Abstract:
A fuel cell system is provided with a fuel cell having an anode and a cathode; a mixing tank containing a mixture of methanol and water; a circulating flow path linking the mixing tank and the anode, the circulating flow path supplying the mixture to the anode and recycling an exhaust fluid exhausted from the anode; and a gas-liquid separator disposed on the circulating flow path, the gas-liquid separator separating a gas phase from a liquid phase of the exhaust fluid.

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. 2004-073062 (filed Mar. 15, 2004); 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, which recycles water from an exhausted fluid.  
         [0004]     2. Description of the Related Art  
         [0005]     A direct methanol fuel cell (DMFC) is one of various types of fuel cells and capable of directly utilizing methanol as a fuel without reforming. The direct methanol fuel cell is ordinarily provided with a fuel cell stack, which includes one or more fuel cells. Each of the fuel cells is provided with a membrane electrode assembly (MEA), which is composed of a cathode catalyst layer, a cathode gas diffusion layer, an anode catalyst layer, an anode gas diffusion layer and an electrolyte membrane put between a cathode catalyst layer and an anode catalyst layer. A mixture of the methanol and water is supplied to the anode and air is supplied to the cathode. As a result of reaction in the fuel cell, water is generated and exhausted from the cathode.  
         [0006]     The water is necessary for generating the reaction in the DMFC and for this purpose the water generated in the reaction is sometimes recycled. Japanese Patent Application Laid-open No. 2002-110199 discloses a related art, in which the water exhausted from the cathode is recycled. According to this related art, the fuel cell system is provided with a mixing tank and the recycled water and fuel supplied from a fuel tank is mixed to form a mixture therein. The recycled water contained in the mixture is supplied to the anode of the DMFC.  
       SUMMARY OF THE INVENTION  
       [0007]     According to a first aspect of the present invention, a fuel cell system is provided with a fuel cell having an anode and a cathode; a mixing tank containing a mixture of methanol and water; a circulating flow path linking the mixing tank and the anode, the circulating flow path supplying the mixture to the anode and recycling an exhaust fluid exhausted from the anode; and a gas-liquid separator disposed on the circulating flow path, the gas-liquid separator separating a gas phase from a liquid phase of the exhaust fluid.  
         [0008]     According to a second aspect of the present invention, a fuel cell system is provided with a fuel cell having an anode and a cathode; a mixing tank supplying a mixture of methanol and water to the anode; a circulating flow path conducting an exhaust fluid exhausted from the anode; and a gas-liquid separator separating a gas phase from a liquid phase of the exhaust fluid. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a schematic illustration of a fuel cell system according to a first embodiment of the present invention; and  
         [0010]      FIG. 2  is a schematic illustration of a fuel cell system according to a second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]     Referring now to  FIG. 1 , a fuel cell system  1  according to a first embodiment of the present invention is provided with a fuel cell (FC) main body  3 , a fuel tank  9 , a mixing tank  11 , an anode-side radiator  29 , a cathode side radiator  33  and an exhaust radiator  43 . The FC main body  3  is composed of one or more fuel cells, each of which is provided with an anode  5 , a cathode  7  and a membrane electrode assembly (MEA) interposed therebetween. The MEA is composed of a cathode catalyst layer, a cathode gas diffusion layer, an anode catalyst layer, an anode gas diffusion layer and an electrolyte membrane put between a cathode catalyst layer and an anode catalyst layer. The anode  5  and the cathode  7  are illustrated as if being separated and the MEA is omitted in  FIG. 1 , however, the anode  5 , the MEA and the cathode  7  are closely accumulated in fact. Moreover, the fuel cell system  1  may include plural anodes  5  and plural cathodes  7 , however, for ease of explanation, the following description will be given to a case where only a pair of anode  5  and cathode  7  are provided.  
         [0012]     The fuel tank  9  contains methanol as a fuel for electricity generation. The mixing tank  11  contains a mixture of methanol and water as will be described later in detail.  
         [0013]     A circulating flow path provided with a connection flow path  13 , an outflow path  17  and a fuel supply path  15  links the fuel tank  9 , the anode-side radiator  29 , the mixing tank  11  and the anode  5 . The connection flow path  13  links the anode  5  and the anode-side radiator  29 . The fuel tank  9  is linked to the connection flow path  13  and is provided with an open-and-closable valve V 1  and a pump P 1  for feeding the fuel. The anode-side radiator  29  is provided with a gas-liquid separation membrane  27  disposed at a side of an outflow port of the anode  5 . The outflow path  17  links the anode-side radiator  29  to the mixing tank  11 . The fuel supply path  15  links the mixing tank  11  to the anode  5  and is provided with a pump P 2  for feeding the mixture to the anode  5 .  
         [0014]     The methanol supplied from the fuel tank  9  is mixed with an exhaust fluid from the anode  5  in the connection flow path  13 , the anode-side radiator  29  and the outflow path  17  in the course of flowing into the mixing tank  11 . Thereby, unreacted methanol contained in the exhaust fluid is recycled.  
         [0015]     The anode-side radiator  29  is provided with a plurality of radiation fins  29 A, which are so dimensioned and configured to receive air fed by a ventilator (not shown in  FIG. 1 ).  
         [0016]     A gas-liquid separation membrane  27  is interposed between the connection flow path  13  and the anode-side radiator  29 . An exhaust flow path  27 A is connected to the gas-liquid separation membrane  27  and the exhaust radiator  43 .  
         [0017]     The exhaust radiator  43  is also provided with a plurality of radiation fins  43 A, which are so dimensioned and configured to receive air fed by the ventilator (not shown in  FIG. 1 ), a water collector tank  45  and an exhaust flow path  47  exposed to the exterior air.  
         [0018]     The exhaust flow path  47  is provided with an adsorbent unit  49  for adsorbing and removing volatile organic compounds (VOC) and an open-and-closable valve V 5  disposed in this order.  
         [0019]     The water collector tank  45  is linked to the mixing tank  11  via a connection flow path  51 . The connection flow path  51  is provided with a pump P 5  for feeding condensed water in the water collector tank  45  to the mixing tank  11  and a check valve CV downstream thereof.  
         [0020]     The gas-liquid separation membrane  27  separates a gas phase, which includes carbon dioxide generated at the anode  5 , from a liquid phase, which includes the methanol supplied from the fuel tank  9  and the unreacted methanol and water exhausted from the anode  5 , of the gas-liquid mixture fluid exhausted from the anode  5 . Thereby, the carbon dioxide does not substantially flow into the anode-side radiator  29 . This leads to suppression of pressure drop in an interior flow path of the anode-side radiator  29  and increase in efficiencies of heat exchange and heat radiation thereof.  
         [0021]     The methanol supplied from the fuel tank  9  and the unreacted methanol and water are sufficiently cooled at the anode-side radiator  29 . The mixing tank  11  receives the sufficiently cooled methanol and water and hence temperature increase of the fluid in the mixing tank  11  is effectively prevented. Moreover, the unreacted methanol and water can be substantially recycled so that fuel efficiency is increased.  
         [0022]     The gas phase separated by the gas-liquid membrane  27  is cooled at the exhaust radiator  43  so as to condense condensable components such as water contained therein. The condensed water is further separated from the gas phase in the exhaust radiator  43  and collected into the water collector tank  45 . The remaining gas phase is exhausted to the exterior air in a sufficiently cooled state. The condensed water is supplied to the mixing tank  11  and mixed with the methanol.  
         [0023]     An air supply path  23  is provided so as to supply air to the cathode  7 . The air supply path  23  is provided with a filter  31 , an open-and-closable valve V 3  and an air pump P 3  disposed in this order.  
         [0024]     The cathode  7  is linked to a cathode-side radiator  33  via a discharging flow path  25 . The cathode-side radiator  33  is provided with a plurality of radiation fins  33 A, which are so dimensioned and configured to receive air fed by the ventilator (not shown in  FIG. 1 ) , a water collector tank  35  and an exhaust flow path  37  exposed to the exterior air.  
         [0025]     The exhaust flow path  37  is provided with an adsorbent unit  39  and an open-and-closable valve V 4  disposed in this order.  
         [0026]     The water collector tank  35  is linked to the mixing tank  11  via a connection flow path  41 . The connection flow path  41  is provided with a pump P 4  for feeding condensed water in the water collector tank  35  to the mixing tank  11  and a check valve CV downstream thereof.  
         [0027]     The exhaust fluid containing water vapor exhausted from the cathode  7  is cooled at the cathode-side radiator  33  so as to condense water and separate a gas phase from the exhaust fluid. The separated gas phase is exhausted to the exterior air in a sufficiently cooled state. The condensed water is supplied to the mixing tank  11  and mixed with the methanol.  
         [0028]     Moreover, the radiators  29 ,  33  and  43  can radiate excessive heat generated in the fuel cell system  1 .  
         [0029]     A second embodiment of the present invention will be described hereinafter with reference to  FIG. 2 . In this drawing and the following description, substantially the same elements as the aforementioned first embodiment are referenced with the same numerals and detailed description thereof will be omitted.  
         [0030]     According to the second embodiment of the present invention, the mixing tank  11  is provided with a gas-liquid membrane  11 A and an exhaust flow path  21  is linked thereto. The exhaust flow path  27 A linked with the gas-liquid membrane  27  is merged with the exhaust flow path  21 . The exhaust flow path  21  is provided with an open-and-closable valve V 2  downstream of the merging portion and further merged with the discharging flow path  25  from the cathode  7 .  
         [0031]     Similarly to the aforementioned first embodiment, the gas-liquid separation membrane  27  separates a gas phase, which includes carbon dioxide generated at the anode  5 , from a liquid phase, which includes methanol supplied from the fuel tank  9  and unreacted methanol and water exhausted from the anode  5 , of the gas-liquid mixture fluid exhausted from the anode  5 . Thereby, the carbon dioxide does not substantially flow into the anode-side radiator  29 . This leads to suppression of pressure drop in an interior flow path of the anode-side radiator  29  and increase in efficiencies of heat exchange and heat radiation thereof.  
         [0032]     The methanol supplied from the fuel tank  9  and the unreacted methanol and water are sufficiently cooled at the anode-side radiator  29 . The mixing tank  11  receives the sufficiently cooled methanol and hence temperature increase of the fluid in the mixing tank  11  is effectively prevented. Moreover, the unreacted methanol can be substantially recycled so that fuel efficiency is increased.  
         [0033]     The gas phase separated by the gas-liquid membrane  27  is cooled at the cathode-side radiator  33  so as to condense condensable components such as water contained therein. The condensed water is further separated from the gas phase in the cathode-side radiator  33  and collected into the water collector tank  35 . The collected water can be conducted into the mixing tank  11  and the remaining gas phase exhausted to the exterior air in a sufficiently cooled state.  
         [0034]     Furthermore, the exhaust fluid containing water vapor exhausted from the cathode  7  is cooled at the cathode-side radiator  33  so as to condense water and separate a gas phase from the exhaust fluid. The separated gas phase is exhausted to the exterior air in a sufficiently cooled state.  
         [0035]     Moreover, the radiators  29  and  33  can radiate excessive heat generated in the fuel cell system  1 .  
         [0036]     According to either embodiment, when a mixture of methanol and water contained in the mixing tank  11  is supplied to the anode  5  and air is supplied to the cathode  7 , an anodic reaction: 
 
CH 3 OH+H 2 O→CO 2 +6H + +6 e   − 
 
 occurs at the anode  5  and a cathodic reaction: 
 
3/2O 2 +6H + +6 e   − →3H 2 O 
 
 occurs at the cathode  7 . The methanol at the anode  5  partly crosses over to the cathode  7  and a combustion reaction thereof: 
 
CH 3 OH+3/2 O 2 →CO 2 +2H 2 O 
 
 may occur at the cathode. 
 
         [0037]     Quantities of methanol consumed by the anodic reaction per unit time (q MeOH   a ), consumed water per unit time (q H2O   a ) and generated carbon dioxide per unit time (q CO2   a ) in each cell can be represented by equations:  
               q   MeOH   a     =     (         I   op       6   ⁢   F       +       I     c   .   o   .         6   ⁢   F         )             (   1   )                 q       H   2     ⁢   O     a     =     (         I   op       6   ⁢   F       +         n   d     ⁢     I   op       F     +   α     )             (   2   )                 q     CO   2     a     =       I   op       6   ⁢   F               (   3   )             
 
 where F is the Faraday constant, I op  is current, I c.o.  is proton current converted from quantity of the crossover methanol, n d  is a number of water molecules which one proton carries and α is a molar flux of moving water by percolation and diffusion. In a case where the FC main body  3  is composed of N fuel cells, those quantities should be multiplied by N. 
 
         [0038]     Quantities of oxygen consumed by the cathodic reaction per unit time (q O2   c ), generated water per unit time (q H2O   c ) and generated carbon dioxide per unit time (q CO2   a ) in each cell can be represented by equations:  
               q     O   2     c     =     (         I   op       4   ⁢   F       +       I     c   .   o   .         4   ⁢   F         )             (   4   )                 q       H   2     ⁢   O     c     =     (         3   ⁢     I   op         6   ⁢   F       +         n   d     ⁢     I   op       F     +       2   ⁢     I     c   .   o   .           6   ⁢   F       +   α     )             (   5   )                 q     CO   2     c     =       I     c   .   o   .       F             (   6   )             
 
 In a case where the FC main body  3  is composed of N fuel cells, those quantities should be multiplied by N. 
 
         [0039]     The carbon dioxide generated by the anodic reaction forms a gas-liquid two-phase flow with a liquid exhausted from the anode  5 . The two-phase flow is dissolved into the gas phase and the liquid phase by means of the gas-liquid membrane  27 . Thereby, in the flow paths for the liquid phase, pressure drop of the fluid therein is suppressed since the fluid does not contain the gas phase. Moreover the flow rate of the fluid in the anode-side radiator  29  is suppressed and hence the heat-radiation efficiency of the anode-side radiator  29  is improved.  
         [0040]     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.