Abstract:
A fuel cell system for powering a vehicle effectively utilizes heat generated by various components of the system. A fuel cell produces electrical power by combining hydrogen and oxygen to form a water by-product. The electrical power is used to charge a battery, which in turn, is used to power an electric motor controlled by a motor driver. The battery, the motor, and the motor driver generate heat, which is captured by water brought in thermal contact with these heat-generating components. The heat-generating components can be immersed in water tanks or surrounded by water jackets to effect the heat transfer to the water. The battery is used to power the motor and generate heat before the fuel cell begins operating. Water heated by the battery, motor, and/or motor driver is provided to the fuel cell in order to bring the fuel cell up to operating temperature.

Description:
RELATED APPLICATIONS  
         [0001]    This application is related to Japanese Patent Application 11-339736, entitled “VEHICLE WITH FUEL CELL DRIVING SYSTEM MOUNTED THEREON,” inventors Mizuno and Kuranishi, attorney docket number P16464, filed Nov. 30, 1999, which has been assigned to the assignee of the present invention and which is hereby incorporated by reference in its entirety.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a vehicle powered by a fuel cell driving system and, more particularly, to a vehicle configured to effectively utilize heat generated by heat-generating devices in the fuel cell driving system.  
           [0004]    2. Description of the Related Art  
           [0005]    Fuel cells create electricity by combining hydrogen and oxygen to form water without combustion. Hydrogen gas can be supplied in raw form, but can alternatively be obtained from hydrogen-rich fuels (feedstock), such as methanol, using a reformer. A reformer is typically an endothermic device that transforms the feedstock into hydrogen gas and other by-products, such as CO and CO 2 . The construction and use of fuel cells and reformers is conventional and well-known.  
           [0006]    The fuel cells can be used to power electric vehicles such as cars, scooters, and motorcycles. In one configuration, the fuel cell is used to power a battery. The battery, in turn, is used to power an electric motor that propels the vehicle.  
           [0007]    Known fuel cell configurations for fuel cell vehicles typically include assemblies to radiate or conduct heat away from the vehicle. These configurations, however, do not generally make effective use of heat generated by various components of a fuel cell system. The present invention seeks to address this deficiency, among others.  
         SUMMARY OF THE INVENTION  
         [0008]    A fuel cell system for powering a vehicle effectively utilizes heat generated by various components of the system. As noted above, a fuel cell produces electrical power by combining hydrogen and oxygen to form a water by-product. The electrical power is used to charge a battery, which in turn, is used to power an electric motor controlled by a motor driver. The battery, the motor, and the motor driver generate heat, which is absorbed by water brought into thermal contact with these heat-generating components. The heat-generating components can be immersed in water tanks or surrounded by water jackets to effect the heat transfer to the water.  
           [0009]    In one embodiment, the battery is used to power the motor and generate heat before the fuel cell begins operating. Water heated by the battery, motor, and/or motor driver is provided to the fuel cell in order to bring the fuel cell up to operating temperature or maintain the operating temperature. Accordingly, the time needed to bring a fuel cell up to operating temperature and to activate the fuel cell can be reduced. After the fuel cell has reached operating temperature and begun operation, the temperature of the fuel cell is regulated such that it does not exceed a maximum temperature. The temperature of the fuel cell can be regulated by regulating the temperature of the water introduced into the fuel cell. The temperature of the water can be controlled through control valves that control whether the water is passed into thermal contact with the heat-generating devices.  
           [0010]    In one embodiment, water is sequentially brought into thermal contact with two or three of the heat generating devices of increasing temperature to further heat the water before the water is provided to the fuel cell. In one embodiment, the water is first heated by a battery, then by an electric motor, then by a motor driver to increasing temperatures.  
           [0011]    In one embodiment, water produced by the fuel cell is passed through a heat exchanger to cool water vapor and recapture liquid water before the vapor is released. The recaptured water is collected in a water tank and then supplied as humidification water to the fuel cell. The recaptured water reduces the amount of water that is lost through the exhaust of the fuel cell.  
           [0012]    In one embodiment, heat generated by one of the heat-generating devices is used to heat fuel through a heat exchanger before the fuel is supplied to a reformer. Since the reforming process is generally endothermic, heat from the heat-generating device is effectively used. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a block diagram of a fuel cell driving system having certain features according to a first embodiment of the present invention.  
         [0014]    [0014]FIG. 2 is a plan view of a scooter provided with the fuel cell driving system of FIG. 1.  
         [0015]    [0015]FIG. 3 is a side view of a scooter provided with the fuel cell driving system of FIG. 1.  
         [0016]    [0016]FIG. 4 is a block diagram of a fuel cell driving system having certain features according to a second embodiment of the present invention.  
         [0017]    [0017]FIG. 5 is a side view of a scooter provided with the fuel cell driving system of FIG. 4.  
         [0018]    [0018]FIG. 6 is a flowchart illustrating the operation of the fuel cell driving system.  
         [0019]    [0019]FIG. 7 is a flowchart illustrating the operation of a water circulation system for the fuel cell driving system. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    In the following description, reference is made to the accompanying drawings, which form a part hereof, and which show, by way of illustration, specific embodiments or processes in which the invention may be practiced. Where possible, the same reference numbers are used throughout the drawings to refer to the same or like components.  
         [0021]    FIGS.  1  to  3  relate to a vehicle incorporating a fuel cell driving system according to a first embodiment of the present invention. FIGS.  4  to  7  relate to a vehicle incorporating a fuel cell driving system according to a second embodiment of the present invention.  
         [0022]    [0022]FIG. 1 is a block diagram of a fuel cell driving system  1  according to a first embodiment of the invention. A fuel cell device  2  supplies electrical power for charging a battery  4 , which in turn provides electrical power to an electric motor  3 . The fuel cell device  2  can also provide power directly to the motor  3 . The electric motor is controlled via a motor driver  3   a.    
         [0023]    A methanol tank  5  supplies methanol (feedstock) to a reformer  8  through a pump  6  with a flow rate being adjusted by an adjusting valve  8   a . The methanol passes through a heat exchanger  7 , which can be used to heat the methanol before it reaches the reformer. The adjusting valve  8   a  can be actuated by a solenoid or other actuation mechanism, which is in turn preferably controlled by a controller or control unit in order to control the generation of power by the fuel cell.  
         [0024]    The reformer  8  produces hydrogen gas from the methanol and the hydrogen gas is supplied via a CO reducing device (not shown) to a cell stack body  9  which produces electrical power. In the reformer  8  a portion of the methanol is mixed with water. The mixture is heated by a burner to evaporate the mixture. The evaporated mixture is transformed by a catalyst to form hydrogen gas and other by-products. A portion of the methanol is also used to drive the burner. Surplus hydrogen, which is not used by the cell stack body, is preferably supplied to the burner for combustion.  
         [0025]    In the cell stack body  9 , the hydrogen gas is combined with oxygen to produce electricity and water or water vapor as a by-product. A water pump  11  supplies water from a water tank  10  for humidification of the cell stack body  9 .  
         [0026]    Additional information relating to the operation of the fuel cell is disclosed in U.S. patent application ______, titled “HYBRID-DRIVEN DEVICE,” attorney docket number YAMAH5.895APC, and filed on Apr. 26, 2001, which has been assigned to the assignee of the present invention and which is hereby incorporated by reference in its entirety.  
         [0027]    The humidification and by-product water from the cell stack  9  are passed through a heat exchanger  12 , which cools the water vapor to recapture moisture. The water and water vapor enter the heat exchanger at a high temperature input and as the water and water vapor are cooled, water vapor is recaptured as liquid. The liquid is output at low temperature output of the heat exchanger  12 . A fan  13  sucks intake air through the heat exchanger  12  from a low temperature input to a high temperature output in order to effect the cooling. The intake air is heated and dried by the heating effect since as air is heated, its relative humidity decreases. The heated intake air is passed into the cell stack body  9  to provide oxygen for the electricity generating reaction.  
         [0028]    In one embodiment, water output from the low temperature output of the heat exchanger  12  is supplied to a low temperature input of the water tank  10 . The water tank  10  heats the water and supplies the heated water through a high temperature output to the cell stack body  9  as humidification water. The water tank  10  can include water channels and/or passageways that transfer heat from a heat generating device to the water. The heat generating device can be, for example, the motor  3 , the motor driver  3   a  or the battery  4 . Accordingly, the water tank  10  can act as a heat exchanger transferring heat from the heat generating device to the water. The water channels and/or passageways can include, for example, a water jacket through which heat is absorbed from the motor  3 . The heated water can also be used to heat the reformer  8 .  
         [0029]    [0029]FIGS. 2 and 3 illustrate a schematic plan view and a side view, respectively, of a vehicle  20  powered by the fuel cell driving system  1 . In the illustrated embodiments, the vehicle is a scooter or motorcycle, but the vehicle can be another type of vehicle, such as a car, truck, ATV, golf cart or “community” car.  
         [0030]    With reference to FIG. 3, the scooter  20  has a body frame  21  including a head pipe  21   a  at the front end, a main pipe  21   b  extending rearward and downward from the head pipe  21   a . A pair of side pipes  21   c  and  21   c  are connected to a lower end of the main pipe  21   b , generally horizontally extending rearward forming low-floor foot rests  21   d  and extending rearward and upward.  
         [0031]    A front fork  22  is steerably supported by the head pipe  21 . A front wheel  23  is rotatably supported at the lower end of the front fork  22 , to which a steering handle  24  is secured at the upper end thereof. A seat  28  is mounted above the side pipes  21   c . A body cover  25  surrounds the front fork  22  and covers the right and the left sides of the body frame  21 .  
         [0032]    A unit swing type motor unit  26  is pivotably mounted at its forward end to a rearward and upward extended portion of the side pipes  21   c . A rear wheel  27  is rotatably supported at the rear end of the motor unit  26 . The motor can directly drive the wheel or a transmission can be used to transfer motor power to the wheel. The motor unit  26  is a combination of the motor  3 , which is preferably transversely mounted, and a transmission case  29  extending rearward along a side of the vehicle. The motor  3  is a water-cooled type surrounded by a water jacket, which preferably functions as the water tank  10 .  
         [0033]    The reformer  8  and the cell stack body  9  are preferably accommodated in a casing  2   a  mounted on supporting frames  21   e  and  21   d  laid between footrests  21   d  and  21   d  on both right and left sides of the body frame  21 . The casing  2   a  is preferably mounted for easy removal for repair and/or replacement, such as, in a “drop-out” configuration in which the casing can be removed from the bottom of the vehicle. The casing  2   a  has an air introduction port  2   b  in the front wall thereof for receiving air forced in from vehicle motion, an air supply port  2   c  in the rear wall thereof, and an air exhaust port  2   d  in the top wall thereof.  
         [0034]    The battery  4  is preferably divided into six parts; two front batteries  4   a , two upper batteries  4   b , and two lower batteries  4   c . The front batteries  4   a  are disposed in an air introduction duct  25   b  through which air is forced by vehicle motion into an air intake  25   a  of the body cover  25 . The forced air is then supplied through the duct  25   b  into the casing  2   a . The upper and lower batteries  4   b  and  4   c  are disposed on a rising part  21   f  at the rear of the body frame  21 . The heat exchanger  7  is preferably interposed between the upper batteries  4   b  and between lower batteries  4   c.    
         [0035]    Cooling fans  14  disposed at the air supply port  2   c  supply cooling air into the casing  2   a . The cooling fans  14  suck air around the upper and lower batteries  4   b  and  4   c  and push the air through the cooling air supply port  2   c  into the casing  2   a . The upper and lower batteries  4   b  and  4   c , the heat exchanger  7  and the cooling fans  14  are surrounded by a rearward extended portion  2   e  of the casing  2   a.    
         [0036]    In the scooter  20  of this embodiment, the fuel cell device  2  is preferably not activated when the vehicle starts running. Thus, at first, electric power is supplied from the batteries  4   a ,  4   b , and  4   c  to the electric motor  3 , by which the rear wheel is driven and the vehicle runs. Methanol, which absorbs heat from the upper and lower batteries  4   b  and  4   c  while passing from the methanol tank  5  through the heat exchanger  7 , is supplied to the reformer  8 . Hydrogen gas produced by the reformer  8  is supplied to the cell stack body  9 . Outside air sucked by the blowing fan  13  (FIG. 1) is dried while passing through the heat exchanger  12  (FIG. 1) and is then provided to the cell stack body  9 . The heat exchanger  12  also recaptures water vapor produced by the cell stack body  9  (as illustrated in FIG. 1). Water warmed by the heat generated by the motor  3  in the water tank  10  is also supplied to the cell stack body  9  by the pump  11  for humidifying water. Water passing through the cell stack body  9  is returned to the water tank  10 .  
         [0037]    The outside air sucked by the cooling fans  14  cools the upper and lower batteries  4   b  and  4   c , and the air whose temperature has been raised by this cooling is supplied to the burner of the heater of the reformer  8 . After the fuel cell device  2  has met conditions required for activation, electric power generated by the fuel cell device  2  is supplied to the electric motor  3 .  
         [0038]    In one embodiment, the electric motor  3  is cooled by a cooling system, which includes the water tank  10  within which the motor  3  is disposed. Fuel cells typically must be raised to an operating temperature (e.g. 80 degreed Celsius) in order to be activated. The time needed to activate the fuel cell device  2  can be shortened since water supplied to the cell stack body  9  is warmed by heat generated by the motor  3 . Since fuel supplied to the reformer  8  is warmed by heat from the batteries  4   b  and  4   c , the amount of heat required to be generated by the burner is reduced.  
         [0039]    [0039]FIG. 4 is a block diagram of a system according to a second embodiment. FIG. 5 illustrates a schematic of a vehicle  20  powered by the system according to the second embodiment.  
         [0040]    In this embodiment, the battery  4 , the electric motor  3  and the motor driver  3   a  are accommodated in a battery water tank  30 , a motor water tank (a water cooling jacket)  10 , and a motor driver water tank  31 , respectively. Alternatively, water passageways or heat exchangers can be used instead of water tanks. The water passageways or heat exchangers can be configured to receive heat from the battery  4 , the motor  3 , and the driver  3   a.    
         [0041]    The battery water tank  30  is disposed in an air introduction duct  25   b  formed in the body cover  25 . The cell stack body  9  and the reformer  8  are preferably contained in a single unit and mounted on or adjacent to the footrests  21   d . Air warmed by heat from the battery water tank  30  is passed around the cell stack body  9  and the reformer  8 .  
         [0042]    The water pump  11  is attached to the left side of the water tank  10  portion of the motor unit  26 . The water pump  11  can be driven by the motor. The driver water tank  31  is disposed on the right side of the rear wheel  27 . A radiator  33   b  is disposed above the rear wheel  27 .  
         [0043]    Water discharged from the water pump  11  is supplied through a water supply passageway  32   a  to the cell stack body  9 . From the cell stack body  9  the water is passed through the heat exchanger  12  and through water supply passageways  32   b  and  32   c . The water is then passed into the battery water tank  30 . The water is then passed through a water supply passageway  32   d , to the motor water tank  10 , and to the driver water tank  31 . The water is then returned to the water pump  11 .  
         [0044]    The water supply passageway  32   a  is preferably disposed along the foot rests  21   d  of the body frame  21 . The water supply passageways  32   b  and  32   c  are preferably disposed along the main pipe  21   b  of the body frame  21 .  
         [0045]    A radiator  33   a  is disposed at the air intake  25   a  in the body frame  25  and is connected to the water supply passageway  32   c . Air is heated as it is passed through the radiator  33   a  and the water tank  30 . A bypass in the water supply passageway  32   c  allows water to be diverted to bypass the radiator  33   a . A three-way valve X, having first and second outlets, is disposed in the water supply passageway  32   c  at the branch point upstream of the radiator  33 . A temperature sensor A is disposed on the upstream side of the three-way valve X and the opening direction of the three-way valve X is controlled on the basis of a temperature detected by the water temperature sensor A.  
         [0046]    A bypass passageway  33   c  is connected to bypass the motor water tank  10  and the driver water tank  31 , and a three-way valve Y is disposed at the upstream branch point of the bypass passageway  33   c . The downstream side of the driver water tank  31  is connected to a three-way valve Z disposed at a branch point downstream of the driver water tank  31 . A first outlet of the valve Z passes water to the cell stack body  9  and a second outlet passes water to a radiator  33   b  and then back to the motor water tank  10 .  
         [0047]    A temperature sensor B is disposed downstream of the downstream branch point of the bypass passageway  33   c  between the valve Z and the cell stack body  9 . The opening directions of the three-way valves Y and Z are controlled on the basis of a temperature detected by the temperature sensor B.  
         [0048]    In this embodiment, water discharged from the pump  11  is switched to the water supply passageway  32   c  when a temperature detected by the temperature sensor A is a predetermined temperature or lower, and to the radiator  33   a  when the temperature is higher the predetermined temperature by the three-way valve X. Water having passed through the battery water tank  30  is switched to the bypass passageway  33   c  when a temperature detected by the temperature sensor B is a predetermined temperature or higher, and to the motor water tank  10  and the driver water tank  31  when the temperature is lower than the predetermined temperature by the three-way valve Y. When water is passing through the bypass passageway  33   c , water in the motor water tank  10  and the driver water tank  31  is circulated through the radiator  33  by the three-way valve Z.  
         [0049]    Though not shown, there may be provided a bypass passageway bypassing the battery water tank, a three-way valve at the upstream branch point thereof and a temperature sensor at the downstream thereof so that water may pass through the bypass passageway when a temperature detected by the temperature sensor is a predetermined temperature or higher.  
         [0050]    Though not shown, the vehicle preferably includes one or more control units or controllers configured to control the operation of the fuel cell  9 ; the reformer  8 ; the pumps  11 ,  13 ; valves X, Y, and Z; and possibly other components. A control unit can be embodied as a hard-wired circuit, a dedicated processor, or a specially programmed general purpose computer. In one embodiment the vehicle is equipped with a fuel cell controller for controlling the fuel cell device  2 , and a vehicle controller for controlling the electric motor  3 .  
         [0051]    Though not shown, the vehicle preferably includes one or more control units or controllers configured to control the operation of the fuel cell  9 ; the reformer  8 ; the pumps  11 ,  13 ; valves X, Y, and Z; and possibly other components. A control unit can be embodied as a hard-wired circuit, a dedicated processor, or a specially programmed general purpose computer. In one embodiment the vehicle is equipped with a fuel cell controller for controlling the fuel cell device  2 , and a vehicle controller for controlling the electric motor  3 . In one embodiment, and the control operation described below is executed while necessary data are sent and received between both controllers.  
         [0052]    [0052]FIG. 6 illustrates a flowchart of a method performed in accordance with the second embodiment.  
         [0053]    At steps S 1 , S 2 , and S 3 , when the control flow starts, various abnormality flags and numeric values are initialized, the battery capacity (ampere-hour [AH]) at the moment is read from an on-board nonvolatile memory, and the system is brought into a low-power standby state. A low-power state here is a state in which a low power necessary to ensure a standby state of the control is applied.  
         [0054]    At step S 4 , the presence or absence of a vehicle activation signal (a main switch, on-off signal, a timer signal, etc.) is determined. When there is no signal, the low-power standby state is continued, and when there is a signal, the low-power state is released at step S 5 . The timer signal can be an activation signal to activate the fuel cell device  2  to fully charge the battery for the next run while the vehicle is not running.  
         [0055]    When the vehicle activation signal is a timer signal, the battery capacity is detected at step S 7 . When the battery is determined not to be in need of charge, an amount of self-discharge thereof is calculated at steps S 8 , S 9 , and S 10 , and the process goes back to step S 3 . When the vehicle activation signal is a signal representing that the main switch is turned on, various registrations, such as a reservation of a user event and a setting of inhibition of activation of the fuel cell device  2  are executed at step S 11 .  
         [0056]    When the battery is determined to be in need of charge, the process goes to step S 12 . At steps S 12  and S 13 , signals from a seat switch, a stand switch, a brake switch, throttle angle sensor and so on are read, and a subroutine for water circulation (described below with reference to FIG. 7) is executed on the basis of the detected values. These switches can be used to indicate that an operator is present. Data on the battery (voltage, current, temperature) are read to calculate the battery capacity, and the optimum target value of electric current to be generated according to the temperature of the battery.  
         [0057]    At steps S 16 , S 17 , and S 18  an indication of the amount of electricity to be generated is sent to the fuel cell device side while data on presence or absence of abnormality in temperature, current value, voltage value etc., on whether the fuel cell device is operating or not, and so on are received from the fuel cell device side.  
         [0058]    At step S 19 , the state of the main switch is determined. When the main switch is on, whether the vehicle is being ridden or not is determined on the basis of the detection results of the seat switch, the stand switch and the brake switch in steps S 12  and S 20 . When the vehicle is being ridden, the presence or absence of an abnormality in the fuel cell device  2  is determined on the basis of detection result in step S 18 . When there is no abnormality, a relay of the fuel cell device is turned on at steps S 21  and S 22 ). Also, the presence or absence of an abnormality in the battery is determined on the basis of the detection result in step S 14 . When no abnormality is in the battery, a battery relay is turned on and an abnormality displaying process is executed at steps S 23 , S 24 , and S 25 .  
         [0059]    When the vehicle is not being ridden in step S 20  both the relays of the battery and the fuel cell device are turned off at step S 20 ′. When there is an abnormality in the fuel cell device  2  in step S 21 , the relay of the fuel cell device is turned off at step S 21 ′. When there is an abnormality in the battery in step S 23 , the battery relay is turned off at step S 23 ′.  
         [0060]    At step S 26 , S 27 , and S 28 , a current value actually flowing in the motor is inputted, a motor current command value is calculated on the basis of the inputted current value and the throttle angle value detected in step S 12  etc., and a duty ratio for outputting the motor current value is outputted.  
         [0061]    At step S 29 , when the main switch is on, or when the main switch is off and the fuel cell device  2  is operating, the process goes to step S 12 . On the other hand, when the main switch is off and the fuel cell device has been stopped at step S 30 , the value of the battery capacity is written into the non-volatile memory at step S 31 . When the battery is in connected state, the process goes back to step S 3 , and when the battery is not in connected state at step S 32 , the process is terminated.  
         [0062]    [0062]FIG. 7 illustrates a subroutine for water circulation. At step S 13 - 1  temperatures are detected by the temperature sensors A and B. At steps S 13 - 2  and S 13 - 3 , when the temperature detected by the temperature sensor A disposed upstream of the battery water tank  30  is a predetermined temperature Ta (80° C., for example) or higher, a first outlet of the three-way valve X is opened so that water is supplied to the battery water tank  30  after having radiated heat in the radiator  33   a . When the temperature detected by the temperature sensor A is not a predetermined temperature Ta or higher, a second outlet of the three-way valve X is opened so that water is directly supplied to the battery water tank  30  at step S 13 - 4 .  
         [0063]    At step S 13 - 5 , when a temperature detected by the temperature sensor B is a predetermined temperature Tb (90° C., for example) or higher, a first outlet of the three-way valve Y and a second outlet of the three-way valve Z are opened at step S 13 - 6  so that water having passed through the battery water tank  30  is supplied through the bypass passageway  33   c  to the cell stack body  9  without passing through the water tanks  10  and  31 . In this case, water in the motor water tank  10  and the motor driver water tank  31  is circulated through the radiator  33   b  and radiates heat. On the other hand, when the temperature detected by the temperature sensor B is not the predetermined temperature Tb or higher, a second outlet of the three-way valve Y and a first outlet of the three-way valve Z are opened at step S 13 - 7  so that water having passed through the battery water tank  30  is supplied to the cell stack body  9  after having passed through the motor water tank  10  and the driver water tank  30  and having been warmed further.  
         [0064]    In the second embodiment, the battery water tank  30 , the motor water tank  10  and the driver water tank  31  are preferably connected in series so that water can be warmed effectively using heat generated by the battery  4 , electric motor  3  and the motor driver  3   a , and the time needed to activate the fuel cell device  2  can be shortened by supplying the warmed water to the cell stack body  9 .  
         [0065]    The bypass passageway  33   c  is provided so that water may be supplied directly to the cell stack body  9  bypassing the water tanks  10  and  31  when the temperature of the water to be supplied to the cell stack body  9  is a predetermined temperature or higher. Accordingly, the temperature of the water to be supplied to the cell stack body  9  can be prevented from abnormally rising. The cell stack body  9  in this embodiment is provided with an ion-exchange resin membrane therein, and the durability of which is reduced when its temperature is a certain temperature or higher.  
         [0066]    The water in the tanks is supplied to the cell stack body  9  and some of the water that is supplied to the cell stack body  9  and some of the water produced in the cell stack body  9  is returned to the water tanks. Accordingly, water to be supplied to the cell stack body can be secured easily.  
         [0067]    Although the invention has been described in terms of certain embodiments, other embodiments that will be apparent to those of ordinary skill in the art, including embodiments which do not provide all of the features and advantages set forth herein, are also within the scope of this invention. Accordingly, the scope of the invention is defined by the claims that follow. In method claims, reference characters are used for convenience of description only, and do not indicate a particular order for performing a method.