Fuel cell system and control method when fuel cell system is stopped

Abstract

To suppress deterioration of a fuel cell when a fuel cell system is stopped.SOLUTION: When a control device 26 detects an operation stop signal for a fuel cell vehicle 11 (fuel cell system 10), it gradually reduces the power generation voltage Vfc of a fuel cell stack 12 at a first voltage reduction rate Vdr1 and charges the surplus generated power to a power storage device 244. Thereafter, it suddenly reduces the power generation voltage Vfc at a second voltage reduction rate Vdr2 that is higher than the first voltage reduction rate Vdr1, thereby suppressing deterioration of the fuel cell stack 12.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a fuel cell system and a method for controlling the fuel cell system during operation stop.

Background Art

[0002] In recent years, in order to ensure that more people have access to affordable, reliable, sustainable, and advanced energy, research and development on fuel cells that contribute to energy efficiency improvement have been carried out.

[0003] A power generation system equipped with a fuel cell stack is referred to as a fuel cell system. The fuel cell stack includes a plurality of power generation cells. Each power generation cell generates electricity through an electrochemical reaction between a fuel gas (hydrogen-containing gas) and an oxidant gas (oxygen-containing gas) supplied from an air compressor.

[0004] For example, Patent Document 1 discloses a fuel cell system that supplies the generated power of a fuel cell stack to an external load (an inverter-driven motor), an internal load (a circulation pump), a smoothing capacitor, and a lithium-ion capacitor (hereinafter referred to as a capacitor) via a diode and a relay (FIG. 1 of Patent Document 1). When the relay is closed, the power of the capacitor is also supplied to the external load and the internal load (paragraph

[0039] of Patent Document 1).

[0005] In the fuel cell system disclosed in Patent Document 1, when a stop operation (shutdown) request is input, the control unit stops the power generation of the fuel cell stack and shuts off the relay (paragraph

[0049] of the same). Further, when a stop operation request is input, the control unit reduces the stack voltage of the fuel cell stack prior to the voltage of the internal load in order to suppress the deterioration of the fuel cell stack (paragraphs

[0050] -

[0052] of the same).

[0006] With such control, in the fuel cell system disclosed in Patent Document 1, when the operation is stopped, the flow of current from the fuel cell stack to the external load is suppressed (ibid., paragraph

[0053] ).

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] In a fuel cell system, when the operation is stopped, the amount of power that can charge the power storage device with surplus generated power is determined. On the other hand, in a fuel cell system, when the operation is stopped, there is a problem that it is desired to lower the voltage of the fuel cell stack as quickly as possible from the viewpoint of suppressing deterioration of the fuel cell stack.

[0009] However, in the fuel cell system disclosed in Patent Document 1, since the relay that connects the fuel cell stack to the capacitor is cut off when the operation is stopped, the above problems cannot be solved.

[0010] An object of the present invention is to solve the above-described problems.

Means for Solving the Problems

[0011] A first invention is a fuel cell system including a fuel cell that generates power by a fuel gas and an oxidant gas supplied from an air compressor, and a control device that controls the fuel cell, wherein the control device includes an acquisition unit that acquires an operation stop signal, and a control unit that, when the acquisition unit acquires the operation stop signal, lowers the power generation voltage of the fuel cell at a first voltage drop rate and then lowers it at a second voltage drop rate higher than the first voltage drop rate.

[0012] The second invention is a method for controlling the operation stop of a fuel cell system including a fuel cell that generates electricity using fuel gas and oxidant gas supplied from an air compressor, and a control device that controls the fuel cell. When the control device acquires an operation stop signal, it decreases the power generation voltage of the fuel cell at a first voltage decrease rate and then decreases it at a second voltage decrease rate higher than the first voltage decrease rate.

Advantages of the Invention

[0013] According to this invention, when the fuel cell system stops operating, by decreasing the power generation voltage at a first voltage decrease rate, it is possible to perform charge control so as not to exceed the power that can charge the surplus power generation into the power storage device. This suppresses the deterioration of the power storage device, and by decreasing the power generation voltage at a second voltage decrease rate higher than the first voltage decrease rate after the charge control, it is possible to suppress the deterioration of the fuel cell.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0015] [Configuration of Fuel Cell System 10] FIG. 1 is a schematic configuration diagram of a fuel cell vehicle 11 equipped with a fuel cell system 10 according to an embodiment. The fuel cell system 10 can be mounted not only on vehicles but also on ships, aircraft, robots, etc. Further, the fuel cell system 10 can also be used as a power source in facilities, homes, etc. In the fuel cell system 10, a fuel gas and an oxidant gas are used as reaction gases. The fuel gas is a hydrogen-containing gas. The oxidant gas is an oxygen-containing gas such as air. Each of the fuel gas and the oxidant gas is supplied to a fuel cell stack 12 (fuel cell) and subjected to an electrochemical reaction. In this specification, the fuel gas discharged from the fuel cell stack 12 without being subjected to the electrochemical reaction is also referred to as fuel off-gas. Also, in this specification, the oxidant gas discharged from the fuel cell stack 12 without being subjected to the electrochemical reaction is also referred to as oxidant off-gas.

[0016] The fuel cell system 10 includes a fuel cell stack 12, a tank 14, an anode system 16, a cathode system 18, a cooling system 20, a load 21, and a power supply system 27. Further, the fuel cell system 10 includes a control device 26.

[0017] The fuel cell stack 12 includes a positive electrode terminal 23a and a negative electrode terminal 23b. A voltage sensor 25 for detecting the generated voltage Vfc of the fuel cell stack 12 is attached between the power lines extending from the positive electrode terminal 23a and the negative electrode terminal 23b. The electricity generated by the fuel cell stack 12 is supplied to the load 21 and the power supply system 27 via the positive electrode terminal 23a and the negative electrode terminal 23b. The tank 14 is filled with high-pressure fuel gas.

[0018] The fuel cell stack 12 includes a fuel gas supply port 22a for supplying fuel gas into the fuel cell stack 12 and a fuel gas discharge port 22b for discharging fuel off-gas from inside the fuel cell stack 12. The fuel cell stack 12 includes an oxidant gas supply port 22c for supplying oxidant gas into the fuel cell stack 12, an oxidant gas discharge port 22d for discharging oxidant off-gas from inside the fuel cell stack 12, and a drain port 22g for draining the generated water of the fuel cell stack 12. The fuel cell stack 12 includes a coolant supply port 22e for supplying a coolant into the fuel cell stack 12 and a coolant discharge port 22f for discharging the coolant from inside the fuel cell stack 12.

[0019] The anode system 16 includes a fuel gas supply passage 84, a fuel gas discharge passage 86, a circulation passage 88, and a drain passage 90. The anode system 16 includes an injector 94, an ejector 96, a gas-liquid separator 98, and a drain valve 100 of an on-off valve. The fuel gas supply passage 84 is connected to the discharge port of the tank 14 and the fuel gas supply port 22a of the fuel cell stack 12. The fuel gas supply passage 84 is provided with the injector 94 and the ejector 96. The inflow port of the ejector 96 is connected to the discharge port of the injector 94, and the discharge port of the ejector 96 is connected to the fuel gas supply port 22a. A pressure sensor 85 is attached to the fuel gas supply passage 84 near the fuel gas supply port 22a. The ejector 96 is arranged between the injector 94 and the fuel gas supply port 22a. The fuel gas discharge passage 86 is connected to the fuel gas discharge port 22b of the fuel cell stack 12 and the supply port of the gas-liquid separator 98. The circulation passage 88 is connected to the exhaust port of the gas-liquid separator 98 and the suction port of the ejector 96. The drain passage 90 is connected to the drain port of the gas-liquid separator 98 and the discharge passage 109. The discharge passage 109 communicates with the atmosphere through a discharge port 109p provided in the fuel cell vehicle 11. The drain passage 90 is provided with the drain valve 100.

[0020] The cathode system 18 includes an oxidant gas supply passage 106, an oxidant gas discharge passage 108 (discharge passage), a bypass passage 110, and a drain passage 91. The cathode system 18 includes an air cleaner 105, an air compressor 112 (oxidant gas supplier), a humidifier 114, an inlet shutoff valve 116 of an on-off valve, an outlet shutoff valve 118 of a valve opening degree adjustment valve which is a back pressure valve, and a bypass valve 120 of the valve opening degree adjustment valve. The oxidant gas supply passage 106 is connected to an air intake port 106p provided in the fuel cell vehicle 11 and an oxidant gas supply port 22c of the fuel cell stack 12. The oxidant gas supply passage 106 is provided with an air cleaner 105, a flow rate sensor 107, an air compressor 112, an inlet shutoff valve 116, and a humidifier supply passage 114A of the humidifier 114. A portion of the oxidant gas supply passage 106 arranged upstream of the humidifier 114 is referred to as an oxidant gas supply passage 106A. A portion of the oxidant gas supply passage 106 arranged downstream of the humidifier 114 is referred to as an oxidant gas supply passage 106B. The inlet shutoff valve 116 is arranged closer to the humidifier 114 than the air compressor 112. A flow rate sensor 107 is attached to the oxidant gas supply passage 106A upstream of the air compressor 112. The flow rate sensor 107 detects the flow rate of the oxidant gas flowing through the air compressor 112.

[0021] Figure 2 is a schematic diagram of a turbo-type air compressor 112 equipped with an air bearing mechanism. The air compressor 112 has a rotor shaft 274, and an impeller 276 is fixedly attached or integrally rotates with the rotor shaft 274. A plurality of magnets are embedded in the cylindrical side surface of the rotor shaft 274 in the axial direction. In the casing of the air compressor 112, stator coils 272 of U-phase, V-phase, and W-phase are attached. In the casing of the air compressor 112, an air bearing 292 through which the rotor shaft 274 is inserted is attached. In the casing of the air compressor 112, a flow path for the oxidant gas is attached, and a supercharger 294 including the impeller 276 is provided. When the impeller 276 rotates as the rotor shaft 274 rotates, the air bearing 292 floats the rotor shaft 274 by compressed air at a predetermined rotational speed or higher. Air is taken in from an intake port 300 of the air compressor 112 communicating with the oxidant gas supply path 106A, and is supercharged (pressurized and compressed) by the supercharger 294. The pressurized and compressed air, that is, the oxidant gas, is discharged from a discharge port 302 to the oxidant gas supply path 106A.

[0022] Air is taken in from an intake port 300 of the air compressor 112 communicating with the oxidant gas supply path 106A, and is supercharged (pressurized and compressed) by the supercharger 294. The air pressurized and compressed by the supercharger 294, that is, the oxidant gas, is discharged from a discharge port 302 to the oxidant gas supply path 106A.

[0023] Figure 3 is a circuit diagram of a switching-type boost converter 240 that boosts the power generation voltage Vfc as the input voltage to the high-voltage power supply voltage Vh of the power storage device 244. The boost converter 240 includes an inductor 262, a power MOSFET 264, a diode 266 (freewheeling diode), a smoothing capacitor 268, and a discharge resistor 270.

[0024] One end of an inductor 262 is connected to a positive electrode terminal 23a of a fuel cell stack 12. The other end of the inductor 262 is connected to a drain terminal of a MOSFET 264 and an anode terminal of a diode 266. A cathode terminal of the diode 266 is connected to high-voltage terminals of inverters 242, 252, a power storage device 244, and a step-down converter 247. Low-voltage terminals of the inverters 242, 252, the power storage device 244, and the step-down converter 247 are connected to a negative electrode terminal 23b of the fuel cell stack 12. A capacitor 268 and a resistor 270 are connected in parallel between the cathode terminal of the diode 266 and the negative electrode terminal 23b of the fuel cell stack 12. A source terminal of the MOSFET 264 is connected to the negative electrode terminal 23b of the fuel cell stack 12.

[0025] Returning to FIG. 1, an oxidant gas discharge passage 108 is connected to an oxidant gas discharge port 22d of the fuel cell stack 12 and a discharge passage 109. The oxidant gas discharge passage 108 is provided with a humidifier discharge passage 114B of a humidifier 114 and an outlet shutoff valve 118. A portion of the oxidant gas discharge passage 108 disposed upstream of the humidifier 114 is referred to as an oxidant gas discharge passage 108A. A portion of the oxidant gas discharge passage 108 disposed downstream of the humidifier 114 is referred to as an oxidant gas discharge passage 108B. A pressure sensor 111 is attached to the oxidant gas discharge passage 108B. A bypass flow passage 110 is connected to an oxidant gas supply passage 106A between an air compressor 112 and an inlet shutoff valve 116 and the oxidant gas discharge passage 108B downstream of the outlet shutoff valve 118. The bypass flow passage 110 is provided with a bypass valve 120. A drain passage 91 is connected to a drain port 22g and the discharge passage 109. The discharge passage 109 is provided with a drain valve 101 of an on-off valve.

[0026] The cooling system 20 includes a cooling medium supply passage 122 and a cooling medium discharge passage 124. The cooling system 20 includes a pump 126 and a radiator 128. The cooling medium supply passage 122 is connected to the cooling medium discharge port of the radiator 128 and the cooling medium supply port 22e of the fuel cell stack 12. The pump 126 is provided in the cooling medium supply passage 122. The cooling medium discharge passage 124 is connected to the cooling medium discharge port 22f of the fuel cell stack 12 and the fluid supply port of the radiator 128. A temperature sensor 130 is attached to the cooling medium discharge passage 124. The temperature sensor 130 detects the temperature of the cooling medium flowing through the cooling medium discharge passage 124. The temperature of the cooling medium flowing through the cooling medium discharge passage 124 is estimated to be the temperature inside the fuel cell stack 12 (stack temperature).

[0027] The fuel cell stack 12 is formed by stacking a plurality of power generation cells 24. The power generation cell 24 includes an electrolyte membrane / electrode structure 32 and separators 28 and 30 that sandwich the electrolyte membrane / electrode structure 32. The electrolyte membrane / electrode structure 32 includes, for example, an MEA (Membrane Electrode Assembly) 36 that is a thin film of perfluorosulfonic acid containing moisture (solid polymer electrolyte membrane), and a cathode electrode 40 and an anode electrode 38 that sandwich the MEA 36. The cathode electrode 40 and the anode electrode 38 have a gas diffusion layer (not shown) made of carbon paper or the like. A porous carbon particle having a platinum alloy supported on its surface is uniformly applied to the surface of the gas diffusion layer to form an electrode catalyst layer (not shown). The electrode catalyst layer is formed on both sides of the MEA 36.

[0028] On the surface of one separator 28 facing the electrolyte membrane / electrode structure 32, a cathode flow path (oxidant gas flow path) 50 that communicates an oxidant gas supply port 22c and an oxidant gas discharge port 22d is formed. The pressure of the oxidant gas flowing through the cathode flow path 50 is controlled by adjusting the opening degree of the outlet sealing valve 118 by the control device 26.

[0029] On the surface of the separator 30 facing the electrolyte membrane-electrode assembly 32 of the other party, an anode flow path (fuel gas flow path) 66 that communicates the fuel gas supply port 22a and the fuel gas discharge port 22b is formed. In the anode electrode 38, when fuel gas (hydrogen) is supplied, hydrogen ions are generated from hydrogen molecules by an electrode reaction with a catalyst, and the hydrogen ions permeate the MEA 36 and move to the cathode electrode 40. At the same time, electrons are released from the hydrogen molecules. The electrons released from the hydrogen molecules move from the separator 30 and the negative electrode terminal 23b, through the load 21, and to the cathode electrode 40 via the positive electrode terminal 23a and the separator 28. At the cathode electrode 40, the hydrogen ions, the electrons, and oxygen contained in the supplied oxidant gas react with each other under the action of the catalyst to generate water. The load 21 includes a motor 246 and an air compressor 112 as a high-voltage load.

[0030] The power supply system 27 includes a power storage device 248 that is a low-voltage power supply for generating a low-voltage power supply voltage Vl, and a power storage device 244 that is a high-voltage power supply for generating a high-voltage power supply voltage Vh. Here, a lead storage battery is used as the power storage device 248. A lithium-ion secondary battery or the like may be used instead of the lead storage battery. Here, a lithium-ion secondary battery is used as the power storage device 244. A capacitor or the like may be used instead of the lithium-ion secondary battery.

[0031] Note that temperature sensors (not shown) for detecting the temperatures of the power storage devices 244 and 248 and SOC sensors (not shown) for detecting the state of charge (SOC), which is the remaining capacity of the power storage devices 244 and 248, are attached to the power storage devices 244 and 248, respectively.

[0032] The inverter 242 that drives the motor 246 is supplied with electricity at a high voltage (power supply voltage Vh) from the fuel cell stack 12 via the boost converter 240. Also, the inverter 242 that drives the motor 246 is supplied with electricity at a high voltage (power supply voltage Vh) from the power storage device 244. That is, the motor 246 is supplied with electricity from the fuel cell stack 12 and / or the power storage device 244. The inverter 242 converts the supplied high DC voltage into a three-phase alternating current to drive the motor 246.

[0033] The power storage device 244 is charged with the electric energy generated by the fuel cell stack 12 when a high DC voltage obtained by boosting the power generation voltage Vfc of the fuel cell stack 12 by the boost converter 240 is applied thereto. When the fuel cell vehicle 11 decelerates and the motor 246 regenerates, the motor 246 generates a three-phase alternating current as a generator. This three-phase alternating current is converted into electric energy at a high DC voltage by the inverter 242 to charge the power storage device 244.

[0034] The control device 26 as a low-voltage load, various sensors, and an air conditioner, an electric power steering device, a lighting device, etc. (not shown) are supplied with electricity at a low DC voltage (power supply voltage Vl) from the power storage device 248. The power storage device 248 is charged with the electric energy obtained by stepping down the high voltage (power supply voltage Vh) of the power storage device 244 to a low voltage (power supply voltage Vl) by the buck converter 247.

[0035] The air compressor 112 is supplied with a three-phase alternating current obtained by converting the high voltage from the power storage device 244 by the inverter 252.

[0036] The control device 26 can be configured by an ECU (Electronic Control Unit). The control device 26 includes an arithmetic unit 136 and a storage unit 138. The arithmetic unit 136 is a processor such as a CPU (Central Processing Unit) or a GPU (Graphics Processing Unit), for example. That is, the arithmetic unit 136 can be constituted by a processing circuitry. The arithmetic unit 136 controls each device by executing computer-executable instructions (programs) stored in the storage unit 138. At least a part of the arithmetic unit 136 may be realized by an integrated circuit such as an ASIC (Application Specific Integrated Circuit) or an FPGA (Field-Programmable Gate Array). At least a part of the arithmetic unit 136 may be realized by an electronic circuit including discrete devices.

[0037] The arithmetic unit 136 includes an acquisition unit 140, a control unit 142, a timing unit 144, and a determination unit 146. The acquisition unit 140 acquires information from electronic components (sensors, ECUs, etc.) other than the control device 26.

[0038] In addition to the illustrated voltage sensor 25, pressure sensors 85 and 111, and temperature sensor 130, the sensors include a current sensor (not shown) for detecting the generated current Ifc, a voltage sensor for detecting the power supply voltage Vh, a voltage sensor for detecting the power supply voltage Vl, etc.

[0039] The control unit 142 executes computer-executable instructions (programs) based on various signals acquired from the sensors through the acquisition unit 140, and controls the operations of the injector 94, air compressor 112, pump 126, motor 246, boost converter 240, inverters 242 and 252, buck converter 247, valves, etc. The timing unit 144 measures the execution time, etc. using a timer (not shown). The determination unit 146 performs various determination processes.

[0040] The memory unit 138 is composed of a volatile memory (not shown) and a non-volatile memory (not shown), which are computer-readable storage media. The volatile memory is, for example, a RAM (Random Access Memory) or the like. The non-volatile memory is, for example, a ROM (Read Only Memory), a flash memory, or the like. Data and the like are recorded in the volatile memory, for example. Programs, tables, maps, and the like are recorded in the non-volatile memory, for example. At least a part of the memory unit 138 may be provided in the above-described processor, integrated circuit, or the like.

[0041] A power switch 280 (operation switch) through which a user instructs the start (power generation start) and stop (operation stop or power generation stop) of the fuel cell vehicle 11 (fuel cell system 10) is connected to the control device 26.

[0042] [Fluid Flow in Fuel Cell System 10] (1) Fluid Flow in Anode System 16 The injector 94 is pulse-width modulated (PWM) by the control device 26 so that the pressure in the anode flow path 66 of the fuel cell stack 12 detected by the pressure sensor 85 becomes the set pressure. The injector 94 injects the fuel gas supplied from the tank 14 downstream of the fuel gas supply path 84 for a prescribed time during which the on-duty INJod (INJod = valve opening time / control cycle) of the PWM control is defined. The fuel gas injected from the injector 94 is supplied to the anode flow path 66 from the fuel gas supply port 22a of the fuel cell stack 12 via the fuel gas supply path 84. The fuel gas that did not react inside the fuel cell stack 12 is discharged as fuel off-gas from the fuel gas discharge port 22b of the fuel cell stack 12. The fuel off-gas contains hydrogen that did not react with oxygen, nitrogen in the oxidant gas that permeated through the MEA 36, and moisture generated by the reaction between oxygen and hydrogen.

[0043] The fuel off-gas is supplied to the gas-liquid separator 98 via the fuel gas discharge passage 86. The gas-liquid separator 98 separates the fuel off-gas into a gas component (fuel off-gas) and a liquid component (water). The fuel off-gas discharged from the gas-liquid separator 98 is supplied to the suction port of the ejector 96 via the circulation passage 88. The ejector 96 merges the fuel off-gas supplied from the suction port with the fuel gas supplied from the input port. The ejector 96 supplies the merged gas as fuel gas from the discharge port through the fuel gas supply passage 84 and the fuel gas supply port 22a to the anode flow passage 66 of the fuel cell stack 12.

[0044] (2) Flow of fluid in the cathode system 18 The air compressor 112 heats and compresses the oxidant gas (air) inhaled from outside the fuel cell vehicle 11 and discharges it downstream of the oxidant gas supply passage 106. The oxidant gas discharged from the air compressor 112 is supplied from the oxidant gas supply port 22c of the fuel cell stack 12 to the cathode flow passage 50 via the oxidant gas supply passage 106 provided with a cooler (not shown). The oxidant gas that did not react inside the fuel cell stack 12 is discharged as oxidant off-gas from the oxidant gas discharge port 22d of the fuel cell stack 12. The oxidant off-gas contains each component mainly composed of nitrogen contained in the oxidant gas and moisture generated by the reaction of oxygen and hydrogen.

[0045] The oxidant off-gas is discharged to the outside of the fuel cell vehicle 11 via the oxidant gas discharge passage 108. The oxidant off-gas contains moisture. In the humidifier 114, a part of the moisture contained in the oxidant off-gas is used to humidify the oxidant gas flowing through the humidifier supply passage 114A through the hollow fiber membrane in the humidifier 114.

[0046] (3) Flow of fluid in the cooling system 20 The pump 126 discharges the cooling medium toward the cooling medium supply port 22e of the fuel cell stack 12. The cooling medium discharged from the pump 126 is supplied to the cooling medium supply port 22e of the fuel cell stack 12 via the cooling medium supply passage 122. The cooling medium that has circulated inside the fuel cell stack 12 is discharged from the cooling medium discharge port 22f of the fuel cell stack 12. The cooling medium discharged from the cooling medium discharge port 22f is supplied to the radiator 128 via the cooling medium discharge passage 124. The cooling medium that has dissipated heat in the radiator 128 is sucked into the pump 126.

[0047] [Operation Explanation by Flowchart] The fuel cell system 10 is basically configured as described above. Next, the operation (operation stop process) when the fuel cell system 10 (fuel cell stack 12) stops operating will be described with reference to the flowchart of FIG. 4.

[0048] In step S1, the determination unit 146 determines whether the operation stop signal output from the power switch 280 has been detected by the acquisition unit 140 and / or whether an operation stop request to stop the power generation operation has occurred. The operation stop signal is output from the power switch 280 when the power switch 280 is switched from the on state to the off state. Even when the power switch 280 is in the on state, a power generation stop request may occur depending on the situation.

[0049] Note that when the power switch 280 is in the on state and the fuel cell vehicle 11 is stopped (vehicle speed is 0) during the power generation operation (during the idling power generation of the fuel cell stack 12), the control unit 142 controls each component as follows.

[0050] The control unit 142 drives the air compressor 112 at the normal rotational speed. The control unit 142 sets the generated voltage Vfc to a predetermined voltage command value (generated voltage command value) Vfccom. The control unit 142 sets the on-duty INJod of the injector 94 to the normal on-duty so that the fuel gas pressure (anode pressure) Fh detected by the pressure sensor 85 and acquired by the acquisition unit 140 becomes a predetermined pressure. The control unit 142 closes the bypass valve 120, opens the inlet sealing valve 116, opens the outlet sealing valve 118 to a predetermined opening degree, closes the drain valve 100, and opens the drain valve (cathode drain valve) 101. The control unit 142 detects the opening degrees of the bypass valve 120 and the outlet sealing valve 118 through the acquisition unit 140.

[0051] When the control unit 142 determines that the determination unit 146 has not generated an operation stop request (step S1: NO), the control unit 142 continues the idling power generation of the fuel cell stack 12 at the vehicle speed 0 (while stopped) of the fuel cell vehicle 11 in step S1.

[0052] On the other hand, when the control unit 142 determines that the determination unit 146 has generated an operation stop request (step S1: YES), the control unit 142 shifts the processing by the control unit 142 to step S2.

[0053] In step S2, the control unit 142 performs initial settings at the time of operation stop. At the stage of operation stop control of the fuel cell system 10, basically, the state of the SOC (remaining capacity) of the power storage device 244 is high. Therefore, in this initial setting, the control unit 142 sets the voltage drop rate Vdr [V / sec], which is the drop rate of the voltage command value Vfccom of the fuel cell stack 12, to the first voltage drop rate Vdr1 [V / sec] so that it becomes equal to or less than the power that the power storage device 244 can accept. The first voltage drop rate Vdr1 is set in consideration of the fact that the remaining oxygen concentration in the cathode flow path 50 is high, and to achieve a gentle charging rate that the power storage device 244 can withstand (without deterioration). With this setting, the power storage device 244 is charged by the suppressed power generation of the fuel cell stack 12. By this control, the surplus power generation power of the fuel cell stack 12 at the time of operation stop can be charged to the power storage device 244 without exceeding the power that can be charged to the power storage device 244.

[0054] Here, the boost ratio (Vh / Vfc) of the boost converter 240 for setting the voltage drop rate Vdr is given by the following equation (1).

[0055] (Vh / Vfc)={1 / (1-D)} …(1) The duty D is the ratio of the on-time ton of the MOSFET 264 to the control period (constant period of PWM) T of the boost converter 240, and is calculated as D=(ton / T). The control unit 142 gently increases the on-time ton while maintaining the first voltage drop rate Vdr1 (approaching the control period T). Since the power supply voltage Vh of the power storage device 244 hardly changes, the power generation voltage Vfc can be decreased at the first voltage drop rate Vdr1 by increasing the boost ratio by the absolute value of the first voltage drop rate Vdr1.

[0056] In step S2, the control unit 142 sets the rotation speed Nac [rpm] of the air compressor 112 to a predetermined rotation speed Nacpd that is slightly higher than the rotation speed at which the rotor shaft 274 can float by the air bearing 292 and lower than the normal rotation speed during traveling. On the other hand, in step S2, the control unit 142 controls the on-duty INJod of the injector 94 to decrease so that the anode pressure Ph becomes a specified hydrogen partial pressure and controls it near a predetermined on-duty INJodpd.

[0057] In step S3, the control unit 142 performs processing at the first voltage drop rate Vdr1 and ends the processing at the first voltage drop rate Vdr1 when the time when the residual oxygen concentration becomes equal to or less than a predetermined value has elapsed.

[0058] In step S4, the determination unit 146 detects the power generation voltage Vfc of the fuel cell stack 12 through the voltage sensor 25 and the acquisition unit 140, and determines whether or not the detected power generation voltage Vfc has dropped to the threshold power generation voltage Vfcth. If it has not dropped to the threshold power generation voltage Vfcth (step 4: NO), the control unit 142 repeats the processing of steps S3 and S4. When the power generation voltage Vfc has dropped to the threshold power generation voltage Vfcth (step 4: YES), the process proceeds to step S5.

[0059] Here, the threshold power generation voltage Vfcth is a voltage obtained by adding a margin voltage to the lower limit voltage at which the fuel cell stack 12 does not deteriorate. In the fuel cell stack 12, it is preferable that the time during which the power generation voltage Vfc stays in the deterioration voltage range between this lower limit voltage and the value of 0 is as short as possible from the viewpoint of deterioration suppression and prevention.

[0060] In step S5, the control unit 142 performs each of the control processes described below related to the drainage preparation process of the cathode-generated water. The inlet shutoff valve 116 is controlled to be in an open valve continuous state. The outlet shutoff valve 118 is controlled to be in a closed valve state. The drain valve 101 is controlled to be in an open valve continuous state. The rotation speed Nac of the air compressor 112 (rotor shaft 274) is controlled to be in a continuous state at the predetermined rotation speed Nacpd set in step S2. The on-duty INJod [%] of the injector 94 is controlled to be in a continuous state at the predetermined on-duty INJodpd [%] for maintaining the hydrogen partial pressure set in step S2.

[0061] In step S6, the control unit 142 performs a forced drainage process of the liquid water including the generated water stored in the cathode flow path 50 for a predetermined time while being timed by the timer unit 144. For this purpose, during the predetermined time, the control unit 142 slightly increases the rotation speed Nac of the air compressor 112 from the predetermined rotation speed Nacpd when the rotation speed Nac of the air compressor 112 was decreased at the first voltage drop rate Vdr1. Also, with the outlet shutoff valve 118 closed, the oxidant gas remaining in the cathode flow path 50 of the fuel cell stack 12 is consumed. To consume the oxidant gas, the control unit 142 slightly increases the predetermined on-duty INJodpd [%] of the injector 94 to maintain the hydrogen partial pressure during the predetermined time. In this way, the cathode-generated water stored in the cathode flow path 50 is discharged from the fuel cell stack 12 by the pressure of the oxidant gas supplied from the air compressor 112 during the predetermined time when the rotation speed Nac [rpm] of the air compressor 112 slightly increases from the predetermined rotation speed Nacpd. That is, the compressed air discharged from the air compressor 112 flows through the oxidant gas supply path 106 and through the cathode flow path 50. The compressed air that has flowed through the cathode flow path 50 is discharged to the outside of the fuel cell vehicle 11 through the drain port 22g, the drain path 91, the drain valve 101, the discharge path 109, and the discharge port 109p, together with the liquid water including the cathode-generated water stored in the cathode flow path 50 of the fuel cell stack 12.

[0062] After the drainage process in step S6, the control unit 142 closes the drain valve 101 and returns the rotation speed Nac of the air compressor 112 to the predetermined rotation speed Nacpd.

[0063] In step S7, the control unit 142 performs an end period setting at the time of operation stop. In the end period setting, the control unit 142 drives the inlet sealing valve 116 to a closed state. At the same time, the voltage drop rate Vdr [V / sec], which is the drop rate of the voltage command value Vfccom of the fuel cell stack 12, is changed (set) to a second voltage drop rate Vdr2 [V / sec] higher than the first voltage drop rate Vdr1 [V / sec]. The second voltage drop rate Vdr2 is a drop rate for reducing the power generation voltage Vfc toward 0 [V] at a high voltage drop rate Vdr [V / sec] in order to avoid deterioration of the fuel cell stack 12 because the outlet sealing valve 118 has already been closed, the residual oxygen concentration in the cathode flow path 50 has decreased, and the electrochemical reaction does not proceed.

[0064] Here, since the voltage drop rate Vdr is a negative value, it is set so that Vdr2 > Vdr1 and |Vdr1| < |Vdr2|. With this setting, the control unit 142 continues the drop process of the power generation voltage Vfc at the second voltage drop rate Vdr2 until the power generation voltage Vfc in step 8 becomes equal to or lower than a predetermined voltage. The predetermined voltage may continue until the determination process for becoming substantially 0 [V] is established. Instead of this, the predetermined voltage may be the lower limit voltage controllable by the boost converter 240. After dropping below the predetermined voltage, the voltage may be further decreased. The drop process of the power generation voltage Vfc at the second voltage drop rate Vdr2 is a process in which the control unit 142 increases the duty D in the above equation (1) in proportion to the absolute value of the second voltage drop rate Vdr2, which is a high rate, up to 100 [%] {D = 1: Vfc = Vh(1 - D)}. That is, as the duty D is increased, the power supply voltage Vh does not change, so the power generation voltage Vfc decreases toward 0 [V].

[0065] In step S8, the determination unit 146 detects the power generation voltage Vfc of the fuel cell stack 12 through the acquisition unit 140. When the detected power generation voltage Vfc has not decreased to a sufficiently low predetermined voltage, for example, 0 [V] (step S8: NO), the processes of steps S7 and S8 are repeated. When the determination unit 146 detects in step S8 that the power generation voltage Vfc has decreased to the predetermined voltage (step S8: YES), the control unit 142 ends the operation stop time processing.

[0066] [Operation Explanation by Time Chart] An example of the operation (operation stop time processing) described by the flowchart of FIG. 4 will be described with reference to the time chart of FIG. 5. In the time chart of FIG. 5, the waveform at the uppermost stage shows an example of the time change state of the power generation power Wfc of the fuel cell stack 12.

[0067] During the idling power generation of the fuel cell system 10 (fuel cell stack 12), at time t0 in FIG. 5, the control unit 142 detects, through the determination unit 146, an operation stop request based on the transition of the power switch 280 from the on state to the off state (corresponding to step S1: YES). Note that the time when the power switch 280 becomes the off state may be a time before time t0. In that case, power generation continues until time t0, and at time t0, a stop request for the fuel cell system 10 is detected.

[0068] At the detected time t0, the control unit 142 performs an initial setting including the setting process of the first voltage decrease rate Vdr1 of the boost converter 240 (corresponding to step S2).

[0069] In the initial setting, the inlet sealing valve 116 is set to the open (continuous) state, the outlet sealing valve 118 is set to an opening degree of a few [%] corresponding to the idling power generation, the drain valve (cathode drain valve) 101 is set to the open (continuous) state, the rotation speed Nac of the air compressor 112 is set to a predetermined rotation speed Nacpd, and the on-duty INJod of the injector 94 is set to a predetermined on-duty INJodpd.

[0070] With this setting, between time point t0 and time point t1, the power storage device 244 is charged with a suppressed power generation power on the order of the number [kW] of the fuel cell stack 12. Since the power generation voltage Vfc is gradually decreased at the first voltage decrease rate Vdr1, the surplus power generation power of the fuel cell stack 12 at the time of operation stop can be charged without deteriorating the power storage device 244 (corresponding to step S3).

[0071] The control unit 142 decreases the power generation voltage Vfc along the voltage command value Vfccom that decreases the power generation voltage Vfc at the first voltage decrease rate Vdr1 from time point t0. When it is detected at time point t1 that the power generation voltage Vfc has decreased to the threshold power generation voltage Vfcth (corresponding to step S4: YES), it is maintained at the threshold power generation voltage Vfcth until time point t5.

[0072] Between time point t1 and time point t3, the control unit 142 performs learning processing of the outlet sealing valve 118 by varying the opening degree of the outlet sealing valve 118 from several [%] to 100 [%] and then returning it to 0 [%] (corresponding to step S5).

[0073] Between time point t3 and time point t4, the control unit 142 slightly increases and holds the on-duty INJod [%] of the injector 94 from the predetermined on-duty INJodpd. When the power generation power between time point t3 and time point t4 increases the rotation speed Nac [rpm] of the air compressor 112 by decreasing the power generation voltage Vfc at the first voltage decrease rate Vdr1, it is increased slightly more than the predetermined rotation speed Nacpd. By increasing the rotation speed Nac of the air compressor 112, the pressure of the oxidant gas discharged from the air compressor 112 and flowing through the cathode flow path 50 is increased. Due to this oxidant gas pressure, the cathode generated water stored in the cathode flow path 50 is discharged from the fuel cell stack 12 and discharged to the outside through the drain valve 101 and the discharge path 109 (corresponding to step S6).

[0074] At time point t4, after the drainage process, the control unit 142 closes the drain valve 101 and returns the rotation speed Nac of the air compressor 112 to the predetermined rotation speed Nacpd. The on-duty INJod of the injector 94 is set to the predetermined on-duty INJodpd.

[0075] At time t5, the control unit 142 drives the inlet sealing valve 116 from the open state to the closed state, and sets a voltage drop rate Vdr at which the generated voltage Vfc drops from the threshold generated voltage Vfcth to 0 to a second voltage drop rate Vdr2 higher than the first voltage drop rate Vdr1 (corresponding to step S7).

[0076] With this setting, between time t5 and time t6, the generated voltage Vfc rapidly drops from the threshold generated voltage Vfcth to 0 along the voltage command value Vfccom set to the second voltage drop rate Vdr2 (corresponding to step S8: YES). By ending the drive of the air compressor 112 at time t6, the rotational speed Nac becomes 0. Also, at time t6, the on-duty INJod of the injector 94 is set to 0 [%] to end the stop process.

[0077] [Description of Effects of Embodiment] When the control device 26 detects an operation stop signal of the fuel cell vehicle 11 (fuel cell system 10), it gently (gradually) decreases the generated voltage Vfc of the fuel cell stack 12 at the first voltage drop rate Vdr1 to charge the surplus generated power to the power storage device 244. Thereafter, by rapidly decreasing the generated voltage Vfc at a second voltage drop rate Vdr2 higher than the first voltage drop rate Vdr1, deterioration of the fuel cell stack 12 is suppressed and power generation is stopped.

[0078] Incidentally, in the processing according to the comparative example before the processing according to this embodiment, the rate of decrease (decrease rate) of the power generation voltage Vfc at the time of operation stop was constant and not variable. Therefore, when setting to a low voltage decrease rate (corresponding to the first voltage decrease rate Vdr1) that prioritizes charging of the power storage device 244, deterioration of the fuel cell stack 12 was caused. On the other hand, when setting to a high (fast) voltage decrease rate (corresponding to the second voltage decrease rate Vdr2) that prioritizes prevention of deterioration of the fuel cell stack 12, the amount of charge to the power storage device 244 was limited to a small amount. In this embodiment, in consideration of this antinomic relationship in the comparative example, at the time of operation stop, by changing the voltage decrease rate Vdr in two steps from low to high, it was possible to achieve both charging of the power storage device 244 and prevention of deterioration of the fuel cell stack 12.

[0079] [Appendix] Regarding the above disclosure, the following appendix is further disclosed. (Appendix 1) The first disclosure is a fuel cell system 10 including a fuel cell (fuel cell stack 12) that generates power by a fuel gas and an oxidant gas supplied from an air compressor 112, and a control device 26 that controls the fuel cell. The control device includes an acquisition unit 140 that acquires an operation stop signal, and a control unit 142 that, when the acquisition unit acquires the operation stop signal, decreases the power generation voltage Vfc of the fuel cell at a first voltage decrease rate Vdr1 and then decreases it at a second voltage decrease rate Vdr2 higher than the first voltage decrease rate.

[0080] The second disclosure is a method for controlling the operation stop of a fuel cell system including a fuel cell that generates power by a fuel gas and an oxidant gas supplied from an air compressor, and a control device that controls the fuel cell. When the control device acquires an operation stop signal, it decreases the power generation voltage of the fuel cell at a first voltage decrease rate and then decreases it at a second voltage decrease rate higher than the first voltage decrease rate.

[0081] According to the above first and second disclosures, when the fuel cell system stops operating, by reducing the power generation voltage at a first voltage reduction rate, charging control can be performed so as not to exceed the power that can charge the surplus power generation into the power storage device. Thereby, deterioration of the power storage device can be suppressed, and deterioration of the fuel cell can be suppressed by reducing the power generation voltage at a second voltage reduction rate after the charging control.

[0082] (Appendix 2) In the fuel cell system described in Appendix 1, there are a boost converter 240 that boosts the power generation voltage of the fuel cell to the power supply voltage Vh, a load 21 to which the power supply voltage is applied, and a power storage device 244. When the control unit is reducing the power generation voltage at the first voltage reduction rate, the boost converter boosts the power generation voltage to the power supply voltage to charge the power storage device. In this way, by boosting the power generation voltage to the power supply voltage of the power storage device by the boost converter, the power storage device can be charged by the power generation power.

[0083] (Appendix 3) In the fuel cell system described in Appendix 1, before the control unit reduces the power generation voltage at a second voltage reduction rate higher than the first voltage reduction rate after reducing the power generation voltage at the first voltage reduction rate, the rotation speed Nac of the air compressor is made higher than the rotation speed when reducing the power generation voltage at the first voltage reduction rate for a predetermined time. According to this configuration, liquid water stored in the cathode flow path of the fuel cell, such as generated water due to power generation occurring during operation stop, can be drained in a short time.

[0084] (Appendix 4) In the fuel cell system described in Appendix 3, during the predetermined time when the control unit keeps the rotation speed of the air compressor high, the fuel gas pressure of the fuel cell is increased by a predetermined amount. According to this configuration, the power for the air compressor required to drain generated water and the like generated during operation stop can be covered by the power generation power of the fuel cell instead of the power of the power storage device for which charging has been completed.

[0085] Note that the present invention is not limited to the above-described disclosure, and various configurations can be adopted without departing from the gist of the present invention.

Explanation of Reference Numerals

[0086] 10…Fuel cell system 11…Fuel cell vehicle 12…Fuel cell stack 16…Anode system 18…Cathode system 101…Drain valve 106(106A)…Oxidant gas supply passage 108(108A)…Oxidant gas discharge passage 109…Discharge passage 110…Bypass flow passage 112…Air compressor 116…Inlet shutoff valve 118…Outlet shutoff valve 120…Bypass valve 240…Boost converter 242, 252…Inverter 244…Power storage device (power source)

Claims

1. A fuel cell system comprising a fuel cell that generates electricity using a fuel gas and an oxidant gas supplied from an air compressor, a control device that controls the fuel cell, wherein the control device includes an acquisition unit that acquires an operation stop signal, and a control unit that, when the acquisition unit acquires the operation stop signal, reduces the power generation voltage of the fuel cell at a first voltage reduction rate and then reduces it at a second voltage reduction rate higher than the first voltage reduction rate. A fuel cell system comprising the above.

2. In the fuel cell system according to Claim 1, a boost converter that boosts the power generation voltage of the fuel cell to a power supply voltage, and a load and a power storage device to which the power supply voltage is applied, wherein the control unit, when reducing the power generation voltage at the first voltage reduction rate, boosts the power generation voltage to the power supply voltage by the boost converter and charges the power storage device. A fuel cell system.

3. In the fuel cell system according to Claim 1, before the control unit reduces the power generation voltage at the second voltage reduction rate higher than the first voltage reduction rate after reducing the power generation voltage at the first voltage reduction rate, the control unit increases the rotation speed of the air compressor for a predetermined time to be higher than the rotation speed when reducing the power generation voltage at the first voltage reduction rate. A fuel cell system.

4. In the fuel cell system according to Claim 3, the control unit increases the fuel gas pressure of the fuel cell by a predetermined amount during the predetermined time when the rotation speed of the air compressor is increased. A fuel cell system.

5. A fuel cell that generates electricity using a fuel gas and an oxidant gas supplied from an air compressor, a control device that controls the fuel cell, and a method for controlling the operation stop of a fuel cell system including the above, wherein when the control device acquires an operation stop signal, the control device reduces the power generation voltage of the fuel cell at a first voltage reduction rate and then reduces it at a second voltage reduction rate higher than the first voltage reduction rate. A method for controlling the operation stop of a fuel cell system.

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