heat Pipe Technology," by K. T. Feldman, Jr., Bureau of Engineering Research, The University of New Mexico, 1978, which is incorporated by reference. The van't Hoff diagrams, which describe the pressure and temperature cycle that the hydrogen gas undergoes in each reactor 51 and 52 are shown in respective Figures 3b and 3c. The van't Hoff diagrams of Figures 3b and 3c show the log of the gas pressure versus the inverse of the temperature in the respective reactors 51 and 52.

With reference first to Figure 3c, the process for the hydride reactor 52 is as follows.

1-2. Compression. The compressor 55 pumps hydrogen gas 53 into the hydride reactor 52, eventually increasing the pressure within the reactor 52 to P _h . As the hydrogen gas 53 is absorbed into the hydride within the reactor 52, the exothermic heat of absorption is given off, increasing the temperature of the reactor 52 to T _h . The gas temperature is also increased slightly due to the compression work needed to raise the pressure of the hydrogen gas to P _h . Because less

hydrogen can be absorbed at the higher temperatures, the cooling fan 63 is turned on during compression to cool the hydride, so the temperature does not increase too much above ambient temperature, T _m . Fins made of heat conducting material are not shown, but may be provided on reactors 51 and 52 (as shown in Figure 1) and on the lower end of heat pipes 56 and 57 to enhance convection heat transfer.

2-3. Cool to Ambient Temperature. The compressor 55 is then stopped and the hydrogen flow stops. The cooling fan 63 remains on and the hydride is cooled to ambient temperature T _m and pressure P _m .

3-4. Decompression. The compressor 55 is reversed and hydrogen gas 53 is pumped out of the hydride within the reactor 52, eventually dropping the pressure to P _c . As hydrogen is desorbed from the hydride within the reactor 52, the heat of desorption is removed from the hydride, cooling it to cold temperature T _c .

4-1. Cooling. The compressor 55 is stopped and no hydrogen flow occurs. The hydride reactor 52 is at temperature T _c , so the one-directional heat pipe 57 turns on and transfers heat 4Q from the cold space 60, cooling the cold space 60, essentially to T _c .

The process for the hydride reactor 51, as shown in Figure 3b, occurs in the opposite sequence to that of reactor 52, as follows.

1-2. Decompression. The compressor 55 pumps hydrogen gas 53 out of the hydride reactor 51, eventually decreasing the pressure to P ₌ . As the hydrogen gas 53 is desorbed from the hydride within the reactor 51, the endothermic heat of desorption is removed from the hydride, cooling the reactor 51 to temperature to T _c .

2-3. Cool the Cold Space. The compressor 55 is stopped and no hydrogen flow occurs. The hydride within the reactor 51 is at temperature T _c , so the one-directional heat pipe 56

turns on and transfers heat 2°- ₃ from the cold space 60, cooling the cold space essentially to T _c .

3-4. Compression. The compressor 55 is then reversed and hydrogen gas 53 is pumped into the hydride within the reactor 51, eventually increasing the pressure to P _h . As hydrogen is absorbed into the hydride within the reactor 51, the heat of absorption is given off, heating the hydride reactor 51 to T _h . The cooling fan 64 is then turned on to dissipate the heat and cool the hydride so the temperature does not increase too much above the ambient temperature, T _m .

4-1. Cooling the Cold Space. The compressor 55 is stopped and no hydrogen flow occurs. The hydride is cooled to ambient temperature T _m and pressure P _m .

With reference now to Figure 4, an ice maker 70 is shown, in accordance with another preferred embodiment of the invention. The ice maker 70 has two hydride reactors 71 and 72, which may be constructed of lightweight aluminum tubes with longitudinal fins 73 and 74, for example. The reactors 71 and 72 are filled with a hydride 76 with integral high thermal conductivity structure provided by multiple aluminum screens 77, for example, to provide enhanced heat transfer in a radial direction. Each reactor 71 and 72 also has a central artery 78 for improved hydrogen flow. The reactors 71 and 72 are interconnected by a pipe 75, which is connected to a hydrogen compressor 79 by a 4-way solenoid valve 82. An insulated container 80, filled with water 81 to be frozen, is shown on the left side in which the reactor 71 is submerged.

In operation, the reactor 71 is initially fully charged with hydrogen and the reactor 72 is evacuated. The hydrogen is evacuated from the reactor 71 on the left side and is com¬ pressed by compressor 79 into the reactor 72 on the right side. The reactor 72 becomes hot as a result of the heat of absorption being released as hydrogen is being absorbed. This heat is transferred to the ambient surroundings by free convection cooling, although other cooling methods may be

used. For example, convection by cool water (not shown) or a fan (not shown) could be added for enhanced cooling.

As hydrogen is evacuated from reactor 71, it becomes cold and absorbs heat from the water 81. Once the water 81 is frozen to ice by the reactor 71 and reactor 72 is cooled to near ambient temperature, the 4-way valve 82 is reversed, reversing the direction of hydrogen flow. As the reactor 71 on the left side begins to heat, the ice filled container 80 can be removed and the ice removed. The container 80 can then be refilled with water and installed on reactor 72 on the right side to freeze another batch of ice. The process may then be repeated. The heat pumps depicted in Figures 1 - 4 are of the "package system" types.

Another embodiment of a hydride air conditioner, heat pump or refrigerator 90 (again, referred to hereinafter simply as the "heat pump") is shown in Figures 5 and 6, in which heat transfer takes place by the flow of hydrogen gas that is circulated between the reactor beds and the heat exchangers. The heat pump 90 has two hydride reactors 91 and 92, each containing a porous hydride bed 93 of similar type, wherein heat transfer takes place by flow of hydrogen. Some of the hydrogen circulated through the porous beds is absorbed (or desorbed) and some passes through the external tubing for convection heat transfer. One advantage of this approach is that good internal heat transfer in the reactor bed is provided by convection of hydrogen, and therefore conduction heat transfer is not required. This simplifies construction of the reactor since loose hydride particles may be used instead of a PMH compact. A compressor 95 and a 4-way valve 96 are used along with four 3-way valves 100 - 103 to control the direction of hydrogen flow between the reactors. Two pressure reducers 104 and 105 and two particle filters 108 and 109 are associated with the reactors 91 and 92. Two dedicated heat exchangers 111 and 112 are included, the heat exchanger 112 always being cold

and the heat exchanger 111 always being hot. One advantage to this design is that parasitic heat losses of the thermal mass of the finned heat exchangers and the finned reactor shell is eliminated. This embodiment also allows the system to be built as a split-system in a manner similar to a split-system air conditioner or a conventional refrigerator in which the cold heat exchanger coil is installed inside the insulated refrig¬ erator box to remove the heat inside the refrigerator and the hot coil is installed outside to reject heat to the surround- ings. Additionally, this embodiment does not require dampers to direct air flow.

The reactors 91 and 92 each are surrounded by an exterior pressure vessel 114 and 115, respectively, with interior thermal insulation 116 to minimize heat transfer with the surroundings. The insulation also may be added on the outside of the reactor wall; however, placing the insulation 116 on the inside of the wall reduces the thermal mass of the reactor, since the mass of the vessel is insulated from the hydride. The result is higher efficiency for the system. The operation of the system may be described by referring first to Figure 5. Initially the reactor 91 is fully charged with hydrogen and reactor 92 is fully discharged. Pressurized hydrogen leaves the compressor 95 in tube 120, through the 4- way valve 96, through the 3-way valve 101, and into the bottom of the high pressure reactor 92 via tube 121. Much of the hydrogen in the reactor 92 is absorbed within the hydride 93, and the remainder flows out of the top, through the filter 109, the tube 123, the 3-way valve 103, and the tube 124 into the hot heat exchanger 111. The heat of absorption in the hot reactor 92 is removed by this hydrogen flow and is transferred to the surroundings by the hot heat exchanger 111. Fans (not shown) may be used to enhance the heat transfer from the heat exchangers 111 and 112.

This convection heat transfer to the surroundings continues until the reactor 92 is fully saturated with

hydrogen, and has thereafter been cooled to near ambient temperature. At the same time, cold hydrogen is desorbed from the hydride bed in the reactor 91 on the left side. The hydrogen in the reactor 91 flows out from the top of the reactor 91 through a tube 130, the filter 108, the tube 135, the 3-way valve 102, and the pipe 131 to the cold heat exchanger 112, where heat is absorbed from the cold space surrounding the cold heat exchanger 112.

A small amount of high pressure hydrogen from the compressor 95 flows through the pressure reducer 104, becoming low pressure hydrogen, which is returned to the bottom of the reactor 91 via tube 134. This flow of hydrogen through the reactor 91 provides the convection heat transfer required to transfer essentially all of the "cold" to the cold heat exchanger 112. A small amount of additional compressor work is required for this excess flow.

Once the reactor 91 is fully discharged, reactor 92 is fully charged and heat is exchanged with the cold space and the surroundings, the 4-way valve 96 is switched, the four 3- way valves 100 - 103 are switched, and the direction of the flow of hydrogen is reversed, as is shown in Figure 6.

In Figure 6, the reactor 91 begins charging with high pressure hydrogen from the compressor 95, through tube 120, 4- way valve 96, 3-way valve 100, and tube 134. The reactor 91 then becomes hot. This heat is transferred to the hot heat exchanger 111 by flow of hot hydrogen through the tube 130, particle filter 108, tube 135, 3-way valve 102, and tube 136. At the same time, the reactor 92 begins discharging cold hydrogen to the cold heat exchanger 112 through the particle filter 109, tube 123, 3-way valve 103, and tube 136.

The high pressure hydrogen flow from the hot heat exchanger 111 passes through the pressure reducer 105, joins the low pressure gas flow from the cold heat exchanger 112 to return through the 3-way valve 101 and 4-way valve 96 to the suction side input tube 138 of the compressor 95.

Once the reactor 91 is fully charged, the reactor 92 is fully discharged, and heat is exchanged with the cold space and the surroundings, the valves are again switched and apparatus is reversed back to ^' the configuration shown in Figure 5. This embodiment can be used for refrigerators, air conditioners, ice makers and other "split-system" applications where it is advantageous to transfer heat with the surround¬ ings through fixed external heat exchangers.

Another embodiment 140 of the invention is shown in Figures 7 and 8, in which the internal heat transfer is enhanced by the flow of a mixture of hydrogen gas and liquid refrigerant. In the embodiment 140, two hydride reactors 141 and 142 are filled with a porous hydride 143 (and optionally a high thermal conductivity structure as described above) , hydrogen, and a volatile liquid, which serves as a refriger¬ ant. In this system heat transfer takes place by flow of hydrogen gas, liquid, and vapor in a two-phase gas-vapor/- liquid mixture flowing through the hydride bed. In Figure 7, the reactor 141 is initially fully charged with hydrogen and a liquid refrigerant with suitable vapor pressure and other properties, while the reactor 142 is fully discharged. The compressor 145 and the 4-way valve 146 are combined with two 3-way valves 148 and 149 and two pressure reducers 151 and 152. In the operation of the system of Figures 7 and 8, hydrogen and vapor are circulated to cause thorough mixing of the gas, liquid and solid hydride particles. Previous research (J. J. Reilly and J. R. Johnson, "Kinetics of Absorption of Hydrogen by LaNi ₅ /n-Undecane Suspension," Journal of Less Common Metals, 90, 1984), incorporated by reference herein, has shown that hydrogen/liquid/hydride slurries using light oils have good heat transfer within the bed, and can function as hydride reactors where hydrogen is absorbed or desorbed. Their advantage is that they have good internal heat transfer and relatively fast reaction rates when highly agitated. The

disadvantage of the oil/hydride slurries is the parasitic mass of the oil, which significantly decreases heat pump effi¬ ciency. In this invention, the use of a volatile liquid instead of passive oil, along with the hydride, adds to the cooling power and efficiency of the system.

Thus, in operation, pressurized hydrogen leaves the compressor 145, flows through the 4-way valve 146, the 3-way valve 149, and the piping 154 into the bottom of the reactor 142. The hydrogen is mostly absorbed by the hydride 143 in the reactor 142, but the excess flow is bled off through a heat exchanger 156, a pressure reducer 151, and 3-way valve 148 to the low pressure side of the compressor 145. The amount of excess hydrogen bled off is that needed to maintain adequate agitation and mixing in the gas/liquid/solid mixture in the reactor 142. The pressure in the reactor 142 increases as hydrogen is compressed and absorbed into the hydride 143 therein.

The pressure is also influenced by the vapor pressure and heat of condensation of the refrigerant. Refrigerant vapor also is compressed and flows along with the hydrogen flow. In the reactor 142, the refrigerant vapor condenses and its liquid resides in the porous hydride bed 143. The hydride heat of absorption and the refrigerant heat of condensation are transferred to the surroundings by forced convection of air 159 over the fins 157 by a fan 158, although other heat transfer means could also be used.

Concurrently, hydrogen is evacuated from the reactor 141 on the right side where it is desorbed from the porous hydride bed 143. Refrigerant vapor is also evaporated and drawn off from the reactor 141 along with the hydrogen. The mixture of low pressure hydrogen and refrigerant vapor flows through the finned heat exchanger 160, through the 3-way valve 148, through the 4-way valve 146 and to the suction side of the compressor 145. Hydrogen bleed flow from the high pressure side of the compressor 145 flows through a pressure reducing

valve 152 and is circulated into the bottom of reactor 141 through the hydride bed 143 and liquid refrigerant in reactor 141 to provide adequate mixing. The pressure in the reactor

141 decreases as hydrogen is desorbed and refrigerant is evaporated and removed. The hydride heat of desorption and the refrigerant heat of vaporization cools the hydride reactor 141, which in turn cools the surrounding cold space by the flow of air 162 from fan 164 over the reactor fins 166. After a period of time, the hydride reactor 141 is fully discharged and all the "cold" is transferred to the cold space and reactor 142 is fully charged with both hydrogen and condensed vapor and is cooled to near ambient temperature by ambient air flow 159 from fan 158, then the valves can be switched.

As shown in Figure 8, after the 4-way valve 146 and the two 3-way valves 148 and 149 are switched, the flows of the mixture of hydrogen and vapor are reversed. Then, the reactor

142 becomes discharged (becoming cold) and reactor 141 becomes charged (becoming hot) . This is a packaged type of system that can be used for refrigerators, air conditioners, and heat pumps, where the transfer of heat to the cold space and surroundings can be managed with the use of dampers in a manner similar to that described above with reference to Figure 3. The system shown in Figures 7 and 8 may, of course, also be used without a refrigerant. Another embodiment of a heat pump system 170 of the invention is shown in Figure 9 in which a slurry 171 comprised of hydrogen gas, a liquid, and solid hydride particles is circulated between two finned reactors 173 and 174. The system 170 has a slurry pump 176 to circulate the mixture of liquid and hydride particles, a hydrogen compressor 175, and an expansion valve 180. The slurry pump 176 may be of the type, like a rotary vane pump, that can pump a two-phase mixture of liquid and hydride particles.

The reactors 173 and 174 are shown as parallel finned tubes with a header at the top in which the slurry 171 forms

a puddle. The slurry 171 is circulated clockwise so that the liquid-hydride mixture is compressed to high pressure on the left side, flows through the expansion valve 180, where a pressure drop occurs, and is expanded to the right side at low pressure. The compressor 175 compresses hydrogen into the left side and evacuates the right side. The high pressure reactor 174 becomes hot as hydrogen is absorbed into the hydride and heat of absorption is released. Also, refrigerant vapor is compressed into reactor 174 where it condenses, giving up its heat of vaporization. This heat is rejected to the surround¬ ings by convection of ambient air 178 by fan 179.

Concurrently, the low pressure reactor 173 becomes cold as the hydrogen is desorbed and refrigerant is evaporated. Air flow 181, from fan 182, over this cold reactor 173 cools the cold space.

Two versions of the system 170 shown in Figure 9 exist, depending on the type of liquid used. The first version is the "passive liquid" concept in which a low vapor pressure liquid, such as silicon oil, is used. In the passive liquid version, the liquid serves to transport the hydride around the loop and to transfer heat to the surroundings. This version requires good agitation of the gas/liquid/hydride slurry 171 so that hydrogen is readily absorbed/desorbed from the slurry. This passive liquid system has a porous hydride bed of loose particles that can be mixed by the flow of hydrogen and liquid.

The second version is the "active liquid" version in which a volatile liquid, such as a refrigerant or liquid like methanol, is used, for example. The latent heat of vaporiza- tion of the active liquid adds to the refrigeration effect provided by the hydride by evaporating in the low pressure reactor and condensing in the high pressure reactor, as is done in a conventional vapor compression refrigeration system. Thus, the cooling power of the refrigeration system is

increased at the expense of slightly higher compressor power requirement.

Another split-system embodiment of a heat pump system 190, according to the invention, is shown in Figure 10. The system 190 is similar to that shown in Figure 9, except that a liquid slurry pump 191 is used, and serpentine finned tube reactor heat exchangers 192 and 193 are used. In the system 190, the slurry pump 191 can be a rotary vane pump that can pump a three-phase mixture of hydrogen gas, liquid and small hydride particles. The operation of this system is similar to that of the system shown in Figure 9 above, where the compres¬ sor/pump 191 compresses the mixture of hydrogen gas, refriger¬ ant vapor and liquid, and hydride particles into the high pressure reactor 193. In the reactor 193 the heat of absorption of hydrogen and the heat of condensation of the refrigerant is transferred to the surroundings through the finned heat exchanger 195 and the mixture is cooled to near ambient temperature. A fan (not shown) may be used to enhance heat transfer to the surround- ings. The high pressure slurry is then expanded through expansion valve 197 to produce a low pressure slurry, which flows through the finned reactor 198 in which the mixture cools due to hydrogen desorption and liquid refrigerant vaporization. The cold space is cooled by heat transfer to the cold reactor 192 through the finned surface 198.

The heat pump system 190 shown in Figure 10 may be modified slightly to provide the heat pump embodiment 260 shown in Figure 15. In the heat pump system 260, the expansion valve 197 of Figure 10 has been replaced with an expander 261. The expander 261 may be, for example, similar to that de¬ scribed by J. B. Jones and G. A. Hawkins in Engineering Thermodynamics, John Wiley & Sons, Inc., 1960, page 627. The expander may be a turbine, a rotary vane engine, or a piston engine that produces work from the expanding gas/vapor. By coupling the output shaft of the expander 261 to the input

shaft of the compressor 262, the work produced by the expander can supply a portion of the work required by the compressor 262, which is driven by a motor 266. Thus, the efficiency of the overall system 260 can be improved. In the construction of the apparatuses and systems described above, each employed a hydride bed in a reactor for reaction with hydrogen. In order to enhance conduction heat transfer within the hydride reactors, with a fixed hydride bed, the hydride metal particles are coated with a thin layer of high conductivity material, such as copper (about 1 to 3 micron thick) , and then compressed and sintered into a porous powder metal hydride compact (PMH compact) , preferably in the shape of a disk.

The process by which the particles are coated with high conductivity material is performed on the hydride forming metal particles before they are activated. The high conductiv¬ ity material coating is sufficiently porous and permeable to hydrogen so that hydrogen absorption and desorption in the hydride is not slowed significantly. The sintered PMH compact disk, press fit into the reactor vessel, has sufficient structural strength, so that even after activation, when the hydride expands in volume, the PMH compact disk retains its shape (while restrained by the reactor vessel) . The fine hydride particles are trapped within the PMH compact and do not migrate through the system.

The high conductivity material in the PMH compact provides good thermal conductivity paths for conduction heat transfer within the hydride bed, thus greatly increasing the effective thermal conductivity of the bed. With the PMH compact disk pressed into the hydride reactor, low contact resistance occurs between the hydride particles, the reactor vessel wall and the external heat transfer means. The PMH compact disks are sufficiently porous so hydrogen can readily flow in and out of the hydride bed.

Additional grooves, called arteries, can be made in the PMH compact disks to further improve hydrogen flow. The PMH compact disk may be, for example, copper or aluminum coated hydride particles compressed and sintered into a porous powder metal hydride compact. Additionally, if desired, a material selected from the group comprising copper, tin, or zinc powder may be mixed with the hydride to improve thermal conductivity and strength and improve ease of sintering. The copper plated hydride particles can also be plated with a. thin layer of tin, zinc or other suitable material to improve low temperature sintering.

An enlarged cross sectional drawing is shown in Figure 11 of a portion 200 of a hydride bed that provides a high thermal conductivity structure 204. The structure 204 is comprised of a plurality of hydride particles 205, each enclosed within a high thermal conductivity coating 206. The hydride coating 206 provides a porous structure, which may be formed, for example, of a high thermal conductivity material selected from the group comprising aluminum, graphite, carbon, copper, diamond- like materials, or other suitable high thermal conductivity material that can encase the hydride particles, yet provide sufficient porosity though which the hydrogen can pass. The hydride bed 200 with high thermal conductivity structure 204 may be manufactured by first coating the hydride particles 205 with the high thermal conductivity coating 206 and then compressing and sintering them into a strong PMH compact, which provides excellent conduction heat transfer and prevents migration of small hydride particles.

I have observed that during repeated cycles of absorption and desorption of hydrogen, metal hydride materials have a tendency to fracture and break up into smaller pieces, even¬ tually becoming a fine powder of hydride material. Without some containment structure, the fine powder becomes compacted and admits less and less hydrogen. Thus, by providing encase- ments for small portions of the hydride material, a porous

supporting structure is provided that allows the hydride material to break up, while preserving the ability of the powdered hydride bed to efficiently admit hydrogen. It has been found, for example, that particles of between about 10 to 500 microns, preferably about 35 to 40 microns, are particu¬ larly suitable for this purpose. It should also be noted that the thickness of the encapsulating material should be suffi¬ ciently thin, perhaps even with discontinuities in its encapsulating skin, to allow hydrogen to penetrate the skin to reach the contained hydride material. This can be accomplished by forming the encapsulating layer, for instance, on the order of a few microns, for example, between about 1 to 5 microns, with such a thin layer of material that it incompletely encases the contained hydride particles, yet nevertheless is sufficiently continuous to constrain the hydride particles or powder. An additional advantage is realized if the encasing material is selected to have a high thermal conductivity, since heat can be more rapidly moved to or from the innermost regions of the hydride bed. The internal high thermal conductivity structure to enhance heat transfer within the hydride reactor may also be made by enclosing the hydride particles within the pores of a porous open-cell foam, made of high thermal conductivity material, such as a foam structure of aluminum, copper, carbon, graphite or diamond-like materials. The best foam would be one with high thermal conductivity but with low mass to minimize parasitic heat transfer losses. Graphite foam is promising because of its interconnected porosity, its high thermal conductivity (several times greater than copper) and its low mass. Graphite foams are discussed in the paper "Processing, Structure, and Morphology of Graphite Carbon Foams Produced from Anisotropic Pitch, " by D. Dutta, C. S. Hill and D. P. Anderson, Proceedings of the Materials Research Society (Novel Forms of Carbon II) , April 1994, incorporated herein by reference. The porous graphite foam has intercon-

nected pores so that hydrogen can flow through to be absorbed and desorbed by the hydride with minimum pressure drop.

Thus, a portion of a hydride compact which may be provided as a porous foam structure 210, is shown in Figure 12. The hydride/foam structure 210 has a foam structure 211 with inter-connected pores 212 enclosing hydride particles 213 within the pores. The foam structure 211 may be, for example, fabricated of a graphite foam material. The high thermal conductivity porous hydride/foam structure 210 may be manufac- tured, for example, by first making the porous foam structure 211 and then melting a hydride forming metal alloy and pressing the molten metal into the foam structure 211. The foam structure 211 may also be made of aluminum, copper, or other high thermal conductivity material. The foam structure 211 with enclosed hydride particles 213 may be further compressed and sintered to form a strong, porous PMH compact with excellent conduction heat transfer and no migration of small hydride particles.

As mentioned, the hydride bed may be formed of a number of PMH compact disks, as shown in Figure 13. In the structure 220 of Figure 13, a hydride reactor 221 has a vessel 222 for containing a plurality of PMH compact disks 226 to form a porous hydride bed 223. The reactor 221 has an axial artery tube 228 for free hydrogen flow, a tube 229 for hydrogen flow in and out, and fins 227 for heat transfer with the surround¬ ings.

An enlarged view of one of the PMH compact disks 226 is seen in Figure 13, exploded from the hydride bed 223 of the reactor 221. As can be seen, the PMH compact disk 226 com- prises a plurality of hydride particles 230 formed into a PMH compact disk structure, having a central artery 228 and radial grooves 232 to additionally improve radial hydrogen flow.

Another way of improving heat transfer and strength in the reactor hydride bed is to assemble porous screens or fibers of high thermal conductivity materials within the PMH

compact or PMH compact disk. The screens or fibers provide enhanced heat conduction paths through the hydride bed, considerably improving the effective thermal conductivity of the hydride bed, as well as its structural strength. Examples of high thermal conductivity materials that may be used for fibers or coatings for the hydride particles include aluminum, graphite, carbon or diamond-like materials as well as copper. Also, copper, tin, or zinc powder may be mixed with the coated hydride particles or plated in a thin layer on the hydride particles to improve thermal conductivity and strength resulting from low temperature sintering.

One embodiment of a structure 240 that incorporates porous screens or fibers of high thermal conductivity materi¬ als is shown in Figure 14. The structure 240 is a PMH compact disk 241 with a central hole 242 for improved axial hydrogen flow and radial grooves 243 for improved radial hydrogen flow. The PMH compact disk 241 is comprised of hydride particles contained within a high thermal conductivity structure of the type described above with respect to Figure 11, additionally with imbedded internal fibers 245. The fibers 245 are of high thermal conductivity material, preferably oriented in the radial direction, as shown, to serve as an internal heat transfer enhancing structure within the hydride bed that enhances radial heat conduction. The fibers 245 may be, for example, graphite fibers mixed within the hydride particles and a binding metal compressed and sintered into a porous powder metal hydride compact.

It will be appreciated that the hydride heat pump of the invention is simple, compact, and reliable. The following analysis demonstrates that it is also highly efficient. As described above, the system has two hydride reactors which alternately provide cooling, so the cold space receives nearly steady cooling to the desired cold temperature, T _c .

The compressor removes hydrogen gas from the desorbing hydride reactor at pressure p _c and compresses it to the

absorbing reactor at p _m , where T _m is the ambient tempera¬ ture. In compressing the gas to p _m the temperature is increased to T _h . After the gas is compressed to p and T _h , it is cooled to T _m before it is absorbed into the hydride. For a frictionless, adiabatic (isentropic) process of an ideal gas with constant specific heats, pv ^k = constant, where the ratio of specific heats k = C _p /C _v = 1.4 for hydrogen, between states 1 and 2 (See Figure 3b) PαV^ = p ₂ v ₂ ^k , and with pv - RT,

The compression work may be determined for an isentropic, steady flow process in an open system from,

where, E _c = the compressor efficiency (which accounts for non- isentropic compression) , v is the specific volume and R is the gas constant. At temperature T _c , the hydrogen gas pressure desorbing from the cold hydride is given by the van't Hoff equation, where ΔH and ΔS are the heat and entropy of desorp- tion respectively,

R lnp C = — r - ΔS (4)

C

and at the temperature T _m the gas pressure absorbing into the warm hydride is,

ΔH

R lnp = — - ΔS (5) m

The preceding two equations (4) and (5) are referred to hereinafter as the van't Hoff equations. Combining the two van't Hoff equations, the pressure ratio is obtained:

For the hydrides, the pressure and temperature of the gas are described by the van't Hoff equations (3) and (4) . Equation (6) may be combined with equation (3), to give,

Jc-1 Δff. 1 _ 1 . _w = _-£(_*_) _[e * * ^r - ^T = -1] (7) ^{c E} E c k - l

Equations 6 and 7 include the assumption that the hydro- gen flow out of the compressor is cooled from T _h to T _m . The amount of cooling produced is given by,

<?c = ∞ ^" < ⁽⁸⁾

per mole of hydrogen, where q^ _s is the parasitic heat loss due to heating and cooling the mass, defined by, qi _oss = Q/n = (1/n) (i^C _p + π C' _p ) (T _m - T _c ) , where the mass of the hydride is m _< - with specific heat C _p , and the mass of the hydride container is _o -,,. with specific heat C _p , the amount of cooling per mole of hydrogen is given by, m-= m/n and n-.-,^ = rn _c /R,., where R-_ accounts for the mass of the container with fins, and n is the number of moles of hydrogen desorbed per cycle. Equation 8 then becomes,

q _c = ΔE - m _c (C _p + C' _P /R (T _B - T _c ) , cal/mole hydrogen. (8)

For example, if a hydride air conditioner operates with hydride LaNi= where the heat of desorption is ΔH = -7380 cal/mole, and C _p = 0.1, C' _p = 0.11, R _a = 5 (for a thin wall

finned reactor), u^ = 175.8 grams/mole hydrogen, T = 35°C (308°K), T _c = 8°C (281°K), the cooling per mole is,

q.. = 7380 - (175.8) (0.1 + 0.11/5) (35 - 8)

q _c = 7380 - 579 = 6801 cal/mole hydrogen.

The compression work, from equation (7) for isentropic compression (efficiency, E _c = 1) and R = 1.987 cal/mole°K is,

1 Δ 1.4 - 1 ₍ . 7380 1 _ 1

C w I = (1 . 987) (281°jq ( ^λ ' ^q ) [ e ¹ - ^{4 1} - ^{987 308 281} -1 ]

1 . 4 - 1

_c w _m = 767 cal/mole

The efficiency is given by the coefficient of performance or, COP, for isentropic compression is,

COP = -^ = ϋ2i = 8.87 (9)

W 767

With a compressor of efficiency E _c = 0.7, COP = 6.2, and with E _c = 0.8, COP = 7.1.

The best existing air conditioners or heat pumps oper- ating with similar temperatures currently have COP _* 3.5, or less. Thus, the present invention has potentially twice the efficiency of the current refrigerators, air conditioners, and heat pumps. Because of its potential high efficiency and environmental cleanness, the present hydride heat pump is commercially attractive.

Although the invention has been described and illus¬ trated with a certain degree of particularity, it is under¬ stood that the present disclosure has been made only by way of example, and that numerous changes in the combination and

arrangement of parts can be resorted to by those skilled in the art without departing from the spirit and scope of the invention, as hereinafter claimed.