Patent Publication Number: US-2010119899-A1

Title: Fuel cell system

Description:
TECHNICAL FIELD 
     The present invention relates to fuel cell system. 
     BACKGROUND 
     A fuel cell typically includes a plurality of membrane-electrode assemblies. Each membrane-electrode assembly is provided on one side with a flow channel-defining member for defining a flow channel for an oxidant gas. In the membrane-electrode assemblies, water evolves in association with generation of electricity. Some of the evolved water is retained in the flow channel-defining members. 
     One known fuel cell of this type is that disclosed in JP-A 2006-221853. 
     The amount of water that is retained in flow channel-defining members will vary. If there is a high level of variation in the amount of water retained in flow channel-defining members, a high level of variation in the power generation capabilities of the membrane-electrode assemblies will result, possibly causing the output voltage of the fuel cell to drop, or the fuel cell to become incapable of continuous power generation. 
     One practice employed in the past to reduce variation in the amount of water retained in flow channel-defining members is to increase the flow of oxidant gas. However, there exists a need for other methods for reducing variation in the amount of water retained in flow channel-defining members. 
     SUMMARY 
     In view of the problem, an advantage of some aspects of the invention is to provide technology for reducing variation in the amount of water retained in flow channel-defining members. 
     An advantage of some aspects of the invention is intended to address this issue at least in part, and can be reduced to practice as described below. 
     (First Aspect) A fuel cell system according to a first aspect of the invention includes: a fuel cell and a process executing unit. The fuel cell includes a plurality of flow channel-defining members and a plurality of membrane-electrode assemblies. The flow channel-defining member is combined with the membrane-electrode assembly and defines a flow channel for supplying a reactant gas to the membrane-electrode assembly. The process executing unit executes a process for increasing the amount of water held in each of the plurality of flow channel-defining members, so as to reduce variation in the amount of water among each of the plurality of flow channel-defining members. 
     In this system, the amount of water held in each of the plurality of flow channel-defining members can be increased by executing the aforementioned process, and then variation in the amount of water among each of the plurality of the flow channel-defining members can be reduced as a result. 
     (Second Aspect) In the fuel cell system according to the first aspect, the process executing unit executes the process when load of the fuel cell decreases. 
     When the load of the fuel cell has decreased, there will be a tendency for variation in the amount of water held in the flow channel-defining members to increase. However, by employing the above strategy, variation in the amount of water held in the flow channel-defining members can be reduced in more efficient manner. 
     (Third Aspect) In the fuel cell system according to the first aspect, the process executing unit executes the process periodically. 
     By so doing, variation in the amount of water held in the flow channel-defining members can be reduced. 
     (Fourth Aspect) In the fuel cell system according to any one of the first, second and third aspects, the process executing unit includes a supply unit that supplies the reactant gas to the fuel cell, and the process includes a process for reducing a flow rate of the reactant gas being supplied to the fuel cell by the supply unit. 
     By so doing, the flow rate of the reactant gas in the flow channels defined by the flow channel-defining members can be decreased, and as a result the amount of water retained in the flow channel-defining members can be increased. 
     (Fifth Aspect) In the fuel cell system according to any one of the first, second and third aspects, the process executing unit includes a valve in a passage through which flows the reactant gas that has been discharged from the fuel cell, and the process includes a process for reducing a opening rate of the valve. 
     By so doing, pressure of the reactant gas in the flow channels defined by the flow channel-defining members can be increased, and as a result the amount of water retained in the flow channel-defining members can be increased. 
     (Other Aspect) In the fuel cell system according to any one of the first, second and third aspects, the process executing unit includes a humidifying unit that humidifies the reactant gas to be supplied to the fuel cell, and the process includes a process for increasing a humidification rate of the reactant gas by the humidifying unit. 
     (Other Aspect) In the fuel cell system according to any one of the first, second and third aspects, the process executing unit includes a cooling unit that cools the fuel cell, and the process includes a process for cooling the fuel cell by the cooling unit. 
     (Other Aspect) In the fuel cell system according to any one of the first, second and third aspects, the process executing unit includes a sensing unit that senses a physical quantity related to variation in the amount of water held in the plurality of flow channel-defining members, and the process executing unit executes the process based on a result of sensing by the sensing unit. 
     There are various possible modes for working the invention, for example, a fuel cell system; a moving body equipped with the fuel cell system; a control method for the fuel cell system; a computer program for carrying out the functions of such a method or device; a recording medium having such a computer program recorded thereon; a data signal including the computer program and carried on a carrier wave; and so on. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration depicting in model form a configuration of a fuel cell system; 
         FIG. 2  is an illustration depicting in model form an internal configuration of a fuel cell  100 ; 
         FIG. 3  is an illustration showing distributions of water retained inside a porous body  130   c;    
         FIG. 4  is a flowchart showing a series of processes for reducing variation of water content of porous bodies; 
         FIG. 5  is an illustration depicting a relationship of load on a fuel cell and internal temperature of the fuel cell; 
         FIG. 6  is an illustration which models a relationship between air stoichiometric ratio and pressure loss; 
         FIG. 7  is a flowchart depicting the specific process of Step S 114  ( FIG. 4 ) in Embodiment 1; 
         FIG. 8  is an illustration depicting air stoichiometric ratio distributions before and after the process of Step S 202  ( FIG. 7 ); 
         FIG. 9  is an illustration which models a relationship between air stoichiometric ratio and cell voltage; and 
         FIG. 10  is a flowchart depicting the specific process of Step S 114  ( FIG. 4 ) in Second Embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A. First Embodiment 
     A-1: Configuration of Fuel Cell System: 
     Certain modes of the invention will be described through preferred embodiments.  FIG. 1  is an illustration depicting in model form a configuration of a fuel cell system. This fuel cell system is intended for installation on board an automobile. 
     As illustrated, the fuel cell system includes a fuel cell  100 ; a fuel gas supply unit  200  for supplying hydrogen gas (fuel gas) to the fuel cell; an oxidant gas supply unit  300  for supplying an oxidant gas (air) to the fuel cell; and a control circuit  600  for controlling operation of the fuel cell system as a whole. 
     To the fuel cell  100  there are connected a fuel gas passage  201  through which the fuel gas may pass, and a fuel off-gas passage  202  through which spent fuel off-gas may pass. Also connected to the fuel cell  100  are an oxidant gas passage  301  through which the oxidant gas may pass, and an oxidant off-gas passage  302  through which spent oxidant off-gas may pass. The fuel off-gas passage  202  and the oxidant off-gas passage  302  connect at the downstream end to a confluent off-gas passage  401 . 
     The fuel gas supply unit  200  includes a hydrogen gas tank  220 , a pressure reducing valve  236 , and a flow rate control valve  238 . The hydrogen gas tank  220  stores hydrogen gas (fuel gas) at relatively high pressure. The pressure reducing valve  236  reduces to a prescribed level the pressure of the fuel gas discharged from the hydrogen gas tank  220 . The flow rate control valve  238  adjusts the flow rate of fuel gas, for supply to the fuel cell  100 . 
     The fuel gas supply unit  200  further includes a gas-liquid separator  240 , a circulation pump  250 , and a shutoff valve  260 . The gas-liquid separator  240  and the shutoff valve  260  are disposed in the fuel off-gas passage  202 . The circulation pump  250  is disposed in a circulation passage  203  that connects the fuel off-gas passage  202  and the fuel gas passage  201 . The circulation passage  203  connects at its upstream end to the fuel off-gas passage  202  at a point between the gas-liquid separator  240  and the shutoff valve  260 , and connects at its downstream end to the fuel gas passage  201  at a point downstream from the flow rate control valve  238 . 
     The gas-liquid separator  240  removes excess water vapor contained in the fuel off-gas. Water removed by the gas-liquid separator  240  is discharged to the fuel off-gas passage  202  via a discharge valve  242 . 
     The circulation pump  250  has the function of returning the fuel off-gas, which has relatively low hydrogen gas concentration, into the fuel gas passage  201  where it serves as fuel gas. For this reason the fuel gas is circulated through an annular passage. By circulating the fuel gas in this way, the flow of hydrogen gas supplied to the fuel cell per unit of time (mol/sec) can be increased, and as a result reaction efficiency in the fuel cell can be improved. However, as the electrochemical reaction in the fuel cell proceeds, the level of hydrogen gas (mol) contained in the fuel gas inside the annular passage will decline. Also, the hydrogen gas concentration (volume percent) in the fuel gas will gradually drop. For this reason, in the present embodiment, the flow rate control valve  238  and the shutoff valve  260  will be intermittently placed in the open state so that fuel gas with a high concentration of hydrogen gas can supplied to the fuel cell, while the fuel off-gas with a low concentration of hydrogen gas can discharged from the fuel cell. The spent fuel off-gas is discharged to the atmosphere via the fuel off-gas passage  202  and the confluent off-gas passage  401 . 
     The oxidant gas supply unit  300  includes a compressor  310 , a humidity level regulating valve  320 , a pressure regulating valve  340 , and a humidifier  350 . The compressor  310  and the humidity level regulating valve  320  are disposed in the oxidant gas passage  301 . The pressure regulating valve  340  and the humidifier  350  are disposed in the oxidant off-gas passage  302 . 
     The compressor  310  supplies an oxidant gas containing oxygen gas (i.e. air) to the fuel cell  100 . The humidity level regulating valve  320  is situated in parallel with the humidifier  350 . If the opening of the humidity level regulating valve  320  is small, a large amount of oxidant gas will pass through the humidifier  350 , so the oxidant gas supplied to the fuel cell  100  will have a high humidity level. On the other hand, if the opening of the humidity level regulating valve  320  is small, a small amount of oxidant gas will pass through the humidifier  350 , so the oxidant gas supplied to the fuel cell  100  will have a low humidity level. 
     The pressure regulating valve  340  has the function of regulating back pressure (pressure at the oxidant off-gas discharge outlet) of the fuel cell  100 . The humidifier  350  utilizes the water and water vapor present in the oxidant off-gas to humidify the oxidant gas. A humidifier of hollow fiber membrane design for example could be used as the humidifier  350 . The oxidant off-gas is discharged to the atmosphere via the oxidant off-gas passage  302  and the confluent off-gas passage  401 . 
     The fuel cell system is provided with a cooling unit  500  for the purpose of cooling the fuel cell  100 . The cooling unit  500  includes a heat exchanger  510  for lowering the temperature of a coolant, and a circulation pump  520  for circulating the coolant. The cooling unit  500  lowers the temperature inside the fuel cell  100  by supplying coolant to the fuel cell  100 . 
       FIG. 2  is an illustration depicting in model form the internal configuration of the fuel cell  100 . The fuel cell  100  is a fuel cell of solid polymer design, which offers exceptional power generation efficiency with relatively compact size. Electricity is generated utilizing the hydrogen gas (fuel gas) supplied by the fuel gas supply unit  200 , and the oxidant gas (air) supplied by the oxidant gas supply unit  300 . 
     The fuel cell  100  is furnished with a multiplicity of generating units  110  and a multiplicity separators  120 , stacked in alternating fashion. 
     Each generating unit  110  includes an electrolyte membrane  112 ; a first electrode catalyst layer (anode)  114   a  and a first gas diffusion layer  116   a  stacked in that order on a first face of the electrolyte membrane  112 ; and a second electrode catalyst layer (cathode)  114   c  and a second gas diffusion layer  116   c  stacked in that order on a second face of the electrolyte membrane  112 . 
     Separators  120  are disposed to either side of each generating unit  110 . Between the generating unit  110  and a first separator  120  there is disposed a first porous body  130   a  that contacts the first gas diffusion layer  116   a;  and between the generating unit  110  and a second separator  120  there is disposed a second porous body  130   c  that contacts the second gas diffusion layer  116   c.    
     The fuel gas supplied by the fuel gas supply unit  200  will flow through a first flow channel that is defined by the first porous body  130   a,  and the oxidant gas supplied by the oxidant gas supply unit  300  will flow through a second flow channel that is defined by the second porous body  130   c.  The fuel gas and the oxidant gas will then be utilized in the electrochemical reaction that takes place in the generating unit  110 . 
     The electrolyte membrane  112  is a membrane made of a solid polymer material, such as a fluororesin or the like. Layers of carbon particles supporting a catalyst such as platinum are used for the electrode catalyst layers  114   a,    114   c.  The gas diffusion layers  116   a,    116   c  are made of a material having gas permeability and electrical conductivity, such as carbon paper. The porous bodies  130   a,    130   a  are components having gas permeability and electrical conductivity, and may be made of metal such as stainless steel or titanium for example. As such metal porous bodies it would be possible to employ sintered metal foam, or sinters obtained through sintering of tiny pieces of metal with spherical or fibrous morphology, for example. 
     In the present embodiment, the separator  120  is composed of three plates. The plate situated in the middle is provided with coolant flow channels  128  through which the coolant supplied by the cooling unit  500  will flow. Each of the plates making up the separator  120  is made of a metal plate having conductivity, such as stainless steel, titanium, or titanium alloy for example. 
     A-2. Water Retention by Porous Bodies: 
     In the present embodiment, the gas diffusion layers  116   a,    116   c  have been subjected to water repellency treatment. In order to increase their conductivity, the porous bodies  130   a,    130   c  have undergone metal plating. Metal plating has the effect of enhancing hydrophilicity of the porous bodies  130   a,    130   c.  The separators  120  also have undergone metal plating in order to increase their conductivity. Metal plating has the effect of enhancing hydrophilicity of the separators  120 . 
     As the electrochemical reaction proceeds in each generating unit  110 , water will evolve within the generating units  110 . Specifically, water (evolved water) will be produced in the electrode catalyst layer  114   c  that is situated on the cathode side of each generating unit  110 . The evolved water will flow into the porous body  130   c  via the gas diffusion layer  116   c.  In the present embodiment, because the gas diffusion layers  116   c  have been subjected to water repellency treatment, the water will be rapidly transported into the porous body  130   c  interior. Some of the water will be retained in the porous body  130   c  interior. 
       FIG. 3  is an illustration showing distributions of water retained inside the porous body  130   c.  Paragraph (A) of  FIG. 3  depicts a distribution of water in an instance in which a relatively small amount of water is retained; and Paragraph (B) of  FIG. 3  depicts a distribution of water in an instance in which a relatively large amount of water is retained. 
     As depicted in Paragraphs (A) and (B) of  FIG. 3 , a disproportionally greater portion of the water that has flowed into the porous body  130   c  is retained in proximity to the face lying towards the separator  120  in the porous body  130   c.  This occurs because the separator  120  situated to one side of the porous body  130   c  is more hydrophilic than the gas diffusion layer  116   c  situated to the other side of the porous body  130   c.    
     The water inside the porous body  130   c  is discharged from the porous body  130   c  in the liquid state, or discharged from the porous body  130   c  in the vapor state. Specifically, in the event that the flow of oxidant gas passing through the porous body  130   c  is high, water will be carried away primarily in the liquid state in response to flow speed of the oxidant gas. On the other hand, in the event that the flow of oxidant gas passing through the porous body  130   c  is low, water will be carried away primarily in the vapor state in response to vapor pressure. 
     The flow channels that have been formed in the porous body  130   c  will never become completely blocked off, even if the maximum amount of water is retained in the porous body  130   c.  For example, water will be retained at most in only about 80% of pores among the multitude of pores present in the porous body  130   c.  For this reason, even when the maximum amount of water is retained in the porous body  130   c,  oxidant gas will continue to be supplied to the electrode catalyst layer  114   c  via the gas diffusion layer  116   c.    
     In the present embodiment, the porous body  130   c  has undergone metal plating, and the separator  120  has undergone metal plating as well; however, even if the porous body  130   c  and the separator  120  had not undergone metal plating, the distribution of water would still be disproportional in proximity to the face towards the separator  120 . That is, it is possible to dispense with metal plating of the porous body  130   c  and the separator  120 . 
     If the separator  120  were to undergo water repellency treatment, water would be retained in the interior of the porous body  130   c,  i.e. in the middle section of the porous body  130   c  between its face on the separator  120  side and its face on the gas diffusion layer  116   c  side. 
     The fuel cell  100  includes a multiplicity of porous bodies  130   c.  In preferred practice, the flow rate of oxidant gas passing through these porous bodies  130   c  will be about the same. Also, in preferred practice the amount of water retained in these porous bodies  130   c  will be about the same. However, for reasons which will be discussed below, in actual practice the flow rate of oxidant gas passing through the porous bodies  130   c  and the amount of water retained (water content) in these porous bodies  130   c  will sometimes differ. 
     The interior of the fuel cell  100  is provided with distribution passages (called a manifold) for distributing the oxidant gas to the several generating units  110 , or more specifically to the several porous bodies  130   c . However, from point of view of the porous bodies  130   c,  these distribution passages differ in structure. Nor is each porous body  130   c  identical in structure to the others. Thus, flow rates of oxidant gas passing through the porous bodies  130   c  will differ, even if no water is currently retained in the porous bodies  130   c.  Consequently, the amounts of water retained in these porous bodies  130   c  (i.e. their water content) in association with the electrochemical reaction proceeding in each generating unit  110  will differ as well. Where the porous bodies  130   c  differ in water content, differences in flow rate of oxidant gas passing through the porous bodies  130   c  will become even greater. 
     Variation of water content of the porous bodies  130   c,  in other words, variation of oxidant gas flow rates through the porous bodies  130   c,  will have an adverse effect on the output characteristics of the fuel cell  100 . Specifically, in the event that some of the porous bodies  130   c  have excessively high water content, the output voltage of the fuel cell  100  drops, or the fuel cell  100  becomes incapable of continuous power generation. 
     Accordingly, it is preferable for variation of water content of the porous bodies  130   c,  in other words, variation of oxidant gas flow rates through the porous bodies  130   c,  to be minimal. Other practice was to increase the oxidant gas flow rate to an excessive degree in order to expel in liquid form the water retained in the porous bodies  130   c,  thereby reducing the amount of water retained in the porous bodies  130   c  and as a result reducing variation of water content of the porous bodies  130   c.  However, in the present embodiment, a different method is employed for reducing variation of water content of the porous bodies  130   c.    
     A-3. Water Content Variation Reducing Process: 
       FIG. 4  is a flowchart showing a series of processes for reducing variation of water content of the porous bodies. In Step S 112 , the control circuit  600  will decide whether a prescribed condition has been met. In the present embodiment, this prescribed condition will be met when the load on the fuel cell  100  has changed from a high load to a low load, or more specifically, when the load on the fuel cell  100  has fallen to or below a prescribed level. 
     It is possible to determine changes in load on the fuel cell  100  on the basis of changes in output voltage required of the fuel cell  100 . The load on the fuel cell  100 , in other words, the output power required of the fuel cell  100 , will vary depending on factors such as the extent to which the accelerator pedal is pressed by the driver of the vehicle. 
       FIG. 5  is an illustration depicting a relationship of load on a fuel cell and internal temperature of the fuel cell. As illustrated, at a time to at which the load on the fuel cell is relatively high, the temperature of the fuel cell will be relatively high as well. On the other hand, at a time tc at which the load on the fuel cell is relatively low, the temperature of the fuel cell will be relatively low as well. If the load on the fuel cell drops, the temperature of the fuel cell will drop also. However, as illustrated, the drop in temperature will be delayed following a drop in load. Thus, at a time tb immediately following a drop in load on the fuel cell, the load on the fuel cell will be relatively low while the temperature of the fuel cell will remain relatively high. As will be discussed later, under such conditions, variation in water content among the porous bodies  130  will gradually increase. For this reason, in the present embodiment, in Step S 112  ( FIG. 4 ) a decision is made as to whether the load on the fuel cell has changed from a high load to a low load. 
       FIG. 6  is an illustration which models a relationship between air stoichiometric ratio and pressure loss. The horizontal axis in the drawing gives the air stoichiometric ratio in relation to the amount of oxidant gas (air) supplied to a generating unit  110 . The vertical axis gives the pressure loss (kPa) of the generating unit  110  (more specifically, of the porous body  130 ). That is,  FIG. 6  depicts change in pressure loss of a single generating unit  110  observed when the air stoichiometric ratio of oxidant gas supplied to that generating unit  110  has changed. 
     Here, the air stoichiometric ratio refers to the ratio of the amount of oxidant gas (air) supplied to a generating unit, to the expected amount of oxidant gas (air) that will be utilized for electricity generation in the generating unit. Where all of the oxygen gas in oxidant gas supplied to the generating unit has been utilized in electricity generation, the air stoichiometric ratio will be 1.0. During operation of the fuel cell system, the air stoichiometric ratio will typically be set to a value greater than 1.0 (e.g. about 1.5). 
     Curve Ca is a graph depicting conditions at time to of  FIG. 5 , that is, conditions of high load on the fuel cell and high temperature (about 80° C.) of the fuel cell. Curve Cc is a graph depicting conditions at time tc of  FIG. 5 , that is, conditions of low load on the fuel cell and low temperature (about 60° C.) of the fuel cell. Curve Cb is a graph depicting conditions at time tb of  FIG. 5 , that is, conditions of low load on the fuel cell and high temperature (about 80° C.) of the fuel cell. Curves Cb and Cc are graphs based on test findings, while curve Ca is a graph based on an estimate. 
     As will be appreciated from curves Ca and Cc, during the time interval that substantially constant load on the fuel cell is maintained, pressure loss of the porous body  130   c  will vary in substantially linear fashion depending on the air stoichiometric ratio. The two curves Ca and Cc are graphs representing scenarios for two mutually different loads, with the oxidant gas flow rate on the curve Ca at a specific air stoichiometric ratio being greater than the oxidant gas flow rate on the curve Cc at that specific air stoichiometric ratio. For this reason, pressure loss on the curve Ca is greater than pressure loss on the curve Cc. 
     On the other hand, as shown by curve Cb, during the time interval immediately after the load on the fuel cell has changed from a high load to a low load, the pressure loss of the porous body  130   c  does not change monotonically with respect to the air stoichiometric ratio. Specifically, whereas in an area of relatively large air stoichiometric ratio (the area to the right side in the drawing) and in an area of relatively small air stoichiometric ratio (the area to the left side in the drawing) pressure loss changes in substantially linear fashion with the air stoichiometric ratio, an inflection point is observed in proximity to an air stoichiometric ratio of about 1.5. The two curves Cb, Cc are graphs representing scenarios for identical load, with the oxidant gas flow rate on the curve Cb at a specific air stoichiometric ratio being the same as the oxidant gas flow rate on the curve Cc at that specific air stoichiometric ratio. 
     Focusing on curves Cb and Cc, at the relatively small first air stoichiometric ratio R 1 , pressure loss on the two curves Cb and Cc assumes substantially equal values; whereas at the relatively large second air stoichiometric ratio R 2 , the pressure loss on the curve Cb is smaller than the pressure loss on the curve Cc. On the curve Cb, pressure loss assumes approximately values at the first air stoichiometric ratio R 1  and the second air stoichiometric ratio R 2 . 
     It is thought that, on curve Cc, in the range of air stoichiometric ratios depicted in  FIG. 6  (approximately 1.1 to approximately 2.0), the interior of the porous body  130   c  will be at saturated vapor pressure. It is also thought that, on curve Cc, in a range of relatively small air stoichiometric ratios (approximately 1.1 to approximately 1.5) depicted in  FIG. 6 , the interior of the porous body  130   c  will be at saturated vapor pressure, while in a range of relatively large air stoichiometric ratios (approximately 1.5 to approximately 2.4) depicted in  FIG. 6 , the interior of the porous body  130   c  will not be at saturated vapor pressure. The phenomenon discussed above is thought to be a result of this. 
     Specifically, for the two curves Cb, Cc, while the load on the fuel cell is low in both cases, the internal temperature of the fuel cell differs. Specifically, in the case of curve Cc the internal temperature of the fuel cell is low, whereas in the case of curve Cb the internal temperature of the fuel cell is high. Thus, in the range of air stoichiometric ratio of curve Cc depicted in  FIG. 6 , vapor inside the porous body  130   c  will be at saturation, and according to the temperature (approximately 60° C.) a relatively small amount of water vapor will be discharged. Similarly, in the range of air stoichiometric ratio of curve Cb depicted in  FIG. 6 , vapor inside the porous body  130   c  will be at saturation, and according to the temperature (approximately 80° C.) a relatively large amount of water vapor will be discharged. On the other hand, in the range of relatively large air stoichiometric ratio of curve Cb depicted in  FIG. 6 , vapor inside the porous body  130   c  will not be at saturation because of the relatively high flow speed of the oxidant gas. Consequently, water retained in the porous body  130  will be rapidly vaporized and discharged. For this reason, in the range of relatively large air stoichiometric ratio depicted in  FIG. 6 , water content on the curve Cb will be lower than water content on the curve Cc. As a result, in the range of relatively large air stoichiometric ratio depicted in  FIG. 6 , pressure loss on the curve Cb will be less than pressure loss on the curve Cc. Additionally, water content on the curve Cb at the second air stoichiometric ratio R 2  will be less than water content on the curve Cb at the first air stoichiometric ratio R 1 . As a result, irrespective of the difference in oxidant gas flow rate, pressure loss on the curve Cb will assume approximately equal values at the first air stoichiometric ratio R 1  and the second air stoichiometric ratio R 2 . 
     In  FIG. 6 , the curve Cc does not include an inflection point, but it is thought to include an inflection point at a higher air stoichiometric ratio (e.g. about 2.5 or above). 
     While  FIG. 6  shows pressure loss observed in the case of change in the air stoichiometric ratio of the oxidant gas supplied to a single porous body  130   c,  if the plurality of porous bodies  130   c  should happen to differ in water content, the air stoichiometric ratio of the oxidant gas supplied to the porous bodies  130   c,  as well as pressure loss of the porous bodies  130   c,  are observed to differ as well. 
     If the plurality of porous bodies  130   c  have different water content, not much oxidant gas will be supplied to those porous bodies that have high water content, and most of the gas will be supplied to the other porous bodies that have low water content. In this event, water will be discharged with difficulty from those porous bodies that have high water content, while water will be discharged easily from the other porous bodies that have low water content. That is, variation in water content among the porous bodies  130   c  will become progressively greater. 
     Accordingly, in the present embodiment, in Step S 114  in  FIG. 4 , the control circuit  600  will execute a reduction process for the purpose of reducing variation in water content among the porous bodies  130   c.  In the present embodiment, variation in water content among the porous bodies  130   c  is reduced by increasing the water content of the porous bodies  130   c.    
       FIG. 7  is a flowchart depicting the specific process of Step S 114  ( FIG. 4 ) in Embodiment 1. In Step S 202 , the control circuit  600  will control the compressor  310  and reduce the flow rate of oxidant gas. Specifically, the control circuit  600  will reduce the speed of the compressor  310 . 
     In Step S 204 , the control circuit  600  will control the pressure regulating valve  340  and increase the pressure at the oxidant gas outlet (back pressure) of the fuel cell  100 . Specifically, the control circuit  600  will decrease the opening of the pressure regulating valve  340 . At this point, pressure inside the porous bodies  130   c  will increase. 
       FIG. 8  is an illustration depicting air stoichiometric ratio distributions before and after the process of Step S 202  ( FIG. 7 ). In the drawing, the horizontal axis shows the air stoichiometric ratio of oxidant gas supplied to the porous bodies  130   c,  and the vertical axis shows the number (frequency) of porous bodies  130   c  being supplied with oxidant gas at the corresponding air stoichiometric ratio. In  FIG. 8 , curves Cb and Cc from  FIG. 6  are shown for reference. 
     Curve D 1  depicts an air stoichiometric ratio distribution before the process of Step S 202 . As shown, prior to the process of Step S 202 , considerable variation of the air stoichiometric ratio of oxidant gas supplied to the porous bodies  130   c,  in other words, variation of the water content of the porous bodies  130   c,  is observed. In the present embodiment, variation of air stoichiometric ratio (i.e. variation of water content) is assumed to follow a standard distribution. 
     Curve D 2  depicts an air stoichiometric ratio distribution after the process of Step S 202 . In Step S 202 , when the oxidant gas flow rate is reduced, the flow rate and air stoichiometric ratio of oxidant gas supplied to the porous bodies  130   c  will be reduced. As a result, as shown by curve D 2 , variation of the air stoichiometric ratio of oxidant gas supplied to the porous bodies  130   c,  in other words, variation of the water content of the porous bodies  130   c,  will become smaller. 
     Specifically, by reducing the flow rate of oxidant gas supplied to the porous bodies  130   c,  the mean value of air stoichiometric ratio of oxidant gas supplied to the porous bodies  130   c  will become smaller. Additionally, because the flow speed of the oxidant gas supplied to the porous bodies  130   c  will be reduced, vapor pressure inside a portion of the porous bodies  130   c  will change from the unsaturated state to the saturated state. That is, saturation vapor pressure will be reached inside these porous bodies  130   c.  As a result, water content of the porous bodies  130   c  will increase to a high level. However, as noted earlier, the flow channels that have been formed in the porous body  130   c  will never become completely blocked off. For this reason, as shown by curve D 2 , variation of the air stoichiometric ratio of oxidant gas supplied to the porous bodies  130   c,  in other words, variation of the water content of the porous bodies  130   c,  will become smaller. 
     When the air stoichiometric ratio of oxidant gas supplied to the porous bodies  130  is reduced in Step S 202 , the output voltage of the fuel cell  100  will drop. 
       FIG. 9  is an illustration which models a relationship between air stoichiometric ratio and cell voltage.  FIG. 9  depicts a range W 1  indicating variation of the air stoichiometric ratio before the process of Step S 202  in  FIG. 9  (i.e. curve D 1  of  FIG. 8 ).  FIG. 9  also depicts a range W 2  indicating variation of the air stoichiometric ratio after the process of Step S 202  in  FIG. 9  (i.e. curve D 2  of  FIG. 8 ). Cell voltage indicates voltage across the two electrode catalyst layers  114   a,    114   b  of the generating unit  110 . 
     As illustrated, due to concentration overpotential, cell voltage will become progressively lower with decreasing air stoichiometric ratio, in other words, with decreasing water content. 
     As the mean value of air stoichiometric ratio and the variation of air stoichiometric ratio become smaller through execution of the process of Step S 202 , mean cell voltage of the plurality of generating units  110  of the fuel cell  100  will become smaller as well. For this reason, in the present embodiment, back pressure is increased in the manner described in Step S 204 . As shown in  FIG. 9 , by increasing the back pressure, the mean cell voltage of the plurality of generating units  110  can be increased, and as a result the drop in output voltage of the fuel cell  100  can be moderated. 
     As described above, in the present embodiment, by reducing the flow rate of the oxidant gas in Step S 202 , the amount of water retained in the porous bodies  130   c  can be increased, and variation in the amount of water held in the porous bodies  130   c  can be reduced as a result. 
     While the present embodiment entails executing the process of Step S 204 , it is possible for the process of Step S 204  to be dispensed with. Variation in the water content of the porous bodies  130   c  can be reduced even where the process of Step S 204  is eliminated. If the process of Step S 204  is carried out, the compressor  310  will consume more energy due to increasing back pressure. Thus, where Step S 204  is omitted, a resultant advantage will be the ability to reduce energy consumption by the compressor  310  in association with carrying out the process of Step S 204 . 
     Also, whereas in the present embodiment the process of Step S 204  takes place after Step S 202 , these processes could take place simultaneously. 
     From the above discussion it will be appreciated that the membrane  112 , the first electrode catalyst layer  114   a,  and the second electrode catalyst layer  114   c  together correspond to the membrane-electrode assembly in the invention. The second porous body  130   c  in the embodiment corresponds to the flow channel-defining member in the invention. The compressor  310  in the embodiment corresponds to the supply unit in the invention; and the compressor  310  and the control circuit  600  together correspond to the process execution section in the invention. 
     In the present embodiment, the control circuit  600  controls the compressor in order to reduce the flow rate of oxidant gas supplied to the fuel cell; however, if a flow regulating valve has been disposed between the compressor and the fuel cell, the control circuit could instead reduce the flow rate of oxidant gas by reducing the opening of the flow regulating valve. In this case, the compressor and the flow regulating valve will together correspond to the supply unit in the invention. 
     B. Second Embodiment 
     The fuel cell system depicted in  FIG. 1  is utilized in Embodiment 2 as well. While the processes of Embodiment 2 are generally similar to the processes of Embodiment 1, the specific process of Step S 114  ( FIG. 4 ) has been changed. 
       FIG. 10  is a flowchart depicting the specific process of Step S 114  ( FIG. 4 ) in Embodiment 2, and corresponds to  FIG. 7 . In Step S 302 , the control circuit  600  will control the pressure regulating valve  340  and increase the pressure at the oxidant gas outlet (back pressure) of the fuel cell  100 . Specifically, the control circuit  600  will decrease the opening of the pressure regulating valve  340 . 
     Once the process of Step S 302  has been executed, pressure will increase in the interior of the porous bodies  130   c.  Thus, water vapor present inside the porous bodies  130   c  will condense to liquid form, causing the water content of the porous bodies  130   c  to increase. Variation of the air stoichiometric ratio of the oxidant gas supplied to the porous bodies  130   c,  in other words, variation of water content of the porous bodies  130   c,  will decrease as a result. 
     However, in the event that the process of Step S 302  is executed, if the compressor  310  is maintained at constant speed, energy consumption by the compressor  310  will increase. 
     For this reason, in the present embodiment, the process of Step S 304  will be executed. In Step S 304 , the control circuit  600  will control the compressor  310  and reduce the flow rate of oxidant gas. Specifically, the control circuit  600  will reduce the speed of the compressor  310 . By so doing it will be possible to moderate the increase in energy consumption by the compressor  310 . 
     As described above, according to the present embodiment, the amount of water retained in the porous bodies  130   c  can be increased by increasing the back pressure in Step S 302 ; and variation in the amount of water retained in the porous bodies  130   c  can be reduced as a result. 
     While the present embodiment entails executing the process of Step S 304 , it is possible for the process of Step S 304  to be eliminated. Variation in the water content of the porous bodies  130   c  can be reduced even if the process of Step S 304  is eliminated. 
     Also, whereas in the present embodiment the process of Step S 304  takes place after Step S 302 , these processes could take place simultaneously. 
     From the above discussion it will be appreciated that the pressure regulating valve  340  corresponds to the valve in the invention; and the pressure regulating valve  340  and the control circuit  600  together correspond to the process execution section in the invention. 
     While the invention has been shown above through certain preferred embodiments, the invention is in no way limited to these embodiments, and without departing from the spirit of the invention may be reduced to practice in various other modes, such as the following modifications for example. 
     (1) In Embodiment 1, variation of water content of the porous bodies  130   c  is reduced by decreasing the rate of oxidant gas in Step S 202  ( FIG. 7 ). In Embodiment 2, variation of water content of the porous bodies  130   c  is reduced by increasing the back pressure in Step S 302  ( FIG. 10 ). However, various other methods could be implemented by way of the process of Step S 114  ( FIG. 4 ). 
     For example, where the fuel cell system has been furnished with a humidity regulating unit adapted to regulate the humidity of the oxidant gas, the control circuit may control the humidity regulating unit and increase the humidification level of the oxidant gas. Specifically, in the fuel cell system depicted in  FIG. 1 , the control circuit  600  may increase the humidification level of the oxidant gas by decreasing the opening of the humidity level regulating valve  320 . In this instance, water vapor present in the oxidant gas supplied to the porous bodies  130   c  will assume liquid form, causing water content of the porous bodies  130   c  to increase. Variation of water content of the porous bodies  130   c  can be reduced as a result. 
     Alternatively, where the fuel cell system has been furnished with a temperature regulating unit adapted to regulate the internal temperature of the fuel cell, the control circuit may control this temperature regulating unit and lower the temperature inside the fuel cell. For example, the fuel cell system depicted in  FIG. 1  could be additionally provided with a cooler for cooling the heat exchanger  510 , and the coolant for supply to the fuel cell may be cooled indirectly by this cooler. In this instance, curve Cb of  FIG. 6  can be brought into approximation with curve Cc. Specifically, in the range of relatively large air stoichiometric ratio (approximately 1.5 to approximately 2.4) shown in  FIG. 6 , the interior of the porous bodies  130   c  will reach saturation vapor pressure, and the water content of the porous bodies  130   c  will increase. Variation of water content of the porous bodies  130   c  can be reduced as a result. 
     In general, it is acceptable to employ any process capable of increasing the amount of water retained in the flow channel-defining members, in order to reduce variation of the amount of water retained in the flow channel-defining members. 
     (2) In the preceding embodiments, the process of Step S 114  ( FIG. 4 ) is carried out in the event that the load on the fuel cell has dropped to or below a prescribed level in Step S 112 ; however, the process of Step S 114  could instead be carried out whenever the load on the fuel cell has dropped, irrespective of the extent of drop of the load. Where the process of Step S 114  is carried out whenever the load on the fuel cell decreases in this way, variation of the amount of water retained in the flow channel-defining members can be reduced in an efficient manner. 
     (3) In the preceding embodiments, as discussed in Step S 112 , the process of Step S 114  ( FIG. 4 ) is carried out the event that the load on the fuel cell has decreased; however, the process could instead be carried out at some other timing. 
     For example, the process of Step S 114  could be carried out periodically, in other words, each time that a prescribed time interval has passed. By so doing, variation of the amount of water retained in the flow channel-defining members can be reduced easily. 
     Alternatively, the process of Step S 114  could be carried out according to the outcome of measurement of some physical quantity that relates to water content of the porous bodies  130   c.  Specifically, the process of Step S 114  could be carried out in the event that an evaluation value indicative of variation of water content of the porous bodies  130   c  and obtained as a result of measuring the physical quantity in question is found to be greater than a prescribed value. By so doing, variation of the amount of water retained in the flow channel-defining members can be reduced in a reliable manner. 
     As the aforementioned physical quantity it would be possible to utilize, for example, the pressure or flow rate measured in proximity to the outlet of the porous body  130   c.  As the aforementioned value indicative of variation, the standard deviation or variance could be used for example. Alternatively, the difference between the maximum and minimum value among multiple measurements could be utilized as the value indicative of variation. 
     Where a physical quantity is to be measured, it will be preferable to select, from among all of the porous bodies, some plural number of porous bodies for the purpose of measurement. Also, if there is a given tendency as regards the distribution of water content of the plurality of porous bodies, it will be preferable to select this plural number of porous bodies according to this tendency. For example, if the porous bodies situated towards the ends of the fuel cell tend to have higher water content than porous bodies situated in the center section of the fuel cell, it will be preferable to select at least porous bodies situated towards the ends and bodies that are situated in the center section, for the purpose of measurement. 
     (4) In the preceding embodiments, the porous bodies are made of metal, but they could be made of other materials (e.g. carbon) instead. 
     In the preceding embodiments, porous bodies are utilized as the flow channel-defining members, but punched metal, wire mesh, or the like could be utilized instead. 
     It is also possible to eliminate the porous bodies that are disposed between the generating units and the separators. For example, where the gas diffusion layers have considerable thickness, the gas diffusion layers could be utilized as flow channel-defining members. Also, where the separators have multiple grooves formed thereon, the separators could be utilized as flow channel-defining members. 
     That is, it is sufficient for the flow channel-defining members to be components that define flow channels for the reactant gases and that are capable of retaining water. In preferred practice, the flow channel-defining members will be ones in which the reactant gas flow channels do not become completely blocked off by water. 
     (5) In the preceding embodiments, the invention was described with a focus on variation in the amount of water retained by the porous bodies  130   c  on the cathode side. However, water evolving at the cathode will migrate to the anode side via the electrolyte membrane  112 . Consequently, the invention also has potential application in instances where it is desired to reduce variation in the amount of water retained by the porous bodies  130   a  on the anode side. 
     (6) While the preceding embodiments described the use of a solid polymer fuel cell, other types of fuel cell could be used as well.