WO2009013595A1

WO2009013595A1 - Fuel cell system

Info

Publication number: WO2009013595A1
Application number: PCT/IB2008/001906
Authority: WO
Inventors: Daigoro Mori
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2007-07-25
Filing date: 2008-07-23
Publication date: 2009-01-29
Also published as: JP2009032460A

Abstract

A fuel cell system (100) includes a fuel cell (1), a hydrogen storage alloy that supplies a hydrogen to the fuel cell (1), and an air compressor (7) that raises the temperature of an off-gas discharged from the fuel cell (1) by compressing the off-gas so that the compressed off-gas heats the hydrogen storage alloy.

Description

FUEL CELLSYSTEM

BACKGROUND OF THE INVENfTION 1. Field of the Invention [0001] The invention relates to a fuel cell system.

2. Description of the Related Art

[0002] In conjunction with a fuel cell system of a fuel cell motor vehicle that travels using as a power the electricity obtained by a fuel cell, a technology described in, for example, Japanese Patent Application Publication No. 2000-12056 (JP-A-2000-12056), is known. This technology uses an exhaust gas (hereinafter, referred to as "off-gas") discharged from the fuel gas to heat a hydrogen storage alloy so that hydrogen is released therefrom, and supplies the released hydrogen to the fuel cell.

[0003] By the way, in the solid polymer type fuel cell, the temperature of the off-gas is 60 to 80⁰C while the temperature needed in order to release hydrogen from a magnesium-base hydrogen storage alloy is 150 to 400⁰C. Therefore, the temperature of the off-gas of the solid polymer type fuel cell is too low to release hydrogen from the magnesium-base hydrogen storage alloy.

[0004] As a measure to overcome this problem, a technology in which a hydrogen storage alloy tank is provided with an electric heater, and the hydrogen storage alloy is heated to high temperature by throwing electric energy into the electric heater is known, as in Japanese Patent Application Publication No. 11-106201 (JP-A-11-106201).

[0005] On the other hand, an advance in the study on the material of the hydrogen storage alloy has made it possible to use a hydrogen storage alloy, such as an amide-base hydrogen storage alloy or the like, that releases hydrogen at a lower heating temperature (at or below 150⁰C) than the magnesium-base hydrogen storage alloy.

[0006] However, the hydrogen storage alloy capable of releasing hydrogen even at relatively low temperature, such as the amide-base hydrogen storage alloy, has a great weight per unit volume. Therefore, a hydrogen storage alloy tank that employs such a hydrogen storage alloy becomes heavy in weight. [0007] Besides, there has been a demand for the provision of a technology that is capable of releasing hydrogen from the hydrogen storage alloy by using an off-gas of the fuel cell without a need to use an electric heater.

. SUMMARY OF THE INVElSfTION

[0008] The invention provides a fuel cell system capable of releasing hydrogen from a hydrogen storage alloy without a need to use an electric heater, while achieving a weight reduction of the hydrogen storage alloy tank.

[0009] A fuel cell system in accordance with the invention includes: a fuel cell; a hydrogen storage alloy that releases a hydrogen when heated and supplies the hydrogen to the fuel cell; compression means for raising temperature of an off-gas discharged from the fuel cell by compressing the off-gas; and heat exchange means for heat-exchanging between the compressed off-gas and the hydrogen storage alloy.

[0010] In the fuel cell system in accordance with the foregoing aspect, the heat exchange means may transfer heat from the compressed off-gas to the hydrogen storage alloy.

[0011] Examples of the compression means include an air compressor. Examples of the heat exchange means include a hydrogen storage alloy tank that contains a hydrogen storage alloy. [0012] When the off-gas is compressed by the compression means, the temperature of the off-gas rises following the Boyle-Charles law. Hence, by adjusting the compression rate of the off-gas by the compression means, the temperature of the off-gas can be changed from the temperature of about 60 to 80⁰C that the off-gas has when discharged from the fuel cell. [0013] In the fuel cell system in accordance with the foregoing aspect, the off-gas may be a cathode off-gas discharged from a cathode that is an air electrode of the fuel cell.

[0014] Besides, in the fuel cell system in accordance with the foregoing aspect, the compression means may compress the off-gas so that the temperature of the off-gas is in a predetermined temperature range.

[0015] In the fuel cell system in accordance with the foregoing aspect, the predetermined temperature range may be 150 to 400⁰C. Hereinafter, the compressed off-gas will be referred to as "the compressed off-gas". [0016] Besides, in the fuel cell system in accordance with the foregoing aspect, the hydrogen storage alloy may be a magnesium-base hydrogen storage alloy.

[0017] If the temperature of the compressed off-gas is 150 to 400⁰C, a hydrogen storage alloy that releases hydrogen in the temperature range of 150 to 400⁰C, for example, a magnesium-base hydrogen storage alloy, can be utilized. Hence, the hydrogen storage alloy used is not limited to a hydrogen storage alloy that releases hydrogen at low temperature, such as an amide-base hydrogen storage alloy or the like.

Besides, the magnesium-base hydrogen storage alloy has a small weight per unit volume.

Therefore, if the hydrogen storage alloy tank is produced by using a hydrogen storage alloy whose weight per unit volume is small, the weight of the hydrogen storage alloy tank can be reduced.

[0018] Besides, since the compressed off-gas whose temperature is brought into the high temperature range is supplied to the hydrogen storage alloy, hydrogen can be reliably released from the hydrogen storage alloy without a need for an electric heater.

[0019] The fuel cell system in accordance with the foregoing aspect may further include hydrogen release determination means for determining that the hydrogen has been released from the hydrogen storage alloy, and a compression rate of the compression means may be raised if it is determined by the hydrogen release determination means that the hydrogen has not been released from the hydrogen storage alloy.

[0020] Besides, in the fuel cell system in accordance with the foregoing aspect, the compression rate of the compression means may be reduced if the compression rate becomes higher than a predetermined permissible compression rate.

[0021] The fuel cell system in accordance with the foregoing aspect may further include a turbine that recovers energy from the compressed off-gas. The turbine is rotated by the compressed off-gas, so that a large amount of air compressed by the rotation of the turbine is fed to the cathode of the fuel cell. Thus, energy for rotating the turbine can be recovered from the compressed off-gas. Thus, the off-gas can be effectively utilized.

[0022] Besides, the fuel cell system in accordance with the foregoing aspect may further include pressure reduction means for reducing pressure of the compressed off-gas after heat is transferred from the compressed off-gas to the hydrogen storage alloy, and energy may be recovered from the compressed off-gas by the turbine after the pressure of the off-gas is reduced by the pressure reduction means.

[0023] In the fuel cell system in accordance with the foregoing aspect, the fuel cell may be a solid polymer type fuel cell.

[0024] In the fuel cell system in accordance with the foregoing aspect, the fuel cell system may be mounted in a vehicle.

[0025] In the fuel cell system in accordance with the foregoing aspect, the compression means may be driven by the fuel cell, except when the fuel cell is at low temperature or has just been started up, and the compression means may be driven by a battery mounted in the vehicle, when the fuel cell is at low temperature or has just been started up.

[0026] According to the invention, a weight reduction of the hydrogen storage alloy tank can be achieved, and hydrogen can be released from the hydrogen storage alloy without a need to use an electric heater.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The foregoing and/or further objects, features and advantages of the invention will become more apparent from the following description of preferred embodiment with reference to the accompanying drawings, in which like numerals are used to represent like elements and wherein:

FIG 1 is a overall construction diagram of a fuel cell system in accordance with the invention;

FIG 2 is a flowchart for describing a series of process step up to a process in which hydrogen is released from the hydrogen storage alloy of a hydrogen storage alloy tank by warming the hydrogen storage alloy through the use of a cathode compressed off-gas in accordance with the invention;

FIG 3 is a graph showing relations between the ejection temperature and the drive force/ejection pressure of an air compressor in accordance with the invention;

FIG 4 is a pressure-temperature graph showing a gas-liquid critical line of water in accordance with the invention; and

FIG 5 is an enlarged partial view of FIG 4.

DETAILED DESCRIPTION OF EMBODIMENTS

[0028] A fuel cell system of the invention is applied to a fuel cell system of a fuel cell motor vehicle that travels by using as power the electricity obtained by the fuel cell.

[0029] In this system, a hydrogen storage alloy of a hydrogen storage alloy tank is heated by an off-gas of the fuel cell, and hydrogen released from the hydrogen storage alloy by the heating is supplied to the fuel cell.

[0030] The off-gas used for the heating is compressed by an air compressor that is compression means. By the compression, the temperature of the off-gas rises. The temperature-raised off-gas is used to heat the hydrogen storage alloy.

[0031] Next, with reference to the accompanying drawings, a concrete construction of a fuel cell system 100 will be described. This system 100 is controlled by an ECU (Electric Control Unit) that is a control unit that includes a CPU (Central Processing Unit).

[0032] As shown in FIG 1, the system 100 has a fuel cell 1 that is supplied with oxygen and hydrogen and that causes oxygen and hydrogen to chemically react via an electrolyte membrane. The system 100 further has a hydrogen storage alloy tank 3 made of a hydrogen storage alloy that supplies hydrogen to the fuel cell 1, a turbocharger 6 that is oxygen supply means for supplying oxygen (air) to the fuel cell 1, an air compressor 7 that is compression means for compressing the off-gas that is discharged from the fuel cell 1 at the time of the chemical reaction, a plurality of channels through which oxygen or hydrogen flows, and other component parts.

[0033] The fuel cell 1 includes an electrolyte membrane where the reaction progresses, an anode as a fuel electrode supplied with hydrogen, that is, a fuel, and a cathode as an air electrode supplied with oxygen which are positioned on two opposite sides of the electrolyte membrane, and a stack (fuel cell body) constructed by stacking a plurality of cells that have separators as partition plates that separate hydrogen and oxygen. Incidentally, the electrolyte membranes, the anodes, the cathodes, the separators, the cells and the stack are omitted from the illustration.

[0034] Besides, the fuel cell 1 has a hydrogen gas channel 14 that supplies hydrogen to the anode, and an oxygen gas channel 16 that supplies oxygen to the cathode.

[0035] The hydrogen gas channel 14 is connected to the hydrogen storage alloy tank 3, and supplies a hydrogen-rich hydrogen gas from the hydrogen storage alloy tank 3 to the anode.

[0036] The oxygen gas channel 16 is linked to a compressor-wheel assembly 61 of the turbocharger 6 that is oxygen supply means, and supplies oxygen from the atmosphere to .the fuel cell 1.

[0037] In the fuel cell 1, when the anode is fed with the hydrogen gas, hydrogen ions are produced from hydrogen of the hydrogen gas (H₂→2H⁺+2e^"). On the other hand, when the cathode is fed with the oxygen gas, water is produced from hydrogen ions and oxygen ((l/2)O₂+2H⁺+2e^~→H₂O), thus generating electricity. Most of the produced water absorbs heat generated in the fuel cell 1, and becomes vapor, which becomes contained in a cathode off-gas that is the cathode-side exhaust gas, and is thus discharged from the fuel cell 1.

[0038] An anode off-gas that is the anode-side exhaust gas is discharged into the atmosphere through an anode off-gas channel 22. The anode off-gas channel 22 and the hydrogen gas channel 14 are interlinked by a link channel 28. The link channel 28 has a pump 30 and a check valve 34.

[0039] The anode off-gas sent into the link channel 28 is sent into the hydrogen gas channel 14 by operation of the pump 30, and is thus sent again to the anode of the fuel cell 1, in which the anode-off gas is re-used. In addition, the check valve 34 prevents the hydrogen gas from flowing from the hydrogen gas channel 14 toward the anode off-gas channel 22. This circulation system is termed the anode circulation system.

[0040] Furthermore, an impurity discharge valve 24 is connected to the anode off-gas channel 22. The impurity discharge valve 24 is opened when the proportion of impurity in the anode off-gas increases as the anode off-gas circulates in the anode circulation system. As the impurity discharge valve 24 is opened, the flow of the anode off-gas into the hydrogen gas channel 14 is restrained, so that the re-use of the anode off-gas containing a large amount of impurity is refrained. [0041] On the other hand, the cathode off-gas, after being discharged from the fuel cell 1, is caused to flow into the hydrogen storage alloy tank 3 via a cathode off-gas channel 36. An upstream side of the cathode off-gas channel 36 is linked to the oxygen gas channel 16 via a link channel 40.

[0042] Via this link channel 40, a portion of the cathode off-gas is returned to the oxygen gas channel 16. The link channel 40 is provided with an air circulation valve 42 that adjusts the amount of flow of the cathode off-gas returned to the oxygen gas channel 16. This circulation system is termed the cathode circulation system.

[0043] In addition, the air compressor 7, compression means, is disposed at a site on the cathode off-gas channel 36 that is downstream of a site from which the link channel 40 branches.

[0044] Ordinarily, the air compressor 7 is driven by the fuel cell 1, and has a drive force (rated output) of about 1 to 5 kw. Besides, by the air compressor 7, the cathode off-gas is compressed to become a high-pressure off-gas with a temperature rise according to the Boyle-Charles law. Therefore, the air compressor 7 can be said to be heating means for warming the cathode off-gas. Incidentally, the cathode off-gas that is compressed with a corresponding temperature rise by the air compressor 7 will be hereinafter termed the cathode compressed off-gas.

[0045] The temperature of the cathode compressed off-gas can be changed by adjusting the compression rate of the cathode off-gas discharged from the fuel cell 1, through the use of the air compressor 7. The cathode off-gas is compressed so that the temperature of the cathode compressed off-gas is in a predetermined range. The predetermined range is the range of 150 to 400⁰C, which is needed in order to release hydrogen from the magnesium-base hydrogen storage alloy. [0046] The temperature of the cathode compressed off-gas discharged from the air compressor 7 is monitored by an exhaust gas temperature sensor 47 that is disposed downstream of the air compressor 7.

[0047] The cathode compressed off-gas is guided to the hydrogen storage alloy tank 3, which is disposed further downstream, by a portion 36a of the cathode off-gas channel 36 that is downstream of a site on which the air compressor 7 is installed. The cathode compressed off-gas guided to the hydrogen storage alloy tank 3 functions as a heating medium that heats the hydrogen storage alloy. Subsequently to the heat transfer to the hydrogen storage alloy, the cathode compressed off-gas is released into the atmosphere from the hydrogen storage alloy tank 3 via an atmospheric release passageway 361. [0048] Thus, since the portion 36a of the cathode off-gas channel 36 that is downstream of the site on which the air compressor 7 is installed guides the cathode compressed off-gas compressed with a corresponding temperature rise by the air compressor 7 to the hydrogen storage alloy tank 3, and is used to heat the hydrogen storage alloy, the portion 36a can be said to be heating means. [0049] Besides, the air compressor 7 is heating means for warming the cathode off-gas as described above. However, the air compressor 7 warms the cathode off-gas, and the warmed cathode off-gas warms the hydrogen storage alloy tank 3. Therefore, the air compressor 7 can also be said to be heating means that is used to heat the hydrogen storage alloy. [0050] In the heating of the hydrogen storage alloy, the cathode compressed off-gas warms the hydrogen storage alloy tank 3 indirectly and/or directly. In the case of indirect warming, it suffices that an external wall (not shown) of the hydrogen storage alloy tank 3 be warmed by the cathode compressed off-gas and heat be caused to conduct from the external wall into the hydrogen storage alloy tank 3. In the case of direct warming, it suffices that an off-gas passageway extending through an interior of the hydrogen storage alloy tank 3 be provided and the cathode compressed off-gas be passed through the off-gas passageway.

[0051] In either case, heat transfers from the cathode compressed off-gas to the hydrogen storage alloy while the cathode compressed off-gas is being guided around or through the hydrogen storage alloy tank 3. Therefore, the hydrogen storage alloy tank 3 having the above-described construction can be said to be heat exchange means.

[0052] Besides, the atmospheric release passageway 361 is provided with a variable pressure nozzle 361n. A downstream side of the variable pressure nozzle 361n is linked to a turbine-wheel assembly 62 of the turbocharger 6. Therefore, the cathode compressed off-gas arriving through the atmospheric release passageway 361 rotates a turbine wheel 62a of the turbine-wheel assembly 62, thus producing drive force to the turbocharger 6.

[0053] The pressure of the cathode compressed off-gas flowing in the atmospheric release passageway 361 is adjusted by adjusting a nozzle opening (not shown) of the variable pressure nozzle 361n (which is pressure reduction means).

[0054] If through this pressure adjustment, the pressure of the cathode compressed off-gas is made low, high-speed flow of the cathode compressed off-gas can be supplied to the turbocharger 6. In consequence, the cathode compressed off-gas rotates the turbine wheel 62a at high speed.

[0055] In the turbocharger 6, which is oxygen supply means, a compressor wheel 61a of the compressor- wheel assembly 61 and the turbine wheel 62a of the turbine- wheel assembly 62 are interlinked by a shaft 63. Therefore, as the turbine wheel 62a rotates, the compressor wheel 61a also rotates. As the compressor wheel 61a rotates, air is drawn in from the atmosphere via the oxygen gas channel 16, and is compressed, and then is fed to the cathode of the fuel cell 1.

[0056] Ordinarily, the air compressor 7 is driven by the fuel cell 1. When the fuel cell 1 is at low temperature or immediately after the fuel cell 1 is started up, however, the fuel cell 1 does not sufficiently function, and therefore the air compressor 7 is not operated by the fuel cell 1. Therefore, when the fuel cell 1 is at low temperature, for example, during a non-operation state thereof, or the like, the air compressor 7 is driven by using a battery that is mounted in the motor vehicle.

[0057] Next, with reference to the flowchart shown in FIG 2, a series of process steps up to a process step in which hydrogen is released from the hydrogen storage alloy of the hydrogen storage alloy tank 3 by warming the hydrogen storage alloy through the use of the cathode compressed off-gas will be described. This series of process steps is repeatedly executed in a main control program executed by the control unit (ECU).

[0058] In step (hereinafter, referred to as "S") 1, the control unit determines whether or not the cathode off-gas is being emitted from the fuel cell 1. If an affirmative determination is made, the process proceeds to S2. If a negative determination is made, Sl is repeated.

[0059] In S2, the control unit determines whether or not the fuel cell 1 is at low temperature or has just been started up. If an affirmative determination is made in S2, the process proceeds to S3. If a negative determination is made, the process proceeds to S4. A reason for performing this determination in S2 is that when the fuel cell 1 is at low temperature or immediately after the fuel cell 1 is started up, the fuel cell 1 does not sufficiently function, and therefore the air compressor 7 is not operated, as described above. [0060] In S3, the control unit drives the air compressor 7 by using the battery mounted in the motor vehicle.

[0061] In S4, the control unit determines whether or not the temperature of the cathode compressed off-gas is in the predetermined range (150 to 400⁰C) by monitoring the exhaust gas temperature sensor 47. When the temperature is not in the predetermined range, the magnesium-base hydrogen storage alloy cannot be used. If an affirmative determination is made in S4, the control unit proceeds to S5. If a negative determination is made, the control unit repeats S4. Alternatively, after a negative determination is made in S4, the control unit may adjust the air compressor 7 to control the compression rate so that the temperature of the cathode compressed off-gas comes into the predetermined range.

[0062] In S5, the control unit introduces the cathode compressed off-gas to the hydrogen storage alloy tank 3.

[0063] In S6, the control unit as hydrogen release determination means determines whether or not the hydrogen storage alloy tank 3 has released hydrogen. If an affirmative determination is made in S6, the process returns to S4. If a negative determination is made, the process proceeds to S7. If the temperature of the cathode compressed off-gas is within the predetermined range, it is assumed that the hydrogen storage alloy supplied with the cathode compressed off-gas should release hydrogen. [0064] However, a case is conceivable in which the whole body of the hydrogen storage alloy tank 3 has not reached a sufficient temperature. In such a case, it can happen that hydrogen is not released or is not readily or sufficiently released from the hydrogen storage alloy even though the hydrogen storage alloy is supplied with the cathode compressed off-gas whose temperature is in the predetermined range. Therefore, although it has been determined that the temperature of the cathode compressed off-gas is in the predetermined range, the determination of S6 is performed.

[006S] In S7, the control unit adjusts the air compressor 7 so as to raise the compression rate of the cathode compressed off-gas. By raising the compression rate, the temperature of the cathode compressed off-gas rises. As a result, the temperature of the whole body of the hydrogen storage alloy tank 3 rises, so that the release of hydrogen from the hydrogen storage alloy more readily occurs. Incidentally, if the pressure of the cathode compressed off-gas becomes higher than a permissible value, the compression rate may be reduced. Besides, the process of FIG 2 may be stopped upon a stop command from a user. [0066] Next, with reference to FIG 3 and Table 1, a relation between the temperature

(ejection temperature) of the cathode compressed off-gas and the pressure (ejection pressure) of the cathode compressed off-gas will be described.

[0067] FIG 3 is an ejection temperature/drive force-ejection pressure graph in which the left-side vertical axis represents the temperature (ejection temperature) of the cathode compressed off -gas ejected from the air compressor 7, and the right-side vertical axis represents the rated output (drive force) of the compressor, and the horizontal axis represents the pressure (ejection pressure) of the cathode compressed off-gas ejected from the air compressor 7. Table 1 shows detailed data regarding FIG 3.

Table 1

[0068] From FIG 3 and Table 1, it can be understood that, for example, when the cathode compressed off-gas of about 400⁰C is needed in order to release hydrogen from the hydrogen storage alloy of the hydrogen storage alloy tank 3, it suffices to drive the air compressor 7 at an intake (inlet) temperature of 100⁰C and an ejection pressure of about 4 bar. Furthermore, it can be understood that at that time, the drive force of the air compressor 7 is about 6.8 kW in the case of the air flow rate is 1000 L/min (see data (A) in Table 1).

[0069] Likewise, it can also be understood that in the case where the cathode compressed off-gas of about 180⁰C is needed, it suffices to drive the air compressor 7 at an intake temperature of 60⁰C and an ejection pressure of about 2 bar. Furthermore, it can also be understood that at that time, the drive force of the air compressor 7 is about 8.2 kW in the case where the air flow rate is 3000 L/min (see data (B) in Table 1).

[0070] In this system 100, as the cathode off-gas is compressed by the air compressor 7 to become a high-pressure cathode compressed off-gas, the temperature of the cathode compressed off-gas rises following the Boyle-Charles law. At this time, by adjusting the compression rate of the cathode off-gas, the temperature of the cathode compressed off-gas can be changed to a suitable temperature.

[0071] Even in the case where the temperature of the off-gas in the non-compressed state is 60 to 80⁰C, if the compression rate of the cathode off-gas is adjusted so that the temperature of the cathode compressed off-gas becomes 150 to 400⁰C, the hydrogen storage alloy used herein is not limited to a hydrogen storage alloy of low hydrogen release temperature, such as an amide-base hydrogen storage alloy or the like, but can be a hydrogen storage alloy of high hydrogen release temperature, for example, a magnesium-base hydrogen storage alloy.

[0072] Besides, the magnesium-base hydrogen storage alloy has a small weight per unit volume. Hence, the weight of the hydrogen storage alloy tank 3 can be reduced. In addition, the effective storage amount of hydrogen of the magnesium-base hydrogen storage alloy is 6.5 mass% in terms of percent by mass, and the effective hydrogen storage amount of a hydrogen storage alloy of an amide base or the like is 2.5 mass%. Hence, utilization of the magnesium-base hydrogen storage alloy will increase the amount of hydrogen that is released from the hydrogen storage alloy tank 3. [0073] As described above, the temperature of the cathode compressed off -gas can be raised by adjusting the compression rate of the cathode off-gas through the use of the air compressor 7. Therefore, hydrogen can be released even from a magnesium-base hydrogen storage alloy that releases hydrogen only at high temperatures as mentioned above, without a need for an electric heater.

[0074] Since the cathode compressed off-gas can also be used to drive the turbocharger 6, a large amount of air in a compressed state can be fed to the cathode of the fuel cell 1. Hence, the effective utilization of the compressed off -gas can be further pursued. In other words, since the energy for driving the turbocharger 6 can be recycled through the use of the compressed off-gas, the intake efficiency can be raised and the fuel economy can be improved.

[0075] It is also conceivable that an air compressor is also provided on the oxygen gas channel 16, specifically, upstream of the cathode of the fuel cell 1, so as to supply oxygen to the cathode. In this case, when the air compressor is operated, oxygen is supplied to the cathode in such a manner that oxygen is pushed in.

[0076] On the other hand, in the case where an air compressor is disposed on the cathode off-gas channel 36 as described above, the air compressor is disposed downstream of the cathode. Then, the cathode off-gas is discharged from the fuel cell 1 in such a manner that the cathode off-gas is sucked to the air compressor side. Correspondingly, oxygen supplied to the cathode is supplied to the fuel cell 1 in such a manner that oxygen is sucked in from the oxygen gas channel 16.

[0077] The force needed to draw a gas into the fuel cell is less than the force needed to push the gas into the fuel cell. Hence, the provision of the air compressor downstream of the cathode allows the rated output of the air compressor to be smaller than the provision thereof upstream of the cathode.

[0078] Furthermore, it is also conceivable that when the cathode is fed with oxygen, water is produced from hydrogen ions and oxygen, and the produced water may reside in the cathode.

[0079] However, if the air compressor is provided downstream of the cathode, the suction from the cathode draws water out. As a result, water does not reside in the cathode. Hence, the freezing of water in the fuel cell during the winter time can be restrained.

[0080] Incidentally, since the cathode off-gas contains water vapor, the water vapor becomes liquid as the cathode off-gas is compressed by the air compressor 7. Therefore, the condensation heat (latent heat) produced as water vapor becomes liquid can contribute to the temperature raise of the cathode off-gas

[0081] FIG 4 is a pressure-temperature graph in which the vertical axis represents pressure and the horizontal axis represents temperature in order to show a gas-liquid critical line of water. FIG 5 is an enlarged diagram of a portion of FIG 4. Table 2 shows detailed data regarding FIG 4.

Table 2

[0082] From FIGS. 4 and 5 and Table 2, it can be understood that a hydrogen storage alloy that can release hydrogen at temperatures shown in Table 2 will allow the exhaust heat of the condensation heat of water (2.3 kJ/g) to be recovered if the pressure of the air compressor 7 is set at about a value of 3 to 8 bar.

[0083] It can also be understood from Table 2 that in the case where the pressure of the air compressor is 4.3688 bar, the temperature of the exhaust of the condensation heat of water is 146.85°C, and that in the case where the pressure of the air compressor is about 7.33 bar, the temperature of the exhaust of the condensation heat of water is 166.85°C. Incidentally, whole-number values are presented in a lower portion of Table 2, showing that in the case where the pressure of the air compressor is 3 to 8 bar, the corresponding temperatures of the exhaust of the condensation heat of water are 132 to 173.

[0084] In this case, since the temperature of the magnesium-base hydrogen storage alloy that allows release of hydrogen is 150 to 400⁰C as mentioned above, it is not quite reasonable to utilize the condensation heat in order to release hydrogen from the magnesium-base hydrogen storage alloy.

[0085] As for the amide-base hydrogen storage alloy, however, the temperature at which hydrogen is released is 150⁰C or lower as mentioned above. Therefore, the condensation heat can be used to release hydrogen from the amide-base hydrogen storage alloy.

[0086] If the temperature of the fuel cell itself can be raised, the temperature of the cathode off-gas will also rise. Therefore, the temperature raise of the fuel cell itself will further contribute to the raising of the temperature of the cathode compressed off-gas.

[0087] It should be apparent that the foregoing embodiments are not meant to limit the scope of the invention.

[0088] While the invention has been described with reference to example embodiments thereof, it should be understood that the invention is not limited to the example embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the example embodiments are shown in various combinations and configurations, which are example, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.

Claims

1. A fuel cell system comprising: a fuel cell; a hydrogen storage alloy that releases a hydrogen when heated and supplies the hydrogen to the fuel cell; compression means for raising temperature of an off-gas discharged from the fuel cell by compressing the off-gas; and heat exchange means for heat-exchanging between the compressed off-gas and the hydrogen storage alloy.

2. The fuel cell system according to claim 1, wherein the heat exchange means transfers heat from the compressed off-gas to the hydrogen storage alloy.

3. The fuel cell system according to claim 1 or 2, wherein the off-gas is a cathode off-gas discharged from a cathode that is an air electrode of the fuel cell.

4. The fuel cell system according to claim any one of claims 1 to 3, wherein the compression means compresses the off-gas so that the temperature of the off-gas is in a predetermined temperature range.

5. The fuel cell system according to claim 4, wherein the predetermined temperature range is 150 to 400⁰C.

6. The fuel cell system according to claim 5, wherein the hydrogen storage alloy is a magnesium-base hydrogen storage alloy.

7. The fuel cell system according to any one of claims 1 to 6, further comprising hydrogen release determination means for determining that the hydrogen has been released from the hydrogen storage alloy, wherein a compression rate of the compression means is raised if it is determined by the hydrogen release determination means that the hydrogen has not been released from the hydrogen storage alloy.

8. The fuel cell system according to any one of claims 1 to 7, wherein the compression rate of the compression means is reduced if the compression rate becomes higher than a predetermined permissible compression rate.

9. The fuel cell system according to any one of claims 1 to 8, further comprising a turbine that recovers energy from the compressed off-gas.

10. The fuel cell system according to claim 9, further comprising pressure reduction means for reducing pressure of the compressed off-gas after heat is transferred from the compressed off-gas to the hydrogen storage alloy, wherein energy is recovered from the compressed off-gas by the turbine after the pressure of the off-gas is reduced by the pressure reduction means.

11. The fuel cell system according to any one of claims 1 to 10, wherein the fuel cell is a solid polymer type fuel cell.

12. The fuel cell system according to any one of claims 1 to 11, wherein the fuel cell system is mounted in a vehicle.

13. The fuel cell system according to claim 12, wherein: the compression means is driven by the fuel cell, except when the fuel cell is at low temperature or has just been started up, and the compression means is driven by a battery mounted in the vehicle, when the fuel cell is at low temperature or has just been started up.