Abstract:
A fuel cell power plant stops anode gas supply to a fuel cell stack  1  by an anode gas supply mechanism  20  when an anode gas pressure in the fuel cell stack  1  reaches an upper limit pressure, and resumes supplying the anode gas by the anode gas supply mechanism  20  when the anode gas pressure in the fuel cell stack  1  lowers to a lower limit pressure. A sensor  52 - 54  detects if a hydrogen supply amount supplied to the fuel cell stack  1  satisfies a required amount to generate a target generated power, and a controller  51  corrects the lower limit pressure in an increasing direction when the hydrogen supply amount does not satisfy the required amount, thereby suppressing a generated power of the fuel cell stack  1  from reducing even when a flooding takes place in the fuel cell stack  1.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to operation control of a fuel cell power plant. 
       BACKGROUND OF THE INVENTION 
       [0002]    JP2007-48479A, published by the Japan Patent Office in 2007, discloses an operation control device for a fuel cell power plant which performs a pressure pulsation operation on an anode gas. A pressure pulsation operation is an operation method in which an internal pressure of a fuel cell stack is caused to pulsate between an upper limit pressure and a lower limit pressure by halting an anode gas supply to the fuel cell stack when a pressure of the anode gas reaches the upper limit pressure and restarting the anode gas supply when the pressure of the anode gas falls to the lower limit pressure. With this prior art technique, it is possible to promote the discharge of water generated in the fuel cell stack along a pressure gradient formed as the pressure falls during the pressure pulsation operation. 
       SUMMARY OF THE INVENTION 
       [0003]    According to research conducted by the inventors, anode gas pressure pulsation may exacerbate an adverse effect that flooding in the fuel cell stack has on a power generation capacity. More specifically, when a pressure reduction is performed toward the lower limit pressure of the anode gas in a condition where the fuel cell stack is flooded with generated water, the amount of anode gas that can be used by an anode decreases. As a result, it may be impossible to obtain the amount of anode gas required to generate a target amount of power, leading to a reduction in generated power. 
         [0004]    It is therefore an object of this invention to realize pressure pulsation operation control in a fuel cell stack with which a generated power of the fuel cell stack can be recovered quickly even when flooding occurs. 
         [0005]    In order to achieve the above object, this invention provides an operation control device for a fuel cell power plant, which controls a generated power of a fuel cell stack that performs power generation using hydrogen on the basis of a target generated power, comprising an anode gas supply mechanism that supplies an anode gas containing hydrogen to the fuel cell stack, a sensor that determines whether or not an amount of hydrogen supplied to the fuel cell stack satisfies an amount required to generate the target generated power, and a programmable controller. 
         [0006]    The programmable controller is programmed to control the anode gas supply mechanism to cause an anode gas pressure in the fuel cell stack to pulsate between an upper limit pressure and a lower limit pressure, and correct the lower limit pressure in an increasing direction when the hydrogen supply amount does not satisfy the amount required to generate the target generated power. 
         [0007]    This invention also provides an operation control method for the fuel cell power plant described above, comprising supplying an anode gas containing hydrogen to a fuel cell stack, determining whether or not a hydrogen supply amount supplied to the fuel cell stack satisfies an amount required to generate a target generated power, controlling an anode gas supply mechanism to cause an anode gas pressure in the fuel cell stack to pulsate between an upper limit pressure and a lower limit pressure, and correcting the lower limit pressure in an increasing direction when the hydrogen supply amount does not satisfy the amount required to generate the target generated power. 
         [0008]    The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic diagram of a fuel cell power plant to which this invention is applied. 
           [0010]      FIGS. 2A to 2C  are timing charts illustrating a normal operation of an anode dead end type fuel cell stack. 
           [0011]      FIG. 3  is a flowchart illustrating a reactant gas pressure control routine executed by a controller according to this invention. 
           [0012]      FIG. 4  is a flowchart illustrating a recovery mode sub-routine executed by the controller. 
           [0013]      FIG. 5  is a flowchart illustrating a recovery mode lower limit anode gas pressure setting sub-routine executed by the controller. 
           [0014]      FIG. 6  is a flowchart illustrating a recovery mode upper limit anode gas pressure setting sub-routine executed by the controller. 
           [0015]      FIG. 7  is a diagram showing characteristics of a map of the recovery mode lower limit anode gas pressure and the recovery mode upper limit anode gas pressure, which is stored by the controller. 
           [0016]      FIGS. 8A and 8B  are timing charts illustrating results of the reactant gas pressure control executed by the controller. 
           [0017]      FIG. 9  is a flowchart illustrating a recovery mode sub-routine executed by a controller according to a second embodiment of this invention. 
           [0018]      FIG. 10  is a flowchart illustrating a recovery mode upper limit anode gas pressure setting sub-routine executed by the controller according to the second embodiment of this invention. 
           [0019]      FIG. 11  is a timing chart illustrating results of reactant gas pressure control executed by the controller according to the second embodiment of this invention. 
           [0020]      FIG. 12  is a flowchart illustrating a recovery mode sub-routine executed by a controller according to a third embodiment of this invention. 
           [0021]      FIG. 13  is a flowchart illustrating a cathode gas pressure control sub-routine executed by the controller according to the third embodiment of this invention. 
           [0022]      FIG. 14  is a timing chart illustrating results of reactant gas pressure control executed by the controller according to the third embodiment of this invention. 
           [0023]      FIG. 15  is a diagram illustrating a variation of the pressure pulsation operation control according to this invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    Referring to  FIG. 1  of the drawings, a fuel cell power plant includes a fuel cell stack  1  having an anode and a cathode, a cathode line  10  that supplies a cathode gas containing oxygen to the cathode of the fuel cell stack  1  and discharges a cathode effluent gas from the fuel cell stack  1 , and an anode line  20  that supplies an anode gas having hydrogen as a main component to the anode of the fuel cell stack  1  and discharges the anode gas from the fuel cell stack  1 . 
         [0025]    The fuel cell power plant is a well-known so-called anode dead end type power plant that performs a pressure-increasing stroke for raising a pressure of the anode gas in the fuel cell stack  1  and a pressure-reducing stroke for lowering the pressure of the anode gas in the fuel cell stack  1  alternately. This operation generates pressure pulsation in the anode gas. The anode gas discharged from the fuel cell stack  1  is resupplied to the anode in the pressure-reducing stroke, and therefore anode gas discharge to the outside can be suppressed to a minimum. 
         [0026]    In the fuel cell power plant, when a nitrogen concentration of the cathode of the fuel cell stack  1  is higher than the nitrogen concentration of the anode, nitrogen gas is transmitted from the cathode to the anode via a polymer electrolyte. In an anode dead end type fuel cell stack, the nitrogen gas transmitted to the anode may accumulate downstream of an anode gas passage, thereby impairing power generation in the fuel cell stack  1 . Therefore, anode gas pressure pulsation is preferable for discharging nitrogen gas that has accumulated downstream of the anode gas passage to the outside of the fuel cell stack  1 . When the nitrogen gas is discharged from the fuel cell stack  1 , a ratio between a total gas amount and a hydrogen gas amount of the anode gas can be brought close to a stoichiometric ratio (SR). This effect cannot be obtained simply by keeping the pressure of the anode gas high. 
         [0027]    To make anode gas pressure pulsation possible, the anode line  20  comprises a high-pressure hydrogen tank  21 , an anode gas supply passage  201 , a hydrogen pressure regulating valve  22 , an anode gas discharge passage  202 , a buffer tank  23 , a water discharge passage  203 , a water discharge valve  24 , a purge passage  204 , and a purge valve  25 . 
         [0028]    The hydrogen tank  21  is disposed at an upstream end of the anode gas supply passage  201 . 
         [0029]    The hydrogen pressure-adjusting valve  22  is disposed in the anode gas supply passage  201  that connects the hydrogen tank  21  to the fuel cell stack  1 . The hydrogen pressure-adjusting valve  22  adjusts the pressure of the anode gas supplied from the hydrogen tank  21  and supplies the anode gas having a favorable pressure to the fuel cell stack  1 . 
         [0030]    The anode gas discharge passage  202  connects the fuel cell stack  1  to the buffer tank  23 . A volume of the buffer tank  23  is set to be identical to or approximately 80% of a volume of the anode gas passage of the fuel cell stack  1 . The buffer tank  23  stores the anode gas discharged from the fuel cell stack  1 . The anode gas discharged from the fuel cell stack  1  contains moisture generated by the cathode and transmitted from the cathode to the anode through the polymer electrolyte. The buffer tank  23  separates the moisture from the anode gas. The accumulated nitrogen gas downstream of the anode gas passage is also intermixed with the anode gas by the pressure pulsation. The buffer tank  23  also has a function for separating the nitrogen gas from the anode gas. 
         [0031]    The water discharge passage  203  is connected to a bottom surface of the buffer tank  23 . The water discharge valve  24  is disposed in the water discharge passage  203 . The water discharge valve  24  discharges liquid water separated from the anode gas and accumulated in the buffer tank  23  to the outside of the buffer tank  23 . 
         [0032]    The purge passage  204  is connected to a crown surface of the buffer tank  23 . The purge valve  25  is disposed in the purge passage  204 . The purge valve  25  purges inert gases such as nitrogen that have accumulated in an upper portion of the buffer tank  23  into a purge passage  104  via the purge passage  204 . A purge flow rate is adjusted by varying an opening of the purge valve  25  continuously or intermittently. 
         [0033]    The cathode line  10  includes a cathode gas passage  101 , a compressor  11 , a humidifier  12 , a cathode gas supply passage  102 , and a cathode gas discharge passage  103 . 
         [0034]    The compressor  11  is disposed at an upstream end of the cathode gas passage  101 . The compressor  11  compresses air and supplies the compressed air to the humidifier  12 . 
         [0035]    The humidifier  12  is connected to the compressor  11  via the cathode gas passage  101 . The cathode gas supply passage  102 , the cathode gas discharge passage  103 , and the aforesaid purge passage  104  are also connected to the humidifier  12 . 
         [0036]    The humidifier  12  humidifies the compressed air sent thereto from the compressor  11  via the cathode gas passage  101  using moisture contained in the cathode effluent gas discharged from the fuel cell stack  1  via the cathode gas discharge passage  103 . The humidified compressed air is supplied to the cathode of the fuel cell stack  1  via the cathode gas supply passage  102  as cathode gas. In the cathode of the fuel cell stack  1 , the cathode gas reacts with hydrogen ions permeating the polymer electrolyte to generate water. Therefore, following the reaction, the cathode effluent gas contains a large amount of moisture. The cathode effluent gas containing the humidified compressed air is discharged into the atmosphere from the purge passage  104 . 
         [0037]    An operation control device according to this invention, which is used together with the fuel cell power plant configured as described above, includes an ammeter  52  that detects a load current I of the fuel cell stack  1 , a voltmeter  53  that detects a stack voltage V of the fuel cell stack  1 , a hydrogen concentration sensor  54  that detects a hydrogen concentration of the anode gas in the fuel cell stack  1 , a pressure sensor  55  that detects an anode gas pressure Pa in the fuel cell stack  1 , and a programmable controller  51 . Detection values from the respective sensors  52  to  55  are input into the controller  51  via a signal circuit. 
         [0038]    The controller  51  is constituted by a microcomputer including a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM), and an input/output interface (I/O interface). The controller may be constituted by a plurality of microcomputers. 
         [0039]    The controller  51  determines whether or not the amount of anode gas supplied to the fuel cell stack  1  satisfies an amount required to realize a target generated power of the fuel cell stack  1  on the basis of a detection signal from one or a plurality of the sensors  52 - 54 . More specifically, when the load current/detected by the ammeter  52  equals or exceeds a predetermined current, when the stack voltage V equals or exceeds a predetermined voltage, or when the hydrogen concentration of the anode gas in the fuel cell stack  1  equals or exceeds a predetermined concentration, it is determined that the amount of anode gas supplied to the fuel cell stack  1  satisfies the amount required to realize the target generated power of the fuel cell stack  1 . When none of these cases is established, it is determined that the amount of anode gas supplied to the fuel cell stack  1  does not satisfy the amount required to realize the target generated power of the fuel cell stack  1 . 
         [0040]    The controller  51  controls the pressure of the anode gas in the fuel cell stack  1  in accordance with the result of the above determination. More specifically, the controller  51  controls openings or opening/closing of the hydrogen pressure regulating valve  22 , the water discharge valve  24 , and the purge valve  25 . Further, the controller  51  controls the pressure of the cathode gas in the fuel cell stack  1  by controlling an operation of the compressor  11 . 
         [0041]    Referring to  FIGS. 2A-2C , a normal operation of the fuel cell stack  1  will be described. 
         [0042]    As shown in  FIG. 2A , when the hydrogen pressure regulating valve  22  is opened to a predetermined opening during a power generation operation in the fuel cell stack  1 , the pressure of the anode gas in the fuel cell stack  1  is increased as shown in  FIG. 2B  by anode gas supplied from the hydrogen tank  21  through the anode gas supply passage  201 . A part of the hydrogen contained in the anode gas is consumed by the fuel cell stack  1 , and the remaining anode gas flows into the buffer tank  23  as the anode gas through the anode gas discharge passage  202 , as shown in  FIG. 2C . In the following description, this flow of anode gas through the anode gas discharge passage  202  will be referred to as a forward flow. 
         [0043]    After passing through the fuel cell stack  1 , the anode gas contains impurities such as generated water and nitrogen transmitted through the fuel cell stack  1  from the cathode to the anode. Of the impurities contained in the anode gas that flows into the buffer tank  23 , the generated water condenses in the buffer tank  23  to form liquid water that accumulates in the lower portion of the buffer tank  23 . The nitrogen and unreacted anode gas accumulate in the upper portion of the buffer tank  23 . The buffer tank  23  has a sufficient volume to store these impurities. 
         [0044]    When the amount of generated water accumulated in the buffer tank  23  increases, the controller  51  opens the water discharge valve  24  to discharge the generated water to the outside of the buffer tank  23  through the water discharge passage  203 . When the amount of nitrogen accumulated in the buffer tank  23  increases, the controller  51  opens the purge valve  25  to discharge the nitrogen to the outside of the buffer tank  23  through the purge passage  204 . 
         [0045]    As shown in  FIG. 2B , the increasing anode gas pressure reaches an upper limit pressure Pmax at a time t 1 . When the anode gas pressure reaches the upper limit pressure Pmax, the controller  51  closes the hydrogen pressure-adjusting valve  22  fully, as shown in  FIG. 2A , to halt the anode gas supply to the fuel cell stack  1 . The anode gas is consumed by the anode during the power generation operation of the fuel cell stack  1 , and therefore the anode gas pressure Pa falls from the time t 1  onward, as shown in  FIG. 2B . When the anode gas pressure Pa of the fuel cell stack  1  falls, anode gas flows from the buffer tank  23  into the fuel cell stack  1  through the anode gas discharge passage  202 , as shown in  FIG. 2C . In the following description, this flow of anode gas through the anode gas discharge passage  202  will be referred to as a backflow. 
         [0046]    When the anode gas pressure Pa in the fuel cell stack  1  falls to a lower limit pressure Pmin at a time t 2  after fully closing the hydrogen pressure adjusting valve  22 , the controller  51  reopens the hydrogen pressure adjusting valve  22  to the predetermined opening, as shown in  FIG. 2A , such that the anode gas supply to the fuel cell stack  1  from the hydrogen tank  21  is restarted. As a result of this operation, the anode gas pressure Pa in the fuel cell stack  1  increases again such that the anode gas discharged from the fuel cell stack  1  flows into the buffer tank  23  through the anode gas discharge passage  202 , as shown in  FIG. 2C . 
         [0047]    At a time t 3 , the anode gas pressure Pa of the fuel cell stack  1  reaches the upper limit pressure Pmax again. Accordingly, the controller  51  fully closes the hydrogen pressure-adjusting valve  22  again to halt the anode gas supply to the fuel cell stack  1  from the hydrogen tank  21 . Thereafter, as shown in  FIG. 2B , the anode gas pressure Pa in the fuel cell stack  1  decreases as the hydrogen contained in the anode gas is consumed in accordance with the power generation operation of the fuel cell stack  1 . In accordance with this pressure reduction, the anode gas stored in the buffer tank  23  flows back into the fuel cell stack  1  through the anode gas discharge passage  202 , as shown in  FIG. 2C . 
         [0048]    Hence, by operating the hydrogen pressure adjusting valve  22  to open and close, the controller  51  performs a pulsation operation in which the anode gas pressure Pa in the fuel cell stack  1  increases and decreases repeatedly. After the anode gas discharged from the fuel cell stack  1  flows into the buffer tank  23  in the forward flow, the anode gas is resupplied to the fuel cell stack  1  in the backflow. As a result, the anode gas, excluding the nitrogen and generated water contained therein, circulates between the fuel cell stack  1  and the buffer tank  23  without being discharged to the outside. Through the processes described above, an anode dead end operation is executed in the fuel cell power plant. 
         [0049]    Incidentally, according to research conducted by the inventors, when the pressure of the anode gas is increased and reduced repeatedly in a condition where the fuel cell stack  1  is flooded with the generated water, the amount of anode gas required to generate the target power may not be supplied to the anode during the pressure reduction, and as a result, the amount of power generated by the fuel cell stack  1  may decrease. 
         [0050]    To prevent a reduction in the generated power of the fuel cell stack  1  occurring when a pressure pulsation operation is performed in a flooded condition, the controller  51  controls a reactant gas pressure by operating the hydrogen pressure adjusting valve  22  to open and close as follows. 
         [0051]    Referring to  FIG. 3 , a reactant gas pressure control routine executed by the controller  51  for this purpose will be described. This routine is executed repeatedly at fixed time intervals of 10 milliseconds, for example, while the fuel cell power plant is operative. 
         [0052]    In a step S 1 , the controller  51  determines whether or not an amount of hydrogen required in an electrochemical reaction for generating the target generated power of the fuel cell stack  1  is being supplied. More specifically, the load current I of the fuel cell stack  1 , detected by the ammeter  52 , the stack voltage V detected by the voltmeter  53 , and the hydrogen concentration of the anode gas, detected by the hydrogen concentration sensor  54 , are compared with their respective predetermined values, and when the predetermined values are not satisfied, it is determined that the amount of hydrogen required in the electrochemical reaction for generating the target generated power is not being supplied. 
         [0053]    The determination may be made on the basis of an output of any one of the sensors  52 - 54 . Alternatively, outputs of a plurality of the sensors may be compared with their respective predetermined values. In the latter case, it may be determined that the anode gas amount is insufficient when the output of any one of the sensors is smaller than its predetermined value or that the anode gas amount is insufficient when the outputs of all of the sensors are smaller than their respective predetermined values. 
         [0054]    Instead of measuring the load current I, respective power generation currents in a plurality of sections of the fuel cell stack  1  may be measured using a plurality of ammeters, and the determination as to whether or not the anode gas amount is insufficient may be made on the basis of a current distribution thereof. Instead of measuring the stack voltage V, voltages of respective cells constituting the fuel cell stack  1  may be measured individually, and the determination as to whether or not the anode gas amount is insufficient may be made on the basis of an average value or a minimum value of the voltages of the respective cells. Further, the determination as to whether or not the anode gas amount is insufficient may be made by measuring the voltage of a specific cell. 
         [0055]    When it is determined in the step S 1  that the hydrogen supply amount is not insufficient, the controller  51  performs anode gas pressure pulsation operation control in a normal mode in a step S 2 . More specifically, the controller  51  controls the opening and closing operations of the hydrogen pressure adjusting valve  22  so that the anode gas pressure Pa pulsates between the upper limit pressure Pmax and the lower limit pressure Pmin, as shown in  FIGS. 2A-2C . This operation is identical to a conventionally performed operation, and therefore description thereof has been omitted. 
         [0056]    When it is determined in the step S 1  that the hydrogen supply amount is insufficient, the controller  51  performs recovery control on the anode gas pressure Pa in steps S 3 -S 5 . 
         [0057]    In the step S 3 , the controller  51  determines whether or not a predetermined time has elapsed following the start of the recovery control. 
         [0058]    When the determination of the step S 3  is negative, the controller  51  executes a recovery mode sub-routine shown in  FIG. 4  in the step S 4 . When the determination of the step S 3  is affirmative, the controller  51  performs recovery impossible processing in the step S 5 . 
         [0059]    Here, the recovery impossible processing is processing performed when the deficiency in the anode gas amount is not resolved even when the recovery mode sub-routine is executed for a predetermined time. Specifically, the controller  51  performs processing to open the purge valve  25  or reduce a load voltage of the fuel cell stack  1 . When the purge valve  25  is opened, the hydrogen concentration of the anode gas increases, leading to an increase in the voltage of each cell. When the load voltage of the fuel cell stack  1  is reduced, the required amount of anode gas decreases, leading to an increase in the voltage of each cell. 
         [0060]    Following the processing of the step S 2 , S 4  or S 5 , the controller  51  terminates the routine. 
         [0061]    Referring to  FIG. 4 , the recovery mode sub-routine executed by the controller  51  in the step S 4  will be described. 
         [0062]    In a step S 401 , the controller  51  determines whether or not the anode gas pressure Pa in the fuel cell stack  1  is decreasing. When the determination of the step S 401  is affirmative, the controller  51  performs processing of steps S 402 -S 406 . When the determination of the step S 401  is negative, the controller  51  performs processing of steps S 407 -S 411 . 
         [0063]    In the step S 402 , the controller  51  sets a recovery mode lower limit anode gas pressure Pamin by executing a recovery mode lower limit anode gas pressure setting sub-routine shown in  FIG. 5 . 
         [0064]    Referring to  FIG. 5 , in a step S 4021 , the controller  51  detects the load current I of the fuel cell stack  1  from an output signal of the ammeter  52 . 
         [0065]    In a step S 4022 , the controller  51  detects the stack voltage V from an output signal of the voltmeter  53 . 
         [0066]    In a step S 4023 , the controller  51  determines the recovery mode lower limit anode gas pressure Pamin from the load current I and the stack voltage V of the fuel cell stack  1  by referring to a map having characteristics shown in  FIG. 7 , which is stored in the ROM in advance. Here, the map is set in advance such that the recovery mode lower limit anode gas pressure Pamin is higher than the lower limit pressure Pmin applied during the pressure pulsation operation in the normal mode. 
         [0067]    Following the processing of the step S 4023 , the controller  51  terminates the recovery mode lower limit anode gas pressure setting sub-routine. 
         [0068]    Referring back to  FIG. 4 , after setting the recovery mode lower limit anode gas pressure Pamin by executing the recovery mode lower limit anode gas pressure setting sub-routine, the controller  51  detects the anode gas pressure Pa in the fuel cell stack  1  from an output signal of the pressure sensor  55  in the step S 403 . 
         [0069]    Next, in the step S 404 , the controller  51  determines whether or not the anode gas pressure Pa is higher than the recovery mode lower limit anode gas pressure Pamin set in the step S 402 . 
         [0070]    When the determination of the step S 404  is affirmative, the controller  51  reduces the anode gas pressure in the step S 405 . More specifically, the controller  51  either closes the hydrogen pressure-regulating valve  22  or maintains the hydrogen pressure-regulating valve  22  in a closed condition. When the hydrogen pressure-regulating valve  22  is closed, the anode gas pressure decreases by a fixed amount at a time as hydrogen is consumed in the fuel cell stack  1 . 
         [0071]    When the determination of the step S 404  is negative, the controller  51  increases the anode gas pressure in the step S 406 . More specifically, the controller  51  opens the hydrogen pressure-regulating valve  22  to the aforementioned predetermined opening. As a result of this processing, the anode gas supply amount from the high-pressure hydrogen tank  21  to the fuel cell stack  1  increases, leading to an increase in the anode gas pressure Pa in the fuel cell stack  1 . 
         [0072]    Following the processing of the step S 405  or S 406 , the controller  51  terminates the recovery mode sub-routine. 
         [0073]    In the step S 407 , on the other hand, the controller  51  sets a recovery mode upper limit anode gas pressure Pamax by executing a recovery mode upper limit anode gas pressure setting sub-routine shown in  FIG. 6 . 
         [0074]    Referring to  FIG. 6 , in a step S 4071 , the controller  51  detects the load current of the fuel cell stack  1  from the output signal of the ammeter  52 . 
         [0075]    In a step S 4072 , the controller  51  detects the stack voltage V from the output signal of the voltmeter  53 . 
         [0076]    In a step S 4073 , the controller  51  determines the recovery mode upper limit anode gas pressure Pamax from the load current I and the stack voltage V of the fuel cell stack  1  by referring to the map having the characteristics shown in  FIG. 7 , which is stored in the ROM in advance. Here, the map is set in advance such that the recovery mode upper limit anode gas pressure Pamax is higher than the upper limit pressure Pmax applied during the pressure pulsation operation in the normal mode. 
         [0077]    Following the processing of the step S 4073 , the controller  51  terminates the recovery mode upper limit anode gas pressure setting sub-routine. 
         [0078]    Referring back to  FIG. 4 , after setting the recovery mode upper limit anode gas pressure Pamax by executing the recovery mode upper limit anode gas pressure setting sub-routine, the controller  51  detects the anode gas pressure Pa in the fuel cell stack  1  from the output signal of the pressure sensor  55  in the step S 408 . 
         [0079]    Next, in the step S 409 , the controller  51  determines whether or not the anode gas pressure Pa is lower than the recovery mode upper limit anode gas pressure Pamax set in the step S 407 . 
         [0080]    When the determination of the step S 409  is affirmative, the controller  51  increases the anode gas pressure in the step S 410 . Specific processing content of the pressure increasing operation is identical to the step S 406 . 
         [0081]    When the determination of the step S 409  is negative, the controller  51  reduces the anode gas pressure in the step S 411 . Specific processing content of the pressure reducing operation is identical to the step S 405 . 
         [0082]    Following the processing of the step S 410  or S 411 , the controller  51  terminates the recovery mode sub-routine. 
         [0083]    The map of the recovery mode lower limit anode gas pressure and the recovery mode upper limit anode gas pressure, which has the characteristics shown in  FIG. 7  and is stored in the ROM of the controller  51  in advance, is created by predetermining, through experiments and simulations, a lower limit pressure and an upper limit pressure of the pressure pulsation at which the amount of generated power does not decrease further with respect to each load current I and stack voltage V in various fuel cell stacks. 
         [0084]    Referring to  FIG. 7 , with respect to the recovery mode lower limit anode gas pressure Pamin on the map, a steadily larger value is applied to the recovery mode lower limit anode gas pressure Pamin as the deficiency in the amount of anode gas increases, or in other words as the reduction in the stack voltage V increases. Further, a steadily larger value is applied to the recovery mode lower limit anode gas pressure Pamin as the load current I of the fuel cell stack  1  increases. 
         [0085]    With respect to the recovery mode upper limit anode gas pressure Pamax, on the other hand, a steadily larger value is applied to the recovery mode upper limit anode gas pressure Pamax as the deficiency in the amount of anode gas increases, or in other words as the reduction in the stack voltage V increases. Further, a steadily larger value is applied to the recovery mode upper limit anode gas pressure Pamax as the load current I of the fuel cell stack  1  increases. 
         [0086]    The characteristics of the map are preferably set such that with respect to a constant deficiency in the amount of anode gas, a differential pressure between the recovery mode upper limit anode gas pressure Pamax and the upper limit pressure Pmax in the normal mode is equal to or greater than a differential pressure between the recovery mode lower limit anode gas pressure Pamin and the lower limit pressure Pmin in the normal mode. By setting the characteristics of the map in this manner, a center value of the pressure pulsation in the recovery mode varies further toward a pressure increasing direction than a center value of the pressure pulsation in the normal mode, and therefore recovery of the reduced generated power can be promoted more forcefully. 
         [0087]    Through the reactant gas pressure control executed by the controller  51  as described above, the anode gas pressure in the fuel cell stack  1  varies as shown in  FIGS. 8A and 8B . 
         [0088]    On these timing charts, the fuel cell stack  1  maintains the target generated power until a time t 13 , as shown in  FIG. 8B . In this condition, the determination of the step S 1  is affirmative, and therefore the controller  51  performs anode gas pressure pulsation operation control in the normal mode in the step S 2 . 
         [0089]    As a result, the anode gas pressure in the fuel cell stack  1  pulsates between the upper limit pressure Pmax and the lower limit pressure Pmin of the normal mode, shown in  FIG. 8A . 
         [0090]    As shown in  FIG. 8B , when the stack voltage V decreases from the time t 13 , the determination of the step S 1  becomes negative, and therefore the controller  51  performs the anode gas pressure recovery control processing in the steps S 3  and S 4 . As shown in  FIG. 8A , an anode gas pressure reduction operation is performed at the time t 13 , and therefore an affirmative result is obtained in the determination of the step S 401  in  FIG. 4 , which is executed during the recovery mode sub-routine of the step S 4 . 
         [0091]    The controller  51  sets the recovery mode lower limit anode gas pressure Pamin by executing the recovery mode lower limit anode gas pressure setting sub-routine of  FIG. 5  in the step S 402 . The controller  51  performs the determination of the step S 404  during every execution of the routine, and while the determination remains affirmative, repeats the pressure reducing processing of the step S 405  in  FIG. 4 . As a result, the anode gas pressure Pa decreases continuously until the anode gas pressure Pa reaches the recovery mode lower limit anode gas pressure Pamin. The recovery mode lower limit anode gas pressure Pamin is set to be higher than the normal mode lower limit pressure Pmin, and therefore the anode gas pressure Pa decreases to a smaller extent during pressure pulsation in the recovery mode than in the normal mode. Hence, this control acts to suppress a reduction in the generated power. 
         [0092]    When the anode gas pressure Pa falls to the recovery mode lower limit anode gas pressure Pamin at a time t 14 , the determination of the step S 404  becomes negative during the next execution of the routine, and therefore the controller  51  increases the anode gas pressure Pa in the step S 406 . During subsequent executions of the routine, the determination of the step S 401  is negative, and therefore the controller  51  performs processing to increase the anode gas pressure Pa in S 408 -S 410  on the basis of the recovery mode upper limit anode gas pressure Pamax set in the step S 407  thereafter. As a result, the anode gas pressure Pa increases continuously until it reaches the recovery mode upper limit anode gas pressure Pamax. The recovery mode upper limit anode gas pressure Pamax is set to be higher than the normal mode upper limit pressure Pmax, and therefore the anode gas pressure Pa increases to a greater extent during pressure pulsation in the recovery mode than in the normal mode. Hence, this control acts to promote recovery of the generated power. 
         [0093]    When the anode gas pressure Pa reaches the recovery mode upper limit anode gas pressure Pamax at a time t 15 , the determination of the step S 409  in  FIG. 4  becomes negative, and therefore the controller  51  reduces the anode gas pressure Pa in the step S 411 . During subsequent executions of the routine, the determination of the step S 401  is affirmative, and therefore the controller  51  performs processing to reduce the anode gas pressure Pa in S 403 -S 405  on the basis of the recovery mode lower limit anode gas pressure Pamin set in the step S 402  thereafter. 
         [0094]    When the anode gas pressure Pa reaches the recovery mode lower limit anode gas pressure Pamin at a time t 16 , the controller  51  sets the recovery mode upper limit anode gas pressure Pamax again in the step S 407  and then performs processing to increase the anode gas pressure Pa in the steps S 408 -S 410 . 
         [0095]    Hence, by executing the routine of  FIG. 3  and the sub-routines of  FIGS. 4-6 , the controller  51  continuously performs anode gas pressure pulsation operation control in the recovery mode. 
         [0096]    When the stack voltage V recovers to a predetermined voltage at a time t 17 , the determination of the step S 1  becomes affirmative. Thereafter, the controller  51  performs anode gas pressure pulsation operation control in the normal mode. 
         [0097]    When the anode gas supply amount is insufficient for the target generated power, the amount of anode gas required to achieve the target generated power can be supplied by performing a pulsation operation while keeping the anode gas pressure in the fuel cell stack  1  slightly high. Therefore, according to this operation control device, a reduction in the generated power of the fuel cell stack  1  is less likely to occur even when a pressure pulsation operation is performed while the fuel cell stack  1  is flooded. 
         [0098]    A water discharge capacity for discharging the generated water from the fuel cell stack  1  is believed to be dependent on a product of an anode gas flow speed and a pressure increasing time. According to this operation control device, the recovery mode upper limit anode gas pressure Pamax is set to be higher than the normal mode upper limit pressure Pmax, and therefore, in the recovery mode, the anode gas flow speed through the fuel cell stack  1  is maintained at a high level. As a result, a favorable water discharge capacity can be maintained. 
         [0099]    The load current I and the stack voltage V of the fuel cell stack  1  decrease greatly as the deficiency in the amount of anode gas supplied to the fuel cell stack  1  increases. On the map having the characteristics shown in  FIG. 7 , steadily greater values are applied respectively to the recovery mode lower limit anode gas pressure Pamin and the recovery mode upper limit anode gas pressure Pamax as the load current I and the stack voltage V decrease, or in other words as the deficiency in the anode gas amount increases. In this operation control device, a pressure pulsation range is increased in accordance with the deficiency in the anode gas amount, and therefore deterioration of a power generation condition can be prevented through a minimal pressure increase. 
         [0100]    Further, in this operation control device, the fuel cell power plant is returned to the normal mode pressure pulsation operation when the deficiency in the anode gas amount is eliminated, and therefore the power generation condition can be maintained, or deterioration thereof can be prevented, by increasing the anode gas pressure by a required minimum. 
         [0101]    According to this operation control device, when the stack voltage V does not return to the predetermined voltage even after performing the recovery mode pressure pulsation operation continuously for a predetermined time, the recovery impossible processing is performed by opening the purge valve  25  or reducing the load voltage of the fuel cell stack  1 . Therefore, when deterioration of the power generation condition cannot be prevented by controlling the anode gas pressure alone, deterioration of the power generation condition can be prevented by other means. 
         [0102]    Referring to  FIGS. 9-11 , a second embodiment of this invention, relating to the recovery mode sub-routine and the recovery mode upper limit anode gas pressure setting sub-routine executed by the controller  51 , will be described. 
         [0103]    This embodiment differs from the first embodiment in that a limitation is added to the recovery mode upper limit anode gas pressure Pamax during the recovery mode pressure pulsation operation to ensure that the anode gas pressure does not become excessive relative to the cathode gas pressure. For this purpose, the operation control device according to this embodiment further includes a pressure sensor  56  that detects a cathode gas pressure Pc, as shown in  FIG. 1 . 
         [0104]    The recovery mode sub-routine according to this embodiment, shown in  FIG. 9 , corresponds to the recovery mode sub-routine of the first embodiment, shown in  FIG. 4 , and is executed in the step S 4  of the reactant gas pressure control routine of  FIG. 3 . 
         [0105]    The recovery mode upper limit anode gas pressure setting sub-routine according to this embodiment, shown in  FIG. 10 , corresponds to the recovery mode upper limit anode gas pressure setting sub-routine of the first embodiment, shown in  FIG. 6 , and is executed in the step S 421  of the recovery mode sub-routine of  FIG. 9 . 
         [0106]    In the following description relating to  FIGS. 9 and 10 , steps in which identical processing to that of the first embodiment is performed have been allocated identical step numbers and description thereof has been omitted. 
         [0107]    Referring to  FIG. 9 , the recovery mode sub-routine according to this embodiment will be described. 
         [0108]    In the step S 401 , the controller  51  determines whether or not the anode gas pressure in the fuel cell stack  1  is decreasing, and when the determination is negative, the controller  51  sets the recovery mode upper limit anode gas pressure Pamax in the step S 421  by executing the recovery mode upper limit anode gas pressure setting sub-routine shown in  FIG. 10 . 
         [0109]    Referring to  FIG. 10 , after executing the processing of the steps S 4071 -S 4073  in a similar manner to the first embodiment, the controller  51  determines in a step S 4211  whether or not a differential pressure between the recovery mode upper limit anode gas pressure Pamax and the cathode gas pressure Pc detected by the pressure sensor  56  exceeds an allowable differential pressure ΔP 0 . The allowable differential pressure ΔP 0  is set in advance through experiments and simulations at a value that does not permit a dramatic reduction in a durability of the fuel cell stack  1 . A typical value of the allowable differential pressure ΔP 0  is between 50 and 200 kilopascals (kPa). 
         [0110]    When the determination of the step S 4211  is affirmative, the controller  51  resets the recovery mode upper limit anode gas pressure Pamax at a value obtained by adding the allowable differential pressure ΔP 0  to the cathode gas pressure Pc in a step S 4212 . Following the processing of the step S 4212 , the controller  51  terminates the recovery mode upper limit anode gas pressure setting sub-routine. 
         [0111]    When the determination of the step S 4211  is negative, on the other hand, the controller  51  terminates the recovery mode upper limit anode gas pressure setting sub-routine without adding a limitation to the recovery mode upper limit anode gas pressure Pamax. 
         [0112]    Referring back to  FIG. 9 , in the step S 409 , the controller  51  determines whether or not the anode gas pressure Pa is lower than the recovery mode upper limit anode gas pressure Pamax, similarly to the first embodiment. The recovery mode upper limit anode gas pressure Pamax used here, however, takes a value limited by the value obtained by adding the allowable differential pressure ΔP 0  to the cathode gas pressure Pc. 
         [0113]    When the determination of the step S 409  is affirmative, the controller  51  increases the anode gas pressure in the step S 410 , similarly to the first embodiment, and then terminates the recovery mode sub-routine. 
         [0114]    When the determination of the step S 409  is negative, the controller  51  determines in a step S 422  whether or not a pressure reducing condition of the anode gas pressure Pa is established. The pressure reducing condition of the anode gas pressure Pa is determined according to whether or not a required time following a point at which the determination of the step S 409  switches from affirmative to negative has reached a predetermined time, for example. 
         [0115]    When the determination of the step S 422  is negative, or in other words when the required time following the point at which the determination of the step S 409  switches from affirmative to negative has not reached the predetermined time, the controller  51  maintains the anode gas pressure Pa as is in a step S 423 . 
         [0116]    When the determination of the step S 422  is affirmative, or in other words when the required time following the point at which the determination of the step S 409  switches from affirmative to negative has reached the predetermined time, the controller  51  reduces the anode gas pressure Pa in the step S 411 . 
         [0117]    Following the processing of the step S 423  or the step S 411 , the controller  51  terminates the recovery mode sub-routine. 
         [0118]    Referring to  FIG. 11 , as a result of the control described above, the recovery mode upper limit anode gas pressure Pamax is suppressed to be lower than that of the first embodiment. On the other hand, during a period between times t 24  and t 26  and a period between times t 27  and t 28 , the anode gas pressure Pa is maintained at the recovery mode upper limit anode gas pressure Pamax. 
         [0119]    According to this embodiment, a limitation is added to the recovery mode upper limit anode gas pressure Pamax so that a differential pressure Pa-Pc between the anode gas and the cathode gas does not exceed the allowable differential pressure ΔP 0 , and when the anode gas pressure Pa reaches the recovery mode upper limit anode gas pressure Pamax, the anode gas pressure Pa is maintained at the recovery mode upper limit anode gas pressure Pamax for a predetermined period rather than reducing the anode gas pressure Pa immediately. 
         [0120]    By setting the differential pressure between the anode gas and the cathode gas at or below the allowable differential pressure ΔP 0 , a reduction in the durability of the fuel cell stack  1  caused by an excessive differential pressure can be prevented. Further, by maintaining the recovery mode upper limit anode gas pressure Pamax within the range of the allowable differential pressure ΔP 0  for a predetermined period, the anode gas amount required to maintain the power generation condition or recover from deterioration of the power generation condition can be secured. 
         [0121]    Referring to  FIGS. 12-14 , a third embodiment of this invention will be described. 
         [0122]    In the first and second embodiments, the controller  51  controls the anode gas pressure alone, but in this embodiment, the controller  51  controls the cathode gas pressure as well as the anode gas pressure. 
         [0123]    A recovery mode sub-routine according to this embodiment, shown in  FIG. 12 , corresponds to the recovery mode sub-routine of the first embodiment, shown in  FIG. 4 , and is executed in the step S 4  of the reactant gas pressure control routine of  FIG. 3 . A step S 431  that does not exist in the other embodiments is provided in the recovery mode sub-routine according to this embodiment. 
         [0124]    Referring to  FIG. 12 , the processing of the steps S 401 -S 411  is identical to that of the first embodiment. Following the processing of one of the steps S 405 , S 406 , S 410 , and S 411 , the controller  51  executes a cathode gas pressure control sub-routine shown in  FIG. 13  in the step S 431 . 
         [0125]    Referring to  FIG. 13 , in a step S 4311 , the controller  51  sets a recovery mode lower limit cathode gas pressure Pc 0  by subtracting the allowable differential pressure ΔP 0  from the anode gas pressure Pa. The allowable differential pressure ΔP 0  is identical to the allowable differential pressure ΔP 0  used in the second embodiment. 
         [0126]    In a step S 4312 , the controller  51  detects the cathode gas pressure Pc from an output signal of the pressure sensor  56 . 
         [0127]    In a step S 4313 , the controller  51  determines whether or not the cathode gas pressure Pc is lower than the recovery mode lower limit cathode gas pressure Pc 0 . 
         [0128]    When the determination of the step S 4313  is affirmative, the controller performs processing to increase the cathode gas pressure Pc by controlling the operation of the compressor  11  in a step S 4314 . Following the processing of the step S 4314 , the controller  51  terminates the cathode gas pressure control sub-routine. 
         [0129]    When the determination of the step S 4313  is negative, on the other hand, the controller  51  determines in a step S 4315  whether or not the cathode pressure Pc is equal to a normal mode cathode pressure. When the cathode pressure Pc is equal to the normal mode cathode pressure, the controller  51  terminates the cathode gas pressure control sub-routine without applying processing to the cathode pressure Pc. 
         [0130]    When the cathode pressure Pc is not equal to the normal mode cathode pressure, the controller  51  reduces the cathode pressure Pc in a step S 4316 . Following the processing of the step S 4316 , the controller  51  terminates the cathode gas pressure control sub-routine. In accordance with termination of the cathode gas pressure control sub-routine, the recovery mode sub-routine of  FIG. 12  is terminated. 
         [0131]    Referring to  FIG. 14 , as a result of the control described above, when the anode gas amount becomes insufficient in a pressure reducing stroke from a time t 32  onward during the normal mode pressure pulsation operation, the controller  51  executes the recovery mode sub-routine of  FIG. 12  in the step S 4  of  FIG. 3 . In the recovery mode sub-routine, processing is performed to reduce the anode gas pressure Pa in the step S 405  such that the anode gas pressure Pa reaches the recovery mode lower limit anode gas pressure Pamin at a time t 33 . As the anode gas pressure Pa decreases, the recovery mode lower limit cathode gas pressure Pc 0  also decreases, and therefore the cathode pressure Pc does not fall below the recovery mode lower limit cathode gas pressure Pc 0 . Hence, as long as the anode gas pressure Pa decreases, the cathode pressure Pc is not increased during the cathode pressure control sub-routine of  FIG. 13 , executed in the step S 431  of the recovery mode sub-routine. 
         [0132]    After the anode gas pressure Pa reaches the recovery mode lower limit anode gas pressure Pamin at the time t 33 , the controller  51  performs processing to increase the anode gas pressure by controlling the operation of the compressor  11  in the step S 410  and the step S 410  of the recovery mode sub-routine shown in  FIG. 12 . As a result, the anode gas pressure Pa increases. 
         [0133]    As the anode gas pressure Pa increases, the recovery mode lower limit cathode gas pressure Pc 0  set in the step S 4311  of the cathode gas pressure control sub-routine shown in  FIG. 13  also increases. As a result, the cathode pressure Pc falls below the recovery mode lower limit cathode gas pressure Pc 0  at a time t 34 , whereby the controller  51  performs processing to increase the cathode pressure Pc in the step S 4314 . Hence, from the time t 34  onward, the cathode pressure Pc increases together with the anode pressure Pa. 
         [0134]    At a time t 35 , the anode gas pressure Pa reaches the recovery mode upper limit anode gas pressure Pamax. Thereafter, the controller  51  performs processing to reduce the anode gas pressure Pa in the steps S 411  and S 406  of the recovery mode sub-routine shown in  FIG. 12 . Further, the controller  51  performs processing to reduce the cathode gas pressure Pc by controlling the operation of the compressor  11  in the step S 4316  of  FIG. 13 . Thereafter, the anode gas pressure Pa and the cathode gas pressure Pc decrease while maintaining the differential pressure ΔP 0 . 
         [0135]    When the cathode gas pressure Pc decreases to a normal mode cathode pressure at a time t 36 , the determination of the step S 4316  becomes affirmative, and therefore subsequent processing to reduce the cathode pressure Pc is stopped such that the cathode pressure Pc is maintained at the normal mode cathode pressure. On the other hand, the processing to reduce the anode gas pressure Pa is continued until the anode gas pressure Pa reaches the recovery mode lower limit anode gas pressure Pamin at a time t 37 . 
         [0136]    The recovery mode pulsation operation control performed from the time t 33  to the time t 37  is repeated likewise after the time t 37 . From a time t 40  onward, when the stack voltage V recovers to the predetermined voltage, the determination of the step S 1  in the reactant gas pressure control routine of  FIG. 3  switches from negative to affirmative. In accordance with this variation, the controller  51  switches the pressure pulsation operation control from the recovery mode to the normal mode such that thereafter, the normal mode pressure pulsation operation control is performed in the step S 2 . 
         [0137]    According to this embodiment, the processing to increase the cathode gas pressure Pc is performed in accordance with the increase in the anode gas pressure Pa so that the differential pressure between the anode gas pressure Pa and the cathode gas pressure Pc does not exceed the allowable differential pressure ΔP 0 . In so doing, a reduction in the durability of the fuel cell stack  1  caused by an excessive differential pressure can be prevented. Further, the anode gas pressure Pa is not prevented from rising to the recovery mode upper limit anode gas pressure Pamax, and therefore the anode gas amount required to maintain the power generation condition or recover from deterioration thereof can also be secured. 
         [0138]    The contents of Tokugan 2009-160528 with a filing date of Jul. 7, 2009 in Japan, are hereby incorporated by reference. 
         [0139]    Although the invention has been described above with reference to certain embodiments, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, within the scope of the claims. 
         [0140]    As shown in  FIG. 15 , for example, with respect to all of the embodiments described above, when the anode gas pressure Pa is increased in the recovery mode, an increase speed of the anode gas pressure is preferably raised such that the flow speed of the anode gas passing through the fuel cell stack  1  exceeds the flow speed thereof in the normal mode. More specifically, the predetermined opening of the hydrogen pressure-adjusting valve  22  applied to the processing for increasing the anode gas pressure Pa in the steps S 406  and S 410  is set at a larger value. 
         [0141]    When the increase speed of the anode gas pressure Pa is raised, a pressure pulsation interval shortens such that a time required to switch from the recovery mode to the normal mode after the stack voltage V recovers to the predetermined voltage is reduced. As a result, the pressure pulsation operation in the recovery mode can be completed within a shorter period. 
       INDUSTRIAL FIELD OF APPLICATION 
       [0142]    As described above, the control device and control method for a fuel cell power plant according to this invention exhibits favorable effects when applied to a fuel cell power plant for a vehicle, but is not limited thereto. 
         [0143]    The embodiments of this invention in which an exclusive property or privilege is claimed are defined as follows: