Abstract:
A fuel cell system is provided with one or more fuel cells, each fuel cell including an anode, electrolyte membranes put on both faces of the anode and cathodes respectively put on the electrolyte membranes and a case enclosing the fuel cells so as to leave an air flow path for air supply to the cathodes. The cathode receives air flowing in the air flow path so as to generate electricity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2003-175514 (filed Jun. 19, 2003); the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a fuel cell system and more particularly to a small-sized fuel cell system capable of generating large electric power.  
         [0004]     2. Description of the Related Art  
         [0005]     A fuel cell is provided with a membrane electrode assembly (“MEA” hereinafter), which is provided with a cathode, an anode and a polymer electrolyte membrane put therebetween. The MEA is further put between a pair of separators having electric conductivity. When generating electric power, an oxidant such as air is supplied to the cathode. There is disclosed an art of a method as such air supply, in which the air is supplied by means of diffusion. An art related to this art is disclosed in Japanese Patent Application Laid-open No. 2000-58072. Another art is further disclosed, in which serpentine flow path is formed in the separator and the air is supplied therein by a pump. An art related to this art is disclosed in Japanese Patent Application Laid-open No. 2003-86230.  
         [0006]     Among the aforementioned related arts, the former method does not need a pump for supplying the air and therefore a fuel cell system can be constituted in a small size. However, it is difficult to supply a large amount of air to the cathode to generate large electric power according to the method. It is further difficult to get heat radiation efficiency corresponding to heat generation and therefore temperature of the fuel cell is uneasy to be regulated in a proper range. Therefore it is difficult to generate the large electric power.  
         [0007]     According to the aforementioned latter method, it is easy to supply an enough amount of air to the cathode to generate large electric power. However, the serpentine flow path causes a large pressure drop so that a pump having large capacity should be provided. Such a pump causes a large power consumption, large noise and difficulty for down-sizing.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention is intended for solving the above problem and providing a small-sized fuel cell system capable of generating large electric power.  
         [0009]     According to a first aspect of the present invention, a fuel cell system is provided with one or more fuel cells, each fuel cell including an anode, one or more electrolyte membranes layered on the anode and one or more cathodes layered on the electrolyte membranes and a case enclosing the fuel cell so as to leave an air flow path for air supply to the cathodes.  
         [0010]     According to a second aspect of the present invention, a fuel cell system is provided with a fuel cell including an anode having first and second sides, en electrolyte membrane layered on the first side and a cathode layered further on the electrolyte membrane and a case, an inner surface of the case being adhered to the second side and the case enclosing the fuel cell so as to leave an air flow path for air supply to the cathode. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a schematic illustration of a fuel cell system according to a first embodiment of the present invention;  
         [0012]      FIG. 2  is a schematic illustration of a fuel cell system according to a second embodiment of the present invention;  
         [0013]      FIG. 3  is a schematic illustration of a fuel cell system according to a third embodiment of the present invention;  
         [0014]      FIG. 4  is a schematic illustration of a fuel cell system according to a fourth embodiment of the present invention;  
         [0015]      FIG. 5  is a schematic illustration of a fuel cell system according to a fifth embodiment of the present invention;  
         [0016]      FIG. 6  is a schematic illustration of a fuel cell system according to a sixth embodiment of the present invention;  
         [0017]      FIG. 7  is a schematic illustration of a fuel cell system according to a seventh embodiment of the present invention;  
         [0018]      FIG. 8  is a perspective view with a partial cross-sectional view of a heat exchanger according to a first version; and  
         [0019]      FIG. 9  is a perspective view with a partial cross-sectional view of a heat exchanger according to a second version. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     Referring to  FIG. 1 , a fuel cell system  1  according to a first embodiment of the present invention is provided with a case  7  formed in a tubular shape, which encloses a space  5 . Both ends of the space  5  are opened so as to permit an airflow through the space  5  for supplying air as an oxidant. A cross-section thereof preferably has a square or rectangular shape, where the rectangular shape is long sideways and the sideways direction is perpendicular to a plane of the paper of  FIG. 1 . The cross-sectional shape is not limited to square and rectangular but can be formed in various shapes.  
         [0021]     The case  7  is preferably provided with a heat insulator  3  along the case  7  for heat insulation between the interior and the exterior thereof. As the heat insulator  3 , various means can be employed. Preferably, heat insulating material such as glass wool, ceramics or foam plastic can be applied. Alternatively, a thin airtight chamber, an interior of which is kept in a vacuum or filled with heat insulating gas such as carbon dioxide or xenon, can be applied.  
         [0022]     An outflow path  9  is connected to one end of the case  7  and has a cross-sectional area less than a cross-sectional area of the space  5 . An outer periphery of the outflow path  9  is provided with a heat exchanging unit  11  such as fins. An inner periphery of the outflow path  9  is provided with a water absorbent member  13  such as a wick.  
         [0023]     A fuel cell  19  is provided with an anode (a fuel electrode)  15  to which fuel is supplied, a pair of electrolyte membranes  21  respectively layered on both sides of the anode  15  and a pair of cathodes (air electrodes)  17  layered further on both sides thereof. The fuel cell  19  is housed in the case  7  and disposed in the space  5  so as to leave an air flow path for air supply to the cathodes  17  between the fuel cell  19  and the case  7 . The anode  15  is provided with a flow path of a conductive material. The fuel flows through the flow path so as to spread all over the anode  15 . The cathode  17  is provided with a current collector (not shown) so that the generated electric power is extracted to an external cable (not shown).  
         [0024]     For supplying the fuel to the anode  15 , a supply flow path  25  such as a pipe connects a mixing tank  23  and the anode  15  and is laid through the outflow path  9 . A pump P 1  for transmitting the fuel is joined in the supply path  25 . A recovery flow path  27  further connects the anode  15  and the mixing tank  23 . The supply flow path  25  and the recovery flow path  27  are connected to and pass through a heat exchanger  29 . Namely, the fuel supplied to the anode  15  via the supply flow path  25  and fluid flowing through the recovery flow path  27  exchange heat at the heat exchanger  29 .  
         [0025]     A fuel tank  31  housing methanol as the fuel is connected to the mixing tank  23  via a pump P 2 . A water tank  33  is further connected to the mixing tank  23  via a pump P 3 . The mixing tank  23  is provided with an exhaust port  35  so as to exhaust gas therein outward. The water tank  33  houses a porous member such as a sponge therein so as to contain water. A connection channel  37  connects the water absorbent member  13  and the water tank  33  so as to conduct the water contained in the water absorbent member  13  to the water tank  33 .  
         [0026]     A fan  39  is disposed at the other end, opposite to the outflow path  9 , of the space  5  so as to produce flow of air as an oxidant supplied to the cathode  17 . An air duct  41  connected to a suction side of the fan  39  is led to the heat exchanging unit  11  and an opening portion  43  of the air duct  41  is disposed at and encircles the heat exchanging unit  11 .  
         [0027]     When driving the pump P 1  so as to supply the fuel in the mixing tank  23  to the anode  15  and also driving the fan  39  so as to supply the air to the cathode  17 , the fuel cell  19  generates electricity. Accompanying the generation of the electricity, the fuel cell  19  generates heat. The space  5  encloses the fuel cell  19  so as to reduce heat transmission to the surroundings. Thereby the temperature of the fuel cell  19  is easily regulated in a proper range and the efficiency of the power generation can be increased.  
         [0028]     The air introduced into the opening portion  43  exchanges heat with the hot air exhausted from the outflow path  9  so as to be heated and is then supplied to the fuel cell  19 . Thereby the fuel cell  19  is prevented from over-cooling and the temperature thereof is further easily regulated.  
         [0029]     The air exhausted from the outflow path  9  contains water vapor generated in the fuel cell  19  and is cooled because of the heat exchange so that the water is condensed. The condensed water is absorbed into the water absorbent member  13  and sucked to the water tank  33  by means of negative pressure, which the pump P 3  generates in the water tank  33  when supplying water to the mixing tank  23 .  
         [0030]     Consequently, the water generated in the fuel cell  19  is conducted to the mixing tank  23  and mixed with the fuel therein. In expectation of being diluted with the water in the mixing tank  23 , highly concentrated fuel (for example, pure methanol) can be utilized and housed in the fuel tank  31 . Therefore the fuel tank  31  as well as the water  33  can be small-sized. Moreover, the fuel cell system as a whole can be small-sized.  
         [0031]     Furthermore, the fuel supplied from the mixing tank  23  by means of the pump P 1  exchanges heat with the hot fluid containing unreacted methanol, water, and carbon dioxide exhausted from the anode  15  through the recovery flow path  27  so as to be heated and is then supplied to the anode  15 . Thereby the fuel cell  19  is prevented from over-cooling and the temperature thereof is further easily regulated.  
         [0032]     The space  5  has a linear configuration so that the flow of the air supplied by the fan  39  is hardly obstructed by anything except for the fuel cell  19  from the inflow opening to the outflow path  9  and has relatively small pressure drop. Therefore the fan  39  can also small-sized and the consumption of the electricity can be reduced. The fuel cell system as a whole can be further small-sized.  
         [0033]     The water introduced from the water tank  33  and the fuel introduced from the fuel tank  31  are so mixed in the mixing tank  23  as to be a proper concentration, which the fuel cell  19  generates power efficiently.  
         [0034]     In contrast to the above description, the air duct  41  connected to the opening portion  43  may be omitted. As well, the aforementioned configuration that the outflow path  9  is configured to collect the water contained in the exhaust gas, may be omitted and another source of the water may be provided instead. Furthermore, the heat exchanger  29  may be omitted.  
         [0035]     A rate of feeding air by the fan  39  may be fixed or, where necessary, changed so as to regulate the temperature of the fuel cell  19  or the amount of water which the fed air carries away from the fuel cell  19 .  
         [0036]     A second embodiment of the present invention will be described hereinafter with reference to  FIG. 2 . In the following description, substantially the same elements as the aforementioned description are referenced with the same numerals and the detailed descriptions are omitted.  
         [0037]     As compared with the above first embodiment, one set of the cathode  17  and the electrolyte membrane  21  is omitted and the anode  15  is supported by the inner surface of the case  7 . Either direct adhesion or any support member interposed therebetween may be employed as an aspect of the support manner. As well as having the same effect as the above first embodiment, the second embodiment makes the fuel cell system as a whole further small-sized because the case  7  can be constituted in a thinner shape.  
         [0038]     A third embodiment of the present invention will be described hereinafter with reference to  FIG. 3 . In the following description, substantially the same elements as the aforementioned description are referenced with the same numerals and the detailed descriptions are omitted.  
         [0039]     As compared with the above first embodiment, the water tank  33 , the connection channel  37  connected thereto, the mixing tank  23  and the pumps P 2 , P 3  are omitted and the fuel tank  31  is directly connected to the pump P 1 . Furthermore, the recovery flow path  27  and the heat exchanger  29  are omitted and CO 2  generated at the anode  15  is separated at a gas-liquid separation membrane  51  provided at end peripheries of the anode  15  and exhausted through the outflow path  59 . Moreover, the heat exchanging unit  11  and the air duct  41  having the opening portion  43  encircling the heat exchanging unit  11  are omitted and the air is supplied to the cathode  17  by means of the fan  39  without heat exchange.  
         [0040]     According to the present embodiment, the fuel housed in the fuel tank  31  is necessary to be diluted in a proper concentration, which corresponds to a mixing ratio of a quantity of fuel consumed at the anode  15  and a quantity of water consumed at the anode  15 , humidifying the electrolyte membranes  21 , and partly percolating to the cathode  17 , in advance. Therefore it is necessary that the fuel tank  31  is made larger or a capacity of the fuel cell is made smaller as compared with the above first embodiment. Instead, the aforementioned elements can be omitted and therefore the fuel cell system as a whole can be further small-sized.  
         [0041]     A fourth embodiment of the present invention will be described hereinafter with reference to  FIG. 4 . In the following description, substantially the same elements as the aforementioned description are referenced with the same numerals and the detailed descriptions are omitted.  
         [0042]     As compared with the above first embodiment, the pump P 1  is omitted and a porous body  55  such as a sponge links the mixing tank  23  with the anode  15  so as to supply the fuel to the anode  15 . Furthermore, the recovery flow path  27  and the heat exchanger  29  are omitted and CO 2  generated at the anode  15  is separated at a gas-liquid separation membrane  51  provided at end peripheries of the anode  15  and exhausted through the outflow path  59 . According to the present embodiment, the fuel is conducted by means of capillary force of the porous body  55  so that the pump P 1  can be omitted. To this extent, the whole constitution can be simplified.  
         [0043]      FIG. 5  schematically shows a fifth embodiment of the present invention, which has a further simplified constitution as compared with the above third and fourth embodiments. The water tank  33 , the heat exchanging unit  11  and such are omitted so that the fuel is directly supplied from the fuel tank  31  to the anode  15 , as is the case with the above third embodiment. A link between the fuel tank  31  and the anode  15  is achieved by the porous body  55 , as is the case with the above fourth embodiment.  
         [0044]     According to the present embodiment, it is necessary to utilize fuel which is diluted in a proper concentration in advance, like as the third embodiment. Therefore it is necessary that the fuel tank  31  is made larger or the cell capacity is made smaller than the case with the first embodiment. Instead, the constitution is simplified, as is the case with the third embodiment, and can be further simplified because the pump P 1  is omitted.  
         [0045]     A sixth embodiment of the present invention will be described hereinafter with reference to  FIG. 6 . In the following description, substantially the same elements as the aforementioned description are referenced with the same numerals and the detailed descriptions are omitted.  
         [0046]     As compared with the above first embodiment, the fuel cell system is provided with a plurality of the fuel cells  19 . The fuel cells  19  are arranged in parallel with each other at proper intervals and disposed in the space  5 . As shown in  FIG. 6 , the supply flow path  25  is connected to the first of the fuel cells  19  and the recovery flow path therefrom is connected to the second. There covery flow path from the second is further connected to the third and the rests of the fuel cells  19  are similarly configured. Thereby the fuel cells  19  are connected in series via the flow paths. Alternatively, as a seventh embodiment shown in  FIG. 7 , a manifold  45  may be provided and the fuel are supplied to the fuel cells  19  via the manifold  45 . According to the sixth or seventh embodiment, larger electricity generation can be obtained because the plural fuel cells  19  are provided.  
         [0047]     The aforementioned embodiments utilize the heat exchanger  29  to exchange heat between the supply flow path  25  and the recovery flow path  27 . As an alternative to such constitutions, to-and-fro tubes so configured as to exchange heat therebetween can be applied.  FIG. 8  shows a first version thereof, which is provided with a tube  61  having a rectangular cross section and a partition  63  having high heat conductivity. An interior of the tube  61  is partitioned into two by the partition  63 .  FIG. 9  shows a second version, which is provided with an outer cylindrical tube  67  and an inner cylindrical tube  69  having high heat conductivity, which are coaxially disposed. A heat insulator  65  may be applied for heat insulation from the surroundings. Respective cavities partitioned by the partition  63  or the inner cylindrical tube  69  serve as the supply flow path  25  and the recovery flow path  27  and exchange heat therebetween.  
         [0048]     As being understood from the above description, according to any of the embodiments of the present invention, pressure drop of the air in the space is suppressed. Therefore a small fan is enough for supplying the air to the cathode and for generating relatively large electricity. Down-sizing and getting higher efficiency of the fuel cell can be achieved.  
         [0049]     Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the above teachings.