Fuel cell system

Abstract

To provide a technique for preventing drying of a stack and reducing fluctuations in an operating point in a fuel cell system.SOLUTION: There is provided a fuel cell system including a fuel cell, an air compressor, a flow rate sensor, a current sensor, a voltage sensor, a load device, and a control unit. The control unit controls the air compressor so as to achieve a required flow rate, and performs either a raising operation of causing an air compressor to supply cathode gas so as to achieve a first air stoichiometric ratio or a lowering operation of causing the air compressor to supply the cathode gas so as to achieve a second air stoichiometric ratio lower than the first air stoichiometric ratio. In the case where the lowering operation is performed, the control unit performs, for a first time, the raising operation when a first condition including an output voltage or an output current being a value included within a first range, and does not perform the raising operation when a second condition including the output voltage or the output current being a value included within a second range smaller than the first range.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present disclosure relates to fuel cell systems. [Background technology]

[0002] To prevent fuel cells from deteriorating, it is necessary to prevent the stack from drying out. The fuel cell system of Patent Document 1 prevents the stack from drying out by adjusting the amount of water generated per unit time during power generation. More specifically, the fuel cell system of Patent Document 1 reduces the amount of cathode gas supplied to the stack, thereby reducing the amount of water removed by the cathode gas. This prevents the stack from drying out.

[0003] In such a case, the air stoichiometric ratio is lower than normal. The air stoichiometric ratio is the ratio of the amount of cathode gas actually supplied to the stack to the minimum amount of cathode gas required to generate the power required by the load. The smaller the air stoichiometric ratio, the less cathode gas is supplied to the stack.

[0004] In the fuel cell system of Patent Document 1, the air stoichiometric ratio is maintained at 1.0 or higher. The fuel cell system adjusts the flow rate of the cathode gas while the anode gas is supplied in an amount necessary to generate the required power. In other words, when the air stoichiometric ratio fluctuates, the power generated by the fuel cell also fluctuates. Note that when the generated power exceeds or falls short of the required power, the secondary battery provided in the fuel cell system discharges the shortage of power or charges the surplus power.

[0005] When the air stoichiometric ratio is lower than normal, moisture is likely to accumulate in the stack. When excess moisture is generated in the stack, flooding occurs. Flooding causes concentration overvoltage, which can cause the fuel cell to be unable to output the required power due to a drop in output voltage. For this reason, the fuel cell system of Patent Document 1 performs a drainage process before flooding occurs. The drainage process increases the amount of cathode gas supplied to the stack, thereby removing moisture that has accumulated in the stack.

[0006] The output voltage of a fuel cell drops due to the occurrence of overvoltages, which are composed of activation overvoltages, resistance overvoltages, and concentration overvoltages. The larger the output current, the larger the overvoltage. Furthermore, the larger the output current, the more significantly the concentration overvoltage in the overvoltage. In other words, the amount of change in output voltage relative to the output current tends to increase as the concentration overvoltage in the overvoltage increases. Fuel cell systems adjust the output power by adjusting the output current of the fuel cell using a DC-DC converter. However, if the concentration overvoltage in the overvoltage changes, it becomes difficult to adjust the output power with consistent accuracy. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2020-4675 Summary of the Invention [Problem to be solved by the invention]

[0008] The wastewater treatment in the fuel cell system of Patent Document 1 tends to dry out the stack, which may accelerate deterioration of the fuel cell. For this reason, the inventors have been studying ways to reduce the wastewater treatment when the air stoichiometric ratio is lower than normal.

[0009] However, if the wastewater treatment is reduced, concentration overvoltage due to flooding becomes more likely to occur. In a fuel cell system, increasing the flow rate of cathode gas increases the power generation. That is, when the current required by the load is constant, the output voltage increases. Therefore, if the output voltage decreases due to concentration overvoltage, the fuel cell system increases the flow rate of cathode gas to restore the decreased output voltage. However, increasing the flow rate of cathode gas removes moisture from the stack. Therefore, the concentration overvoltage due to flooding is eliminated. This increases the output voltage of the fuel cell, so the fuel cell system returns the flow rate of cathode gas to the state before the concentration overvoltage due to flooding occurred. However, because the wastewater treatment is reduced when the air stoichiometric ratio is lower than normal, flooding occurs and disappears again.

[0010] That is, the operating point determined by the output voltage and output current of the fuel cell fluctuates between a region where the proportion of concentration overvoltage in the overvoltage is relatively large and a region where the proportion of concentration overvoltage is relatively small.

[0011] Fluctuations in the operating point affect the amount of change in the output voltage relative to the fuel cell's output current, making it difficult to adjust the output power with a consistent level of precision, as mentioned above. Therefore, there was a need for technology that could prevent the stack from drying out and reduce fluctuations in the operating point. [Means for solving the problem]

[0012] The present disclosure can be realized in the following forms.

[0013] (1) According to one aspect of the present disclosure, there is provided a fuel cell system comprising: a fuel cell; an air compressor for adjusting the flow rate of a cathode gas flowing into the fuel cell; a flow rate sensor for acquiring the flow rate; a current sensor for acquiring an output current of the fuel cell; a voltage sensor for acquiring an output voltage of the fuel cell; a load device for consuming the output power of the fuel cell; and a control unit for controlling the fuel cell system, wherein the control unit controls the air compressor so that the flow rate becomes a predetermined required flow rate for outputting the output voltage corresponding to a required current determined based on a required power required by the load device and the output power, and controls the air compressor to obtain a first air stoichiometric ratio that is equal to or greater than 1. The control device performs either a power-up operation in which the air compressor is caused to supply the cathode gas, or a power-down operation in which the air compressor is caused to supply the cathode gas so as to achieve a second air stoichiometric ratio that is one or more of the air stoichiometric ratios and is lower than the first air stoichiometric ratio. When the power-down operation is being performed, if a first condition is satisfied, including that the output voltage or the output current is a value included in a first range, the control device performs the power-up operation for a predetermined first time, and if a second condition is satisfied, including that the output voltage or the output current is a value included in a second range that is lower than the first range, the control device does not perform the power-up operation. By adopting this configuration, the fuel cell system of the present disclosure performs power-up operation for a first time when the output current or output voltage is within a first range. During power-up operation, the air stoichiometric ratio is higher than during power-down operation, so a greater amount of water is removed from the fuel cell than during power-down operation. In other words, during power-up operation, concentration overvoltage due to flooding is less likely to occur than during power-down operation. Therefore, the fuel cell system of the present disclosure can prevent fluctuations in the operating point. Furthermore, since the fuel cell system of the present disclosure increases the cathode gas flow rate in response to concentration overvoltage due to flooding, it can prevent deterioration of the fuel cell due to drying out, compared to a configuration in which the flow rate of the cathode gas is increased to prevent flooding. (2) According to another aspect of the present disclosure, there is provided a fuel cell system comprising: a fuel cell; an air compressor that adjusts the flow rate of a cathode gas flowing into the fuel cell; a flow rate sensor that acquires the flow rate; a current sensor that acquires an output current of the fuel cell; a voltage sensor that acquires an output voltage of the fuel cell; a load device that consumes the output power of the fuel cell; an output adjustment unit that adjusts the output current to a required current determined based on the required power of the load device and the output power; and a control unit that controls the fuel cell system, wherein the control unit controls the air compressor so that the flow rate becomes a predetermined required flow rate for outputting the output voltage according to the required current, and controls the air compressor to supply the cathode gas to a first air stoichiometric ratio that is equal to or greater than 1. and a reduction operation in which the air compressor supplies the cathode gas to the air compressor so that the air stoichiometric ratio is one or more and is a second air stoichiometric ratio that is lower than the first air stoichiometric ratio. When the reduction operation is being performed and the requested current is controlled so that the response speed of the output current becomes a first response speed, when a first condition is satisfied that includes the output voltage or the output current being a value included in a first range, a first control is performed to control the response speed of the output current to a second response speed that is slower than the first response speed, and when a second condition is satisfied that includes the output voltage or the output current being a value included in a second range that is lower than the first range, the first control is not performed. By adopting this configuration, the fuel cell system of the present disclosure controls the required current so that the response speed of the output current becomes a second response speed slower than the first response speed when the output current or output voltage is within a first range. The air compressor is controlled to achieve a required flow rate for outputting an output voltage corresponding to the required current. In other words, when the required current fluctuates slowly, the flow rate of the cathode gas also fluctuates slowly. This suppresses fluctuations in the amount of water contained in the fuel cell, thereby making it less likely for the proportion of concentration overvoltage in the overvoltage to fluctuate. Therefore, the fuel cell system of the present disclosure suppresses fluctuations in concentration overvoltage due to flooding, thereby preventing fluctuations in the operating point during pull-down operation. Furthermore, because the fuel cell system of the present disclosure increases the cathode gas flow rate in response to concentration overvoltage due to flooding, deterioration due to dryness of the fuel cell can be prevented compared to a configuration in which the flow rate of the cathode gas is increased to prevent flooding. (3) In the fuel cell system of the above aspect, the first condition may include that the amplitude of the output voltage or the output current in a predetermined cycle is included in the first range. By adopting such a configuration, the fuel cell system of the present disclosure can prevent the control that would be performed when the first condition is satisfied from being erroneously executed due to a momentary fluctuation in output voltage or output current that is shorter than a predetermined cycle. (4) In the fuel cell system of the above aspect, the control unit may execute the reduction operation after the first time period has elapsed. By adopting such a configuration, the fuel cell system of the present disclosure can prevent the fuel cell from drying out after the first time period has elapsed more effectively than a configuration in which the reduction operation is not performed. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is an explanatory diagram showing the configuration of a fuel cell system according to a first embodiment. [Figure 2] FIG. 3 is an explanatory diagram showing operating points of a fuel cell stack. [Figure 3] FIG. 3 is an explanatory diagram showing operating points of a fuel cell stack. [Figure 4] 3 is a flowchart showing a control method for the fuel cell system of the first embodiment. [Figure 5] 6 is a flowchart showing a control method for a fuel cell system according to a second embodiment. [Figure 6] 10 is a waveform showing the output power of the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0015] A. First embodiment: A-1. Fuel cell system configuration: 1 is an explanatory diagram showing the configuration of a fuel cell system 10 according to a first embodiment. The fuel cell system 10 includes a fuel cell stack 100, a cathode gas supply / discharge system 200, an anode gas supply / discharge system 300, an output circuit 400, and a control unit 500. The fuel cell system 10 according to this embodiment is mounted on, for example, an electric vehicle and used as a driving power source or a power source for various devices.

[0016] The fuel cell stack 100 generates electricity by receiving anode gas and cathode gas as reactant gases. For example, the anode gas is hydrogen and the cathode gas is air. The fuel cell stack 100 is a polymer electrolyte fuel cell, and has a stack structure in which multiple unit cells serving as power generation bodies are stacked. In this specification, the "fuel cell stack" is also simply referred to as a "fuel cell" or a "stack."

[0017] A single cell includes an MEA (Membrane Electrode Assembly), a pair of gas diffusion layers arranged to sandwich the MEA, and a pair of gas separators arranged on the outside of each gas diffusion layer. The MEA includes an electrolyte membrane and an anode and a cathode, which are catalytic electrode layers formed on each side of the electrolyte membrane. In each single cell, an anode gas flow channel 120 through which anode gas flows is formed on the anode side, and a cathode gas flow channel 110 through which cathode gas flows is formed on the cathode side, with the electrolyte membrane interposed therebetween. In FIG. 1, the single cell is not shown to facilitate understanding of the technology.

[0018] In the cathode, water is generated as the electrochemical reaction progresses. This increases the amount of water contained in the cathode gas flow channel 110. However, since the water in the cathode gas flow channel 110 is removed by the cathode gas, the amount of water contained in the cathode gas flow channel 110 varies depending on the amount of water generated and the flow rate of the cathode gas. Therefore, depending on the flow rate of the cathode gas, the cathode gas flow channel 110 may dry out or flood. Adjustment of the water amount in the cathode gas flow channel 110 by the fuel cell system 10 will be described in detail later. In this specification, the water amount in the fuel cell stack 100 refers to the water amount in the cathode gas flow channel 110.

[0019] The cathode gas supply / discharge system 200 supplies cathode gas to the fuel cell stack 100 and discharges cathode gas from the fuel cell stack 100. The cathode gas supply / discharge system 200 includes a cathode gas supply pipe 210, a flow rate sensor 220, an air compressor 230, and a cathode gas discharge pipe 240.

[0020] The cathode gas supply pipe 210 supplies cathode gas from outside the fuel cell system 10 to the fuel cell stack 100. The cathode gas supply pipe 210 is composed of a first cathode gas supply pipe 211 that connects the outside to the inlet of the air compressor 230, and a second cathode gas supply pipe 212 that connects the outlet of the air compressor 230 to the inlet of the cathode gas flow path 110. As a result, the cathode gas supply pipe 210 supplies the cathode gas supplied from the outside by the air compressor 230 to the cathode gas flow path 110.

[0021] The flow rate sensor 220 acquires the flow rate of the cathode gas. More specifically, the flow rate sensor 220 measures the flow rate of the cathode gas flowing from the first cathode gas supply pipe 211 toward the air compressor 230. The flow rate sensor 220 sends the acquired flow rate to the control unit 500.

[0022] The air compressor 230 adjusts the flow rate of the cathode gas that flows into the fuel cell stack 100. More specifically, the air compressor 230 compresses the cathode gas from the first cathode gas supply pipe 211 and discharges the cathode gas to the second cathode gas supply pipe 212. The air compressor 230 adjusts the flow rate of the cathode gas by compressing the cathode gas in accordance with a command from the control unit 500.

[0023] Adjusting the flow rate of the cathode gas adjusts the output voltage Vf of the fuel cell stack 100. The control of power of the fuel cell system 10 will be described in detail later. Furthermore, adjusting the flow rate of the cathode gas adjusts the amount of water in the fuel cell stack 100, as described above.

[0024] The cathode gas discharge pipe 240 discharges the cathode gas discharged from the fuel cell stack 100 to the outside of the fuel cell system 10.

[0025] The anode gas supply / discharge system 300 supplies anode gas to the fuel cell stack 100 and discharges the anode gas from the fuel cell stack 100. The anode gas supply / discharge system 300 includes an anode gas tank, an anode gas pump, and the like. The anode gas supply / discharge system 300 also includes a supply pipe and an exhaust pipe that allow the anode gas supplied from the anode gas tank to flow through the anode gas flow path 120. However, to facilitate understanding of the technology, the configuration of the anode gas supply / discharge system 300 is not shown in FIG. 1.

[0026] In this specification, the fuel cell system 10 operates in a state where the anode gas supply / discharge system 300 is supplied with an amount of anode gas necessary to generate the required power.

[0027] The output circuit 400 supplies power from the fuel cell stack 100 in accordance with the power required by the load device 440. The output circuit 400 includes a voltage sensor 410, a current sensor 420, an output adjustment unit 430, and the load device 440.

[0028] The voltage sensor 410 acquires the output voltage Vf of the fuel cell stack 100. The voltage sensor 410 sends the acquired output voltage Vf to the control unit 500.

[0029] The current sensor 420 acquires the output current If of the fuel cell stack 100. The current sensor 420 sends the acquired output current If to the control unit 500.

[0030] The output adjustment unit 430 adjusts the output power P1 of the fuel cell stack 100 in accordance with the power required by the load. More specifically, the output adjustment unit 430 adjusts the output current If so that it becomes a required current determined based on the power required by the load device 440 and the output power P1. The control of the output adjustment unit 430 will be described in detail later. The output adjustment unit 430 is specifically a DC-DC converter. The output adjustment unit 430 is connected to the power output unit of the fuel cell stack 100.

[0031] Furthermore, the output adjustment unit 430 transforms the output voltage Vf of the fuel cell stack 100 to a voltage required by the load device 440. For example, a drive device for a motor included in the load device 130 requires a voltage higher than the output voltage Vf of the fuel cell stack 100. Therefore, the output adjustment unit 430 boosts the output voltage Vf of the fuel cell stack 100 to the voltage required by the drive device for the motor. The "output adjustment unit" is also referred to as an "FDC."

[0032] The load device 440 consumes an output power P1. More specifically, the load device 130 consumes the output power P1 adjusted by the output adjustment unit 430 in accordance with the required power. In this specification, the load power P2 is the output power P1 adjusted by the output adjustment unit 430. Furthermore, the load power P2 required by the load device 130 is referred to as the "required power." The load device 440 includes, for example, a vehicle drive device including a motor and a motor drive circuit, an air compressor 230, an anode gas pump, and the like. The load device 440 sends information about the required load power P2 to the control unit 500.

[0033] The control unit 500 controls the fuel cell system 10. The control unit 500 is configured as a logic circuit centered around a microcomputer. More specifically, the control unit 500 includes a CPU, a ROM, a RAM, and input / output ports for inputting and outputting various signals. The CPU executes a preset control program. The ROM stores in advance control programs and control data required for the CPU to execute various arithmetic processes. The RAM temporarily reads and writes various data required for the CPU to execute various arithmetic processes. The functions of the control unit 500 are described below.

[0034] A-2. Raising and lowering operations: The control unit 500 performs either an operation to increase the air stoichiometric ratio or an operation to decrease the air stoichiometric ratio by controlling the flow rate of the cathode gas using the air compressor 230. The air stoichiometric ratio is the ratio of the amount of cathode gas actually supplied to the fuel cell stack 100 to the minimum amount of cathode gas necessary to generate the required power. In other words, the air stoichiometric ratio is 1.0 or greater.

[0035] The air stoichiometric ratio raising operation is an operation in which the air compressor 230 is caused to supply cathode gas so that the air stoichiometric ratio becomes a first air stoichiometric ratio. Specifically, the first air stoichiometric ratio is 1.5. The first air stoichiometric ratio is based on the air stoichiometric ratio at which the power consumption of the vehicle's drive device is maximized. In other words, the first air stoichiometric ratio is based on the air stoichiometric ratio at which the power obtained by subtracting the power consumption of the air compressor 230, the cathode pump, etc. from the output power P1 of the fuel cell stack 100 is maximized.

[0036] The operation of decreasing the air stoichiometric ratio is an operation in which the air compressor 230 supplies cathode gas so that the air stoichiometric ratio becomes a second air stoichiometric ratio that is lower than the first air stoichiometric ratio. For example, when the first air stoichiometric ratio is 1.5, the second air stoichiometric ratio is a value included in the range of 1.2 to 1.3.

[0037] By operating the air stoichiometric ratio downward, the flow rate of the cathode gas is reduced compared to the case of the first air stoichiometric ratio. The reduction in the flow rate of the cathode gas reduces the amount of water removed from the fuel cell stack 100. In other words, the amount of water contained in the fuel cell stack 100 increases, preventing the fuel cell stack 100 from drying out. However, the increased amount of water in the fuel cell stack 100 makes flooding more likely to occur. Note that the "air stoichiometric ratio raising operation" is also simply referred to as the "raising operation," and the "air stoichiometric ratio lowering operation" is also simply referred to as the "lowering operation."

[0038] Even in the case of operation to increase the air stoichiometric ratio, the cathode gas supplied from the air compressor 230 is dry, so the fuel cell stack 100 may become partially dry. In particular, the inlet portion of the cathode gas flow path 110 may become dry. In such a case, by performing operation to decrease the air stoichiometric ratio, the fuel cell stack 100 is prevented from drying out.

[0039] A-3. Equal power operation: The control unit 500 further performs power operation, such as controlling the output power P1 of the fuel cell stack 100 in accordance with the power required by the load device 440. More specifically, the control unit 500 acquires information about the required power from the load device 130. The control unit 500 controls the air compressor 230 based on a predetermined reference value for the output voltage Vf in accordance with the required power. When cathode gas is supplied to the fuel cell stack 100, the fuel cell stack 100 starts to generate power, thereby generating the output voltage Vf. Furthermore, the control unit 500 controls the output adjustment unit 430 based on a predetermined reference value for the output current If in accordance with the required power. That is, the output adjustment unit 430 extracts the output current If from the fuel cell stack 100 based on the reference value for the output current If, thereby outputting the output current If in accordance with the reference value for the output current If. Furthermore, the control unit 500 acquires the output current If and the output voltage Vf using the voltage sensor 410 and the current sensor 420. The control unit 500 calculates the output power P1 based on the acquired output current If and output voltage Vf. The control unit 500 calculates the required current to be extracted by the output adjustment unit 430 based on the difference between the output power P1 and the required power and the reference value of the output current If. That is, the required current varies depending on the difference between the output power P1 and the required power. The control unit 500 sets the required current as a command value for controlling the output adjustment unit 430. As a result, the output adjustment unit 430 adjusts the output current If so that it becomes the required current determined based on the required power of the load device 440 and the output power. That is, the output adjustment unit 430 causes the fuel cell stack 100 to output an output power P1 that satisfies the required power. Such operation of the fuel cell system 10 is called "constant power operation."

[0040] As described above, the air stoichiometric ratio is the ratio of the amount of cathode gas actually supplied to the fuel cell stack 100 to the minimum amount of cathode gas required to generate the required power. During operation to increase or decrease the air stoichiometric ratio, it is necessary to vary the flow rate of the cathode gas according to the required power. For this reason, the control unit 500 controls the air compressor 230 so that the flow rate of the cathode gas becomes the required flow rate. The required flow rate is a predetermined flow rate for outputting an output voltage Vf according to the required current.

[0041] For example, during operation to reduce the air stoichiometric ratio and when constant power operation is being performed, the control unit 500 increases the flow rate of the cathode gas by the air compressor 230 in response to an increase in the required current. Note that, as described above, an increase in the required current occurs in response to the difference between the output power P1 and the required power. For example, an increase in the required current occurs when the required power increases or when the output power P1 decreases in response to a decrease in output voltage Vf due to concentration overvoltage caused by flooding.

[0042] A-4. Operating point fluctuation: FIG. 2 is an explanatory diagram showing the operating points of the fuel cell stack 100. In FIG. 2, the horizontal axis represents the current density of the unit cell of the fuel cell stack 100, and the vertical axis represents the voltage of the unit cell. The output current If depends on the current density of the unit cell. The output voltage Vf depends on the voltage of the unit cell. In FIG. 2, the theoretical electromotive force is shown by a dashed line. As shown by a solid line in FIG. 2, the voltage of the unit cell decreases as the current density of the unit cell increases because the overvoltage increases. Overvoltage is divided into activation overvoltage, resistance overvoltage, and concentration overvoltage. As shown in FIG. 2, the proportion of concentration overvoltage in the overvoltage increases as the current density of the unit cell increases. Because the output current If depends on the current density of the unit cell, the change in concentration overvoltage relative to the output current If increases as the output current If increases. The region in FIG. 2 where the change in concentration overvoltage is small is called the "first region." The region in FIG. 2 where the change in concentration overvoltage is large is called the "second region." The first region is also the region where the change in output voltage Vf relative to the output current If is large. The second region is also a region where the amount of change in output voltage Vf with respect to output current If is small. In FIG. 2, the point on the solid line defined by the voltage and current density of the single cell is the operating point of the single cell. In FIG. 2, the operating point is located in the first region. The power density output by the single cell corresponds to the area on the plane with the operating point as its vertex. The output power P1 depends on the power density of the single cell. In this specification, to facilitate understanding of the technology, the operating points shown in FIG. 2 or FIG. 3 are treated as the operating points of the fuel cell stack 100.

[0043] FIG. 3 is an explanatory diagram showing the operating point of the fuel cell stack 100. The operating point of the fuel cell stack 100 during the operation of decreasing the air stoichiometric ratio and constant power operation will be described. When the operation of decreasing the air stoichiometric ratio causes a concentration overvoltage due to flooding, the operating point is located in the second region as shown in FIG. 3. As described above, an increase in the required current occurs when the output power P1 decreases in response to a decrease in the output voltage Vf due to the concentration overvoltage caused by flooding. Therefore, the control unit 500 increases the flow rate of the cathode gas by the air compressor 230 in response to the increase in the required current. This removes moisture from the fuel cell stack 100, thereby eliminating the concentration overvoltage due to flooding. That is, as the output voltage Vf increases, the operating point is located in the first region as shown in FIG. 2. However, as the output voltage Vf increases, a difference occurs between the output power P1 and the required power, resulting in a decrease in the required current. In response to the decrease in the required current, the control unit 500 reduces the flow rate of the cathode gas by the air compressor 230. That is, the possibility of flooding occurring again increases. Therefore, the operating point fluctuates between the first region and the second region. When the operating point fluctuates, the amount of change in the output voltage Vf relative to the output current If changes, making it difficult for the control unit 500 to control the output power P1 with a constant accuracy.

[0044] In general, the response speed of the output current If adjusted by the output adjustment unit 430, which is a DC-DC converter, is faster than the response speed of the cathode gas flow rate adjusted by the air compressor 230. Therefore, when the operating point fluctuates, the response of the cathode gas flow rate cannot follow the response of the output current If. Therefore, even during constant power operation, the fuel cell stack 100 may not be able to output the output power P1 that satisfies the required power due to a delay in the response of the cathode gas flow rate. In other words, the output power P1 may be insufficient. The response speed of the output current If is the time from when a fluctuation in the required current occurs until the output current If reaches the required current. The response speed of the cathode gas flow rate is the time from when a fluctuation in the required flow rate occurs until the cathode gas flow rate reaches the required flow rate.

[0045] A-5. Control method for fuel cell system: 4 is a flowchart showing a control method for the fuel cell system 10 of the first embodiment. The control method for the fuel cell system 10 will be described below. The control unit 500 repeatedly executes the following process while the fuel cell system 10 is in operation.

[0046] 4, the control unit 500 determines whether or not an operation to decrease the air stoichiometric ratio is being performed. That is, if the air stoichiometric ratio is not the second air stoichiometric ratio, the control unit 500 advances the process to step S160. If the air stoichiometric ratio is the second air stoichiometric ratio, the control unit 500 advances the process to step S110.

[0047] 4, the control unit 500 determines whether constant power operation is being performed. If constant power operation is not being performed, the control unit 500 proceeds to step S170. If constant power operation is being performed, the control unit 500 proceeds to step S120.

[0048] In step S120 of Fig. 4, the control unit 500 detects fluctuations in the operating point. Note that in step S120, an operation to reduce the air stoichiometric ratio is being performed. Specifically, the control unit 500 advances the process to step S130 when a first condition is satisfied, including that the voltage sensor 410 detects that the output voltage Vf of the fuel cell stack 100 is a value included in a first range. The control unit 500 advances the process to step S180 when the voltage sensor 410 detects that the output voltage Vf of the fuel cell stack 100 is a value included in a second range lower than the first range. Note that the output voltage Vf in step S120 specifically refers to the amplitude of the output voltage Vf.

[0049] The "second range lower than the first range" means that the upper limit of the second range is lower than the lower limit of the first range. The first range and the second range are defined by a predetermined threshold. More specifically, the first range is equal to or greater than the threshold, and the second range is equal to or less than the threshold. The threshold is set experimentally, for example, based on the maximum amplitude of the output voltage Vf that can be reached due to fluctuations in the load power P2.

[0050] The first condition further includes the condition that the amplitude of the output voltage in a predetermined period falls within a first range. Specifically, the predetermined period is experimentally set based on the response speed of the cathode gas flow rate. By adopting this configuration, the fuel cell system 10 of the present disclosure can prevent erroneous control from being performed due to momentary fluctuations in the output voltage Vf that are shorter than the predetermined period. The momentary fluctuations in the output voltage Vf occur, for example, due to a sudden change in the load power P2.

[0051] 4, the control unit 500 executes an operation to increase the air stoichiometric ratio. That is, the control unit 500 increases the flow rate of the cathode gas by the air compressor 230 so that the air stoichiometric ratio becomes the first air stoichiometric ratio.

[0052] In step S140 of FIG. 4, the control unit 500 continues to perform the pull-up operation for a predetermined first time period so that the air stoichiometric ratio is maintained at the first air stoichiometric ratio. The first time period is the time required to resolve flooding. In this specification, the "first time period" is also referred to as the "drainage time period." The drainage time period is experimentally set according to the specifications of the fuel cell stack 100. For example, the drainage time period is 30 minutes.

[0053] 4, the control unit 500 executes an operation to reduce the air stoichiometric ratio after the first time has elapsed. That is, the control unit 500 controls the air compressor 230 to reduce the flow rate of the cathode gas so that the air stoichiometric ratio becomes the second air stoichiometric ratio. By adopting this configuration, the fuel cell system 10 of the present disclosure can prevent the fuel cell from drying out more effectively than in a configuration in which the reduction operation is not executed after the first time has elapsed.

[0054] In step S160 of FIG. 4, the control unit 500 continues operation with the air stoichiometric ratio increased.

[0055] In step S170 in Fig. 4, the control unit 500 continues operation with the air stoichiometric ratio reduced. The same process is carried out in step S180 in Fig. 4.

[0056] The above process is repeated while the fuel cell stack 100 outputs the output power P1. That is, after the process of any one of steps S150 to S180 in Fig. 4, the control unit 500 starts the process of step S100.

[0057] Regarding the above, in this embodiment, when the air stoichiometric ratio becomes the second air stoichiometric ratio, the flow rate of the cathode gas is reduced compared to when the air stoichiometric ratio is the first air stoichiometric ratio. This reduces the amount of water removed from the fuel cell, preventing the fuel cell from drying out. However, when the air stoichiometric ratio becomes the second air stoichiometric ratio, the amount of water contained in the fuel cell increases, increasing the possibility of concentration overvoltage occurring due to flooding. Concentration overvoltage reduces the output voltage Vf of the fuel cell.

[0058] The fuel cell system 10 of the present disclosure controls the air compressor 230 so that the flow rate of the cathode gas becomes the required flow rate for outputting the output voltage Vf corresponding to the required current. When the output voltage Vf decreases due to concentration overvoltage, the air compressor 230 increases the flow rate of the cathode gas. This increases the amount of water removed from the fuel cell, thereby reducing the concentration overvoltage caused by flooding. As the concentration overvoltage decreases, the output voltage Vf increases. In this case, the air compressor 230 reduces the flow rate of the cathode gas. In other words, flooding occurs and disappears again. The occurrence and disappearance of flooding changes the proportion of the concentration overvoltage in the overvoltage. Therefore, the operating point defined by the output voltage Vf and output current If of the fuel cell fluctuates.

[0059] When detecting a fluctuation in the operating point, the fuel cell system 10 of the present disclosure performs a pull-up operation for a first time. This reduces the amount of water contained in the fuel cell, thereby eliminating concentration overvoltage due to flooding. During pull-up operation, the air stoichiometric ratio is higher than during pull-down operation, so more water is removed from the fuel cell than during pull-down operation. In other words, during pull-up operation, concentration overvoltage due to flooding is less likely to occur than during pull-down operation. Therefore, the fuel cell system 10 of the present disclosure can prevent fluctuations in the operating point. Furthermore, because the fuel cell system 10 of the present disclosure increases the cathode gas flow rate in response to concentration overvoltage due to flooding, it can prevent deterioration of the fuel cell due to drying out, compared to a configuration in which the cathode gas flow rate is increased to prevent flooding.

[0060] Furthermore, by defining the first condition, the fuel cell system 10 of the present disclosure can prevent erroneous execution of control when the first condition is satisfied due to a momentary fluctuation in the output voltage Vf that is shorter than a predetermined period. Note that the same effect can be obtained in the second embodiment.

[0061] Furthermore, by performing the reduction operation after the first time has elapsed, the fuel cell system 10 of the present disclosure can prevent the fuel cell from drying out more effectively than a configuration in which the reduction operation is not performed after the first time has elapsed.

[0062] Furthermore, the fuel cell system 10 of the present disclosure can prevent the output power P1 from becoming insufficient by preventing fluctuations in the operating point. Specifically, the same effect as that of the second waveform W2 in Fig. 6 of the second embodiment can be obtained.

[0063] B. Second embodiment: FIG. 5 is a flowchart showing a control method for the fuel cell system 10 of the second embodiment. When a fluctuation in the operating point is detected, the fuel cell system 10 of the first embodiment performs an operation to increase the air stoichiometric ratio. This prevents the fluctuation in the operating point. However, the fluctuation in the operating point may be prevented by other methods. The control method for the fuel cell system 10 of the second embodiment will be described below. Note that the configuration of the fuel cell system 10 of the first embodiment is the same as that of the fuel cell system 10 of the second embodiment. In the second embodiment, the control unit 500 repeatedly executes the following process while the fuel cell system 10 is in operation.

[0064] The processing in steps S200 to S220 in FIG. 5 is the same as the processing in steps S100 to S120 in FIG.

[0065] In step S230 of FIG. 5, the control unit 500 slows down the response speed of the output current If. More specifically, the control unit 500 controls the requested current so that the response speed of the output current If becomes a second response speed that is slower than the first response speed. This control is referred to as "first control." Note that in step S220, the control unit 500 is performing a step-down operation and controlling the requested current so that the response speed of the output current If becomes the first response speed.

[0066] The first response speed is the response speed of the output current If when the first control is not performed during constant power operation. The second response speed is the response speed of the cathode gas flow rate during constant power operation. That is, the control unit 500 controls the output adjustment unit 430 so that the response speed of the output current If approximately matches the response speed of the cathode gas flow rate.

[0067] In step S230 of FIG. 5, the control unit 500 executes the first control and then ends the process.

[0068] The process of step S240 in FIG. 5 is the same as the process of step S160 in FIG.

[0069] The processes in steps S250 and S260 in FIG. 5 are similar to those in steps S170 and S180 in FIG.

[0070] The above process is repeated while the fuel cell stack 100 outputs the output power P1. That is, after the process of any one of steps S230 to S260 in Fig. 5, the control unit 500 starts the process of step S200.

[0071] In this embodiment, as in the first embodiment, fluctuations occur in the operating point defined by the fuel cell output voltage Vf and output current If. When the output voltage Vf is within a first range, the fuel cell system 10 of the second embodiment controls the required current so that the response speed of the output current If becomes a second response speed slower than the first response speed. The air compressor 230 is controlled to achieve a required flow rate for outputting the output voltage Vf corresponding to the required current. That is, when the required current fluctuates gradually, the flow rate of the cathode gas also fluctuates gradually. This suppresses fluctuations in the amount of water contained in the fuel cell, thereby making it difficult for the proportion of concentration overvoltage in the overvoltage to fluctuate. Therefore, the fuel cell system 10 of the second embodiment prevents fluctuations in the operating point during step-down operation by suppressing fluctuations in concentration overvoltage due to flooding. Furthermore, the fuel cell system 10 of the second embodiment increases the cathode gas flow rate in response to the concentration overvoltage due to flooding, thereby preventing deterioration due to dryness of the fuel cell compared to a configuration in which the flow rate of the cathode gas is increased to prevent flooding.

[0072] FIG. 6 shows a waveform illustrating the output power P1 of the second embodiment. In FIG. 6, the horizontal axis represents the passage of time, and the vertical axis represents the magnitude of the output power P1. The first waveform W1 is the waveform of the output power P1 when the first control is not performed. The second waveform W2 is the waveform of the output power P1 when the first control is performed. As explained in the first embodiment, when the operating point fluctuates, the output power P1 fluctuates due to a delay in the response of the cathode gas flow rate. In other words, a shortage of the output power P1 occurs. However, the fuel cell system 10 of the second embodiment can prevent a shortage of the output power P1 by preventing the operating point from fluctuating.

[0073] C. Variations: (1) In the above embodiment, the first air stoichiometric ratio is 1.5. However, the first air stoichiometric ratio may be any value as long as it is 1.5 or greater. Furthermore, in the above embodiment, the second air stoichiometric ratio is a value within the range of 1.2 to 1.3. However, the second air stoichiometric ratio may be any value as long as it is 1.0 or greater and less than 1.5. (2) In the above embodiment, the first condition is a condition that the amplitude of the output voltage Vf or the output current If in a predetermined period falls within a first range. However, the first condition does not have to include such a condition. Furthermore, in the above embodiment, the predetermined period is experimentally set based on the response speed of the cathode gas flow rate. However, the predetermined period may also be experimentally set regardless of the response speed of the cathode gas flow rate. This configuration facilitates control and setting of the fuel cell system 10 of the present disclosure. (3) In the above embodiment, the control unit 500 executes the reduction operation after the first time has elapsed. However, the control unit 500 does not have to execute the reduction operation after the first time has elapsed. Even if the reduction operation is not executed, the drying of the fuel cell stack 100 is reduced because the reduction operation has already been executed. (4) In the above embodiment, the control unit 500 may be configured with a plurality of microcomputers. More specifically, the function of the control unit 500 to increase or decrease the air stoichiometric ratio or to control the output adjustment unit 430 does not have to be realized by a single microcomputer. (5) In the above embodiment, the control unit 500 detects fluctuations in the operating point based on the output voltage Vf. However, the control unit 500 may detect fluctuations in the operating point based on the output current If. More specifically, when the current sensor 420 determines that a first condition is satisfied, including that the output current If of the fuel cell stack 100 is a value included in a first range, the control unit 500 proceeds to step S130 in FIG. 4 or step S230 in FIG. 5. When the current sensor 420 determines that a second condition is satisfied, including that the output voltage Vf of the fuel cell stack 100 is a value included in a second range that is smaller than the first range, the control unit 500 proceeds to step S180 in FIG. 4 or step S260 in FIG. 5. The first and second ranges are set in the same way as for the output voltage Vf. [Explanation of symbols]

[0074] 10... fuel cell system, 100... fuel cell stack, 110... cathode gas flow path, 120... anode gas flow path, 130... load device, 200... cathode gas supply / discharge system, 210... cathode gas supply pipe, 211... first cathode gas supply pipe, 212... second cathode gas supply pipe, 220... flow rate sensor, 230... air compressor, 240... cathode gas discharge pipe, 300... anode gas supply / discharge system, 400... output circuit, 410... voltage sensor, 420... current sensor, 430... output adjustment unit, 440... load device, 500... control unit, If... output current, P1... output power, P2... load power, Vf... output voltage, W1... first waveform, W2... second waveform

Claims

1. 1. A fuel cell system, comprising: A fuel cell; an air compressor for adjusting the flow rate of the cathode gas flowing into the fuel cell; a flow rate sensor for acquiring the flow rate; a current sensor for acquiring an output current of the fuel cell; a voltage sensor for acquiring an output voltage of the fuel cell; a load device that consumes the output power of the fuel cell; a control unit that controls the fuel cell system, The control unit controlling the air compressor so that the flow rate becomes a predetermined required flow rate for outputting the output voltage corresponding to a required current determined based on the required power required by the load device and the output power; performing either a raising operation in which the air compressor is caused to supply the cathode gas so as to achieve a first air stoichiometric ratio that is equal to or greater than 1, or a lowering operation in which the air compressor is caused to supply the cathode gas so as to achieve a second air stoichiometric ratio that is equal to or greater than 1 and is lower than the first air stoichiometric ratio; When the lowering operation is being performed, When a first condition is satisfied, the first condition includes that the output voltage or the output current is a value included in a first range, and the pull-up operation is performed for a predetermined first time period; The fuel cell system does not perform the boost operation when a second condition is satisfied, the second condition including that the output voltage or the output current is a value included in a second range lower than the first range.

2. 1. A fuel cell system, comprising: A fuel cell; an air compressor for adjusting the flow rate of the cathode gas flowing into the fuel cell; a flow rate sensor for acquiring the flow rate; a current sensor for acquiring an output current of the fuel cell; a voltage sensor for acquiring an output voltage of the fuel cell; a load device that consumes the output power of the fuel cell; an output adjustment unit that adjusts the output current so that the output current becomes a required current determined based on the required power of the load device and the output power; a control unit that controls the fuel cell system, The control unit controlling the air compressor so that the flow rate becomes a predetermined required flow rate for outputting the output voltage corresponding to the required current; performing either a raising operation in which the air compressor is caused to supply the cathode gas so as to achieve a first air stoichiometric ratio that is equal to or greater than 1, or a lowering operation in which the air compressor is caused to supply the cathode gas so as to achieve a second air stoichiometric ratio that is equal to or greater than 1 and is lower than the first air stoichiometric ratio; When the step-down operation is performed and the requested current is controlled so that the response speed of the output current becomes a first response speed, When a first condition is satisfied, including the output voltage or the output current being a value included in a first range, a first control is executed to control a response speed of the output current to a second response speed that is slower than the first response speed; A fuel cell system that does not execute the first control when a second condition is satisfied, the second condition including that the output voltage or the output current is a value included in a second range lower than the first range.

3. 3. The fuel cell system according to claim 1, The fuel cell system, wherein the first condition includes that the amplitude of the output voltage or the output current in a predetermined period is included in the first range.

4. 2. The fuel cell system according to claim 1, The control unit executes the reduction operation after the first time period has elapsed.

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