Abstract:
A fuel cell system is provided, which does not malfunction when started below a freezing point nor require a large force to open a purge gas introduction valve. The purge gas introduction valve  42  that introduces a cathode gas into the anode is installed in the bypass pipe that bypasses a humidifier. A plunger  44  that is supported and kept movable in the purge gas introduction valve is kept exposed to the cathode gas atmosphere when the purge gas introduction valve  42  is kept closed. The valve body  45  in the purge gas introduction valve  42  comprises a first pressed portion  45   a  to act for closing the purge gas introduction valve  42  and a second pressed portion  45   b  to act for opening the purge gas introduction valve  42.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the foreign priority benefit under Title 35, United States Code, § 119(a)-(c), of Japanese Patent Application No. 2007-131367, filed on May 17, 2007 with the Japan Patent Office, the disclosure of which is herein incorporated by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to the fuel cell system equipped with a scavenge system to scavenge the anode of fuel cells. 
         [0003]    In the fuel cell system electric power is generated through electrochemical reaction with hydrogen gas supplied to the anode of the fuel cells and air supplied to the cathode of the fuel cells. As a result of this electro-chemical reaction, water is produced at the cathode and this produced water permeates an electrolyte membrane between the anode and the cathode and comes from the cathode to the anode. Accordingly if the fuel cell system is used in a low temperature environment and is cooled below the freezing point, this produced water freezes while the fuel cell is out of operation. In order to prevent the produced water from freezing, there is an idea that not only the cathode but also the anode is scavenged with air to discharge the produced water outside the fuel cell system. A fuel system has been put forward to scavenge the anode with air. According to this fuel cell system a pipe is installed to connect the anode with the cathode and a scavenge gas introduction valve is installed to keep and stop air flowing in this pipe and is opened to keep air flowing when the anode is scavenged. 
         [0004]    However when the anode is scavenged with air in this fuel cell system while the fuel cell system is out of operation, water diffuses from the anode to a plunger in the scavenge gas introduction valve after the scavenging is finished and stays between the plunger and a guide that supports the plunger. If this water freezes while the fuel cell is out of operation, there is a problem that the scavenge gas introduction valve does not work when the fuel cell system is started below the freezes point because the plunger is stuck. 
         [0005]    In order to prevent the scavenge gas introduction valve from being stuck, Japanese Laid-Open Paten Application 2005-265036 (especially FIG. 2 and FIG. 3 are referred to ) and Japanese Laid-Open Paten Application 2005-273704 (especially FIG. 2 and FIG. 3 are referred to) propose a fuel cell system in which the plunger in the scavenge gas introduction valve is kept separated from water diffused from the anode by a diaphragm or something like the diaphragm. 
         [0006]    However if a diaphragm is used to keep the scavenge gas introduction valve separated from water as proposed by Japanese Laid-Open Patent Application 2005-265036 and Japanese Laid-Open Patent Application 2005-273704, there is another problem with the scavenge gas introduction valve stopping working due because a suction effect caused by the plunger room being compressed and expanded during operation that is separated with the diaphragm. Therefore in this fuel cell system a breathing hole has to be formed to have the plunger room communicate with the atmosphere. But there is a problem with the scavenge gas introduction valve due to this breathing hole. If this breathing hole is formed, a driving force needed to open the scavenge gas introduction valve becomes large because the whole diaphragm is pressurized by the cathode gas pressure while the anode is being scavenged with the plunger room&#39;s pressure equal to the atmosphere pressure. 
         [0007]    It is also problematic to have more components in a fuel cell system for the diaphragm and the breathing hole because the number of components increases, which results in a difficulty in packing all of them in a case, as well as cost increase and weight increase. 
       SUMMARY OF THE INVENTION 
       [0008]    A first aspect of the present invention provides a fuel cell system comprising a fuel cell comprising a cathode into which a cathode gas is introduced and an anode which is connected through a bypass pipe with the cathode and scavenged with the cathode gas supplied through the bypass pipe, the anode into which an anode gas is introduced, a humidifier that humidifies the cathode gas to be introduced into the cathode, a scavenge gas introduction valve which is installed in the bypass pipe and enables the cathode gas to be supplied through the bypass pipe to the anode when opened and a scavenge gas introduction valve control device that controls the scavenge gas introduction valve to be opened and closed, wherein the bypass pipe bypasses the humidifier and wherein the scavenge gas introduction valve comprises a plunger that is attached therein, held movable and exposed to an atmosphere of the cathode gas when the scavenge gas introduction valve is kept closed. 
         [0009]    According to the first aspect of the present invention, the produced water remaining at the anode is prevented by the humidifier from diffusing and the cathode gas (air) which is dry and fed through the air inlet comes and fills the cathode side space in the scavenge gas introduction valve when electricity generation is out of operation. As a result the cathode side space in the scavenge gas introduction valve is kept dry. 
         [0010]    A second aspect of the present invention provides a fuel cell system of the first aspect, wherein the scavenge gas introduction valve is opened by the plunger moving forward and closed by the plunger moving backward and includes a valve body that comprises a first pressed portion which is pressed by an anode gas pressure so as to keep the scavenge gas introduction valve closed at least when the scavenge gas introduction valve is kept closed and a second pressed portion which is pressed by a cathode gas pressure so as to open the scavenge gas introduction valve. 
         [0011]    According to the second aspect of the present invention, a differential pressure between the cathode gas and the anode gas is applied to the valve body while electricity is being generated so as to have the valve body in contact with the valve seat do close the scavenge gas introduction valve, since the anode gas pressure is usually kept higher than the cathode gas pressure. As a result the driving force to open the scavenge gas introduction valve is made smaller and does not have to be so large as for the prior fuel cell system, because a force of a spring that presses the valve body can be reduced. 
         [0012]    A third aspect of the present invention provides a fuel cell system of the second aspect, wherein the plunger is installed on a side of the cathode in the bypass pipe with respect to the valve body of the scavenge gas introduction valve. 
         [0013]    According to the third aspect of the present invention, the plunger is installed on the cathode side in the scavenge gas introduction valve where the dry cathode gas is introduced. As a result the plunger is kept dry when electricity generation is out of operation and no water comes and freezes between the plunger and its guide. 
         [0014]    A fourth aspect of the present invention provides a fuel cell system of the first aspect, further comprising an air-compressor which supplies the cathode gas into the cathode. 
         [0015]    According to the fourth aspect of the present invention, the cathode gas is supplied not only to the cathode but also to the anode through the bypass pipe to scavenge the anode. As a result the cathode side space in the scavenge gas introduction valve installed in the bypass pipe is kept dry. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The object and features of the present invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0017]      FIG. 1  indicates schematically a whole configuration of an embodiment of the present invention; 
           [0018]      FIG. 2  is a cross sectional view of a scavenge gas introduction valve used for the present invention and indicates an inner structure of the scavenge gas introduction valve; 
           [0019]      FIG. 3A  is a cross sectional views of a scavenge gas introduction valve which is open when the anode is being scavenged. 
           [0020]      FIG. 3B  is a cross sectional views of a scavenge gas introduction valve which is close 
       
    
    
       [0021]    The same or corresponding elements or parts are designated with like references throughout the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Taking a look at  FIG. 1 to 3A  and  3 B, the embodiment of the present invention is explained hereinafter. The fuel cell system of the this embodiment is applied to such fixed power sources as used for a fuel cell automobile, a ship, an aircraft and a domestic use. 
         [0023]    As indicated in  FIG. 1 , the fuel cell system of this embodiment comprises a fuel cell  10 , a cathode system  30 , an anode scavenge system  40 , and ECU (Control Device)  50 . 
         [0024]    The fuel cell  10  includes a plurality of single cells, each of which is constituted by separators  14 ,  15  both of which are conductive and a membrane electrode assembly (referred to as MEA hereinafter) which is constituted an anode  12 , a cathode  13  and a proton conductive solid polymer electrolyte film  11  sandwiched by the anode  12  and the cathode  13 . Both the anode  12  and the cathode  13  include a catalyst. The plural single cells are stacked in series in the single cell&#39;s thickness direction in the fuel cell  10 . In each separator  14  attached to the anode  12  is formed an anode gas flow passage  14   a  is, through which hydrogen gas flows, and in each separator  15  attached to cathode  13  is formed a cathode gas passage  15   a , through which air flows. In  FIG. 1  there is indicated only one single cell for convenience of explanation. 
         [0025]    The anode system  20  is intended to supply the hydrogen gas to the fuel cell  10  and discharge the hydrogen gas from the fuel cell  10 , and comprises a hydrogen tank  21 , a shutoff valve  22 , a regulator  23 , an ejector  24 , a discharge valve  25 , and pipes  26   a  to  26   f.    
         [0026]    The hydrogen tank  21  is constituted by a tank chamber and a cover. The tank chamber (not shown), in which highly pure hydrogen gas that is highly pressurized is stored, is made of an aluminum alloy and covered by the cover that is made of a material such as CFRP (Carbon Fiber Reinforced Plastic) or GFRP (Glass Fiber Reinforced Plastic). 
         [0027]    The shutoff valve  22  can be, for instance, an electromagnetic valve that is installed near an outlet of the hydrogen tank  21  and in communication with the hydrogen tank  21  through the pipe  26   a.    
         [0028]    The regulator  23  is connected with the shutoff valve  22  through the pipe  26   b  and has a function to reduce the pressure of the highly pressurized hydrogen gas supplied from the hydrogen tank  21 . 
         [0029]    The ejector  24  is a kind of a vacuum pump to recirculate unreacted hydrogen gas discharged from the anode gas outlet of the anode  12  in the fuel cell  10  and is in communication with the regulator  23  through the pipe  26   c . Through the pipe  26   d  the ejector  24  is in communication with the anode gas inlet of the anode  12  in the fuel cell  10 . Through the pipe  26   e  the anode gas outlet of the anode  12  is in communication with the discharge valve to be explained later. Through the pipe  26   f  the ejector  24  is in communication with an intermediate portion of the pipe  26   e . As a result, hydrogen gas supplied into the anode system  20  is circulated through the pipes  26   d ,  26   e  and  26   f.    
         [0030]    The discharge valve  25  is attached at a portion of the pipe  26   e  near the outlet of the pipe  26   e  and functions to become opened to have the produced water discharged from the anode system  20 . Here the produced water is water remaining in the anode system  20  including the anode gas passage  14   a  and the pipes  26   d ,  26   e  and  26   f  and the anode scavenging refers to a process in which an air-compressor  31  is driven to introduce air (cathode gas) into the anode system  20  and have the remaining produced water discharged outside. 
         [0031]    The cathode system  30  comprises the air-compressor  31 , a humidifier  32 , a back-pressure control valve  33  and pipes  34   a  to  34   d  and is intended to supply air (oxygen gas) to the cathode  13  in the fuel cell  10  and discharge a cathode-off gas that is discharged out of the outlet of the cathode  13  in the fuel cell  10 . 
         [0032]    The air-compressor  31  can be an apparatus like a super-charger driven by a motor, and has a function to take in air from the air inlet  31   a , compress the air and pump out the compressed air to be supplied to the cathode  13 . 
         [0033]    The humidifier  32  has a function to collect water contained in the cathode-off gas and humidify the cathode gas (air). This humidifier  32  includes, for instance, a hollow fiber membrane filter module that is constituted by a case in which a plurality of bundled hollow membrane fiber filters that are water permeable are accommodated. The pumped compressed air from the air-compressor  31  is supplied to and made to flow in either an inner space inside each membrane filter in the hollow fiber membrane filter module or an outer space outside each membrane filter in the hollow fiber membrane filter module. On the other hand the cathode-off gas which contains a lot of water derived from the produced water is supplied to and made to flow in the space which is either an inner space inside each membrane filter in the hollow fiber membrane filter module or an outer space outside each membrane filter in the hollow fiber membrane filter module and the pumped compressed air is not supplied to and made to flow in. As a result, the cathode gas from the air-compressor  31  is humidified. 
         [0034]    The back pressure control valve  33  is constituted by, for instance, a butterfly valve and has a function to keep appropriately the pressure in the cathode system  30 . 
         [0035]    Through the pipe  34   a  the air-compressor  31  is in communication with the cathode gas inlet  32   a  through which the cathode gas supplied from air-compressor  31  is introduced. Through the pipe  34   b  the cathode gas outlet  32   b  of the humidifier  32  out of which the humidified cathode-off gas in the humidifier  13  is discharged is in communication with the gas inlet of the cathode  13 . Through the pipe  34   c  the gas outlet of the cathode  13  is in communication with the cathode-off gas inlet  32   c  of the humidifier  32  through which the cathode-off gas is introduced into the humidifier  32 . Through the pipe  34   d  the cathode-off gas outlet  32   d  out of which the cathode-off gas in the humidifier  32  is discharged is in communication with the back pressure control valve  33 . 
         [0036]    The anode scavenge system  40  comprises a bypass pipe  41  and a scavenge gas introduction valve  42 . 
         [0037]    The bypass pipe  41  is a gas passage through which the cathode gas supplied from the air-compressor  31  is introduced into the anode  12  in the fuel cell  10 . One end of the bypass pipe  41  is connected with the pipe  34   a  which is located on the upstream side from the humidifier  32  while the other end of the bypass pipe  41  is connected with the pipe  26   d  that is connected with the gas inlet of the anode  12  of the fuel cell  10 . 
         [0038]    The scavenge gas introduction valve  42  is installed in a portion of the bypass pipe  41  and opened to have the cathode gas coming from the air-compressor  31  introduced through the bypass pipe  41  into the anode  12  when the anode  12  is scavenged. 
         [0039]    As is shown in  FIG. 2  which indicates the cross sectional view of the scavenge gas introduction valve  42  that is closed, the scavenge gas introduction valve  42  is constituted by such components as a plunger  44  and a valve body  45  accommodated in a case  43 . 
         [0040]    The case  43  is constituted by, for instance, a plurality of cases which are engaged with each other and has a scavenge gas inlet  43   a  through which the cathode gas as a scavenge gas is introduced and a scavenge gas outlet  43   b  out of which the cathode gas is discharged. Inside the case  43  there is installed a valve seat  43   c . When a valve body  45  (to be explained later in this description) comes in contact with the valve seat  43   c , the scavenge gas introduction valve  42  is closed. On the other hand when the valve body  45  comes off the valve seat  43   c , the scavenge gas introduction valve  42  is opened. 
         [0041]    The plunger  44  is installed in the case  43  and held movable in the up-and-down direction in  FIG. 2  by a drive part  46 . When the plunger  44  is moved forward (which means upward in  FIG. 2 ), the scavenge gas introduction valve  42  is opened. On the other hand when the plunger  43  is moved backward (which means downward in  FIG. 2 ), the scavenge gas introduction valve  42  is closed. The drive part  46  comprises a bobbin  46   b  on which a coil is wound, a fixed iron core  46   c , and a guide member  46   d  which supports the plunger  44  so that the plunger  44  is held movable in the up-and-down direction in  FIG. 2 . There is a shaft  47  installed in the case  43 . One end of a shaft  47  (which means the lower end in  FIG. 2 ) is inserted and secured in the plunger  44  while the valve body  45  is fixed on the other end of the shaft  47  (which means the upper end in  FIG. 2 ). There is a spring  48   a  which is installed between the lower end of the plunger  44  and the case  43  and kept pressing the valve body  45  in the valve opening direction (which means upward in  FIG. 2 ). 
         [0042]    The valve body  45  has a first pressed portion  45   a  and a second pressed portion  45   b . The first pressed portion  45   a  is a whole portion of the valve body  45  that is in contact with the anode gas and the second pressed portion  45   b  is a whole portion of the valve body  45  that is in contact with the cathode gas. Both the contact portion  45   c   1  in the valve body  45  with which the valve seat  43   c  comes in contact is made of an elastic material  45   c  and the extremity portion (upper end portion)  45   c   2  is also made of the elastic material  45   c . There is another spring  48   b  which is disposed between the valve body  45  and the case  43  and kept pressing the valve body in the valve closing direction (which means downward in  FIG. 2 ). The shape of the valve body  45  indicated in  FIG. 2  is just an example and can be modified within the scope of the present invention. 
         [0043]    As shown in  FIG. 2 , there is a scavenge gas inlet  43   a  disposed in the middle portion of the case  43  in the up-and down direction. There is a scavenge gas outlet  43   b  in the case  43  disposed across the valve seat  43   c  from the scavenge gas inlet  43   a . There are a couple of spaces into which the inner space of the case  43  is divided by the valve body  45 , one is a cathode side space and the other is an anode side space. As is indicated by  FIG. 2 , the plunger  44  is held in the cathode side space in the case  43 . 
         [0044]    ECU (Control Device)  50  as indicated in  FIG. 1  has a CPU (Central Processing Unit), a memory and a program and is connected with the shutoff valve  22 , the discharge valve  25 , the air-compressor  31 , the back pressure control valve  33  and the scavenge gas introduction valve  42 . ECU  50  functions as a scavenge gas introduction valve control device. The scavenge gas introduction valve control device controls the scavenge gas introduction valve  42  so that scavenge gas introduction valve is opened when the anode scavenging starts and closed when the anode scavenging ends. Furthermore ECU  50  controls the open and close operations of the shutoff valve  22 , the discharge valve  25  and the rotation speed of the air-compressor and the valve opening of the back pressure control valve  33 . 
         [0045]    Then the operation of the fuel cell system  1  is to be explained hereinafter. To begin with, while the fuel cell system  1  is in operation, the anode  12  in the fuel cell  10  is supplied with hydrogen gas with the shutoff valve  22  opened and the cathode  13  in the fuel cell  10  is supplied with air including oxygen gas which is humidified with the humidifier  32  with the air-compressor  31  being driven. Under this condition hydrogen gas is oxidized with the help of a catalyst on the anode  12  in the fuel cell  10  to discharge electrons that flow through a outer load to the cathode  13 . On the cathode  13  oxygen gas in air that is supplied is reduced with electrons with the help of a catalyst to combine with hydrogen ions permeating through a solid polymer electrolyte membrane  11  from the anode  12  and produce water. In this way electricity is generated in the fuel cell system  1 . 
         [0046]    However while electricity generation is under way, the water produced on the cathode  13  can permeate through the solid polymer electrolyte membrane  1  to the anode  12 . Therefore when the fuel cell system  1  stops electricity generation, both the scavenge introduction valve  42  and the discharge valve  25  are opened, the air-compressor  31  is driven, according to the directions of ECU  50 . As a result the cathode gas fed by the air-compressor  31  is supplied to the anode  12  through the bypass pipe  41 . In this operation the scavenge gas introduction valve  42  is opened the following way. As indicated in  FIG. 3A , as soon as the coil  46   a  is electrified and excited, the plunger  44  is moved upward in  FIG. 3A  and the valve body  45  is lifted up off the valve seat  43   c . Then the cathode gas can flow through the scavenge introduction valve  42  as indicated by an arrow in  FIG. 3A . As the cathode gas flowing from the air-compressor  31  is supplied to the anode  12 , the produced water that permeates from the cathode  13  and remains on the anode  12  is blown off and discharged through the discharge valve  25  out of the fuel cell system  1 . When this anode scavenging is finished, the scavenge gas introduction valve  42  and the discharge valve  25  are closed and the air-compressor  31  is stopped according to control directions of ECU  50 . However the produced water remaining inside MEA can diffuse out of MEA after the anode scavenging is finished and come up to the scavenge gas introduction valve  42  through the pipes  26   d  to  26   f  and the bypass pipe. 
         [0047]    In order to have the fuel cell system  1  unaffected by this produced water diffusing out of MEA, the fuel cell system  1  of the present invention has a structure to have the plunger  44  exposed to the atmosphere of air (cathode gas) when the scavenge gas introduction valve  42  is kept closed as shown in  FIG. 3B . To be specific, the plunger  44  is disposed in the cathode side space on the cathode side from the valve body  45  as shown in  FIG. 2  and the cathode gas to be used for the anode scavenging is introduced from the pipe  34   a  disposed upstream from the humidifier  32  which collects water from the cathode-off gas as shown in  FIG. 1 . Because of this structure of the fuel cell system  1  as explained above, the produced water remaining at the cathode  13  is collected by the humidifier  31  and can not diffuse up to the pipe  34   a  when the fuel cell system  1  is kept stopped. As a result the cathode side space in the scavenge gas introduction valve  42  is filled with the air supplied from the air fed through the air inlet  31   a  and is kept dry, when the fuel cell system  1  is kept stopped. 
         [0048]    Accordingly the plunger  44  is kept exposed to the cathode gas which is dry when the fuel cell system  1  is kept stopped and the produced water on the cathode  13  can not diffuse up to a space between the plunger  44  and the guide member  46   d  through a space between the fixed iron core  46   e  and the shaft  47 . Hence it is possible to prevent water from coming between the plunger  44  and the guide member  46   d  and freezing. As a result it is possible to prevent the scavenge gas introduction valve  42  from malfunctioning when the fuel cell system  1  starts below the freezing point. 
         [0049]    In the fuel cell system  1  of the present invention, the scavenge gas introduction valve  42  is opened by the valve body  45  being pushed off the valve seat  43   c  by the plunger  44 . Since the gas pressure Pa in the anode  12  (as indicated in  FIG. 3B ) is controlled to be higher than the gas pressure Pc in the cathode  13  (as indicated in  FIG. 3B ) and the differential pressure, Pa−Pc (&gt;0) is applied to the valve body  45  to keep the valve body in contact with the valve seat  43   c  when electricity is being generated in the fuel cell system  1 , it is possible to make smaller the spring force of the spring  48   b  to secure the gas isolation between the cathode side and the anode side by preventing a gas from leaking from between the valve body  45  and the valve seat  43   c . As a result the plunger  44  is driven and moved by a smaller driving force when the scavenge gas introduction valve  42  is opened and it is possible to make the scavenge gas introduction valve  42  smaller. 
         [0050]    Furthermore in the fuel cell system of the present invention it is possible to prevent the scavenge gas introduction valve  42  from malfunctioning when the fuel cell system  1  starts below the freezing point without a diaphram and a breathing hole. As a result it is possible to improve component lay-out due to the reduced number of used components and reduce the weight and the cost of the fuel cell system. 
         [0051]    A fuel cell system according to claim  2 , 
         [0052]    wherein the plunger is installed on a side of the cathode in the bypass pipe with respect to the valve body of the scavenge gas introduction valve.