Patent Publication Number: US-2009239111-A1

Title: Fuel Cell Humidifier and Fuel Cell System Having the Same

Description:
TECHNICAL FIELD 
     The present invention relates to a fuel cell humidifier used for a fuel cell, and to a fuel cell system equipped with the fuel cell humidifier. 
     BACKGROUND ART 
     There is a type of fuel cell humidifier, conventionally used for a fuel cell, that exchanges moisture between off-gas discharged from a fuel cell and gas (or reactant gas) supplied to the fuel cell via a water vapor exchange film. 
     An example of such a fuel cell humidifier, JP-A-6-132038, discloses a reactant gas humidifier including a water vapor permeation film, and a humidifying gas chamber and a humidified gas chamber defined by the water vapor permeation film. This reactant gas humidifier humidifies reactant gas where the reacted off-gas discharged from the fuel cell is a humidifying gas and the reactant gas (or supplied gas) to be supplied to the fuel cell is a humidified gas. 
     Another example, JP-A-2004-165062, discloses a fuel cell humidifier composed of an anode humidifier including a plurality of hollow fiber membrane modules, and a cathode humidifier including a plurality of hollow fiber membrane modules. This fuel cell humidifier is equipped with a pair of heads holding both ends of the hollow fiber membrane modules, a connecting member for connecting the heads, and a hot water vaporizer for warming the supplied gas (or reactant gas) outlet in the head and the supplied gas inlet in the head. 
     The output performance of a fuel cell, particularly a solid polymer fuel cell, depends largely on the humidification state of the supplied gas. However, the humidification characteristics of the aforementioned conventional fuel cell humidifiers are easily affected by, for example, the conditions and environment under which they operate. Also, their humidification value can easily change due to load changes in the fuel cells. 
     Specifically speaking, in the configuration of a conventional fuel cell humidifier, changes in the ambient temperature where the humidification cells or the fuel cell humidifier is located, and other changes, for example, those in the gas flow status caused by water condensation in the fuel cell humidifier due to a reduction of water exchange efficiency between the supplied gas and the off-gas in the fuel cell humidifier, tend to occur easily. Therefore, it is difficult to maintain performance stability. Accordingly, a fuel cell humidifier that cannot be easily affected by internal or external causes such as an output level of a fuel cell, and that can exhibit stable humidification capability is preferable. 
     In a method for having the supplied gas or the off-gas bypass the fuel cell humidifier according to the opening or closing of a bypass valve, if the bypass valve opens and closes at a high frequency, a bypass valve with an extended practical life is necessary. Moreover, there is the possibility that power consumption owing to the opening and closing action of the bypass valve may increase and system efficiency may decrease. 
     DISCLOSURE OF THE INVENTION 
     This invention was devised in view of the circumstances described above. It is an object of the invention to provide a fuel cell humidifier, and a fuel cell system equipped with the fuel cell humidifier, that can appropriately adjust the humidification value and the heat exchange amount, prevent humidification characteristics from being affected by ambient temperature changes, and exhibit enhanced reliability, stability, and control. 
     In order to achieve the above-described object, the invention provides a fuel cell humidifier for performing humidification via a water exchange film by bringing together supplied gas to be supplied to a fuel cell, and off-gas discharged from the fuel cell. The fuel cell humidifier includes: a humidification cell including the water exchange film, a supplied gas passage provided on one surface of the water exchange film to allow the supplied gas to flow through, and an off-gas passage provided on the other surface of the water exchange film to allow the off-gas to flow through; and a gas flow unit that is formed independently from the humidification cell, that includes a gas passage connected to either the supplied gas passage or the off-gas passage to allow the supplied gas or the off-gas to flow through, and is placed adjacent to the humidification cell. 
     In the fuel cell humidifier having the above-described configuration, either the supplied gas or the off-gas flows (or passes) through the gas flow unit. Therefore, neither water exchange nor heat exchange between the off-gas and the supplied gas takes place in the gas flow unit. Consequently, if the off-gas flows through the gas flow unit, off-gas at a temperature almost equivalent to the internal temperature of the fuel cell will be introduced into the gas flow unit. As a result, it is possible to thermally insulate the fuel cell humidifier and prevent heat radiation from the fuel cell humidifier or heat absorption from the ambient environment. 
     If the off-gas flows through the gas flow unit, and if the gas utilization rate is constant, the amount of off-gas flowing through the humidification cells decreases by the amount of off-gas passing through the gas passage (or bypassing the humidification cells). Accordingly, the supplied gas amount increases relative to the off-gas amount in the humidification cells, and the relative ratio of the supplied gas to the off-gas can be increased. Consequently, the water exchange efficiency ratio of the fuel cell humidifier (the ratio of water [mol/sec] used to humidify the supplied gas via the water exchange film to water [mol/sec] in the off-gas) can be increased, i.e., the ratio of water used to humidify the supplied gas to water in the off-gas can be brought closer to 1:1. Therefore, water can be exchanged efficiently between the supplied gas and the off-gas in the humidification cells. As a result, it is possible to prevent water condensation (something that would happen if water exchange in the humidification cells were conducted insufficiently), and to enhance operational stability of the fuel cell humidifier. 
     Meanwhile, if the supplied gas flows through the gas flow unit, the supplied gas, whose temperature has become high to a certain degree through compression by, for example, a pump or a compressor causing the supplied gas to flow, passes through the gas flow unit. Accordingly, in this case as well, it is possible to thermally insulate the fuel cell humidifier and prevent heat radiation from the fuel cell humidifier or heat absorption from the ambient environment. 
     The gas flow unit can be composed of a gas flow cell. This gas flow cell can be placed side by side with at least either the supplied gas passage or the off-gas passage of the humidification cell. Moreover, the gas flow cell may be placed at one end or both ends of the humidification cell. If the gas flow cells are placed at both ends of the humidification cell, heat radiation from the ends of the humidification cell can be prevented more effectively. 
     Also in the fuel cell humidifier according to this invention, a plurality of humidification cells may be placed side by side with each other and the gas flow cell may be placed within the humidification cells. In addition to the advantageous effects mentioned above, the above-described arrangement can further enhance the heat retaining property of the fuel cell humidifier and perform water exchange in the humidification cells more efficiently. 
     Furthermore, the gas flow cell may be placed at least at one end of the humidification cell in the direction perpendicular to the direction of the side-by-side alignment of the humidification cells. In this case, the gas flow cell can have a flow port for allowing the supplied gas or the off-gas to flow through, and the flow port can be provided independently from the supplied gas inlet and supplied gas outlet of the humidification cell. If the humidification cells are piled together in their side-by-side alignment direction, the flow port can constitute a gas flow manifold. 
     This invention provides a fuel cell system including: a fuel cell; a gas supply passage for supplying supplied gas to the fuel cell; a gas discharge passage for allowing off-gas discharged from the fuel cell to pass through; and the fuel cell humidifier described above. 
     The fuel cell system having the above-described configuration can thermally insulate the fuel cell humidifier and prevent heat radiation from the fuel cell humidifier and heat absorption from the ambient environment. The fuel cell system can also conduct water exchange efficiently in the humidification cells. 
     Moreover, the fuel cell system according to the invention can be configured so that the gas discharge passage branches off between the fuel cell and the fuel cell humidifier, and a branch flow member for distributing the off-gas to a branch passage is provided. In this case, the branch flow member is, for example, a valve, and the off-gas can be made to flow into the branch passage according to the opening or closing of the valve. 
     This configuration allows excessive off-gas to be discharged from the branch passage. Therefore, it is possible to change the amount of off-gas introduced to the fuel cell humidifier according to changes in the load (such as changes in the gas flow rate) on the fuel cell, and to control the humidification value in the fuel cell humidifier. When this happens, since the gas flow unit (or gas flow cell) can absorb the excessive amount of off-gas (or have the off-gas bypass the humidification cells) while the valve is closed (that is, while the off-gas does not flow into the branch passage), the valve operation frequency can be reduced. Accordingly, the practical life of the valve can be extended. Also, the robustness of the control of the humidification value can be enhanced. When the valve is open, the total sum of the amount of off-gas discharged from the branch passage and the amount of off-gas passing through the gas flow unit represents the actual bypass amount. Therefore, the amount of off-gas discharged from the branch passage, i.e., the amount of off-gas passing through the valve can be reduced. As a result, it is unnecessary to use a valve with a large bore and it is possible to conserve the power required to drive the valve. 
     The branch flow member may be placed in the branch passage, or at a position in the gas discharge passage downstream from a point where the gas discharge passage branches into the branch passage. Also, the branch flow member may be placed at a point where the gas flow passage branches into the branch passage. In this case, the branch flow member may be a three-way valve. 
     Since neither water exchange nor heat exchange between the off-gas and the supplied gas takes place in the gas flow unit of the fuel cell humidifier according to the invention, it is possible to thermally insulate the fuel cell humidifier and prevent heat radiation from the fuel cell humidifier and heat absorption from the ambient environment. As a result, any influence ambient temperature changes may have on the humidification characteristics can be prevented. Moreover, water exchange can be conducted efficiently in the humidification cells. As a result, it is possible to provide a fuel cell humidifier with enhanced reliability, stability, and control. 
     The fuel cell system according to the invention can thermally insulate the fuel cell humidifier, and prevent heat radiation from the fuel cell humidifier and heat absorption from the ambient environment. Moreover, water exchange can be conducted efficiently in the humidification cells. As a result, it is possible to provide a fuel cell system with enhanced reliability, stability, and control. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a fuel cell humidifier according to the first embodiment of this invention. 
         FIG. 2  is a sectional view of part of a humidification cell belonging to the fuel cell humidifier shown in  FIG. 1 . 
         FIG. 3  is a sectional view of part of the humidification cell and a gas flow cell belonging to the fuel cell humidifier shown in  FIG. 1 . 
         FIG. 4  is a plan view of the inner surface of the gas flow cell shown in  FIG. 3 . 
         FIG. 5  is a schematic diagram illustrating part of a fuel cell system equipped with the fuel cell humidifier shown in  FIG. 1 . 
         FIG. 6  is a schematic diagram illustrating flows of supplied gas and off-gas in the fuel cell humidifier shown in  FIG. 1 . 
         FIG. 7  is a sectional view of a fuel cell humidifier according to another embodiment of the invention. 
         FIG. 8  is a schematic view illustrating part of a fuel cell system according to the second embodiment of the invention. 
         FIG. 9  is a chart showing the relationship between the humidification value for the supplied air and the supply air temperature in the fuel cell system according to the second embodiment. 
         FIG. 10  is a flowchart explaining valve control for the fuel cell system according to the second embodiment. 
         FIG. 11  is a chart showing the relationship between the valve status of the fuel cell system and the flow rate (actual bypass flow rate) of discharged air that does not pass through humidification cells, according to the second embodiment. 
         FIG. 12  is a chart showing the relationship between the valve status of a conventional fuel cell system and the flow rate (actual bypass flow rate) of discharged air that does not pass through the humidification cells in that fuel cell system. 
         FIG. 13  is a sectional view of a fuel cell humidifier according to another embodiment of the invention. 
         FIG. 14  is a plan view of the inner surface of a gas flow cell shown in  FIG. 13 . 
         FIG. 15  is a schematic view illustrating part of a fuel cell system according to yet another embodiment of the invention. 
         FIG. 16  is a schematic view illustrating part of a fuel cell system according to a further embodiment of the invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     A fuel cell humidifier and a fuel cell system equipped with the fuel cell humidifier according to preferred embodiments of this invention are described below in detail with reference to the attached drawings. The embodiments described below are for the purpose of describing this invention, but the invention is not limited only to these embodiments. Accordingly, this invention can be utilized in various ways unless the utilizations depart from the gist of the invention. 
     First Embodiment 
       FIG. 1  is a sectional view of a fuel cell humidifier according to the first embodiment of this invention.  FIG. 2  is a sectional view of part of a humidification cell belonging to the fuel cell humidifier shown in  FIG. 1 .  FIG. 3  is a sectional view of part of the humidification cell and a gas flow cell belonging to the fuel cell humidifier shown in  FIG. 1 .  FIG. 4  is a plan view of the inner surface of the gas flow cell shown in  FIG. 3 .  FIG. 5  is a schematic diagram illustrating part of a fuel cell system equipped with the fuel cell humidifier shown in  FIG. 1 .  FIG. 6  is a schematic diagram illustrating flows of supplied gas and off-gas in the fuel cell humidifier shown in  FIG. 1 . 
     A fuel cell humidifier  1  according to the first embodiment is incorporated into a fuel cell system as shown in  FIG. 5 . The fuel cell humidifier  1  is connected to a supply source (not shown in the drawing) for supplied gas and is also connected to a gas supply passage  50  for supplying the supplied gas (oxidized gas and/or fuel gas), and to a gas discharge passage  60  for discharging off-gas ejected from a fuel cell  100 . 
     As shown in  FIGS. 1 to 4 , the fuel cell humidifier  1  includes: a humidification cell group  10  composed of a plurality of humidification cells  11  installed side by side; and gas flow cells  20  placed at both ends of the humidification cell group along the direction of the side-by-side alignment of the humidification cells  11 . 
     As shown in  FIG. 2 , the humidification cell  11  includes: a supplied gas passage board  12  for allowing the supplied gas from the fuel cell  100  (see  FIG. 5 ) to flow through; an off-gas passage board  13  placed opposite the supplied gas passage board  12 , for allowing the off-gas discharged from the fuel cell  100  to flow through; and a water exchange film  14  interposed between the supplied gas passage board  12  and the off-gas passage board  13 . 
     On the surface of the supply gas passage board  12  facing the water exchange film  14 , a plurality of partitions  15  are placed parallel to each other, with a certain distance between adjacent partitions  15 . These partitions  15  form a plurality of supply gas passages  16  (many parallel grooves). On the surface of the off-gas passage board  13  facing the water exchange film  14 , a plurality of partitions  17  are placed parallel to each other, with a certain distance between adjacent partitions  17 . These partitions  17  form a plurality of off-gas passages  18  (many parallel grooves). The supply gas passage board  12  and the off-gas passage board  13  are preferably made of metals, carbons, plastic, resins, rubber or the like. 
     The water exchange film  14  serves to exchange moisture between the supplied gas and the off-gas, and may preferably be composed of an ion exchange resin film, or a porous film or the like. 
     The gas flow cell  20  is composed of a gas passage board  21 , as specifically shown in  FIG. 3 . On the surface of the gas passage board  21  facing the humidification cell  11 , a plurality of partitions  22  are placed parallel to each other, with a certain distance between adjacent partitions  22 . These partitions  22  form a plurality of gas passages  23 . Moreover, as shown in  FIG. 4 , a gas inlet  24  connected to the gas passages  23  and a gas outlet  25  for discharging the gas that has been introduced from the gas inlet  24  and passed through the gas passages  3  are formed in the gas flow cell  20 . 
     The gas inlet  24  for the gas flow cell  20  is connected either the gas supply passage  50  or the gas discharge passage  60 . Consequently, either the supplied gas or the off-gas flows through the gas passages  23 . The first embodiment is designed so that the gas inlet  24  is connected to the gas discharge passage  60  (see  FIG. 6 ) and only the off-gas flows through the gas passages  23 . 
     Specifically speaking, the off-gas is introduced from the gas inlet  24  of the gas flow cell  20  and off-gas inlets (not shown) of the respective humidification cells  11  as shown in  FIG. 6 . On the other hand, the supplied gas is introduced from supplied gas inlets (not shown) belonging to the respective humidification cells  11 . As explained above, only the off-gas, and not the supplied gas, is introduced into the gas flow cells  20  according to the first embodiment. This means that the supplied gas is introduced into the respective humidification cells  11  without passing through the gas flow cells  20 . On the other hand, both the off-gas and the supplied gas are introduced into the respective humidification cells  11 , where moisture exchange between the supplied gas and the off-gas is conducted via the water exchange film  14 . 
     The off-gas introduced into the gas flow cell  20  flows through the gas passages  23  and is then discharged externally from the gas outlet  25 . Therefore, neither heat exchange nor water exchange between the off-gas and the supplied gas takes place in the gas flow cell  20 . As a result, since the off-gas, at a temperature almost equivalent to that of the internal temperature of the fuel cell  100 , flows within the gas flow cell  20 , it is possible to thermally insulate the fuel cell humidifier  1 . It is also possible to prevent heat radiation from the ends of the fuel cell humidifier  1  or heat absorption from the ambient environment. 
     Moreover, in the fuel cell humidifier  1 , the amount of off-gas supplied to the humidification cells  11  decreases by the amount of off-gas passing through the gas flow cell  20  gas passage  23  (or the amount of off-gas bypassing the humidification cells  11 ). Accordingly, the amount of supplied gas in the humidification cells  11  increases relatively, so that the relative ratio of the supplied gas to the off-gas increases. Consequently, in the fuel cell humidifier  1 , the ratio of water [mol/sec] used to humidify the supplied gas to water [mol/sec] in the off-gas can be brought closer to 1:1. Therefore, water can be exchanged efficiently between the supplied gas and the off-gas in the humidification cells  11 . As a result, it is possible to prevent generation of condensed water in the humidification cells  11  and enhance the operational stability of the fuel cell humidifier  1 . 
     Also, in the fuel cell humidifier  1  according to the first embodiment, configured in such a way that the gas flow cell  20  is placed at both ends of the humidification cell group  10 , the proportion of the amount of off-gas flowing through the gas flow cells  20  to the total amount of off-gas changes depending on an increase or decrease in the off-gas flow rate caused by uneven flow distribution rates of the humidification cells  11 . Therefore, the fuel cell humidifier  1  can autonomously respond to changes in the off-gas flow rate caused by load changes. 
     Incidentally, the fuel cell humidifier  1  according to this invention may be placed in an oxidant gas system in order to humidify an oxidant gas, or be placed in a fuel gas system in order to humidify a fuel gas. Also, the fuel cell humidifier  1  may be placed in both the oxidant gas system and the fuel gas system in order to humidify both the oxidant gas and the fuel gas. 
     The first embodiment described the gas inlet  24  of the gas flow cell  20  connected to the gas discharge passage  60  so that only the off-gas flows through the gas passages  23 . However, the invention is not limited to the above-described configuration, and the fuel cell humidifier  1  may be configured so that the gas inlet  24  is connected to the gas supply passage  50 , and only the supplied gas flows through the gas passages  23 . 
     The first embodiment also described one gas flow cell  20  placed at both ends of the humidification cell group  10 . However, the invention is not limited to the above-described configuration, and a plurality of gas flow cells  20  may be placed side by side as desired. Also, the positions of the gas flow cells  20  are not particularly limited. 
     For example, the gas flow cell  20  may be placed within the humidification cell group  10  as shown in  FIG. 7 . Referring to  FIG. 7 , the gas flow cell  20  is located in the middle of the humidification cell group  10 . However, the positions of the gas flow cells  20  are not limited to the above-described example, and the gas flow cells  20  and the humidification cells  11  may be placed alternately or a gas flow cell  20  may be inserted every certain number of humidification cells  11 , for example, one gas flow cell  20  every two or three humidification cells  11 . When a gas flow cell  20  is placed in a humidification cell group  10 , a gas flow cell  20  may not necessarily be placed at both ends of the humidification cell group  10 . As described above, the heat retaining property of the fuel cell humidifier  1  can be further enhanced by placing the gas flow cell(s)  20  in the humidification cell group  10 . Also, the water exchange in the humidification cells  11  can be conducted more efficiently. 
     Furthermore, the first embodiment described gas flow cells  20  placed at both ends of the humidification cell group  10  along the direction of the side-by-side alignment of the humidification cells  11 . However, the positions of the gas flow cells  20  are not limited to those in the above-described example, and the gas flow cells  20  may be placed at the end of the humidification cell group  10  in the direction perpendicular to the direction of the side-by-side alignment of the humidification cells  11 , as shown in  FIG. 13 . In this case, as shown in  FIG. 14 , flow ports  26  for allowing the supplied gas or the off-gas to flow through may be formed independently from the supplied gas inlet  24  and the supplied gas outlet  25  of the humidification cell  11 . The flow ports  26  formed independently from the supplied gas inlet and the supplied gas outlet of the humidification cell  11  serve as gas flow manifolds when the humidification cells  11  are piled together. 
     Second Embodiment 
     Next, a fuel cell system according to the second embodiment of this invention will be described with reference to the relevant drawings. The elements used in the second embodiment the same as those explained in the first embodiment are given the same reference numerals as in the first embodiment, and any detailed description thereof is omitted. 
       FIG. 8  is a schematic view illustrating part of a fuel cell system according to the second embodiment of the invention. The second embodiment describes the case where the fuel cell humidifier  1  explained in the first embodiment is placed in an oxidant gas system in order to humidify an oxidant gas (or air). 
     As shown in  FIG. 8 , the difference between the fuel cell system according to the second embodiment and the fuel cell system according to the first embodiment is point A (branch point A) between the fuel cell  100  and the fuel cell humidifier  1 , where the gas discharge passage  60  branches off at the branch point A and a valve  71  is provided in a branch passage  70 . 
     In this fuel cell system, a temperature sensor  72  is placed in the gas supply passage  50  at a position upstream of the fuel cell humidifier  1  in order to measure the temperature T l1  of the supplied air (or supplied gas) passing there. A temperature sensor  73  is placed at the fuel cell humidifier  1  in order to measure the surface temperature T h1  of the fuel cell humidifier  1 . Also, a temperature sensor  74  is placed in the gas supply passage  50  at a position downstream of the fuel cell humidifier  1  in order to measure the temperature T l2  of the supplied air discharged from the fuel cell humidifier  1 . 
     Meanwhile a temperature sensor  75  is placed in the gas discharge passage  60  at a position downstream of the fuel cell  100  and upstream of the branch point A in order to measure the temperature T E1  of the discharged air (or off-gas) ejected from the fuel cell  100 . Also, a temperature sensor  76  is placed in the gas discharge passage  60  at a position downstream of the fuel cell humidifier  1  in order to measure the temperature T E2  of the discharged air ejected from the fuel cell humidifier  1 . 
     Furthermore, a temperature sensor  77  for measuring refrigerant temperature is placed at the fuel cell  100  in order to measure the temperature T c  of a refrigerant. 
     This fuel cell system includes a control unit (ECU)  80 . This control unit  80  receives the temperatures measured by the respective temperature sensors  72  to  77  and controls the opening and closing of the valve  71  according to these temperatures. 
     In the fuel cell system with the above-described configuration, the supplied air (or supplied gas) provided by an air supply source  90  is introduced via the gas supply passage  50  into, and humidified by, the fuel cell humidifier  1 , and then supplied to the fuel cell  100 . Fuel gas is also supplied from a fuel gas system (not shown) to the fuel cell  100 . An electrochemical reaction occurs at the fuel cell  100  that receives these gases, and the fuel cell  100  discharges the high-temperature and high-humidity air (off-gas) to the gas discharge passage  60 . Unreacted hydrogen is also discharged to the gas discharge passage of the fuel gas system (not shown). 
     The high-temperature and high-humidity air discharged to the gas discharge passage  60  is introduced into the fuel cell humidifier  1 . The fuel cell humidifier  1  performs water exchange and heat exchange to transfer moisture and heat from the discharged air to the supplied air via the water exchange film  14 . The discharged air is then ejected from the fuel cell humidifier  1  into the gas discharge passage  60 . In the water exchange and the heat exchange, the amount of heat exchange to the supplied air increases based on an increase in the amount of water exchanged. In other words, correlations are found between the temperature T l2  of the supplied air discharged from the fuel cell humidifier  1  and the humidification value W for the supplied air, as shown in  FIG. 9 . 
     Referring to  FIG. 9 , it is apparent that the relationship between the humidification value W and the temperature T l2  changes depending on the supplied air flow rate Q 1 . When the temperature T l1  of the supplied air introduced into the fuel cell humidifier  1 , the surface temperature T h1  of the fuel cell humidifier  1 , and the refrigerant temperature T c  of the fuel cell  100  are maintained at constant values under specified conditions, the temperature T l2  of the supplied air ejected from the fuel cell humidifier  1  is an indicator of the humidification value W for the supplied air. Specifically speaking, the above relationship is represented by the following formula: 
         T   l2   =f ( W,T   l1   ,T   h1   ,T   c   ,Q   1 )  [Formula 1] 
     Because of the same reason, the temperature T E2  of the discharged air that has passed through the fuel cell humidifier  1  is also a control target value for the humidification value W for the supplied air. Specifically speaking, that relationship is represented by the following formula: 
         T   E2   =f ( W,T   l1   ,T   h1   ,T   c   ,Q   1 )  [Formula 1] 
     The humidification value W for the supplied air, and the temperature T l2  or T E2  as the control target value for the humidification value W are controlled by opening and closing the valve  71  placed in the branch passage  70 . As the amount of discharged air passing through the branch passage  70  (or bypassing the humidification cells  11 ) increases, the net amount of the discharged air introduced into the respective humidification cells  11  (or the humidification cell group  10 ) of the fuel cell humidifier  1 , giving moisture and heat to the supplied air decreases, and the humidification value W for the supplied air then decreases proportionately under the influence of the decrease in the net amount of discharged air. As the net amount of the discharged air decreases, the amount of heat exchanged between the discharged air and the supplied air also decreases proportionately. As a result, the temperature T l2  of the supplied air that has passed through the fuel cell humidifier  1 , or the temperature T E2  of the discharged air that has passed through the fuel cell humidifier  1  decreases according to the decrease in the net amount of discharged air passing through the humidification cell group  10  of the fuel cell humidifier  1 . For the same reason, when the amount of discharged air passing through the valve  71  (or bypassing the humidification cells  11 ) is reduced, the temperature T l2  of the supplied air that has passed through the fuel cell humidifier  1 , or the temperature T E2  of the discharged air that has passed through the fuel cell humidifier  1  increases. 
     The valve  71  may be a variable valve or an on/off valve. If the valve  71  is a variable valve, the size of the valve  71  opening is adjusted to a specified level so that the temperature T l2  or T E2  required by the humidification value in order to humidify the supplied air reaches a control target value T lW . As a result, it is possible to obtain the amount of discharged air that should bypass the humidification cells  11  and pass through the branch passage  70 , and to secure the requested required humidification value for the supplied air. 
     On the other hand, if the valve  71  is an on/off valve, the opening and closing of the valve  71  is controlled so that the temperature T l2  or T E2  required by the humidification value in order to humidify the supplied air becomes the control target value or enters a control target range. If the temperature T l2  is used for this control, it is apparent from  FIG. 9  that when the temperature T l2  is 60° C.≦T l2 ≦62° C., the humidification value W for the supplied air corresponds to a molar ratio of 0.18 to 0.22. Accordingly, the control unit (ECU)  80  controls the valve  71 , opening it when the temperature T l2  reaches 62° C., and closing it when the temperature T l2  becomes lower than 60° C., so that the humidification value W corresponding to a molar ratio of 0.18 to 0.22 will be applied to the supplied air. 
     Next, the case where the valve  71  is a variable valve and the size of the valve  71  opening is controlled will be described in more detail by using the value of the temperature T l2  and referring to the flowchart in  FIG. 10 . 
     First, the required humidification value W for the supplied air in the fuel cell humidifier  1  is input into the control unit (ECU)  80  (step S 101 ). When the values T l1 , T h1  and T c  measured by the respective temperature sensors  72 ,  73  and  77 , as well as the supplied air flow rate Q 1  are input into the control unit (ECU)  80  (step S 102 ), the control unit (ECU)  80  applies these values to the aforementioned formula 1 and decides on the control target value T lW  for the temperature T l2  of the supplied air that has passed through the fuel cell humidifier  1  (S 103 ). 
     Subsequently, the temperature sensor  74  measures the temperature T l2  of the supplied air that has passed through the fuel cell humidifier  1 , and the obtained value (actual measurement value) is then input into the control unit (ECU)  80  (step S 104 ). 
     The control unit (ECU)  80  compares the control target value T lW  with the temperature T l2  (step S 105 ). If the temperature T l2  is lower than the control target value T lW  (step S 105 : YES), the control unit (ECU)  80  controls the valve  71 , decreasing the size of the valve  71  opening (step S 106 ). The control unit (ECU)  80  then judges whether or not the temperature T l2  is the same value as the control target value T lW  (step S 107 ). If the temperature T l2  is the same value as the control target value T lW  (step S 107 : YES), the control unit (ECU)  80  maintains the size of the valve  71  opening (step S 108 ). On the other hand, if the temperature T l2  is not the same value as the control target value T lW , the processing returns to step S 105  (step S 107 : NO). 
     If the temperature T l2  is higher than the control target value T lW  at step S 105  (step S 105 : NO), the control unit (ECU)  80  controls the valve  71 , to increasing the size of the valve  71  opening (step S 109 ). The control unit (ECU)  80  then judges whether or not the temperature T l2  is the same value as the control target value T lW  (step S 107 ). If the temperature T l2  is the same value as the control target value T lW  (step S 107 : YES), the control unit (ECU) maintains the size of the valve  71  opening (step S 108 ). On the other hand, if the temperature T l2  is not the same value as the control target value T lW , the processing returns to step S 105  (step S 107 : NO). 
     If the valve  71  is an on/off valve, the control unit (ECU)  80  judges whether the temperature T l2  is within the range of the lower and upper limits of the control target value T lW  or whether the temperature T l2  has exceeded the upper limit of the control target value T lW . If the temperature T l2  is within the range of the lower and upper limits of the control target value T lW , the control unit (ECU)  80  controls the valve  71 , closing it. If the temperature T l2  has exceeded the upper limit of the control target value T lW , the control unit (ECU)  80  controls the valve  71 , opening it. 
     In the fuel cell system according to the second embodiment, the fuel cell humidifier  1  and the valve  71  placed in the branch passage  70  control the amount of discharged air to be introduced into the respective humidification cells  11  (or the humidification cell group  10 ). The gas flow cell  20  in the fuel cell humidifier  1  is like a bypass passage that is always open. Accordingly, as shown in  FIG. 11 , the closed state (or the OFF state) of the valve  71  is set so that the discharged air will be supplied to the fuel cell humidifier  1  at a maximum flow rate (in a full load state). Therefore, the operation mode using the valve  71  with low discharged air flow rates can be employed. As a result, the operational stability, water exchange efficiency, and heat exchange efficiency of the fuel cell humidifier  1  can be enhanced. 
     On the other hand, if a conventional fuel cell humidifier having no gas flow cell  20  is used instead of the fuel cell humidifier  1 , the humidification value for the supplied air is controlled only by the opening and closing of the valve  71 . If so, as shown in  FIG. 12 , a wider range of the air supply amount to the fuel cell  100  or the required humidification value according to a load level can be applied. In order to respond to the full range of the required humidification value, the flow rate of discharged air passing through the branch passage  70  ranges from several NL/min to several tens of NL/min. Therefore, a valve with a large bore and larger valve drive power are required, and there is the possibility that responsiveness or controllability may degrade when the discharged air flow rate is low. There is also the possibility that pressure fluctuations may increase owing to flow fluctuations in the discharged air, thereby adversely affecting auxiliary machines, such as an air blower. 
     The second embodiment described the control target value T lW  for the temperature T l2  of the supplied air that has passed through the fuel cell humidifier  1  as decided according to formula 1. However, without limitation to this example, the case where a control target range indicating a certain range for the control target value T lW  may be decided and whether the temperature T l2  (actual measurement value) is within the control target range or not is judged is also possible. 
     Moreover, according to the second embodiment, the situation where a control target value T EW  or a control target range for the discharged air that has passed through the fuel cell humidifier  1  may be decided according to the aforementioned formula 2, and whether the temperature T E2  (actual measurement value) is the same as the control target value T EW  or within the control target range or not is judged is also possible. 
     The fuel cell humidifier  1  and the valve  71  according to this invention may be placed in an oxidant gas system or a fuel gas system, or they may be placed in both the oxidant gas system and the fuel gas system. 
     Furthermore, the second embodiment described the valve  71  placed in the branch passage  70 . However, the position of the valve  71  is not limited to that in the above-described example, and the valve  71  may be placed at a position in the gas discharge passage  60  downstream from the branch point A as shown in  FIG. 15 . Also, a three-way valve may be placed at the branch point A as shown in  FIG. 16 .