Fuel cell system

Abstract

To properly set the anode target pressure of the fuel cell system during vehicle deceleration. [Solution] The fuel cell system 100 includes an anode control unit 62 and a cathode control unit 61 that set the anode target pressure and cathode target pressure according to the power generation current detected by the current detection unit 56, and control the injector 22 and air pump 31 according to the anode target pressure and cathode target pressure. When the current detection unit 56 detects a predetermined decrease in the output current, the anode control unit 62 sets the anode target pressure to decrease at a first rate, and then to decrease at a second rate that is smaller in rate than the first rate.

Description

Technical Field

[0001] The present invention relates to a fuel cell system.

Background Art

[0002] In recent years, in order to enable more people to access affordable, reliable, sustainable and advanced energy, technological developments related to fuel cells that contribute to energy efficiency have been carried out. As a technology related to this type of fuel cell, a fuel cell system is known that includes an exhaust valve for discharging fuel gas to the outside and a circulation pump for circulating the fuel gas (see, for example, Patent Document 1). In the system described in Patent Document 1, when the operation load suddenly decreases from a high load to a medium load or the like, the supply of fuel gas to the injector is stopped and the circulation pump is operated to circulate the fuel gas.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, when the supply of fuel gas to the injector is stopped when the operation load suddenly decreases as in the fuel cell system described in Patent Document 1 above, there is a risk that the flow path of the fuel gas will be filled with generated water and the power generation performance will deteriorate.

Means for Solving the Problems

[0005] A fuel cell system according to one aspect of the present invention includes: a current detection unit that detects the power generation current of a fuel cell that generates electricity by being supplied with anode gas and cathode gas; an anode gas supply unit that supplies anode gas to the fuel cell via an anode supply channel; a cathode gas supply unit that supplies cathode gas to the fuel cell via a cathode supply channel; a gas circulation unit that circulates the anode gas discharged from the fuel cell through the anode supply channel; and a gas control unit that sets the anode target pressure and cathode target pressure, which are target pressures for the anode gas and cathode gas, according to the power generation current detected by the current detection unit, and controls the anode gas supply unit and cathode gas supply unit according to the anode target pressure and cathode target pressure. When the current detection unit detects a predetermined decrease in the power generation current, the gas control unit sets the anode target pressure to decrease by a first percentage, and then to decrease it by a second percentage, which is a smaller percentage than the first percentage. [Effects of the Invention]

[0006] According to the present invention, good power generation performance can be obtained while suppressing the increase in differential pressure between the fuel gas and the oxidizer gas. [Brief explanation of the drawing]

[0007] [Figure 1] A diagram showing a schematic configuration of a fuel cell system according to an embodiment of the present invention. [Figure 2] Figure 1 shows a cross-sectional view of the fuel cell stack at the waist of the fuel cell system. [Figure 3] A block diagram showing the control configuration of a fuel cell system according to an embodiment of the present invention. [Figure 4A] A time chart showing an example of operation according to a first reference example of a fuel cell system according to an embodiment of the present invention. [Figure 4B] A time chart showing an example of operation according to a second reference example of a fuel cell system according to an embodiment of the present invention. [Figure 5A] A time chart showing an example of operation by a fuel cell system according to an embodiment of the present invention. [Figure 5B]A diagram showing a magnified portion of Figure 5A. [Figure 6] A time chart showing an example of operation by the valve control unit in Figure 3. [Figure 7] This diagram illustrates a modified example of the anode control unit shown in Figure 3. [Figure 8] A flowchart showing an example of the processing performed in the anode control unit in Figure 3. [Modes for carrying out the invention]

[0008] Embodiments of the present invention will be described below with reference to Figures 1 to 8. Figure 1 is a diagram showing a schematic configuration of a fuel cell system 100 according to an embodiment of the present invention. The fuel cell system 100 in Figure 1 is mounted on a vehicle (fuel cell vehicle), for example, and generates electricity to be supplied to a drive motor.

[0009] As shown in Figure 1, the fuel cell system 100 includes a fuel cell stack 1, a fuel gas supply and discharge unit 2 that supplies fuel gas (anode gas) to the fuel cell stack and discharges fuel gas from the fuel cell stack, an oxidant gas supply and discharge unit 3 that supplies oxidant gas (cathode gas) to the fuel cell stack 1 and discharges oxidant gas from the fuel cell stack 1, and a cooling medium supply and discharge unit 4 that supplies a cooling medium to the fuel cell stack 1 and discharges the cooling medium from the fuel cell stack. The fuel gas is, for example, hydrogen. The oxidant gas is, for example, air containing oxygen. The cooling medium is, for example, water or a coolant liquid containing ethylene glycol or propylene glycol.

[0010] Figure 2 is a cross-sectional view of the main part of the fuel cell stack 1. As shown in Figure 2, the fuel cell stack 1 has a cell stack 110 formed by stacking a plurality of power generation cells 101. Each power generation cell 101 has an electrode assembly (UEA) 102 having a membrane electrode assembly (MEA) including an electrolyte membrane and an electrode, and separators 103 arranged alternately with the electrode assembly 102. The separator 103 integrally includes an anode separator 103a positioned facing one side of the electrode assembly 102 and a cathode separator 103b positioned facing the other side of the electrode assembly 102.

[0011] A cooling channel PAw is formed inside the separator 103, which is surrounded by the anode separator 103a and the cathode separator 103b, through which a cooling medium flows. The flow of the cooling medium cools the power generation surface of the power generation cell 101. The surface of the separator 103 facing the electrode assembly 102 is made uneven by press molding or the like to form a gas channel between it and the electrode assembly 102. Between the electrode assembly 102 and the anode separator 103a, an anode channel PAa is formed by a recess through which fuel gas flows. Between the electrode assembly 102 and the cathode separator 103b, a cathode channel PAc is formed by a recess through which oxidizer gas flows.

[0012] Figure 2 includes a cross-sectional view of the membrane electrode assembly 104 as a cross-sectional view of the electrode assembly 102. As shown in the detailed view of part A in Figure 2, the membrane electrode assembly 104 has an electrolyte membrane 105, an anode electrode 106 provided on one side of the electrolyte membrane 105, and a cathode electrode 107 provided on the other side of the electrolyte membrane 105. The electrolyte membrane 105 is, for example, a solid polymer electrolyte membrane, and a thin film of a water-containing perfluorosulfonic acid polymer can be used. Not limited to fluorine-based electrolyte membranes, hydrocarbon-based electrolyte membranes can also be used.

[0013] The anode electrode 106 is formed on one side of the electrolyte membrane 105 and has an electrode catalyst layer 106a that serves as the reaction field for the electrode reaction, and a gas diffusion layer 106b provided on the surface of the electrode catalyst layer 106a opposite to the electrolyte membrane 105 and supplies fuel gas by diffusion. The cathode electrode 107 is formed on the other side of the electrolyte membrane 105 and has an electrode catalyst layer 107a that serves as the reaction field for the electrode reaction, and a gas diffusion layer 107b provided on the surface of the electrode catalyst layer 107a opposite to the electrolyte membrane 105 and supplies oxidizing gas by diffusion. An intermediate layer (underlayer) may also be provided between the electrode catalyst layers 106a, 107a and the gas diffusion layers 106b, 107b.

[0014] The electrode catalyst layers 106a and 107a contain a catalytic metal that promotes the electrochemical reaction between hydrogen in the fuel gas and oxygen in the oxidizer gas, a proton-conducting electrolyte (such as an ionomer), and electron-conducting carbon particles. The gas diffusion layers 106b and 107b are composed of a gas-permeable conductive material, such as a porous carbon body. The gas diffusion layers 106b and 107b can hold the fuel gas and the oxidizer gas.

[0015] At the anode electrode 106, fuel gas (hydrogen) supplied via the anode channel PAa is ionized by the action of a catalyst and moves to the cathode electrode side through the electrolyte membrane 105. The electrons generated at this time pass through an external circuit and are extracted as electrical energy. At the cathode electrode 107, oxidizing gas (oxygen) supplied via the cathode channel PAc reacts with hydrogen ions introduced from the anode electrode 106 and electrons that have moved from the anode electrode 106 to produce water. The produced water (called generated water) provides appropriate humidity to the electrolyte membrane 105, and excess water is discharged to the outside of the electrode assembly 102 along the gas flow. The generated water on the cathode side also flows to the anode side by reverse diffusion through the electrolyte membrane 105. Therefore, generated water is contained in both the fuel gas and the oxidizing gas. Condensed water is also contained in both the fuel gas and the oxidizing gas.

[0016] As shown in Fig. 1, the fuel gas supply / discharge unit 2 includes a fuel gas tank 21 in which fuel gas (anode gas) is stored, a fuel gas supply passage PA21 that guides the fuel gas in the fuel gas tank to the fuel gas inlet 21a of the fuel cell stack 1, and a fuel gas discharge passage PA22 through which the fuel gas (fuel exhaust gas) discharged from the fuel gas outlet 21b of the fuel cell stack 1 flows. An injector 22 and an ejector 23 are arranged in the fuel gas supply passage PA21. A gas-liquid separator 24 is connected to the fuel gas discharge passage PA22.

[0017] The injector 22 is composed of a single or a plurality of electromagnetic injectors connected in parallel. By driving the injector 22, fuel gas is injected toward the ejector 23. The ejector 23 has a nozzle portion, a suction portion, a confluence portion, and a diffuser portion. The fuel gas injected from the injector 22 passes through the nozzle portion with a small diameter and then flows into the diffuser portion through the confluence portion. The fuel gas that has passed through the ejector 23 is supplied to the fuel cell stack 1 through the fuel gas inlet 21a.

[0018] The fuel gas discharged from the fuel gas outlet 21b, that is, the fuel exhaust gas (anode off-gas), is separated into fuel gas and water by the gas-liquid separator 24. The water separated by the gas-liquid separator 24 is discharged to the outside through an electromagnetic drain valve 25 and a drain passage PA23. The fuel gas separated by the gas-liquid separator 24 is guided to a circulation passage PA24. The ejector 23 is connected to the circulation passage PA24, and a purge valve 26 is connected through a purge passage PA25. The purge valve 26 is an electromagnetic valve device that can be opened and closed, and the fuel gas in the circulation passage PA24 can be discharged to the outside through the purge passage PA25 and the purge valve 26.

[0019] The fuel gas, separated into gas and liquid form by the gas-liquid separator 24 due to the flow of fuel gas injected from the injector 22, is drawn into the ejector 23 via the circulation channel PA24. The drawn-in fuel gas merges with the fuel gas that has passed through the nozzle section of the ejector 23 at the confluence section of the ejector 23, and is then made into a uniform flow in the diffuser section of the ejector 23 before being supplied to the fuel cell stack 1 via the fuel gas inlet 21a.

[0020] The oxidant gas supply and discharge section 3 includes an electrically operated air pump 31 that generates high-pressure oxidant gas (cathode gas), an oxidant gas supply channel PA 31 that guides the oxidant gas generated by the air pump 31 to the oxidant gas inlet 31a of the fuel cell stack 1, and an oxidant gas discharge channel PA 32 through which the oxidant gas (oxidant exhaust gas) discharged from the oxidant gas outlet 31b of the fuel cell stack 1 flows. The air pump 31 functions as a gas supply section that compresses air taken in from the atmosphere to generate high-pressure oxidant gas. The air pump 31 may also be configured as a compressor.

[0021] A humidifier 32 is positioned across the oxidizer gas supply channel PA31 and the oxidizer gas discharge channel PA32. In the humidifier 32, humidity exchange occurs between the oxidizer gas and the oxidizer exhaust gas, and the moisture (water vapor) contained in the oxidizer exhaust gas in the oxidizer gas discharge channel PA32 humidifies the oxidizer gas in the oxidizer gas supply channel PA31.

[0022] The oxidizer gas supply and discharge section 3 further includes a bypass channel PA33. The bypass channel PA33 is connected to the oxidizer gas supply channel PA31 upstream of the humidifier 32 and the oxidizer gas discharge channel PA32 downstream of the humidifier 32. Through the bypass channel PA33, the oxidizer gas can be supplied while bypassing the humidifier 32 and the fuel cell stack 1.

[0023] In the oxidizer gas supply channel PA31, an adjustable electromagnetic control valve 33 is provided between the bypass channel PA33 and the humidifier 32. In the oxidizer gas discharge channel PA32, an adjustable electromagnetic control valve 34 is provided between the bypass channel PA33 and the humidifier 32. An adjustable electromagnetic control valve 35 is provided in the bypass channel PA33. By controlling the air pump 31 and the control valves 33-35, the supply amount and pressure of the oxidizer gas supplied to the fuel cell stack 1 can be adjusted. Furthermore, by controlling the control valves 33-35, the amount of oxidizer gas bypassing the fuel cell stack 1 can be adjusted.

[0024] A diluent 36 is connected to the downstream end of the oxidizer gas discharge channel PA32. The ends of the drain channel PA23 and the purge channel PA25 are also connected to the diluent 36. In the diluent 36, the fuel exhaust gas introduced via the purge channel PA25 is diluted with the oxidizer exhaust gas. The diluted fuel exhaust gas, along with the liquid water introduced via the drain channel PA23, is discharged to the outside (into the atmosphere) via the drain channel PA34.

[0025] The cooling medium supply and discharge section 4 includes a cooling device 41, a cooling medium supply channel PA41 connecting the cooling device 41 to the cooling medium inlet 41a of the fuel cell stack 1, and a cooling medium discharge channel PA42 connecting the cooling device 41 to the cooling medium outlet 41b of the fuel cell stack 1. Although not shown in the figures, the cooling device 41 includes a pump that pressurizes the cooling medium toward the fuel cell stack 1, a heat exchanger (radiator) that cools the cooling medium that has been heated after passing through the fuel cell stack 1, and a cooling fan that blows cooling air to the heat exchanger.

[0026] A pressure sensor 51 is connected downstream of the ejector 23 in the fuel gas supply channel PA21. The pressure sensor 51 detects the inlet pressure (anode pressure Pa) of the fuel gas supplied to the fuel cell stack 1. A pressure sensor 52 is connected downstream of the humidifier 32 in the oxidizer gas supply channel PA31. The pressure sensor 52 detects the inlet pressure (cathode pressure Pc) of the oxidizer gas supplied to the fuel cell stack 1. Furthermore, a pressure sensor 53 is connected upstream of the air pump 31 in the oxidizer gas supply channel PA31. The pressure sensor 53 detects the atmospheric pressure P0.

[0027] A temperature sensor 54 is connected to the cooling medium supply channel PA41 to detect the temperature of the cooling medium (refrigerant inlet temperature). A temperature sensor 55 is connected to the cooling medium discharge channel PA42 to detect the temperature of the cooling medium (refrigerant outlet temperature). The refrigerant inlet temperature and refrigerant outlet temperature are collectively referred to as the refrigerant temperature Te. The refrigerant temperature Te refers to either or both of the refrigerant inlet temperature and / or refrigerant outlet temperature.

[0028] Figure 3 is a block diagram showing the control configuration of a fuel cell system 100 according to an embodiment of the present invention. As shown in Figure 3, the fuel cell system 100 includes a controller 60, current detection units 56 connected to the controller 60, pressure sensors 51-53, temperature sensors 54, 55, an injector 22, an air pump 31, and a purge valve 26.

[0029] The current detection unit 56 is composed of, for example, a current sensor that detects the output current of the fuel cell stack 1. The fuel cell system 100 is configured to calculate the amount of power generation required by the vehicle (required power generation amount) and to generate power according to the required power generation amount. For this reason, the current detection unit 56 may calculate the output current using the required power generation amount. In other words, instead of the current detection unit 56 detecting the output current with a sensor, the output current may be calculated using parameters that have a correlation with the output current. To put it another way, the current detection unit 56 may be provided inside the controller 60. Thus, detecting the output current by the current detection unit 56 also includes calculating the output current.

[0030] The controller 60 receives signals from the output current detected by the current detection unit 56, the anode pressure Pa detected by the pressure sensor 51, the cathode pressure Pc detected by the pressure sensor 52, the atmospheric pressure P0 detected by the pressure sensor 53, and the refrigerant temperature Te detected by the temperature sensors 54 and 55. The refrigerant temperature Te is correlated with the temperatures of the fuel gas and oxidizer gas. Therefore, the refrigerant temperature Te is used as a parameter representing the temperatures of the fuel gas and oxidizer gas. Based on these input signals, the controller 60 performs predetermined processing and outputs control signals to the injector 22, the air pump 31, and the purge valve 26.

[0031] The controller 60 is a computer comprising a processing unit having a CPU, ROM, RAM, and peripheral circuits. Functionally, the controller 60 has a cathode control unit 61, an anode control unit 62, a valve control unit 63, and a storage unit 64. The storage unit 64 stores various maps, thresholds, calculation formulas, programs, etc. The storage unit 64 also stores the volume of the anode flow path PAa and the volume of the gas diffusion layer 106b (Figure 2) capable of holding fuel gas.

[0032] The controller 60 calculates the required power generation amount. Specifically, the controller 60 calculates the target drive torque of the drive motor based on the signal from the accelerator pedal position sensor that detects the opening degree of the accelerator pedal, and calculates the required power generation amount necessary for the drive motor to generate the target drive torque. Alternatively, the controller 60 calculates the required power generation amount based on the signal from the battery sensor that detects the remaining capacity (SOC) of the battery, so that the remaining capacity of the battery reaches a predetermined value.

[0033] The cathode control unit 61 calculates a target flow rate of oxidizer gas according to the required power generation amount and outputs a control signal to the air pump 31 so that the oxidizer gas according to the target flow rate is supplied to the fuel cell stack 1. As a result, the cathode pressure Pc detected by the pressure sensor 52 approaches the target cathode pressure Pct corresponding to the target flow rate. In reality, the cathode control unit 61 outputs control signals not only to the air pump 31 but also to control valves 33-35, etc., but for convenience, the control of components other than the air pump will be omitted from the explanation.

[0034] Similar to the cathode control unit 61, the anode control unit 62 calculates a target flow rate of fuel gas according to the required power generation amount and outputs a control signal to the injector 22 so that fuel gas according to the target flow rate is supplied to the fuel cell stack 1. More specifically, the anode control unit 62 calculates the target injection period and target injection amount (duty cycle) of the injector 22 and controls the injector 22 according to the target injection period and target injection amount.

[0035] In this case, the anode control unit 62 stops fuel injection when the anode pressure Pa is higher than the anode target pressure Pat. Then, when the anode pressure Pa falls below the anode target pressure Pat, it outputs a control signal to the injector 22, causing the injector 22 to inject fuel. In other words, the anode control unit 62 injects fuel from the injector 22 only when the anode pressure Pa falls below the anode target pressure Pat. As a result, the anode pressure Pa detected by the pressure sensor 51 approaches the anode target pressure Pat.

[0036] The air supplied to the cathode channel PAc contains nitrogen. The valve control unit 63 uses the anode pressure Pa, cathode pressure Pc, and refrigerant temperature Te to calculate the amount of nitrogen permeating from the cathode channel PAc to the anode channel PAa. By dividing this amount of nitrogen by the amount of fuel gas in the anode channel PAa, the valve control unit 63 calculates the nitrogen concentration in the anode channel PAa. Furthermore, the valve control unit 63 multiplies the nitrogen concentration by the anode pressure Pa to calculate the nitrogen partial pressure. As the nitrogen concentration increases, the power generation performance decreases. Therefore, when the nitrogen partial pressure exceeds a predetermined value, the valve control unit 63 outputs a control signal to the purge valve 26 and opens the purge valve 26. As a result, the fuel gas containing nitrogen flows out of the anode channel PAa, and the nitrogen partial pressure can be kept below the predetermined value.

[0037] In such a fuel cell system 100, for example, if a moving vehicle suddenly decelerates, the required power generation decreases sharply. At this time, the cathode pressure Pc changes responsively in response to changes in the rotational speed of the air pump 31. Therefore, if the cathode target pressure Pct is rapidly reduced, the cathode pressure Pc can be reduced accordingly. On the other hand, since the fuel gas circulates through the circulation channel PA24, it is difficult to rapidly reduce the anode pressure Pa. As a result, the following problems arise.

[0038] Figure 4A is a time chart showing an example of the changes over time of the anode pressure Pa (dotted line), anode target pressure Pat (solid line), cathode pressure Pc (dotted line), cathode target pressure Pct (solid line), and differential pressure ΔP (anode target pressure Pat minus cathode target pressure Pct) during vehicle deceleration. This time chart is a first reference example of this embodiment, illustrating the problems that occur during deceleration. The fuel gas is intermittently injected from the injector 22 by duty cycle control, but in Figure 4A, for convenience, the change in anode pressure Pa is shown as a series of triangular pulse waveforms.

[0039] The required power generation changes according to the operation of the accelerator pedal, and the output current and vehicle speed change accordingly. Time t0 in Figure 4A is the point at which deceleration begins, when the vehicle starts to decelerate rapidly. As shown in Figure 4A, when the vehicle starts to decelerate at time t0, the output current decreases. At this time, the cathode target pressure Pct decreases at a rate corresponding to the decrease in output current, and consequently the cathode pressure Pc also decreases. On the other hand, the anode target pressure Pat also decreases from the start of deceleration, but as mentioned above, the anode pressure Pa has poor responsiveness. For this reason, the rate of decrease (slope) of the anode target pressure Pat is smaller than the rate of decrease (slope) of the cathode target pressure Pct.

[0040] In this case, the larger the rate of decrease of the anode target pressure Pat per unit time (referred to as the reference rate Pat0), the later the start time t1 of fuel gas injection by the injector 22, which is determined by the intersection point PT1 of the anode target pressure Pat and the anode pressure Pa, is delayed. Although the water generated in the anode flow path PAa can be swept out by the injection of fuel gas, if the start time t1 of fuel gas injection is delayed, the water generated cannot be swept out, and the entire anode flow path PAa may be filled with the water generated, potentially adversely affecting power generation. For this reason, the reference rate Pat0 needs to be set to a value such that the anode flow path PAa is not filled with the water generated. Thus, the reference rate Pat0 is the rate of decrease of the anode target pressure Pat, which is set to determine the start time t1 of fuel gas injection by the injector 22 when the required power generation decreases.

[0041] Here, α is defined as the rate of decrease in anode pressure Pa due to the consumption of hydrogen contained in the fuel gas during power generation (decrease per unit time), Pax is defined as the degree of increase in anode pressure Pa due to one injection by injector 22 (pulsation), and Ta is defined as the target time from the start of injection by injector 22 to the start of the next injection. In this case, the reference rate Pat0 is calculated by the following equation (I). Pat0 = α - Pax / Ta ···(I)

[0042] In equation (I) above, α is a value parameterized by the amount of power generated, and can be calculated using the instantaneous output current detected by the current detection unit 56. In equation (I) above, Pax can be calculated by multiplying the degree of increase in the anode pressure Pa during fuel gas injection, which has been determined experimentally in advance, by the valve opening time of the injector 22 corresponding to the load (output current) acting on the fuel cell system 100. In equation (I) above, Ta is the sum of the time required to use up the hydrogen held in the power generation unit (gas diffusion layer 106b) (hydrogen consumption time) Ta1 and the time required for generated water to accumulate in the anode channel PAa (full water time) Ta2. That is, after the hydrogen in the power generation unit is consumed, generated water begins to accumulate in the anode channel PAa, but if injection is performed within the target time Ta, the generated water can be swept out before the anode channel PAa is filled with generated water, and the anode channel PAa can be prevented from being filled with generated water.

[0043] The hydrogen consumption time Ta1 can be calculated by dividing the amount of hydrogen held in the power generation unit by the amount of hydrogen consumed per unit time during power generation. Here, the amount of hydrogen held in the power generation unit can be calculated using the volume of the power generation unit stored in the memory unit 64, the refrigerant temperature Te detected by the temperature sensors 54 and 55, and the anode pressure Pa detected by the pressure sensor 51. More specifically, it can be calculated by multiplying the volume of the power generation unit by a parameter corresponding to the refrigerant temperature Te and a parameter corresponding to the anode pressure Pa. The amount of hydrogen consumed per unit time during power generation can be calculated using the instantaneous output current detected by the current detection unit 56. More specifically, it can be calculated by multiplying the output current by a predetermined coefficient.

[0044] The full water time Ta2 can be calculated by dividing the volume of the anode channel PAa, which is stored in the memory unit 64 beforehand, by the amount of water that permeates (backdiffuses) per unit time from the cathode channel PAc to the anode channel PAa. The amount of water that permeates per unit time can be calculated using the instantaneous output current detected by the current detection unit 56.

[0045] By calculating the reference rate Pat0 using the above equation (I) and setting the anode target pressure Pat in accordance with the reference rate Pat0, the adverse effects on power generation due to the accumulation of generated water in the anode channel PAa can be suppressed. However, in this case, because the rate of decrease (slope) of the anode target pressure Pat is small, the differential pressure ΔP between the anode target pressure Pat and the cathode target pressure Pct gradually increases after the deceleration start time t0. If the differential pressure ΔP increases too much, the membrane electrode assembly 104 may not be able to withstand the differential pressure ΔP and may be damaged. For this reason, the differential pressure ΔP must be kept below a predetermined value ΔP1. Figure 4A shows an example in which pressure protection control is started after the differential pressure ΔP reaches a predetermined value ΔP1, and the differential pressure ΔP decreases.

[0046] Pressure protection control is a control mechanism that protects the membrane electrode assembly 104 by suppressing the increase in differential pressure ΔP. Although not shown in Figure 4A, when pressure protection control is initiated, injection from the injector 22 is stopped or limited. As a result, in the first reference example in Figure 4A, it is not possible to inject fuel gas at the desired timing corresponding to the anode target pressure Pat, leading to a problem of reduced power generation performance.

[0047] Figure 4B is a time chart of a second reference example of this embodiment. The operation from the start of deceleration t0 to time t2 is the same as in Figure 4A. Time t2 is earlier than time t1 in Figure 4A, and from time t0 to time t2, the anode target pressure Pat decreases along the reference rate Pat0. However, in Figure 4B, when the differential pressure ΔP reaches a predetermined value ΔP2 at time t2, the anode target pressure Pat is reduced by a predetermined amount, and then kept constant until time t4. Furthermore, at time t4, the anode target pressure Pat decreases again along the same rate as the initial reference rate Pat0 (the rate obtained by extending the reference rate Pat0 from time t0 to t2). The predetermined value ΔP2 is smaller than the predetermined value ΔP1 in Figure 4A.

[0048] In Figure 4B, the anode target pressure Pat is reduced by a predetermined amount at time t2, so the differential pressure ΔP does not increase to a predetermined value ΔP1 (Figure 4A), thus avoiding pressure protection control. However, in Figure 4B, the injection start time t3, determined by the intersection point PT2 between the anode target pressure Pat and the anode pressure Pa, is delayed compared to the injection start time t1 in Figure 4A. As a result, the anode flow path PAa is filled with generated water, causing the humidity of the electrolyte membrane 105 to rise excessively, leading to a problem of reduced power generation performance.

[0049] Thus, in both the first and second reference examples, there is a risk that power generation performance will decrease after the deceleration start time t0. Therefore, in this embodiment, in order to suppress the decrease in power generation performance after the deceleration start time t0, the fuel cell system 100 is configured as follows.

[0050] Figure 5A is a time chart showing an example of the operation of the fuel cell system 100 according to this embodiment during vehicle deceleration (when the output current decreases). The difference between Figure 5A and Figure 4A is the anode target pressure Pat from the deceleration start time t0 to the injection start time t1, which is set by the anode control unit 62.

[0051] In other words, in this embodiment, the anode control unit 62 sets the anode target pressure Pat so that it decreases at a first rate (first percentage) Pat1 from the start of deceleration t0 to time t5, decreases at a second rate (second percentage) Pat2 from time t5 to the start of injection t1, and then decreases at the same reference rate Pat0 as in Figure 4A from time t1 onward. This process of setting the anode target pressure Pat is called deceleration processing.

[0052] The deceleration process is performed when the degree of decrease in the output current, as detected by the current detection unit 56 and corresponding to the load acting on the vehicle, is greater than or equal to a predetermined level. Specifically, it is performed when the output current decreases from a state where it is greater than or equal to a first predetermined value to a second predetermined value or lower than the first predetermined value. For example, the deceleration process is performed when the load acting on the vehicle decreases from a high load to a low load, or from a high load to a medium load. The anode control unit 62 may also perform the deceleration process when the rate of decrease in output current (rate of decrease in load) is greater than or equal to a predetermined value, and the degree of decrease in output current is greater than or equal to a predetermined level.

[0053] The setting of the first rate Pat1 and the second rate Pat2 through the deceleration process will be explained in more detail. Figure 5B is an enlarged view of the time chart showing the changes in anode pressure Pa and anode target pressure Pat from time t0 to time t1 in Figure 5A. As shown in Figure 5B, the anode control unit 62 sets the first rate Pat1 to a value greater than the reference rate Pat0 and the second rate Pat2 to a value less than the reference rate Pat0. In other words, the slope of the first rate Pat1 is set to a value greater than (steeper than) the slope of the reference rate Pat0, and the slope of the second rate Pat2 is set to a value less than (gentler than) the slope of the reference rate Pat0.

[0054] The procedure for setting the first rate Pat1 and the second rate Pat2 through deceleration processing is as follows. First, at time t0, the anode control unit 62 sets the reference rate Pat0 using the same method as in the first reference example (Figure 4A), and calculates the injection start time t1 and the anode target pressure Pat (intersection PT1) at injection start time t1. Furthermore, the anode control unit 62 reads the cathode target pressure Pct set by the cathode control unit 61 and sets the first rate Pat1 to the same value as the decrease rate of the cathode target pressure Pct (Figure 5A). The decrease rate of the cathode target pressure Pct (slope in Figure 5A) is larger the larger the output current detected by the current detection unit 56. Therefore, the first rate Pat1 (slope) is set to a larger value the larger the output current.

[0055] Next, the anode control unit 62 refers to the value detected by the current detection unit 56 and determines whether the output current has decreased to or below a predetermined value. The predetermined value is a value smaller than the second predetermined value, which is the starting condition for the deceleration process, and is a value larger than the output current corresponding to the anode target pressure Pat (intersection PT1) at the injection start time t1. If it is determined that the output current is greater than the predetermined value, the anode control unit 62 sets the anode target pressure Pat according to the first rate Pat1.

[0056] If the anode control unit 62 determines at time t5 that the output current is below a predetermined value, it switches the rate of decrease of the anode target pressure Pat from the first rate Pat1 to the second rate Pat2. In this case, the anode control unit 62 sets the second rate Pat2 by drawing a straight line between the anode target pressure Pat at time t5 (point PT3), when the output current is determined to be below a predetermined value, and the anode target pressure Pat at injection start time t1 (point PT1). The anode control unit 62 sets the anode target pressure Pat along the second rate Pat2 from time t5 to time t1. The second rate Pat2 is set to a smaller value as the output current decreases. From time t1 onward, the anode control unit 62 sets the anode target pressure Pat along the reference rate Pat0.

[0057] In this embodiment, immediately after the vehicle starts to decelerate, the anode target pressure Pat is reduced along a first rate Pat1, which has a larger reduction rate than the reference rate Pat0. As a result, as shown in Figure 5A, the differential pressure ΔP between the anode target pressure Pat and the cathode target pressure Pct can be kept below a predetermined value ΔP1, thus avoiding the execution of pressure protection control. Furthermore, when the output current falls below a predetermined value, the anode target pressure Pat is reduced at a second rate Pat2, which has a smaller reduction rate than the reference rate Pat0. This avoids delays in the start of fuel gas injection and prevents the anode flow path PAa from being filled with generated water. As a result, a decrease in power generation performance can be suppressed.

[0058] Figure 6 is a time chart showing an example of the control operation by the valve control unit 63. In addition to the changes over time of the anode pressure Pa, anode target pressure Pat, cathode pressure Pc, and cathode target pressure Pct shown in Figure 5A, Figure 6 also shows an example of the changes over time of the nitrogen partial pressure (N2 partial pressure) Pn and the opening and closing operation of the purge valve 26.

[0059] As shown in Figure 6, until the deceleration process begins at time t0, the valve control unit 63 sets an opening threshold Na1 for opening the purge valve 26 and a closing threshold Nb1 for closing the purge valve 26. In this state, when the nitrogen partial pressure Pn rises above the opening threshold Na1, the valve control unit 63 outputs a control signal to the purge valve 26, opening the purge valve 26. This causes the fuel gas to be discharged to the outside, and the nitrogen partial pressure Pn decreases. At this time, although not shown in the figure, the anode pressure Pa also decreases simultaneously.

[0060] When the nitrogen partial pressure Pn falls below the closing threshold Pb1, the valve control unit 63 outputs a control signal to the purge valve 26, closing the purge valve 26. This stops the discharge of fuel gas to the outside. In this way, by opening and closing the purge valve 26 in accordance with the nitrogen partial pressure Pn, the nitrogen partial pressure Pn can be maintained within a predetermined range between the closing threshold Nb1 and the opening threshold Na1.

[0061] When deceleration control is initiated by the anode control unit 62, the open threshold Na1 and the closed threshold Nb1 are changed. Specifically, at time t0, the valve control unit 63 sets an open threshold Na2 lower than Na1 and a closed threshold Nb2 lower than Nb1, according to the output current detected by the current detection unit 56. In this case, the valve control unit 63 reads the atmospheric pressure detected by the pressure sensor 53, and sets the thresholds Na2 and Nb2 to smaller values ​​as the atmospheric pressure decreases.

[0062] In this way, by setting the threshold values ​​Na2 and Nb2 low at the start of deceleration control, the nitrogen partial pressure Pn reaches the opening threshold Na2 at the start time t0 of deceleration control. Therefore, the valve control unit 63 opens the purge valve 26 simultaneously with the start of deceleration control. As a result, fuel gas is discharged to the outside, and the anode pressure Pa can be rapidly reduced. This speeds up the timing at which the anode pressure Pa becomes below the anode target pressure Pat, and the injection start time t1 can be advanced. As a result, the generated water in the anode flow path PAa can be swept out earlier, improving power generation performance. After the purge valve 26 is opened, when the nitrogen partial pressure Pn falls below the closing threshold Nb2, the valve control unit 63 closes the purge valve 26.

[0063] The above describes an example in which the anode control unit 62 calculates the reference rate Pat0 using the above equation (I). However, the reference rate Pat0 can also be determined by referring to a map M1 that is pre-stored in the storage unit 64. Figure 7 shows an example of map M1. Map M1 defines the relationship between the reference rate Pat0 corresponding to the output current (load) detected by the current detection unit 56 and the anode pressure Pa detected by the pressure sensor 51. That is, the larger the output current and the larger the anode pressure Pa, the greater the reference rate Pat0 (slope), and the smaller the output current and the smaller the anode pressure Pa, the smaller the reference rate Pat0 becomes. Map M1 can be determined in advance by conducting experiments or analyses.

[0064] By referring to such a map M1, the anode control unit 62 can easily determine the reference rate Pat0 shown in Figure 5A. The anode control unit 62 may also determine either or both of the first rate Pat1 and the second rate Pat2 by referring to map M1, in addition to the reference rate Pat0.

[0065] Figure 8 is a flowchart showing an example of processing performed by the anode control unit 62. The processing shown in this flowchart is started, for example, when the output current decreases from a state of being above a first predetermined value to below a second predetermined value, and deceleration processing is performed, and is repeated at a predetermined cycle. As shown in Figure 8, first, in step S1, the anode control unit 62 reads signals from sensors 51-55 and the current detection unit 56. Furthermore, it reads the cathode target pressure Pct set by the cathode control unit 61.

[0066] Next, in step S2, the anode control unit 62 sets the rate of decrease (decrease rate) of the anode target pressure Pat based on the signal read in step S1. That is, the first rate Pat1 immediately after the start of the deceleration process is set to the same value as the cathode target pressure Pct. Furthermore, when the output current falls below a predetermined value, the anode target pressure Pat at that time and the anode target pressure Pat corresponding to the injection start time t1 on the reference rate Pct0 are connected by a straight line, and the second rate Pct2 is set. Furthermore, from the injection start time t1 onward, the reference rate Pat0 is set as the decline rate. The anode control unit 62 sets the anode target pressure Pat based on these decline rates (first rate Pat1, second rate Pat2, reference rate Pat0).

[0067] Next, in step S3, the anode control unit 62 calculates the differential pressure ΔP (=Pat-Pct) between the anode target pressure Pat set in step S2 and the cathode target pressure Pct read in step S1. Next, in step S4, the anode control unit 62 determines whether the differential pressure ΔP is greater than or equal to a predetermined value ΔP1. If the result in step S4 is positive, the process proceeds to step S5; otherwise, the process proceeds to step S6.

[0068] In step S5, pressure protection control is performed and the process ends. This stops or limits the injection from the injector 22. Meanwhile, in step S6, it is determined whether the anode target pressure Pat set in step S2 is greater than or equal to the anode pressure Pa detected by the pressure sensor 51. If the result in step S6 is positive, the process proceeds to step S7; otherwise, the process ends. In step S7, a control signal is output to the injector 22 to inject fuel gas.

[0069] The flowchart in Figure 8 includes pressure protection control (step S5). However, in this embodiment, by performing a predetermined deceleration process, the differential pressure ΔP during vehicle deceleration is kept below a predetermined value ΔP1 (Figure 5A). Therefore, pressure protection control is not performed, and the injector 22 can inject fuel gas at the desired timing (step S7).

[0070] This embodiment can provide the following effects and advantages. (1) The fuel cell system 100 includes a current detection unit 56 that detects the output current of a fuel cell stack 1 (fuel cell) which generates electricity by being supplied with fuel gas (anode gas) and oxidant gas (cathode gas), an injector 22 that supplies anode gas to the fuel cell stack 1 via a fuel gas supply channel PA21, an air pump 31 that supplies cathode gas to the fuel cell stack 1 via an oxidant gas supply channel PA31, a circulation channel PA24 that circulates the anode gas discharged from the fuel cell stack 1 back to the fuel gas supply channel PA21, and an anode control unit 62 and a cathode control unit 61 that set the anode target pressure Pat and cathode target pressure Pct, which are the target pressures of the anode gas and cathode gas, according to the output current detected by the current detection unit 56, and control the injector 22 and the air pump 31 according to the anode target pressure Pat and cathode target pressure Pct (Figures 1 and 3). When the current detection unit 56 detects a predetermined decrease in the output current (generated current), that is, when the output current decreases from a value of a first predetermined value or higher to a value of a second predetermined value or lower, the anode control unit 62 sets the anode target pressure Pat to decrease at a first rate Pat1, and then to decrease at a second rate Pat2, which has a smaller decrease rate than the first rate Pat1 (Figure 5A).

[0071] With this configuration, immediately after the start of deceleration, the rate of decrease in the anode target pressure Pat is large, and then the rate of decrease decreases. As a result, the differential pressure ΔP between the anode target pressure Pat and the cathode target pressure Pct can be kept below a first predetermined value ΔPA1, thus avoiding pressure protection control. In addition, the delay in the injection start time t1 can be suppressed, thereby preventing the anode flow path PAa from being filled with generated water. As a result, the decrease in power generation performance during deceleration can be suppressed.

[0072] (2) The anode control unit 62 sets the first rate Pat1 (slope) to a larger value the greater the decrease in output current detected by the current detection unit 56. This makes it possible to suppress the increase in the differential pressure ΔP between the anode target pressure Pat and the cathode target pressure Pct immediately after the start of deceleration.

[0073] (3) When the output current decreases from a value above a first predetermined value to a value below a second predetermined value, the anode control unit 62 sets the rate of decrease of the cathode target pressure Pct to decrease the cathode target pressure Pct, and sets the first rate Pat1 to a value equal to the rate of decrease of the cathode target pressure Pct (Figure 5A). This makes it easy to set the first rate Pat1 and effectively suppresses the increase in differential pressure ΔP immediately after the start of deceleration.

[0074] (4) The anode control unit 62 sets the second rate Pat2 to a smaller value as the output current decreases. When the degree of vehicle deceleration is large, the output current decreases significantly, but by setting the second rate Pat2 to a smaller value as the output current decreases, the discrepancy between the anode pressure Pa and the anode target pressure Pat can be suppressed, and the delay in the fuel gas injection timing by the injector 22 can be suppressed. In other words, when the output current is small, the amount of hydrogen consumed decreases, so the degree of decrease in anode pressure Pa is small. In this case, if the second rate Pat2 is set to a small value, the anode pressure Pa is more likely to fall below the anode target pressure Pat, and the fuel gas can be injected quickly.

[0075] (5) The fuel cell stack 1 has an anode channel PAa through which anode gas flows, and a power generation section (gas diffusion layer 106b) facing the anode channel PAa that can hold anode gas (Figure 2). The anode control unit 62 calculates a target time Ta, which is the sum of a hydrogen consumption time Ta1 required for anode gas consumption, which is determined according to the output current and the volume of the power generation section, and a full water time Ta2 required for the anode channel PAa to be filled with generated water, which is determined according to the output current and the volume of the anode channel PAa. The anode target pressure Pat is set so that the decrease of the anode target pressure Pat at the first rate Pat1 and the decrease at the second rate Pat2 are completed within the target time Ta. This makes it possible to inject fuel gas from the injector 22 before the anode channel PAa is filled with generated water.

[0076] (6) The anode control unit 62 calculates the amount of hydrogen consumed per unit time, which is determined by the output current, and also calculates the amount of permeate water per unit time that permeates from the cathode channel PAc through which the cathode gas flows to the anode channel PAa, based on the output current. Furthermore, the anode control unit 62 calculates the hydrogen consumption time Ta1 by dividing the amount of hydrogen held in the power generation unit, which is determined according to the volume of the power generation unit and the pressure of the anode gas (anode pressure Pa), by the amount of hydrogen consumed per unit time, and calculates the full-water time Ta2 by dividing the volume of the anode channel PAa by the amount of permeate water per unit time. As a result, the hydrogen consumption time Ta1 and the full-water time Ta2 can be calculated with high accuracy, and fuel gas can be reliably injected from the injector 22 before the anode channel PAa is filled with generated water.

[0077] (7) After the target time Ta has elapsed, the anode target pressure Pat is set to a reference rate Pat0, which is lowered by a smaller percentage than the first rate Pat1 and by a larger percentage than the second rate Pat2 (Figure 5A). In this case, since the cathode target pressure Pct has already decreased, the differential pressure ΔP between the anode target pressure Pat and the cathode target pressure Pct is small. For this reason, even if the anode target pressure Pat is lowered by the reference rate Pat0, pressure protection control is not executed, and fuel gas can be injected from the injector 22 at a good timing.

[0078] (8) The fuel cell system 100 further includes a purge valve 26 provided in a purge channel PA25 (discharge channel) that discharges anode gas discharged from the fuel cell stack 1 to the outside, and a valve control unit 63 that controls the purge valve 26 to discharge anode gas to the outside when the output current drops from a value of a first predetermined value or more to a value of a second predetermined value or less (Figure 3). This makes it possible to reduce the anode pressure Pa to the anode target pressure Pat or less at an early stage when the vehicle is decelerating. As a result, it is possible to suppress the increase in the differential pressure between the anode pressure Pa and the cathode pressure Pc, and further suppress the delay in fuel gas injection.

[0079] (9) The valve control unit 63 calculates the nitrogen partial pressure Pn of the anode gas and sets threshold values ​​Na1, Na2, Nb1, and Nb2 for the nitrogen partial pressure Pn according to the output current (Figure 6). That is, the smaller the output current, the smaller the threshold value is set (Figure 6). Furthermore, when the nitrogen partial pressure Pn becomes equal to or greater than the threshold values ​​Na1 and Na2, the valve control unit 63 opens the purge valve 26. This makes it possible to reduce the anode pressure Pa in conjunction with the start of deceleration control.

[0080] (10) The fuel cell system 100 is further equipped with a pressure sensor 53 (pressure detection unit) for detecting atmospheric pressure (Figure 1). The valve control unit 63 sets the threshold values ​​Na2 and Nb2 to smaller values ​​as the atmospheric pressure detected by the pressure sensor 53 decreases. This allows for accurate discharge of nitrogen from the anode flow path PAa, taking atmospheric pressure into consideration.

[0081] The above embodiment can be modified into various forms. Several modifications are described below. In the above embodiment, the generated current is detected by a current sensor, but the generated current may be detected by other sensors or by calculation, and the configuration of the current detection unit 56 is not limited to that described above. In the above embodiment, the injector 22 controlled by the controller 60 is used as an anode gas supply unit that supplies anode gas to the fuel cell via the fuel gas supply passage PA21 (anode supply passage), but the configuration of the anode gas supply unit is not limited to that described above. In the above embodiment, the air pump 31 controlled by the controller 60 is used as a cathode gas supply unit that supplies cathode gas to the fuel cell via the oxidizer gas supply passage PA31 (cathode supply passage), but the configuration of the cathode gas supply unit is not limited to that described above. In the above embodiment, the fuel gas discharged from the fuel cell stack 1 is circulated to the fuel gas supply passage PA21 via the circulation passage PA24 and the ejector 23, but the configuration of the gas circulation unit is not limited to that described above. The anode gas may be circulated via a circulation pump. However, since this embodiment does not have a circulation pump, the fuel cell system 100 can be constructed at a low cost, and in this respect it is more preferable.

[0082] In the above embodiment, the cathode control unit 61 and the anode control unit 62, which act as gas control units, set the cathode target pressure Pct and the anode target pressure Pat, respectively, and control the injector 22 and the air pump 31 according to the cathode target pressure Pct and the anode target pressure Pat. In this regard, the configuration of the gas control unit can be anything as long as, when a predetermined decrease in the generated current is detected, the anode target pressure Pat is set to decrease by a first rate, and then decrease by a second rate which is smaller than the first rate. The first rate does not have to be the same as the rate of decrease of the cathode target pressure Pct, and therefore the first rate is not limited to the first rate Pat1 described above. The second rate is also not limited to the second rate Pat2 described above.

[0083] In the above embodiment, the deceleration process is initiated when the output current drops from a value greater than or equal to a first predetermined value to a value less than or equal to a second predetermined value. However, the predetermined decrease in the generated current, which is the starting condition for the deceleration process, is not limited to this. For example, the starting condition for the deceleration process may be that the rate of decrease (slope) of the generated current is greater than or equal to a predetermined value. In the above embodiment, the decrease in the anode target pressure Pat at the first rate Pat1 and the decrease at the second rate Pat2 are completed within the target time Ta (predetermined time), which is the sum of the hydrogen consumption time Ta1 (first time) required for the consumption of anode gas, determined according to the output current and the volume of the power generation unit, and the full-water time Ta2 (second time) required for the anode flow path PAa to be filled with generated water, determined according to the output current and the volume of the anode flow path PAa. However, the method for calculating the first time and the second time is not limited to that described above.

[0084] In the above embodiment, after the target time Ta has elapsed, the anode target pressure Pat is reduced according to the reference rate Pat0 (third rate), but the third rate is not limited to the one described above. In the above embodiment, when the deceleration process is started, the threshold values ​​Na1 and Na2 of the nitrogen partial pressure Pn are lowered to open the purge valve 26 provided in the purge flow path PA25, but the configuration of the valve device provided in the discharge flow path is not limited to the one described above. When the deceleration process is started, the purge valve 26 may be opened regardless of the setting of the threshold value of the nitrogen partial pressure Pn, and the configuration of the valve control unit is not limited to the one described above.

[0085] The above describes an example of applying the fuel cell system 100 to a fuel cell vehicle, but the fuel cell system of the present invention can also be applied to vehicles other than fuel cell vehicles.

[0086] The above description is merely an example, and the present invention is not limited by the embodiments and modifications described above, as long as the features of the present invention are not impaired. It is also possible to arbitrarily combine one or more of the above embodiments and modifications, and to combine modifications with each other. [Explanation of symbols]

[0087] 1 Fuel cell stack, 22 Injector, 26 Purge valve, 31 Air pump, 51-53 Pressure sensors, 54, 55 Temperature sensors, 60 Controller, 61 Cathode control unit, 62 Anode control unit, 63 Valve control unit, 100 Fuel cell system, PA21 Fuel gas supply channel, PA24 Circulation channel, PA31 Oxidizer gas supply channel, Pat0 Reference rate, Pat1 First rate, Pat2 Second rate, Ta Target time, Ta1 Hydrogen consumption time, Ta2 Full water time, Na1, Na2 Open threshold, Nb1, Nb2 Closed threshold

Claims

1. A current detection unit for detecting the power generation current of a fuel cell that generates electricity by being supplied with anode gas and cathode gas, an anode gas supply unit that supplies the anode gas to the fuel cell via an anode supply channel, A cathode gas supply unit that supplies the cathode gas to the fuel cell via a cathode supply channel, A gas circulation unit that circulates the anode gas discharged from the fuel cell to the anode supply channel, The system includes a gas control unit that sets the anode target pressure and cathode target pressure, which are the target pressures of the anode gas and cathode gas, according to the power generation current detected by the current detection unit, and controls the anode gas supply unit and the cathode gas supply unit according to the anode target pressure and the cathode target pressure, respectively. A fuel cell system characterized in that, when the gas control unit detects a predetermined decrease in the power generation current by the current detection unit, it lowers the anode target pressure by a first percentage, and then sets the anode target pressure to be lowered by a second percentage that is smaller than the first percentage.

2. In the fuel cell system according to claim 1, The fuel cell system is characterized in that the gas control unit sets the first ratio to a larger value as the generated current detected by the current detection unit increases.

3. In the fuel cell system according to claim 1, The fuel cell system is characterized in that, when the current detection unit detects a predetermined decrease in the power generation current, the gas control unit sets a rate of decrease in the cathode target pressure to lower the cathode target pressure, and sets the first rate to a value equal to the rate of decrease in the cathode target pressure.

4. In the fuel cell system according to claim 3, The fuel cell system is characterized in that the gas control unit sets the second ratio to a smaller value as the generated current detected by the current detection unit decreases.

5. In the fuel cell system according to claim 1, The fuel cell has an anode channel through which the anode gas flows, and a power generation unit facing the anode channel that is capable of holding the anode gas. The fuel cell system is characterized in that the gas control unit calculates a predetermined time which is the sum of a first time required for the consumption of the anode gas, which is determined according to the power generation current detected by the current detection unit and the volume of the power generation unit, and a second time required until the anode flow path is filled with generated water, which is determined according to the power generation current detected by the current detection unit and the volume of the anode flow path, and sets the anode target pressure so that the first rate of decrease and the second rate of decrease of the anode target pressure are completed within the predetermined time.

6. In the fuel cell system according to claim 5, The fuel cell system is characterized in that the gas control unit calculates the amount of hydrogen consumed per unit time determined by the power generation current, calculates the amount of permeate water per unit time that permeates from the cathode channel through which the cathode gas flows to the anode channel based on the power generation current detected by the current detection unit, calculates the first time by dividing the amount of hydrogen held in the power generation unit, which is determined according to the volume of the power generation unit and the pressure of the anode gas, by the amount of hydrogen consumed per unit time, and calculates the second time by dividing the volume of the anode channel by the amount of permeate water per unit time.

7. In the fuel cell system according to claim 5, A fuel cell system characterized in that, after the elapsed time, the gas control unit sets the anode target pressure so that the anode target pressure is reduced by a third rate, which is smaller than the first rate and larger than the second rate.

8. In the fuel cell system according to any one of claims 1 to 7, A valve device provided in a discharge channel for discharging the anode gas discharged from the fuel cell to the outside, A fuel cell system further comprising: a valve control unit that controls the valve device to discharge the anode gas to the outside when the current detection unit detects a predetermined decrease in the generated current.

9. In the fuel cell system according to claim 8, The fuel cell system is characterized in that the valve control unit calculates the nitrogen partial pressure of the anode gas, sets a threshold for the nitrogen partial pressure according to the power generation current detected by the current detection unit, and opens the valve device when the nitrogen partial pressure exceeds the threshold.

10. In the fuel cell system according to claim 9, It further includes a pressure detection unit that detects atmospheric pressure, The fuel cell system is characterized in that the valve control unit sets the threshold value to a smaller value the smaller the atmospheric pressure detected by the pressure detection unit.

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