Fuel cell system

Abstract

The invention provides a fuel cell system. A fuel cell system is provided with a fuel cell, an air compressor, a flow rate sensor, a current sensor, a voltage sensor, a load device, and a control unit that controls the air compressor so as to achieve a required flow rate. When the cathode gas is supplied at a first air stoichiometric ratio, either a pull-up operation in which the cathode gas is supplied at a first air stoichiometric ratio or a pull-down operation in which the cathode gas is supplied at a second air stoichiometric ratio lower than the first air stoichiometric ratio is executed, and when a first condition is satisfied when the pull-down operation is executed, the cathode gas is supplied at a second air stoichiometric ratio lower than the first air stoichiometric ratio during a first time period. When the first condition is satisfied, the pull-up operation is executed, and when the second condition is satisfied, the pull-up operation is not executed.

Description

Technical Field

[0001] The present disclosure relates to a fuel cell system. Background Art

[0002] In order to prevent deterioration, a fuel cell needs to prevent drying of a battery pack. The fuel cell system of Japanese Patent Application Laid-Open No. 2020-4675 prevents drying of the battery pack by adjusting the amount of generated water per unit time generated by power generation. More specifically, the fuel cell system of Japanese Patent Application Laid-Open No. 2020-4675 reduces the amount of moisture removed by the cathode gas by reducing the cathode gas supplied to the battery pack. Thereby, drying of the battery pack can be prevented.

[0003] In this case, the air stoichiometry is in a state lower than normal. The air stoichiometry is the ratio of the amount of cathode gas actually supplied to the battery pack to the minimum amount of cathode gas required to generate the required power for the load. The smaller the ratio of the air stoichiometry, the less cathode gas is supplied to the battery pack.

[0004] In the fuel cell system of Japanese Patent Application Laid-Open No. 2020-4675, the air stoichiometry is maintained at 1.0 or more. The fuel cell system adjusts the flow rate of the cathode gas in a state where the anode gas is supplied in an amount required to generate the required power. That is, when the air stoichiometry changes, the power generation power of the fuel cell also changes. In addition, when the power generation power exceeds the required power or is insufficient with respect to the required power, the secondary battery included in the fuel cell system discharges the insufficient power or charges the excess power.

[0005] A state where the air stoichiometry is lower than normal is a state where moisture easily stays in the battery pack. When too much moisture is generated in the battery pack, overflow occurs. Since the overflow causes concentration overvoltage, there is a case where the fuel cell cannot output the required power due to a decrease in the output voltage. Therefore, the fuel cell system of Japanese Patent Application Laid-Open No. 2020-4675 performs a drainage process before overflow occurs. Due to the drainage process, the amount of cathode gas supplied to the battery pack increases, and thus the moisture staying in the battery pack is removed.

[0006] The output voltage of a fuel cell decreases due to the generation of overvoltages composed of activation overvoltage, resistance overvoltage, and concentration overvoltage. The larger the output current, the larger the overvoltage. Moreover, regarding the overvoltage, the larger the output current, the more significantly the proportion of the concentration overvoltage in the overvoltage increases. That is, the change amount of the output voltage with respect to the output current shows a tendency to increase as the proportion of the concentration overvoltage in the overvoltage increases. A fuel cell system adjusts the output power by adjusting the output current of the fuel cell using a DCDC converter. However, when the proportion of the concentration overvoltage in the overvoltage changes, it is difficult to adjust the output power with a certain accuracy.

[0007] The drainage treatment of the fuel cell system disclosed in Japanese Unexamined Patent Application Publication No. 2020 - 4675 easily dries the battery pack, so there is a possibility of promoting the deterioration of the fuel cell. Therefore, the inventors studied a method of reducing the drainage treatment in a state where the air stoichiometric ratio is lower than normal.

[0008] However, when reducing the drainage treatment, it is easy to generate concentration overvoltage caused by overflow. The fuel cell system increases the power generation by increasing the flow rate of the cathode gas. That is, when the current required by the load is constant, the output voltage rises. Therefore, when the output voltage decreases due to the concentration overvoltage, the fuel cell system returns the decreased output voltage to its original state by increasing the flow rate of the cathode gas. However, the moisture in the battery pack is removed due to the increase in the flow rate of the cathode gas. Therefore, the concentration overvoltage caused by overflow is eliminated. As a result, since the output voltage of the fuel cell rises, the fuel cell system returns the flow rate of the cathode gas to before the generation of the concentration overvoltage caused by overflow. However, since the drainage treatment is reduced in a state where the air stoichiometric ratio is lower than normal, the generation and disappearance of overflow occur again.

[0009] That is, the operating point defined by the output voltage and output current of the fuel cell varies between a region where the proportion of the concentration overvoltage in the overvoltage is relatively large and a region where the proportion is relatively small.

[0010] Since the change amount of the output voltage of the fuel cell with respect to the output current changes due to the variation of the operating point, as described above, it is difficult to adjust the output power with a certain accuracy. Therefore, a technique for reducing the variation of the operating point while preventing the drying of the battery pack is required. Summary of the Invention

[0011] The present disclosure can be implemented in the following manner.

[0012] (1) According to one aspect of the present disclosure, a fuel cell system is provided.

[0013] The fuel cell system includes:

[0014] Fuel cell;

[0015] An air compressor that adjusts the flow rate of the cathode gas flowing into the fuel cell above;

[0016] A flow rate sensor that obtains the above flow rate;

[0017] A current sensor that obtains the output current of the fuel cell above;

[0018] A voltage sensor that obtains the output voltage of the fuel cell above;

[0019] A load device that consumes the output power of the fuel cell above; and

[0020] A control unit that controls the fuel cell system above,

[0021] The control unit controls the air compressor such that the flow rate becomes a predetermined required flow rate for outputting the output voltage corresponding to the required current determined based on the required power required by the load device and the output power above,

[0022] The control unit performs either a pull-up operation in which the air compressor supplies the cathode gas in such a way as to achieve a first air stoichiometric ratio or a pull-down operation in which the air compressor supplies the cathode gas in such a way as to achieve a second air stoichiometric ratio, where the first air stoichiometric ratio is an air stoichiometric ratio of 1 or more, the second air stoichiometric ratio is an air stoichiometric ratio of 1 or more, and is lower than the first air stoichiometric ratio,

[0023] In the case of performing the above pull-down operation,

[0024] When a first condition including the case where the output voltage or the output current is a value included in a first range is satisfied, the above pull-up operation is performed during a predetermined first time period,

[0025] When a second condition including the case where the output voltage or the output current is a value included in a second range lower than the first range is satisfied, the above pull-up operation is not performed.

[0026] By being in this way, when the output current or output voltage of the fuel cell system of the present disclosure is a value included in the first range, during the first period of time, pull-up operation is performed. During the pull-up operation, since the air stoichiometric ratio is higher than that during the pull-down operation, more moisture is removed from the fuel cell than during the pull-down operation. That is, during the pull-up operation, it is more difficult to generate concentration overvoltage caused by overflow than in the case of the pull-down operation. Therefore, the fuel cell system of the present disclosure can prevent fluctuations in the operating point. And since the cathode gas is increased corresponding to the concentration overvoltage caused by overflow, the fuel cell system of the present disclosure can prevent deterioration of the fuel cell caused by drying compared with the method of increasing the flow rate of the cathode gas in advance to prevent overflow.

[0027] (2) According to another aspect of the present disclosure, a fuel cell system is provided.

[0028] The fuel cell system includes:

[0029] A fuel cell;

[0030] An air compressor that adjusts the flow rate of the cathode gas flowing into the fuel cell;

[0031] A flow rate sensor that obtains the flow rate;

[0032] A current sensor that obtains the output current of the fuel cell;

[0033] A voltage sensor that obtains the output voltage of the fuel cell;

[0034] A load device that consumes the output power of the fuel cell;

[0035] 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; and

[0036] A control unit that controls the fuel cell system,

[0037] 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 the required current,

[0038] The above control unit executes either a pull-up operation in which the air compressor supplies the cathode gas so as to achieve a first air stoichiometric ratio or a pull-down operation in which the air compressor supplies the cathode gas so as to achieve a second air stoichiometric ratio. The first air stoichiometric ratio is an air stoichiometric ratio of 1 or more, and the second air stoichiometric ratio is an air stoichiometric ratio of 1 or more and lower than the first air stoichiometric ratio.

[0039] When executing the above pull-down operation and controlling the required current so that the response speed of the output current becomes a first response speed,

[0040] When a first condition including the case where the output voltage or the output current is a value included in a first range is satisfied, a first control is executed in which the control is performed so that the response speed of the output current becomes a second response speed slower than the first response speed.

[0041] When a second condition including the case where the output voltage or the output current is a value included in a second range lower than the first range is satisfied, the above first control is not executed.

[0042] By being configured in this way, in the fuel cell system of the present disclosure, when the output current or the output voltage is a value included in the first range, the required current is controlled so that the response speed of the output current becomes a second response speed slower than the first response speed. The air compressor is controlled so as to achieve a required flow rate for outputting an output voltage corresponding to the required current. That is, when the required current changes slowly, the flow rate of the cathode gas also changes slowly. As a result, by suppressing the change in the amount of moisture contained in the fuel cell, the ratio of the concentration overvoltage in the overvoltage is also less likely to change. Therefore, the fuel cell system of the present disclosure suppresses the change in the concentration overvoltage caused by overflow to prevent the change in the operating point during the pull-down operation. And since the cathode gas is increased corresponding to the concentration overvoltage caused by overflow, the fuel cell system of the present disclosure can prevent deterioration of the fuel cell due to drying compared to the case where the flow rate of the cathode gas is increased in advance to prevent overflow.

[0043] (3) In the fuel cell system of the above-described manner, it may also be configured such that the first condition includes the case where the amplitude of the output voltage or the output current in a predetermined period is included in the first range.

[0044] By being configured in this way, the fuel cell system of the present disclosure can prevent the control at the time of satisfying the first condition from being erroneously executed due to the change in the output voltage or the output current for an instant shorter than the predetermined period.

[0045] (4) In the fuel cell system in the above-described manner, the control unit may also perform the pull-down operation after the above first time has elapsed.

[0046] By adopting this manner, compared with the manner in which the pull-down operation is not performed after the first time has elapsed, the fuel cell system of the present disclosure can prevent drying of the fuel cell. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Hereinafter, the features, advantages, and technical and industrial significance of the embodiments of the present invention will be described with reference to the drawings, and the same reference numerals denote the same elements, where

[0048] Figure 1 is an explanatory diagram showing the structure of the fuel cell system according to the first embodiment.

[0049] Figure 2 is an explanatory diagram showing the operating point of the fuel cell stack.

[0050] Figure 3 is an explanatory diagram showing the operating point of the fuel cell stack.

[0051] Figure 4 is a flowchart showing the control method of the fuel cell system according to the first embodiment.

[0052] Figure 5 is a flowchart showing the control method of the fuel cell system according to the second embodiment.

[0053] Figure 6 is a waveform diagram showing the output power of the second embodiment. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0054] A. First Embodiment:

[0055] A-1. Structure of the Fuel Cell System:

[0056] Figure 1 is an explanatory diagram showing the structure of the fuel cell system 10 according to the 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 of the present embodiment is used, for example, as a driving power source, a power source for various devices, etc. by being mounted on a battery electric vehicle.

[0057] The fuel cell stack 100 receives supplies of anode gas and cathode gas as reaction gases to generate electricity. 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 stacked structure in which a plurality of single cells serving as power generation elements are stacked. In this specification, the "fuel cell stack" is also simply referred to as the "fuel cell" or the "stack".

[0058] The single cell includes a membrane electrode assembly (MEA: Membrane Electrode Assembly), a pair of gas diffusion layers arranged to sandwich the MEA, and a pair of gas separators arranged outside each gas diffusion layer. The MEA includes an electrolyte membrane, and an anode and a cathode formed on respective surfaces of the electrolyte membrane as catalyst electrode layers. In each single cell, an anode gas flow path 120 for allowing the anode gas to flow is formed on the anode side via the electrolyte membrane, and a cathode gas flow path 110 for allowing the cathode gas to flow is formed on the cathode side. In Figure 1 order to facilitate understanding of the technology, the illustration of the single cell is omitted.

[0059] In addition, in the cathode, water is generated as the electrochemical reaction proceeds. As a result, the amount of moisture contained in the cathode gas flow path 110 increases. However, since the moisture in the cathode gas flow path 110 is removed by the cathode gas, the amount of moisture contained in the cathode gas flow path 110 varies according to the amount of generated water and the flow rate of the cathode gas. Therefore, depending on the flow rate of the cathode gas, drying of the cathode gas flow path 110 or overflow in the cathode gas flow path 110 is likely to occur. Adjustment of the amount of moisture in the cathode gas flow path 110 in the fuel cell system 10 will be described in detail later. In addition, in this specification, the amount of moisture in the fuel cell stack 100 refers to the amount of moisture in the cathode gas flow path 110.

[0060] The cathode gas supply / discharge system 200 supplies the cathode gas to the fuel cell stack 100 and discharges the 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.

[0061] The cathode gas supply pipe 210 supplies the cathode gas 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 connecting the outside to the inlet of the air compressor 230 and a second cathode gas supply pipe 212 connecting the outlet of the air compressor 230 to the inlet of the cathode gas flow path 110. Thus, the cathode gas supply pipe 210 supplies the cathode gas supplied from the outside through the air compressor 230 to the cathode gas flow path 110.

[0062] The flow sensor 220 obtains the flow rate of the cathode gas. More specifically, the flow 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 sensor 220 sends the obtained flow rate to the control unit 500.

[0063] The air compressor 230 adjusts the flow rate of the cathode gas flowing 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 compresses the cathode gas according to the instruction of the control unit 500, thereby adjusting the flow rate of the cathode gas.

[0064] By adjusting the flow rate of the cathode gas, the output voltage Vf of the fuel cell stack 100 is adjusted. The control of the power of the fuel cell system 10 will be described in detail later. Also, by adjusting the flow rate of the cathode gas, the moisture content of the fuel cell stack 100 can be adjusted as described above.

[0065] 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.

[0066] The anode gas supply / discharge system 300 supplies the 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, etc. Also, the anode gas supply / discharge system 300 includes a supply pipe, an exhaust pipe, etc. for allowing the anode gas supplied from the anode gas tank to flow in the anode gas flow path 120. However, for the sake of easy understanding of the technology, in Figure 1 the illustration of the structure of the anode gas supply / discharge system 300 is omitted.

[0067] In addition, in this specification, the fuel cell system 10 operates in a state where the amount of anode gas required to generate the required power is supplied to the anode gas supply / discharge system 300.

[0068] The output circuit 400 supplies power from the fuel cell stack 100 according to the required power of the load device 440. The output circuit 400 includes a voltage sensor 410, a current sensor 420, an output adjustment unit 430, and a load device 440.

[0069] The voltage sensor 410 obtains the output voltage Vf of the fuel cell stack 100. The voltage sensor 410 sends the obtained output voltage Vf to the control unit 500.

[0070] The current sensor 420 obtains the output current If of the fuel cell stack 100. The current sensor 420 sends the obtained output current If to the control unit 500.

[0071] The output adjustment unit 430 adjusts the output power P1 of the fuel cell stack 100 according to the required power of the load. More specifically, the output adjustment unit 430 adjusts the output current If to be a required current determined based on the required power of the load device 440 and the output power P1. The control of the output adjustment unit 430 will be described in detail later. Specifically, the output adjustment unit 430 is a DCDC converter. The output adjustment unit 430 is connected to the power output unit of the fuel cell stack 100.

[0072] Moreover, the output adjustment unit 430 transforms the output voltage Vf of the fuel cell stack 100 into the voltage required by the load device 440. For example, the drive device of the 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 of the motor. In addition, the "output adjustment unit" is also referred to as "FDC".

[0073] The load device 440 consumes the output power P1. More specifically, the load device 130 consumes the output power P1 adjusted by the output adjustment unit 430 according to the required power. In this specification, the load power P2 is the output power P1 adjusted by the output adjustment unit 430. And 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 drive device of a vehicle including a motor and a drive circuit of the motor, an air compressor 230, an anode gas pump, etc. The load device 440 sends the information of the required load power P2 to the control unit 500.

[0074] The control unit 500 controls the fuel cell system 10. The control unit 500 is configured as a logic circuit centered on a microcomputer. More specifically, the control unit 500 includes a CPU, a ROM, a RAM, and an input / output port for inputting and outputting various signals. The CPU executes a preset control program. The ROM stores in advance control programs, control data, etc. required for performing various arithmetic processes in the CPU. The RAM temporarily reads and writes various data required for performing various arithmetic processes in the CPU. Hereinafter, the functions of the control unit 500 will be described.

[0075] A-2. Pull-up operation and pull-down operation:

[0076] The control unit 500 performs either the air stoichiometry pull-up operation or the air stoichiometry pull-down operation by controlling the flow rate of the cathode gas using the air compressor 230. The air stoichiometry refers to 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. That is, the air stoichiometry is 1.0 or more.

[0077] The air stoichiometry pull-up operation is an operation in which the air compressor 230 supplies the cathode gas in such a way that the air stoichiometry becomes the first air stoichiometry. Specifically, the first air stoichiometry is 1.5. The first air stoichiometry is based on the air stoichiometry at which the power consumption of the drive device of the vehicle is maximized. That is, the first air stoichiometry is based on the air stoichiometry 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.

[0078] The air stoichiometry pull-down operation is an operation in which the air compressor 230 supplies the cathode gas in such a way that the air stoichiometry becomes a second air stoichiometry lower than the first air stoichiometry. For example, when the first air stoichiometry is 1.5, the second air stoichiometry is a value included in the range of 1.2 or more and 1.3 or less.

[0079] By the air stoichiometry pull-down operation, the flow rate of the cathode gas is reduced compared to the case where the first air stoichiometry is achieved. By reducing the flow rate of the cathode gas, the amount of moisture removed from the fuel cell stack 100 is reduced. That is, since the amount of moisture contained in the fuel cell stack 100 increases, drying of the fuel cell stack 100 can be prevented. However, since the amount of moisture in the fuel cell stack 100 increases, overflow is likely to occur. In addition, the "air stoichiometry pull-up operation" is simply referred to as the "pull-up operation", and the "air stoichiometry pull-down operation" is simply referred to as the "pull-down operation".

[0080] Even in the case of the air stoichiometry pull-up operation, since the cathode gas supplied from the air compressor 230 is in a dry state, the fuel cell stack 100 may sometimes be partially dried. In particular, the inlet portion of the cathode gas flow path 110 may be dry. In this case, by performing the air stoichiometry pull-down operation, drying of the fuel cell stack 100 can be prevented.

[0081] A-3. Equal power operation:

[0082] The control unit 500 further performs an isopower operation of controlling the output power P1 of the fuel cell stack 100 according to the required power of the load device 440. More specifically, the control unit 500 obtains information on the required power through the load device 130. The control unit 500 controls the air compressor 230 based on the required power and a reference value of the output voltage Vf determined in advance. By supplying the cathode gas to the fuel cell stack 100, the fuel cell stack 100 starts generating power, and thus the output voltage Vf is generated. Also, the control unit 500 controls the output adjustment unit 430 based on the required power and a reference value of the output current If determined in advance. That is, the output adjustment unit 430 extracts the output current If from the fuel cell stack 100 based on the reference value of the output current If and outputs the output current If corresponding to the reference value of the output current If. On this basis, the control unit 500 obtains the output current If and the output voltage Vf through the voltage sensor 410 and the current sensor 420. The control unit 500 calculates the output power P1 based on the obtained output current If and output voltage Vf. The control unit 500 calculates the required current that the output adjustment unit 430 should extract 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 according to the difference between the output power P1 and the required power. The control unit 500 makes the required current the command value for controlling the output adjustment unit 430. Thereby, the output adjustment unit 430 adjusts the output current If to be the required current determined based on the required power of the load device 440 and the above output power. That is, the output adjustment unit 430 causes the fuel cell stack 100 to output the output power P1 that satisfies the required power. The operation of such a fuel cell system 10 is referred to as "isopower operation".

[0083] As described above, the air stoichiometry ratio refers to the ratio of the amount of the cathode gas actually supplied to the fuel cell stack 100 to the minimum amount of the cathode gas required to generate the required power. In the up-pull operation or the down-pull operation of the air stoichiometry ratio, it is necessary to vary the flow rate of the cathode gas according to the required power. Therefore, 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 flow rate determined in advance for outputting the output voltage Vf corresponding to the required current.

[0084] For example, in the down-pull operation of the air stoichiometry ratio and during the isopower operation, the control unit 500 increases the flow rate of the cathode gas through the air compressor 230 according to the increase in the required current. In addition, the increase in the required current occurs as described above according to the difference between the output power P1 and the required power. For example, the increase in the required current occurs when the required power increases, or when the output power P1 decreases corresponding to the decrease in the output voltage Vf due to the concentration overvoltage caused by the overflow.

[0085] A - 4. Variation of the operating point:

[0086] Figure 2 This is an explanatory diagram showing the operating point of the fuel cell stack 100. In Figure 2 the horizontal axis represents the current density of the single cells of the fuel cell stack 100, and the vertical axis represents the voltage of the single cells. The output current If depends on the current density of the single cells. The output voltage Vf depends on the voltage of the single cells. In Figure 2 the theoretical electromotive force is represented by a dashed line. As Figure 2 shown by the solid line, the larger the current density of the single cell, the larger the overvoltage, and thus the smaller the voltage of the single cell. The overvoltage is divided into activation overvoltage, resistance overvoltage, and concentration overvoltage. As Figure 2 shown, the larger the current density of the single cell, the larger the proportion of the concentration overvoltage in the overvoltage. Since the output current If depends on the current density of the single cell, the larger the output current If, the larger the change amount of the concentration overvoltage with respect to the output current If. The region with a small change amount of the concentration overvoltage in Figure 2 is called the "first region". The region with a large change amount of the concentration overvoltage in Figure 2 is called the "second region". The first region is also the region where the change amount of the output voltage Vf with respect to the output current If is large. The second region is also the region where the change amount of the output voltage Vf with respect to the output current If is small. In Figure 2 the points on the solid line defined by the voltage and current density of the single cell are the operating points of the single cell. In Figure 2 the operating point is in the first region. The power density output by the single cell is equivalent to the area on the plane with the operating point as the vertex. The output power P1 depends on the power density of the single cell. In this specification, for the sake of easy understanding of the technology, the operating points shown in Figure 2 or Figure 3 are treated as the operating points of the fuel cell stack 100.

[0087] Figure 3 This is an explanatory diagram showing the operating point of the fuel cell stack 100. The operating point of the fuel cell stack 100 in the case of under - pull operation of the air stoichiometry and under constant - power operation is explained. In the case where concentration overvoltage caused by overflow occurs due to the under - pull operation of the air stoichiometry, the operating point is as shown in Figure 3As shown, it is located in the second region. As described above, an increase in the required current is generated when the output power P1 decreases corresponding to a decrease in the output voltage Vf due to the concentration overvoltage caused by the overflow. Therefore, corresponding to the increase in the required current, the control unit 500 increases the flow rate of the cathode gas through the air compressor 230. As a result, since the moisture contained in the fuel cell stack 100 is removed, the concentration overvoltage caused by the overflow is eliminated. That is, since the output voltage Vf rises, the operating point is as Figure 2 shown and is located in the first region. However, since the output voltage Vf rises, a difference between the output power P1 and the required power is generated, so the required current decreases. Corresponding to the decrease in the required current, the control unit 500 decreases the flow rate of the cathode gas through the air compressor 230. That is, the possibility of the overflow occurring again becomes high. Therefore, the operating point varies between the first region and the second region. When the operating point varies, the change amount of the output voltage Vf with respect to the output current If changes, so it is difficult for the control unit 500 to control the output power P1 with a certain accuracy.

[0088] Generally, the response speed of the output current If adjusted by the output adjustment unit 430 as a DCDC converter is faster than the response speed of the flow rate of the cathode gas adjusted by the air compressor 230. Therefore, when the variation of the operating point occurs, the response of the flow rate of the cathode gas cannot follow the response of the output current If. Therefore, even during the constant power operation, due to the delay in the response of the flow rate of the cathode gas, the fuel cell stack 100 sometimes cannot output the output power P1 that satisfies the required power. That is, there is a situation where the output power P1 is insufficient. In addition, the response speed of the output current If refers to the time from the generation of the change in the required current until the output current If reaches the required current. The response speed of the flow rate of the cathode gas refers to the time from the generation of the change in the required flow rate until the flow rate of the cathode gas reaches the required flow rate.

[0089] A - 5. Control Method of Fuel Cell System:

[0090] Figure 4 It is a flowchart showing the control method of the fuel cell system 10 of the first embodiment. Hereinafter, the control method of the fuel cell system 10 will be described. The control unit 500 repeatedly executes the following processes during the operation of the fuel cell system 10.

[0091] In Figure 4 S100, the control unit 500 determines whether to execute the pull - down operation of the air stoichiometry. That is, when the air stoichiometry is not the second air stoichiometry, the control unit 500 makes the process enter S160. When the air stoichiometry is the second air stoichiometry, the control unit 500 makes the process enter S110.

[0092] InFigure 4 In S110, the control unit 500 determines whether to perform equal-power operation. When the control unit 500 does not perform equal-power operation, the process proceeds to S170. When the control unit 500 performs equal-power operation, the process proceeds to S120.

[0093] In Figure 4 In S120, the control unit 500 detects changes in the operating point. In addition, in S120, a pull-down operation of the air stoichiometric ratio is performed. Specifically, when the first condition is satisfied, including the case where the output voltage Vf of the fuel cell stack 100 is determined by the voltage sensor 410 to be a value included in the first range, the process proceeds to S130. When the second condition is satisfied, including the case where the output voltage Vf of the fuel cell stack 100 is determined by the voltage sensor 410 to be a value included in the second range lower than the first range, the process proceeds to S180. In addition, specifically, the output voltage Vf in S120 refers to the amplitude of the output voltage Vf.

[0094] 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 value. More specifically, above the threshold value is the first range, and less than the threshold value is the second range. The threshold value is set by experiment, for example, based on the maximum amplitude of the output voltage Vf that can be achieved according to the change in the load power P2.

[0095] The first condition is a condition that also includes the case where the amplitude of the output voltage in a predetermined period is included in the first range. Specifically, the predetermined period is set by experiment based on the response speed of the cathode gas flow rate. By being in this way, the fuel cell system 10 of the present disclosure can prevent the situation where control is erroneously executed due to a momentary change in the output voltage Vf shorter than the predetermined period. A momentary change in the output voltage Vf is generated, for example, due to a sharp change in the load power P2.

[0096] In Figure 4 In S130, the control unit 500 performs a pull-up operation of the air stoichiometric ratio. That is, the control unit 500 increases the flow rate of the cathode gas through the air compressor 230 so that the air stoichiometric ratio becomes the first air stoichiometric ratio.

[0097] In Figure 4In S140, the control unit 500 continues to perform the pull-up operation in such a manner that the air stoichiometric ratio is maintained at the first air stoichiometric ratio during a pre-determined first period of time. The first period of time is the time required to eliminate the overflow. In this specification, the "first period of time" is also referred to as the "drainage time". The drainage time is set through experiments according to the specifications of the fuel cell stack 100. For example, the drainage time is 30 minutes.

[0098] In Figure 4 In S150, after the first period of time has elapsed, the control unit 500 performs the pull-down operation of the air stoichiometric ratio. That is, the control unit 500 reduces the flow rate of the cathode gas through the air compressor 230 in such a manner that the air stoichiometric ratio becomes the second air stoichiometric ratio. By doing so, compared with the method of not performing the pull-down operation after the first period of time has elapsed, the fuel cell system 10 of the present disclosure can prevent the drying of the fuel cell.

[0099] In Figure 4 In S160, the control unit 500 continues the operation in the state of pulling up the air stoichiometric ratio.

[0100] In Figure 4 In S170, the control unit 500 continues the operation in the state of pulling down the air stoichiometric ratio. In addition, Figure 4 The same processing is performed in S180.

[0101] During the period when the fuel cell stack 100 outputs the output power P1, the above processing is repeated. That is, after any one of the processes in Figure 4 S150 to S180, the control unit 500 starts the process of S100.

[0102] As described above, in this manner, when the air stoichiometric ratio becomes the second air stoichiometric ratio, the flow rate of the cathode gas decreases compared with the case where the air stoichiometric ratio is the first air stoichiometric ratio. Therefore, since the amount of moisture removed from the fuel cell decreases, the drying of the fuel cell can be prevented. However, when the air stoichiometric ratio is the second air stoichiometric ratio, since the amount of moisture contained in the fuel cell increases, the possibility of concentration overvoltage caused by overflow is high. The concentration overvoltage reduces the output voltage Vf of the fuel cell.

[0103] The fuel cell system 10 of the present disclosure controls the air compressor 230 so that the flow rate of the cathode gas becomes a required flow rate for outputting an 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. As a result, since the amount of water removed from the fuel cell increases, the concentration overvoltage caused by overflow becomes smaller. Since the concentration overvoltage becomes smaller, the output voltage Vf rises. In this case, the air compressor 230 decreases the flow rate of the cathode gas. That is, the generation and disappearance of overflow occur again. Due to the generation and disappearance of overflow, the ratio of the concentration overvoltage in the overvoltage changes. Therefore, the operating point defined by the output voltage Vf and the output current If of the fuel cell changes.

[0104] When the fuel cell system 10 of the present disclosure detects a change in the operating point, it performs a pull-up operation during the first period. As a result, since the amount of water contained in the fuel cell decreases, the concentration overvoltage caused by overflow is eliminated. During the pull-up operation, since the air stoichiometry is higher than that during the pull-down operation, more water is removed from the fuel cell than during the pull-down operation. That is, during the pull-up operation, it is more difficult to generate the concentration overvoltage caused by overflow than in the case of the pull-down operation. Therefore, the fuel cell system 10 of the present disclosure can prevent the change in the operating point. In addition, since the fuel cell system 10 of the present disclosure increases the cathode gas corresponding to the concentration overvoltage caused by overflow, it can prevent the deterioration of the fuel cell due to drying compared to the method of increasing the flow rate of the cathode gas in advance to prevent overflow.

[0105] By further determining the above first condition, the fuel cell system 10 of the present disclosure can prevent the situation where the control when the first condition is satisfied is erroneously executed due to a change in the output voltage Vf for an instant shorter than a predetermined period. In addition, the same effect can also be obtained in the second embodiment.

[0106] On this basis, by performing a pull-down operation after the first period, the fuel cell system 10 of the present disclosure can prevent the drying of the fuel cell compared to the method of not performing a pull-down operation after the first period.

[0107] In addition, the fuel cell system 10 of the present disclosure can prevent the situation where the output power P1 is insufficient by preventing the change in the operating point. Specifically, the same effect as the second waveform W2 in the second embodiment can be obtained. in

[0108] B. Second Embodiment:

[0109] Figure 6This is a flowchart showing the control method of the fuel cell system 10 of the second embodiment. When the fuel cell system 10 of the first embodiment detects a change in the operating point, it performs a pull-up operation of the air stoichiometric ratio. Thereby, the change in the operating point can be prevented. However, the change in the operating point can also be prevented by other methods. Hereinafter, the control method of the fuel cell system 10 of the second embodiment will be described. In addition, the structure of the fuel cell system 10 of the first embodiment is the same as the structure of the fuel cell system 10 of the second embodiment. In the second embodiment, the control unit 500 repeatedly executes the following processes during the operation of the fuel cell system 10.

[0110] Figure 5 The processes of S200 to S220 are the same as Figure 5 the processes of S100 to S120.

[0111] In Figure 4 S230, the control unit 500 slows down the response speed of the output current If. More specifically, the control unit 500 controls the demanded current such that the response speed of the output current If becomes a second response speed slower than the first response speed. This control is referred to as "the first control". In addition, in S220, the control unit 500 is in a state of performing a pull-down operation and controlling the demanded current such that the response speed of the output current If becomes the first response speed.

[0112] The first response speed is the response speed of the output current If when the first control is not performed during the constant power operation. The second response speed is the response speed of the flow rate of the cathode gas during the constant power operation. That is, the control unit 500 controls the output adjustment unit 430 such that the response speed of the output current If is substantially the same as the response speed of the flow rate of the cathode gas.

[0113] In Figure 5 S230, after the control unit 500 executes the first control, it ends the process.

[0114] Figure 5 The process of S240 is the same as Figure 5 the process of S160.

[0115] Figure 4 The processes of S250 and S260 are the same as Figure 5 S170 and S180.

[0116] During the period when the fuel cell stack 100 outputs the output power P1, the above processes are repeated. That is, after any one of the processes of S230 to S260, the control unit 500 starts the process of S200. Figure 4 After any one of the processes of S230 to S260, the control unit 500 starts the process of S200.

[0117] As described above, in this manner, similar to the first embodiment, variations in the operating point defined by the output voltage Vf and output current If of the fuel cell occur. When the output voltage Vf of the fuel cell system 10 of the second embodiment is a value included in the first range, the required current is controlled in such a way 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 in such a way that the required flow rate for outputting the output voltage Vf corresponding to the required current is achieved. That is, when the required current changes slowly, the flow rate of the cathode gas also changes slowly. As a result, by suppressing variations in the moisture content contained in the fuel cell, the proportion of concentration overvoltage in the overvoltage is also less likely to change. Therefore, the fuel cell system 10 of the second embodiment prevents variations in the operating point during pull-down operation by suppressing variations in the concentration overvoltage caused by flooding. Also, the fuel cell system 10 of the second embodiment increases the cathode gas in response to the concentration overvoltage caused by flooding. Therefore, compared with the method of increasing the flow rate of the cathode gas in advance to prevent flooding, deterioration of the fuel cell due to drying can be prevented.

[0118] Figure 5 is a waveform showing the output power P1 of the second embodiment. In Figure 6 it, 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 described in the explanation of the first embodiment, when the operating point changes, due to the delay in the response of the flow rate of the cathode gas, the output power P1 changes. That is, 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 variations in the operating point.

[0119] C. Modification example:

[0120] (1) In the above embodiment, the first air stoichiometry is 1.5. However, the first air stoichiometry may be 1.5 or more. Also, in the above embodiment, the second air stoichiometry is a value included in the range of 1.2 or more and 1.3 or less. However, the second air stoichiometry may be 1.0 or more and less than 1.5.

[0121] (2) In the above-described embodiment, the first condition is a condition including the case where the amplitude of the output voltage Vf or the output current If in a predetermined cycle is included in the first range. However, the first condition may not include such a condition. Also, in the above-described embodiment, the predetermined cycle is set through experiments based on the response speed of the cathode gas flow rate. However, the predetermined cycle may be set through experiments regardless of the response speed of the cathode gas flow rate. By being in this way, the control and setting of the fuel cell system 10 of the present disclosure become easier.

[0122] (3) In the above-described embodiment, the control unit 500 performs the pull-down operation after the first time has elapsed. However, the control unit 500 may not perform the pull-down operation after the first time has elapsed. Even when the pull-down operation is not performed, since the pull-down operation has been performed, the drying of the fuel cell stack 100 is alleviated.

[0123] (4) In the above-described embodiment, the control unit 500 may be constituted by a plurality of microcomputers. More specifically, the function of the control unit 500 that performs the pull-up operation or the pull-down operation of the air stoichiometry or controls the output adjustment unit 430 may not be realized by one microcomputer.

[0124] (5) In the above-described embodiment, the control unit 500 detects the change in the operating point based on the output voltage Vf. However, the control unit 500 may also detect the change in the operating point based on the output current If. More specifically, when the control unit 500 determines through the current sensor 420 that the first condition including the case where the output current If of the fuel cell stack 100 is a value included in the first range is satisfied, the process proceeds to Figure 6 S130 or Figure 4 S230. When the control unit 500 determines through the current sensor 420 that the second condition including the case where the output voltage Vf of the fuel cell stack 100 is a value included in the second range smaller than the first range is satisfied, the process proceeds to Figure 5 S180 or Figure 4 Figure 5 S260. In addition, for the first range and the second range, they are set in the same manner as in the case of the output voltage Vf.

Claims

1. A fuel cell system, wherein, Comprising: A fuel cell; An air compressor that adjusts the flow rate of the cathode gas flowing into the fuel cell; A flow rate sensor that obtains the flow rate; A current sensor that obtains the output current of the fuel cell; A voltage sensor that obtains the output voltage of the fuel cell; A load device that consumes the output power of the fuel cell; And A control unit that controls the fuel cell system, The control unit controls the air compressor in such a way that the flow rate becomes a predetermined required flow rate for outputting the output voltage corresponding to the required current determined based on the required power required by the load device and the output power. The control unit executes either a pull-up operation in which the air compressor supplies the cathode gas at a first air stoichiometric ratio or a pull-down operation in which the air compressor supplies the cathode gas at a second air stoichiometric ratio. The first air stoichiometric ratio is an air stoichiometric ratio of 1 or more, and the second air stoichiometric ratio is an air stoichiometric ratio of 1 or more and lower than the first air stoichiometric ratio. When performing the pull-down operation, When a first condition including the case where the output voltage or the output current is a value included in a first range is satisfied, the pull-up operation is executed during a predetermined first time period. When a second condition including the case where the output voltage or the output current is a value included in a second range lower than the first range is satisfied, the pull-up operation is not executed.

2. A fuel cell system, wherein, Comprising: A fuel cell; An air compressor that adjusts the flow rate of the cathode gas flowing into the fuel cell; A flow rate sensor that obtains the flow rate; A current sensor that obtains the output current of the fuel cell; A voltage sensor that obtains the 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 in such a way that the output current becomes 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, The control unit controls the air compressor in such a way that the flow rate becomes a predetermined required flow rate for outputting the output voltage corresponding to the required current. The control unit executes either a pull-up operation in which the air compressor supplies the cathode gas at a first air stoichiometric ratio or a pull-down operation in which the air compressor supplies the cathode gas at a second air stoichiometric ratio. The first air stoichiometric ratio is an air stoichiometric ratio of 1 or more, and the second air stoichiometric ratio is an air stoichiometric ratio of 1 or more and lower than the first air stoichiometric ratio. When performing the pull-down operation and controlling the required current in such a manner that the response speed of the output current becomes the first response speed, when a first condition including a case where the output voltage or the output current is a value included in the first range is satisfied, perform first control that controls in such a manner that the response speed of the output current becomes a second response speed slower than the first response speed, when a second condition including a case where the output voltage or the output current is a value included in a second range lower than the first range is satisfied, do not perform the first control.

3. The fuel cell system according to claim 1 or 2, wherein the first condition includes a case where the amplitude of the output voltage or the output current in a predetermined period is included in the first range.

4. The fuel cell system according to claim 1, wherein the control unit performs the pull-down operation after the first time has elapsed.

Images