Fuel cell system

Abstract

It achieves good power generation performance even when the operating load suddenly decreases. [Solution] The fuel cell system 100 includes a controller that sets the anode target pressure Pat and cathode target pressure Pct according to the output current, and controls the injector and air pump according to the anode target pressure Pat and cathode target pressure Pct. When the controller detects an increase in output current, it performs differential pressure protection control to maintain the differential pressure between the anode target pressure Pat and the cathode target pressure Pct below a predetermined value, provided that the pump limiting condition is met. When the controller detects a decrease in output current, it performs anode pressure adjustment control to make the rate of decrease in anode target pressure Pat smaller than the rate of decrease in cathode target pressure Pct, regardless of whether the pump limiting condition is met.

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). When the operating load suddenly decreases from a high load to a medium load or the like, the system described in Patent Document 1 stops supplying fuel gas to the injector and operates the circulation pump 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, if the supply of fuel gas to the injector is stopped when the operating 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 the cathode gas supply unit according to the anode target pressure and cathode target pressure. When the current detection unit detects an increase in the generated current, the gas control unit performs a first control to maintain the differential pressure between the anode target pressure and the cathode target pressure below a predetermined value, provided that predetermined conditions are met. When the current detection unit detects a decrease in the generated current, the gas control unit performs a second control to make the rate of decrease in the anode target pressure smaller than the rate of decrease in the cathode target pressure, regardless of whether the predetermined conditions are met. [Effects of the Invention]

[0006] According to the present invention, good power generation performance can be obtained even when the operating load decreases sharply. [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 the operation of a fuel cell system according to an embodiment of the present invention when the output current increases. [Figure 4B] A time chart showing an example of operation of a fuel cell system according to an embodiment of the present invention when the output current decreases. [Figure 5]This diagram illustrates an example of determining the rate of decrease in the anode target pressure. [Figure 6] A flowchart showing an example of the process executed by the controller 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 6. 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 Figure 1, the fuel gas supply and discharge section 2 includes a fuel gas tank 21 in which fuel gas (anode gas) is stored, a fuel gas supply channel 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 channel PA22 through which 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 channel PA21. A gas-liquid separator 24 is connected to the fuel gas discharge channel 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 small-diameter nozzle portion 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 in the gas-liquid separator 24. The water separated in the gas-liquid separator 24 is discharged to the outside through the electromagnetic drain valve 25 and the drain flow path PA23. The fuel gas separated in the gas-liquid separator 24 is led to the circulation flow path PA24. The ejector 23 is connected to the circulation flow path PA24, and the purge valve 26 is connected through the purge flow path PA25. The purge valve 26 is an electromagnetic valve device that can be opened and closed, and the fuel gas in the circulation flow path PA24 can be discharged to the outside through the purge flow path PA25 and the purge valve 26.

[0019] Due to the flow of the fuel gas injected from the injector 22, the fuel gas gas-liquid separated in the gas-liquid separator 24 is sucked into the ejector 23 through the circulation flow path PA24. The sucked fuel gas merges with the fuel gas that has passed through the nozzle portion of the ejector 23 at the confluence portion of the ejector 23, and after being made into a uniform flow in the diffuser portion of the ejector 23, it is supplied to the fuel cell stack 1 through the fuel gas inlet 21a.

[0020] The oxidant gas supply / discharge section 3 includes an electric air pump 31 that generates high-pressure oxidant gas (cathode gas), an oxidant gas supply passage PA31 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 passage PA32 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 the air taken in from the atmosphere to generate high-pressure oxidant gas. The air pump 31 may be configured as a compressor.

[0021] In the oxidant gas supply passage PA31 and the oxidant gas discharge passage PA32, a humidifier 32 is disposed so as to intersect these passages PA31 and PA32. In the humidifier 32, humidity exchange is performed between the oxidant gas and the oxidant exhaust gas, and the oxidant gas in the oxidant gas supply passage PA31 is humidified by the moisture (water vapor) contained in the oxidant exhaust gas in the oxidant gas discharge passage PA32.

[0022] The oxidant gas supply / discharge section 3 further has a bypass passage PA33. The bypass passage PA33 is connected to the oxidant gas supply passage PA31 upstream of the humidifier 32 and the oxidant gas discharge passage PA32 downstream of the humidifier 32. Through the bypass passage PA33, the oxidant gas can flow bypassing the humidifier 32 and the fuel cell stack 1.

[0023] An electromagnetic control valve 33 with adjustable opening is provided in the oxidant gas supply passage PA31 between the bypass passage PA33 and the humidifier 32. An electromagnetic control valve 34 with adjustable opening is provided in the oxidant gas discharge passage PA32 between the bypass passage PA33 and the humidifier 32. An electromagnetic control valve 35 with adjustable opening is provided in the bypass passage PA33. By controlling the air pump 31 and the control valves 33 to 35, the supply amount and pressure of the oxidant gas supplied to the fuel cell stack 1 can be adjusted. Also, by controlling the control valves 33 to 35, the bypass amount of the oxidant 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] The power generated by the fuel cell stack 1 is supplied to the drive motor 67 via the power control unit 65. A battery 66, which is a rechargeable and dischargeable energy storage device, is connected to the power control unit 65. The power control unit 65 includes a DC-DC converter that boosts or decompresses the power supplied from the fuel cell stack 1 and the battery 66, and an inverter that converts the DC power into three-phase AC power and supplies it to the drive motor 67.

[0029] 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, a battery sensor 57, pressure sensors 51-53, temperature sensors 54, 55, an injector 22, and an air pump 31.

[0030] 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.

[0031] The battery sensor 57 is installed on the battery 66 and detects the remaining capacity SOC (State of Charge) of the battery 66. The remaining capacity SOC of the battery 66 can also be calculated by a battery ECU (not shown) that monitors, manages, and controls the state of the battery 66, rather than being detected by the battery sensor 57.

[0032] The controller 60 receives signals from the output current detected by the current detection unit 56, the remaining capacity SOC detected by the battery sensor 57, 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 and the air pump 31.

[0033] 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, and a storage unit 63. The storage unit 63 stores various maps, thresholds, programs, etc. The storage unit 63 also stores the volume of the anode flow path PAa and the volume of the gas diffusion layer 106b (Figure 2) that can hold fuel gas.

[0034] The controller 60 calculates the required power generation amount. Specifically, the controller 60 calculates the target drive torque of the drive motor 67 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 67 to generate the target drive torque. Alternatively, the controller 60 calculates the required power generation amount based on the signal from the battery sensor 57 so that the remaining capacity (SOC) of the battery 66 is equal to or greater than a predetermined value.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] The fuel cell system 100 according to this embodiment is characterized by the manner in which the cathode target pressure Pct and anode target pressure Pat are set by the cathode control unit 61 and the anode control unit 62. In particular, it is characterized by setting the target pressures Pat and Pct in different manners when the vehicle is accelerating and decelerating, when the load (output current) on the fuel cell system 100 increases and decreases.

[0039] Figure 4A is a time chart showing an example of the changes over time in anode pressure Pa (dotted line), anode target pressure Pat (solid line), cathode pressure Pc (dotted line), and cathode target pressure Pct (solid line) during rapid acceleration of the vehicle. Although the fuel gas is intermittently injected from the injector 22 by duty cycle control, in Figure 4A, for convenience, the change in anode pressure Pa is shown as a series of triangular pulse waveforms.

[0040] Figure 4A is a time chart for the case where the increase in cathode pressure Pc is limited due to the driving restriction of the air pump 31 (referred to as the pump-limited state). The pump-limited state is a state in which the driving of the air pump 31 is limited when the pump-limited condition is met. The pump-limited condition is met, for example, when the remaining capacity SOC of the battery 66 is below a predetermined value. In this case, the cathode control unit 61 limits the power consumption of the air pump 31. Therefore, even if the vehicle is commanded to accelerate rapidly, the rotational speed of the air pump 31 cannot be rapidly increased, resulting in the pump-limited state.

[0041] The pump limiting condition is also met when surging of the air pump (compressor) 31 is estimated to occur. Surging is more likely to occur when driving at high altitudes or in high humidity conditions, and when surging occurs, it becomes difficult to increase the rotational speed of the air pump 31. The occurrence of surging can be estimated by the cathode control unit 61 based on 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.

[0042] At time t1 (start of acceleration) in Figure 4A, when a command is issued for rapid acceleration of the vehicle, the required power generation increases rapidly. As a result, the anode control unit 62 increases the anode target pressure Pat at an increase rate Pat1 corresponding to the increase in the required power generation. At this time, because the pump is limited, the cathode control unit 61 limits the increase in the cathode target pressure Pct. Therefore, the cathode control unit 61 increases the cathode target pressure Pct at an increase rate Pct2 that is more limited than the increase rate Pct1 (dash-dot line) corresponding to the required power generation. Consequently, the rate of increase of the cathode target pressure Pct immediately after the acceleration command (slope of the increase rate Pct2) is smaller than the rate of increase of the anode target pressure Pat (slope of the increase rate Pat1).

[0043] As a result, the differential pressure ΔP between the anode target pressure Pat and the cathode target pressure Pct gradually increases. 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. Therefore, in this embodiment, when the pump limiting condition is met during vehicle acceleration, the anode control unit 62 performs pressure protection control to keep the differential pressure ΔP below a predetermined value ΔP1. Pressure protection control is a control that protects the membrane electrode assembly 104 by suppressing the increase in the differential pressure ΔP.

[0044] More specifically, at time t2, when the differential pressure ΔP reaches a predetermined value ΔP1, the anode control unit 62 lowers the anode target pressure P, thereby reducing the differential pressure ΔP. Subsequently, at time t3, the anode control unit 62 sets the rate of increase of the anode target pressure Pat to the same value (Pat2) as the rate of increase of the cathode target pressure Pct Pct Pct2. This keeps the differential pressure ΔP below the predetermined value ΔP1.

[0045] The differential pressure protection control shown in Figure 4A is executed when the pump limiting conditions are met and the degree of increase in output current exceeds a predetermined level, that is, when the differential pressure ΔP exceeds a predetermined value ΔP1 at the time of the predetermined increase in output current. The time of the predetermined increase in output current is, for example, when the output current changes from a state where it is less than or equal to a first predetermined value to a state where it is greater than or equal to a second predetermined value, which is greater than the first predetermined value. The time of the predetermined increase in output current may also be considered when the rate of increase in output current is greater than or equal to a predetermined value.

[0046] Figure 4B is a time chart showing an example of the changes over time in anode pressure Pa (dotted line), anode target pressure Pat (solid line), cathode pressure Pc (dotted line), and cathode target pressure Pct (solid line) when a command is issued to rapidly decelerate a vehicle.

[0047] When a sudden deceleration of the vehicle is commanded at time t4 (the start of deceleration), the required power generation decreases rapidly. As a result, the cathode control unit 61 reduces the rotational speed of the air pump 31 in accordance with the decrease in the required power generation. Consequently, as shown in Figure 4B, the cathode target pressure Pct decreases at a rate corresponding to the decrease in output current, and the cathode pressure Pc also decreases accordingly. In this case, the cathode pressure Pc decreases responsively in response to changes in the required power generation.

[0048] On the other hand, the anode target pressure Pat also decreases from the start of deceleration t4, but since the fuel gas circulates through the circulation channel PA24, the degree of decrease in the anode pressure Pa is small, and the responsiveness to the decrease in anode pressure Pa is poor. For this reason, the descent rate Pat4 (slope), which is the rate of decrease of the anode target pressure Pat, is smaller than the descent rate Pct3 (slope), which is the rate of decrease of the cathode target pressure Pct. For reference, Figure 4B shows a descent rate Pat3 (dotted line) with the same slope as the descent rate Pct3.

[0049] In this case, the greater the rate of decrease of the anode target pressure Pat, the later the start time of fuel gas injection by the injector 22, which is determined by the intersection of the anode target pressure Pat and the anode pressure Pa. Specifically, when the slope of the decrease rate is small (Pat4), the injection start time corresponding to the intersection PT1 of the anode target pressure Pat and the anode pressure Pa after the start of deceleration is t5. In contrast, when the slope of the decrease rate is large (Pat3), the injection start time corresponding to the intersection PT2 of the anode target pressure Pat and the anode pressure Pa is t6.

[0050] The water generated in the anode channel PAa can be swept out by fuel gas injection, but if the start of fuel gas injection is delayed, the generated water cannot be swept out, and the entire anode channel PAa may be filled with generated water, potentially adversely affecting power generation. For this reason, the drop rate Pat4 immediately after deceleration needs to be set to a value such that the anode channel PAa is not filled with generated water. Taking this into consideration, in this embodiment, when the vehicle is decelerating, anode pressure adjustment control is performed such that the drop rate Pat4 of the anode target pressure Pat is set to a value smaller than the drop rate Pct3 (Pat3) of the cathode target pressure Pct.

[0051] Here, we define α 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 as the degree of increase in anode pressure Pa due to one injection from injector 22 (pulsation), and Ta as the target time from the start of injection by injector 22 to the start of the next injection. At this time, the rate of decrease Pat4 of the anode target pressure Pat can be calculated by the following equation (I). Pat0 = α - Pax / Ta ···(I)

[0052] 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.

[0053] 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.

[0054] 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.

[0055] When the vehicle is ordered to decelerate, the anode control unit 62 performs anode pressure adjustment control to calculate the drop rate Pat4 using equation (I) above, and sets the anode target pressure Pat in accordance with the drop rate Pat4. This suppresses the adverse effects on power generation caused by the accumulation of generated water in the anode flow path PAa.

[0056] The anode voltage regulation control shown in Figure 4B is executed when the degree of output current reduction exceeds a predetermined level, that is, when the output current drops to a predetermined level. A predetermined output current reduction occurs, for example, when the output current drops from a state where it is below a first predetermined value to a state where it is below a second predetermined value, which is smaller than the first predetermined value. Alternatively, the rate of output current reduction may be considered to be above a predetermined value.

[0057] The above describes an example in which the anode control unit 62 calculates the descent rate Pat4 during deceleration using the above equation (I). However, the descent rate Pat4 can also be determined by referring to a map stored in the memory unit 64 beforehand. Figure 5 shows an example of map M1. Map M1 defines the relationship between the descent rate Pat4 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 descent rate Pat4 (slope), and the smaller the output current and the smaller the anode pressure Pa, the smaller the descent rate Pat4 becomes. Map M1 can be determined beforehand by conducting experiments or analyses. By referring to such a map M1, the anode control unit 62 can easily determine the descent rate Pat4 shown in Figure 4B.

[0058] Figure 6 is a flowchart showing an example of the process performed by the controller 60 in Figure 3. The process shown in this flowchart starts, for example, when the vehicle's power switch is turned on, and is repeated at a predetermined cycle. As shown in Figure 6, first, in step S1, the controller 60 reads signals from the pressure sensors 51-53, temperature sensors 54, 55, current detection unit 56, and battery sensor 57.

[0059] Next, in step S2, the controller 60 determines whether the increase in output current corresponding to the requested power generation amount is above a predetermined level, that is, whether there has been a predetermined increase in output current. If affirmed in step S2, the process proceeds to step S3; if denied, steps S3 to S5 are skipped and the process proceeds to step S6. In step S3, the controller 60 determines whether the pump limiting condition has been met. If affirmed in step S3, the process proceeds to step S5; if denied, the process proceeds to step S4.

[0060] In step S4, the controller 60 sets the anode target pressure Pat and cathode target pressure Pct according to the required power generation amount. Furthermore, it outputs control signals to the injector 22 and air pump 31 so that the anode pressure Pa and cathode pressure Pc become the anode target pressure Pat and cathode target pressure Pct, respectively. This is called normal control. During normal control, the above-mentioned driving restriction of the air pump 31 is not performed, so the differential pressure ΔP does not exceed a predetermined value ΔP1, and differential pressure protection control is unnecessary.

[0061] Meanwhile, in step S5, the controller 60 performs differential pressure protection control. In this case, the controller 60 calculates the differential pressure ΔP between the anode target pressure Pat and the cathode target pressure Pct, and sets the anode target pressure Pat and cathode target pressure Pct according to the required power generation amount, as in normal control, until the differential pressure ΔP becomes equal to or greater than a predetermined value ΔP1. For example, if the anode target rise rate Pat1 is small, the differential pressure ΔP will not become equal to or greater than the predetermined value ΔP1, and in this case, the same processing as in normal control is performed. When the differential pressure ΔP becomes equal to or greater than the predetermined value ΔP1, the controller 60 lowers the anode target pressure Pat and sets the rise rate of the anode target pressure PAt to the same value (Pat2) as the rise rate Pct2 of the cathode target pressure Pct. This keeps the differential pressure ΔP below the predetermined value ΔP1.

[0062] Next, in step S6, the controller 60 determines whether the degree of decrease in output current corresponding to the requested power generation is above a predetermined level, that is, whether a predetermined decrease in output current has occurred. If the result in step S6 is negative, the process proceeds to step S7; if it is positive, the process proceeds to step S8. In step S7, the controller 60 sets the anode target pressure Pat and cathode target pressure Pct according to the requested power generation. Furthermore, it outputs control signals to the injector 22 and air pump 31 so that the anode pressure Pa and cathode pressure Pc become the anode target pressure Pat and cathode target pressure Pct, respectively. In other words, the controller 60 performs normal control similar to that in step S4.

[0063] Meanwhile, in step S8, the controller 60 performs anode pressure regulation control. Specifically, it calculates the anode target pressure Pat drop rate Pat4 using the predetermined calculation formula (I) described above, or by referring to map M1 (Figure 5). Then, it sets the anode target pressure Pat according to the drop rate Pat4. This allows the injector 22 to inject fuel gas at the desired timing without delay, and prevents the anode flow path PAa from being filled with generated water. As for the cathode target pressure Pct, the controller 60 sets the cathode target pressure Pct in the same way as in normal control (step S7).

[0064] The operation of the fuel cell system according to this embodiment can be summarized as follows. When a vehicle acceleration command is issued, if a pump limiting condition is met due to insufficient remaining capacity (SOC), the rise in the cathode target pressure Pct is restricted, and the differential pressure ΔP between the anode target pressure Pat and the cathode target pressure Pct may increase. At this time, if the differential pressure ΔP becomes greater than or equal to a predetermined value ΔP1, the rate of increase (slope) of the anode target pressure Pat is set to the same value (Pat2) as the rate of increase (slope) of the cathode target pressure Pct (step S5). This keeps the differential pressure ΔP below the predetermined value ΔP1, preventing damage to the membrane electrode assembly 104 due to an increase in the differential pressure ΔP.

[0065] When a vehicle deceleration command is issued, regardless of whether the pump limiting conditions are met, the anode target pressure Pat drop rate Pat4 is set based on a predetermined calculation formula (I) or map M1 (step S8). As a result, the anode target pressure Pat approaches the anode pressure Pa (actual pressure), and the injector 22 can inject fuel gas at the desired timing. Consequently, it is possible to suppress the accumulation of generated water in the anode flow path PAa, which would otherwise reduce power generation performance.

[0066] 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 (generated current) of a fuel cell stack 1 (fuel cell) which generates electricity by being supplied with anode gas and 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 a controller 60 that sets 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 controls 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 an increase in output current, the controller 60 performs differential pressure protection control to maintain the differential pressure ΔP between the anode target pressure Pat and the cathode target pressure Pct at or below a predetermined value ΔP1, provided that the pump limiting conditions are met. When the current detection unit 56 detects a decrease in output current, the controller 60 performs anode pressure regulation control to make the rate of decrease in anode target pressure Pat smaller than the rate of decrease in cathode target pressure Pct, regardless of whether the pump limiting conditions are met (Figures 4A, 4B, 6).

[0067] This configuration, while limiting the rise in cathode pressure Pc, suppresses the increase in differential pressure ΔP between the anode target pressure Pat and the cathode target pressure Pct during vehicle acceleration, thereby protecting the membrane electrode assembly 104. Furthermore, it suppresses the discrepancy between the anode pressure Pa and the anode target pressure Pat during vehicle deceleration, allowing the injector 22 to inject fuel gas at a good timing. As a result, it prevents the anode flow path PAa from being filled with generated water, which would reduce power generation performance. In other words, good power generation performance can be obtained even when the operating load is rapidly reduced.

[0068] (2) When the current detection unit 56 detects a decrease in output current, the controller 60 reduces the rate of decrease in the anode target pressure Pat as the output current detected by the current detection unit 56 decreases. When the degree of vehicle deceleration is large, the output current decreases significantly, but by setting the anode target pressure Pat drop rate Pat4 to a small value as the output current decreases, the discrepancy between the anode pressure Pa and the anode target pressure Pat can be suppressed effectively, and the timing of fuel gas injection by the injector 22 can be prevented from being delayed. 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 anode target pressure Pat drop rate Pat4 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.

[0069] (3) When the current detection unit 56 detects a decrease in output current, the controller 60 sets the anode target pressure Pat drop rate Pat4 to a value smaller than the cathode target pressure Pct drop rate Pct3, and sets the anode target pressure Pat based on the drop rate Pat4 (Figure 4B). This suppresses the accumulation of generated water in the anode flow path PAa due to the fuel gas injection delay.

[0070] (4) 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). When the controller 60 detects a decrease in output current by the current detection section 56, it calculates a target time Ta, which is the sum of the hydrogen consumption time Ta1 required to consume anode gas, determined according to the output current and the volume of the power generation section, and the full-water time Ta2 required until the anode channel PAa is filled with generated water, determined according to the output current and the volume of the anode channel PAa, and sets the anode target pressure Pat until the target time Ta has elapsed based on the drop rate Pat4 (Figure 4B). This makes it possible to inject fuel gas from the injector 22 before the anode channel PAa is filled with generated water.

[0071] (5) The controller 60 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 controller 60 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.

[0072] (6) When the current detection unit 56 detects an increase in output current, the controller 60 determines whether it is possible to increase the cathode pressure Pc to the cathode target pressure Pct (increase rate Pct1 in Figure 4A) corresponding to the anode target pressure Pat (increase rate Pat1 in Figure 4A) (Figure 6). The pump limiting condition is met when it is determined that it is not possible to increase the cathode pressure Pc to the cathode target pressure Pct corresponding to the anode target pressure Pat. This allows the pump limiting condition to be set optimally and prevents unnecessary pressure protection control during acceleration.

[0073] (7) The fuel cell system 100 further includes a battery 66 that stores the electricity generated by the fuel cell stack 1 (Figure 1). The pump limiting condition is met when the controller 60 determines that the remaining capacity (SOC) of the battery 66 is less than a predetermined value. If the remaining capacity (SOC) of the battery 66 is insufficient, the rotational speed of the air pump 31 cannot be increased, and in this case, the pump limiting condition is met, enabling effective differential pressure protection control.

[0074] (8) The fuel cell system 100 has an air pump 31 (compressor) (Figure 1). The controller 60 estimates whether or not surging occurs in the air pump 31, and the pump limiting condition is met when the controller 60 estimates that surging will occur. When surging occurs, the rotational speed of the air pump 31 cannot be increased, and in this case, the pump limiting condition is met, which enables effective differential pressure protection control.

[0075] 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.

[0076] 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 it is configured to execute differential pressure protection control (first control) when an increase in the generated current is detected, provided that the pump limiting condition (predetermined condition) is met, and to execute anode pressure adjustment control (second control) when a decrease in the generated current is detected, regardless of whether the pump limiting condition is met or not. The first control can be anything as long as it keeps the differential pressure between the anode target pressure and the cathode target pressure below a predetermined value, and the second control can be anything as long as it makes the rate of decrease in the anode target pressure smaller than the rate of decrease in the cathode target pressure. Therefore, the first control as differential pressure protection control and the second control as anode pressure adjustment control are not limited to those described above.

[0077] In the above embodiment, when a decrease in output current is detected, the anode target pressure Pat drop rate Pat4 (predetermined drop rate) is set. However, the predetermined drop rate to be set is not limited to the one described above, as long as it is set to be smaller than the cathode target pressure Pct drop rate Pct3. In the above embodiment, a target time Ta (predetermined time) is calculated as the sum of the hydrogen consumption time Ta1 (first time) required for anode gas consumption, which is 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, which is determined according to the output current and the volume of the anode flow path PAa. Based on the drop rate Pat4, the anode target pressure Pat is set until the target time Ta has elapsed. However, the method for setting the predetermined time is not limited to the one described above.

[0078] In the above embodiment, the fuel cell system 100 is equipped with a battery 66 as an energy storage unit, but the configuration of the energy storage unit is not limited to that described above. In the above embodiment, it was determined that the pump limiting condition was met when it was determined that the remaining capacity (SOC) of the battery 66 was less than a predetermined value, or when it was estimated that surging was occurring in the air pump 31 (compressor). However, the pump limiting condition (predetermined condition) can be any condition as long as it determines whether or not it is possible to increase the cathode pressure to the cathode target pressure corresponding to the anode target pressure.

[0079] 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.

[0080] 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]

[0081] 1 Fuel cell stack, 22 Injector, 31 Air pump, 51-53 Pressure sensors, 54, 55 Temperature sensors, 57 Battery sensor, 60 Controller, 61 Cathode control unit, 62 Anode control unit, 66 Battery, 100 Fuel cell system, PA21 Fuel gas supply channel, PA24 Circulation channel, PA31 Oxidizer gas supply channel, Rise rate Pat2, Pct2, Fall rate Pat4, Pct3, Ta Target time, Ta1 Hydrogen consumption time, Ta2 Full water time

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. The gas control unit, When the current detection unit detects an increase in the generated current, a first control is executed to maintain the differential pressure between the anode target pressure and the cathode target pressure below a predetermined value, provided that predetermined conditions are met. A fuel cell system characterized in that, when the current detection unit detects a decrease in the generated current, it performs a second control to make the rate of decrease in the anode target pressure smaller than the rate of decrease in the cathode target pressure, regardless of whether the predetermined conditions are met or not.

2. In the fuel cell system according to claim 1, The fuel cell system is characterized in that when the current detection unit detects a decrease in the generated current, the gas control unit reduces the rate of decrease in the anode target pressure as the amount of generated current detected by the current detection unit decreases.

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 decrease in the power generation current, the gas control unit sets the rate of decrease of the anode target pressure to a predetermined rate of decrease that is smaller than the rate of decrease of the cathode target pressure, and sets the anode target pressure based on the predetermined rate of decrease.

4. In the fuel cell system according to claim 3, 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, when the current detection unit detects a decrease in the power generation current, 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 until the predetermined time has elapsed based on the predetermined drop rate.

5. In the fuel cell system according to claim 4, 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.

6. In the fuel cell system according to any one of claims 1 to 5, When the current detection unit detects an increase in the generated current, the gas control unit determines whether it is possible to increase the cathode pressure to the cathode target pressure corresponding to the anode target pressure. The fuel cell system is characterized in that the predetermined condition is met when the gas control unit determines that it is not possible to increase the cathode pressure to the cathode target pressure corresponding to the anode target pressure.

7. In the fuel cell system according to claim 6, The system further comprises a power storage unit for storing electricity generated by the aforementioned fuel cell, The fuel cell system is characterized in that the predetermined condition is met when the gas control unit determines that the charge rate in the energy storage unit is less than a predetermined value.

8. In the fuel cell system according to claim 6, The cathode gas supply unit has a compressor, The gas control unit estimates whether or not surging is occurring in the compressor. The fuel cell system is characterized in that the predetermined conditions are met when the gas control unit estimates that surging will occur.

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