Fuel cell system and its control method

Abstract

To suppress the decrease in cell voltage and degradation of dry fuel cells without compromising the marketability of the fuel cell system. [Solution] The fuel cell system comprises a fuel cell stack that generates electricity when anode gas and cathode gas are supplied, an impedance sensor that measures the impedance of the fuel cell stack, a power circuit that connects the fuel cell stack and the drive motor, and an ECU that controls the output of the fuel cell stack by operating this power circuit. If the impedance measurement value is above a protection control start threshold, the ECU increases the cooling capacity of the fuel cell stack by the cooling system and the pressure of the cathode gas flow path, and then performs current increase rate limiting control that limits the rate of increase of the output current of the fuel cell stack (the amount of increase of output current per unit time) to below an upper limit of the current increase rate set based on the impedance measurement value.

Description

Technical Field

[0001] The present invention relates to a fuel cell system and a control method therefor.

Background Art

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

[0003] If a fuel cell continues power generation when its electrolyte membrane is in a dry state, the electrolyte membrane may deteriorate. Therefore, in the invention described in Patent Document 1, when it is determined that the fuel cell has a drying tendency, a current limiting process is performed to limit the output current output from the fuel cell to a load to be equal to or less than a maximum limiting current value determined based on a cell voltage.

Prior Art Literature

Patent Literature

[0004]

Patent Document 1

Summary of the Invention

Problem to be Solved by the Invention

[0005] According to the invention described in Patent Document 1, it is possible to suppress an excessive load from being applied to a fuel cell in a dry state, and further suppress the decrease and deterioration of the cell voltage of the fuel cell. However, if the output current of the fuel cell is rapidly limited in this manner, the user may feel a sense of discomfort, which is not preferable from the viewpoint of marketability.

[0006] The present invention aims to achieve a fuel cell system capable of suppressing the decrease and deterioration of the cell voltage of a fuel cell in a dry state without reducing marketability, and further contributes to energy efficiency.

Means for Solving the Problem

[0007] (1) The fuel cell system according to the present invention (for example, fuel cell system 1 described later) comprises a fuel cell (for example, fuel cell stack 2 described later) that generates electricity when anode gas and cathode gas are supplied, and comprises impedance acquisition means (for example, impedance sensor 25 described later) for acquiring the impedance of the fuel cell, and output control means (for example, ECU 6 and power circuit 7 described later) for controlling the output of the fuel cell, wherein the output control means performs current increase rate limiting control which limits the rate of increase of the output current of the fuel cell to a predetermined current increase rate upper limit value or less when the acquired value of the impedance is greater than or equal to a predetermined threshold (for example, protection control start threshold described later).

[0008] (2) In this case, it is preferable that the output control means set the upper limit of the current increase rate to a smaller value as the acquired value of the impedance increases.

[0009] (3) In this case, the fuel cell system further comprises a current detection means (for example, a current sensor 26 described later) for detecting the output current, and it is preferable that the output control means sets the upper limit of the current increase rate to a smaller value as the current detected value by the current detection means increases.

[0010] (4) In this case, it is preferable that the output control means initiates current upper limit limit control to limit the output current to a current upper limit value determined based on the decrease in the cell voltage after the current increase rate limit control has been initiated, in response to the decrease in the cell voltage exceeding a predetermined width (for example, a width threshold described later).

[0011] (5) In this case, it is preferable that the output control means set the current upper limit to a smaller value as the decrease in the current increases.

[0012] (6) In this case, the fuel cell system further comprises a cooling system for cooling the fuel cell (for example, the cooling system 5 described later), and it is preferable that the output control means increases the cooling capacity of the fuel cell by the cooling system and the pressure in the cathode gas flow path of the fuel cell (for example, the cathode flow path 22 described later) before starting the current increase rate limiting control.

[0013] (7) A control method for a fuel cell system according to the present invention is a method for controlling the output of a fuel cell that generates electricity when an anode gas and a cathode gas are supplied, comprising the steps of: acquiring the impedance of the fuel cell; and, if the acquired value of the impedance is greater than or equal to a predetermined threshold, performing current increase rate limiting control to limit the rate of increase of the output current of the fuel cell to less than or equal to a predetermined current increase rate upper limit. [Effects of the Invention]

[0014] (1) When the output control means determines that the acquired impedance value of the fuel cell is above a predetermined threshold, that is, that the fuel cell is in a dry state, it performs current increase rate limiting control to limit the rate of increase of the fuel cell's output current (i.e., the amount of increase per unit time) to below a predetermined upper limit of the current increase rate. Therefore, according to the present invention, a sudden increase in load on a fuel cell in a dry state can be suppressed, and thus a decrease in the cell voltage and deterioration of the fuel cell can be suppressed. Furthermore, according to the present invention, although a sudden increase in output current is suppressed, the output current is not suddenly limited against the user's intention, nor is power generation stopped due to an excessive decrease in cell voltage, so the marketability of the fuel cell system is not reduced. Therefore, according to the present invention, a decrease in the cell voltage and deterioration of a fuel cell in a dry state can be suppressed without reducing the marketability of the fuel cell system, and in turn, it can contribute to energy efficiency.

[0015] (2) The output control means sets the upper limit of the current increase rate to a smaller value as the acquired impedance value increases. Therefore, according to the present invention, the upper limit of the current increase rate can be set to an appropriate value according to the dry state of the fuel cell so that the cell voltage does not drop suddenly while the rate of increase of the output current is being limited, thereby suppressing a decline in the marketability of the fuel cell system.

[0016] (3) The output control means sets the upper limit of the current increase rate to a smaller value as the acquired impedance value and the detected current value increase. Therefore, according to the present invention, the upper limit of the current increase rate can be set to an appropriate value according to the dry state of the fuel cell so that the cell voltage does not drop suddenly while the rate of increase of the output current is being limited, thereby suppressing a decline in the marketability of the fuel cell system.

[0017] (4) As described above, by performing current increase rate limiting control, the decrease in cell voltage can be suppressed in most cases, but there are cases in which the cell voltage may decrease for some reason. In such cases, the output control means starts current upper limit limiting control, which limits the output current to a current upper limit value determined based on the decrease in cell voltage, in response to the decrease in cell voltage exceeding a predetermined range after the start of current increase rate limiting control. This can suppress further decreases in cell voltage. Furthermore, in the present invention, as long as the decrease in cell voltage does not exceed a predetermined range after the start of current increase rate limiting control, the output current is not limited, so a decrease in the marketability of the fuel cell system can be suppressed.

[0018] (5) The output control means sets an upper limit of the current based on the decrease in the cell voltage after the start of the current increase rate limit control, thereby limiting the output current of the fuel cell to an amount appropriate to the state of the fuel cell at that time, which can suppress further decreases in the cell voltage and ultimately prevent power generation from stopping.

[0019] (6) Before starting current increase rate limit control, the output control means can wet the electrolyte membrane of the fuel cell by increasing the cooling capacity of the fuel cell by the cooling system and the pressure of the cathode gas flow channel. Therefore, according to the present invention, the increase rate of the output current is not limited more than necessary, so that a decrease in the marketability of the fuel cell system can be suppressed.

[0020] (7) According to the control method for a fuel cell system of the present invention, a decrease in cell voltage and deterioration of the fuel cell can be suppressed without reducing the marketability of the fuel cell system, and thus the invention can contribute to improvement in energy efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] [Figure 1] It is a diagram showing a configuration of a fuel cell system according to an embodiment of the present invention. [Figure 2] It is a flowchart showing a specific procedure of drying protection control. [Figure 3] It is a flowchart showing a specific procedure of output restriction processing. [Figure 4] It is a diagram showing an example of a map for determining an upper limit value of a current increase rate. [Figure 5] It is a diagram showing a result of a test for determining whether or not a cell voltage has decreased. MODE FOR CARRYING OUT THE INVENTION

[0022] Hereinafter, a fuel cell system according to an embodiment of the present invention will be described with reference to the drawings.

[0023] Figure 1 shows the configuration of the fuel cell system 1 according to this embodiment. The fuel cell system 1 comprises a fuel cell stack 2 that generates electricity when anode gas and cathode gas are supplied, an anode gas supply device 3 that supplies hydrogen as anode gas to the fuel cell stack 2, a cathode gas supply device 4 that supplies air as cathode gas to the fuel cell stack 2, a cooling system 5 that cools the fuel cell stack 2, a battery B that stores the electricity generated by the fuel cell stack 2, a drive motor M that rotates tires (not shown) using the electricity supplied from the fuel cell stack 2 and battery B, a power circuit 7 that electrically connects the battery B, the drive motor M and the fuel cell stack 2, and an ECU 6 which is a computer that controls them. In the following description, the case in which this fuel cell system 1 is mounted on a fuel cell vehicle that runs using the above-mentioned tires as drive wheels will be described.

[0024] The fuel cell stack 2 is, for example, a stack structure in which tens to hundreds of fuel cell cells are stacked. Each fuel cell is constructed by sandwiching a membrane electrode structure (MEA) between a pair of separators. The membrane electrode structure consists of two electrodes, an anode electrode (cathode) and a cathode electrode (anode), and a solid polymer electrolyte membrane sandwiched between these electrodes. Typically, both electrodes are formed from a catalyst layer that performs oxidation-reduction reactions in contact with the solid polymer electrolyte membrane, and a gas diffusion layer in contact with this catalyst layer. In this fuel cell stack 2, when hydrogen is supplied to the anode channel 21 formed on the anode electrode side and oxygen-containing air is supplied to the cathode channel 22 formed on the cathode electrode side, electricity is generated by these electrochemical reactions. The electricity generated by the fuel cell stack 2 is supplied to loads such as the driving motor M and battery B via the power circuit 7.

[0025] The anode gas supply device 3 includes a hydrogen tank 31 for storing hydrogen gas at high pressure, a hydrogen supply pipe 32 from the hydrogen tank 31 to the inlet of the anode flow path 21 of the fuel cell stack 2, a hydrogen discharge pipe 33 from the discharge of the anode flow path 21 to a diluent (not shown) provided in the cathode gas supply device 4, and a hydrogen reflux pipe 34 that branches off from the hydrogen discharge pipe 33 and leads to the hydrogen supply pipe 32.

[0026] The hydrogen supply pipe 32 is equipped with, in order from the hydrogen tank 31 side toward the fuel cell stack 2 side, a shut-off valve 321, an injector 322 that injects hydrogen gas supplied via the shut-off valve 321 toward the fuel cell stack 2, and an ejector 323 that circulates the gas discharged from the anode flow path 21 (hereinafter also referred to as "anode off gas") toward the fuel cell stack 2. The shut-off valve 321 is a solenoid valve that opens and closes in response to a command signal from the ECU 6. The amount of hydrogen gas injected from the injector 322 is controlled by PWM control by the ECU 6.

[0027] A hydrogen circulation pump 341 is provided in the hydrogen reflux pipe 34. The hydrogen circulation pump 341 pumps anode-off gas from the hydrogen discharge pipe 33 side to the hydrogen supply pipe 32 side, thereby circulating hydrogen-containing gas within the circulation path composed of the hydrogen supply pipe 32, anode flow path 21, hydrogen discharge pipe 33, hydrogen reflux pipe 34, and ejector 323. The hydrogen circulation pump 341 operates in response to command signals from the ECU 6.

[0028] The hydrogen discharge pipe 33 is equipped with a catch tank 331 for storing water contained in the anode off gas and a purge valve 332 for discharging the anode off gas to the cathode gas supply device 4, in that order from the fuel cell stack 2 side toward the cathode gas supply device 4 side. The catch tank 331 is also provided with a drain pipe 35 for discharging the accumulated water. This drain pipe 35 runs from the catch tank 331 to the downstream side of the hydrogen discharge pipe 333 beyond the purge valve 332. The drain pipe 35 is equipped with a drain valve 351. When this drain valve 351 is opened, the water accumulated in the catch tank 331 is discharged through the hydrogen discharge pipe 33 to a diluent (not shown). The purge valve 332 and the drain valve 351 are solenoid valves that open and close in response to command signals from the ECU 6.

[0029] The cathode gas supply device 4 includes an air compressor 41, an air supply pipe 42 leading from the air compressor 41 to the inlet of the cathode flow path 22, an air discharge pipe 43 leading from the discharge of the cathode flow path 22 to a diluent (not shown), an air return pipe 45 branching from the air discharge pipe 43 to the air supply pipe 42, and a humidifier 46 connecting the air discharge pipe 43 and the air supply pipe 42.

[0030] The air compressor 41 supplies outside air to the cathode channel 22 of the fuel cell stack 2 via the air supply pipe 42. The air compressor 41 operates in response to command signals from the ECU 6. The humidifier 46 recovers water contained in the gas discharged from the cathode channel 22 (hereinafter also referred to as "cathode-off gas") and uses the recovered water to humidify the air supplied by the air compressor 41. Due to the function of this humidifier 46, the MEA of the fuel cell stack 2 during power generation is maintained in a state suitable for power generation.

[0031] The air supply pipe 42 is provided with a bypass pipe 47 that bypasses the humidifier 46. This bypass pipe 47 is provided with a bypass valve 471. When the bypass valve 471 is opened, most of the air supplied from the air compressor 41 bypasses the humidifier 46 and is supplied to the fuel cell stack 2. The bypass valve 471 is a solenoid valve that opens and closes in response to a command signal from the ECU 6. The air discharge pipe 43 is provided with a back pressure control valve 432 for adjusting the back pressure in the cathode flow path of the fuel cell stack 2. The back pressure control valve 432 is a solenoid valve that opens and closes in response to a command signal from the ECU 6.

[0032] Furthermore, the air supply pipe 42 and the air discharge pipe 43 are equipped with an inlet sealing valve 421 and an outlet sealing valve 431, respectively. When these sealing valves 421 and 431 are closed, the inside of the cathode flow path 22 is isolated from the outside air. These sealing valves 421 and 431 are solenoid valves that open and close in response to command signals from the ECU 6.

[0033] The air recirculation pipe 45 is equipped with an EGR pump 48 that pressurizes the gas from the air discharge pipe 43 to the air supply pipe 42 and circulates the oxygen-containing gas within the oxygen circulation channel. The EGR pump 48 operates in response to command signals from the ECU 6. The rotation speed of the EGR pump 48 is controlled by the ECU 6. When this EGR pump 48 is driven, a portion of the gas discharged from the outlet side of the cathode channel 22 of the stack 2 is recirculated to the inlet side of the cathode channel 22. Therefore, this EGR pump 48 is used when it is desired to reduce the oxygen concentration of the gas in the cathode channel 22, etc.

[0034] The cooling system 5 includes a refrigerant circulation path 51 that includes the inside of the fuel cell stack 2 as part of its flow path, a cooling pump 52 that circulates refrigerant within the refrigerant circulation path 51, a radiator 53 provided upstream of the cooling pump 52 in the refrigerant circulation path 51, a thermovalve 54 provided downstream of the cooling pump 52 in the refrigerant circulation path 51, and a bypass pipe 55 that connects the thermovalve 54 to the upstream side of the radiator 53 in the refrigerant circulation path 51.

[0035] The fuel cell stack 2 is cooled by heat exchange with the refrigerant flowing through its internal passages. The radiator 53 cools the refrigerant by heat exchange with the outside air. The cooling pump 52 operates in response to command signals from the ECU 6. The rotational speed of the cooling pump 52 is controlled by the ECU 6. Increasing the rotational speed of the cooling pump 52 increases the flow rate of the refrigerant circulating in the refrigerant circulation path 51, which includes the fuel cell stack 2 and the radiator 53 in its refrigerant passages, thereby increasing the cooling capacity of the fuel cell stack 2.

[0036] The thermovalve 54 is a three-way valve that opens and closes in response to a command signal from the ECU 6. The opening ratio of the thermovalve 54 (the ratio of the opening on the refrigerant circulation path 51 side (100% to 0%) to the opening on the bypass pipe 55 side (0% to 100%)) is controlled by the ECU 6. When the opening ratio of the thermovalve 54 is set to the maximum (i.e., "1"), all the refrigerant discharged from the cooling pump 52 is supplied to the fuel cell stack 2, thereby increasing the cooling capacity of the fuel cell stack 2. When the opening ratio of the thermovalve 54 is set to the minimum (i.e., "0"), all the refrigerant discharged from the cooling pump 52 is supplied to the bypass pipe 55.

[0037] Battery B is a secondary battery capable of both discharging, which converts chemical energy into electrical energy, and charging, which converts electrical energy into chemical energy. In the following description, a so-called lithium-ion battery, which charges and discharges by the movement of lithium ions between electrodes, is used as Battery B, but the present invention is not limited to this. Battery B may also be a capacitor, for example.

[0038] The power circuit 7 consists of power lines connecting the fuel cell stack 2 to the drive motor M and battery B, a DC-DC converter installed in these power lines to step up or step down the DC power output from the fuel cell stack 2, and an inverter installed in these power lines to convert the DC power output from the DC-DC converter into three-phase AC power for supply to the drive motor M, and to convert the three-phase AC power supplied from the drive motor M into DC power for supply to the battery B. Multiple switching elements constituting these DC-DC converters and inverters are driven on / off according to gate drive signals generated at predetermined timings from a gate drive circuit (not shown) of the ECU 6. Therefore, the ECU 6 can control the flow of power between the fuel cell stack 2, battery B, and drive motor M in the power circuit 7 by operating the DC-DC converters and inverters using the gate drive circuit.

[0039] The ECU6 is a computer equipped with multiple functions, including power generation control, temperature control, output control, and protection control. The power generation control function refers to the function in which the ECU6 controls the power generation state of the fuel cell stack 2 by operating the anode gas supply device 3 and the cathode gas supply device 4, etc. The temperature control function refers to the function in which the ECU6 controls the temperature of the fuel cell stack 2 by operating the cooling system 5.

[0040] The output control function refers to the function in which the ECU 6 controls the output of the fuel cell stack 2 (more specifically, the output current and output power from the fuel cell stack 2 to loads such as the drive motor M and battery B) by operating the DC-DC converter and inverter of the power circuit 7. The protection control function refers to the function in which the ECU 6 prevents the fuel cell stack 2 from drying out excessively while suppressing a decrease in the cell voltage and deterioration of the fuel cell stack 2 by executing the drying protection control, current increase rate limiting control, and current upper limit limiting control described later.

[0041] The ECU6 is connected to several sensors, including a cell voltage sensor 24, an impedance sensor 25, a current sensor 26, and a cathode pressure sensor 27, to monitor the state of the fuel cell stack 2 during power generation.

[0042] The cell voltage sensor 24 detects the voltage (so-called cell voltage) of each fuel cell constituting the fuel cell stack 2 and transmits a signal approximately proportional to the detected value to the ECU 6. The impedance sensor 25 detects the impedance resistance value of the fuel cell stack 2 and transmits a signal approximately proportional to the detected value to the ECU 6. The impedance resistance value of the fuel cell stack 2 is correlated with the water content of the MEA inside the fuel cell stack 2. More specifically, the lower the water content inside the fuel cell stack 2, in other words, the drier the inside of the fuel cell stack 2, the higher the impedance resistance value tends to be.

[0043] The current sensor 26 detects the output current of the fuel cell stack 2, more specifically the current output from the fuel cell stack 2 to loads such as the drive motor M and battery B via the power circuit 7, and transmits a signal approximately proportional to the detected value to the ECU 6. The cathode pressure sensor 27 is installed, for example, in the air supply pipe 42. The cathode pressure sensor 27 detects the pressure of the cathode gas supplied to the cathode flow path 22 (so-called cathode pressure) and transmits a signal approximately proportional to the detected value to the ECU 6.

[0044] Figure 2 is a flowchart showing the specific procedure for drying protection control to prevent excessive drying of the fuel cell stack 2 during power generation. This drying protection control is repeatedly performed by the ECU 6 at a predetermined control cycle while the fuel cell stack 2 is generating power.

[0045] First, in step ST1, the ECU 6 determines whether the impedance resistance value detected by the impedance sensor 25 (hereinafter referred to as the "impedance measurement value") is equal to or greater than a predetermined protection control initiation threshold. If the result of the determination in step ST1 is NO, the ECU 6 determines that the fuel cell stack 2 is in a moderately wet state and terminates the dry protection control shown in Figure 2 without executing the process described below. If the result of the determination in step ST1 is YES, the ECU 6 determines that the fuel cell stack 2 is in a dry state and proceeds to step ST2.

[0046] Next, in step ST2, the ECU6 determines whether the opening ratio of the thermovalve 54 is at its maximum. As described above, when the opening ratio of the thermovalve 54 is maximized, all the refrigerant discharged from the cooling pump 52 is supplied to the fuel cell stack 2. If the result of the determination in step ST2 is NO, the ECU6 moves to step ST3, increases the cooling capacity of the cooling system 5 by maximizing the opening ratio of the thermovalve 54, and then terminates the dry protection control shown in Figure 2. If the result of the determination in step ST2 is YES, the ECU6 moves to step ST4.

[0047] Next, in step ST4, the ECU 6 determines whether the rotational speed of the cooling pump 52 is at its maximum. If the result of the determination in step ST4 is NO, the ECU 6 proceeds to step ST5, where it increases the cooling capacity of the cooling system 5 by maximizing the rotational speed of the cooling pump 52, and then terminates the drying protection control shown in Figure 2. In this way, the drying protection control first maximizes the opening ratio of the thermovalve 54, and then maximizes the rotational speed of the cooling pump 52, thereby maximizing the cooling capacity of the fuel cell stack 2 by the cooling system 5 and rapidly lowering the temperature of the fuel cell stack 2. This lowers the saturated water vapor pressure of the air inside the fuel cell stack 2, and consequently promotes an increase in the water content inside the fuel cell stack 2. Also, if the result of the determination in step ST4 is YES, that is, if the cooling capacity of the fuel cell stack 2 by the cooling system 5 has already been maximized, the ECU 6 proceeds to step ST6.

[0048] In step ST6, the ECU6 determines whether the cathode pressure value detected by the cathode pressure sensor 27 (hereinafter referred to as the "cathode pressure detection value") is equal to or greater than a predetermined upper limit. If the result of the determination in step ST6 is NO, the ECU6 proceeds to step ST7. In step ST7, the ECU6 increases the cathode pressure detection value to the upper limit by operating the back pressure control valve 432. More specifically, the ECU6 increases the cathode pressure detection value to the upper limit by adjusting the opening of the back pressure control valve 432 to the closed side. This increases the vapor pressure of the air inside the fuel cell stack 2, thereby promoting an increase in the water content inside the fuel cell stack 2.

[0049] Furthermore, if the result of step ST6 is YES, that is, if the cooling capacity of the fuel cell stack 2 by the cooling system 5 has already been maximized and the cathode pressure of the fuel cell stack 2 has already been increased to the upper limit, the ECU 6 proceeds to step ST8. In step ST8, the ECU 6 performs output limiting processing (see Figure 3 described later) to limit the output of the fuel cell stack 2, and then terminates the dry protection control shown in Figure 2. In this way, if the impedance measurement value exceeds the protection control start threshold, the ECU 6 increases the cooling capacity of the fuel cell stack 2 by the cooling system 5 and the cathode pressure of the fuel cell stack 2 before starting the output limiting processing shown in Figure 3, thereby promoting an increase in the water content inside the fuel cell stack 2. This avoids, as much as possible, the execution of output limiting processing, which involves limiting the rate of increase of the output current of the fuel cell stack 2, as will be explained later.

[0050] Figure 3 is a flowchart showing the specific steps of the output limiting process.

[0051] In step ST11, the ECU 6 obtains the current impedance measurement value and the output current value detected by the current sensor 26 (hereinafter referred to as the "current detection value"), and then proceeds to step ST12.

[0052] In step ST12, the ECU 6 sets the current increase rate upper limit [A / sec] based on the impedance measurement and current detection value obtained in step ST11, and then proceeds to step ST13. Here, the current increase rate upper limit corresponds to the upper limit of the increase rate [A / sec] (i.e., the amount of increase in output current per unit time) of the output current supplied from the fuel cell stack 2 to the load via the power circuit 7. The ECU 6 sets the current increase rate upper limit according to the current impedance measurement and current detection value by referring to a map such as the one shown in Figure 4.

[0053] Figure 4 shows an example of a map for determining the upper limit of the current increase rate. As shown in Figure 4, it is preferable for the ECU6 to set the upper limit of the current increase rate to a smaller value as the impedance measurement value increases. It is also preferable for the ECU6 to set the upper limit of the current increase rate to a smaller value as the current detection value increases.

[0054] The map shown in Figure 4 is determined based on the results of tests that determine whether or not there is a decrease in cell voltage while changing the power generation conditions of the fuel cell stack 2 (output current, impedance, and rate of increase of output current).

[0055] Figure 5 shows the results of a test to determine whether or not the cell voltage has decreased. Figure 5 shows whether or not the cell voltage has decreased when the rate of increase of the output current of the fuel cell stack 2 is decreased in the order of b1, b2, b3, b4 (b1>b2>b3>b4) when the measured impedance of the fuel cell stack 2 is a1, a2, a3, a4 (a1>a2>a3>a4). In Figure 5, a "×" indicates that the cell voltage decreased under each power generation condition, and a "○" indicates that the cell voltage did not decrease. As shown in Figure 5, when the measured impedance of the fuel cell stack 2 is a4, a3, a2, a1, power generation of the fuel cell stack 2 can be continued without decreasing the cell voltage by reducing the rate of increase of the output current to b1, b2, b3, b4, respectively. Also, as shown in Figure 5, in order to continue power generation of the fuel cell stack 2 while preventing a decrease in cell voltage, the upper limit of the rate of increase of the output current needs to be set to a smaller value as the measured impedance of the fuel cell stack 2 increases, that is, as the degree of dryness inside the fuel cell stack 2 increases. While I will omit a detailed explanation of the test results, in order to continue generating power from fuel cell stack 2 while preventing a drop in cell voltage, the larger the output current of fuel cell stack 2, the smaller the upper limit for the rate of increase of the output current needs to be set.

[0056] The map used to determine the upper limit of the current increase rate (see Figure 4) is created based on the test results described above. In other words, it is preferable to determine the upper limit of the output current increase rate that does not cause a decrease in cell voltage under each power generation condition by performing tests as shown in Figure 5 in advance, and then set this upper limit as the upper limit of the current increase rate.

[0057] Returning to Figure 3, in step ST13, the ECU 6 performs current increase rate limiting control, which limits the rate of increase of the output current to the upper limit of the current increase rate set in step ST12, and then proceeds to step ST14. More specifically, in this current increase rate limiting control, the ECU 6 operates the power circuit 7 to limit the rate of increase of the output current output from the fuel cell stack 2 to the drive motor M, battery B, etc. via the power circuit 7 to less than or equal to the upper limit of the current increase rate.

[0058] Next, in step ST14, the ECU 6 calculates the decrease in cell voltage from the start of the current increase rate limiting control to the present time, and then proceeds to step ST15. More specifically, the ECU 6 calculates the decrease in the cell voltage detection value (cell voltage detection value at the start of the current increase rate limiting control - cell voltage detection value at the present time) based on the cell voltage value detected by the cell voltage sensor 24 at the time the current increase rate limiting control was started (hereinafter referred to as the "cell voltage detection value"). In other words, if the cell voltage decreases after the current increase rate limiting control is started, this decrease will be a positive value, and if the cell voltage increases after the current increase rate limiting control is started, this decrease will be a negative value.

[0059] Next, in step ST15, the ECU6 determines whether the drop in cell voltage calculated in step ST14 is greater than or equal to a positive width threshold. If the result of the determination in step ST15 is NO, the ECU6 terminates the output limiting process shown in Figure 3. If the result of the determination in step ST15 is YES, that is, if the cell voltage drops by more than the width threshold after the current increase rate limiting control has started, the ECU6 determines that limiting the increase rate of the output current alone is not enough to suppress the drop in cell voltage, and proceeds to step ST16.

[0060] In step ST16, the ECU6 sets the current limit value, which will be referenced in the current limit control described later, based on the value of the cell voltage drop calculated in step ST14, and then proceeds to step ST17. More specifically, the ECU6 sets the current limit value to a smaller value as the cell voltage drop increases.

[0061] In step ST17, the ECU 6 executes current limit control to restrict the output current of the fuel cell stack 2 to a current limit value set in step ST16, and terminates the output limiting process shown in Figure 3. More specifically, in this current limit control, the ECU 6 operates the power circuit 7 to limit the output current that is output from the fuel cell stack 2 to the drive motor M, battery B, etc., via the power circuit 7 to a current limit value.

[0062] As explained above with reference to Figures 2 and 3, when the impedance measurement value exceeds the protection control initiation threshold, the ECU 6 first increases the cooling capacity of the fuel cell stack 2 by the cooling system 5 and the cathode pressure of the fuel cell stack 2, thereby promoting an increase in the water content inside the fuel cell stack 2. Subsequently, while promoting an increase in the water content inside the fuel cell stack 2, the ECU 6 executes current increase rate limiting control to limit the rate of increase of the output current of the fuel cell stack 2 to below the current increase rate upper limit in order to prevent a decrease in the cell voltage of the fuel cell stack 2 and deterioration of the fuel cell stack 2. Subsequently, in response to the decrease in cell voltage after the start of current increase rate limiting control exceeding a predetermined threshold, the ECU 6 starts current upper limit limiting control to limit the output current to below the current upper limit in order to prevent further decrease in cell voltage and deterioration. In this way, when the impedance measurement value exceeds the protection control initiation threshold, the ECU 6 can suppress a decline in the marketability of the fuel cell system 1 by executing the process of promoting an increase in water content, current increase rate limiting control, and current upper limit limiting control in that order in steps.

[0063] The fuel cell system 1 according to this embodiment provides the following effects. (1) When the ECU6 determines that the impedance measurement value of the fuel cell stack 2 is above the protection control start threshold, i.e., that the fuel cell stack 2 is in a dry state, it executes current increase rate limiting control to limit the rate of increase of the output current of the fuel cell stack 2 to below a predetermined upper limit of the current increase rate. Therefore, the fuel cell system 1 can suppress a sudden increase in load on the fuel cell stack 2 in a dry state, thereby suppressing a decrease in the cell voltage and deterioration of the fuel cell stack 2. Furthermore, although a sudden increase in output current is suppressed by the fuel cell system 1, the output current is not suddenly limited against the user's intention, nor is power generation stopped due to an excessive decrease in cell voltage, so the marketability of the fuel cell system 1 is not reduced. Therefore, the fuel cell system 1 can suppress a decrease in the cell voltage and deterioration of the fuel cell stack 2 in a dry state without reducing its marketability, and in turn contribute to energy efficiency.

[0064] (2) The ECU 6 sets the upper limit of the current increase rate to a smaller value as the impedance measurement value increases. Therefore, with the fuel cell system 1, the upper limit of the current increase rate can be set to an appropriate value according to the dry state of the fuel cell stack 2 so that the cell voltage does not drop suddenly while the rate of increase of the output current is being limited, thereby suppressing a decrease in the marketability of the fuel cell system 1.

[0065] (3) The ECU 6 sets the upper limit of the current increase rate to a smaller value as the impedance measurement value and the current detection value increase. Therefore, with the fuel cell system 1, the upper limit of the current increase rate can be set to an appropriate value according to the dry state of the fuel cell stack 2 so that the cell voltage does not drop suddenly while the rate of increase of the output current is being limited, thereby suppressing a decrease in the marketability of the fuel cell system 1.

[0066] (4) As described above, by performing current increase rate limiting control, the decrease in cell voltage can be suppressed in most cases, but there are cases where the cell voltage decreases for some reason. In such cases, the ECU 6 starts current upper limit limiting control, which limits the output current to a current upper limit value determined based on the decrease in cell voltage, in response to the decrease in cell voltage exceeding a predetermined threshold after the start of current increase rate limiting control. This suppresses further decreases in cell voltage. In addition, in the fuel cell system 1, the output current is not limited as long as the decrease in cell voltage does not exceed a predetermined range after the start of current increase rate limiting control, thus suppressing a decrease in the marketability of the fuel cell system 1.

[0067] (5) The ECU 6 sets a current upper limit based on the amount of decrease in cell voltage after the start of current increase rate limit control, thereby limiting the output current of the fuel cell stack 2 to an amount appropriate to the state of the fuel cell stack 2 at that time. This suppresses further decreases in cell voltage and ultimately prevents power generation from stopping.

[0068] (6) Before initiating current increase rate limiting control, the ECU 6 can increase the cooling capacity of the fuel cell stack 2 by the cooling system 5 and the cathode pressure of the fuel cell stack 2, thereby wetting the electrolyte membrane of the fuel cell stack 2. Therefore, with the fuel cell system 1, the rate of increase of the output current is not limited more than necessary, and thus a decrease in the marketability of the fuel cell system 1 can be suppressed.

[0069] Although one embodiment of the present invention has been described above, the present invention is not limited thereto. Within the scope of the spirit of the present invention, the details of the configuration may be modified as appropriate. [Explanation of Symbols]

[0070] 1…Fuel cell system 2…Fuel cell stack 21... Anode channel 22... Cathode channel 24... Cell voltage sensor 25…Impedance sensor (means for acquiring impedance) 26...Current sensor (current detection means) 27… Cathode pressure sensor 3…Anode gas supply device 4… Cathode gas supply device 5…Cooling system (cooling system) 6…ECU (Output Control Unit) 7…Power circuit (output control means) B... Battery M... Driving motor

Claims

1. A fuel cell system comprising a fuel cell that generates electricity when anode gas and cathode gas are supplied, An impedance acquisition means for acquiring the impedance of the fuel cell, The system includes output control means for controlling the output of the fuel cell, The fuel cell system is characterized in that the output control means performs current increase rate limiting control, which limits the rate of increase of the output current of the fuel cell to a predetermined upper limit of the current increase rate when the acquired value of the impedance is greater than or equal to a predetermined threshold.

2. The fuel cell system according to claim 1, characterized in that the output control means sets the upper limit of the current increase rate to a smaller value as the acquired impedance value increases.

3. The system further comprises current detection means for detecting the output current, The fuel cell system according to claim 2, characterized in that the output control means sets the upper limit of the current increase rate to a smaller value as the current detected value by the current detection means increases.

4. The fuel cell system according to any one of claims 1 to 3, characterized in that the output control means starts current upper limit limit control to limit the output current to a current upper limit value determined based on the decrease in the cell voltage after the current increase rate limit control has been started, in response to the decrease in the cell voltage exceeding a predetermined amount.

5. The fuel cell system according to claim 4, characterized in that the output control means sets the current upper limit to a smaller value as the decrease in magnitude increases.

6. The system further comprises a cooling system for cooling the aforementioned fuel cell, The fuel cell system according to any one of claims 1 to 3, characterized in that the output control means increases the cooling capacity of the fuel cell by the cooling system and the pressure in the cathode gas flow path of the fuel cell before starting the current increase rate limiting control.

7. A control method for controlling the output of a fuel cell that generates electricity when anode gas and cathode gas are supplied, The steps include obtaining the impedance of the fuel cell, A method for controlling a fuel cell system, comprising the step of performing current increase rate limiting control to limit the rate of increase of the output current of the fuel cell to a predetermined upper limit of the current increase rate when the acquired value of the impedance is greater than or equal to a predetermined threshold.

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