Fuel cell system and control method for the same

Abstract

To provide a fuel cell system capable of suppressing degradation of a fuel cell stack while limiting the output of a fuel cell to a predetermined value or below when stopping operation of the fuel cell.SOLUTION: The fuel cell system (FC system 1) includes a fuel cell stack (FC stack 2) that generates electricity through the reaction of fuel gas and oxidant gas, an oxidant gas supply device 3 that supplies oxidant gas to the fuel cell stack, an oxygen concentration acquisition unit 51 that acquires the oxygen concentration within the fuel cell stack, and a voltage control unit 52 that controls the voltage of the fuel cell stack. The voltage control unit 52 varies the rate of decrease in the fuel cell stack voltage according to the oxygen concentration when stopping the fuel cell system.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a fuel cell system that can efficiently reduce the voltage when the operation of the fuel cell is stopped, and a control method thereof. [Background technology]

[0002] In recent years, research and development into fuel cells has been conducted to contribute to energy efficiency, ensuring that more people have access to affordable, reliable, sustainable and advanced energy.

[0003] In fuel cell systems that generate electricity using fuel cells, there is a problem that when the operation of the fuel cell is stopped, oxygen remaining in the cathode system can cause deterioration of the electrodes and electrolyte membrane. To address this problem, a technology has been developed in which, when the fuel cell is stopped, an appropriate amount of fuel gas such as hydrogen is supplied to the anode to react with the oxygen remaining in the cathode, thereby consuming the remaining oxygen and suppressing fuel cell deterioration.

[0004] For example, Patent Document 1 discloses a technology in which, while the fuel cell system is stopped, it is determined based on the voltage value of the fuel cell stack whether or not oxygen in the cathode is being consumed normally, and based on the determination result, the pressure of the fuel gas supplied to the anode is changed to supply a sufficient amount of fuel gas to consume the remaining oxygen in the cathode. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2020-087529 Summary of the Invention [Problem to be solved by the invention]

[0006] Conventionally, when the operation of a fuel cell is stopped, control is performed to reduce the voltage of the fuel cell stack at a fixed reduction rate in accordance with a voltage command value from a control unit. Here, when the voltage of the fuel cell stack is reduced, there is a demand to reduce the voltage of the fuel cell stack as quickly as possible in order to suppress deterioration of the fuel cell stack. On the other hand, due to the IV and IP characteristics of a typical fuel cell, a decrease in the voltage of the fuel cell stack increases the current, and an increase in current increases the output (amount of power generated). For example, in a system in which the power generated by the fuel cell is stored in a battery, there is an upper limit to the power that can be supplied to the battery due to the battery's charging characteristics. Therefore, it is necessary to keep the output of the fuel cell stack below a certain value, which can make it difficult to rapidly decrease the voltage of the fuel cell stack.

[0007] As described above, there are two conflicting requirements when reducing the voltage of a fuel cell stack. Conventionally, it has been difficult to satisfy both of these requirements, so the reduction rate of the voltage command value has been set by compromising on one requirement while satisfying the other. That is, when prioritizing suppression of fuel cell stack degradation, the voltage is reduced quickly by setting a high voltage reduction rate, knowing that the upper limit of power that can be supplied to the battery may be exceeded.On the other hand, when prioritizing the upper limit of power that can be supplied to the battery, the voltage is reduced gradually by setting a low voltage reduction rate, knowing that this will accelerate degradation of the fuel cell stack. Therefore, there has been a demand for a fuel cell system that can suppress deterioration of the fuel cell stack while suppressing the output of the fuel cell to below the upper limit of power that can be supplied to the battery when the operation of the fuel cell is stopped.

[0008] The present invention has been made to solve these problems, and aims to provide a fuel cell system that can suppress deterioration of the fuel cell stack while suppressing the output of the fuel cell below a certain value when the operation of the fuel cell is stopped, thereby contributing to energy efficiency. [Means for solving the problem]

[0009] In order to achieve this object, the fuel cell system (FC system 1) according to claim 1 of the present invention comprises a fuel cell stack (FC stack 2) that generates electricity by reacting fuel gas with an oxidant gas, an oxidant gas supply device 3 that supplies the oxidant gas to the fuel cell stack, an oxygen concentration acquisition unit 51 that acquires the oxygen concentration in the fuel cell stack, and a voltage control unit 52 that controls the voltage of the fuel cell stack, and when the fuel cell system is stopped, the voltage control unit 52 changes the rate at which the voltage of the fuel cell stack decreases depending on the oxygen concentration.

[0010] In this fuel cell system, the oxygen concentration acquisition unit acquires the oxygen concentration in the fuel cell stack, and the voltage control unit changes the rate at which the voltage of the fuel cell stack drops when the fuel cell system is stopped in accordance with the acquired oxygen concentration. In the present invention, the oxygen concentration in the fuel cell stack after the fuel cell system has been shut down is used as an index showing the amount of power generated after the shutdown process of the fuel cell system has begun. For example, when the oxygen concentration in the fuel cell stack is high, it is determined that the amount of power generated in the fuel cell stack is large and the output of the fuel cell stack is likely to become excessive. On the other hand, when the oxygen concentration in the fuel cell stack is low, it is determined that the amount of power generated in the fuel cell stack is small and the risk of the output of the fuel cell stack becoming excessive is low. Furthermore, instead of using a constant (fixed) voltage reduction rate as in the past, by changing the voltage reduction rate according to the oxygen concentration, it is possible to use different voltage reduction rates depending on the power generation state of the fuel cell stack.

[0011] That is, by changing the voltage reduction rate according to the oxygen concentration in the fuel cell stack, it is possible to set the voltage reduction rate according to the amount of power generated by the fuel cell stack. As a result, for example, when the oxygen concentration is high, the voltage reduction rate can be set low to suppress an increase in output, and when the oxygen concentration is low, the voltage can be set high to quickly reduce the voltage and suppress deterioration of the fuel cell stack. In this way, by changing the rate at which the fuel cell stack voltage drops depending on the oxygen concentration when the fuel cell system is stopped, it is possible to suppress deterioration of the fuel cell stack while keeping the fuel cell output below a certain value.

[0012] The invention of claim 2 of the present invention is characterized in that, in the fuel cell system described in claim 1, it further comprises a flow rate acquisition unit (flow rate sensor 37, control device 5) that acquires the flow rate of the oxidant gas supplied to the fuel cell stack as an oxidant gas flow rate, and a current value acquisition unit (current sensor 25) that acquires the current value of the fuel cell stack, and the oxygen concentration acquisition unit estimates and acquires the oxygen concentration based on the oxidant gas flow rate and the current value.

[0013] According to this configuration, the oxygen concentration acquisition unit estimates the oxygen concentration based on the flow rate of the oxidant gas supplied to the fuel cell stack and the current value of the fuel cell stack. In other words, in this configuration, the oxidant gas flow rate is used as an indicator of the amount of oxidant gas supplied to the fuel cell stack, and the fuel cell stack current value is used as an indicator of the amount of oxidant gas consumed in the fuel cell stack.The oxygen concentration is estimated based on the oxidant gas flow rate and the current value, so the concentration of oxygen remaining in the fuel cell stack can be estimated with high accuracy.

[0014] The invention according to claim 3 of the present invention is characterized in that, in the fuel cell system according to claim 1 or 2, the voltage control unit reduces the rate of voltage reduction when stopping the fuel cell system, the higher the oxygen concentration.

[0015] With this configuration, when shutting down the fuel cell system, the voltage control unit controls the rate at which the fuel cell stack voltage is reduced so that the higher the oxygen concentration in the fuel cell stack, the lower the rate at which the fuel cell stack voltage is reduced. Therefore, for example, immediately after the start of the shut-down process for the fuel cell system, when a large amount of oxygen remains in the fuel cell stack and the fuel cell stack is generating a large amount of power, making it easy for the output to become excessive, the voltage reduction rate can be set low. This prevents the fuel cell stack voltage from dropping too much, and more reliably keeps the fuel cell output below a certain value.

[0016] The invention of claim 4 of the present invention is characterized in that, in the fuel cell system described in claim 1 or 2, the voltage control unit increases the rate of voltage reduction when stopping the fuel cell system, the lower the oxygen concentration.

[0017] According to this configuration, when shutting down the fuel cell system, the voltage control unit controls the fuel cell stack voltage reduction rate so that the lower the oxygen concentration in the fuel cell stack, the higher the rate at which the fuel cell stack voltage is reduced. Therefore, for example, if a certain amount of time has passed since the start of the fuel cell system shutdown process and the oxygen in the fuel cell stack has been consumed by reaction with the fuel gas, causing the oxygen concentration to decrease, the voltage reduction rate can be set high. This allows the fuel cell stack voltage to be reduced quickly when there is no risk of excessive output, making it possible to more effectively suppress deterioration of the fuel cell stack.

[0018] A control method for a fuel cell system according to claim 5 of the present invention is a control method for a fuel cell system having a fuel cell stack that generates electricity by reacting a fuel gas with an oxidant gas, an oxidant gas supply means that supplies the oxidant gas to the fuel cell stack, an oxygen concentration acquisition means that acquires the oxygen concentration in the fuel cell stack, and a voltage control means that controls the voltage of the fuel cell stack, wherein when the fuel cell system is stopped, the voltage control means performs control to change the rate at which the voltage of the fuel cell stack decreases depending on the oxygen concentration.

[0019] In the fuel cell system control method of the present invention, the voltage reduction rate can be set according to the amount of power generated by the fuel cell stack by changing the voltage reduction rate according to the oxygen concentration in the fuel cell stack, which makes it possible to set the voltage reduction rate low when the oxygen concentration is high to suppress an increase in output, and high when the oxygen concentration is low to suppress deterioration of the fuel cell stack. In this way, by changing the rate at which the fuel cell stack voltage drops depending on the oxygen concentration when the fuel cell system is stopped, it is possible to suppress deterioration of the fuel cell stack while keeping the fuel cell output below a certain value. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a schematic diagram of a fuel cell vehicle equipped with a fuel cell system according to an embodiment of the present invention; [Figure 2] FIG. 2 is a diagram showing the configuration of a control system of the fuel cell system according to the embodiment. [Figure 3] FIG. 4 is a diagram showing the relationship between the oxidant gas flow rate and the oxygen concentration in a fuel cell stack. [Figure 4] FIG. 4 is a diagram showing the relationship between the current value and the oxygen concentration in the fuel cell stack. [Figure 5] 4 is a flowchart showing a stop-time voltage control process of the fuel cell system according to the embodiment. [Figure 6] FIG. 10 is a diagram showing the relationship between the oxygen concentration and the voltage drop rate in voltage control according to the embodiment, in comparison with a conventional example. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, a preferred embodiment of the fuel cell system of the present invention will be described in detail while referring to the drawings. The fuel cell system according to the illustrated embodiment is mounted on a fuel cell vehicle and functions as one of the power sources of the fuel cell vehicle. The fuel cell vehicle may be an automobile such as a two-wheeled, three-wheeled, or four-wheeled vehicle. Note that the configurations described below are examples of the present invention, and the present invention is not limited thereto. For example, the fuel cell system of the present invention may be mounted on a moving body such as a ship or an airplane in addition to a vehicle, or may be used in stationary equipment such as a house or a building. <Omitted>[ <Omitted>[

[0022] <Omitted>[ FIG. 1 is a schematic configuration diagram of a fuel cell vehicle 100 on which an FC (Fuel Cell) system 1 according to an embodiment is mounted. The fuel cell vehicle 100 is, for example, a fuel cell electric vehicle, and as shown in the figure, includes an FC system 1, a battery 200, a motor 300, and the like. Other components of the fuel cell vehicle 100 are not shown in the figure. <Omitted>[ <Omitted>[

[0023] <Omitted>[ The motor 200 is, for example, a three-phase AC motor and is driven using the electric power supplied from the FC system 1 or the battery 300 as a power source. The rotor of the motor 200 is connected to a drive wheel (not shown), and the motor 200 outputs a driving force used for the running of the fuel cell vehicle 100 to the drive wheel under the control of a higher-level controller (not shown) mounted on the fuel cell vehicle 100. Further, the motor 200 performs regenerative power generation using the kinetic energy of the vehicle when the vehicle decelerates. <Omitted>[ <Omitted>[<Omitted>[

[0024] <Omitted>[<Omitted>[ The battery 300 is a secondary battery such as a lithium-ion battery, for example. The battery 300 stores the electric power generated in the FC system 1 or the motor 200 and supplies the electric power for the running of the fuel cell vehicle 100 to the motor 200 under the control of a higher-level controller. <Omitted>[ Sensors such as a current sensor, a voltage sensor, and a temperature sensor (not shown) are provided in the battery 300, and the current value, voltage value, temperature, etc. detected by these sensors are output to a higher-level controller. <Omitted>[ <Omitted>[

[0025] [[ID=I7]]<Omitted>[ <Configuration of the FC System 1><Omitted>[ The following describes a specific configuration of the FC system 1. Note that the configuration described below is merely an example, and any system configuration that generates power using an anode and a cathode may be used. The FC system 1 includes an FC (fuel cell) stack 2, an oxidant gas supply device 3, a hydrogen gas supply device 4, a control device 5, a VCU (Voltage Control Unit) 7, and a cooling system 8.

[0026] The FC stack 2 is a structure in which multiple power generation cells 21 are stacked. The FC stack 2 is provided with an oxidant gas inlet 2a, an oxidant gas outlet 2b, a hydrogen gas inlet 2c, a hydrogen gas outlet 2d, and an electrode 2e.

[0027] Each power generating cell 21 is a battery that generates electricity through an electrochemical reaction between a fuel gas supplied to the anode and an oxidant gas supplied to the cathode. In this embodiment, hydrogen gas is used as the fuel gas, and air containing oxygen is used as the oxidant gas. Each power generation cell 21 has a configuration in which a solid polymer electrolyte membrane (hereinafter simply referred to as the electrolyte membrane) 22 made of a cation exchange membrane such as a thin film of water-containing perfluorosulfonic acid is sandwiched between an anode electrode 23 and a cathode electrode 24. As the electrolyte membrane 22, a fluorine-based electrolyte or a hydrocarbon-based electrolyte can be used.

[0028] Hydrogen gas, a fuel gas containing hydrogen, is supplied to the anode electrode 23 from a hydrogen gas supply device 4. Air, an oxidizing gas containing oxygen, is supplied to the cathode electrode 24 from an oxidizing gas supply device 3. The hydrogen supplied to the anode electrode 23 is ionized by a catalytic reaction on an anode catalyst (not shown), and the generated hydrogen ions permeate the electrolyte membrane 22 and move to the cathode electrode 24. Electrons released as the hydrogen is ionized move to an external circuit via electrode 2e, generating a current, which then generates electricity. The hydrogen ions that move from the anode electrode 23 to the cathode electrode 24 react with the oxygen supplied to the cathode electrode 24 to produce water.

[0029] The oxidant gas supply device 3 includes an air pump 31 that compresses air from the atmosphere and supplies it to the FC stack 2, and the air pump 31 is disposed in an air supply flow path 35. The air pump 31 is controlled by the control device 5. A humidifier 33 is provided in the air supply passage 35. The air supply passage 35 communicates with the oxidant gas inlet 2a of the FC stack 2. Further, a flow rate sensor 37 is disposed in the air supply flow path 35 to detect the air flow rate at the outlet of the air pump 31. Information relating to the air flow rate detected by the flow rate sensor 37 is transmitted to the control device 5. The air supply passage 35 is connected to a purge passage 49, which will be described later, via a bypass valve 38.

[0030] The oxidant gas outlet 2b is connected to an air discharge flow path 36 that passes through a humidifier 33. The humidifier 33 recovers moisture from the post-reaction air (including post-reaction gas and off-gas) that is discharged from the oxidant gas outlet 2b and passes through the air discharge flow path 36, and uses this moisture to humidify the air that passes through the air supply flow path 35. This makes it possible to maintain the electrolyte membrane 22 in each power generation cell 21 of the FC stack 2 at a humidity suitable for power generation.

[0031] A supply-side sealing valve 32 is provided on the air supply flow path 35 downstream of the air pump 31. The supply-side sealing valve 32 is opened and closed under the control of the control device 5, thereby switching the air supply flow path 35 between open and closed states. In addition, a discharge-side seal valve 34 is provided in the air discharge flow path 36. The discharge-side seal valve 34 is opened and closed under the control of the control device 5, thereby switching between opening and closing of the air discharge flow path 36. The downstream side of the discharge-side seal valve 34 is connected to a purge flow path 49, which will be described later.

[0032] The hydrogen gas supply device 4 has a hydrogen tank 41 that stores high-pressure hydrogen gas. The hydrogen tank 41 communicates with the hydrogen gas inlet 2c of the FC stack 2 via a hydrogen supply flow path 47. An injector 42 and an ejector 43 are provided in series in the hydrogen supply flow path 47. The opening degree of the injector 42 is controlled by the control device 5, and determines the flow rate and supply timing of the hydrogen gas supplied to the FC stack 2. The ejector 43 creates a negative pressure inside, and sucks in the off-gas discharged from the hydrogen gas outlet 2d to the off-gas flow path 48, and recirculates it to the hydrogen supply flow path 47.

[0033] An off-gas passage 48 communicates with the hydrogen gas outlet 2d of the FC stack 2, and a gas-liquid separator 44 is connected to the off-gas passage 48. The gas-liquid separator 44 separates the off-gas discharged from the hydrogen gas outlet 2d of the FC stack 2 into a gas component and a liquid component. The liquid component separated from the off-gas is discharged to a purge flow path 49 via a drain valve 45, the opening and closing of which is controlled by the control device 5. In addition, part of the gas component separated from the off-gas is recirculated via the ejector 43, and the other part is discharged to the purge flow path 49 via a purge valve 46, the opening and closing of which is controlled by the control device 5.

[0034] The purge flow path 49 is a flow path that communicates with the outside of the fuel cell vehicle 100. The off-gas containing hydrogen that flows through the purge flow path 49 is mixed with the post-reaction air (including the post-reaction gas and the off-gas) discharged from the oxidant gas outlet 2b and the air bypassed from the air supply flow path 35 via the bypass valve 38, diluted, and then discharged to the outside.

[0035] The VCU 7 is, for example, a step-up DC-DC converter. The VCU 7 is disposed between the anode electrode 23 and the cathode electrode 24 of the FC stack 2 and an electrical load external to the FC system 1. The VCU 7 has a function to change the output state of the FC stack 2 under the control of the control unit 5. Specifically, the VCU 7 has a function to set the output voltage when the FC stack 2 generates power. The VCU 7 also has a function to boost the output voltage to a desired voltage when supplying the power generated by the FC stack 2 to loads such as the motor 200, the battery 300, the air pump 31, and various other auxiliary devices. A current sensor 25 is disposed between the VCU 7 and the FC stack 2. The current sensor 25 detects the power generation current of the fuel cell stack 2 and transmits the detected value to the control device 5.

[0036] The control device 5 is an ECU configured with a microcomputer including a CPU, RAM, ROM, and an I / O interface (none of which are shown). The control device 5 is configured to be able to acquire information about the state of the FC system 1 based on the detection values ​​of various sensors (not shown). The acquired state of the FC system includes, for example, the current power generation status, power generation amount, power generation time, number of starts (or number of stops), etc.

[0037] The control device 5 also controls the opening and closing of various valves in the FC system 1, the drive of various auxiliary equipment (such as the air pump 31), and the amount of power generated by the FC stack 2 via the VCU 7. The control device 5 also controls the amount and timing of hydrogen gas supplied to the FC stack 2 by controlling the opening of the injector 42 while referencing the value of a pressure sensor (not shown) provided on the anode electrode 23. The control device 5 also operates the cooling system 8 to perform control related to temperature adjustment of the FC stack 2. The control device 5 may also perform charge / discharge control of the battery 300 and power running / regenerative drive control of the motor 300 .

[0038] As described above, the flow rate sensor 37 and the current sensor 25 are connected to the control device 5, and their detection signals are sequentially input. Based on the inputs from these sensors, the control device 5 reads and executes a program stored in the ROM or RAM, thereby realizing the functions of the oxygen concentration acquisition unit 51 and the voltage control unit 52, which will be described later.

[0039] The cooling system 8 cools the FC stack 2 under the control of the control device 5. For example, the cooling system 8 cools the FC stack 2 by circulating a refrigerant such as pure water or ethylene glycol through a refrigerant flow path (not shown) provided inside the FC stack 2.

[0040] <Power generation operation of the FC system 1> The power generation operation of the FC system 1 configured as described above (power generation operation in the FC stack 2) will be described below.

[0041] The oxidant gas supply device 3 supplies air as an oxidant gas to the air supply passage 35 via the air pump 31. This air is humidified through the humidifier 33 and then supplied to the FC stack 2 from the oxidant gas inlet 2a.

[0042] On the other hand, the hydrogen gas supply device 4 supplies hydrogen gas from the hydrogen tank 41 to the hydrogen supply passage 47 based on the opening degree control of the injector 42 by the control device 5. This hydrogen gas is supplied to the fuel cell stack 2 from the hydrogen gas inlet 2c after passing through the ejector 43.

[0043] The air supplied to the FC stack 2 from the oxidant gas inlet 2a is supplied to the cathode electrode 24 of each power generation cell 21, and the hydrogen gas supplied to the FC stack 2 from the hydrogen gas inlet 2c is supplied to the anode electrode 23 of each power generation cell 21. As a result, in each power generation cell 21, hydrogen and oxygen in the air are consumed by an electrochemical reaction, and power generation is performed. The electric power generated by power generation is supplied to the motor 200, the battery 300, and other various auxiliary machines via the VCU 7 based on the control of the control device 5.

[0044] The air after the reaction at the cathode electrode 24 of each power generation cell 21 (including the reacted gas and off-gas) is discharged from the oxidant gas outlet 2b to the air discharge passage 36. The discharged air is recovered of moisture when passing through the humidifier 33 and then discharged to the outside through the purge passage 49. The moisture recovered by the humidifier 33 is used for humidifying the air passing through the air supply passage 35 as described above, thereby adjusting the humidity of the electrolyte membrane 22 in each power generation cell 21 of the FC stack 2.

[0045] Furthermore, hydrogen gas after the reaction at the anode electrode 23 of each power generation cell 21 is discharged as off-gas (partially consumed fuel gas) from the hydrogen gas outlet 2d to the off-gas passage 48. The discharged off-gas is introduced from the off-gas passage 48 into the gas-liquid separator 44 where liquid water is separated, and then the discharged off-gas is recirculated via the ejector 43, or mixed with air and diluted as it passes through the purge passage 49, and then discharged to the outside.

[0046] During the execution of the series of power generation operations described above, the cooling system 8 operates in accordance with the temperature of the FC stack 2 under the control of the control device 5 to cool the FC stack 2 .

[0047] <Configuration of the control device 5> Next, the configuration of the control device 5 will be described. As shown in Fig. 2, in this embodiment, the control device 5 includes an oxygen concentration acquisition unit 51 and a voltage control unit 52. These functional units are realized, for example, by a hardware processor such as a CPU of the control device 5 reading and executing a program (software). Such a program may be stored in a ROM or RAM included in the control device 5, or may be stored in an external storage device (a storage device including a non-transitory storage medium such as an HDD or flash memory).

[0048] During the shutdown voltage control process of the FC system 1, which will be described later, the oxygen concentration acquisition unit 51 estimates the oxygen concentration in the cathode system of the FC stack 2 based on the air flow rate value acquired based on the detection value of the flow rate sensor 37 and the value of the power generation current of the FC stack 2 acquired from the current sensor 25. In this embodiment, the amount of air supplied to the cathode electrode 24 of the FC stack 2 is calculated by setting an approximate value of the leakage flow rate before it enters the FC stack 2 for the air flow rate at the outlet of the air pump 31 detected by the flow rate sensor 37, and subtracting this approximate value from the air flow rate detected by the flow rate sensor 37.

[0049] Here, the relationship between the amount of air supplied to the FC stack 2, the current value of the FC stack 2, and the oxygen concentration will be described with reference to FIGS. Figure 3 shows the relationship between the amount of supplied air and the oxygen concentration when the current value in the FC stack 2 is controlled to be approximately constant. As shown in the figure, when the amount of supplied air is gradually reduced while the current value is kept approximately constant, the oxygen concentration decreases accordingly. This correlation is affected by the amount of oxygen consumed in the FC stack 2, i.e., the progress of the reaction between oxygen and fuel gas, but it can be seen that, in general, the greater the amount of supplied air, the higher the oxygen concentration in the FC stack 2, and the smaller the amount of supplied air, the lower the oxygen concentration in the FC stack 2.

[0050] Next, Figure 4 shows the relationship between oxygen concentration and current value when the amount of supplied air is controlled to be approximately constant in the FC stack 2. As the figure shows, with the amount of supplied air kept approximately constant, as oxygen is consumed through the reaction between oxygen and fuel gas within the FC stack 2 and the oxygen concentration decreases, the value of the generated current of the FC stack 2 also decreases accordingly. This correlation is influenced by the amount of oxygen consumed in the FC stack 2, i.e., how the reaction between oxygen and fuel gas progresses, but it can be seen that generally, when the detected current value is large, the oxygen concentration tends to be high, and when the current value is small, the oxygen concentration tends to be low.

[0051] The oxygen concentration acquisition unit 51 of this embodiment estimates and acquires the current oxygen concentration from the acquired air flow rate and current value using estimation logic (relational equation) constructed based on the relationship between the supply air volume, current value, and oxygen concentration. In this embodiment, the oxygen concentration acquisition unit 51 is configured to acquire the oxygen concentration by estimation, but if a device or instrument that can directly measure the oxygen concentration in the FC stack 2 is available, the oxygen concentration may be acquired by measurement.

[0052] In the voltage control process during shutdown of the FC system 1, which will be described later, the voltage control unit 52 determines the voltage reduction rate of the FC stack 2 based on the oxygen concentration in the FC stack 2 acquired by the oxygen concentration acquisition unit 51. Here, the voltage reduction rate of the FC stack 2 refers to the rate of change in voltage when reducing the voltage of the FC stack 2. In other words, the higher the voltage reduction rate, the more rapid the voltage reduction, and the lower the voltage reduction rate, the more gradual the voltage reduction.

[0053] In this embodiment, the oxygen concentration in the FC stack 2 after the start of the shutdown process of the FC system 1 is used as an index showing the amount of power generated in the FC system 1 after shutdown. For example, when the oxygen concentration in the FC stack 2 is high, it is assumed that the amount of power generated in the FC stack 2 is large. Therefore, if the current increases by lowering the voltage of the FC stack 2, it is determined that the output (amount of power generated) of the FC stack 2 becomes excessive and is likely to exceed the upper limit of power that the battery 300 can accept. On the other hand, when the oxygen concentration in the FC stack 2 is low, it is assumed that the amount of power generated in the FC stack 2 is small. Therefore, even if the current increases by lowering the voltage of the FC stack 2, it is determined that there is little risk that the power generated by the FC stack 2 will exceed the upper power limit on the battery 300 side.

[0054] The voltage reduction rate based on the oxygen concentration can be determined, for example, by storing in advance in a storage unit (not shown) a data table that defines optimal voltage reduction rates for each oxygen concentration, and by having the voltage control unit 52 refer to the data table. The optimal voltage reduction rate can be set, for example, as the highest possible reduction rate within a range that does not cause the amount of power generated by the FC stack 2 to exceed the upper power limit of the battery 300. Such optimal voltage reduction rates for each oxygen concentration can be set based on the results of experiments or simulations conducted in advance. By determining the voltage reduction rate in this way, it is possible to reduce the voltage as quickly as possible and suppress deterioration of the FC stack 2 while preventing the amount of power generated by the FC stack 2 from exceeding a predetermined value (for example, the upper power limit that can be tolerated by the battery 300). When setting the optimal voltage reduction rate, the voltage reduction rate may be determined based only on the oxygen concentration, or a correction factor or the like based on other parameters such as the voltage value of the FC stack 2 may be set in addition to the oxygen concentration, and then the voltage reduction rate may be determined.

[0055] <Stopping Voltage Control of FC System 1> Subsequently, the stopping voltage control in the FC system 1 of the present embodiment will be described with reference to FIGS. 5 and 6. FIG. 5 is a flowchart showing the stopping voltage control process of the FC system 1. This process is repeatedly executed at a predetermined timing or a predetermined cycle in a predetermined period after the start of the stopping process of the power generation operation (power generation operation in the FC stack 2) in the FC system 1, for example. Note that the power generation operation in the FC system 1 may be stopped when the ignition switch of the fuel cell vehicle 100 is turned off, or may be automatically stopped according to the state of charge of the battery 200 and the driving state of the fuel cell vehicle 100.

[0056] In this control process, first, the oxygen concentration acquisition unit 51 of the control device 5 calculates and acquires an air flow rate (oxidant gas flow rate) as an index indicating the supply air amount in the FC stack 2 based on the current detection value of the flow rate sensor 37 and the approximate value of the leakage flow rate described above (step 501 (illustrated as "S501". The same applies hereinafter)). Subsequently, the oxygen concentration acquisition unit 51 acquires the value of the power generation current of the FC stack 2 based on the current detection value of the current sensor 25 (step 502). The order of executing the acquisition of the air flow rate (step 501) and the acquisition of the current value (step 502) may be reversed or may be simultaneous. After acquiring the air flow rate and the current value, the oxygen concentration acquisition unit 51 calls a previously constructed estimation logic, and uses this estimation logic to estimate the current oxygen concentration in the FC stack 2 from the acquired air flow rate and current value (step 503). <A

[0057] After the oxygen concentration acquisition unit 51 estimates the oxygen concentration, the voltage control unit 52 of the control device 5 determines the voltage reduction rate of the FC stack 2 based on the estimated oxygen concentration (step 504). As described above, this determination is made by reading out a data table created in advance and searching for the voltage reduction rate associated with the estimated oxygen concentration. It is also possible to store a relational expression between the oxygen concentration and the voltage drop rate in advance in a memory unit, rather than reading out a data table created in advance, and derive the voltage drop rate by calculation using this relational expression for the acquired oxygen concentration.

[0058] After determining the voltage reduction rate, the voltage control unit 52 sets the new voltage command value to a value obtained by multiplying the immediately preceding voltage command value by the determined voltage reduction rate, and then controls the VCU 7 so that the voltage of the FC stack 2 matches the voltage command value (step 505), terminating this process.

[0059] FIG. 6 is a diagram showing the relationship between the oxygen concentration and the voltage drop rate in the shutdown voltage control of this embodiment, in comparison with a conventional example. As mentioned above, conventionally, when an FC system is shut down, the voltage of the FC stack is reduced using a constant voltage reduction rate. Therefore, as shown by the dashed line in Figure 6, the voltage reduction rate is constant regardless of the oxygen concentration in the FC stack, and the voltage command value decreases linearly according to the constant reduction rate. On the other hand, in the voltage control of this embodiment, the voltage reduction rate is changed according to the oxygen concentration in the FC stack. That is, the voltage reduction rate is set so that the higher the oxygen concentration, the lower the voltage reduction rate, and the lower the oxygen concentration, the higher the voltage reduction rate. This makes it possible to suppress deterioration of the FC stack while keeping the output (power generation amount) of the FC stack below a certain value.

[0060] <Effects of this embodiment> The effects of this embodiment will be described below. In the FC system 1 of this embodiment, the oxygen concentration acquisition unit 51 acquires the oxygen concentration in the FC stack 2, and the voltage control unit 52 changes the voltage reduction rate of the FC stack 2 when the FC system 1 is stopped according to the acquired oxygen concentration. That is, in the FC system 1, the oxygen concentration inside the FC stack 2 after the FC system 1 is shut down is used as an index showing the amount of power generation after the FC system 1 is shut down, and it is determined that when the oxygen concentration is high, the amount of power generation is large and the output of the FC stack 2 is likely to become excessive. On the other hand, when the oxygen concentration inside the FC stack 2 is low, the amount of power generation is small and there is little risk of the output of the FC stack 2 becoming excessive. And instead of using a fixed voltage reduction rate as in the past, it is possible to use different voltage reduction rates depending on the state of power generation of the FC stack 2.

[0061] This makes it possible, for example, to set the voltage reduction rate low to suppress an increase in output when the oxygen concentration is high, and to set the voltage reduction rate high to suppress deterioration of the FC stack 2 when the oxygen concentration is low. In this way, when the FC system 1 is shut down, the rate at which the voltage of the FC stack 2 drops can be changed depending on the oxygen concentration, thereby suppressing deterioration of the FC stack 2 while keeping the output of the FC stack 2 below a certain value.

[0062] The oxygen concentration acquisition unit 51 also estimates the oxygen concentration based on the flow rate of the oxidant gas (air) supplied to the FC stack 2 and the value of the power generation current of the FC stack 2. That is, the FC system 1 uses the air flow rate calculated based on the detection value of the flow rate sensor 37 as an index indicating the amount of air supplied to the FC stack 2, and uses the value of the power generation current of the FC stack as an index indicating the amount of oxygen consumed in the FC stack 2. Then, because the oxygen concentration is estimated based on these two indexes, the concentration of oxygen remaining in the FC stack 2 can be estimated with high accuracy.

[0063] In addition, when the FC system 1 is stopped, the voltage control unit 2 controls the voltage reduction rate of the FC stack 2 to be lower the higher the oxygen concentration in the FC stack 2, and to be higher the voltage reduction rate of the FC stack 2 the lower the oxygen concentration in the FC stack 2.

[0064] Therefore, the voltage reduction rate can be set low when a large amount of oxygen remains in the FC stack 2, such as immediately after the FC system 1 is shut down, and the amount of power generated by the FC stack 2 is large, making it easy for output to become excessive. This prevents the voltage of the FC stack 2 from dropping too much and the output from increasing too much, making it possible to more reliably keep the output of the FC stack 2 below a certain value.

[0065] Furthermore, after the FC system 1 is stopped, when a certain amount of time has passed and the oxygen in the FC stack 2 is consumed by reaction with hydrogen, the oxygen concentration drops, the amount of power generated by the FC stack 2 is small, and there is little risk of excessive output, the voltage reduction rate can be set high. This allows the voltage of the FC stack 2 to be reduced quickly when there is no risk of excessive output from the FC stack 2, making it possible to more effectively suppress deterioration of the FC stack 2.

[0066] The present invention is not limited to the embodiments described above, and can be implemented in various forms. In addition, the detailed configuration can be appropriately changed within the scope of the spirit of the present invention. [Explanation of symbols]

[0067] 1...FC system (fuel cell system) 2...FC stack (fuel cell stack) 25...Current sensor (current value acquisition unit) 3...Oxidant gas supply device (oxidant gas supply means) 37...Flow sensor (flow rate acquisition unit) 4...Hydrogen gas supply device 5...Control device (flow rate acquisition unit) 51...Oxygen concentration acquisition unit (oxygen concentration acquisition means) 52...Voltage control unit (voltage control means) 7...VCU 300…Battery

Claims

1. 1. A fuel cell system, comprising: a fuel cell stack that generates electricity by reacting a fuel gas with an oxidant gas; an oxidant gas supply device that supplies the oxidant gas to the fuel cell stack; an oxygen concentration acquisition unit that acquires an oxygen concentration in the fuel cell stack; a voltage control unit that controls the voltage of the fuel cell stack, the voltage control unit changes a rate of decrease in the voltage of the fuel cell stack in accordance with the oxygen concentration when the fuel cell system is stopped. Fuel cell system.

2. a flow rate acquisition unit that acquires the flow rate of the oxidant gas supplied to the fuel cell stack as an oxidant gas flow rate; a current value acquisition unit that acquires a current value of the fuel cell stack, The oxygen concentration acquisition unit estimates and acquires the oxygen concentration based on the oxidant gas flow rate and the current value.

2. The fuel cell system according to claim 1, wherein:

3. 3. The fuel cell system according to claim 1, wherein the voltage control unit, when shutting down the fuel cell system, reduces the rate at which the voltage is reduced as the oxygen concentration increases.

4. 3. The fuel cell system according to claim 1, wherein the voltage control unit, when shutting down the fuel cell system, increases the rate at which the voltage is reduced as the oxygen concentration decreases.

5. a fuel cell stack that generates electricity by reacting a fuel gas with an oxidant gas; an oxidant gas supply means for supplying the oxidant gas to the fuel cell stack; an oxygen concentration acquisition means for acquiring an oxygen concentration in the fuel cell stack; a voltage control means for controlling a voltage of the fuel cell stack, the voltage control means executes control to change the rate of voltage reduction of the fuel cell stack in accordance with the oxygen concentration when the fuel cell system is stopped. A method for controlling a fuel cell system.

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