Abstract:
A heat exchange system includes a fuel cell that receives a specified gas and generates electric power, a heat exchange device that exchanges heat with a heat exchange medium, a heat exchange medium passage, and a gas detector. The heat exchange medium passage allows the heat exchange medium to circulate between the heat exchange device and the fuel cell such that the heat exchange medium can exchange heat with the heat exchange device and the fuel cell. The gas detector is disposed at at least one of the heat exchange device and the heat exchange medium passage to detect the specified gas that leaks into the heat exchange medium.

Description:
INCORPORATION BY REFERENCE  
         [0001]    The disclosure of Japanese Patent Application No. 2000-060806 filed on Mar. 6, 2000 including the specification, drawings and abstract, is incorporated herein by reference in its entirety.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention relates to a heat exchange system which feeds a heat exchange medium to a fuel cell so as to exchange heat with the fuel cell, or which feeds a heat exchange medium warmed through heat exchange with a heating element, to a gas absorbing device such as a hydrogen gas absorbing alloy tank, so as to heat the gas absorbing device.  
           [0004]    2. Description of Related Art  
           [0005]    In general, a fuel cell generates power in the manner as follows: hydrogen-containing fuel gas and oxygen-containing oxidizing gas are supplied to a fuel cell, so that electrochemical reactions take place at an anode and a cathode of the cell, according to reaction formulas as indicated below.  
           [0006]    To be more specific, when the fuel gas and the oxidizing gas are supplied to the anode and the cathode, respectively, the reactions as represented by formulas (1) and (2) take place at the anode side and the cathode side, respectively, such that the fuel cell as a whole undergoes a reaction as represented by formula (3).  
           H 2 →2H + +2e −   (1)  
           2H + +2e − +(½)O 2 →H 2 O  (2)  
           [0007]     H 2 +(½)O 2 →H 2 O  (3)  
           [0008]    Since these electrochemical reactions are heat generating or exothermic reactions, the inside of the fuel cell must be cooled in order to prevent the temperatures at the anode and the cathode from rising excessively. To this end, a heat exchange system is usually provided for feeding the fuel cell with cooling water as a heat exchange medium cooled by a radiator, through a cooling water passage, thereby to cool the inside of the fuel cell. One such type of heat exchange system for a fuel cell is disclosed in Japanese Patent Publication No. HEI 7-66828.  
           [0009]    In some cases, the fuel gas to be fed to the fuel cell is supplied from a hydrogen absorbing alloy tank containing a hydrogen absorbing alloy. In general, hydrogen absorbing alloys have the property of releasing hydrogen through an endothermic reaction when heated, and of absorbing hydrogen through an exothermic reaction when cooled. Therefore, in order to extract hydrogen from the hydrogen absorbing alloy, the hydrogen absorbing alloy inside the hydrogen absorbing alloy tank must be heated as needed. To this end, the heat exchange system feeds the hydrogen absorbing alloy tank with cooling water that is a heat exchange medium warmed by heat exchange with a heating element such as a fuel cell, through a cooling water passage, thereby to heat the inside of the hydrogen absorbing alloy tank.  
           [0010]    Thus, the heat exchange system feeds cooling water serving as a heat exchange medium to the fuel cell in order to cool it and to the hydrogen absorbing alloy tank in order to heat it.  
           [0011]    In the fuel cell, the cooling water supplied to the cell is completely separated from the fuel gas and the oxidizing gas by separators in each single cell. When the fuel cell is used for an extended period of time, however, the sealing member that seals the periphery of each separator may deteriorate, causing the fuel gas or oxidizing gas to leak into the cooling water.  
           [0012]    In the hydrogen absorbing alloy tank, the supplied cooling water runs through a tube while circulating within the tank, and is thus completely separated from hydrogen gas (that is, fuel gas). In some cases, the wall surface of the tube deteriorates after an extended period of use, and the hydrogen gas leaks into the cooling water.  
           [0013]    In the conventional heat exchange system, however, no countermeasure has been taken against leakage of the fuel gas or oxidizing gas into the cooling water as the heat exchange medium. Thus, the heat exchange system may suffer from deterioration of heat exchange performance due to the presence of gas in the cooling water.  
         SUMMARY OF THE INVENTION  
         [0014]    It is an object of the invention to provide a heat exchange system which can minimize the possibility of a specified gas leaking into a heat exchange medium.  
           [0015]    To accomplish at least a part of the above object, a heat exchange system according to the first aspect of the invention includes a fuel cell that receives a specified gas and generates electric power, a heat exchange device that performs heat exchange with a heat exchange medium, a heat exchange medium passage, and a gas detector. The heat exchange medium passage circulates the heat exchange medium between the heat exchange device and the fuel cell such that the heat exchange medium can exchange heat with the heat exchange device and the fuel cell. A gas detector is provided at at least one of the heat exchange device and the heat exchange medium passage at a location to detect the specified gas that leaks into the heat exchange medium.  
           [0016]    According to a second aspect of the invention, there is provided a heat exchange system which includes an exothermic body capable of generating heat, a gas absorbing device comprising a gas absorbing alloy that is able to absorb or release a specified gas, a heat exchange device configured and positioned to perform heat exchange with a heat exchange medium, a heat exchange medium passage and a gas detector. The heat exchange medium passage circulates the heat exchange medium among the heat exchange device, the exothermic body, and the gas absorbing device such that the heat exchange medium can exchange heat with the heat exchange device, the exothermic body and the gas absorbing device. The gas detector is provided at at least one of the heat exchange device and the heat exchange medium passage at a location to detect the specified gas that leaks into the heat exchange medium.  
           [0017]    In the heat exchange system of the invention as described above, even where a specified gas leaks into the heat exchange medium, the gas detector immediately detects leakage of the gas, of which the driver can be promptly informed. Thus, the leakage of the gas into the heat exchange medium will not be left as it is, and otherwise possible deterioration of the heat exchange performance due to bubbling of the specified gas can be advantageously avoided.  
           [0018]    The heat exchange system may further include a heat exchange medium storage device for storing at least an excess of the heat exchange medium when the amount of the heat exchange medium that circulates through the heat exchange system becomes excessive. In this case, the gas detector is provided at at least one of the heat exchange device, the heat exchange medium passage and the heat exchange medium storage device. The provision of the gas detector at the heat exchange medium storage device also yields the same advantage as described above.  
           [0019]    Preferably, the gas detector is located at a portion of the heat exchange device or the heat exchange medium passage, which portion is higher in position than the other portions thereof or has a larger volume than the other portions thereof.  
           [0020]    Since gas is normally likely to collect at a location that is higher in position or has a larger volume or capacity, the gas detector is preferably disposed at such a location so that leakage of the specified gas into the heat exchange medium can be more quickly and surely detected.  
           [0021]    In one preferred embodiment of the invention, the heat exchange device comprises a radiator with a radiator cap located at the top thereof, and the gas detector is attached to the radiator cap.  
           [0022]    In another preferred embodiment of the invention, the heat exchange medium storage device comprises a reserve tank, and the gas detector is attached to an upper portion of the reserve tank.  
           [0023]    Where the radiator is used as the heat exchange device, and the reserve tank is used as the heat exchange medium storage device, the gas detector is located at the upper portion of the radiator or the reserve tank which is higher in position and has a larger volume or capacity and at which the specified gas leaking into the heat exchange medium is likely to collect. Also, the gas detector provided at such a location can be relatively easily detached or removed, thus facilitating maintenance or replacement of the gas detector.  
           [0024]    The heat exchange system of the invention is preferably installed in a vehicle. In the case where a fuel cell and a hydrogen absorbing alloy tank are installed in an electric vehicle or a hybrid vehicle, for example, the heat exchange system installed in the vehicle permits early detection of any leakage of a specified gas into the heat exchange medium.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    [0025]FIG. 1 is a schematic view showing a heat exchange system according to a first embodiment of the invention;  
         [0026]    [0026]FIGS. 2A and 2B are sectional views schematically showing a stack structure and a single cell structure, respectively, of the fuel cell of FIG. 1;  
         [0027]    [0027]FIG. 3 is a schematic view showing a heat exchange system according to a second embodiment of the invention; and  
         [0028]    [0028]FIG. 4 is a view showing an example of another location at which a hydrogen sensor may be installed.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0029]    Hereinafter, presently preferred embodiments of the invention will be described. FIG. 1 is a schematic view showing a heat exchange system according to a first embodiment of the invention.  
         [0030]    The heat exchange system of this embodiment can cool a fuel cell  30  and heat a hydrogen absorbing alloy tank  40 . The heat exchange system is installed in an electric vehicle or a hybrid vehicle or the like having the fuel cell  30  and the hydrogen absorbing alloy tank  40 .  
         [0031]    As shown in FIG. 1, the heat exchange system mainly includes a radiator  10 , cooling water passages  60  to  64 , water pumps  70  and  76 , valves  72  and  74 , and a reserve tank  20 , and uses cooling water as a heat exchange medium flowing through the system. As the cooling water, normal water can be used, but it is preferable to use water to which anticorrosive and/or antifreeze treatment(s) have been applied.  
         [0032]    The radiator  10  is a heat exchange device for cooling the cooling water warmed by the fuel cell  30 , and includes an upper tank  12  and a lower tank  14  for temporarily storing the cooling water, and a core  16  for passing the cooling water. Although not shown in FIG. 1, the core  16  is composed of a combination of narrow water tubes through which the cooling water runs and wavy metal plates called corrugated fins, the combination being in the form of a network.  
         [0033]    The cooling water warmed by the fuel cell  30  flows to the upper tank  12  to be temporarily stored therein and then led to the lower tank  14  through the water tubes in the core  16  to be stored in the lower tank  14 . While the cooling water passes through the water tube, the fins that are in contact with the tubes take away or dissipate the heat, to thus cool the cooling water. The fins are cooled by the breeze while the vehicle is running, or by a cooling fan (not shown) provided behind the radiator  10 .  
         [0034]    In this manner, the cooling water cooled and stored in the lower tank  14  flows out from the lower tank  14  to reach the fuel cell  30  through the cooling water passage  60 . A water pump  70  is provided midway in the cooling water passage  60  so as to forcibly circulate the cooling water flowing through the cooling water passage  60 . The water pump  70  and another water pump  76  which will be described later are both electrically driven.  
         [0035]    The cooling water which has reached the fuel cell  30  enters a manifold (not shown) that allows cooling water to flow into the fuel cell  30 , and is then divided into streams flowing into cooling water channels within respective single cells so as to cool the anode and cathode of each single cell. During the flow through the fuel cell  30 , the cooling water itself is warmed by taking heat away from the anode and the cathode of each cell. The streams of cooling water that have passed through these cooling water channels again join together to reach a manifold (not shown) which allows the cooling water to flow out from the fuel cell  30 .  
         [0036]    The cooling water that flows out from the fuel cell  30  passes through the cooling water passage  61  and is then divided into two flow paths, one of which is led to a valve  72  and the other of which is led to a valve  74 . These valves  72  and  74  selectively switch between a flow path leading the cooling water warmed by the fuel cell  30  to the hydrogen absorbing alloy tank  40  so as to heat the hydrogen absorbing alloy tank  40 , and a flow path bypassing the hydrogen absorbing alloy tank  40 .  
         [0037]    For example, when the valve  72  is closed and the valve  74  is open, the warmed cooling water flows through the cooling water passage  62  into the hydrogen absorbing alloy tank  40  so as to heat the hydrogen absorbing alloy tank  40 . On the contrary, when the valve  72  is open and the valve  74  is closed, the warmed cooling water bypasses the hydrogen absorbing alloy tank  40  without being used to heat the hydrogen absorbing alloy tank  40 .  
         [0038]    The hydrogen absorbing alloy tank  40  contains a hydrogen absorbing alloy  42 . As is well known in the art, the hydrogen absorbing alloy  42  has the property of releasing hydrogen through an endothermic reaction when heated, and absorbing hydrogen through an exothermic reaction when cooled. Therefore, when it is desired to extract or take out absorbed hydrogen from the hydrogen absorbing alloy tank  40 , warmed cooling water is supplied to the hydrogen absorbing alloy tank  40  so as to heat the hydrogen absorbing alloy  42  in the hydrogen absorbing alloy tank  40  as described above. On the other hand, when it is desired to store hydrogen in the hydrogen absorbing alloy tank  40 , the temperature of the hydrogen absorbing alloy  42  in the tank  40  is lowered by stopping the supply of the warmed cooling water to the hydrogen absorbing alloy tank  40 .  
         [0039]    When the warmed cooling water is supplied to the hydrogen absorbing alloy tank  40 , the cooling water flows through a cooling water tube  44  circulating within the hydrogen absorbing alloy tank  40  so as to heat the hydrogen absorbing alloy  42  in the hydrogen absorbing alloy tank  40 .  
         [0040]    After flowing out from the hydrogen absorbing alloy tank  40 , the cooling water that heated the hydrogen absorbing alloy  42  is returned to the upper tank  12  of the radiator  10  through cooling water passages  63  and  64 . Midway in the cooling water passage  63 , the water pump  76  is provided for forcibly circulating the cooling water which has passed through the hydrogen absorbing alloy tank  40 . Thus, the water pump  76  is driven when the valve  72  is closed and the valve  74  is open.  
         [0041]    When the cooling water is not supplied to the hydrogen absorbing alloy tank  40 , on the other hand, the warmed cooling water that flows out from the fuel cell  30  is returned to the upper tank  12  of the radiator  10  after passing through the valve  72  and the cooling water passage  64 .  
         [0042]    A radiator cap  18 , which also serves as a pressure regulating valve, is mounted on the top of the upper tank  12 , and a cooling water tube  65  extends from the radiator cap  18  to a reserve tank  20 .  
         [0043]    As shown in FIG. 1, the reserve tank  20  is a simple sealed type reserve tank, and an air intake tube  66  connects to the reserve tank  20  to maintain atmospheric pressure inside the reserve tank  20 .  
         [0044]    When the temperature of the cooling water in the upper tank  12  rises to such an extent that part of the water boils and the pressure within the upper tank  12  exceeds a predetermined level, cooling water and steam emitted from the tank  12  are pushed out through the cooling water tube  65  into the reserve tank  20 . In the reserve tank  20 , the steam liquefies and returns to water  22  without being actively cooled because of the low ambient temperature. Later, when the pressure inside the upper tank  12  becomes lower than the atmospheric pressure due to a decrease in the temperature of the cooling water in the upper tank  12 , the cooling water flows out from the reserve tank  20  and runs back to the upper tank  12  through the cooling water tube  65 .  
         [0045]    The reserve tank  20  has a cooling water supply cap  24  mounted atop it. The cooling water supply cap  24  can be opened so that the cooling water  22  in the reserve tank  20  can be replenished when it falls below a predetermined amount.  
         [0046]    The heat exchange system shown in FIG. 1 has been schematically described above. Hydrogen sensors  50  and  52  and so forth, which are characteristic features of the invention, will be described in detail later.  
         [0047]    Next, a circulation path of fuel gas to be supplied from the hydrogen absorbing alloy tank  40  to the fuel cell  30  will be briefly described.  
         [0048]    As shown in FIG. 1, a hydrogen gas is first supplied from outside to the hydrogen absorbing alloy tank  40  through a hydrogen gas inflow passage  80 . At this time, if the supply of heated cooling water to the hydrogen absorbing alloy tank  40  is stopped, and the temperature of the hydrogen absorbing alloy tank  40  falls as described above, the supplied hydrogen gas is absorbed in the hydrogen absorbing alloy  42 . Then, if the supply of the heated cooling water to the hydrogen absorbing alloy tank  40  is started, and the temperature inside the tank  40  rises, the hydrogen gas absorbed in the hydrogen absorbing alloy  42  is released therefrom. At this moment, a valve  82  is opened, and the released hydrogen gas is supplied to the fuel cell  30  through fuel gas passages  81  and  83  to serve as fuel gas in the cell. Midway in the fuel gas passage  83  are provided a hydrogen gas compressor  84  for circulating the hydrogen gas, a valve  85  for stopping the supply of the hydrogen gas to the fuel cell  30 , and a throttle valve  86  for adjusting the amount of flow of the hydrogen gas to be supplied to the fuel cell  30 . The hydrogen gas supplied to the fuel cell  30  enters a manifold for fuel gas inflow and is then divided into streams flowing into fuel gas channels within respective single cells so that the hydrogen gas is supplied to the anode of each single cell, as will be described later. The remaining hydrogen gas that was not supplied to the anode is re-collected into a manifold for fuel gas outflow and flows out from the fuel cell  30 . The hydrogen gas thus discharged is returned again to the fuel gas passage  81  through a fuel gas passage  87  and circulated.  
         [0049]    The schematic structure of the fuel cell  30  will be described hereinafter with reference to FIGS. 2A and 2B. FIGS. 2A and 2B are sectional views schematically showing stack structure and single cell structure, respectively, of the fuel cell  30  as shown in FIG. 1. FIG. 2A shows a section of the stack structure, and FIG. 2B shows a section of the single cell structure which is an enlargement of a portion of FIG. 2A including a single cell.  
         [0050]    As shown in FIG. 2B, a single cell is composed of an electrolyte film  35 , an anode  36  and a cathode  37  which are diffusion electrodes that sandwich the film  35  from both sides, and two separators  34  which sandwich the electrodes from both sides. The separators  34  have mutually opposed surfaces in which recesses are formed, and cooperate with the anode  36  and cathode  37  sandwiched between the separators  34  to form gas channels within the single cell. Of the gas channels thus formed, gas channels  32  formed between the separator  34  and the anode  36  allow hydrogen gas supplied as described above as fuel gas to pass therethrough, and gas channels  33  allow oxygen containing air, serving as oxidizing gas, to pass therethrough.  
         [0051]    In the present embodiment, as shown in FIG. 2A, two adjacent separators  34 , which are located at intervals of two single cells, are in direct contact with each other, and have recesses formed in their opposed surfaces such that cooling water channels  31  are formed between the adjacent separators  34 . The cooling water supplied to the fuel cell  30  as described above is caused to flow through the cooling water channels  31 .  
         [0052]    As shown in FIG. 2A, the cooling water flowing through the cooling water channels  31  is usually completely separated from the hydrogen gas and oxidizing gas respectively flowing through the gas channels  32  and  33 . However, as the fuel cell  30  is used for an extended period of time, cracks may be formed in the separators  34 , or a sealing member (not shown) sealing the periphery of the separators  34  may deteriorate, causing the hydrogen gas (and/or the oxidizing gas) flowing through the gas channels  32  (and  33 ) to leak into the cooling water flowing through the cooling water channels  31 .  
         [0053]    In the hydrogen absorbing alloy tank  40 , the supplied cooling water normally flows through the cooling water tube  44  circulating in the tank  40  while being completely separated from the hydrogen gas, as shown in FIG. 1. In some cases, however, the wall surface of the cooling water tube  44  may deteriorate after a long period of use, and the hydrogen gas present in the upper portion of the hydrogen absorbing alloy tank  40  may leak into the cooling water passing through the cooling water tube  44 .  
         [0054]    If hydrogen gas leaks into the cooling water in the above manner, the hydrogen gas turns into bubbles in the cooling water, which may possibly result in deterioration of the heat exchange performance of the entire heat exchange system.  
         [0055]    In view of the above problem, the present embodiment adopts the following structure for detecting leakage of hydrogen gas into the cooling water early and informing the driver of the vehicle of the gas leakage.  
         [0056]    In the heat exchange system of the present embodiment as shown in FIG. 1, the hydrogen sensor  50  is mounted in the radiator cap  18  at the top of the radiator  10 , and the hydrogen sensor  52  is mounted at the top portion of the reserve tank  20 . Each of the hydrogen sensors  50  and  52  detects even a very small amount of hydrogen if it is contained in the air, and outputs a detection signal.  
         [0057]    The heat exchange system of the present embodiment further includes a control unit  90  and a hydrogen gas leakage warning lamp  92  provided on the dashboard of the driver&#39;s seat. The control unit  90  detects the leakage of hydrogen gas into the cooling water from a detection signal received from the hydrogen sensors  50  and  52 , and outputs a driving signal. The hydrogen gas leakage warning lamp  92  lights up when the driving signal is received from the control unit  90 .  
         [0058]    When hydrogen gas leaks into the cooling water, the hydrogen gas turns into bubbles, which then flow through the cooling water passage together with the cooling water and collect at a portion within the heat exchange system which is higher in position and has a relatively large capacity. To be more specific, the hydrogen gas in the form of bubbles collects at the top portion of the upper tank  12  of the radiator  10 , or around the radiator cap  18 , which is located at the highest position in the heat exchange system. If the pressure inside the upper tank  12  is high, the cooling water is pushed out as described above from the upper tank  12  into the reserve tank  20  through the cooling water tube  65  so that the hydrogen gas caught within the upper tank  12  is also pushed out into the reserve tank  20  along with the cooling water. The hydrogen gas pushed out together with the cooling water turns into bubbles in the cooling water  22  and floats up to the surface of the water, to be present at the top of the reserve tank  20 .  
         [0059]    As described heretofore, the hydrogen sensors  50  and  52  mounted in the radiator cap  18  of the radiator  10  and in the reserve tank  20 , respectively, detect hydrogen gas collected at the top of the upper tank  12  or at the top of the reserve tank  20  due to the leakage of the hydrogen gas into the cooling water, and output detection signals. Upon detecting the leakage of the hydrogen gas into the cooling water from the detection signals, the control unit  90  outputs a driving signal to the hydrogen gas leakage warning lamp  92 . The lamp  92  then lights up to inform the driver that hydrogen gas is leaking into the cooling water.  
         [0060]    Thus, in the heat exchange system of the present embodiment, if hydrogen gas leaks into the cooling water, the hydrogen sensors  50  and  52  immediately detect the leakage, and the hydrogen gas leakage warning lamp  92  informs the driver of the leakage. Once the driver notices the lighting of the lamp  92 , the driver can ask for an inspection of the vehicle soon in order to get repairs or replacements and so forth as necessary. The hydrogen gas collected in the upper tank  12  of the radiator  10  and the hydrogen gas collected at the top of the reserve tank  20  can be easily discharged into the air by opening the radiator cap  18  and the cooling water supply cap  24 , respectively. Moreover, the hydrogen sensors  50  and  52  are installed at sites which allow the sensors to be comparatively easily detached, which facilitates the maintenance or replacement of these hydrogen sensors.  
         [0061]    [0061]FIG. 3 is a block diagram showing the structure of a heat exchange system according to a second embodiment of the invention. The heat exchange system of the present embodiment differs from the system of the first embodiment shown in FIG. 1 in that a completely sealed type reserve tank  100  is used instead of the simple sealed type reserve tank  20 . Since the other components are identical to those shown in FIG. 1, the description of these components will be omitted.  
         [0062]    When the pressure in the upper tank  12  exceeds a predetermined level due to a rise in the temperature of the cooling water in the upper tank  12  of the radiator  10 , the cooling water and steam emitted from the tank  12  flow into the reserve tank  100  through a cooling water tube  68  in the same manner as with the reserve tank  20  shown in FIG. 1. However, since the reserve tank  100  is of the completely sealed type unlike the reserve tank  20 , the cooling water never returns to the upper tank  12  from the reserve tank  100  through the cooling water tube  68  even if the pressure in the upper tank  12  falls due to a decrease in the temperature of the cooling water in the upper tank  12 . Instead, the cooling water  22  in the reserve tank  100  is led to the cooling water passage  60 , not through the cooling water tube  68 , but through a cooling water passage  67  after leaving an outlet formed at the bottom of the reserve tank  100 .  
         [0063]    Since hydrogen gas that leaks into the cooling water may collect at the top of the reserve tank  100  in the present embodiment, a hydrogen sensor  52  is provided at the top of the reserve tank  100  for detecting the leakage of the hydrogen gas. Thus, the present embodiment provides the same advantages as the first embodiment. In addition, the use of the reserve tank of the completely sealed type in the present embodiment eliminates a possibility that impurities contained in the air may be introduced into the cooling water.  
         [0064]    While the hydrogen sensors are mounted in the radiator cap  18  of the radiator  10  and at the top of the reserve tank  20 ,  100  in the illustrated embodiments, such a hydrogen sensor may be installed midway in a cooling water passage connecting the radiator  10  and the fuel cell  30  or the hydrogen absorbing alloy tank  40  as shown in FIG. 4.  
         [0065]    [0065]FIG. 4 shows an example of a location at which a hydrogen sensor may be installed. In FIG. 4, a portion of the cooling water passage  64  through which the cooling water flows into the upper tank  12  of the radiator  10  forms a circuit that projects upwards so as to bypass an obstacle(s) or the like. Since the circuit portion of the passage  64  is higher in position than the other portions, it is considered that hydrogen gas that leaks into the cooling water and turns into bubbles is likely to collect at the circuit portion. In this modified example, therefore, another hydrogen sensor  54  is provided at the circuit portion of the cooling water passage  64 .  
         [0066]    Thus, the same advantages as provided in the illustrated embodiments may be obtained by providing an additional hydrogen sensor at a portion of the cooling water passage which is higher in position than the other portions.  
         [0067]    It is to be understood that the invention is not limited to details of the illustrated embodiments, but may be embodied with various changes or improvements without departing from the scope of the invention.  
         [0068]    In the heat exchange system of each of the above embodiments, the fuel cell  30  is cooled by using the cooling water, and the hydrogen absorbing alloy tank  40  is heated by using the cooling water that has been warmed through the cooling of the fuel cell  30 . However, the invention is not restricted to this type of system. For instance, the invention is applicable to a system in which cooling water is used only to cool the fuel cell  30 . In another example of the heat exchange system, the hydrogen absorbing alloy tank  40  can be heated by cooling water that has been warmed not by taking heat away from the fuel cell  30  but by cooling another heat-generating or exothermic body (auxiliary equipment or an engine in the case of a hybrid car, for example).  
         [0069]    In the illustrated embodiments, the hydrogen sensors  50 ,  52 , and  54  detect the presence of hydrogen in the air. However, if a sensor capable of detecting the presence of hydrogen in a liquid is developed, such a sensor could also be used. In that case, sensors could be installed at any location in the path through which the cooling water flows, without taking account of the height in position or the likelihood of collection of hydrogen gas in the form of bubbles.  
         [0070]    While leakage of hydrogen gas into cooling water is detected by the hydrogen sensors in the illustrated embodiments, leakage of, for example, oxidizing gas into cooling water may be detected by using a gas sensor for detecting oxidizing gas.  
         [0071]    In the illustrated embodiments, cooling water is used as a heat exchange medium. However, the invention is not restricted to this, but may use a heat exchange medium other than water.  
         [0072]    In the above embodiments, the warning lamp  92  is used to visually inform the driver that hydrogen gas is leaking into the cooling water. Alternatively, a beeper or a speaker can be used to give notification by sound.