FUEL CELL SYSTEM

Abstract

Fuel cell system, comprising: a plurality of fuel cell stacks (10, 11), including a first fuel cell stack (10) and a second fuel cell stack (11); a power generation control unit (22) configured to control power generation of the plurality of fuel cell stacks (10, 11) based on a required power for the plurality of fuel cell stacks (10, 11); and a refresh control unit (24) configured to perform a refresh process of reducing a voltage across the plurality of fuel cell stacks (10, 11), wherein the power generation control unit (22) is configured to stop power generation of the first fuel cell stack (10) when the required power is greater than or equal to a first threshold and less than a second threshold that is greater than the first threshold, and to allow the first fuel cell stack (10) to generate electric power when the required power is greater than or equal to the second threshold, and wherein the refresh control unit (24) is configured to perform the refresh process on the first fuel cell stack (10) when the required power changes from a state in which the required power is less than a first predetermined value, which is greater than the first threshold value and less than the second threshold value, to a state in which the required power is greater than or equal to the first predetermined value, and when the required power is in a range that is greater than or equal to the first predetermined value and less than the second threshold value.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] The invention relates to a fuel cell system. 2. Description of the related art

[0002] In a fuel cell including a membrane-electrode assembly in which electrodes are arranged on both surfaces of an electrolyte membrane, it is known to perform a refreshing process of temporarily reducing a voltage of the fuel cell to remove an oxide layer formed on a surface of a catalyst included in the electrodes and impurities attached thereto. For example, in a fuel cell system including a plurality of fuel cell stacks, it is known that a refreshing process can be performed sequentially on the plurality of fuel cell stacks when the fuel cell system is started or stopped (see, for example, JP 2009-59610 A). JP 2009-259408 A describes a method for controlling a fuel cell system including multiple fuel cells.In this case, two stacks are operated either individually or jointly, depending on the required power. A method for controlling a fuel cell system with multiple fuel cells is also the subject of DE 10 2019 117 255 A1. Here, too, two stacks are operated either individually or jointly, depending on the required power. Finally, DE 10 2019 123 562 A1 describes a method for refreshing a fuel cell stack in a fuel cell system comprising multiple stacks. SUMMARY OF THE INVENTION

[0003] When the voltage of a fuel cell stack reaches a high potential after the oxide layer and impurities on the catalyst surface are removed by the rejuvenation process, the catalyst is likely to be flushed out, and the catalyst amount gradually decreases due to the flushing, thus causing deterioration in power generation capability. If the fuel cell stack does not generate electrical power for a long time after the oxide layer and impurities on the catalyst layer are removed by the rejuvenation process, an oxide layer will re-form on the catalyst surface and / or impurities will be deposited on the catalyst surface, thus causing long-term deterioration in power generation capability.

[0004] The invention enables a refresh process to be carried out at a suitable time.

[0005] According to one aspect of the invention, there is provided a fuel cell system including: a plurality of fuel cell stacks including a first fuel cell stack and a second fuel cell stack; a power generation control unit configured to control power generation of the plurality of fuel cell stacks based on a required power for the plurality of fuel cell stacks; and a refresh control unit configured to perform a refresh process of decreasing a voltage across the plurality of fuel cell stacks.The power generation control unit is configured to stop power generation of the first fuel cell stack when the required power is greater than or equal to a first threshold and less than a second threshold that is greater than the first threshold, and to allow the first fuel cell stack to generate electric power when the required power is greater than or equal to the second threshold.The refresh control unit is configured to perform the refresh process on the first fuel cell stack when the required power changes from a state where the required power is less than a first predetermined value, which is greater than the first threshold value and less than the second threshold value, to a state where the required power is greater than or equal to the first predetermined value, and when the required power is in a range that is greater than or equal to the first predetermined value and less than the second threshold value.

[0006] The refresh control unit may be configured to determine whether the refresh process is to be performed on the first fuel cell stack and to perform the refresh process on the first fuel cell stack when it is determined that the refresh process is to be performed.

[0007] The fuel cell system may further include a plurality of switches connected between the plurality of fuel cell stacks and an auxiliary machine into which a current supplied from the plurality of fuel cell stacks flows. The power generation control unit may be configured to set one of the plurality of switches connected between the first fuel cell stack and the auxiliary machine to a disconnected state when power generation of the first fuel cell stack is stopped. The refresh control unit may be configured to change a state of the switch from the disconnected state to a connected state when the refresh process is to be performed on the first fuel cell stack.

[0008] The refresh control unit may be configured to perform the refresh process by passing a current through the first fuel cell stack without supplying a cathode gas thereto.

[0009] The first predetermined value may be an intermediate value between the first threshold value and the second threshold value.

[0010] The first threshold can be 0.

[0011] The power generation control unit may be configured to stop power generation of the second fuel cell stack when the required power is greater than or equal to a third threshold that is greater than or equal to the second threshold and less than a fourth threshold that is greater than the third threshold, and to allow the second fuel cell stack to generate electric power when the required power is greater than or equal to the fourth threshold and when the required power is less than the third threshold.The refresh control unit may be configured to perform the refresh process on the second fuel cell stack when the required power changes from a state where the required power is less than a second predetermined value that is greater than the third threshold and less than the fourth threshold to a state where the required power is greater than or equal to the second predetermined value, and when the required power is in a range that is greater than or equal to the second predetermined value and less than the fourth threshold.

[0012] The power generation control unit may be configured to stop power generation of the second fuel cell stack when the required power is greater than or equal to a third threshold that is greater than or equal to the second threshold and less than a fourth threshold that is greater than the third threshold, and to allow the second fuel cell stack to generate electric power when the required power is greater than or equal to the fourth threshold and when the required power is less than the third threshold.The refresh control unit may be configured to perform the refresh process on the second fuel cell stack when the required power changes from a state where the required power is greater than a third predetermined value that is greater than the third threshold and less than the fourth threshold to a state where the required power is less than or equal to the third predetermined value, and when the required power is in a range that is greater than or equal to the third threshold and less than or equal to the third predetermined value.

[0013] A maximum output power of the first fuel cell stack may be greater than that of the second fuel cell stack.

[0014] According to the invention, it is possible to carry out a refresh process at a suitable time. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Features, advantages, and technical and economic significance of exemplary embodiments of the invention are described below with reference to the accompanying drawings, in which like reference numerals designate like elements and in which: Fig. 1 is a diagram schematically illustrating a configuration of a fuel cell system according to a first embodiment; Fig. 2 is a current-power characteristic diagram illustrating a relationship between an output current and an output power of a first fuel cell stack and a second fuel cell stack; Fig. 3 is a diagram illustrating a maximum output power when a maximum allowable current and a minimum allowable voltage are set in the first fuel cell stack; Fig. 4 is a diagram schematically illustrating an electrical configuration of the fuel cell system according to the first embodiment; Fig. 5 is a flowchart illustrating power generation control according to the first embodiment; Fig. 6 is a timing chart illustrating power generation control and refresh control according to the first embodiment; Fig. 7 is a diagram illustrating power generation control according to the first embodiment; Fig. 8 is a flowchart illustrating refresh control according to the first embodiment; Fig. 9 is a flowchart illustrating refresh control according to a second embodiment; Fig. 10 is a timing chart illustrating power generation control and refresh control according to the second embodiment; Fig. 11 is a flowchart illustrating refresh control according to Modified Example 1 of the second embodiment; and Fig. 12 is a timing chart illustrating power generation control and refresh control according to Modified Example 1 of the second embodiment. DETAILED DESCRIPTION OF EMBODIMENTS

[0016] Embodiments of the invention will be described below with reference to the accompanying drawings.

[0017] Fig. 1 is a diagram schematically illustrating a configuration of a fuel cell system according to a first embodiment. The fuel cell system is a power generation system used for a fuel cell vehicle, a stationary fuel cell device, and the like, and outputs electric power in accordance with a required power. In the following embodiment, it is assumed that the fuel cell system is mounted in a vehicle. As shown in Fig. 1, a fuel cell system 100 includes a first fuel cell stack 10 (hereinafter also referred to as a first FC stack 10), a second fuel cell stack 11 (hereinafter also referred to as a second FC stack 11), a control unit 20, cathode gas piping systems 30 and 40, and anode gas piping systems 50 and 70. The fuel cell system 100 also includes a coolant piping system, which is neither illustrated nor described herein.

[0018] The first FC stack 10 and the second FC stack 11 are solid polymer fuel cells that are supplied with hydrogen (an anode gas) and air (a cathode gas) as reactant gases and generate electrical power. The first FC stack 10 and the second FC stack 11 have a stacked structure in which a plurality of cells are stacked. Each cell includes a membrane-electrode assembly, which is a power generator comprising electrodes arranged on both surfaces of an electrolyte membrane and a pair of separators between which the membrane-electrode assembly is sandwiched.

[0019] The electrolyte membrane is a solid polymer membrane formed from a fluorine-based resin or a hydrocarbon-based resin containing a sulfonate group, which exhibits excellent proton conductivity in a wet state. The electrodes include carbon supports. The carbon supports in the electrodes support a catalyst (e.g., platinum or platinum-cobalt alloy) to promote a power generation reaction. A manifold for allowing the flow of reactant gases is provided in each cell. Reactant gases flowing in the manifold are supplied to a power generation region of each cell via gas flow passages provided in each cell.

[0020] The first FC stack 10 has a larger maximum output power than the second FC stack 11. For example, the first FC stack 10 has a larger number of stacked cells than the second FC stack 11, and thus its maximum output power is larger.

[0021] Fig. Figure 2 is a current-power characteristic diagram illustrating a relationship between an output current and an output power of the first FC stack and the second FC stack. As shown in Fig. 2, a maximum output power P1 of the first FC stack 10 is greater than a maximum output power P2 of the second FC stack 11. The first FC stack 10 and the second FC stack 11 are different in maximum output power because they have a different number of cells formed of the same material and have the same power generation area. Accordingly, an output current at the maximum output power P1 of the first FC stack 10 and an output current at the maximum output power P2 of the second FC stack 11 are the same (e.g., the same) output current A. The first FC stack 10 has the same number of stacked cells as the second FC stack 11, but its maximum output power may be greater due to different materials and / or power generation areas.

[0022] When a maximum allowable current and / or a minimum allowable voltage is set for the output current and / or the output voltage in the first FC stack 10 and the second FC stack 11 for the purpose of avoiding a rapid voltage drop, suppressing heat radiation in the FC stack, or the like, the maximum output powers of the first FC stack 10 and the second FC stack 11 can be set to maximum output powers within an allowable range.

[0023] Fig. Figure 3 is a diagram illustrating maximum output power when a maximum allowable current and a minimum allowable voltage are set in the first FC stack. When a maximum allowable current LA and a minimum allowable voltage LV are set as in Fig. 3, a maximum power P1a within a permissible current range and a permissible voltage range can be set as the maximum output power of the first FC stack 10. The same applies to the second FC stack 11.

[0024] As in Fig. 1, the control unit 20 serves as a power generation control unit 22 and a refresh control unit 24. An accelerator pedal operation amount signal is transmitted to the control unit 20 from an accelerator pedal sensor 67 that detects an operation amount of an accelerator pedal 66 (that is, a depression amount of the accelerator pedal 66 by a driver).

[0025] The power generation control unit 22 calculates a required power based on the accelerator operation amount signal and controls the component units of the fuel cell system 100 described later in accordance with the calculated required power, so as to control the power generation of the first FC stack 10 and the second FC stack 11. The refresh control unit 24 calculates a required power based on the accelerator operation amount signal and controls the component units of the fuel cell system 100 described later in accordance with the calculated required power, so as to perform a refresh process of temporarily reducing a voltage across the first FC stack 10 and the second FC stack 11.Here, the required power for the entire fuel cell system 100 including the first FC stack 10 and the second FC stack 11 is calculated based on the accelerator pedal operation amount. If the fuel cell system 100 includes a secondary battery, a state of charge of the secondary battery may be detected, and the required power for the first FC stack 10 and the second FC stack 11 may be calculated considering an electric power charged / discharged by the secondary battery. The refresh control unit 24 may use the required power calculated by the power generation control unit 22 instead of calculating the required power based on the accelerator pedal operation amount signal.

[0026] The cathode gas piping system 30 supplies a cathode gas to the first FC stack 10 and exhausts a cathode exhaust gas that has not been consumed in the first FC stack 10. The cathode gas piping system 30 includes a cathode gas pipe 31, an air compressor 32, a switching valve 33, a cathode exhaust pipe 34, and a pressure regulating valve 35. The cathode gas pipe 31 is a pipe connected to a cathode inlet of the first FC stack 10. The air compressor 32 is connected to a cathode of the first FC stack 10 via the cathode gas pipe 31, draws in outside air, and supplies compressed air to the first FC stack 10 as a cathode gas. The control unit 20 controls a flow rate of air supplied to the first FC stack 10 by controlling the driving of the air compressor 32. The switching valve 33 is provided between the air compressor 32 and the first FC stack 10 and is opened and closed depending on an air flow in the cathode gas tube 31.For example, the switching valve 33 is normally closed and opens when air at a predetermined pressure is supplied to the cathode gas pipe 31 from the air compressor 32. The cathode exhaust pipe 34 is a pipe connected to a cathode outlet of the first FC stack 10 and discharges a cathode exhaust gas to the outside of the fuel cell system 100. The pressure regulating valve 35 regulates a pressure of the cathode exhaust gas in the cathode exhaust pipe 34.

[0027] The cathode gas piping system 40 supplies a cathode gas to the second FC stack 11 and discharges a cathode exhaust gas that has not been consumed in the second FC stack 11. The cathode gas piping system 40 includes a cathode gas pipe 41, an air compressor 42, a switching valve 43, a cathode exhaust pipe 44, and a pressure regulating valve 45. The cathode gas pipe 41, the air compressor 42, the switching valve 43, the cathode exhaust pipe 44, and the pressure regulating valve 45 have the same functions as the cathode gas pipe 31, the air compressor 32, the switching valve 33, the cathode exhaust pipe 34, and the pressure regulating valve 35 of the cathode gas piping system 30, respectively. Accordingly, the control unit 20 controls a flow rate of air supplied to the second FC stack 11 by controlling the driving of the air compressor 42.

[0028] The anode gas piping system 50 supplies an anode gas to the first FC stack 10 and discharges an anode exhaust gas that has not been consumed in the first FC stack 10. The anode gas piping system 50 includes an anode gas pipe 51, a switching valve 52, a regulator 53, an injector 54, an anode exhaust pipe 55, a gas-liquid separator 56, an anode gas circulation pipe 57, a circulation pump 58, an anode drainage pipe 59, and a drainage valve 60. The anode gas pipe 51 is a pipe that connects a hydrogen tank 65 to an anode inlet of the first FC stack 10. That is, the hydrogen tank 65 is connected to the anode of the first FC stack 10 via the anode gas pipe 51 and supplies hydrogen stored in the tank to the first FC stack 10. The switching valve 52, the regulator 53, and the injector 54 are arranged in this order from upstream in the anode gas pipe 51.The switching valve 52 is switched in accordance with a command from the control unit 20 and controls a flow of hydrogen from the hydrogen tank 65 to an upstream side of the injector 54. The regulator 53 is a decompression valve that regulates a pressure of hydrogen upstream of the injector 54. The injector 54 is an electromagnetically controlled switching valve whose valve body is electromagnetically controlled based on a drive cycle and a valve opening time set by the control unit 20. The control unit 20 controls a flow rate of hydrogen supplied to the first FC stack 10 by controlling the drive cycle and / or the valve opening time of the injector 54.

[0029] The anode exhaust pipe 55 is a pipe that connects an anode outlet of the first FC stack 10 to the gas-liquid separator 56 and supplies an anode exhaust gas containing unreacted gas (such as hydrogen and nitrogen) that has not been used for a power generation reaction to the gas-liquid separator 56. The gas-liquid separator 56 separates the anode exhaust gas into a gas component and moisture, supplies the gas component to the anode gas circulation pipe 57, and supplies the moisture to the anode drainage pipe 59. The anode gas circulation pipe 57 is connected to the anode gas pipe 51 downstream of the injector 54. The circulation pump 58 is provided in the anode gas circulation pipe 57. Hydrogen contained in the gas component separated by the gas-liquid separator 56 is supplied to the anode gas pipe 51 by the circulation pump 58.The circulation pump 58 operates in accordance with a command from the control unit 20. The anode drain pipe 59 is a pipe that discharges the moisture separated by the gas-liquid separator 56 to the outside of the fuel cell system 100. The drain valve 60 is provided in the anode drain pipe 59 and is opened or closed in accordance with a command from the control unit 20.

[0030] The anode gas piping system 70 supplies an anode gas to the second FC stack 11 and discharges an anode exhaust gas that has not been consumed in the second FC stack 11. The anode gas piping system 70 includes an anode gas pipe 71, a switching valve 72, a regulator 73, an injector 74, an anode exhaust pipe 75, a gas-liquid separator 76, an anode gas circulation pipe 77, a circulation pump 78, an anode drainage pipe 79, and a drainage valve 80. The anode gas pipe 71, the switching valve 72, the regulator 73, the injector 74, the anode exhaust pipe 75, the gas-liquid separator 76, the anode gas circulation pipe 77, the circulation pump 78, the anode drainage pipe 79, and the drainage valve 80 have the same functions as the anode gas pipe 51, the switching valve 52, the regulator 53, the Injector 54, the anode exhaust pipe 55, the gas-liquid separator 56, the anode gas circulation pipe 57, the circulation pump 58, the anode drainage pipe 59 respectively.the drain valve 60 of the anode gas piping system 50. Accordingly, the control unit 20 controls a flow rate of hydrogen supplied to the second FC stack 11 by controlling a drive cycle and / or a valve opening time of the injector 74.

[0031] Fig. 4 is a diagram schematically illustrating an electrical configuration of the fuel cell system according to the first embodiment. The fuel cell system 100 includes FDCs 81a and 81b, an inverter 82, a motor generator 83, a BDC 84, a battery 85, and switches 86a and 86b in addition to the control unit 20.

[0032] The FDCs 81a and 81b are DC-DC converters. The FDC 81a converts the output voltage of the first FC stack 10 and supplies the converted output voltage to the inverter 82 and the BDC 84. The FDC 81b converts the output voltage of the second FC stack 11 and supplies the converted output voltage to the inverter 82 and the BDC 84. The BDC 84 is a DC-DC converter. The battery 85 is a rechargeable and dischargeable secondary battery. The BDC 84 can adjust a DC voltage from the battery 85 and output the adjusted DC voltage to the inverter 82, and can adjust DC voltages from the first FC stack 10 and the second FC stack 11 and a voltage from the motor generator 83 converted to a DC voltage by the inverter 82 and output the adjusted voltages to the battery 85.The inverter 82 is a DC / AC converter, converts DC voltage output from the first FC stack 10, the second FC stack 11, and the battery 85 into AC voltage, and supplies the AC voltage to the motor generator 83. The motor generator 83 drives vehicle wheels 68. The switches 86a and 86b are opened and closed in accordance with a command from the control unit 20, and switch between electrical connection and disconnection of the first FC stack 10, the second FC stack 11, the motor generator 83, and the battery 85.

[0033] The control unit 20 is an electronic control unit (ECU) formed by a microcomputer including a central processing unit (CPU), a random access memory (RAM), a read-only memory (ROM), and a storage unit. The storage unit is a non-volatile memory such as a hard disk drive (HDD) or a flash memory. The control unit 20 comprehensively controls the component units of the fuel cell system 100 and controls the operation of the fuel cell system 100.

[0034] The control unit 20 acquires an accelerator operation amount signal from the accelerator pedal sensor 67 that detects an operation amount of the accelerator pedal 66, and calculates a required power for the first FC stack 10 and the second FC stack 11 from the acquired accelerator operation amount signal. Then, the control unit 20 controls gas supply flow rates to the first FC stack 10 and the second FC stack 11, duty ratios of the FDCs 81a and 81b, and the like based on the calculated required power, and controls the power generation of the first FC stack 10 and the second FC stack 11. In this way, the control unit 20 serves as the power generation control unit 22, which calculates the required power for the first FC stack 10 and the second FC stack 11 from the accelerator operation amount signal and controls the power generation of the first FC stack 10 and the second FC stack 11 based on the calculated required power.

[0035] For example, the power generation control unit 22 controls a flow rate of a cathode gas supplied to the first FC stack 10 and the second FC stack 11 by controlling the air compressors 32 and 42 and the like, and controls a flow rate of an anode gas supplied to the first FC stack 10 and the second FC stack 11 by controlling the injectors 54 and 74, the circulation pumps 58 and 78, and the like. The power generation control unit 22 turns the switches 86a and 86b ON (a connected state) when the first FC stack 10 and the second FC stack 11 are allowed to generate electric power, and turns the switches 86a and 86b OFF (a disconnected state) when the power generation of the first FC stack 10 and the second FC stack 11 is stopped.In the first embodiment, a configuration in which the switches 86a and 86b are provided separately from the FDCs 81a and 81b is adopted, but the invention is not limited to this. For example, by providing switching elements in the FDCs 81a and 81b and causing the control unit 20 to control the switching elements of the FDCs 81a and 81b, switching between electrical connection and disconnection of the first FC stack 10, the second FC stack 11, the motor generator 83, and the battery 85 can be performed.

[0036] As described above, the control unit 20 calculates a required power for the first FC stack 10 and the second FC stack 11 based on the accelerator pedal operation amount signal. Then, the control unit 20 performs a refresh process on the first FC stack 10 and the second FC stack 11 based on the calculated required power. In this way, the control unit 20 serves as the refresh control unit 24, which calculates a required power for the first FC stack 10 and the second FC stack 11 from the accelerator pedal operation amount signal and performs a refresh process on the first FC stack 10 and the second FC stack 11 based on the calculated required power. For example, the refresh control unit 24 controls a flow rate of a cathode gas supplied to the first FC stack 10 and the second FC stack 11 by controlling the air compressors 32 and 42 and the like.In addition, the refresh control unit 24 performs a refresh process of temporarily lowering the voltages of the first FC stack 10 and the second FC stack 11 by turning the switches 86a and 86b ON to pass a current through the first FC stack 10 and the second FC stack 11.

[0037] Fig. 5 is a flowchart illustrating power generation control according to the first embodiment. Fig. 6 is a timing chart illustrating power generation control and refresh control according to the first embodiment. As shown in Fig. 5, the control unit 20 waits until an accelerator operation amount signal output from the accelerator pedal sensor 67 with an operation amount other than zero is acquired (step S10). When an accelerator operation amount signal with an operation amount other than zero is acquired (YES in step S10), the control unit 20 calculates a required power based on the accelerator operation amount signal (step S12). For example, the control unit 20 calculates the required power for the first FC stack 10 and the second FC stack 11 from the acquired accelerator operation amount signal by referring to a map stored in the storage unit that indicates a correlation between the accelerator operation amount signal and the required power.

[0038] Subsequently, the control unit 20 determines whether the calculated required power is greater than or equal to 0 (a first threshold) and less than a second threshold greater than 0 (step S14). For example, a value greater than or equal to 70% of the maximum output power of the second FC stack 11 and less than or equal to 100% of the same can be used as the second threshold. The second threshold can be determined from the maximum output power in an initial state of the second FC stack 11 or can be determined from the maximum output power obtained at a predetermined time after the second FC stack 11 has been operating.

[0039] If it is determined in step S14 that the required power is greater than or equal to 0 (the first threshold) and less than the second threshold (YES in step S14), the control unit 20 controls the component units of the fuel cell system 100 such that the power generation of the first FC stack 10 is stopped and the second FC stack 11 generates electric power to provide the required power using the second FC stack 11 (step S16). At this time, the control unit 20 turns the switch 86b ON so that the second FC stack 11 is electrically connected to the motor generator 83, and turns the switch 86a OFF so that the first FC stack 10 is electrically disconnected from the motor generator 83.The control unit 20 drives the air compressor 42, the injector 74, and the like so that air and hydrogen necessary for power generation to provide the required power are supplied to the second FC stack 11. The control unit 20 can stop the driving of the air compressor 32, the injector 54, and the like so that air and hydrogen are not supplied to the first FC stack 10, or can drive the air compressor 32, the injector 54, and the like so that air and hydrogen are supplied to the first FC stack 10. Even when air and hydrogen are supplied to the first FC stack 10, the switch 86a electrically connecting the first FC stack 10 to the motor generator 83 is in the OFF state, and thus the first FC stack 10 does not generate electric power.

[0040] By performing the control of steps S14 and S16 as shown in Fig. 6, the power generation of the second FC stack 11 provides the required power in a time in which the required power for the first FC stack 10 and the second FC stack 11 is between 0 (the first threshold) and the second threshold.

[0041] If it is determined in step S14 that the required power is not in the range greater than or equal to 0 (the first threshold) and less than the second threshold (NO in step S14), the control unit 20 determines whether the required power is greater than or equal to the second threshold and less than a third threshold that is greater than the second threshold (step S18).

[0042] If it is determined in step S18 that the required power is greater than or equal to the second threshold and less than the third threshold (YES in step S18), the control unit 20 controls the component units of the fuel cell system 100 such that both the first FC stack 10 and the second FC stack 11 generate electric power to provide the required power (step S20).

[0043] That is, the control unit 20 drives the air compressor 32, the injector 54, and the like to supply air and hydrogen to the first FC stack 10. The control unit 20 drives the air compressor 42, the injector 74, and the like to supply air and hydrogen to the second FC stack 11. At this time, the control unit 20 turns the switches 86a and 86b ON, so that the first FC stack 10 and the second FC stack 11 are electrically connected to the motor generator 83. Accordingly, as shown in Fig. 6, the required power is provided by power generation of both the first FC stack 10 and the second FC stack 11 during the time in which the required power is greater than or equal to the second threshold and less than the third threshold.

[0044] If it is determined in step S18 that the required power is not within the range greater than or equal to the second threshold and less than the third threshold (NO in step S18), the control unit 20 determines whether the required power is greater than or equal to the third threshold and less than a fourth threshold greater than the third threshold (step S22). For example, a value greater than or equal to 70% of the maximum output power of the first FC stack 10 and less than or equal to 100% of the same may be used as the fourth threshold. The fourth threshold may be determined from the maximum output power in the initial state of the first FC stack 10 or may be determined from the maximum output power obtained at a predetermined time after the first FC stack 10 has been operating.

[0045] If it is determined in step S22 that the required power is greater than or equal to the third threshold and less than the fourth threshold (YES in step S22), the control unit 20 controls the component units of the fuel cell system 100 such that the power generation of the second FC stack 11 is stopped and the first FC stack 10 is caused to generate electric power to provide the required power with the aid of the first FC stack 10 (step S24). At this time, the control unit 20 turns the switch 86a ON so that the first FC stack 10 is electrically connected to the motor generator 83, and turns the switch 86b OFF so that the second FC stack 11 is electrically disconnected from the motor generator 83.The control unit 20 drives the air compressor 32, the injector 54, and the like so that air and hydrogen necessary for power generation to provide the required power are supplied to the first FC stack 10. The control unit 20 can stop the driving of the air compressor 42, the injector 74, and the like so that air and hydrogen are not supplied to the second FC stack 11, or can drive the air compressor 42, the injector 74, and the like so that air and hydrogen are supplied to the second FC stack 11. Even when air and hydrogen are supplied to the second FC stack 11, the switch 86b electrically connecting the second FC stack 11 to the motor generator 83 is in the OFF state, and thus the second FC stack 11 does not generate electric power. Accordingly, as shown in FIG. Fig. 6 illustrates, the required power is provided by power generation of the first FC stack 10 during the time in which the required power is greater than or equal to the third threshold and less than the fourth threshold.

[0046] The control of causing both the first FC stack 10 and the second FC stack 11 to generate electric power when the required power is greater than or equal to the second threshold and less than the third threshold is performed to switch between a case where the required power is less than the second threshold, in which the required power is provided exclusively by the second FC stack 11, and a case where the required power is greater than or equal to the third threshold, in which the required power is provided exclusively by the first FC stack 10.When the power generation of the first FC stack 10 and the second FC stack 11 is stopped by turning the switches 86a and 86b OFF while a sufficient amount of reaction gas is supplied to the first FC stack 10 and the second FC stack 11, the output power of the first FC stack 10 and the second FC stack 11 can be rapidly increased by turning the switches 86a and 86b ON. In this case, the second threshold and the third threshold may be the same value.

[0047] If it is determined in step S22 that the required power is greater than or equal to the fourth threshold (NO in step S22), the control unit 20 causes both the first FC stack 10 and the second FC stack 11 to generate electric power so that the required power is provided (step S26). Accordingly, as shown in Fig. 6, the required power is provided by power generation of both the first FC stack 10 and the second FC stack 11 during the time the required power is greater than or equal to the fourth threshold. At this time, the control unit 20 turns the switches 86a and 86b ON, so that the first FC stack 10 and the second FC stack 11 are electrically connected to the motor generator 83.

[0048] Subsequently, the control unit 20 determines whether an accelerator operation amount signal with an operation amount other than zero is acquired from the accelerator pedal sensor 67 (step S28). If an accelerator operation amount signal with an operation amount other than zero is acquired (YES in step S28), the control unit 20 returns to step S12. On the other hand, if an accelerator operation amount signal with an operation amount other than zero is not acquired (NO in step S28), the control unit 20 stops the power generation of the first FC stack 10 and the second FC stack 11 (step S30) and terminates the power generation control.

[0049] Fig. Fig. 7 is a diagram illustrating power generation control according to the first embodiment. When Fig. 7, the total sum of the maximum output power of the first FC stack 10 and the maximum output power of the second FC stack 11 (hereinafter also referred to as a maximum total power) is defined as 100%, the maximum output power of the first FC stack 10 is assumed to be 70% and the maximum output power of the second FC stack 11 is assumed to be 30%. The second threshold is assumed to be 100% of the maximum output power of the second FC stack 11, that is, 30% of the maximum total power, and the fourth threshold is assumed to be 100% of the maximum output power of the first FC stack 10, that is, 70% of the maximum total power. The third threshold is assumed to be 35% of the maximum total power.

[0050] As in Fig. As illustrated in Figure 7, when the required power is less than 30% of the maximum total power (less than the second threshold), the required power is provided by power generation of the second FC stack 11. When the required power is greater than or equal to 35% of the maximum total power and less than 70% of the maximum total power (greater than or equal to the third threshold and less than the fourth threshold), the required power is provided by power generation of the first FC stack 10.When the required power is greater than or equal to 30% of the maximum total power and less than 35% of the maximum total power (greater than or equal to the second threshold and less than the third threshold), and when the required power is greater than or equal to 70% of the maximum total power (greater than or equal to the fourth threshold), the required power is provided by power generation of both the first FC stack 10 and the second FC stack 11.

[0051] For example, when the maximum output power of the first FC stack 10 and the maximum output power of the second FC stack 11 are equal and the required power is greater than 50% of the maximum total power, the required power is provided by power generation of both the first FC stack 10 and the second FC stack 11. That is, the required power can be provided by power generation of only one of the first FC stack 10 and the second FC stack 11 only when the required power is less than or equal to 50% of the maximum total power. Accordingly, in the first embodiment, since the maximum output power of the first FC stack 10 is greater than the maximum output power of the second FC stack 11, a time during which the first FC stack 10 and / or the second FC stack 11 alone generates electric power can be extended, as shown in Fig. 7. In other words, the time for which at least one of the first FC stack 10 and the second FC stack 11 stops power generation can be extended. Accordingly, it is possible to suppress deterioration due to a potential change at the time of power generation of the first FC stack 10 and / or the second FC stack 11 and improve the durability of the first FC stack 10 and / or the second FC stack 11.

[0052] Fig. Fig. 8 is a flowchart illustrating refresh control according to the first embodiment. The refresh control is repeatedly performed at predetermined time intervals during operation of the fuel cell system. Here, the predetermined time intervals may be, for example, 1 second, 1 minute, or 1 hour. As shown in Fig. As illustrated in Figure 8, the control unit 20 determines whether a refresh process is necessary for the first FC stack 10 (step S40). Whether a refresh process is necessary can be determined, for example, based on at least one of (1) to (4). (1) It is determined that a refresh process is necessary when a voltage value of the first FC stack 10 at a predetermined current density is less than a threshold value. (2) It is determined that a refresh process is necessary when a time elapsed since a previous refresh process on the first FC stack 10 is greater than or equal to a predetermined time. (3) It is determined that a refresh process is necessary when an operation time of the first FC stack 10 since a previous refresh process on the first FC stack 10 is greater than or equal to a predetermined time. (4) It is determined that a refresh process is necessary when a travel distance of a vehicle in which the fuel cell system 100 is mounted since a previous refresh process on the first FC stack 10 is greater than or equal to a predetermined distance.

[0053] If it is determined that a refresh process is not necessary for the first FC stack 10 (NO in step S40), the control unit 20 terminates the refresh control. On the other hand, if it is determined that a refresh process is necessary for the first FC stack 10 (YES in step S40), the control unit 20 performs step S42.

[0054] In step S42, the control unit 20 calculates a required power for the first FC stack 10 and the second FC stack 11 based on an acquired accelerator pedal operation amount signal. Subsequently, the control unit 20 determines whether the required power for the first FC stack 10 and the second FC stack 11 has increased to or above a first predetermined value from a state where the required power is less than the first predetermined value, which is greater than 0 (the first threshold value) and less than the second threshold value (step S44). For example, the first predetermined value may be set to an intermediate value between 0 (the first threshold value) and the second threshold value, as shown in Fig. 6 illustrates.

[0055] If it is determined that the required power has increased from a value smaller than the first predetermined value to or above the first predetermined value (YES in step S44), the control unit 20 performs a refresh process on the first FC stack 10 until the required power reaches the second threshold value (step S46). That is, in Fig. 6, the refresh process is performed on the first FC stack 10 for a time A. The refresh process can be performed, for example, by turning the switch 86a ON to allow a predetermined output current to flow through the first FC stack 10 in a state where a supply of air to the first FC stack 10 is stopped or an amount of air to be supplied is reduced, and by temporarily reducing the voltage of the first FC stack 10 to a target voltage. The refresh process can be performed by turning the switch 86a ON to allow a large current to flow in the first FC stack 10 in a state where a sufficient amount of reactant gas is supplied to the first FC stack 10, and by temporarily reducing the voltage of the first FC stack 10 to a target voltage.

[0056] If it is determined that the required power has not increased to or above the first predetermined value (NO in step S44), the control unit 20 determines whether an accelerator operation amount signal with an operation amount other than zero is continuously acquired (step S48). If an accelerator operation amount signal with an operation amount other than zero is continuously acquired (YES in step S48), the control unit 20 returns to step S42. On the other hand, if an accelerator operation amount signal with an operation amount other than zero is not acquired (NO in step S48), the control unit 20 ends the refresh control.

[0057] According to the first embodiment, as shown in Fig. 5 and Fig. 6, the control unit 20 stops the power generation of the first FC stack 10 when the required power is greater than or equal to 0 (the first threshold) and less than the second threshold, and causes the first FC stack 10 to generate electric power when the required power is greater than or equal to the second threshold. If, as shown in Fig. 6 and Fig. 8 illustrates, the required power has increased to or above the first predetermined value from a state where the required power is less than the first predetermined value, which is greater than the first threshold value and less than the second threshold value, and the required power is in a range greater than or equal to the first predetermined value and less than the second threshold value, the control unit 20 performs a refresh process on the first FC stack 10. When the required power has increased to or above the first predetermined value, there is a high probability that the first FC stack 10 will start power generation.Accordingly, by performing the refresh process on the first FC stack 10 when the required power is in the range greater than or equal to the first predetermined value and less than the second threshold, there is a high probability that the first FC stack 10 will start power generation shortly after an oxide layer and impurities on a catalyst surface are removed by the refresh process. Accordingly, the first FC stack 10 is prevented from being maintained at a high potential for a long time in a state in which an oxide layer and impurities on the catalyst surface are removed by the refresh process.Since there is a high probability that the first FC stack 10 will start power generation shortly after an oxide layer and contaminants on the catalyst surface are removed by the rejuvenation process, the first FC stack 10 generates electric power in a state where the formation of an oxide layer on the catalyst surface and the deposition of contaminants thereon are inhibited. Thus, a rejuvenation process can be performed on the first FC stack 10 at an appropriate time when deterioration of the power generation capability is unlikely.Since a refresh process is performed on the first FC stack 10 when there is a high probability that the first FC stack 10 will start power generation, a refresh process can be prevented from being performed prematurely, and losses due to heat radiation and cooling of the first FC stack 10 at the time of the refresh process can be prevented.

[0058] As in Fig. As illustrated in FIG. 8, the control unit 20 preferably determines whether a refresh process is necessary for the first FC stack 10, and performs a refresh process on the first FC stack 10 when it is determined that a refresh process is necessary. Accordingly, since a refresh process is not performed when a refresh process is not necessary, power consumption and fuel consumption for performing a refresh process can be inhibited.

[0059] Preferably, the control unit 20 turns the switch 86a OFF when power generation of the first FC stack 10 is to be stopped, and turns the switch 86a from OFF to ON when a refresh process is to be performed on the first FC stack 10. Accordingly, it is possible to perform a refresh process on the first FC stack 10 while improving the durability of the first FC stack 10 by more reliably stopping the power generation of the first FC stack 10.

[0060] The refresh process on the first FC stack 10 is preferably performed by passing a current through the first FC stack 10 without supplying air thereto. Accordingly, since the output power of the first FC stack 10 can be kept low at the time of the refresh process, it is possible to suppress an influence of the output power of the first FC stack 10 on the required power.

[0061] In the first embodiment, the first predetermined value is set to the middle value between the first threshold and the second threshold, but may be set to a smaller value than the middle value or a larger value than the middle value if the value is larger than the first threshold and smaller than the second threshold. For example, the first predetermined value may be set to a larger power value among power values ​​on two lines that divide values ​​between the first threshold and the second threshold into three parts, may be set to a largest power value among power values ​​on three lines that divide values ​​between the first threshold and the second threshold into four parts, or may be set to a largest power value among power values ​​on four lines that divide values ​​between the first threshold and the second threshold into five parts.Since a refresh process on the first FC stack 10 can preferably be performed when a power generation start of the first FC stack 10 is approaching, the first predetermined value is preferably set close to the second threshold.

[0062] In the first embodiment, a refresh process is performed on the first FC stack 10, but in a second embodiment, a refresh process is performed on the second FC stack 11 in addition to the first FC stack 10. The configuration of a fuel cell system according to the second embodiment is the same as that shown in Fig. 1 illustrated in the first embodiment, its electrical configuration is the same as that shown in Fig. 4 in the first embodiment, and thus, a repetition of their description will be omitted. The power generation control in the second embodiment is the same as in Fig. 5 illustrated in the first embodiment, the refresh control for the first FC stack 10 is the same as that shown in Fig. 8 illustrated in the first embodiment, and thus, repetition of their description will be omitted.

[0063] Fig. 9 is a flowchart illustrating refresh control according to a second embodiment. Fig. 10 is a timing chart illustrating power generation control and refresh control according to the second embodiment. As shown in Fig. 9, the control unit 20 determines whether a refresh process is necessary for the second FC stack 11 (step S60). Whether a refresh process is necessary for the second FC stack 11 can be determined in the same manner as in step S40 of Fig. 8 determines whether a refresh process is necessary for the first FC stack 10.

[0064] If it is determined that a refresh process is not necessary for the second FC stack 11 (NO in step S60), the control unit 20 terminates the refresh control. On the other hand, if it is determined that a refresh process is necessary for the second FC stack 11 (YES in step S60), the control unit 20 performs step S62.

[0065] In step S62, the control unit 20 calculates a required power for the first FC stack 10 and the second FC stack 11 based on an acquired accelerator pedal operation amount signal. The control unit 20 determines whether the required power for the first FC stack 10 and the second FC stack 11 has increased to or above a second predetermined value from a state where the required power is less than the second predetermined value, greater than the third threshold value, and less than the fourth threshold value (step S64). For example, the second predetermined value may be set to an intermediate value between the third threshold value and the fourth threshold value, as shown in Fig. 10 illustrates.

[0066] If it is determined that the required power has increased from a value smaller than the second predetermined value to or above the second predetermined value (YES in step S64), the control unit 20 performs a refresh process on the second FC stack 11 until the required power reaches the fourth threshold value (step S66). That is, in Fig. 10, the refresh process is performed on the second FC stack 11 for a time B. Analogous to the refresh process on the first FC stack 10, the refresh process on the second FC stack 11 can be performed, for example, by turning the switch 86b ON to pass a predetermined output current through the second FC stack 11 in a state where the supply of air to the second FC stack 11 is stopped or an air amount is reduced, and by temporarily reducing the voltage of the second FC stack 11 to a target voltage. The refresh process on the second FC stack 11 can be performed by turning the switch 86b ON to allow a large current to flow in the second FC stack 11 in a state where a sufficient amount of reaction gas is supplied to the second FC stack 11, and by temporarily reducing the voltage of the second FC stack 11 to a target voltage.

[0067] If it is determined that the required power has not increased from a value smaller than the second predetermined value to or above the second predetermined value (NO in step S64), the control unit 20 determines whether an accelerator operation amount signal with an operation amount other than zero is continuously acquired (step S68). If an accelerator operation amount signal with an operation amount other than zero is continuously acquired (YES in step S68), the control unit 20 returns to step S62. On the other hand, if an accelerator operation amount signal with an operation amount other than zero is not acquired (NO in step S68), the control unit 20 ends the refresh control.

[0068] According to the second embodiment, as shown in Fig. 10, the control unit 20 stops the power generation of the second FC stack 11 when the required power is greater than or equal to the third threshold and less than the fourth threshold, and causes the second FC stack 11 to generate electric power when the required power is greater than or equal to the fourth threshold and when the required power is less than the third threshold. If, as shown in Fig. 9 and Fig. 10, the required power has increased to or above the second predetermined value from a state where the required power is less than the second predetermined value, which is greater than the third threshold, and less than the fourth threshold, and the required power is in a range greater than or equal to the second predetermined value and less than the fourth threshold, the control unit 20 performs a refresh process on the second FC stack 11. Accordingly, since a refresh process is performed on the second FC stack 11 when there is a high probability that the second FC stack 11 will start power generation, it is possible to perform a refresh process on the second FC stack 11 at an appropriate timing where deterioration in power generation capability is not likely.

[0069] In the second embodiment, the second predetermined value is set to the middle value between the third threshold value and the fourth threshold value, but may be set to a smaller value than the middle value or to a larger value than the middle value as long as the value is larger than the third threshold value and smaller than the fourth threshold value.For example, the second predetermined value may be set to a larger power value among power values ​​on two lines dividing values ​​between the third threshold and the fourth threshold into three parts, may be set to a largest power value among power values ​​on three lines dividing values ​​between the third threshold and the fourth threshold into four parts, or may be set to a largest power value among power values ​​on four lines dividing values ​​between the third threshold and the fourth threshold into five parts. Since a refresh process can preferably be performed on the second FC stack 11 when a power generation start of the second FC stack 11 is approaching, the second predetermined value is preferably set close to the fourth threshold.

[0070] Fig. 11 is a flowchart illustrating refresh control according to Modified Example 1 of the second embodiment. Fig. 12 is a timing chart illustrating power generation control and refresh control according to Modified Example 1 of the second embodiment. As shown in Fig. 11, the control unit 20 determines whether a refresh process is necessary for the second FC stack 11 (step S70). Whether a refresh process is necessary for the second FC stack 11 can be determined in the same manner as in step S40 of Fig. 8 determines whether a refresh process is necessary for the first FC stack 10.

[0071] If it is determined that a refresh process is not necessary for the second FC stack 11 (NO in step S70), the control unit 20 ends the refresh control. On the other hand, if it is determined that a refresh process is necessary for the second FC stack 11 (YES in step S70), the control unit 20 performs step S72. In step S72, the control unit 20 calculates a required power for the first FC stack 10 and the second FC stack 11 based on an acquired accelerator pedal operation amount signal. The control unit 20 determines whether the required power has decreased to or below a third predetermined value from a state where the required power is greater than the third predetermined value, greater than the third threshold, and less than the fourth threshold (step S74).For example, the third predetermined value may be set to an intermediate value between the third threshold value and the fourth threshold value, as shown in . Fig. 12 illustrates.

[0072] If it is determined that the required power has decreased from a value greater than the third predetermined value to or below the third predetermined value (YES in step S74), the control unit 20 performs a refresh process on the second FC stack 11 until the required power reaches the third threshold value (step S76). That is, in Fig. 12, the refresh process is performed for a time C on the second FC stack 11.

[0073] On the other hand, if it is determined that the required power has not decreased from a value greater than the third predetermined value to or below the third predetermined value (NO in step S74), the control unit 20 determines whether an accelerator operation amount signal with an operation amount other than zero is continuously acquired (step S78). If an accelerator operation amount signal with an operation amount other than zero is continuously acquired (YES in step S78), the control unit 20 returns to step S72. On the other hand, if an accelerator operation amount signal with an operation amount other than zero is not acquired (NO in step S78), the control unit 20 ends the refresh control.

[0074] According to Modified Example 1 of the second embodiment, as shown in Fig. 12, the control unit 20 stops the power generation of the second FC stack 11 when the required power is greater than or equal to the third threshold and less than the fourth threshold, and causes the second FC stack 11 to generate electric power when the required power is greater than or equal to the fourth threshold and when the required power is less than the third threshold. If, as shown in Fig. 11 and Fig. 12, the required power has decreased to or below the third predetermined value from a state where the required power is greater than the third predetermined value, which is greater than the third threshold value and less than the fourth threshold value, and the required power is in a range greater than or equal to the third threshold value and less than or equal to the third predetermined value, the control unit 20 performs a refresh process on the second FC stack 11. Accordingly, since a refresh process is performed on the second FC stack 11 when there is a high probability that the second FC stack 11 will start power generation, a refresh process can be performed on the second FC stack 11 at an appropriate timing where deterioration in power generation capability is not likely.In Modified Example 1 of the second embodiment, unlike the second embodiment, it is possible to perform a refresh process at an appropriate timing on the second FC stack 11 even if the required power does not rise to or above the fourth threshold.

[0075] In Modified Example 1 of the second embodiment, the third predetermined value is set to the middle value between the third threshold value and the fourth threshold value, but may be set to a value smaller than the middle value or to a value larger than the middle value as long as the value is larger than the third threshold value and smaller than the fourth threshold value.For example, the third predetermined value may be set to a smaller power value among power values ​​on two lines dividing values ​​between the third threshold and the fourth threshold into three parts, may be set to a smallest power value among power values ​​on three lines dividing values ​​between the third threshold and the fourth threshold into four parts, or may be set to a smallest power value among power values ​​on four lines dividing values ​​between the third threshold and the fourth threshold into five parts. Since a refresh process can preferably be performed on the second FC stack 11 when a power generation start of the second FC stack 11 is approaching, the third predetermined value is preferably set close to the third threshold.

[0076] The refresh control according to the second embodiment and the refresh control according to Modified Example 1 of the second embodiment can be combined. That is, the refresh process can be performed on the second FC stack 11 in both the Fig. 10 illustrated time B as well as in the Fig. 12 illustrated time C.

[0077] In the first embodiment, the second embodiment, and Modified Example 1 of the second embodiment, the maximum output power of the first FC stack 10 is greater than the maximum output power of the second FC stack 11, but the maximum output power of the first FC stack 10 and the maximum output power of the second FC stack 11 may be the same. In this case, by appropriately setting the thresholds used to switch power generation between the first FC stack 10 and the second FC stack 11, it is possible to perform the same power generation control and refresh control.

[0078] In the first embodiment, the second embodiment, and Modified Example 1 of the second embodiment, two FC stacks (the first FC stack 10 and the second FC stack 11) are provided as a plurality of FC stacks, but three or more FC stacks may be provided. Even if three or more FC stacks are provided, it is also possible to perform the same power generation control and refresh control.

[0079] That is, when the required power is greater than or equal to 0 (the first threshold) and less than the second threshold, power generation of a first FC stack among the plurality of FC stacks is stopped, and the required power is supplied by the FC stacks other than the first FC stack. When the required power is greater than or equal to the second threshold and less than the third threshold, the required power is supplied by power generation of a plurality of FC stacks including a first FC stack and a second FC stack among the plurality of FC stacks. When the required power is greater than or equal to the third threshold and less than the fourth threshold, power generation of a second FC stack among the plurality of FC stacks is stopped, and the required power is supplied by the FC stacks other than the second FC stack.If the required power is greater than or equal to the fourth threshold, the required power is provided by a plurality of FC stacks including the second FC stack among the plurality of FC stacks.

[0080] The switching between power generation of a plurality of FC stacks and stopping power generation is not limited to the case described in the first embodiment, but is applicable to a case where switching between power generation and stopping power generation of an FC stack is performed based on a required power. The first threshold is set to 0, but the invention is not limited to this, and the first threshold may be set to a value greater than 0 and less than the second threshold. It is only necessary to include control for stopping power generation of the first FC stack 10 when the required power is greater than or equal to the first threshold and less than the second threshold, and for causing the first FC stack 10 to perform power generation when the required power is greater than or equal to the second threshold.

[0081] Depending on a method for controlling power generation and stopping power generation of the first FC stack 10 and the second FC stack 11, the power generation of the first FC stack 10 may be stopped even when the required power is not within a range greater than or equal to the first threshold and less than the second threshold. For example, the first FC stack 10 may stop power generation when the required power is greater than or equal to an X-th threshold that is greater than the second threshold and less than a Y-th threshold that is greater than the X-th threshold, and may perform power generation when the required power is greater than or equal to the Y-th threshold.In this case, a refresh process may be performed on the first FC stack 10 until the required power reaches the Y-th threshold after the required power has increased to or above a power value at a bisector that divides values ​​between the X-th threshold and the Y-th threshold into two parts.

[0082] Although embodiments of the invention have been described above, the invention is not limited to a specific embodiment and can be modified and changed in various forms without departing from the gist of the invention.

Claims

[1] Fuel cell system, comprising: a plurality of fuel cell stacks (10, 11), including a first fuel cell stack (10) and a second fuel cell stack (11); a power generation control unit (22) configured to control power generation of the plurality of fuel cell stacks (10, 11) based on a required power for the plurality of fuel cell stacks (10, 11); and a refresh control unit (24) configured to perform a refresh process of reducing a voltage across the plurality of fuel cell stacks (10, 11), wherein the power generation control unit (22) is configured to stop power generation of the first fuel cell stack (10) when the required power is greater than or equal to a first threshold and less than a second threshold that is greater than the first threshold, and to allow the first fuel cell stack (10) to generate electric power when the required power is greater than or equal to the second threshold, and wherein the refresh control unit (24) is configured to perform the refresh process on the first fuel cell stack (10) when the required power changes from a state in which the required power is less than a first predetermined value, which is greater than the first threshold value and less than the second threshold value, to a state in which the required power is greater than or equal to the first predetermined value, and when the required power is in a range that is greater than or equal to the first predetermined value and less than the second threshold value. [2] The fuel cell system according to claim 1, wherein the refresh control unit (24) is configured to determine whether the refresh process is to be performed on the first fuel cell stack (10), and to perform the refresh process on the first fuel cell stack (10) when it is determined that the refresh process is to be performed. [3] The fuel cell system according to claim 2, wherein the refresh control unit (24) determines that the refresh process is to be performed on the first fuel cell stack (10) when at least one of the following conditions (1) to (4) is satisfied: (1) a voltage value of the first fuel cell stack (10) at a predetermined current density is less than a threshold value, (2) a time elapsed since a previous refresh process on the first fuel cell stack (10) is greater than or equal to a first predetermined time, (3) an operating time of the first fuel cell stack (10) since the previous refresh process on the first fuel cell stack (10) is greater than or equal to a second predetermined time, and (4) a travel distance of a vehicle in which the fuel cell system (100) is mounted since the previous refresh process on the first fuel cell stack (10) is greater than or equal to a predetermined distance. [4] A fuel cell system according to any one of claims 1 to 3, further comprising a plurality of switches (86a, 86b) connected between the plurality of fuel cell stacks (10, 11) and an auxiliary machine into which a current supplied from the plurality of fuel cell stacks (10, 11) flows, wherein the power generation control unit (22) is configured to set a switch (86a) of the plurality of switches (86a, 86b) connected between the second fuel cell stack (11) and the auxiliary machine into a non-connected state when the power generation of the first fuel cell stack (10) is stopped, and wherein the refresh control unit (24) is configured to change a state of the switch (86a) from the unconnected state to a connected state when the refresh process is to be performed on the first fuel cell stack (10). [5] The fuel cell system according to any one of claims 1 to 4, wherein the refresh control unit (24) is configured to perform the refresh process by passing a current through the first fuel cell stack (10) without supplying a cathode gas thereto. [6] The fuel cell system according to any one of claims 1 to 5, wherein the first predetermined value is an average value between the first threshold value and the second threshold value. [7] Fuel cell system according to one of claims 1 to 6, wherein the first threshold value is 0. [8] The fuel cell system according to any one of claims 1 to 7, wherein the power generation control unit (22) is configured to stop the power generation of the second fuel cell stack (11) when the required power is greater than or equal to a third threshold value that is greater than or equal to the second threshold value and less than a fourth threshold value that is greater than the third threshold value, and to allow the second fuel cell stack (11) to generate electric power when the required power is greater than or equal to the fourth threshold value and when the required power is less than the third threshold value, and wherein the refresh control unit (24) is configured to perform the refresh process on the second fuel cell stack (11) when the required power changes from a state in which the required power is less than a second predetermined value,which is greater than the third threshold and less than the fourth threshold, changes to a state in which the required power is greater than or equal to the second predetermined value, and when the required power is in a range that is greater than or equal to the second predetermined value and less than the fourth threshold. [9] The fuel cell system according to any one of claims 1 to 8, wherein the power generation control unit (22) is configured to stop the power generation of the second fuel cell stack (11) when the required power is greater than or equal to a third threshold value that is greater than or equal to the second threshold value and less than a fourth threshold value that is greater than the third threshold value, and to allow the second fuel cell stack (11) to generate the electric power when the required power is greater than or equal to the fourth threshold value and when the required power is less than the third threshold value, and wherein the refresh control unit (24) is configured to perform the refresh process on the second fuel cell stack (11) when the required power changes from a state in which the required power is greater than a third predetermined value,which is greater than the third threshold and less than the fourth threshold, changes to a state in which the required power is less than or equal to the third predetermined value, and when the required power is in a range that is greater than or equal to the third threshold and less than or equal to the third predetermined value. [10] The fuel cell system according to claim 8 or 9, wherein a maximum output power of the first fuel cell stack (10) is greater than that of the second fuel cell stack (11).

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