Abstract:
The present invention provides a cooling system of fuel cell, which solves the gas biting noise problem by trapping bubble in a coolant, gives protection to a stack (MEA) and the membrane of a humidifier by regulating the coolant pressure of fuel cell and improves the life of a ion exchanger by preventing foreign materials (carbonate ion etc.) from dissolving into the coolant.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a cooling system of fuel cell, which employs a coolant, and a process for cooling the fuel cell.  
         BACKGROUND OF THE INVENTION  
         [0002]    In recent years, extensive research and development works have been undertaken to provide a polymer electrode membrane type fuel cell. The polymer electrode membrane type fuel cell (PEFC) is capable of generating power under the room temperature conditions and has been coming into wide use. And this type of a fuel cell system does not require the compression of intake gas (air-fuel mixture), allowing the height of the whole system to be smaller than an internal combustion engine. Therefore, it can be disposed in a small space such as the under-floor of a vehicle compartment etc., and the space efficiency is improved.  
           [0003]    This type of a fuel cell system comprises a cathode electrode and an anode electrode, which interpose a solid polymer membrane (electrode membrane). It drives an external load with the electrical power generated by the chemical reaction between the oxygen supplied to the cathode electrode and the hydrogen supplied to the anode electrode. However, because the chemical reaction for generating power is an exothermic reaction, a cooling system is essential to constantly perform a stable operation regardless of the output fuel cell.  
           [0004]    As this type of a cooling system, a two-step heat radiation system is known, in which the heat generated during the power generation by the fuel cell is released by a radiator and the cooling line of the fuel cell is cooled through an intermediate heat exchanger. FIG. 10 shows said cooling system. As shown in FIG. 10, a cooling system  211  comprises; a cooling path  213  laid out in a fuel cell  212  to cool said fuel cell  212 ; a primary coolant circulating path  214  to circulate the primary coolant to the fuel cell  212 ; a secondary coolant circulating path  215  to cool the primary coolant circulating in said primary coolant circulating path  214 ; a heat exchanger  216  to cool the primary coolant by exchanging heat between the primary and secondary coolant. The inlet and outlet of said primary coolant circulating path  214  are respectively connected to the inlet and outlet of the cooling path  213  laid out in said fuel cell, and a radiator  217  is disposed within the secondary coolant circulating path  215 . The secondary coolant cooled by said radiator  217  cools the primary coolant, thus cooling the fuel cell  212 . A bypass path  218  bypassing the heat exchanger  216  is placed in the primary coolant-circulating path  214 . A thermostat valve  220  is disposed at a connection  219  between said bypass path  218  and the downstream location of the primary coolant-circulating path  214  with regard to said heat exchanger  216 . The switching of the thermostat valve  220  controls the primary coolant temperature to be appropriate for the power generation of the fuel cell  212 . The primary and secondary coolants are a mixture of ethylene glycol and water, the ratio of the two fluids being determined as required.  
           [0005]    It is necessary to humidify the electrode membrane to operate the fuel cell and there is a known device to collect the water for humidification (off-gas collection) from the generated water by the power generation of fuel cell. FIG. 11 depicts this type of a system. A cooling system  200  basically comprises; a coolant path  201   c  laid out in a fuel cell  201  to cool the same, a circulating path  202  and a circulating pump  202   a  connected to said coolant path  201   c  to circulate the coolant in the fuel cell  201 , a heat exchanger  202   b  to cool the coolant, a thermo regulator  202   c  to control the temperature of the coolant supplied to the fuel cell  201 . An outlet  202   g  and an inlet  202   f  of said circulating path  202  are connected to an inlet  201   a  and an outlet  201   b  of the coolant path  201   c  laid out in said fuel cell  201  and the fuel cell  201  is cooled by circulating the coolant with the circulating pump  202   a.    
           [0006]    Generally speaking in the cooling system of fuel cell, pure water or dielectric coolant is employed as coolant for the fuel cell cooling to prevent the phenomenon of fluid short circuit. (An off-gas is discharged from the fuel cell  201  as a mixture of vapor and water, which may cause a short circuit with the structure supporting the fuel cell  201  through said water. This short circuit is referred to as “fluid short circuit”.)  
           [0007]    In the circulating path  202 , which is for the coolant of the cooling system of fuel cell  200 , a bypassing path  202   e  is provided bypassing the heat exchanger  202   b  to directly supply the coolant to a heat exchanger  202   c  which is placed at a downstream in the bypassing path  202   e  when the coolant does not require cooling by the heat exchanger  202   b . Thus, the cooling system  200  controls the coolant temperature appropriate for the power generation by the fuel cell  201 .  
           [0008]    Further, an ion exchanger  202   d  is provided in a bypassing path  202   h , which connects the outlet-side path of the thermo regulator  202   c  and the upstream path of the circulating pump  202   a , maintaining the low electrical conductivity of coolant.  
           [0009]    In the cooling system of fuel cell  211 , circulating pumps  221  and  222  are provided for the primary coolant-circulating path  214  and the secondary coolant-circulating path  215  respectively to circulate the coolant. The circulating pumps  221  and  222  forcefully circulate the primary coolant and the secondary coolant respectively. However, when the vertical difference of the primary coolant-circulating path  214  is small and the gas venting of the path is not sufficient, the biting noise of the circulating pump  221  may occur.  
           [0010]    It is feared that the intake pressure of the cooling path  213  varies and imposes the overload, which releases the contact of the separators, on the contact surface of cells of the fuel cell  212 , namely the contact surface of the separators providing the cooling path  213 , thus giving rise to the fluid leak and electrical conductivity failure of the coolant path  213 , etc.  
           [0011]    In said cooling system  200 , when for instance the power output of the fuel cell  201  becomes large, the fluid pressure of the circulating path  202  will increase due to; (i) the coolant expansion by the temperature increase, (ii) the pressure loss inherent to the flow and (iii) the increase of operating pressure of the circulating pump  202   a  circulating the coolant. If the pressure of the coolant is too high, it may cause the following problems:  
           [0012]    (1) As shown in FIG. 12, in a cell  300  of the fuel cell  201 , the pressure of the coolant releases the contact surface S of separators  302 ,  302  providing the coolant path  201   c , resulting in the cause of fluid leak or the electrical conductivity failure.  
           [0013]    (2) When a humidifier  400  (humidifying by water chamber) with hollow fiber membranes  401 ,  401  shown in FIG. 13 is employed to humidify the gas supplied to the fuel cell  201 , an excessive pressure will be imposed on the hollow fiber membrane  401  as the humidification is performed in a closed system.  
           [0014]    As countermeasures, following are anticipated to solve the excessive-pressure problem; (a) part of the cooling system releasing to the atmosphere, (b) regulating the pressure of the cooling system to a predetermined pressure by connecting to the atmosphere to breath through a pressure regulating valve and (c) regulating the pressure of the coolant by balancing the cathode gas pressure and the coolant pressure with the connection of the cathode gas tube and the cooling system.  
           [0015]    However, because the pressure regulation is performed by introducing the air and the cathode gas, which are directly exposed to the coolant, the carbon dioxide in the air or the cathode gas will dissolve into the coolant, ionizing the coolant to increase the electrical conductivity (deteriorating the electrical insulation). Though the carbonate ion is absorbed by the ion exchange resin and the increase in the electrical conductivity of the coolant (insulation deterioration) will be prevented when an ion exchanger  202   d  shown in FIG. 11 is prepared in the cooling system, there still remains a problem that the life of the ion exchanger  202   d  is reduced as the operating hour of the ion exchange resin is lessened.  
           [0016]    Humidifiers  203   a ,  203   b  are used to humidify the gas supplied to the fuel cell  201 , which humidify through a membrane. Because there is no pressure regulating means in the circulating path  202  of the coolant used for humidification, the pressure of the coolant will not be regulated even if the output of the fuel cell  201  increases and the fluid pressure of the circulating path  202  gets higher.  
         SUMMARY OF THE INVENTION  
         [0017]    The first object of the invention is to remove the vapor in the coolant of the fuel cell.  
           [0018]    Also the second object of the invention is to protect the stack (MEA) by regulating the pressure of the coolant of fuel cell.  
           [0019]    Further the third object of the invention is to protect the membrane of the humidifier for humidifying the supplied gas of the fuel cell by regulating the pressure of the coolant of fuel cell.  
           [0020]    Further, the fourth object of the invention is to extend the life of the ion exchanger by preventing foreign materials (carbonic acid ion etc.) from mixing into the coolant when the pressure of coolant is regulated.  
           [0021]    According to the invention defined in the appended claim  1 , there is provided a cooling system of fuel cell, comprising a circulating path connected to the inlet and outlet of fuel cell coolant to cool the inside of fuel cell with layered structure, and a circulating pump to circulate a coolant in said circulating path, said system further comprising a vapor-fluid separator provided in a relatively high pressure point of said circulating path, a tank interconnected with said vapor-fluid separator through a gas venting path, said tank being interconnected with a relatively low pressure point of said circulating path through a coolant retuning path, a pressure regulator placed in one of said gas venting path or said coolant-returning path, which is interconnected with the point that is more remote from the coolant inlet of said fuel cell.  
           [0022]    Consequently, the bubble (gas) is trapped by the vapor-fluid separator and said bubble accumulated in the upper portion of the vapor-fluid separator is directed to the tank with the coolant as mixture of vapor and fluid. Collecting the bubble, the noise of circulating pump due to the bubble biting will disappear. In this case, all the bubble is not necessarily collected in the tank at one time. If the tank is interconnected with said circulating path through the coolant returning path and the cross section of said gas venting path is configured for gas venting, said bubble coming into the tank will be subsequently collected in the tank by repeating the circulating cycle, vapor-fluid separator→gas venting path→tank→coolant returning path→circulating path→cooing path→circulating path→vapor-fluid separator, though the bubble is not collected at one time. As a result, when all the bubble is collected with a plural number of circulating cycles, the noise of circulating pump due to the gas biting will disappear.  
           [0023]    In this case a restriction would be preferably selected for said pressure regulator shown as the invention defined in the appended claim  2 .  
           [0024]    According to the invention defined in the appended claim  3 , there is provided a cooling system of fuel cell, comprising a circulating path connected to the inlet and outlet of fuel cell coolant to cool the inside of fuel cell with layered structure, and a circulating pump to circulate a coolant in said circulating path, said system further comprising a vapor-fuel separator provided in a relatively high pressure point of said circulating path, a tank interconnected with the vapor-fluid separator through the gas-venting path, and a pressure-regulating valve mounted to said tank, which opens when the inner pressure of the tank exceeds a given pressure corresponding to the allowable inlet pressure of fuel cell coolant.  
           [0025]    In this way, the connecting path gives a connection between the coolant circulating path to circulate the coolant and the tank and the relief valve opens when the tank inner pressure exceeds a given pressure, which corresponds to the allowable inlet pressure of coolant of said fuel cell inlet. Consequently, the inlet pressure of coolant of fuel cell inlet is maintained no greater than the allowable inlet pressure of fuel cell coolant. In this case, a diameter-extended portion may be provided in the connecting point between the interconnecting path and the primary coolant-circulating path to improve the gas trapping, which will terminate the gas biting noise of the circulating pump  11  in a short time.  
           [0026]    According to the invention defined in the appended claim  4 , there is provided a cooling system of fuel cell, in which said pressure regulating valve is configured so that it is interconnected with the atmosphere or the cathode gas supplying path, which supplies the oxidant gas to the fuel cell.  
           [0027]    Because sufficient differential pressure is obtained this way, the gas trapping will be efficiently performed and the inlet pressure of coolant will be maintained no grater than the allowable inlet pressure of said fuel cell.  
           [0028]    In this case, said pressure regulating valve would preferably comprise a relief valve shown as the invention defined in the appended claim  5 .  
           [0029]    According to the invention defined in the appended claim  6 , there is provided a cooling system of fuel cell, composed so that the coolant pressure of fuel cell is regulated with the cathode pressure of the cathode gas-supplying path, which supplies the oxidant gas to the fuel cell.  
           [0030]    Because the coolant pressure of the fuel cell is regulated with the cathode pressure, the fluid leak and the electrical conductivity failure caused by the abnormal increase in the coolant pressure will be prevented.  
           [0031]    According to the invention defined in the appended claim  7 , there is provided a cooling system of fuel cell, comprising a circulating path connected to the inlet and outlet of fuel cell coolant to cool the inside of fuel cell with layered structure, and a circulating pump to circulate a coolant in said circulating path, said cooling system further comprising a pressure regulator one room of which being connected to the atmospheric pressure and the other room being interconnected with said circulating path.  
           [0032]    Because one room of the pressure regulator is interconnected with the atmospheric pressure and the other room is connected to the circulating path of the coolant, no direct contact between the coolant and the air will occur. So the soluble materials existing in the atmosphere will not be dissolved into the coolant. As a result, even if the ion exchanger is prepared in the cooling system to prevent the increase in the electrical conductivity of the coolant, the operating hour of ion exchange resin will not be reduced and the life of the ion exchanger of cooling system will be improved. Also, the pressure regulator functions so that the air is discharged when the coolant pressure is high and the air is taken in when it is low, thus regulating the coolant pressure. Therefore, the fluid leak and the electrical conductivity failure of the fuel cell caused by the coolant pressure in the circulating path when the fuel cell generates large output power can be solved.  
           [0033]    According to the invention defined in the appended claim  8 , there is provided a cooling system of fuel cell, in which said humidifier defined in the appended claim  7  has the construction that said coolant flows in one side of the water vapor permeable membrane and said gas flows in the other side for humidification.  
           [0034]    In the humidifier for humidification of the gas supplied to the fuel cell, the coolant flows in one side of the water vapor permeable membrane and said gas flows in the other side. Because said coolant pressure and said gas pressure are balanced by the pressure regulator, it is possible to prevent an excessive pressure from imposing on the humidifier membrane, which has been a problem when the fuel cell generates large power.  
           [0035]    In this case, said pressure regulator would preferably include a pressure vessel and a flexible bellows that gives separate two rooms for said pressure vessel shown as the invention defined in the appended claim  9 , or a pressure vessel and a membrane that gives separate two rooms for said pressure vessel shown as the invention defined in the appended claim  10 . This type of construction will give a good balance between said coolant pressure and said gas pressure. Also said pressure regulator would include a cylindrical pressure vessel and a piston type structure that moves from one end to the other along the inner surface of said pressure vessel shown as the invention defined in the appended claim  11 . If the pressure vessel is configured so that the volume varies interactively between one room and the other with the piston movement, it will be highly reliable with the high strength of the piston.  
           [0036]    According to the invention defined in the appended claim  12 , there is provided a cooling system of fuel cell, comprising a circulating path connected to the inlet and outlet of fuel cell coolant to cool the inside of fuel cell with layered structure, and a circulating pump to circulate a coolant in said circulating path, said cooling system further comprising a pressure regulator one room of which being interconnected with the cathode gas path of said fuel cell and the other room being connected to said circulating path.  
           [0037]    Because one room of the pressure regulator is interconnected with the cathode gas path of said fuel cell and the other room to said circulating path, the coolant and the cathode gas will not directly contact. Therefore, the soluble materials existing in the cathode gas will not be dissolved into the coolant. As a result, even if an ion exchanger is prepared in the cooling system to prevent the increase in the electrical conductivity of the coolant, the operating hour of ion exchange resin will not be reduced and the life of the ion exchanger of cooling system will be improved. Also, the pressure regulator functions so that the pressure of cathode gas side is increased when the coolant pressure is high and the pressure of cathode gas side is decreased when the coolant pressure is low, thus regulating the balance between the cathode gas pressure and the coolant pressure. Therefore, the fluid leak and the electrical conductivity failure of the fuel cell caused by the coolant pressure in the circulating path when the fuel cell generates large output power can be solved.  
           [0038]    In this case, said pressure regulator would preferably include a pressure vessel and a flexible bellows that gives separate two rooms for said pressure vessel shown as the invention defined in the appended claim  14 , or a pressure vessel and a membrane that gives separate two rooms for said pressure vessel shown as the invention defined in the appended claim  15 . This type of construction will give a good balance between said coolant pressure and said gas pressure. Also said pressure regulator would include a cylindrical pressure vessel and a piston that moves from one end to the other along the inner surface of said pressure vessel shown as the invention defined in the appended claim  16 . If said pressure vessel is configured so that the volume varies interactively between one room and the other with the piston movement, it will be highly reliable with the high strength of the piston. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0039]    Preferred embodiments of the present invention will be described below, by way of example only, with reference to the accompanying drawings, in which:  
         [0040]    [0040]FIG. 1 is a schematic view illustrating a fuel cell system according to the first embodiment of the invention.  
         [0041]    [0041]FIG. 2 is a schematic view illustrating a fuel cell system according to the second embodiment of the invention.  
         [0042]    [0042]FIG. 3 is a cross sectional view illustrating the inner construction of the fuel cell of a fuel cell system according to the present invention.  
         [0043]    [0043]FIG. 4 is a cross sectional view illustrating a fuel cell system according to the third embodiment of the invention.  
         [0044]    [0044]FIG. 5 is a schematic view of an overall structure of a cooling system of fuel cell according to the fourth embodiment of the invention.  
         [0045]    [0045]FIG. 6 is an illustrative view of pressure regulator structure used in a cooling system of fuel cell according to the fourth embodiment of the invention.  
         [0046]    [0046]FIG. 7 is a schematic view of an overall structure of a cooling system of fuel cell according to the fifth embodiment of the invention.  
         [0047]    [0047]FIG. 8 is an illustrative view of pressure regulator structure used in a cooling system of fuel cell according to the fifth embodiment of the invention.  
         [0048]    [0048]FIG. 9 is an illustrative view of pressure regulator structure used in a cooling system of fuel cell according to the sixth embodiment of the invention.  
         [0049]    [0049]FIG. 10 is a schematic view illustrating a fuel cell system according to the prior art.  
         [0050]    [0050]FIG. 11 is a schematic view of an overall structure of a cooling system of fuel cell according to the prior art.  
         [0051]    [0051]FIG. 12 is a cross sectional view illustrating the inner construction of the fuel cell of a fuel cell.  
         [0052]    [0052]FIG. 13 is a cross sectional view illustrating a humidifier with hollow fiber membrane. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0053]    To describe the present invention more in detail, preferred embodiments of a cooling system of fuel cell according to the present invention will be described below in detail with reference to the accompanying figures.  
         [0054]    (First Aspect)  
         [0055]    The first aspect of the invention will be described in detail (First embodiment through third embodiment).  
         [0056]    (First Embodiment)  
         [0057]    [0057]FIG. 1 shows the cooling system of fuel cell defined in a first embodiment of the invention. As shown FIG. 1 in cooling system of fuel cell, the inlet and outlet of a primary coolant circulating path  4  are respectively connected to the inlet and outlet of a cooling path  3 , which is laid out in a fuel cell  2 . Heat exchange is performed by a heat exchanger  7  between the secondary coolant cooled by a radiator  6  disposed in a secondary coolant circulating path  5  and a primary coolant. The inside of fuel cell  2  is cooled by the primary coolant thus cooled with heat exchange.  
         [0058]    A bypass path 8  is prepared in said primary coolant circulating path  4  bypassing the primary side of heat exchanger  7 . A thermostat valve  10  is disposed at a interconnecting point  9  between the downstream side of the primary coolant circulating path  4  with regard to the heat exchanger  7  and the bypass path  8 . The temperature of primary coolant is controlled to the temperature appropriate for the power generation of fuel cell by switching the thermostat valve  10 .  
         [0059]    The primary coolant circulating path  4  and the secondary coolant circulating path  5  are provided with circulating pumps  11  and  12  respectively. The primary and secondary coolants are circulated by respective pumps  11  and  12 . A vapor-fluid separator  13  serving as a gas reservoir is provided in the primary coolant-circulating path  4 . A tank  15  is interconnected with the vapor-fluid separator  13  through a gas venting path  14 . An orifice  18  is disposed in the gas venting path  14  as restriction in order to prevent the variation in the pressure and flow rate of the primary coolant-circulating path  4 .  
         [0060]    The vapor-fluid separator  13  is located in the point of the primary coolant-circulating path  4 , where the pressure is relatively high. The vapor-fluid separator is not needed to locate in the geometrically highest point of the primary coolant circulating path  4  as it is requested only to trap and separate the gas flowing in the primary coolant circulating path  4 .  
         [0061]    In this embodiment of the invention, the vapor-fluid separator  13  is located in the direct downstream of the circulating pump to improve the performance of gas trapping. The primary coolant circulating path  4  is bent in the vertical direction so that the outlet of circulating pump  11  is located lower than the inlet of the same.  
         [0062]    On the other hand as shown in FIG. 1, the point of the primary coolant-circulating path  4  where the fluid pressure is relatively high, or the inlet vicinity of primary coolant of the fuel cell  2  and the bottom of the tank  15  is interconnected by a coolant-returning path  16 . In this embodiment of the invention, a pressure valve  17  as relief valve is mounted to the tank  15  to allow the use of tank as pressure monitoring vessel to maintain the tank pressure no greater than the allowable inlet pressure of the cooling path  3  (see FIG. 3).  
         [0063]    In this case, the size of the tank  15  is determined to be large enough in preparation for an increase in the flow rate of the primary coolant-circulating path  4  and the flow rate of the cooling path  3 , and an accidental pressure increase of inlet pressure of the cooling path  3 . The valve opening pressure of the pressure valve is set so that it opens when the tank inner pressure exceeds a given pressure which corresponds to the allowable inlet pressure of the fuel cell coolant, purging the pressure of the tank  15  outside.  
         [0064]    The operation of cooling system of fuel cell according to the first embodiment of the invention will be described below.  
         [0065]    As shown in FIG. 1, when the circulating pumps  11  and  12  of the primary coolant circulating path  4  and the secondary coolant circulating path  5  are operated, the primary coolant circulates in the primary coolant circulating path  4  and the cooling path  3 , and the secondary coolant circulates in the secondary coolant circulating paths and a water jacket (not shown) of the radiator  6 , cooling the fuel cell  2  with the primary coolant cooled by heat exchange.  
         [0066]    The gas circulating in the primary coolant circulating path  4  moves from the gas venting path  14  to the tank  15  in a mixture of gas and fluid as well as it is trapped by the vapor-fluid separator  13 .  
         [0067]    As mentioned before the gas venting path  14  is equipped with the orifice  18  to create the pressure loss, which neither induce backward flow nor affect the pressure of the primary coolant flowing in the primary coolant-circulating path  4 . Therefore, the pressure of the primary coolant flowing in the primary coolant-circulating path  4  will be constantly maintained to the discharge pressure of the circulating pump  11 .  
         [0068]    If all the gas coming into the tank  15  is not collected by the tank  15  at one time, it will be collected with a couple of numbers of the circulating cycles of the primary coolant by the tank  15 . Therefore, when all the gas is collected, the gas biting noise of the circulating pump will completely disappear.  
         [0069]    On the other hand, the pressure valve  17  continuously monitors the inlet pressure of the cooling path  3  through the tank  15  and the coolant-returning path  16 .  
         [0070]    When the inner pressure of the tank  15  corresponds to the inlet pressure of the fuel cell  2 , namely a given pressure, in which the inlet pressure of the cooling path  3  exceeds the allowable inlet pressure, the pressure valve opens and releases the tank inner pressure outside.  
         [0071]    As a result, the inlet pressure of the cooling path  3  is constantly maintained no greater than the allowable inlet pressure of the cooling path  3 . For example, given the allowable inlet pressure of the cooling path  3  of the fuel cell  2  is 100 kPa, the valve opening pressure of the pressure valve  17  is set for 100 kPa. In this case the pressure is selected from the range of 70 kPa through 100 kPa, and the fluid leak could be lessen if a lower pressure is selected.  
         [0072]    Therefore, as shown in FIG. 3, the pressure of the cooling path  3  laid out on the contact surface of separators  2   b ,  2   b  of a cell  2   a  of the fuel cell  2  is constantly maintained no greater than the pressure set by the pressure valve  17 , namely the allowable inlet pressure of the cooling path  3 . As a result, the coolant leak caused by the contact release of the separators  2   b ,  2   b  and the damage to a narrow holding portion  2   d   1  caused by the stress concentration to the narrow holding portion  2   d   1  of the electrode membrane  2   d  against a stack case (not shown) are avoided. In FIG. 3 reference characters  2   e  and  2   f  refer to the catalyst layer,  2   g  gas diffusion layer,  2   h  the path directing the hydrogen as fuel and  2   i  the path directing the oxygen (oxidant gas) as fuel.  
         [0073]    (Second Embodiment)  
         [0074]    A second preferred embodiment of a cooling system of fuel cell according to the present invention will be described in detail with reference to FIG. 2. The same reference character is used for the same item as that of the first preferred embodiment, and the detail explanation would be omitted.  
         [0075]    The differences associated with a second preferred embodiment of the invention are:  
         [0076]    The vapor-fluid separator  13  is disposed before the inlet of the primary coolant of the fuel cell  2 . The primary coolant-circulating path  4 , from the outlet of the primary coolant of the fuel cell  2  to the circulating pump  11 , and the bottom of the tank  15  is interconnected by the coolant-returning path  16 , in which the orifice  18  is prepared.  
         [0077]    In this second preferred embodiment, the pressure valve  17  regulates the gas venting and the pressure of the primary coolant at the inlet of the primary coolant of the fuel cell  2  as well as the first preferred embodiment of the invention shown in FIG. 1.  
         [0078]    Therefore, the pressure of the cooling path  3  is constantly maintained no greater than the pressure set by the pressure valve  17 , namely the allowable inlet pressure of the cooling path  3  as well as the first preferred embodiment of the invention. Thus the coolant leak caused by the contact release of the separators  2   b ,  2   b  and the damage to the narrow holding portion  2   d   1  due to the stress concentration of the narrow holding portion  2   d   1  of the electrolyte  2   d  against the stack case is avoided.  
         [0079]    In this second preferred embodiment of the invention, a diameter-extended portion may be provided in the connecting point between the gas venting path  14  and the primary coolant circulating path  4  to improve the gas trapping, which will efficiently trap the gas to collect in the tank  15  and make disappear the gas biting noise of the circulating pump  11  in a short time.  
         [0080]    (Third Embodiment)  
         [0081]    [0081]FIG. 4 shows the third preferred embodiment of a cooling system of fuel cell according to the present invention. The same reference character is used for the same item as that of the former preferred embodiments, and the detail explanation would be omitted.  
         [0082]    As shown in FIG. 4, in the cooling system of fuel cell according to this embodiment, the vapor-liquid separator  13 , the gas venting path  14  and the coolant-returning path  16  are deleted. The tank  15  is interconnected with the outlet side of the primary coolant-circulating path  4  with regard to the inlet of the cooling path  3  through the interconnecting path  20 . A pressure valve  17   a  is mounted to the tank  15 .  
         [0083]    The diameter of the interconnecting path  20  is set the same as that of the gas venting path  14 . The pressure valve  17   a  basically includes a check valve  19   a  and a return spring  19   b  regulating the valve opening pressure of the valve  19   a.    
         [0084]    The set force of the return spring  19   b  is adjusted so that the valve opens and maintains the inlet pressure of cooling path  3  no greater than the allowable inlet pressure when the inner pressure of the tank  15  exceeds the inlet pressure of the primary coolant of the fuel cell  2 , namely the allowable inlet pressure of the cooling path  3 , releasing the coolant of the tank  15  and the gas trapped in the tank  15  outside. It is possible to select an approach, which allows the adjustment of the total spring pressure with the addition of other assist force beside the return spring  19   b.    
         [0085]    For example, it is possible to introduce the air pressure of the air supply path  21 , which supplies the air as fuel to the anode electrode of the battery  2 , as assist force for the return spring  19   b . The valve opening pressure, the summation of this air pressure and the spring force of the return spring  19   b , is adjusted so that the valve opens when it exceeds the inlet pressure of the fuel cell  2 , namely the allowable inlet pressure of the coolant path  3 .  
         [0086]    In this case, the purge gas and the primary coolant handled by the pressure valve  17   a  may be returned to the air supply path  21 . Thus the spring fatigue is relaxed to prevent the chattering due to the spring fatigue, and the solid polymer electrode membrane of the fuel cell will be humidified by the purged coolant. In FIG. 4, S/C refers to a super charger to supply the fuel air compressed, numeral  22  a hydrogen supply path to supply the hydrogen as fuel to the anode electrode of the fuel cell  2 , and numeral  23  a pump (vacuum pump) to supply the air as oxidant gas to the cathode electrode.  
         [0087]    In this third preferred embodiment of the invention, a diameter-extended portion may be provided in the connecting point between the interconnecting path  20  and the primary coolant circulating path  4  to improve the gas trapping, which will efficiently trap the gas to collect in the tank and make disappear the gas biting noise of the circulating pump  11  in a short time.  
         [0088]    So also in the third preferred embodiment of the invention, the pressure increase of the cooling path  3  laid out on the contact surface of separators  2   b ,  2   b  can be prevented. As a result, the coolant leak and the damage caused by the stress concentration to the narrow holding portion  21  of the electrode membrane  2   d  can be avoided.  
         [0089]    The cooling system of fuel cell defined in the first or third embodiment provides the benefits:  
         [0090]    The biting noise of the circulating pump  11  is prevented by purging the gas from the primary coolant-circulating path  4 , which supplies the primary coolant to the fuel cell  2 .  
         [0091]    The fluid leak and the damage to the electrode membrane of fuel cell caused by abnormal increase in the inlet pressure of the primary coolant are prevented.  
         [0092]    The gas is efficiently trapped by placing only the tank  15  relatively high location of the vehicle, which enables the mounting of the cooling system under the vehicle floor etc. to give a remarkable improvement for the space efficiency.  
         [0093]    A small tube can be used for the gas venting path  14  and the coolant returning path  16  compared with the primary coolant circulating path  4 , which will make the bending of tube easier and give better workability and layout flexibility.  
         [0094]    Although in the first embodiment shown in FIG. 1 the orifice  18  is provided in the gas venting path  14 , the orifice  18  may be deleted by using a smaller tube for the gas venting path  14  compared with the primary coolant circulating path  4 , which circulates the primary coolant.  
         [0095]    Although in the second embodiment shown in FIG. 2 the orifice  18  is provided in the coolant-returning path  16 , the coolant-returning path may be configured smaller instead of employing the orifice  18 .  
         [0096]    Further, in the third embodiment shown in FIG. 4, the pressure valve  17  shown in FIG. 1 and FIG. 2 may be employed instead of the pressure valve  17   a.    
         [0097]    In each of the embodiments, it has been described that the fuel cell is cooled by the primary coolant, which is cooled by the heat exchanger  7 . However, the coolant for the fuel cell  2  may be directly cooled by the radiator  6 , or it may be cooled in a multi-step manner with the plural number of heat exchangers  6 .  
         [0098]    (Second Aspect)  
         [0099]    Second aspect of the invention will be described in detail (Fourth embodiment through sixth embodiment).  
         [0100]    (Fourth Embodiment)  
         [0101]    The cooling system of fuel cell defined in the fourth embodiment will be described in detail with reference to FIG. 5 and FIG. 6. FIG. 5 is a schematic view of an overall structure of a cooling system of fuel cell according to the fourth embodiment of the invention. FIG. 6 is an illustrative view of pressure regulator structure used in a cooling system of fuel cell according to the fourth embodiment of the invention.  
         [0102]    As shown in FIG. 5, the cooling system of fuel cell defined in the fourth embodiment basically comprises; a fuel cell  101 , two humidifiers  102   a ,  102   b  humidifying the fuel gas (anode gas) and the oxidant gas (cathode gas) supplied to the fuel cell  101 , and a coolant circulating supply system  103  which circulates and supplies the coolant for the fuel cell  101  and the humidifiers  102   a.    
         [0103]    The coolant circulating supply system  103  basically comprises; a circulating pump  103   a , a heat exchanger  103   b  for cooling the coolant, a thermo regulator  103   c  for controlling the coolant temperature, a pressure regulating vessel  103   d  which has a flexible bellows  103   d   3  (FIG. 6) inside and is the main portion of the pressure regulator PR, and a circulating path  103   p  where the devices/units are placed in a given order to supply the coolant to the fuel cell  101  an the humidifiers  102   a ,  102   b.    
         [0104]    In the cooling system of fuel cell configured as above mentioned, the coolant for the fuel cell  101  circulates in the circulating path  103   p  with the circulating pump  103   a  and is cooled with the heat exchanger  103   b  (heat exchanger such as radiator), controlled to an appropriate temperature for the fuel cell  101  operation and then introduced into the pressure regulator in the after step.  
         [0105]    When the coolant does not require cooling with the heat exchanger  103   b , it will be directly introduced into the thermo regulator  103   c  through a bypass path  103   g , which bypasses the heat exchanger  103   b.    
         [0106]    As shown in FIG. 6, the pressure of the coolant directed by the pressure regulator PR is appropriately regulated with the pressure variation absorbing mechanism for the coolant, which consists of the bellows  103   d   3  inside the PR and pressure control valves  103   d   1 ,  103   d   2 . The coolant is thus supplied to a coolant inlet  101   a  of the fuel cell  101 . The details of structure and operation of the pressure regulator PR will be described later.  
         [0107]    The pressure regulator should be placed interconnected with somewhere in the cooling system. It would be preferably placed near the tubes or devices, which are susceptible to the pressure influence, to avoid an excessive pressure to be imposed on them.  
         [0108]    The coolant supplied to the coolant inlet  101   a  of the fuel cell  101  cools it while passing the coolant path  101   c  so that the fuel cell  101  can operate stably, and then is discharged from the coolant outlet of the fuel cell  101 .  
         [0109]    Either a part or all the coolant discharged from the coolant outlet  101   b  of the fuel cell  101  is supplied to the humidifier  102   a  of the fuel gas (anode gas) and the humidifier  102   b  of oxidant gas (cathode gas) through two branched paths  103   p   1 ,  103   p   2 , which are branched from the circulating path  103   p  at the point that is located in the downstream of the fuel cell  101  and, at the same time, the upstream of the circulating pump  103   a.    
         [0110]    The coolant respectively supplied to the humidifiers  102   a  and  102   b  humidifies the fuel gas (anode gas) supplied to the fuel cell  101  by the high-pressure hydrogen source and the oxidant gas (cathode gas) supplied by a compressor  104  with the water vapor passing through the water vapor permeable membrane.  
         [0111]    The coolant having humidified the fuel gas (anode gas) and the oxidant gas (cathode gas) is returned to the circulating path  103   p  from the humidifiers  102   a  and  102   b  through the paths  103   p   3  and  103   p   4 , then circulated again by the circulating pump  103   a.    
         [0112]    Part of the coolant circulating the circulating path  103   p  is directed to an ion exchanger  103   e , which prevents the increase in the electrical conductivity of the coolant (insulation deterioration).  
         [0113]    The structure and operation of the pressure regulator PR used for the cooling system of fuel cell defined in the fourth embodiment of the invention will be described in detail with reference to FIG. 5 and FIG. 6.  
         [0114]    As shown in FIG. 6, in the pressure-regulating vessel  103   d  forming the main body of the pressure regulator PR, there is the bellows which provides two rooms sp 1  and sp 2 . The pressure control valves for expiring  103   d   1  and for inspiring  103   d   2  are connected to the room sp 1 , and the coolant inlet nozzle  103   d   4  and the coolant outlet nozzle  103   d   5  are connected to the other room sp 2 .  
         [0115]    The room sp 1  is connected to the atmosphere outside the pressure regulating vessel  103   d  through the pressure control valves  103   d   1  and  103   d   2 . The other room sp 2  is interconnected with the circulating path  103   p  (FIG. 5) for the coolant of the fuel cell  101  through the inlet and outlet nozzles  103   d   4  and  103   d   5 .  
         [0116]    Because the pressure regulating vessel  103   d  forming the main body of the pressure regulator PR is separated into the two rooms sp 1  and sp 2 , in which the air and the coolant flowing in the circulating path  103   p  are directed into the respective rooms to regulate the coolant pressure, no direct contact between the coolant and the air will occur.  
         [0117]    Because the carbon dioxide in the air does not dissolve into the coolant, the operating hour of ion exchange resin will not be reduced and the life of the ion exchanger  103   e  of cooling system will be improved.  
         [0118]    The coolant inlet nozzle  103   d   4  is placed bottom on the side wall, on the other hand the coolant outlet nozzle top on the side wall.  
         [0119]    Placing the inlet nozzle  103   d   4  and outlet nozzle  103   d   5  this way, the bubble is unlikely to accumulate in the coolant within the pressure regulator vessel  103   d  and the coolant pressure will be transferred accurately to the bellows  103   d   3 . A membrane (refer the broken line in FIG. 9) or a piston type structure may be employed to separate the pressure-regulating vessel instead of the bellows  103   d   3 .  
         [0120]    A check valve will be used for the pressure control valves  103   d   1  and  103   d   2  connected to the other room sp 1 .  
         [0121]    The pressure regulator PR configured this way and used in the cooling system of fuel cell defined in the fourth embodiment functions so that the room air (atmosphere) is expired outside from the room sp 1  when the pressure of the coolant within the pressure regulating vessel  103   d  (=the pressure of the other room sp 2 ) exceeds a given value for the pressure regulating valve  103   d   1 , on the other hand the atmosphere is inspired into the room sp 1  when the coolant pressure within the vessel falls below a given value for the pressure regulating valve  103   d   2 . Therefore, the coolant pressure is regulated appropriately by the pressure variation absorbing mechanism consisting of the bellows  103   d   3  and the two pressure regulating valves  103   d   1  and  103   d   2 .  
         [0122]    As a result, the problems associated with the fluid leak and the electrical conductivity failure caused by the increase in the fluid pressure in the circulating path, which occur when the fuel cell generates large output power, are solved.  
         [0123]    (Fifth Embodiment)  
         [0124]    The cooling system of fuel cell defined in the fifth embodiment will be described in detail with reference to FIG. 7 and FIG. 8. FIG. 7 is a schematic view of an overall structure of a cooling system of fuel cell according to the fifth embodiment of the invention. FIG. 8 is an illustrative view of pressure regulator structure used in a cooling system of fuel cell according to the fifth embodiment of the invention.  
         [0125]    The significant difference in the cooling system of fuel cell between the fourth embodiment and the fifth embodiment lies in the fact that the room sp 1 , one of the two rooms sp 1  and sp 2  of the pressure regulating vessel  103   d  of the pressure regulator PR separated by the bellows  103   d   3 , is connected to the cathode gas path  104   p  through an interconnecting path  104   p   1 , as shown in FIG. 7 and FIG. 8.  
         [0126]    Here, the description will be limited to the structure and operation of the pressure regulator used in the cooling system of fuel cell defined in the fifth embodiment. The same numerals as those for the cooling system of fuel cell of the fourth embodiment are used for the same items.  
         [0127]    The structure and operation of the pressure regulator used in the cooling system defined in the fifth embodiment will be described in detail with reference to FIG. 7 and FIG. 8.  
         [0128]    As shown in FIG. 8, in the pressure regulating vessel  103   d  forming the main body of the pressure regulator PR, there is the bellows  103   d   3  changing in shape flexibly according to the pressure, which is placed at the bottom and provides two rooms sp 1  and sp 2 . The interconnecting path  104   p   1  is connected to the room sp 1 , and the coolant inlet nozzle  103   d   4  and the coolant outlet nozzle  103   d   5  are connected to the other room sp 2 .  
         [0129]    The room sp 1  is connected to the cathode gas path  104   p  (FIG. 7) connecting the humidifier  102   b  and the compressor  104  of the fuel cell  101  through the interconnecting path  104   p   1 . The other room sp 2  is interconnected with the circulating path  103   p  (FIG. 7) for the coolant of the fuel cell  101  through the inlet and outlet nozzles  103   d   4  and  103   d   5 .  
         [0130]    Because the pressure regulating vessel  103   d  forming the main body of the pressure regulator PR is separated into the two rooms sp 1  and sp 2 , in which the air (pressure is higher than the atmosphere), namely the cathode gas supplied by the compressor  104  and the coolant flowing in the circulating path  103   p  are directed into the respective rooms to regulate the coolant pressure, no direct contact between the coolant and the air will occur.  
         [0131]    Because the carbon dioxide in the air does not dissolve into the coolant, the operating hour of ion exchange resin will not be reduced and the life of the ion exchanger  103   e  of cooling system will be improved.  
         [0132]    The coolant inlet nozzle  103   d   4  is placed bottom on the side wall of the pressure regulating vessel  103   d , on the other hand the coolant outlet nozzle  103   d   5  top on the side wall.  
         [0133]    Placing the inlet nozzle  103   d   4  and outlet nozzle  103   d   5  this way, the bubble is unlikely to accumulate in the coolant within the pressure regulator vessel  103   d  and the coolant pressure will be transferred accurately to the bellows  103   d   3 . A membrane (see the broken line in FIG. 9) or a piston type structure maybe employed to separate the pressure-regulating vessel  103   d  instead of the bellows  103   d   3 .  
         [0134]    The pressure regulator PR configured this way and used in the cooling system of fuel cell defined in the fifth embodiment functions so that the pressure of the cathode gas is increased by pressing the bellows  103   d   3  (decrease in volume) when the coolant pressure within the pressure regulating vessel  103   d  exceeds the pressure of cathode gas (air), on the other hand the pressure of the cathode gas is decreased by expanding the bellows  103   d   3  (increase in volume) when the coolant pressure falls below the cathode gas pressure. Therefore, the balance between the coolant pressure and the cathode gas pressure is constantly regulated.  
         [0135]    As a result, the problems associated with the fluid leak and the electrical conductivity failure caused by the increase in the fluid pressure in the circulating path, which occur when the fuel cell generates large output power, are solved.  
         [0136]    (Sixth Embodiment)  
         [0137]    The cooling system of fuel cell defined in the sixth embodiment will be described in detail with reference to FIG. 7 and FIG. 9. FIG. 9 is an illustrative view of pressure regulator structure used in a cooling system of fuel cell according to the sixth embodiment of the invention.  
         [0138]    The significant difference in the cooling system of fuel cell between the fifth embodiment and the sixth embodiment lies in the fact that the room sp 1 , one of the two rooms sp 1  and sp 2  of a pressure regulating vessel  105   d  of the pressure regulator PR separated by a piston type structure  105   d   3 , is connected to the cathode gas path  104   p  through the interconnecting path  104   p   1  as shown in FIG. 7 and FIG. 9.  
         [0139]    Here, the description will be limited to the structure and operation of the pressure regulator used in the cooling system of fuel cell defined in the sixth embodiment. The same numerals as those for the cooling system of fuel cell of the fifth embodiment are used for the same items.  
         [0140]    The structure and operation of the pressure regulator used in the cooling system defined in the sixth embodiment will be described in detail with reference to FIG. 7 and FIG. 9.  
         [0141]    As shown in FIG. 9, in the pressure regulating vessel  105   d  forming the main body of the pressure regulator PR, there is the piston type structure  105   d   3  moving along the inner wall in the vertical direction, which provides two rooms sp 1  and sp 2 . The interconnecting path  104   p   1  is connected to the room sp 1 , and a coolant inlet nozzle  105   d   4  and a coolant outlet nozzle  105   d   5  are connected to the other room sp 2 .  
         [0142]    The room sp 1  is connected to the cathode gas path  104   p  (FIG. 7), which connects the humidifier  102   b  and the compressor  104  of the fuel cell  101 , through the interconnecting path  104   p   1 . The other room sp 2  is interconnected with the circulating path  103   p  (FIG. 7) for the coolant of the fuel cell  101  through the inlet and outlet nozzles  105   d   4  and  105   d   5 .  
         [0143]    Because the pressure regulating vessel  105   d  forming the main body of the pressure regulator PR is separated into the two rooms sp 1  and sp 2  by the piston type structure  105   d   3 , in which the air (pressure is higher than the atmosphere), namely the cathode gas supplied by the compressor  104  and the coolant flowing in the circulating path  103   p  are directed into the respective rooms to regulate the coolant pressure, no direct contact between the coolant and the air will occur.  
         [0144]    Because the carbon dioxide in the air does not dissolve into the coolant, the operating hour of ion exchange resin will not be reduced and the life of the ion exchanger  103   e  of cooling system will be improved.  
         [0145]    The coolant inlet nozzle  105   d   4  is placed down on the side wall of the pressure regulating vessel  105 , on the other hand the coolant outlet nozzle  105   d   5  is placed lower than the middle of the vessel and higher than the inlet nozzle  105   d   4  at the same time.  
         [0146]    Placing the inlet nozzle  105   d   4  and outlet nozzle  105   d   5  this way, the bubble is unlikely to accumulate in the coolant within the pressure regulator vessel  105   d  and the coolant pressure will be transferred accurately to the piston type structure  105   d   3 . A membrane (see the broken line in FIG. 9) or a bellows maybe employed to separate the pressure regulating vessel  105   d  instead of the piston type structure  105   d   3 .  
         [0147]    Though not in shown in a figure, the piston type structure  105   d   3  may be equipped with a spring so that it can be returned to the initial position when the operation of fuel cell  101  (FIG. 7) is terminated. Alternatively, appendices on the inner wall of the vessel for supporting the bottom of the piston type structure  105   d   3  at the both ends will work.  
         [0148]    The pressure regulator PR configured this way and used in the cooling system of fuel cell defined in the sixth embodiment functions so that the pressure of the cathode gas is increased by moving the piston type structure  105   d   3  upward when the coolant pressure within the pressure regulating vessel  105   d  exceeds the pressure of cathode gas (air), on the other hand the pressure of the cathode gas is decreased by moving the piston type structure  105   d   3  downward when the coolant pressure falls below the cathode gas pressure. Therefore, the balance between the coolant pressure and the cathode gas pressure is constantly regulated.  
         [0149]    As a result, the problems associated with the fluid leak and the electrical conductivity failure caused by the increase in the fluid pressure in the circulating path, which occur when the fuel cell generates large output power, are solved.  
         [0150]    In the humidifiers  102   a  and  102   b , which humidify the gas supplied to the fuel cell  101  in the cooling system of fuel cell defined in the fourth through sixth embodiments, the coolant is directed into one side of the water vapor permeable membrane and the gas is directed into the other side for humidification. The inside of the pressure regulating vessels  103   d  and  105   d  are separated into two rooms with the bellows  103   d   3  or the piston type structure  105   d   3  to form the pressure regulator PR, thus regulating the balance between the coolant pressure and the gas pressure. As a result, the problem associated with the excess pressure imposed on the humidifier membrane caused by the increase in the fluid pressure in the circulating path, which has occurred in the prior arts when the fuel cell generates large output power, is solved.  
         [0151]    While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.  
         [0152]    For example, the present invention is based on a technical idea that during the course of the fuel cell in a cooling system of the fuel cell comprising a circulating path and a circulating pump, the troubles described previously can be prevented if the temperature of coolant is regulated based on a prescribed pressure (e.g., spring set force, pressure of supply gas, atmospheric pressure, or such). Consequently, the process of cooling the fuel cell based on such a technical idea is also within the scope of the present invention.  
         [0153]    Also, while regulation based upon the pressure of anode, which is gas supplied to the fuel cell has been mainly described, the regulation based on the pressure of cathode is also within the scope of the present invention.