Abstract:
A fuel cell stack is provided with a pair of refrigerant inlet ports and a pair of refrigerant outlet ports. The refrigerant inlet ports are disposed in the vicinity of an oxidant gas inlet port and a fuel gas inlet port in a manner such that one of the refrigerant inlet ports is disposed on the side of the oxidant gas inlet port and the other refrigerant inlet port is disposed on the side of the fuel gas inlet port. The refrigerant outlet ports are disposed in the vicinity of an oxidant gas outlet port and a fuel gas outlet port in a manner such that one of the refrigerant outlet ports is disposed on the side of the oxidant gas outlet port and the other refrigerant outlet port is disposed on the side of the fuel gas outlet port.

Description:
TECHNICAL FIELD  
       [0001]    The present invention relates to an operation method for a fuel cell formed by stacking an electrolyte electrode assembly and a separator having rectangular flat surfaces. The electrolyte electrode assembly includes a pair of electrodes and an electrolyte interposed between the electrodes. Further, the present invention relates to a fuel cell system. 
       BACKGROUND ART  
       [0002]    For example, a solid polymer electrolyte fuel cell employs a solid polymer electrolyte membrane. The electrolyte membrane is a polymer ion exchange membrane, and interposed between an anode and a cathode to form a membrane electrode assembly (MEA). The membrane electrode assembly and a pair of separators sandwiching the membrane electrode assembly make up a unit cell (power generation unit) for generating electricity. A plurality of unit cells are stacked together to form a fuel cell stack, e.g., mounted in a vehicle. 
         [0003]    In the fuel cell, a fuel gas flow field (reactant gas flow field) is formed on a surface of one of adjacent separators that faces the anode, for allowing fuel gas to flow along the fuel gas flow field, and an oxygen-containing gas flow field (reactant gas flow field) is formed on a surface of the other of the adjacent separators that faces the cathode, for allowing oxygen-containing gas to flow along the oxygen-containing gas flow field. Further, a coolant flow field is formed between adjacent separators for allowing a coolant to flow along surfaces of the separators. 
         [0004]    In the fuel cell, in order to maintain a desired humidified state of the solid polymer electrolyte membrane, the oxygen-containing gas and the fuel gas supplied to the fuel cell are humidified beforehand, and water is produced in power generation reaction. Thus, condensation of the water produced in the power generation reaction may occur undesirably in the reactant gas flow field. 
         [0005]    In the fuel gas flow field, the water produced in the oxygen-containing gas flow field permeates through the thin solid polymer electrolyte membrane, and the water tends to be diffused backward. Therefore, the membrane electrode assembly is partially clogged with the dew condensation water, and power generation cannot be performed in the clogged portion. The temperature in the area of the oxygen-containing gas flow field on the downstream side corresponding to the clogged portion is decreased, and the dew condensation may occur. Further, the area of the oxygen-containing gas flow field on the downstream side is clogged with water, and the water produced in the oxygen-containing gas flow field may not be diffused backward to the fuel gas flow field. Thus, the water may be retained as stagnant water also on the downstream side of the oxygen-containing gas flow field undesirably. 
         [0006]    Some fuel cells adopt the so-called skip cooling structure where the coolant flow fields are formed at intervals of a certain number of unit cells. In the structure, the temperature in the fuel gas flow field provided in the unit cell adjacent to the coolant flow field may be decreased significantly, and the condensed water may be retained easily as stagnant water. 
         [0007]    As a result, the fuel gas flow field may be clogged with the stagnant water undesirably, and the flow of the fuel gas becomes non-uniform (so-called flooding occurs). Consequently, a desired power generation performance cannot be achieved. Further, also in the oxygen-containing gas flow field, clogging of the flow field may occur undesirably. 
         [0008]    In this regard, for example, a fuel cell system disclosed in Japanese Laid-Open Patent Publication No. 2004-185938 is known. As shown in  FIG. 15 , in the fuel cell system, an air humidifier  2  is connected to a heat medium discharge port  1   a  of a fuel cell  1 , and a fuel humidifier  3  is connected to the air humidifier  2 . A heat exchanger  4  is connected to the fuel humidifier  3 , and a heat medium supply port  1   b  of the fuel cell  1  is connected to the heat exchanger  4  to form a water circulation channel  5  for water as heat medium. 
         [0009]    A total heat exchanger  6  is connected to an oxygen-containing gas discharge port  1   c  of the fuel cell  1 , and the air humidifier  2  is connected to the total heat exchanger  6 . An oxygen-containing gas supply port  1   d  of the fuel cell  1  is connected to the air humidifier  2  to form an air supply channel  7  for air as the oxygen-containing gas. 
         [0010]    A fuel reformer  8  is connected to the fuel humidifier  3 , and produces reformed gas chiefly containing hydrogen using raw fuel such as the city gas. After this reformed gas is humidified by the fuel humidifier  3 , the humidified reformed gas is supplied to a fuel supply port  1   e  of the fuel cell  1 . 
         [0011]    Thus, in an inlet area of the reactant gas, if the dew point of the reactant gas is set to be lower than the temperature of the heat medium (water discharged from the fuel cell  1 ), the reactant gas is heated in the inlet area by the heat medium. According to the disclosure, in the structure, it becomes possible to prevent condensation of water vapor in the humidified reactant gas in the inlet area, and the reactant gas can stat to flow smoothly without any condensed water retained in the inlet area. 
       SUMMARY OF INVENTION  
       [0012]    In the fuel cell system, in an outlet area of the reactant gas, the dew point of the reactant gas is set to be higher than the temperature of the heat medium. Therefore, the reactant gas is cooled by the heat medium in the outlet area, and condensation of the water vapor in the reactant gas may occur. However, according to the disclosure, the water droplets can be ejected easily under the pressure applied uniformly to each heat medium flow field, and the condensed water can be discharged into an outlet header area in a short period of time. 
         [0013]    However, in the outlet area of the reactant gas, the reactant gas is consumed in the reaction. Therefore, the condensed water may not be discharged suitably from the outlet area of the reactant gas into the outlet header area. In particular, during low load power generation, the flow rate of the reactant gas is small, and removal of the condensed water becomes difficult. Since a large amount of the condensed water is produced easily due to the decrease in the temperature, water may be retained as stagnant water undesirably. 
         [0014]    The present invention has been made to solve the problems of this type, and an object of the present invention is to provide an operation method for a fuel cell, and a fuel cell system in which it is possible to reliably prevent condensed water from being retained as stagnant water in a reactant gas flow field, and a desired power generation state is achieved. 
         [0015]    The present invention relates to an operation method for a fuel cell formed by stacking an electrolyte electrode assembly and a separator having rectangular flat surfaces in a stacking direction. The electrolyte electrode assembly includes a pair of electrodes and an electrolyte interposed between the electrodes. A reactant gas supply passage and a reactant gas discharge passage extend through one pair of two opposite sides of the separator in the stacking direction, the reactant gas supply passage and the reactant gas discharge passage being connected to a reactant gas flow field for allowing a reactant gas to flow along an electrode surface. A pair of coolant supply passages and a pair of coolant discharge passages for allowing a coolant to flow therethrough extend through the other pair of two opposite sides of the separator at least at positions adjacent to the reactant gas supply passage or the reactant gas discharge passage, the pair of coolant supply passages are disposed separately on the two opposite sides, and the pair of coolant discharge passages are disposed separately on the two opposite sides. 
         [0016]    The operation method includes the steps of: detecting whether or not at least a portion of a fuel gas flow field serving as the reactant gas flow field where a fuel gas as the reactant gas flows has been clogged with water; and if it is determined that at least a portion of the fuel gas flow field has been clogged with the water, limiting the flow of the coolant to the coolant discharge passage adjacent to the reactant gas discharge passage. 
         [0017]    Further, the present invention relates to an operation method for a fuel cell and a fuel cell system including the fuel cell. The fuel cell is formed by stacking an electrolyte electrode assembly and a separator having rectangular flat surfaces in a stacking direction. The electrolyte electrode assembly includes a pair of electrodes and an electrolyte interposed between the electrodes. A reactant gas supply passage and a reactant gas discharge passage extend through one pair of two opposite sides of the separator in the stacking direction, the reactant gas supply passage and the reactant gas discharge passage being connected to a reactant gas flow field for allowing a reactant gas to flow along an electrode surface. At least a pair of coolant supply passages and at least a pair of coolant discharge passages for allowing a coolant to flow therethrough extend through the other pair of two opposite sides of the separator in the stacking direction at least at positions adjacent to the reactant gas supply passage or the reactant gas discharge passage, the pair of coolant supply passages are disposed separately on the two opposite sides, and the pair of coolant discharge passages are disposed separately on the two opposite sides. 
         [0018]    The operation method includes the steps of: detecting whether or not at least a portion of the reactant gas flow field has been clogged with water; and if it is determined that at least a portion of the reactant gas flow field has been clogged with the water, at least implementing control to supply the coolant to the coolant supply passages at different flow rates or discharge the coolant from the coolant discharge passages at different flow rates. 
         [0019]    Further, the fuel cell system includes a first supply channel and a first discharge channel connected respectively to the coolant supply passage and the coolant discharge passage that are disposed on a first side of the other pair of two opposite sides of the separator, a second supply channel and a second discharge channel connected respectively to the coolant supply passage and the coolant discharge passage that are disposed on a second side of the other pair of two opposite sides of the separator, a first branch channel connected to a middle portion of the first supply channel and a middle portion of the first discharge channel, a second branch channel connected to a middle portion of the second supply channel and a middle portion of the second discharge channel, valve mechanisms provided at least in the first branch channel and the second branch channel, respectively, and a controller for determining whether or not at least a portion of the reactant gas flow field has been clogged with water. 
         [0020]    In the present invention, if it is determined that at least a portion of the fuel gas flow field has been clogged with water, the flow of the coolant in the coolant discharge passage adjacent to the reactant gas discharge passage is limited. 
         [0021]    Thus, in the power generation surface, the flow rate of the coolant flowing in the area adjacent to the reactant gas discharge passage is reduced, and the temperature in the area adjacent to the reactant gas discharge passage is increased. Thus, the condensed water retained as stagnant water in the area adjacent to the reactant gas discharge passage is discharged easily and suitably, and it is possible to remove the stagnant water. 
         [0022]    Accordingly, in particular, during low load power generation, it becomes possible to reliably prevent the condensed water from being retained as stagnant water in the area adjacent to the reactant gas discharge passage such as the fuel gas discharge passage, and a desired power generation state is achieved. 
         [0023]    Further, in the present invention, if it is determined that at least a portion of the fuel gas flow field on the downstream side has been clogged with water, control at least to supply the coolant to the coolant supply passages at different flow rates or discharge the coolant from the coolant discharge passages at different flow rates is implemented. 
         [0024]    Thus, in the power generation surface, the flow of the coolant can be limited depending on a portion where the condensed water is produced. Thus, it is possible to reliably prevent the condensed water from being retained as stagnant water in the reactant gas flow field, and a desired power generation state is achieved. 
         [0025]    Further, in the present invention, if it is determined that at least a portion of the reactant gas flow field has been clogged with water, the valve mechanisms opens, e.g., the first branch cannel and the second branch channel. Therefore, by bypassing operation, the coolant chiefly flows from the middle portions of the first supply channel and the second supply channel to the first branch channel and the second branch channel, and then, flows into the first discharge channel and the second discharge channel. Thus, it is possible to limit the flow of the coolant through the coolant flow field in the fuel cell. 
         [0026]    Therefore, it is possible to increase the temperature of the fuel cell, and the condensed water retained as stagnant water in the reactant gas flow field is evaporated, and removed from the reactant gas flow field. As a result, the condensed water retained as stagnant water in the area adjacent to the reactant gas discharge passage can be discharged easily and suitably, and it is possible to remove the stagnant water. 
         [0027]    Accordingly, in particular, during low load power generation, it is possible to reliably prevent the condensed water from being retained as stagnant water in the area adjacent to the reactant gas discharge passage, and a suitable power generation state is achieved. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS  
         [0028]      FIG. 1  is a diagram schematically showing a fuel cell system including a fuel cell stack to which an operation method according to a first embodiment of the present invention is applied; 
           [0029]      FIG. 2  is an exploded perspective view showing main components of a power generation unit of the fuel cell stack; 
           [0030]      FIG. 3  is a cross sectional view showing the fuel cell stack; 
           [0031]      FIG. 4  is a front view showing a first separator of the power generation unit; 
           [0032]      FIG. 5  is a diagram showing flows of a coolant for illustration of the operation method; 
           [0033]      FIG. 6  is a diagram showing flows of the coolant for illustration of the operation method; 
           [0034]      FIG. 7  is a view showing the state where stagnant water is produced on the downstream side of a first fuel gas flow field; 
           [0035]      FIG. 8  is a diagram showing the flows of a coolant for illustration of the operation method; 
           [0036]      FIG. 9  is a diagram showing the flows of the coolant for illustration of the operation method; 
           [0037]      FIG. 10  is a diagram schematically showing a fuel cell system including a fuel cell stack to which operation methods according to first and second embodiments of the present invention are applied; 
           [0038]      FIG. 11  is a diagram schematically showing a fuel cell system including a fuel cell stack to which an operation method according to a third embodiment of the present invention is applied; 
           [0039]      FIG. 12  is a diagram showing flows of a coolant for illustration of the operation method; 
           [0040]      FIG. 13  is a diagram showing flows of the coolant for illustration of the operation method; 
           [0041]      FIG. 14  is a diagram showing flows of the coolant for illustration of the operation method; and 
           [0042]      FIG. 15  is a diagram showing a fuel cell system disclosed in Japanese Laid-Open Patent Publication No. 2004-185938. 
       
    
    
     DESCRIPTION OF EMBODIMENTS  
       [0043]    As shown in  FIG. 1 , a fuel cell stack (fuel cell)  10  to which an operation method according to a first embodiment of the present invention is applied is included in a fuel cell system  12 . For example, the fuel cell system  12  is used in an automobile application, and mounted in a vehicle (not shown). 
         [0044]    The fuel cell system  12  includes an oxygen-containing gas supply apparatus (not shown) for supplying an oxygen-containing gas to the fuel cell stack  10 , a fuel gas supply apparatus (not shown) for supplying a fuel gas to the fuel cell stack  10 , a coolant supply apparatus  14  for supplying a coolant to the fuel cell stack  10 , and a controller  16  for controlling the fuel cell system  12  as a whole. 
         [0045]    As shown in  FIGS. 2 and 3 , the fuel cell stack  10  is formed by stacking a plurality of power generation units  18  in a horizontal direction indicated by an arrow A. Each of the power generation units  18  is elongated in a longitudinal direction, and includes a first separator  20 , a first membrane electrode assembly (electrolyte electrode assembly) (MEA)  22   a,  a second separator  24 , a second membrane electrode assembly (electrolyte electrode assembly) (MEA)  22   b,  and a third separator  26 . For example, the first separator  20 , the second separator  24 , and the third separator  26  are metal separators. Alternatively, for example, carbon separators may be used as the first separator  20 , the second separator  24 , and the third separator  26 . 
         [0046]    In the illustrated embodiment, the power generation unit  18  includes the three separators and the two membrane electrode assemblies. However, the present invention is not limited in this respect. For example, the power generation unit  18  may include four separators and three membrane electrode assemblies. 
         [0047]    As shown in  FIG. 2 , at an upper end portion of the power generation unit  18  in the longitudinal direction indicated by an arrow C, an oxygen-containing gas supply passage  30   a  for supplying the oxygen-containing gas, and a fuel gas supply passage  32   a  for supplying the fuel gas such as a hydrogen-containing gas are provided. The oxygen-containing gas supply passage  30   a  and the fuel gas supply passage  32   a  extend through the power generation unit  18  in the direction indicated by the arrow A. 
         [0048]    At a lower end portion of the power generation unit  18  in the longitudinal direction indicated by the arrow C, a fuel gas discharge passage  32   b  for discharging the fuel gas and an oxygen-containing gas discharge passage  30   b  for discharging the oxygen-containing gas are provided. The fuel gas discharge passage  32   b  and the oxygen-containing gas discharge passage  30   b  extend through the power generation unit  18  in the direction indicated by the arrow A. 
         [0049]    At upper positions on both sides of the power generation unit  18  in a lateral direction indicated by an arrow B, at least a pair of coolant supply passages  34   a  for supplying the coolant are provided. At lower positions on both sides of the power generation unit  18  in the lateral direction, at least a pair of coolant discharge passages  34   b  for discharging the coolant are provided. The coolant supply passages  34   a  and the coolant discharge passages  34   b  extend through the power generation unit  18  in the direction indicated by the arrow A. 
         [0050]    The coolant supply passages  34   a  are positioned adjacent to the oxygen-containing gas supply passage  30   a  and the fuel gas supply passage  32   a  on both sides in the direction indicated by the arrow B respectively. The coolant discharge passages  34   b  are positioned adjacent to the oxygen-containing gas discharge passage  30   b  and the fuel gas discharge passage  32   b  on both sides in the direction indicated by the arrow B respectively. Three or more coolant supply passages  34   a  and three or more coolant discharge passages  34   b  may be provided. 
         [0051]    The first separator  20  has a first fuel gas flow field  36  on its surface  20   a  facing the first membrane electrode assembly  22   a.  The first fuel gas flow field  36  is connected to the fuel gas supply passage  32   a  and the fuel gas discharge passage  32   b.  The first fuel gas flow field  36  includes a plurality of flow grooves  36   a  extending in the direction indicated by the arrow C. An inlet buffer  38  is provided adjacent to the inlet of the first fuel gas flow field  36 , and an outlet buffer  40  is provided adjacent to the outlet of the first fuel gas flow field  36 . A plurality of bosses are formed on the inlet buffer  38 , and a plurality of bosses are formed on the outlet buffer  40 . 
         [0052]    As shown in  FIG. 4 , a plurality of flow grooves  44   a  as part of the coolant flow field  44  connected to the coolant supply passages  34   a  and the coolant discharge passages  34   b  are formed on a surface  20   b  of the first separator  20 . An inlet buffer  46   a  is provided adjacent to the inlet of the flow grooves  44   a,  and an outlet buffer  48   a  is provided adjacent to the outlet of the flow grooves  44   a.  A plurality of bosses are formed on the inlet buffer  46   a,  and a plurality of bosses are formed on the outlet buffer  48   a.    
         [0053]    As shown in  FIG. 2 , the second separator  24  has a first oxygen-containing gas flow field  50  on its surface  24   a  facing the first membrane electrode assembly  22   a.  The first oxygen-containing gas flow field  50  is connected to the oxygen-containing gas supply passage  30   a  and the oxygen-containing gas discharge passage  30   b.  The first oxygen-containing gas flow field  50  includes a plurality of flow grooves  50   a  extending in the direction indicated by the arrow C. An inlet buffer  52  is provided adjacent to the inlet of the first oxygen-containing gas flow field  50 , and an outlet buffer  54  is provided adjacent to the outlet of the first oxygen-containing gas flow field  50 . 
         [0054]    The second separator  24  has a second fuel gas flow field  58  on its surface  24   b  facing the second membrane electrode assembly  22   b.  The second fuel gas flow field  58  is connected to the fuel gas supply passage  32   a  and the fuel gas discharge passage  32   b.  The second fuel gas flow field  58  includes a plurality of flow grooves  58   a  extending in the direction indicated by the arrow C. An inlet buffer  60  is provided adjacent to the inlet of the second fuel gas flow field  58 , and an outlet buffer  62  is provided adjacent to the outlet of the second fuel gas flow field  58 . The fuel gas flows along the first fuel gas flow field  36  and the second fuel gas flow field  58  in the direction of gravity. 
         [0055]    The third separator  26  has a second oxygen-containing gas flow field  66  on its surface  26   a  facing the second membrane electrode assembly  22   b.  The second oxygen-containing gas flow field  66  is connected to the oxygen-containing gas supply passage  30   a  and the oxygen-containing gas discharge passage  30   b.  The second oxygen-containing gas flow field  66  includes a plurality of flow grooves  66   a  extending in the direction indicated by the arrow C. An inlet buffer  68  is provided adjacent to the inlet of the second oxygen-containing gas flow field  66 , and an outlet buffer  70  is provided adjacent to the outlet of the second oxygen-containing gas flow field  66 . The oxygen-containing gas flows along the first oxygen-containing gas flow field  50  and the second oxygen-containing gas flow field  66  in the direction of gravity. 
         [0056]    A plurality of flow grooves  44   b  as part of the coolant flow field  44  are formed on a surface  26   b  of the third separator  26 . An inlet buffer  46   b  is provided adjacent to the inlet of the flow grooves  44   b,  and an outlet buffer  48   b  is provided adjacent to the outlet of the flow grooves  44   b.  A plurality of bosses are formed on the inlet buffer  46   b,  and a plurality of bosses are formed on the outlet buffer  48   b.    
         [0057]    A first seal member  74  is formed integrally with each surface  20   a,    20   b  of the first separator  20 , around the outer circumferential end portion of the first separator  20 . Alternatively, a member separate from the first separator  20  may be provided as the first seal member  74  on each surface  20   a,    20   b  of the first separator  20 . A second seal member  76  is formed integrally with each surface  24   a,    24   b  of the second separator  24 , around the outer circumferential end portion of the second separator  24 . Alternatively, a member separate from the second separator  24  may be provided as the second seal member  76  on each surface  24   a,    24   b  of the second separator  24 . A third seal member  78  is formed integrally with each surface  26   a,    26   b  of the third separator  26 , around the outer circumferential end portion of the third separator  26 . Alternatively, a member separate from the third separator  26  may be provided as the third seal member  78  on each surface  26   a,    26   b  of the third separator  26 . 
         [0058]    The first separator  20  includes outer supply holes  80   a  and inner supply holes  80   b  connecting the fuel gas supply passage  32   a  and the first fuel gas flow field  36 , and outer discharge holes  82   a  and inner discharge holes  82   b  connecting the fuel gas discharge passage  32   b  and the first fuel gas flow field  36 . 
         [0059]    The second separator  24  includes supply holes  84  connecting the fuel gas supply passage  32   a  and the second fuel gas flow field  58 , and discharge holes  86  connecting the fuel gas discharge passage  32   b  and the second fuel gas flow field  58 . 
         [0060]    The power generation units  18  are stacked together, and the coolant flow field  44  is formed between the first separator  20  of one of the power generation units  18  that are adjacent to each other and the third separator  26  of the other of the adjacent power generation units  18 . The coolant flow field  44  extends in the direction indicated by the arrow B. 
         [0061]    As shown in  FIG. 2 , the surface area of the first membrane electrode assembly  22   a  is smaller than the surface area of the second membrane electrode assembly  22   b.  Each of the first and second membrane electrode assemblies  22   a,    22   b  includes an anode  92 , a cathode  94 , and a solid polymer electrolyte membrane (electrolyte)  90  interposed between the anode  92  and the cathode  94 . The solid polymer electrolyte membrane  90  is formed by impregnating a thin membrane of perfluorosulfonic acid with water, for example. The surface area of the anode  92  is smaller than the surface area of the cathode  94 . It is called a stepped-type MEA. 
         [0062]    Each of the anode  92  and the cathode  94  has a gas diffusion layer (not shown) such as a carbon paper, and an electrode catalyst layer (not shown) of platinum alloy supported on porous carbon particles. The carbon particles are deposited uniformly on the surface of the gas diffusion layer. The electrode catalyst layer of the anode  92  and the electrode catalyst layer of the cathode  94  are fixed to both surfaces of the solid polymer electrolyte membrane  90 , respectively. 
         [0063]    As shown in  FIG. 1 , the coolant supply apparatus  14  includes a coolant circulation channel  100 , and a circulation pump  102  and a radiator  104  having a tank are provided in the coolant circulation channel  100 . The coolant circulation channel  100  includes a supply channel  100   a  connected to a branch supply channel  106  and a discharge channel  100   b  connected to a branch discharge channel  108 . 
         [0064]    The supply channel  100   a  and the discharge channel  100   b  are connected to the coolant supply passage  34   a  and the coolant discharge passage  34   b  provided on one of long sides through three-way valves  110   a,    110   b,  respectively. The branch supply channel  106  and the branch discharge channel  108  are connected to the coolant supply passage  34   a  and the coolant discharge passage  34   b  provided on the other of the long sides through three-way valves  112   a,    112   b,  respectively. A branch channel  114   a  is connected between the three-way valves  110   a,    110   b,  and a branch channel  114   b  is connected between the three-way valves  112   a,    112   b.    
         [0065]    Instead of the three-way valves  110   a,    110   b,    112   a,  and  112   b,  variable valves (not shown) with adjustable opening angle, i.e., capable of regulating the flow rate of the coolant may be used. Further, the variable valve may be provided in any one of the pair of coolant supply passages  34   a  and the pair of coolant discharge passages  34   b.    
         [0066]    The controller  16  is capable of detecting whether or not at least a portion of the first fuel gas flow field  36  (e.g., an end of the first fuel gas flow field  36  on the downstream side adjacent to the fuel gas discharge passage  32   b  or the oxygen-containing gas discharge passage  30   b ) has been clogged with water. The determination of whether or not water clogging has occurred is made, for example, by performing CDD (current density distribution) measurement on the power generation surface. If electrical current is concentrated on the power generation surface or in an upper area of the power generation surface, it is determined that water is retained as stagnant water at least in a portion of the first fuel gas flow field  36 . 
         [0067]    Further, the cell voltage in the fuel cell stack  10  may be detected. In this case, upon detection of decrease in the cell voltage, it is determined that water is retained as stagnant water at least in a portion of the first fuel gas flow field  36 . 
         [0068]    Further, the electric potential at the anode can be measured by a plurality of potential sensors disposed in the power generation surface. In this case, it is possible to determine that hydrogen shortage, i.e., water clogging, has occurred in a portion where the electric potential of the anode is increased. 
         [0069]    Further, whether or not water clogging has occurred may be determined by detecting pressure loss or the like in the fuel gas. Alternatively, whether or not water has been retained as stagnant water may be determined by storing data of the optimum flow distribution for each load, and mapping based on the stored data. 
         [0070]    Operation of the fuel cell stack  10  having the above structure will be described below in relation to an operation method according to the first embodiment. 
         [0071]    Firstly, an oxygen-containing gas is supplied to the oxygen-containing gas supply passage  30   a,  and a fuel gas such as a hydrogen-containing gas is supplied to the fuel gas supply passage  32   a.  Further, a coolant such as pure water, ethylene glycol, oil, or the like is supplied to the pair of coolant supply passages  34   a.    
         [0072]    Thus, the oxygen-containing gas flows from the oxygen-containing gas supply passage  30   a  to the first oxygen-containing gas flow field  50  of the second separator  24  and the second oxygen-containing gas flow field  66  of the third separator  26 . The oxygen-containing gas moves along the first oxygen-containing gas flow field  50  in the direction of gravity indicated by the arrow C, and the oxygen-containing gas is supplied to the cathode  94  of the first membrane electrode assembly  22   a.  Further, the oxygen-containing gas moves along the second oxygen-containing gas flow field  66  in the direction indicated by the arrow C, and the oxygen-containing gas is supplied to the cathode  94  of the second membrane electrode assembly  22   b.    
         [0073]    In the meanwhile, the fuel gas flows from the fuel gas supply passage  32   a  through the outer supply holes  80   a  to the surface  20   b  of the first separator  20 . Further, after the fuel gas flows from the inner supply holes  80   b  to the surface  20   a,  the fuel gas flows along the first fuel gas flow field  36  in the direction of gravity indicated by the arrow C, and the fuel gas is supplied to the anode  92  of the first membrane electrode assembly  22   a.    
         [0074]    Further, the fuel gas flows through the supply holes  84  to the surface  24   b  of the second separator  24 . Then, the fuel gas flows along the second fuel gas flow field  58  on the surface  24   b  in the direction indicated by the arrow C, and the fuel gas is supplied to the anode  92  of the second membrane electrode assembly  22   b.    
         [0075]    Thus, in each of the first and second membrane electrode assemblies  22   a,    22   b,  the oxygen-containing gas supplied to the cathode  94  and the fuel gas supplied to the anode  92  are consumed in the electrochemical reactions at electrode catalyst layers of the cathode  94  and the anode  92  for generating electricity. 
         [0076]    Then, the oxygen-containing gas consumed at the cathode  94  of each of the first and second membrane electrode assemblies  22   a,    22   b  is discharged into the oxygen-containing gas discharge passage  30   b  in the direction indicated by the arrow A. 
         [0077]    After the fuel gas is consumed at the anode  92  of the first membrane electrode assembly  22   a,  the fuel gas flows through the inner discharge holes  82   b  to the surface  20   b  of the first separator  20 . After the fuel gas reaches the surface  20   b,  the fuel gas flows through the outer discharge holes  82   a,  and moves again to the surface  20   a.  Then, the fuel gas is discharged into the fuel gas discharge passage  32   b.    
         [0078]    Further, the fuel gas consumed at the anode  92  of the second membrane electrode assembly  22   b  flows through the discharge holes  86  to the surface  24   a.  Then, the fuel gas is discharged into the fuel gas discharge passage  32   b.    
         [0079]    Further, as shown in  FIG. 1 , in the coolant supply apparatus  14 , the three-way valves  110   a,    110   b,    112   a,  and  112   b  are operated. Thus, the supply channel  100   a  and the discharge channel  100   b  are connected respectively to the coolant supply passage  34   a  and the coolant discharge passage  34   b  provided on one of the long sides, and the branch supply channel  106  and the branch discharge channel  108  are connected respectively to the coolant supply passage  34   a  and the coolant discharge passage  34   b  provided on the other of the long sides. 
         [0080]    Therefore, the coolant supplied to the fuel cell stack  10  is supplied to the pair of left and right coolant supply passages  34   a  (see  FIGS. 1 and 2 ). The coolant flows into the coolant flow field  44  formed between the first separator  20  of one of the power generation units  18  which are adjacent to each other and the third separator  26  of the other of the adjacent power generation units  18 . 
         [0081]    As shown in  FIG. 4 , the pair of coolant supply passages  34   a  are provided separately at upper positions on both of left and right sides of the power generation unit  18 , adjacent to the oxygen-containing gas supply passage  30   a  and the fuel gas supply passage  32   a.    
         [0082]    In the structure, the coolant from one of the coolant supply passages  34   a  and the coolant from the other of the coolant supply passages  34   a  are supplied to the coolant flow field  44  in the direction indicated by the arrow B, and move toward each other. After the coolants moving toward each other collide with each other at the center of the coolant flow field  44  in the direction indicated by the arrow B, the coolants move in the direction of gravity (in the downward direction indicated by the arrow C). Then, the coolants are discharged into the coolant discharge passages  34   b  provided separately at lower positions on both sides of the power generation unit  18 . 
         [0083]    In the first embodiment, the controller  16  detects whether or not at least a portion of the first fuel gas flow field  36  (in particular, an end of the first fuel gas flow field  36  on the downstream side adjacent to the fuel gas discharge passage  32   b ) has been clogged with water. It is because, in the first fuel gas flow field  36 , water produced in the first oxygen-containing gas flow field  50  tends to permeate through the thin solid polymer electrolyte membrane  90 , and the produced water tends to be diffused backward easily. Further, it is because, as shown in  FIG. 3 , since the first fuel gas flow field  36  is adjacent to the coolant flow field  44 , in particular, during low load power generation, the temperature of the first fuel gas flow field  36  is decreased significantly, and the condensed water tends to be retained easily as stagnant water. 
         [0084]    Thus, if it is determined that at least a portion of the first fuel gas flow field  36  has been clogged with water, the controller  16  operates the three-way valve  110   b  to disconnect the discharge channel  100   b  from the coolant discharge passage  34   b  (see  FIG. 5 ). Therefore, it becomes possible to limit the flow of the coolant in the coolant discharge passage  34   b  adjacent to the fuel gas discharge passage  32   b.    
         [0085]    Thus, in the power generation surface, the flow rate of the coolant flowing in an area adjacent to the fuel gas discharge passage  32   b  is reduced, and it is possible to increase the temperature in the area adjacent to the fuel gas discharge passage  32   b.  Thus, the condensed water which is retained as stagnant water in the area adjacent to the fuel gas discharge passage  32   b  is discharged easily and suitably, and it is possible to remove the stagnant water. 
         [0086]    Accordingly, in particular, during low load power generation, it is possible to reliably prevent the condensed water from being retained as stagnant water in the area adjacent to the fuel gas discharge passage  32   b,  and a desired power generation state is achieved advantageously. 
         [0087]    Further, in the first embodiment, the three-way valves  110   a  and  110   b  may be operated as shown in  FIG. 6  if it is determined that at least a portion of the first fuel gas flow field  36  has been clogged with water. 
         [0088]    That is, the supply channel  100   a  is disconnected from the coolant supply passage  34   a  by the three-way valve  110   a,  and the discharge channel  100   b  is disconnected from the coolant discharge passage  34   b  by the three-way valve  110   b.    
         [0089]    Therefore, the supply channel  100   a  and the discharge channel  100   b  are connected to the branch channel  114   a  bypassing the coolant flow field  44 . In the structure, the coolant flows from the supply channel  100   a  through the branch channel  114   a,  and then, the coolant is discharged into the discharge channel  100   b.  Thus, in the power generation surface, the coolant is restricted from flowing through one of the long sides where the oxygen-containing gas supply passage  30   a  and the fuel gas discharge passage  32   b  are provided. 
         [0090]    Thus, in the power generation surface, the coolant flows along the other of the long sides where the fuel gas supply passage  32   a  and the oxygen-containing gas discharge passage  30   b  are provided, and it is possible to increase the temperature in the area adjacent to the fuel gas discharge passage  32   b.  Thus, the same advantages as described above are obtained. For example, in particular, during low load power generation, it is possible to reliably prevent the condensed water from being retained as stagnant water in the area adjacent to the fuel gas discharge passage  32   b,  and a desired power generation state is achieved. 
         [0091]    Next, an operation method according to a second embodiment of the present invention will be described using a fuel cell stack  10 . 
         [0092]    In the second embodiment, the controller  16  detects whether or not at least a portion of the first fuel gas flow field  36  on the downstream side (in particular, an end of the first fuel gas flow field  36  adjacent to the oxygen-containing gas discharge passage  30   b ) has been clogged with water. 
         [0093]    As shown in  FIG. 7 , in the first fuel gas flow field  36 , the humidified fuel gas is supplied to the fuel gas supply passage  32   a,  and the dew condensation water flows into the first fuel gas flow field  36  easily depending on the degree of humidification or the like. The dew condensation water flows in the direction of gravity, and tends to be retained as stagnant water easily in a portion of the first fuel gas flow field  36  on the downstream side, specifically, in the area adjacent to (above) the oxygen-containing gas discharge passage  30   b.  Moreover, as shown in  FIG. 3 , since the first fuel gas flow field  36  is adjacent to the coolant flow field  44 , in particular, during low load power generation, the temperature of the first fuel gas flow field  36  is decreased significantly, and the condensed water tends to be retained as stagnant water easily. 
         [0094]    Therefore, if it is determined that at least a portion of the first fuel gas flow field  36  on the downstream side has been clogged with water, the controller  16  operates the three-way valve  112   b  to disconnect the coolant discharge passage  34   b  from the branch discharge channel  108  (see  FIG. 8 ). Accordingly, it becomes possible to limit the flow of the coolant in the coolant discharge passage  34   b  adjacent to the oxygen-containing gas discharge passage  30   b.    
         [0095]    Thus, in the power generation surface, the flow rate of the coolant flowing in the area adjacent to the oxygen-containing gas discharge passage  30   b  is reduced, and it is possible to increase the temperature in the area adjacent to the oxygen-containing gas discharge passage  30   b.  Thus, in the first fuel gas flow field  36 , the condensed water retained as stagnant water in the area adjacent to the oxygen-containing gas discharge passage  30   b  is discharged easily and suitably, and it becomes possible to remove the stagnant water. 
         [0096]    Accordingly, in particular, during low load power generation, in the first fuel gas flow field  36 , it is possible to reliably prevent the condensed water from being retained as stagnant water in the area adjacent to the oxygen-containing gas discharge passage  30   b.  Thus, a desired power generation state is achieved, and it is possible to effectively suppress the stagnant water from being produced on the downstream side of the first oxygen-containing gas flow field  50  advantageously. 
         [0097]    Also in the second fuel gas flow field  58 , the stagnant water tends to be produced easily in the area adjacent to the oxygen-containing gas discharge passage  30   b  depending on the degree of humidification of the fuel gas or the like. In an attempt to address the problem, in the second embodiment, as described above, the flow of the coolant in the coolant discharge passage  34   b  adjacent to the oxygen-containing gas discharge passage  30   b  is limited. Therefore, the condensed water which is retained as stagnant water on the downstream side of the second fuel gas flow field  58  is discharged easily and suitably, and it is possible to remove the stagnant water. 
         [0098]    Further, in the second embodiment, the three-way valves  112   a  and  112   b  may be operated as shown in  FIG. 9  if it is determined that at least a portion of the first fuel gas flow field  36  on the downstream side has been clogged with water. 
         [0099]    That is, the branch supply channel  106  is disconnected from the coolant supply passage  34   a  by the three-way valve  112   a,  and the branch discharge channel  108  is disconnected from the coolant discharge passage  34   b  by the three-way valve  112   b.    
         [0100]    Thus, the branch supply channel  106  and the branch discharge channel  108  are connected to the branch channel  114   b  bypassing the coolant flow field  44 . In the structure, the coolant flows from the branch supply channel  106  through the branch channel  114   b,  and then, the coolant is discharged into the branch discharge channel  108 . Thus, in the power generation surface, it is possible to reliably limit the flow of the coolant through the other long side where the fuel gas supply passage  32   a  and the oxygen-containing gas discharge passage  30   b  are provided. 
         [0101]    Thus, in the power generation surface, it is possible to increase the temperature in the area adjacent to the oxygen-containing gas discharge passage  30   b  further rapidly, and increase the temperature in the area on the downstream side of the first fuel gas flow field  36  and adjacent to the oxygen-containing gas discharge passage  30   b.  Accordingly, the same advantages as described above can be obtained. For example, in particular, during low load power generation, it is possible to reliably prevent the condensed water from being retained as stagnant water in the area on the downstream of the first fuel gas flow field  36 , and a desired power generation state is achieved. 
         [0102]      FIG. 10  is a diagram schematically showing a fuel cell system  122  including a fuel cell stack (fuel cell)  120  to which operation methods according to first and second embodiments of the present invention are applied. 
         [0103]    The constituent elements that are identical to those of the fuel cell system  12  according to the first embodiment are labeled with the same reference numerals, and description thereof will be omitted. Also in a third embodiment as described later, the constituent elements that are identical to those of the fuel cell system  12  according to the first embodiment are labeled with the same reference numerals, and description thereof will be omitted. 
         [0104]    The fuel cell  120  has a laterally elongated shape, and the fuel gas and the oxygen-containing gas flow in a horizontal direction perpendicular to the vertical direction. In this case, when power generation of the fuel cell  120  is performed for a long period of time in the low load state, water tends to be retained easily as stagnant water on the upper side of the MEA. Therefore, by operating the three-way valves  110   a  and  110   b  to reduce the flow rate of the coolant flowing on the upper side of the flow field, the water can be discharged from the upper side of the MEA smoothly. 
         [0105]    During the transition period of power generation, the water tends to be retained easily as stagnant water in the flow field on the lower side of the MEA, e.g., due to injection of water from the fluid passage into the fluid flow field. Therefore, during power generation after the transition period, by operating the three-way valves  112   a  and  112   b,  it becomes possible to smoothly discharge the water from the lower side of the MEA. 
         [0106]      FIG. 11  is a diagram schematically showing a fuel cell system  130  to which an operation method according to the third embodiment of the present invention is applied. Though the fuel cell system  130  uses the fuel cell  10 , the present invention is not limited in this respect. For example, the fuel cell  120  may be used. 
         [0107]    In a coolant circulation channel  100  of the fuel cell system  130 , a second supply channel  106   b  is branched from a first supply channel  106   a,  and a second discharge channel  108   b  is branched from a first discharge channel  108   a.    
         [0108]    The first supply channel  106   a  and the first discharge channel  108   a  are connected to the coolant supply passage  34   a  and the coolant discharge passage  34   b  provided on one of long sides (first side) through variable throttle valves (valve mechanisms)  132   a,    132   b,  respectively. The second supply channel  106   b  and the second discharge channel  108   b  are connected to the coolant supply passage  34   a  and the coolant discharge passage  34   b  provided on the other of the long sides (second side) through variable throttle valves (valve mechanisms)  132   c,    132   d,  respectively. 
         [0109]    A first branch channel  134   a  is connected to a middle portion of the first supply channel  106   a  and a middle portion of the first discharge channel  108   a,  and a second branch channel  134   b  is connected to a middle portion of the second supply channel  106   b  and a middle portion of the second discharge channel  108   b.  A variable throttle valve (valve mechanism)  132   e  is connected to the first branch channel  134   a,  and a variable throttle valve (valve mechanism)  132   f  is connected to the second branch channel  134   b.    
         [0110]    In the third embodiment, the variable throttle valves  132   a  to  132   f  are used. However, the number of valves may be increased or decreased as necessary. For example, only the variable throttle valves  132   a  and  132   b  may be used. 
         [0111]    The controller  16  is capable of detecting whether or not, for example, at least a portion of the first fuel gas flow field  36  (e.g., an end of the first fuel gas flow field  36  on the downstream side adjacent to the fuel gas discharge passage  32   b ) and/or at least a portion of the first oxygen-containing gas flow field  50  (e.g., an end of the first oxygen-containing gas flow field  50  on the downstream side adjacent to the oxygen-containing gas discharge passage  30   b ) has been clogged with water. 
         [0112]    Operation of the fuel cell system  130  having the above structure will be described in relation to an operation method according to the third embodiment of the present invention. 
         [0113]    The controller  16  detects, for example, whether or not at least a portion of the first fuel gas flow field  36  (see  FIG. 2 , etc.) (an end of the first fuel gas flow field  36  on the downstream side adjacent to the fuel gas discharge passage  32   b  and/or an end of the first fuel gas flow field  36  on the downstream side adjacent to the oxygen-containing gas discharge passage  30   b ) has been clogged with water. 
         [0114]    Further, if it is determined that the end of the first fuel gas flow field  36  on the downstream side adjacent to the fuel gas discharge passage  32   b  has been clogged with water, the controller  16  firstly closes the variable throttle valve  132   e,  and reduces the opening degree of the variable throttle valve  132   b.  Thus, the flow of the coolant discharged into the coolant discharge passage  34   b  adjacent to the fuel gas discharge passage  32   b  is limited, and it is possible to increase the temperature in the area adjacent to the fuel gas discharge passage  32   b.    
         [0115]    In a case where, even after the above processing, the condensed water which is retained as stagnant water in the area adjacent to the fuel gas discharge passage  32   b  is not removed, as shown in  FIG. 12 , the controller  16  opens the variable throttle valve  132   e  to connect the first supply channel  106   a  and the first discharge channel  108   a  through the first branch channel  134   a.  In the meanwhile, the opening degree of the variable throttle valve  132   b  is reduced. 
         [0116]    Therefore, the coolant flows easily along the first branch channel  134   a  where the pressure loss is small in comparison with the inside of the fuel cell stack  10 , and it is possible to limit the flow of the coolant in the coolant discharge passage  34   b  adjacent to the fuel gas discharge passage  32   b.    
         [0117]    Thus, in the power generation surface, the flow rate of the coolant flowing in the area adjacent to the fuel gas discharge passage  32   b  is reduced, and it is possible to increase the temperature in the area adjacent to the fuel gas discharge passage  32   b.  As a result, the condensed water which is retained as stagnant water in the area adjacent to the fuel gas discharge passage  32   b  is discharged easily and suitably, and it is possible to remove the stagnant water. 
         [0118]    Accordingly, in particular, during low load power generation, it is possible to reliably prevent the condensed water from being retained as stagnant water in the area adjacent to the fuel gas discharge passage  32   b.  In the structure, a desired power generation state is achieved advantageously. Further, also in the second fuel gas flow field  58 , the same advantages as in the case of the first fuel gas flow field  36  are obtained. 
         [0119]    Further, in the third embodiment, if it is determined that an end of the first fuel gas flow field  36  on the downstream side adjacent to the fuel gas discharge passage  32   b  has been clogged with water, the variable throttle valves  132   a,    132   b  may be closed as shown in  FIG. 13 . Therefore, the coolant flows from the first supply channel  106   a  through the first branch channel  134   a,  and then, the coolant is discharged into the first discharge channel  108   a.  The flow of coolant through one of long sides where the oxygen-containing gas supply passage  30   a  and the fuel gas discharge passage  32   b  are provided is limited. 
         [0120]    Thus, in the power generation surface, the coolant flows along the other of the long sides where the fuel gas supply passage  32   a  and the oxygen-containing gas discharge passage  30   b  are provided, and it is possible to increase the temperature in the area adjacent to the fuel gas discharge passage  32   b.  Thus, the same advantages as described above are obtained. For example, in particular, during low load power generation, it is possible to reliably prevent the condensed water from being retained as stagnant water in the area adjacent to the fuel gas discharge passage  32   b,  and a desired power generation state is achieved. 
         [0121]    Further, in the first fuel gas flow field  36 , the humidified fuel gas is supplied to the fuel gas supply passage  32   a,  and dew condensation water tends to flow into the first fuel gas flow field  36  easily depending on the degree of humidification or the like. The dew condensation water flows downward in the direction of gravity, and tends to be retained as stagnant water in a portion of the first fuel gas flow field  36  on the downstream side, specifically, in the area adjacent to (above) the oxygen-containing gas discharge passage  30   b.    
         [0122]    Thus, if it is determined that an end of the first fuel gas flow field  36  on the downstream side corresponding to the oxygen-containing gas discharge passage  30   b  has been clogged with water, the controller  16  opens the variable throttle valve  132   f,  as shown in  FIG. 14 , to connect the second supply channel  106   b  and the second discharge channel  108   b  through the second branch channel  134   b.  At this time, the opening degree of the variable throttle valve  132   d  is reduced. 
         [0123]    Therefore, the coolant tends to flow easily along the second branch channel  134   b  where the pressure loss is relatively small, and it becomes possible to limit the flow of the coolant in the coolant discharge passage  34   b  adjacent to the oxygen-containing gas discharge passage  30   b.    
         [0124]    Thus, in the power generation surface, the flow rate of the coolant flowing in the area adjacent to the oxygen-containing gas discharge passage  30   b  is reduced, and it is possible to increase the temperature in the area adjacent to the oxygen-containing gas discharge passage  30   b.  Thus, in the first fuel gas flow field  36  (and the second fuel gas flow field  58 ), the condensed water which is retained as stagnant water in the area adjacent to the oxygen-containing gas discharge passage  30   b  is discharged easily and suitably, and it is possible to remove the stagnant water. Further, as necessary, the variable throttle valves  132   c,    132   d  may be closed. 
         [0125]    Further, in the third embodiment, the opening degrees of the variable throttle valves  132   a  to  132   f  are controlled suitably. Thus, for example, in the case where the voltage during low load power generation becomes unstable, by opening the variable throttle valves  132   e,    132   f  to change the temperature of the fuel cell stack  10 , it becomes possible to suppress dew condensation or flooding of water. 
         [0126]    Further, in the third embodiment, the coolant supply apparatus  14  is provided at one end of the fuel cell stack  10  in the stacking direction. In the structure, the coolant may not be supplied sufficiently to the power generation units  18  at the other end side of fuel cell stack  10  in the stacking direction. 
         [0127]    In an attempt to address the problem, in the third embodiment, the variable throttle valves  132   e,    132   f  are closed by the controller  16 , and the opening degrees of the variable throttle valves  132   b,    132   d  are reduced. Thus, in each of the coolant flow fields  44 , the pressure loss at the pair of coolant discharge passages  34   b  becomes high, and it becomes possible to sufficiently supply the coolant to the coolant flow fields  44  provided at the other end side of the fuel cell stack  10  in the stacking direction. 
         [0128]    In the structure, for example, during high load power generation, in the power generation units  18  at the other end side of the fuel cell stack  10  in the stacking direction, the power generation performance is not degraded due to insufficient cooling. Further, it is possible to effectively suppress temperature increase in the area adjacent to the oxygen-containing gas discharge passage  30   b.