Fuel cell system and method for starting fuel cell system

Abstract

To improve durability of a fuel cell stack by suppressing freezing when removing residual water in the fuel cell stack.SOLUTION: The fuel cell system includes a fuel cell stack, a pressure reducer configured to reduce a pressure of a cathode gas flow path, a temperature acquirer configured to acquire a stack temperature, and a pressure reduction controller configured to control the pressure reducer. The pressure reduction control unit performs at least one of (i) a first control of reducing the pressure of the cathode gas flow path at a first pressure reduction rate that is a pressure reduction rate at which freezing of the cathode gas flow path does not occur when a start-up temperature that is a stack temperature at the time of start-up of the fuel cell system is equal to or lower than a predetermined threshold temperature, and (ii) a second control of reducing the pressure of the cathode gas flow path at a second pressure reduction rate that is a pressure reduction rate at which the stack temperature can be reduced by using heat of vaporization of liquid water present in the cathode gas flow path when the start-up temperature is higher than the threshold temperature.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a fuel cell system and a method for starting up a fuel cell system. [Background technology]

[0002] For example, Patent Document 1 discloses a fuel cell system in which residual water in the cathode gas flow path is removed by connecting the cathode gas flow path to a pressure reduction section that reduces the pressure to a level lower than atmospheric pressure when the fuel cell stack is stopped. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-97993 Summary of the Invention [Problem to be solved by the invention]

[0004] In Patent Document 1, freezing is prevented by removing residual water from the fuel cell stack. However, reducing the pressure to remove the water lowers the temperature of the fuel cell stack and the gas flow paths connected to it, which can cause the residual water to freeze. In addition, Patent Document 1 only considers the removal of residual water, and does not consider improving the durability of the fuel cell stack. [Means for solving the problem]

[0005] (1) According to a first aspect of the present disclosure, there is provided a fuel cell system. The fuel cell system includes a fuel cell stack that generates power through a chemical reaction between an anode gas and a cathode gas, the fuel cell stack having a cathode gas flow path through which the cathode gas or cathode off-gas flows, a pressure reduction unit that reduces the pressure in the cathode gas flow path, a temperature acquisition unit that acquires a stack temperature that indicates the temperature of the fuel cell stack, and a pressure reduction control unit that controls the pressure reduction unit. The pressure reduction control unit executes at least one of: (i) a first control that reduces the pressure in the cathode gas flow path at a first pressure reduction rate that does not cause freezing of the cathode gas flow path when a startup temperature that indicates the stack temperature at startup of the fuel cell system is equal to or lower than a predetermined threshold temperature; and (ii) a second control that reduces the pressure in the cathode gas flow path at a second pressure reduction rate that can reduce the stack temperature by utilizing the heat of vaporization of liquid water present in the cathode gas flow path when the startup temperature is higher than the threshold temperature. This fuel cell system executes at least one of the following: a first control, in which the cathode gas flow channel is depressurized at a first pressure reduction rate that does not cause freezing of the cathode gas flow channel when the stack temperature (start-up temperature) at startup of the fuel cell system is equal to or lower than a predetermined threshold temperature; and a second control, in which the cathode gas flow channel is depressurized at a second pressure reduction rate that can reduce the stack temperature by utilizing the heat of vaporization of liquid water present in the cathode gas flow channel when the startup temperature is higher than the threshold temperature. This makes it possible to achieve at least one of suppressing freezing of the cathode gas flow channel due to residual water in low-temperature environments and improving durability of the fuel cell stack by adjusting the startup temperature to an appropriate temperature by lowering the stack temperature in high-temperature environments. In other words, it is possible to suppress problems associated with removing residual water in the cathode gas flow channel, such as freezing of residual water and reduced durability of the fuel cell stack. (2) In the fuel cell system of the above aspect, the first pressure reduction rate may be 5 kPa / s (kilopascals per second) or less. In the fuel cell system of this aspect, the first pressure reduction rate is 5 kPa / s or less, so that the cathode gas flow channel can be reduced in pressure to a degree that does not cause freezing of the cathode gas flow channel. (3) In the fuel cell system of the above aspect, the second pressure reduction rate may be 500 kPa / s (kilopascals per second) or less. In this fuel cell system, the second pressure reduction rate is 500 kPa / s or less, so the cathode gas flow path can be reduced in pressure to a level that utilizes the heat of vaporization of liquid water in the cathode gas flow path to reduce the stack temperature, and the amount of carbon oxidation caused by the carbon oxidation reaction on the cathode side of the fuel cell stack can be suppressed, thereby improving the durability of the fuel cell stack. (4) The fuel cell system of the above aspect may further include a cooling water circulation channel for adjusting the temperature of the fuel cell stack, and a heat exchanger for exchanging heat between the cooling water circulation channel and the cathode gas channel. According to this aspect of the fuel cell system, the cooling water circulation flow path and the heat exchanger are provided, so that the cooling water in the cooling water flow path can be cooled by utilizing the heat of the cathode gas flow path whose temperature has been reduced by reducing the pressure. (5) According to a second aspect of the present disclosure, there is provided a method for starting a fuel cell system including a fuel cell stack that generates power through a chemical reaction between an anode gas and a cathode gas and that has a cathode gas flow path through which a cathode gas or a cathode off-gas flows. The method for starting a fuel cell system includes the steps of: acquiring a stack temperature, which is the temperature of the fuel cell stack; and controlling a pressure reducing unit that reduces the pressure in the cathode gas flow path. The step of controlling the pressure reducing unit includes the steps of: (i) performing a first control to reduce the pressure in the cathode gas flow path at a first pressure reducing rate that does not cause freezing of the cathode gas flow path when a startup temperature, which is the stack temperature at startup of the fuel cell system, is equal to or lower than a predetermined threshold temperature; and (ii) performing a second control to reduce the pressure in the cathode gas flow path at a second pressure reducing rate that can reduce the stack temperature by utilizing the heat of vaporization of liquid water present in the cathode gas flow path when the startup temperature is higher than the threshold temperature. According to this form of fuel cell system startup method, at least one of the following is executed: when the startup temperature of the fuel cell system is below a predetermined threshold temperature, a first control is executed to depressurize the cathode gas flow path at a first depressurization rate, which is a depressurization rate that does not cause freezing of the cathode gas flow path; and when the startup temperature is higher than the threshold temperature, a second control is executed to depressurize the cathode gas flow path at a second depressurization rate, which is a depressurization rate that can reduce the stack temperature by utilizing the heat of vaporization of liquid water present in the cathode gas flow path.Therefore, it is possible to achieve at least one of suppressing freezing of the cathode gas flow path due to residual water in low-temperature environments, and improving the durability of the fuel cell stack by lowering the stack temperature in high-temperature environments to adjust the startup temperature to an appropriate temperature. [Brief explanation of the drawings]

[0006] [Figure 1] 1 is a block diagram showing a schematic configuration of a fuel cell system; [Figure 2] FIG. 2 is a cross-sectional view showing a schematic configuration of a fuel cell. [Figure 3] 10 is a flowchart showing a procedure for a system shutdown process. [Figure 4] 10 is a flowchart showing a procedure for a system startup process. [Figure 5] FIG. 10 is an explanatory diagram showing an example of experimental results of measuring the amount of carbon oxidation in the cathode catalyst layer and the membrane resistance value of the electrolyte membrane when the pressure in the cathode gas flow channel is changed. [Figure 6] 10 is a flowchart showing the detailed procedure of S13. [Figure 7] FIG. 10 is an explanatory diagram showing an example of experimental results in which the temperature of residual water was measured when the decompression pressure was changed. [Figure 8] FIG. 10 is an explanatory diagram showing an example of experimental results in which changes in freezing time were measured when the decompression rate was changed. [Figure 9] FIG. 10 is a block diagram showing a schematic configuration of a fuel cell system according to a second embodiment. [Figure 10] FIG. 10 is a block diagram showing a schematic configuration of a fuel cell system according to a third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0007] A. First embodiment: A-1. System Configuration: 1 is a block diagram showing a schematic configuration of a fuel cell system 100. In this embodiment, the fuel cell system 100 is mounted on a fuel cell electric vehicle (FCEV) and supplies power to a traction motor. The fuel cell system 100 includes a fuel cell stack 30, a temperature sensor 40, an anode gas supply / discharge system 60, a cathode gas supply / discharge system 70, a load 80, and a control unit 90.

[0008] The fuel cell stack 30 includes a plurality of fuel cell units 10 and a pair of end terminals 21, 22. Each of the plurality of fuel cell units 10 is plate-shaped and stacked in a stacking direction, which is the thickness direction. Each fuel cell unit 10 is a power-generating element that can generate power on its own. In this embodiment, the fuel cell unit 10 is configured as a polymer electrolyte fuel cell. The fuel cell unit 10 receives anode gas and cathode gas as reactant gases and generates power through an electrochemical reaction between them. For example, the anode gas is hydrogen and the cathode gas is air. The anode gas is also called fuel gas, and the cathode gas is also called oxidant gas.

[0009] 2 is a cross-sectional view showing a schematic configuration of a fuel cell 10. The fuel cell 10 includes a membrane electrode assembly (MEA) 11, an anode diffusion layer 15, a cathode diffusion layer 16, an anode separator 17, and a cathode separator 18.

[0010] The membrane electrode assembly 11 includes an electrolyte membrane 12, an anode catalyst layer 13, and a cathode catalyst layer 14. The electrolyte membrane 12 is a proton-conductive ion-exchange membrane formed from a solid polymer material, such as a fluororesin containing perfluorocarbon sulfonic acid, and exhibits good electrical conductivity in a wet state. The anode catalyst layer 13 and the cathode catalyst layer 14 are formed by coating conductive particles, such as carbon particles, carrying a catalyst such as platinum or a platinum alloy with a proton-conductive polymer electrolyte. The anode catalyst layer 13 is formed on one side of the electrolyte membrane 12, and the cathode catalyst layer 14 is formed on the other side of the electrolyte membrane 12.

[0011] The anode diffusion layer 15 and the cathode diffusion layer 16 are made of gas-permeable and conductive materials such as carbon cloth or carbon paper. The anode diffusion layer 15 is formed on the surface of the anode catalyst layer 13 opposite to the surface that contacts the electrolyte membrane 12. The cathode diffusion layer 16 is formed on the surface of the cathode catalyst layer 14 opposite to the surface that contacts the electrolyte membrane 12.

[0012] The anode separator 17 and the cathode separator 18 are made of gas-blocking and electrically conductive materials, such as dense carbon made by compressing carbon particles to make them gas-impermeable, or press-formed metal materials such as stainless steel or titanium steel. The anode separator 17 is disposed on the surface of the anode diffusion layer 15 opposite to the surface that contacts the anode catalyst layer 13. The anode separator 17 includes an anode-side flow path 27 through which anode gas flows. The cathode separator 18 is disposed on the surface of the cathode diffusion layer 16 opposite to the surface that contacts the cathode catalyst layer 14. The cathode separator 18 includes a cathode-side flow path 28 through which cathode gas flows.

[0013] 1 are arranged at both ends in the stacking direction of the fuel cell stack 30. Specifically, the first end terminal 21 is arranged at one end of the fuel cell stack 30, and the second end terminal 22 is arranged at the other end of the fuel cell stack 30. The first end terminal 21 has openings that connect to a cathode gas supply flow path 71 and a cathode gas discharge flow path 74, which will be described later. The second end terminal 22 has openings that connect to an anode gas supply flow path 62 and an anode gas discharge flow path 64, which will be described later.

[0014] The temperature sensor 40 measures the temperature of the fuel cell stack 30 (hereinafter also referred to as "stack temperature"). In the example shown in Fig. 1, the temperature sensor 40 is connected to the second end terminal 22, but the temperature sensor 40 may be disposed in any location so as to be able to measure the temperature of the fuel cell stack 30. The temperature sensor 40 is able to transmit the measured temperature to the control unit 90.

[0015] The anode gas supply / discharge system 60 supplies anode gas to the fuel cell stack 30 and discharges anode off-gas from the fuel cell stack 30. The anode off-gas contains anode gas that was not used in the power generation of the fuel cell stack 30, moisture, and the like. The anode gas supply / discharge system 60 includes an anode gas tank 61, an anode gas supply flow path 62, an anode gas inlet valve 63, an anode gas discharge flow path 64, and an anode gas outlet valve 65.

[0016] The anode gas tank 61 is a container that stores the anode gas to be supplied to the fuel cell stack 30. The anode gas tank 61 stores, for example, high-pressure hydrogen gas.

[0017] The anode gas supply flow path 62 is a flow path that guides the anode gas from the anode gas tank 61 to the fuel cell stack 30. One end of the anode gas supply flow path 62 is connected to the anode gas tank 61, and the other end of the anode gas supply flow path 62 is connected to an opening of the second end terminal 22.

[0018] The anode gas inlet valve 63 adjusts the amount of anode gas supplied from the anode gas tank 61. The anode gas inlet valve 63 is provided in the anode gas supply flow path 62. In addition to the anode gas inlet valve 63, the anode gas supply flow path 62 is also provided with a pressure adjustment valve and an injector (not shown).

[0019] The anode gas discharge flow path 64 is a flow path that discharges anode off-gas from the fuel cell stack 30. One end of the anode gas discharge flow path 64 is connected to an opening of the second end terminal 22. The other end of the anode gas discharge flow path 64 is open, and the anode off-gas is discharged through this opening.

[0020] The anode gas outlet valve 65 opens and closes the anode gas discharge flow path 64. The anode gas outlet valve 65 is provided in the anode gas discharge flow path 64. The anode gas supply flow path 62 and the anode gas discharge flow path 64 may be connected to each other, and the anode off-gas may be supplied to the fuel cell stack 30 again via the anode gas supply flow path 62.

[0021] The cathode gas supply / discharge system 70 supplies cathode gas to the fuel cell stack 30 and discharges cathode off-gas from the fuel cell stack 30. The cathode off-gas contains cathode gas not consumed in the power generation of the fuel cell stack 30 and moisture and the like generated in the power generation of the fuel cell stack 30. The cathode gas supply / discharge system 70 includes a cathode gas supply passage 71, an air pump 72, a cathode gas inlet valve 73, a cathode gas discharge passage 74, a cathode gas outlet valve 75, a cathode gas discharge branch passage 76, a cathode gas branch outlet valve 77, and a pressure reducing unit 50.

[0022] The cathode gas supply flow path 71 is a flow path that introduces cathode gas to the fuel cell stack 30. One end of the cathode gas supply flow path 71 is open. Cathode gas can be taken in through this opening. The other end of the cathode gas supply flow path 71 is connected to an opening in the first end terminal 21. An air pump 72 and a cathode gas inlet valve 73 are provided in the cathode gas supply flow path 71. A filter may be provided upstream of the air pump 72 in the cathode gas supply flow path 71.

[0023] The air pump 72 compresses the cathode gas and supplies the cathode gas to the fuel cell stack 30 via the cathode gas supply passage 71 .

[0024] The cathode gas inlet valve 73 adjusts the amount of cathode gas supplied from the air pump 72 .

[0025] The cathode gas discharge flow path 74 is a flow path that discharges cathode off-gas from the fuel cell stack 30. One end of the cathode gas discharge flow path 74 is connected to an opening of the first end terminal 21. The other end of the cathode gas discharge flow path 74 is open, and the cathode off-gas is discharged through this opening.

[0026] The cathode gas outlet valve 75 opens and closes the cathode gas discharge passage 74. The cathode gas outlet valve 75 is provided in the cathode gas discharge passage 74.

[0027] The cathode gas discharge branch channel 76 is a channel branching from the cathode gas discharge channel 74 between the fuel cell stack 30 and the cathode gas outlet valve 75. The end of the cathode gas discharge branch channel 76 is open.

[0028] The cathode gas branch outlet valve 77 opens and closes the cathode gas discharge branch channel 76. The cathode gas branch outlet valve 77 is provided in the cathode gas discharge branch channel 76.

[0029] The pressure reducing unit 50 includes a vacuum pump 78 and a throttle unit AP. The pressure reducing unit 50 is provided in the cathode gas discharge branch passage 76 and is configured to reduce the pressure in the passage through which the cathode off-gas flows (hereinafter also referred to as the "cathode gas passage"). In this embodiment, the "cathode gas passage" includes the cathode-side passage 28 in the fuel cell stack 30, the cathode gas discharge passage 74, and the cathode gas discharge branch passage 76. The throttle unit AP is provided in the cathode gas discharge branch passage 76 between the cathode gas branch outlet valve 77 and the vacuum pump 78. The throttle unit AP is configured to finely adjust the opening area of ​​the cathode gas discharge branch passage 76. Therefore, fine adjustment of the throttling amount of the throttle unit AP when the vacuum pump 78 is operated results in fine adjustment of the pressure reducing speed of the cathode gas discharge passage 74, etc.

[0030] The pressure measuring unit 79 measures the pressure in the cathode gas flow path. The pressure measuring unit 79 is provided in the cathode gas discharge flow path 74 between the fuel cell stack 30 and the cathode gas outlet valve 75.

[0031] The load 80 is connected to the fuel cell stack 30 via an electric circuit (not shown) that includes a DC / DC converter. The load 80 is, for example, a traction motor of a vehicle equipped with the fuel cell system 100. The electric circuit is provided with a voltage measurement unit 81 and a current measurement unit 82. The voltage measurement unit 81 and the current measurement unit 82 are capable of measuring the output voltage and output current of the fuel cell stack 30 and transmitting the measurement values ​​to the control unit 90.

[0032] The control unit 90 is configured as a computer including a CPU 91 and a memory 92. The control unit 90 is electrically connected to and controls the above-mentioned valves 63, 65, 73, 75, 77, air pump 72, vacuum pump 78, and throttle unit AP. The CPU 91 reads and executes control programs pre-stored in the memory 92, thereby functioning as a stop control unit 93, a start control unit 94, a temperature acquisition unit 95, and a pressure reduction control unit 96.

[0033] The stop control unit 93 controls the execution of predetermined processing (system stop processing, which will be described later) when stopping the fuel cell system 100. The start control unit 94 controls the execution of predetermined processing (system start processing, which will be described later) when starting up the fuel cell system 100.

[0034] The temperature acquisition unit 95 acquires the stack temperature by receiving the detection result of the temperature sensor 40. The pressure reduction control unit 96 controls the pressure reduction unit 50. Specifically, it controls the operation of the vacuum pump 78 and the throttling amount of the throttling unit AP. In this embodiment, the pressure reduction control unit 96 selectively performs either control (hereinafter referred to as "first control") to reduce the pressure in the cathode gas flow path at a pressure reduction rate (hereinafter referred to as "first pressure reduction rate") that does not cause freezing of the cathode flow path, or control (hereinafter referred to as "second control") to reduce the pressure in the cathode gas flow path at a pressure reduction rate (hereinafter referred to as "second pressure reduction rate") that can reduce the stack temperature by utilizing the heat of vaporization of liquid water present in the cathode gas flow path. Details of the first control and the second control will be described later.

[0035] In the fuel cell system 100 having the above configuration, liquid water may remain in the cathode gas flow path after the system is shut down, for example, when the fuel cell vehicle is stopped. Examples of such liquid water include water generated on the cathode side due to electrochemical reactions in each fuel cell 10 and liquid water generated when water vapor present in the cathode gas flow path liquefies. Such residual water may lead to flooding or blockage of the gas flow path due to freezing. Therefore, the fuel cell system 100 executes a system startup process described below to reduce the pressure in the cathode gas flow path and evaporate and discharge the residual water. Furthermore, the fuel cell system 100 can improve the durability of the fuel cell 10 by executing a system startup process described below. Below, the system shutdown process executed when the fuel cell system 100 is shut down is described, followed by the system startup process.

[0036] A-2. System shutdown procedure: 3 is a flowchart showing the procedure for the system shutdown process. When a power generation stop signal is input to the control unit 90 from a control unit of a vehicle (not shown) while the fuel cell system 100 is in operation, the system shutdown process is executed.

[0037] In step S1, the stop control unit 93 stops the operation of the air pump 72 to stop the supply of cathode gas to the fuel cell stack 30. Hereinafter, "step S" will be simply referred to as "S".

[0038] In S2, the stop control unit 93 closes the cathode gas inlet valve 73 and the anode gas inlet valve 63. In S3, the stop control unit 93 closes the cathode gas outlet valve 75, the cathode gas branch outlet valve 77, and the anode gas outlet valve 65. Note that the cathode gas branch outlet valve 77 is in a closed state while the fuel cell system 100 is in operation, and therefore this state is maintained in S3.

[0039] After the completion of S3, the cathode gas flow path is sealed, and the anode gas flow path is also sealed.

[0040] A-3. System startup process: 4 is a flowchart showing the procedure of the system startup process. When the fuel cell system 100 is in a stopped state and a power generation start signal is input to the control unit 90 from a control unit of the vehicle (not shown), the system startup process is executed.

[0041] In S11, the temperature acquisition unit 95 acquires the stack temperature. The stack temperature acquired in S11 is the stack temperature at the time of startup of the fuel cell system 100, and is also called the "start-up temperature."

[0042] In S12, the pressure reduction control unit 96 calculates a pressure reduction target pressure Ptg for the cathode gas flow channel. The pressure reduction target pressure Ptg refers to a target pressure when reducing the pressure in the cathode gas flow channel, which will be described later. In this embodiment, the pressure reduction control unit 96 calculates 50% of the saturated water vapor pressure at the current stack temperature as the pressure reduction target pressure Ptg. For example, the saturated water vapor pressure when the stack temperature is 60°C is approximately 20 kPa. Therefore, if the stack temperature acquired in S11 is 60°C, the pressure reduction control unit 96 sets the pressure reduction target pressure Ptg to approximately 10 kPa, which is 50% of the saturated water vapor pressure when the stack temperature is 60°C. Furthermore, for example, if the stack temperature acquired in S11 is 25°C, the pressure reduction control unit 96 sets the pressure reduction target pressure Ptg to approximately 1.3 kPa, which is 50% of the saturated water vapor pressure when the stack temperature is 25°C.

[0043] FIG. 5 is an explanatory diagram showing an example of the experimental results of measuring the amount of carbon oxidation in the cathode catalyst layer 14 and the membrane resistance of the electrolyte membrane 12 when the pressure in the cathode gas flow channel is changed. In FIG. 5, the horizontal axis represents the pressure (kPa) in the cathode gas flow channel. The vertical axis on the right represents the amount of carbon oxidation (mg), and the vertical axis on the left represents the membrane resistance (mΩ cm 2 5 shows the experimental results when the stack temperature is 60°C. The thick solid curve La indicates the change in the amount of carbon oxidation, and the thin dashed curve Lb indicates the membrane resistance of the electrolyte membrane 12.

[0044] 5, when the pressure in the cathode gas flow channel is reduced and the pressure in the cathode gas flow channel becomes less than approximately 20 kPa, which is the saturated water vapor pressure at 60°C, the amount of carbon oxidation decreases. This is thought to be because as the pressure in the cathode gas flow channel decreases, liquid water in the fuel cell 10 evaporates (vaporizes) and is removed from the cathode catalyst layer 14, suppressing the carbon oxidation reaction shown in formula (1) below. C+2H2O→CO2+4H + +4e ― ···(1)

[0045] When the pressure in the cathode gas flow channel reaches 50% of the saturated water vapor pressure, the amount of carbon oxidation decreases significantly compared to when the pressure in the cathode gas flow channel is at the saturated water vapor pressure. The membrane resistance of the electrolyte membrane 12 increases as the liquid water in the fuel cell 10 decreases due to the reduced pressure in the cathode gas flow channel. When the pressure in the cathode gas flow channel reaches 25% of the saturated water vapor pressure, the membrane resistance of the electrolyte membrane 12 increases significantly compared to when the pressure in the cathode gas flow channel is at the saturated water vapor pressure. An increase in membrane resistance is undesirable because it causes a voltage drop during startup of the fuel cell 10. Therefore, in this embodiment, the reduced pressure target pressure Ptg is set to "50% of the saturated water vapor pressure," which is a pressure that can suppress the amount of carbon oxidation while also suppressing a voltage drop during startup.

[0046] In S13 shown in Fig. 4, the pressure reduction control unit 96 calculates the pressure reduction speed Vp. Fig. 6 is a flowchart showing the detailed procedure of S13. In S105, the pressure reduction control unit 96 determines whether the stack temperature acquired in S11 is equal to or lower than a predetermined threshold temperature. In this embodiment, the threshold temperature in S105 is set to 50°C.

[0047] If it is determined that the stack temperature is equal to or lower than the threshold temperature (S105: YES), the pressure reduction control unit 96 sets a pressure reduction rate (first pressure reduction rate) that does not cause freezing as the pressure reduction rate Vp (S110). On the other hand, if it is determined that the stack temperature is not equal to or lower than the threshold temperature (S105: NO), the pressure reduction control unit 96 sets a rate (second pressure reduction rate) that allows the stack temperature to decrease due to the heat of vaporization of the remaining water as the pressure reduction rate Vp (S115).

[0048] Figure 7 is an explanatory diagram showing an example of experimental results in which the temperature of the residual water was measured when the decompression pressure was changed. Figure 7 shows the results of an experiment in which a pedestal was placed in a vacuum chamber, and the temperature of the pedestal and the temperature of the water were measured when the pressure was reduced with water on the pedestal. Specifically, the dashed curve L1 represents the pressure inside the vacuum chamber, the thin solid curve L2 represents the temperature of the pedestal, and the thick solid curve L3 represents the temperature of the water on the pedestal. In Figure 7, the horizontal axis represents time. The right vertical axis represents temperature, and the left vertical axis represents pressure inside the vacuum chamber.

[0049] The pressure, which was approximately 100 kPa, decreases as depressurization begins at time t0, reaching approximately 0 kPa at time t1. The slope of curve L1 at this time corresponds to the depressurization rate. The temperature of the pedestal and the temperature of the water on the pedestal were also approximately 40°C at time t0, but they decrease as the vacuum vessel is depressurized, remaining in a supercooled state below 0°C. At time t2, the heat of solidification is released, the temperature rises to 0°C, and the water freezes. Therefore, in this embodiment, the period from time t0 to time t2 is defined as the freezing time Tf. This freezing time Tf can be evaluated as the time from the start of depressurization until the remaining water freezes.

[0050] FIG. 8 is an explanatory diagram showing an example of experimental results measuring the change in freezing time Tf when the depressurization rate is changed. In FIG. 8, the horizontal axis represents the depressurization rate of the vacuum vessel, and the vertical axis represents the freezing time Tf. The present inventors performed the experiment described with reference to FIG. 7 by varying the depressurization rate of the vacuum vessel and measuring the freezing time Tf at each rate. As a result, the present inventors found that the freezing time Tf can vary depending on the depressurization rate. As shown in FIG. 8, when the depressurization rate is approximately 8 kPa / s (kilopascals per second) or higher, the freezing time Tf was within 1 minute. In contrast, when the depressurization rate is 5 kPa / s or lower, the freezing time Tf was 100 minutes or longer. If the freezing time Tf is 100 minutes or longer, the remaining water will not freeze even if the depressurization is continued for, for example, 1 minute to several minutes in the procedure described below, during which the cathode gas flow channel is depressurized to vaporize and exhaust the remaining water. Therefore, in the above-mentioned S110, a rate of 5 kPa / s or lower is set as the first depressurization rate.

[0051] The present inventors conducted an experiment to measure the change in the amount of carbon oxidation in the cathode catalyst layer 14 when the stack temperature was changed, and found that the amount of carbon oxidation increases as the stack temperature increased. Therefore, if the stack temperature is high during startup of the fuel cell system 100, for example, when the fuel cell system 100 is started up to start the vehicle immediately after a temporary stop, the amount of carbon oxidation may increase. Therefore, when the stack temperature is close to the normal operating temperature range of 50 to 80°C, step S115 is executed to reduce the pressure in the cathode gas flow channel and set a pressure reduction rate that can reduce the stack temperature by the heat of vaporization of the residual water as the second pressure reduction rate. The present inventors found that a second pressure reduction rate of 500 kPa / s or less can vaporize the residual water in the cathode gas flow channel and reduce the stack temperature, and that this pressure reduction rate does not cause freezing of water in the membrane electrode assembly 11, which evaporates relatively slowly. Therefore, in this embodiment, the second depressurization rate in S115 is set to a depressurization rate of 500 kPa / s or less.

[0052] 4, the pressure reduction control unit 96 sets the throttling amount of the throttling unit AP to achieve the pressure reduction speed set in S13 (S14). For example, the relationship between the throttling amount of the throttling unit AP and the pressure reduction speed may be identified and mapped in advance through experiments or simulations, and the throttling amount of the throttling unit AP corresponding to the pressure reduction speed may be identified and set by referring to the map. In S15, the pressure reduction control unit 96 starts the vacuum pump 78. In S16, the start-up control unit 94 opens the cathode gas branch outlet valve 77.

[0053] The start-up control unit 94 determines whether the pressure P in the cathode gas flow channel measured by the pressure measurement unit 79 is lower than the pressure reduction target pressure Ptg calculated in S12 (S17). The start-up control unit 94 waits until it is determined that the measured pressure in the cathode gas flow channel is lower than the pressure reduction target pressure Ptg, and if it is determined that the measured pressure in the cathode gas flow channel is lower than the pressure reduction target pressure Ptg (S17: YES), it closes the cathode gas branch outlet valve 77 (S18). Therefore, the pressure reduction control unit 96 selectively performs control to reduce the pressure in the cathode gas flow channel at a first pressure reduction rate (first control) when the stack temperature is equal to or lower than the threshold temperature, and control to reduce the pressure in the cathode gas flow channel at a second pressure reduction rate (second control) when the stack temperature is higher than the threshold temperature, depending on the stack temperature.

[0054] In S19, the pressure reduction control unit 96 stops the vacuum pump 78. In S20, the startup control unit 94 starts the fuel cell stack 30. Specifically, the supply of anode gas from the anode gas tank 61 is started, and then the air pump 72 is started to start the supply of cathode gas, and the supply of power to the load 80 is started.

[0055] According to the fuel cell system 100 of the first embodiment described above, when the stack temperature (start-up temperature) at the time of startup of the fuel cell system 100 is equal to or lower than a predetermined threshold temperature, a first control is performed to depressurize the cathode gas flow path at a first depressurization rate, which is a depressurization rate that does not cause freezing of the cathode gas flow path, and when the startup temperature is higher than the threshold temperature, a second control is performed to depressurize the cathode gas flow path at a second depressurization rate, which is a depressurization rate that can reduce the stack temperature by utilizing the heat of vaporization of liquid water present in the cathode gas flow path.These controls are selectively performed depending on the stack temperature.Therefore, in low-temperature environments, it is possible to suppress freezing of the cathode gas flow path due to residual water, and in high-temperature environments, it is possible to adjust the startup temperature to an appropriate temperature by lowering the stack temperature, thereby improving the durability of the fuel cell stack.

[0056] Furthermore, since the first pressure reduction rate is 5 kPa / s (kilopascals per second) or less, the pressure in the cathode gas flow channel can be reduced to a degree that does not cause freezing of the cathode gas flow channel.

[0057] Furthermore, since the second pressure reduction rate is 500 kPa / sec or less, the cathode gas flow path can be reduced in pressure to a degree that allows the stack temperature to be reduced by utilizing the heat of vaporization of liquid water present in the cathode gas flow path, and the amount of carbon oxidation caused by the carbon oxidation reaction on the cathode side of the fuel cell stack 30 can be suppressed, thereby improving the durability of the fuel cell stack 30.

[0058] B. Second embodiment: 9 is a block diagram showing a schematic configuration of a fuel cell system 100a of the second embodiment. The fuel cell system 100a of the second embodiment differs from the fuel cell system 100 of the first embodiment shown in FIG. 1 in that it includes a cathode gas supply branch passage 71a and a cathode gas branch inlet valve 73a. The other configuration of the fuel cell system 100a of the second embodiment is the same as that of the fuel cell system 100, so the same components are given the same reference numerals and detailed descriptions thereof will be omitted. Furthermore, the system shutdown process of the second embodiment is the same as that of the first embodiment, so detailed descriptions thereof will be omitted.

[0059] The cathode gas supply branch channel 71a branches off from the cathode gas supply channel 71 between the cathode gas inlet valve 73 and the fuel cell stack 30, and leads to the cathode gas discharge branch channel 76. One end of the cathode gas supply branch channel 71a is located between the cathode gas inlet valve 73 and the fuel cell stack 30 in the cathode gas supply channel 71, and the other end is located between the cathode gas branch outlet valve 77 and the throttle portion AP in the cathode gas discharge branch channel 76.

[0060] The cathode gas branch inlet valve 73a opens and closes the cathode gas supply branch passage 71a. The cathode gas branch inlet valve 73a is provided in the cathode gas supply branch passage 71a. The cathode gas branch inlet valve 73a is not opened except during system startup processing. The cathode gas branch inlet valve 73a is electrically connected to the control unit 90 and can be controlled by the startup control unit 94 and pressure reduction control unit 96.

[0061] The following describes the differences between the system startup process of the second embodiment and that of the first embodiment. In S16 of the system startup process, the startup control unit 94 opens the cathode gas branch inlet valve 73a as well as the cathode gas branch outlet valve 77. Therefore, in the second embodiment, the "cathode gas flow path" to be depressurized includes the cathode side flow path 28, the cathode gas discharge flow path 74, and the cathode gas discharge branch flow path 76 in the fuel cell stack 30, as well as the cathode gas supply branch flow path 71a and the cathode gas supply flow path 71 that communicates with the cathode gas supply branch flow path 71a.

[0062] In S18, the startup control unit 94 closes the cathode gas branch outlet valve 77 and also closes the cathode gas branch inlet valve 73a.

[0063] The fuel cell system 100a of the second embodiment described above has the same effects as the fuel cell system 100 of the first embodiment. In addition, because the cathode gas supply flow path 71 is depressurized as part of the cathode gas flow path, evaporation of residual water in the cathode-side flow path 28 and each fuel cell 10 can be promoted from the cathode gas supply side as well. This allows residual water to be discharged more efficiently.

[0064] C. Third embodiment: Fig. 10 is a block diagram showing a schematic configuration of a fuel cell system 100b of the third embodiment. The fuel cell system 100b of the third embodiment differs from the fuel cell system 100 of the first embodiment shown in Fig. 1 in that it includes a cooling water circulation system 55. Since the other configurations of the fuel cell system 100b of the third embodiment are the same as those of the fuel cell system 100, the same components are given the same reference numerals and detailed descriptions thereof will be omitted. Furthermore, the system shutdown processing and system startup processing of the third embodiment are the same as those of the first embodiment, and detailed descriptions thereof will be omitted.

[0065] The cooling water circulation system 55 cools the fuel cell stack 30. The cooling water circulation system 55 includes a cooling water flow path 56, a cooling water pump 57, and a heat exchanger 58.

[0066] The cooling water flow path 56 is composed of a cooling water pipe through which cooling water flows. One end of the cooling water flow path 56 is connected to the fuel cell stack 30 at the first end terminal 21, and the other end is connected to the fuel cell stack 30 at the second end terminal 22. A cooling water distribution manifold (not shown) is formed within the fuel cell stack 30, through which each fuel cell 10 passes. The cooling water flow path 56 communicates with this cooling water distribution manifold, thereby forming a cooling water circulation system 55. Note that any type of cooling medium, such as air or ethylene glycol, may be used instead of cooling water.

[0067] The cooling water pump 57 forms a flow of cooling water in the cooling water passage 56. The heat exchanger 58 is connected to the cooling water passage 56 and the cathode gas discharge passage 74. Specifically, the heat exchanger 58 is connected to the cooling water passage 56 between the cooling water pump 57 and the fuel cell stack 30. The heat exchanger 58 is also connected to the cathode gas discharge passage 74 between the fuel cell stack 30 and a cathode gas outlet valve 75. The heat exchanger 58 exchanges heat between the cathode off-gas flowing through the cathode gas discharge passage 74 and the cooling water flowing through the cooling water passage 56.

[0068] As described above, when the vacuum pump 78 is started in S15 of the system startup process and the cathode gas branch outlet valve 77 is opened in S16, the cathode gas flow path is depressurized. At this time, the heat of vaporization of the residual water cools the air in the cathode gas flow path and the piping that forms the cathode gas flow path. Therefore, the cooling water in the cooling water flow path 56 is also cooled via the heat exchanger 58, making it possible to cool the fuel cell stack 30.

[0069] The fuel cell system 100b of the third embodiment described above has the same effects as the fuel cell system 100 of the first embodiment. In addition, since the fuel cell system 100b is provided with a cooling water circulation flow path and a heat exchanger 58, the cooling water in the cooling water flow path 56 can be cooled by utilizing the heat of the cathode gas flow path whose temperature has been reduced by reducing the pressure.

[0070] D. Other Embodiments: (D1) In each embodiment, the pressure reduction control unit 96 selectively performs control to reduce the pressure in the cathode gas flow channel at a first pressure reduction rate when the stack temperature is equal to or lower than a threshold temperature (first control) and control to reduce the pressure in the cathode gas flow channel at a second pressure reduction rate when the stack temperature is higher than the threshold temperature (second control) depending on the stack temperature, but the present disclosure is not limited to this. The pressure reduction control unit 96 may be configured to perform only the first control or only the second control.

[0071] (D2) In each embodiment, the first depressurization rate is 5 kPa / s or less and the second depressurization rate is 500 kPa / s or less, but the present disclosure is not limited to this. Only one of the first depressurization rate being 5 kPa / s or less and the second depressurization rate being 500 kPa / s or less may be realized.

[0072] (D3) In each embodiment, the fuel cell systems 100, 100a, and 100b are mounted on a fuel cell vehicle, but the present disclosure is not limited to this. They may be mounted on any type of moving object other than an automobile, such as a ship or an aircraft. Furthermore, they may be fixedly installed on a building or the like without being mounted on a moving object.

[0073] The present disclosure is not limited to the above-described embodiments and can be realized in various configurations without departing from the spirit thereof. For example, the technical features of each embodiment corresponding to the technical features in each aspect described in the Summary of the Invention section can be appropriately replaced or combined to solve some or all of the above-described problems or achieve some or all of the above-described effects. Furthermore, if a technical feature is not described as essential in this specification, it can be appropriately deleted. [Explanation of symbols]

[0074] 10... fuel cell, 11... membrane electrode assembly, 12... electrolyte membrane, 13... anode catalyst layer, 14... cathode catalyst layer, 15... anode diffusion layer, 16... cathode diffusion layer, 17... anode separator, 18... cathode separator, 21... first end terminal, 22... second end terminal, 27... anode side flow path, 28... cathode side flow path, 30... fuel cell stack, 40... temperature sensor, 50... pressure reduction section, 55... cooling water circulation system, 56... cooling water flow path, 57... cooling water pump, 58... heat exchanger, 60... anode gas supply / discharge system, 61... anode gas tank, 62... anode gas supply flow path, 63... anode gas inlet valve, 64... anode gas discharge flow path, 65... anode gas outlet valve, 70... cathode gas supply / discharge system, 71 ...cathode gas supply flow path, 71a...cathode gas supply branch flow path, 72...air pump, 73...cathode gas inlet valve, 73a...cathode gas branch inlet valve, 74...cathode gas exhaust flow path, 75...cathode gas outlet valve, 76...cathode gas exhaust branch flow path, 77...cathode gas branch outlet valve, 78...vacuum pump, 79...pressure measurement unit, 80...load, 81...voltage measurement unit, 82...current measurement unit, 90...control unit, 91...CPU, 92...memory, 93...stop control unit, 94...start control unit, 95...temperature acquisition unit, 96...pressure reduction control unit, 100, 100a, 100b...fuel cell system, AP...throttle unit, L1 to L3, La, Lb...curve, Ptg...pressure reduction target pressure, Tf...freezing time, Vp...pressure reduction rate, t0 to t2...time

Claims

1. 1. A fuel cell system, comprising: a fuel cell stack that generates electricity by a chemical reaction between an anode gas and a cathode gas, the fuel cell stack having a cathode gas flow path through which the cathode gas or cathode off-gas flows; a pressure reducing unit that reduces the pressure in the cathode gas flow channel; a temperature acquisition unit that acquires a stack temperature, which is the temperature of the fuel cell stack; a pressure reduction control unit that controls the pressure reduction unit; Equipped with The pressure reduction control unit (i) a first control for depressurizing the cathode gas flow path at a first depressurization rate that is a depressurization rate at which freezing of the cathode gas flow path does not occur when a startup temperature, which is the stack temperature at startup of the fuel cell system, is equal to or lower than a predetermined threshold temperature; (ii) a second control for depressurizing the cathode gas flow channel at a second depressurization rate that is a depressurization rate capable of reducing the stack temperature by utilizing heat of vaporization of liquid water present in the cathode gas flow channel when the startup temperature is higher than the threshold temperature; and Execute at least one of the following: Fuel cell system.

2. 2. The fuel cell system according to claim 1, A fuel cell system, wherein the first pressure reduction rate is 5 kPa / s (kilopascals per second) or less.

3. 2. The fuel cell system according to claim 1, A fuel cell system, wherein the second pressure reduction rate is 500 kPa / s (kilopascals per second) or less.

4. 4. The fuel cell system according to claim 1, a cooling water circulation channel for adjusting the temperature of the fuel cell stack; a heat exchanger that exchanges heat between the cooling water circulation channel and the cathode gas channel; The fuel cell system further comprises:

5. A method for starting a fuel cell system including a fuel cell stack that generates electricity by a chemical reaction between an anode gas and a cathode gas and has a cathode gas flow path formed therein through which the cathode gas or cathode off-gas flows, comprising: acquiring a stack temperature, which is the temperature of the fuel cell stack; controlling a pressure reducing unit that reduces the pressure in the cathode gas flow channel; Equipped with The step of controlling the pressure reducing unit includes: (i) a first control for depressurizing the cathode gas flow path at a first depressurization rate that is a depressurization rate at which freezing of the cathode gas flow path does not occur when a startup temperature, which is the stack temperature at startup of the fuel cell system, is equal to or lower than a predetermined threshold temperature; (ii) a second control for depressurizing the cathode gas flow channel at a second depressurization rate that is a depressurization rate capable of reducing the stack temperature by utilizing heat of vaporization of liquid water present in the cathode gas flow channel when the startup temperature is higher than the threshold temperature; and performing at least one of the steps of: A method for starting a fuel cell system.

Images