Technical Field The invention disclosed herein relates to continuously-operated metal hydride hydrogen compressor driven by a reversible heat pumping system on the basis of thermoelectric elements, and a method of operating such a compressor.

In particular the invention relates to a hydrogen thermal sorption compression device that utilizes a metal hydride material, the heating of which is accompanied by release of high-pressure hydrogen and the cooling of which is accompanied by absorption of low-pressure hydrogen by means of a reversible heat pumping system. The device of the invention can be used in gas services for filling gas cylinders with high-pressure hydrogen gas and in cryogenic engineering for providing hydrogen liquefaction and re-liquefaction systems for high-pressure hydrogen gas. The best use of this device is micro-cryogenic hydrogen re-liquefaction systems in space engineering.

Background Art

A conventional way of applying mechanical compressors has a number of drawbacks when used for the compression of hydrogen. First of all, the combination of intensive mechanical motion and high-pressure media of the explosive gas, entails an intrinsic potential danger. Also, mechanical hydrogen compressors may have reliability problems because of the possible escape of H ₂ through their moving seals, as well as due to hydrogen corrosion and brittleness of the constituent materials. The problem of contamination of the output hydrogen with a lubricant may also be encountered.

A promising method of hydrogen compression which does not require the use of moving parts, is one applying metal hydrides. This method uses the reversible interaction of hydride-forming metals or alloys with hydrogen gas, according to the scheme below:

sorption

M (s) + χ/2 H ₂ Cg)^ZZTMH _x (s) + Q (E ^q . D desorption

where M denotes a metal or alloy, MH _x a correspondi ng metal hydride, and (s) and (g) solid and gas phases, respectively. The direct process, i.e. hydrogen sorption, is accompanied by the formation of metal hydride. This is accomplished by removing a certain amount of heat Q of the exothermic reaction of the hydride formation, at a lower temperature T _L . The reverse process, i.e. endothermic decomposition of the hydride accompanied by the evolution of desorbed hydrogen, is stimulated by the supply of the same amount of heat Q at a higher temperature T _H . The equilibrium hydrogen pressure for the reaction according to Eq. 1 increases exponentially with temperature, such that the sorption takes place at lower hydrogen pressure P _L and the desorption occurs at a higher pressure P _H similarly to suction and discharge processes in a mechanical compressor.

Various engineering approaches for achieving hydrogen thermal sorption compression by the use of metal hydrides have been described in a number of patents and other publications. One example is the method of storing hydrogen described by R. H. Wiswall and JJ. ReMIy ¹ , whereby hydrogen gas is absorbed by a titanium-iron alloy at the lower temperature T _L =10 ⁰ C and lower pressure P _L ~35 bar, and then desorbed at the higher pressure of up to P _H = 250 bar, while being heated to the higher temperature of up to T _H ~200°C. The authors also estimated the capability of generating in such a way, hydrogen pressures of up to P _H = 690 bar.

Another example described by J.J. Reilly, A. Holtz and R. H. Wiswall ² , is a laboratory gas circulation pump powered by decomposition and regeneration of vanadium dihydride, alternatively heated and cooled by hot (T _H ~50°C) and cold (T _L =18°C) water, the operation taking place at pressures between P _L =7 bar and PH = 24 bar.

Both approaches above result in a periodically operated hydrogen compression that limits their possible application for continuous technological processes.

Hence, there is a need for a metal hydride hydrogen compressor that allows continuous operation. On the accompanying drawings, Figure 3 illustrates the principle of the operation of the simplest continuously-operated metal hydride hydrogen compressor. The compressor comprises at least two compression modules 1, 2, each of which com- prising a metal hydride bed A of a hydrogen storage alloy being thermally coupled to a heat supply / removal accessory B. The metal hydride bed A is placed in a container C comprising a gas pipeline D. All the gas pipelines D are connected to a gas distributing system E equipped with a port F for the supply of hydrogen at low pressure P _L and with a port G for the output of hydrogen at high pressure P _H .

In particular, the gas distributing system E can take the form of a set of shut-off valves (including remotely controlled ones), or check valves which are connected to the pipelines D of the compression modules 1, 2 and the hydrogen input and output ports F and G, in such a way that the low-pressure hydrogen flows from the input port F to a compression module 1 or 2 which at one moment operates in the suction (absorption) mode, while the high-pressure hydrogen flows from a compression module 2 or 1 which at the same time operates in the discharge (desorption) mode, to the output port G. The hydrogen suction (absorption) at P _L in the metal hydride bed Al or A2 located in compression module 1 or 2, is achieved by removing heat Q at a lower temperature level T _L using a heat supply / removal accessory Bl or B2 which at one moment operates in the low-temperature heat removal mode, while the hydrogen discharge (desorption) at P _H in the metal hydride bed A2 or Al located in compression module 2 or 1, is achieved by supplying heat Q at a higher temperature level T _H using a heat supply / removal accessory B2 or Bl which at the same time operates in the high-temperature heat supply mode.

Thus, the heat supply / removal accessories Bl and B2 operate in mutually opposite modes, so that when accessory Bl removes heat Q at T _L from the metal hydride bed Al (cooling), accessory B2 supplies the same amount of heat Q at T _H to the metal hydride bed A2 (heating), and vice versa. The continuous operation of the compressor is provided for by a periodic reversal of the operating modes of the heat supply / removal accessories Bl and B2. The relation between the values of P _L , T _L , P _H and T _H is defined by the kind of hydrogen storage alloy contained in the metal hydride bed A, or more specifically, by the thermal stability of the metal hydride formed in the course of the reversible interaction of the associated hydrogen storage alloy with hydrogen gas. The productivity of the compressor mainly depends on the rate of heat supply at the high-temperature level dQ _H /dt and also on the rate of heat removal at the low- temperature level dQ _L /dt, such that when the slower of these two dQ/dt-s increases, the productivity increases as well.

The basic engineering approach described above, to the continuously-operated metal hydride hydrogen compressor, is disclosed in a number of patents and publications ³ ' ⁴ ' ^etc -, and further improvements thereof mainly concern: • The kind of hydrogen storage alloy, the number of compression elements (two or more), their gas connections and the sequence of operation of the associated heat supply / removal accessories.

As an example, the hydrogen compressor disclosed by P.M. Golben and MJ. Rosso ⁵ includes six compression elements arranged in two groups, each of which being thermally coupled to its own heat supply / removal accessory, wherein the metal hydride beds of the compression elements within each group are based on three different hydrogen storage alloys giving the associated metal hydrides different thermal stabilities. The gas distributing system which is based on check valves, is made in a manner that allows multistage operation of the compressor, whereby the heated metal hydride bed characterized by higher thermal stability of the associated metal hydride feeds the cooled metal hydride bed characterized by lower thermal stability of the associated metal hydride, with higher pressure hydrogen.

With this approach it is possible to achieve a higher compression ratio PH/P _| _ using a narrow temperature range (T _H - T _L ), but, at the same time, it has a more complicated design and lower reliability than a single-stage hydrogen compressor. This is due to the large number of check valves, the malfunction of which may result in back flow of hydrogen and disruption of the compressor's operation. Moreover, as shown by thermodynamic analysis ⁶ , the efficiency of multistage metal hydride hydrogen compressors is lower than that of single-stage compressors.

• The methods and accessories for heat supply / removal.

The most commonly used method of supplying heat Q _H at a higher temperature T _H to a metal hydride bed to stimulate hydrogen desorption therefrom at a higher pressure P _H comprises the use of a heat transfer fluid, such as hot water, which is passed through a heat exchanger located in a metal hydride container and being in thermal contact with a metal hydride bed. Similarly, the removal of heat Q _L at a lower temperature T _L from a metal hydride bed, providing absorption of low- pressure (P _| _) hydrogen therein, is achieved by the use of a heat transfer fluid (i.e., cold water), which is passed through the same heat exchanger. In particular, such a method is utilized in the above-mentioned hydrogen circulation pump by J. J. ReMIy, A.Holtz and R. H. Wiswal ² , and hydrogen compressor by P.M. Golben and MJ. Rosso, Jr ⁵ , etc.

For the methods mentioned above, the rate of heat supply or removal dQ/dt to / from a metal hydride bed is determined by Newton's law of cooling ⁷ to be proportional to the difference between the available temperature T' _H or T' _L of the heat transfer fluid, and the temperature T _H or T _L of the metal hydride bed, required to produce high- pressure hydrogen desorption and low-pressure hydrogen absorption. This means that when the temperature of the metal hydride bed approaches the temperature of the heat transfer fluid, the rate of heat supply / removal which is the driving force of hydrogen discharge / charge, decreases dramatically, resulting in loss of productivity.

To increase the productivity of a metal hydride hydrogen compressor in high-pressure discharge mode, one possibility is to use electric heating to generate a forced high- temperature heat supply to the metal hydride bed. One example is the hydrogen com- pressor patented by P.M. Golben ⁴ , where a metal hydride container comprises a built-in electric heater and a cooling jacket for passing a coolant to provide low- temperature heat removal from the metal hydride bed. Another example is an industrial-scale metal hydride hydrogen compressor (P _L =3 ... 5 bar, P _H = 150 bar, productivity: 10 m ³ /h) described by Yu. F. Shmal'ko et al. ⁸ , where metal hydride containers are heated by built-in electric heaters and externally cooled by forced air convection. Since convective cooling is still used for this kind of engineering, such techniques are characterized by a sharp decrease in productivity when the temperature of the metal hydride bed T _L that is required for low-pressure (P _L ) hydrogen absorption, approaches the available temperature T' _L of a coolant.

The aim of the present invention is to intensify the processes of both the heat supply and the heat removal in a continuously-operated metal hydride hydrogen compressor, and in this way maintain high productivity in a single compression stage, at higher discharge pressures P _H as well as at lower suction pressures P _L . This may be achieved by applying a heat pumping system that is able to provide forced heating as well as forced cooling of the metal hydride bed.

One of these approaches is applied in the method and apparatus for compressing hydrogen gas patented by C.Halene ⁹ . The method and apparatus involves two metal hydride containers equipped with heat exchange jackets for heating / cooling the metal hydride beds in the containers. The containers are connected to the source of low- pressure hydrogen and receiver of high-pressure hydrogen through a system of gas pipelines and shut-off valves. In turn, the heat exchange jackets of the metal hydride containers are connected via the system of pipelines and shut-off valves to heating and cooling circuits of an evaporation / condensation heat pump providing the permanent heating of a heat transfer fluid in the heating circuit to the higher temperature T _H and the permanent cooling of the heat transfer fluid in the cooling circuit to the lower temperature T _L .

The operation of the compressor is maintained by opening and closing the valves in such a manner that at one moment the first metal hydride container is connected to a receiver of high-pressure hydrogen and its heat exchange jacket is connected to a heating circuit, while at the same time the second container is connected to a source of low-pressure hydrogen and its heat exchange jacket is connected to a cooling circuit. The permanent operation of the compressor is provided for by periodically switching the valves, so that, at another moment, the first container is connected to a source of low-pressure hydrogen and its heat exchange jacket is connected to a cooling circuit, while at the same time the second metal hydride container is connected to a receiver of high-pressure hydrogen and its heat exchange jacket is connected to a heating circuit. To increase the efficiency of the operation, the applied approach also envisages some improvements, like heat regeneration between the cooled and heated metal hydride containers, the possibility of passing hydrogen from a heated container to a cooled one after completion of a compression cycle, etc.

The above-described approach has the following similar features with the approach proposed by the present invention:

(i) at least two metal hydride compression modules connected through pipelines with a gas-distribution system comprising a port for the input of low-pressure hydrogen and a port for the output of high-pressure hydrogen, (ii) thermal coupling of the metal hydride compression modules with a heat pump, and

(iii) the presence of higher-temperature heating means, lower-temperature cooling means and medium-temperature heat sink means.

However, with the prior art approach said heating, cooling and heat sink means are fixed in space and time, i.e. the heating and cooling circuits of the heat pump are continuously operated without altering their operating mode. This results in the necessity of involving a system of valves to switch the flows of hot and cold heat transfer fluid between the heat exchange jackets of the metal hydride containers, and this switching needs to be synchronised with the operation of valves in the gas distribution system. This arrangement complicates very much both the design and the operation of the compressor, and lowers its reliability.

One promising way of implementing the proposed approach is to make the heat pumping system as a set of thermoelectric modules (Peltier elements) properly connected (both thermally and electrically) with each other and with other elements of the metal hydride hydrogen compressor.

There exist two similar engineering approaches involving thermoelectric modules (Peltier elements) to provide compression of hydrogen gas using metal hydrides. The first one described by V.A.Vasin ¹⁰ uses one Peltier element, one heating / cooling side of which is covered by a hydrogen storage alloy (LaNi ₅ or TiFe). Powering of the Peltier element causes fast heating of the hydrogen storage alloy in contact with the associated hot side of the element, and fast high-pressure hydrogen desorption therefrom. Powering of the element with reversed polarity results in fast cooling of the hydrogen storage alloy and fast low-pressure hydrogen absorption therein. Since this approach has no special means for heat dissipation from the opposite side of the Peltier element, it can be used for periodic operations only (e.g. for pneumatic actuators in vacuum engineering).

An advanced approach was patented by M. J. Rosso ¹¹ ', who discloses a hydride- thermoelectric pneumatic actuation system, where both sides of one thermoelectric element are thermally coupled with their own metal hydride beds. The DC powering of the thermoelectric element causes heating of its one side and high-pressure hydrogen desorption from the associated metal hydride bed, while simultaneous cooling of the other side causes low-pressure hydrogen absorption in the associated metal hydride bed. The periodic reversing of the polarity of the DC powering of the thermoelectric element reverses the absorption / desorption processes and, in principle, provides permanent alternate motion of a piston, one side of which being in communication with the hydrogen line of the first metal hydride container, the other in communication with the hydrogen line of the second metal hydride container.

Although this approach produces periodic changes of the hydrogen pressure (from P _L to P _H and vice versa) in the chambers of a pressure responsive mechanism by the use of metal hydride and thermoelectric module, it is suitable only for pneumatic actuation and can not be directly applied for permanent hydrogen compression.

As it can be shown from a thermodynamic consideration, any heat pump (including the thermoelectric module) will supply more heat Q _H to the high-temperature level, T _H , than the amount Q _L being absorbed by it at the low-temperature level T _L . The difference (Q _H - Q _L ) will be equal to the external work W necessary to provide the transportation of the specified amount of heat from low to high temperature level. For the ideal heat pump having a reverse Carnot cycle, the value of W is determined by W = QH( I - T _| _/T _H ), and for a real heat pump the value will be higher. So, for the modern thermoelectric coolers operating between T _L = -30 ... -1O ⁰ C and T _H = 30 ... 5O ⁰ C the value of W is approximately 1.5 times more than the cooling capacity Q _L ¹² . Therefore, the amount of heat Q _H supplied to the high-temperature level will be 2.5 more than Q _L .

Since it is necessary to remove the same amount of heat from the low-temperature level as that supplied to the high-temperature level, to obtain normal operation of a metal hydride hydrogen compressor, the prior art approach being used for the continuously-operated hydrogen compression will result in a decrease in productivity or, alternatively, a decrease in the compression ratio, PH/P _Q during prolonged operation, this being due to the difficulty of properly managing the residual heat dissipation. Restricted to the layout proposed in the prior art, the only one way to solve this problem is by controlling the periodic removal of excessive heat from the heated side of the thermoelectric module, which substantially complicates the implementation of the approach considered for permanent hydrogen compression.

Disclosure of the Invention The present invention relates to a permanently-operating metal hydride hydrogen compressor which uses two or more compression modules. Each module contains a metal hydride (MH) bed placed in a MH container equipped with a gas pipeline and auxiliary means providing heat exchange between the MH bed and the heating / cooling side of the container. The heating / cooling side is thermally coupled to one of two or more sides of a reversible heat pumping system, each of which being capable of operating as a cooler at one moment and as a heater at another. One part of each MH container is coupled to the side of the heat pumping system that at one moment is operating as a cooler or heater, while the other part is coupled to the other side, this other side being operated as a heater or cooler at the same moment. In doing so, the two parts of the MH containers operate in mutually opposite modes, that is: when one part is cooled, the other one is heated, and vice versa. The reversible heat pumping system also comprises a medium-temperature heat sink side providing for the removal of excessive heat. The gas pipelines of the MH containers are connected to the input port of low pressure hydrogen and output port of high-pressure hydrogen via a gas- distributing system containing gas collectors, check valves and connecting pipelines.

According to the invention the reversible heat pumping system comprises a set of two or more thermoelectric modules (Peltier elements), one of two sides of each being thermally coupled to heat sink accessory, the other side to the heating / cooling side of a MH container. The thermoelectric elements are divided into two or more groups, one of each being powered at one moment by direct polarity producing cooling of the associated MH containers, while, at the same time, the other is powered by reverse polarity producing heating of the associated MH containers.

The method of operating the compressor according to the invention comprises the following steps: one cooling / heating side of said heat pumping system is operated at lower temperature level providing heat removal from the metal hydride bed coupled to this side, that stimulates absorption of low-pressure hydrogen therein, while at the same time the other cooling / heating side of said heat pumping system is operated at higher temperature level providing heat supply to the metal hydride bed coupled to this side, that stimulates desorption of high-pressure hydrogen therefrom; and the excessive heat is removed at a medium temperature level from said medium- temperature side of the heat pumping system. Permanent operation of the compressor is achieved by periodically switching between the heating / cooling modes of the reversible heat pump, in particular, by changing the powering polarity of the thermoelectric modules. In doing so, the power supplied to the modules operating as coolers can be equal to or exceed the power supplied to the modules operating as heaters. The latter mode can be used to increase the compressor's productivity or for saving power, which, in addition, can be achieved by switching the heating thermoelectric modules off for a period that depends on the change of the compressor's productivity, before reversing their polarity, resulting in switching from heating to cooling mode.

The advantages of the invention include fast dynamic performances of the hydrogen compressor, higher productivity, reliable and stable, prolonged operation due to its thermal self-balancing, as well as easy and flexible control of the operation, including the availability of safe power managing.

Brief Description of Drawings

Figure 1 schematically illustrates the design and operation of a basic metal hydride hydrogen compressor driven by a reversible heat pumping system, Figure 2 shows an embodiment of the invention, in which a set of thermoelectric elements is used as the reversible heat pumping system,

Figure 3 shows the principle of the operation of the simplest continuously-operated metal hydride hydrogen compressor,

Figure 4 schematically shows a setup on which tests were performed, Figure 5 presents the results of the tests, showing respectively, in Figure 5A, the output pressure and productivity, and in Figure 5B, the bottom temperature of the metal hydride containers, as a function of time, Figure 6 schematically shows a modified test setup, Figure 7 presents in a manner similar to Figure 5, the results of the tests performed on the modified setup illustrated in Figure 6, and Figure 8 presents a comparison of the maximum productivities of the compressors according to the prior art approach and the present invention, respectively, operated at the same conditions.

Embodiments of the Invention The present invention relates to a reversible heat pumping system (Figure 1, item A) comprising two ore more alternatively operated heating / cooling sides B, C, each of which being capable of being used as a heater at one moment, and as a cooler at another. The respective heating / cooling sides are in a permanent thermal contact with its own metal hydride bed E of an associated metal hydride container F. The heat pumping system also comprises a heat sink side D operating at a medium temperature T _sink , where T _L <T _sink <T _H , and permanently connected to heat dissipation means.

Referring to Figure 2, the described embodiment of the reversible heat pumping system comprises a set of two or more thermoelectric modules (Peltier elements), each of which being thermally coupled to a heat sink accessory and, also to the heating / cooling side of a MH container. One of the two heating / cooling faces of each element (item A) is thermally coupled to its own metal hydride bed D placed in an associated container E, while the other heating / cooling face of the thermoelectric element is thermally coupled to a heat sink accessory L.

The thermoelectric elements are divided into two or more groups. One of the groups is powered by DC in such a manner that the sides of the thermoelectric elements coupled to the associated metal hydride beds are heated to provide hydrogen desorption there- from at a higher pressure, while the opposite sides of the thermoelectric elements are cooled to absorb heat from the heat sink accessory. At the same time, another group of the elements is powered by DC of reverse polarity, so that the sides of the thermoelectric elements coupled to the associated metal hydride beds are cooled to provide hydrogen absorption therein at a lower pressure, while the opposite sides of the thermoelectric elements are heated to dissipate heat to the heat sink accessory. This mode of operation is periodically reversed by the change of DC polarity of the power supplied to all the elements, and in doing so, the previously heated metal hydride beds are now cooled, providing low-pressure hydrogen absorption, and previously cooled metal hydride beds are simultaneously heated, providing high-pressure hydrogen desorption therefrom. The switching of the gas flows from the hydrogen input line H to the cooled metal hydride beds and from the heated metal hydride beds to the hydrogen output line K, respectively, is accomplished by a gas distribution system G, the particular configuration of which can be a set of check valves properly connected by gas pipelines to hydrogen input H and output K lines, and gas pipelines F of the metal hydride containers.

As noted hereinbefore the power dissipating from the hot side of a thermoelectric element always exceeds the power absorbed by the cold side of the thermoelectric element. At the same time, both high-pressure / high temperature desorption and low-pressure / low temperature absorption do require approximately the same amount of heat being supplied to or removed from the metal hydride bed. Therefore, the higher productivity of the compressor according to this invention can be achieved by the increase of the power supplied to the thermoelectric elements operating at one moment as coolers of the associated metal hydride beds, as compared to the power supplied to the thermoelectric elements operating at the same moment as heaters of the associated metal hydride beds. Alternatively, reducing the power supplied to the thermoelectric elements operating at one moment in the heating mode, as compared to the power supplied to the thermoelectric elements operating at the same moment in the cooling mode, will result in a reduction of of the total power consumption without lowering the compressor's productivity. In addition, the power saving operation can be achieved by switching off the thermoelectric elements operating in the heating mode, before being switched to the cooling mode for the period. The output / discharge productivity then falls to zero, while the suction / input productivity does not.

The operation and performance of the compressor according to the present invention compared to the prior art approach, is illustrated by Examples 1 and 2 below, Example 1 being concerned with the realization of the prior art approach, and Example 2 describing the operation of the proposed approach under the same conditions as in Example 1.

Example 1

Experiments were performed on a setup as schematically shown in Figure 4, comprising two compression module containers 1, 2 made of aluminium. Each container (of a total weight of 110 g) equipped with an internal heat exchanger was loaded with 90 g of the AB ₅ -type hydrogen storage alloy IA, 2A, and was hermetically closed by a flange connected to a gas pipeline ending in a compression module gas collector IB, 2B. The bottoms of the containers 1, 2 were thermally coupled to both of two sides of Marlow XLT2385-03AC thermoelectric modules 3 connected to a DC power supply unit 4 allowing output power control and reversal of its polarity. Such a layout corresponds to the prior art engineering approach ¹¹ . To provide the same power supply conditions as for the testing of the proposed approach (Example 2), two thermoelectric modules connected in sequence, both thermally and electrically, were used in the Example 1 as well. The temperatures of the bottoms of the containers Tl, T2 were monitored using K-type thermocouples. Heat sink was provided by a fan 5, and the ambient temperature was about 25 ⁰ C.

The gas collectors IB, 2B of the compression modules 1, 2, were connected via check valves 6 (arrows indicate the gas flow direction) to the input and output gas collectors 7 and 8, comprising the gas distributing system of the compressor. The low-pressure hydrogen supply line connected to the input collector 7 contains a hydrogen cylinder 9 and a reducer 10 allowing to set the input pressure, P _L . The latter was monitored using a low-pressure sensor 11. The output collector 8 was connected to the high- pressure hydrogen receiver 12. The output pressure, P _H , was monitored by a pressure sensor 13. The output productivity, Q, was calculated starting from the time dependency of P _H , the total volume (~3.5 I) and the temperature of the high-pressure hydrogen receiver using the standard procedure of volumetric data processing.

The testing conditions were as follows: the lower (suction) hydrogen pressure was maintained at the level of P _L = 10 bar; the total DC power supplied to both of the thermoelectric modules was 130 to 160 W (30 - 35% of the rated value), the supplied power to both of the thermoelectric modules was equal (since they were connected in series). In addition, when the temperature of the hot side of a module Tl or T2 approached 100 ⁰ C, the operating cycle was interrupted followed by reversal of the polarity. The latter was done to avoid overheating of the thermoelectric modules having an upper allowed limit of their hot side temperature (~130°C).

The test results are presented in Figure 5A,B showing the time dependencies of the output pressure, P _H (bar), output productivity, Q (l _H2 /min, reduced to normal conditions) (Figure 5A), as well as temperatures, Tl and T2 ( ⁰ C) of the bottoms of the metal hydride containers (Figure 5B).

The results show that under the operating conditions the maximum hydrogen pressure in the receiver achieved during the tests bar), was reached in 120 minutes, corresponding to eight complete heating / cooling cycles for each metal hydride container. The average output productivity of the compressor gradually decreases during the operation. Despite of the forced air cooling, a significant increase of the average temperature of the thermoelectric modules / metal hydride containers was observed already after the first cycle. The established values of the minimum cold side temperature (T _L / _M IN) and the average temperature of the compressor were 40° and 6O ⁰ C, respectively. As already noted, the maximum hot side temperature was T _H/MAX =100°C, whereby the maximum temperature difference between cold and hot sides of the compressor was (T _H -T _L ) _MAX ~60 degrees.

Lowering the input pressure value (P _L < 10 bar) results in a sharp decrease of the compressor's productivity, this being explainable by a too high value of its cold side temperature, T _L >40°C. Increasing the supplied power results in the rapid overheating of the hot side and decreased productivity on subsequent cycles, because of reduction of the reversible hydrogen capacity of metal hydride in the containers.

Example 2

A modified test setup is shown in Figure 6. The same component parts as in Example 1 (Figure 4) were used, thus, most of the duplicated captions are omitted. As distinct from Example 1, heat sink was provided by a special water-cooled heat-sink plate (Figure 6, item 5) where both metal hydride containers and thermoelectric modules were installed. As in Example 1, the thermoelectric modules powered by power supply unit 4 were electrically connected in series, but in the actual case by the opposing polarity. The layout used (Figure 6) is one of the variants of the realization of the approach proposed by this invention, having identical common components, as to a realization of the prior art approach (Example 1). Also, the same operating parameters as in Example 1, including electric power supplied to the thermoelectric modules, were kept. The only difference in the operation was that in this Example the switching of heating / cooling modes (reversing polarity) was performed when the output productivity fell to zero, since in the actual case overheating of the thermoelectric modules above 100 ⁰ C was not observed during the operation.

The test results are presented in Figure 7A,B, similar to the Example 1 (Figure 5). As it can be seen, the maximum hydrogen pressure in the receiver bar) was also reached in 120 minutes, but by use of only 6 complete heating / cooling cycles. The compressor is characterized by stable long-term operation, and its output productivity decreases to a lesser extent than in Example 1. The established value of the minimum cold side temperature is T _L y _MIN = -10 ... -5 ⁰ C. The maximum temperature difference between the cold and hot sides of the compressor in operation was (T _H -T _L ) _MAX ~90 degrees.

Lowering the input pressure value (P _L <10 bar) results in a gradual decrease of the compressor's productivity, but the decrease is much smaller than for the case described in Example 1. Even at P _L ~1 bar hydrogen compression was observed, due to a rather low cold side temperature. Increasing the supplied power results in shortening of the cycle time and, in turn, in an increase of the total productivity.

Figure 8 presents a comparison of the maximum productivities of the compressor according to the prior art approach and the present invention, respectively, operated under the same conditions. It appears that the present invention provides the higher output productivity, especially at lower discharge pressures. Moreover, the present invention provides a lower cold side temperature and a larger temperature difference (T _H -T _| _) that, in turn, makes it possible to operate at lower suction pressures and provide higher compression ratios.

The asymmetric heat management with the possibility of the heat sink at the medium temperature level realised by this invention also provides heat self-balancing of the metal hydride compressor driven by thermoelectric elements. This arrangement results in steady thermal conditions and, consequently, stable long-term operation. Moreover, due to the self-balancing, the compressor is characterized by the flexibility in control, allowing the use of asymmetric power supply; which means that less power can be supplied to the thermoelectric modules heating the metal hydride bed than to those producing cooling. This makes it possible either to increase the compressor's productivity by increasing the power supplied to the cooling thermoelectric modules, or to decrease power consumption by decreasing the power supplied to the heating modules (even to zero at the end of the operation cycle when hydrogen is already released from the heated metal hydride bed, but is not yet completely absorbed in the cooled bed).

TABLE OF REFERENCES

1 R. H. Wiswall, JJ. Reilly, Jr. : Method of storing hydrogen, US Patent No. 3 516 263 (1970)

2 JJ. Reilly, A. Holtz and R. H. Wiswall: A new laboratory gas circulation pump for intermediate pressures, Rev. Sci. Instr., 42, 1985 (1971)

3 J. P. Powell, FJ. Salzano: Hydride compressor, US Patent No. 4 085 590 (1978)

4 P.M. GoI ben: Hydrogen compressor, US Patent No. 4 505 120 (1985)

5 P.M. Golben, MJ. Rosso, Jr. : Hydrogen compressor, US Patent No. 4 402 187 (1983)

6 V.Z. Mordkovich, Yu. K. Baichtok, M. Kh. Sosna, N.V. Dudakova, N.N. Korostyshevsky: Efficiency analysis for use of intermetallic compounds in hydrogen isolation and compression, Teoreticheskie Osnovy Khimicheskoi tekhnologii (Foundations of Chemical Technology), 24, No.6 (1990) 769 - 774 (Translated into English by Plenum Publishing Corporation, 1991)

7 Y. A. ςengel, M. A. Boles: Thermodynamics: An engineering approach, McGraw Hill, 1998

8 Yu. F. Shmal'ko, A.I. Ivanovsky, M. V. Lototsky, V.I. Kolosov, D. V. Volosnikov: Sample pilot plant of industrial metal-hydride compressor, International Journal of Hydrogen Energy 24 (1999) 645 - 648

9 C.Halene: Method and apparatus for compressing hydrogen gas, US Patent No. 4 995 235 (1991)

10 V. A. Vasin: Non-delay thermal sorption compressors for autonomous supply of actuators of the controlled resilience for the functional mechanisms of high-vacuum facilities, 6th R&D Conference "Vacuum Science and Technology", Gurzuf, October 1999, p.33 - 34 (in Russian)

11 MJ. Rosso, Jr. : Hydride-thermoelectric pneumatic actuation system, US Patent No. 6 128 904 (2000)

12 Marlow Industries, Inc., www.marlow.com

LJST OF REFERENCE NUMERALS

Ih Figure 1:

11-13 - first group of compression modules 14-16 - second group of compression modules

17 - metal hydride bed (MH)

18 - metal hydride container

21 - reversible heat pumping system (RHP)

22 - low-temperature (LT) cooling / high-temperature (HT) heating side of the RHP

23 - HT / LT side of the RHP

24 - medium-temperature (MT) heat sink side of the RHP

25 - power input to the heat pump

26 - heat removal / supply to the side 22 of the RHP

27 - heat supply / removal from side 23 of the RHP

28 - heat removal from the RHP

31 - gas pipeline

32 - gas distributing system

33 - input pipeline

34 - output pipeline

35 - low-pressure hydrogen flows

36 - high-pressure hydrogen flows

LIST OF REFERENCE NUMERALS (cont.d)

In Figure 2:

11-15 - compression modules

17 - metal hydride bed (MH)

18 - metal hydride container

21 - thermoelectric element (TE)

22 - low-temperature (LT when +) cooling / high-temperature (HT when -) heating side of the TE

23 - high -temperature (HT when +) cooling / low-temperature (LT when -) heating side of the TE

24 - heat sink accessory

31 - gas pipeline

32 - gas distributing system:

33 - input pipeline

34 - output pipeline

35 - low-pressure hydrogen flows

36 - high-pressure hydrogen flows

37 - gas collectors

38 - check valves

39 - pipelines

25 - power supply / control block

26 - heat removal / supply to the side 22 of the TE

27 - heat supply / removal from side 23 of the TE

28 - heat removal