Fuel cell system and valve control method for fuel cell system

Abstract

To provide a fuel cell system capable of preventing deterioration of an electrode while stabilizing power generation of the fuel cell by optimizing hydrogen concentration.SOLUTION: A control unit (96) of a fuel cell system (10) measures or estimates an amount of nitrogen in an anode flow path (36) and performs a first comparison by comparing the amount of nitrogen with a first threshold. When the amount of nitrogen is above the first threshold in the first comparison, the control unit performs a second comparison by comparing a differential pressure obtained by subtracting a cathode pressure value, which indicates a pressure in a cathode flow path, from an anode pressure value, which indicates a pressure in the anode flow path, with a second threshold. The control unit controls opening / closing of a valve (82) based on the result of the first comparison and the result of the second comparison.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a fuel cell system that prevents a decrease in hydrogen concentration in an anode flow channel and a valve control method for the fuel cell system. [Background technology]

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

[0003] Patent Document 1 discloses a fuel cell system mounted on a fuel cell vehicle. Hereinafter, this fuel cell system will also be referred to as the first system. In the first system, anode gas is supplied from an anode supply flow path to an anode flow path in a fuel cell stack. The main component of the anode gas is hydrogen. In the first system, cathode gas is supplied from a cathode supply path to a cathode flow path in the fuel cell stack. The cathode gas is air (oxygen, nitrogen, etc.). The fuel cell stack generates electricity through a reaction between the hydrogen in the anode gas and the oxygen in the cathode gas. Anode off-gas (hydrogen, nitrogen, moisture, etc.) is discharged from the anode flow path. The anode off-gas is supplied to a gas-liquid separator. The gas-liquid separator separates the anode off-gas into gas (hydrogen, nitrogen, etc.) and liquid (water).

[0004] The anode off-gas from the gas-liquid separator can be supplied to the anode supply flow path via the circulation flow path, or can be discharged to the outside of the fuel cell system via the purge flow path and the diluter. Furthermore, the anode off-gas from the gas-liquid separator can be discharged to the outside of the fuel cell system together with water via the drain flow path and the diluter.

[0005] A new fuel cell system is currently under development. Hereinafter, this fuel cell system will be referred to as the "second system." In the second system, a connecting flow path is provided instead of the purge flow path of the first system. The connecting flow path branches off from the circulation flow path and connects to the cathode supply flow path. In other words, in the second system, anode off-gas from the gas-liquid separator can be supplied not only to the anode supply flow path but also to the cathode supply flow path. Hydrogen in the anode off-gas reacts with oxygen on the catalyst of the cathode electrode of the fuel cell stack and is consumed. Therefore, in the second system, the amount of hydrogen discharged from the anode system to the outside is reduced, and therefore the amount of air required to dilute the hydrogen in the diluter is also reduced. Therefore, the second system allows for a lower rotation speed of the air pump that supplies air to the diluter, resulting in improved fuel economy compared to the first system. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-35436 Summary of the Invention [Problem to be solved by the invention]

[0007] In the second system, when the pressure in the cathode supply flow path is higher than the pressure in the circulation flow path, the cathode gas flows through the connecting flow path toward the circulation flow path. This causes air to mix with the anode gas, resulting in a relative decrease in the hydrogen concentration in the anode gas. This causes unstable power generation in the fuel cell stack. Furthermore, the lack of hydrogen causes deterioration of the anode electrode.

[0008] The present invention aims to solve the above-mentioned problems. [Means for solving the problem]

[0009] A first aspect of the present invention is a fuel cell system comprising: a fuel cell stack that generates power using anode gas in an anode flow path and cathode gas in a cathode flow path; an anode supply flow path that supplies the anode gas to the anode flow path; a cathode supply flow path that supplies the cathode gas to the cathode flow path; a circulation flow path that supplies a discharge fluid discharged from the anode flow path to the anode supply flow path; a connection flow path that supplies the discharge fluid discharged from the anode flow path to the cathode supply flow path; a valve that opens and closes the connection flow path; and a control unit that controls the opening and closing of the valve, wherein the fuel cell system comprises: a fuel cell stack that generates power using anode gas in an anode flow path and cathode gas in a cathode flow path; an anode supply flow path that supplies the anode gas to the anode flow path; the control unit further includes a memory unit that stores a first threshold value of the nitrogen amount used to determine whether to perform control to control the anode flow path, and a second threshold value of the pressure value used to determine whether to open or close the valve, wherein the control unit measures or estimates the nitrogen amount in the anode flow path, performs a first comparison in which the nitrogen amount is compared with the first threshold value, and if the nitrogen amount exceeds the first threshold value in the first comparison, performs a second comparison in which the differential pressure obtained by subtracting a cathode pressure value indicating the pressure in the cathode flow path from an anode pressure value indicating the pressure in the anode flow path is compared with the second threshold value, and controls the opening and closing of the valve based on the results of the first comparison and the second comparison.

[0010] A second aspect of the present invention is a valve control method for a fuel cell system including: a fuel cell stack that generates power using anode gas in an anode flow path and cathode gas in a cathode flow path; an anode supply flow path that supplies the anode gas to the anode flow path; a cathode supply flow path that supplies the cathode gas to the cathode flow path; a circulation flow path that supplies a discharge fluid discharged from the anode flow path to the anode supply flow path; a connecting flow path that supplies the discharge fluid discharged from the anode flow path to the cathode supply flow path; a valve that opens and closes the connecting flow path; and a computer that controls the opening and closing of the valve, a first threshold value of the amount of nitrogen used to determine whether to perform control to reduce nitrogen in the anode flow path, and a second threshold value of the pressure value used to determine whether to open or close the valve; the amount of nitrogen in the anode flow path is measured or estimated, and a first comparison is performed to compare the estimated amount of nitrogen with the first threshold value; if the amount of nitrogen exceeds the first threshold value in the first comparison, a second comparison is performed to compare a differential pressure obtained by subtracting a cathode pressure value indicating the pressure in the cathode flow path from an anode pressure value indicating the pressure in the anode flow path with the second threshold value; and the opening and closing of the valve is controlled based on the results of the first comparison and the second comparison. [Effects of the Invention]

[0011] The present invention performs first and second comparisons to open and close the valve at appropriate timing. This prevents cathode gas from flowing through the connecting flow path, eliminating the decrease in hydrogen concentration that would otherwise occur with air mixing into the anode gas, thereby stabilizing power generation in the fuel cell stack. Furthermore, this invention can prevent deterioration of the anode electrode due to hydrogen deficiency. This ultimately contributes to improved energy efficiency. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic diagram of a fuel cell system according to the present invention. [Figure 2] FIG. 2 is a flowchart of the valve control process. DETAILED DESCRIPTION OF THE INVENTION

[0013] [1 Configuration of fuel cell system 10] FIG. 1 is a schematic diagram of a fuel cell system 10 according to the present invention. The fuel cell system 10 is mounted on a vehicle (a fuel cell automobile). Alternatively, the fuel cell system 10 can be mounted on, for example, a ship, an aircraft, or a robot. The fuel cell system 10 includes a fuel cell stack 12, a hydrogen tank 14, an anode system 16, a cathode system 18, and a cooling system 20. The fuel cell system 10 also includes a control device 94. The output (electric power) of the fuel cell stack 12 is supplied to a load (not shown), such as a motor.

[0014] The fuel cell stack 12 has a plurality of power-generating cells 22 stacked in one direction. Each power-generating cell 22 has a membrane electrode assembly 24 (also simply referred to as an electrode assembly 24) and a pair of separators 26, 28. The pair of separators 26, 28 sandwich the electrode assembly 24.

[0015] The electrode structure 24 has a solid polymer electrolyte membrane 30 (also referred to as electrolyte membrane 30), an anode electrode 32, and a cathode electrode 34. The electrolyte membrane 30 is, for example, a thin film of perfluorosulfonic acid containing water. The anode electrode 32 and the cathode electrode 34 sandwich the electrolyte membrane 30. The anode electrode 32 and the cathode electrode 34 have a gas diffusion layer made of carbon paper or the like. An electrode catalyst layer is formed by uniformly applying porous carbon particles to the surface of the gas diffusion layer. A platinum alloy is supported on the surface of the porous carbon particles. The electrode catalyst layers are formed on both sides of the electrolyte membrane 30.

[0016] An anode flow channel 36 is formed on the surface of the separator 26 facing the electrode structure 24. The anode flow channel 36 is connected to the anode supply flow channel 40 via the anode inlet 17A. The anode flow channel 36 is connected to the anode discharge flow channel 42 via the first anode outlet 17B. The anode flow channel 36 is also connected to the second drain flow channel 48 via the second anode outlet 17C. The second anode outlet 17C is located lower than the first anode outlet 17B. A cathode flow channel 38 is formed on the surface of the separator 28 facing the electrode structure 24. The cathode flow channel 38 is connected to the cathode supply flow channel 62 via the cathode inlet 19A. The cathode flow channel 38 is connected to the cathode discharge flow channel 64 via the cathode outlet 19B.

[0017] Anode gas (hydrogen) is supplied to the anode electrode 32. At the anode electrode 32, hydrogen ions and electrons are generated from hydrogen molecules through an electrode reaction caused by a catalyst. The hydrogen ions permeate the electrolyte membrane 30 and move to the cathode electrode 34. The electrons move in the following order: the negative terminal (not shown) of the fuel cell stack 12, a load such as a motor, the positive terminal (not shown) of the fuel cell stack 12, and then the cathode electrode 34. At the cathode electrode 34, the hydrogen ions and electrons react with oxygen contained in the supplied air due to the action of the catalyst to produce water.

[0018] The anode system 16 has components for supplying an anode gas to the anode electrode 32 and components for discharging a fluid from the anode electrode 32. The anode system 16 has an anode supply flow path 40, an anode discharge flow path 42, a circulation flow path 44, a first drain flow path 46, and a second drain flow path 48. The anode system 16 also has an injector 50, an ejector 52, an anode pressure sensor 59, a gas-liquid separator 54, a first drain valve 56, and a second drain valve 58.

[0019] The anode supply flow path 40 connects the outlet of the hydrogen tank 14 with the anode inlet 17A. An injector 50, an ejector 52, and an anode pressure sensor 59 are provided in the anode supply flow path 40. The ejector 52 is disposed closer to the anode inlet 17A than the injector 50. The anode pressure sensor 59 is disposed closer to the anode inlet 17A than the ejector 52. The anode pressure sensor 59 detects the pressure in the anode supply flow path 40. The pressure in the anode supply flow path 40 corresponds to the pressure in the anode flow path 36. The anode pressure sensor 59 may be provided in the anode exhaust flow path 42 or the circulation flow path 44. Multiple anode pressure sensors 59 may be provided.

[0020] The anode discharge flow path 42 communicates between the first anode outlet 17B and the intake port of the gas-liquid separator 54. The circulation flow path 44 communicates between the exhaust port of the gas-liquid separator 54 and the ejector 52. The first drain flow path 46 communicates between the drain port of the gas-liquid separator 54 and the inlet of the diluter 60. A first drain valve 56 is provided in the first drain flow path 46. The second drain flow path 48 communicates between the second anode outlet 17C and a portion of the first drain flow path 46 downstream of the first drain valve 56. A second drain valve 58 is provided in the second drain flow path 48.

[0021] The cathode system 18 has components for supplying a cathode gas to the cathode electrode 34 and components for discharging a fluid from the cathode electrode 34. The cathode system 18 has a cathode supply flow path 62, a cathode discharge flow path 64, and a bypass flow path 66. The cathode system 18 also has a compressor 68, a cathode pressure sensor 79, a humidifier 70, a first shut-off valve 74, a second shut-off valve 76, and a bypass valve 78.

[0022] The cathode supply flow path 62 communicates with an air intake port (not shown) and the cathode inlet 19A. The cathode supply flow path 62 is provided with a compressor 68, a first sealing valve 74, a cathode pressure sensor 79, and a flow path 72A for the humidifier 70. The portion of the cathode supply flow path 62 upstream of the humidifier 70 is referred to as the cathode supply flow path 62A. The portion of the cathode supply flow path 62 downstream of the humidifier 70 is referred to as the cathode supply flow path 62B. The cathode supply flow path 62A is provided with a compressor 68 and a first sealing valve 74. The first sealing valve 74 is disposed closer to the humidifier 70 than the compressor 68. The cathode pressure sensor 79 is disposed closer to the humidifier 70 than the first sealing valve 74. The cathode pressure sensor 79 detects the pressure of the cathode supply flow path 62. The pressure of the cathode supply flow path 62 corresponds to the pressure of the cathode flow path 38. The cathode pressure sensor 79 may be provided in the cathode exhaust flow path 64. A plurality of cathode pressure sensors 79 may be provided.

[0023] The cathode discharge flow path 64 connects the cathode outlet 19B and the inlet of the diluter 60. The cathode discharge flow path 64 is provided with a flow path 72B of the humidifier 70 and a second shutoff valve 76. The portion of the cathode discharge flow path 64 upstream of the humidifier 70 is referred to as a cathode discharge flow path 64A. The portion of the cathode supply flow path 62 downstream of the humidifier 70 is referred to as a cathode discharge flow path 64B. The cathode discharge flow path 64B is provided with a second shutoff valve 76.

[0024] The bypass flow path 66 communicates between the cathode supply flow path 62A and the cathode discharge flow path 64B. For example, the bypass flow path 66 communicates between a portion of the cathode supply flow path 62A between the compressor 68 and the first seal valve 74 and a portion of the cathode discharge flow path 64B downstream of the second seal valve 76. A bypass valve 78 is provided in the bypass flow path 66.

[0025] The anode system 16 and the cathode system 18 are connected by a connection flow path 80. The connection flow path 80 communicates the circulation flow path 44 of the anode system 16 with the cathode supply flow path 62B of the cathode system 18. A bleed valve 82 is provided in the connection flow path 80.

[0026] The cooling system 20 has components for supplying a coolant to the fuel cell stack 12 and components for discharging the coolant from the fuel cell stack 12. The cooling system 20 has a coolant supply passage 84 and a coolant discharge passage 86. The cooling system 20 also has a coolant pump 88, a radiator 90, and a temperature sensor 92.

[0027] A coolant flow path (not shown) for cooling the fuel cell stack 12 is formed inside the fuel cell stack 12. A coolant supply flow path 84 connects the outlet of the radiator 90 with the inlet of the coolant flow path. Coolant supply passage 84 A coolant pump 88 is provided in the fuel cell stack 12. The coolant discharge flow path 86 connects the outlet of the coolant flow path with the inlet of the radiator 90. A temperature sensor 92 is provided in the coolant discharge flow path 86. The temperature sensor 92 detects the temperature of the coolant discharged from the fuel cell stack 12.

[0028] The control device 94 is a computer (e.g., a vehicle ECU). The control device 94 has a control unit 96 and a memory unit 98. The control unit 96 has a processing circuit. The processing circuit may be a processor such as a CPU. The processing circuit may be an integrated circuit such as an ASIC or FPGA. The processor is capable of performing various processes by executing programs stored in the memory unit 98. At least some of the processes may be performed by electronic circuits including discrete devices.

[0029] The control unit 96 controls the operation of the fuel cell system 10. For example, the control unit 96 receives detection signals from various sensors provided in the fuel cell system 10. Based on each detection signal, the control unit 96 outputs control signals for controlling each valve, the injector 50, the compressor 68, the refrigerant pump 88, etc. Each valve, the injector 50, the compressor 68, the refrigerant pump 88, etc. operates in response to the control signals.

[0030] The storage unit 98 has a volatile memory and a non-volatile memory. Examples of the volatile memory include RAM. The volatile memory is used as a working memory for the processor. The volatile memory temporarily stores data required for processing or calculation. Examples of the non-volatile memory include ROM and flash memory. The non-volatile memory is used as a storage memory. The non-volatile memory stores programs, tables, maps, etc. At least a part of the storage unit 98 may be provided in the processor, integrated circuit, etc. described above.

[0031] The nonvolatile memory stores a first threshold value and a second threshold value. The first threshold value is a threshold value of the amount of nitrogen used to determine whether to perform control to reduce nitrogen in the anode flow path 36. Specifically, the first threshold value is the allowable amount of nitrogen in the anode flow path 36. The second threshold value is a threshold value of the pressure value used to determine whether to open or close the bleed valve 82. Each of the first threshold value and the second threshold value is set in advance by the user.

[0032] [2 Fluid Flow] 2-1 Fluid flow in the anode system 16 The injector 50 injects anode gas (hydrogen) from the hydrogen tank 14 downstream into the anode supply flow path 40. The anode gas injected from the injector 50 flows through the anode supply flow path 40 and is supplied to the anode flow path 36. The anode gas flows through the anode flow path 36 and is discharged from the first anode outlet 17B as anode off-gas. The anode off-gas contains hydrogen that has not reacted with oxygen, nitrogen in the cathode gas that has permeated the electrolyte membrane 30, and moisture produced by the reaction between oxygen and hydrogen.

[0033] The anode off-gas flows through the anode discharge flow path 42 and is supplied to the gas-liquid separator 54. The gas-liquid separator 54 separates the anode off-gas into a gas component (anode off-gas) and a liquid component (water). The anode off-gas discharged from the gas-liquid separator 54 flows through the circulation flow path 44 and is supplied to the ejector 52. In the ejector 52, the anode off-gas and the anode gas injected from the injector 50 merge together.

[0034] The water separated in the gas-liquid separator 54 is temporarily stored at the bottom of the gas-liquid separator 54. When the first drain valve 56 is open, the water stored in the gas-liquid separator 54 flows through the first drain passage 46 and is discharged to the diluter 60. When the first drain valve 56 is opened after the water in the gas-liquid separator 54 has run out, the anode off-gas in the gas-liquid separator 54 flows through the first drain passage 46 and is discharged to the diluter 60.

[0035] When the inside of the fuel cell stack 12 is highly humid, water accumulates at the bottom of the anode flow path 36. When the second drain valve 58 is open, the water accumulated in the anode flow path 36 flows through the second drain flow path 48 and the first drain flow path 46 and is discharged to the diluter 60. When the second drain valve 58 is opened after the water in the anode flow path 36 has run out, the anode off-gas in the anode flow path 36 flows through the second drain flow path 48 and the first drain flow path 46 and is discharged to the diluter 60.

[0036] [2-2 Fluid flow in the cathode system 18] The compressor 68 discharges cathode gas (air) drawn in from outside the vehicle downstream of the cathode supply flow path 62. With the first sealing valve 74 open, the cathode gas discharged from the compressor 68 flows through the cathode supply flow path 62 and is supplied to the cathode flow path 38. The cathode gas flows through the cathode flow path 38 and is discharged from the cathode outlet 19B as cathode off-gas. The cathode off-gas contains the components contained in the air and moisture produced by the reaction of oxygen and hydrogen.

[0037] With the second shutoff valve 76 open, the cathode off-gas flows through the cathode discharge passage 64 and is discharged to the diluter 60. The cathode off-gas contains moisture. In the humidifier 70, the moisture in the cathode off-gas is used to humidify the cathode gas.

[0038] With the bypass valve 78 open, the cathode gas flows through the bypass flow path 66 and the cathode discharge flow path 64 and is discharged to the diluter 60. The bypass flow path 66 is used when the amount of cathode gas supplied to the fuel cell stack 12 is reduced.

[0039] [2-3 Fluid flow in the connecting flow path 80] With the bleed valve 82 open, a portion of the anode off-gas flowing through the circulation flow path 44 flows through the connection flow path 80 and is supplied to the cathode supply flow path 62B. However, the bleed valve 82 is opened only when the pressure in the anode flow path 36 is higher than the pressure in the cathode flow path 38.

[0040] [3 Reasons for opening bleed valve 82] The control unit 96 suppresses a decrease in the hydrogen concentration in the anode flow channel 36 and maintains the hydrogen concentration at a certain level or higher. Possible factors that cause the hydrogen concentration in the anode flow channel 36 to decrease are (a) and (b) below. (a) Hydrogen in the anode flow channel 36 is consumed by the power generation in the fuel cell stack 12. (b) Nitrogen contained in the cathode gas permeates the electrolyte membrane 30 and enters the anode flow channel 36, causing a relative increase in the nitrogen concentration in the anode flow channel 36.

[0041] In response to the above factor (a), the control unit 96 controls the injector 50. As a result, the amount of hydrogen in the anode flow path 36 increases, and the hydrogen concentration in the anode flow path 36 increases. In response to the above factor (b), the control unit 96 opens the second drain valve 58 or the bleed valve 82. As a result, anode off-gas containing nitrogen is discharged from the anode flow path 36. Hydrogen is appropriately supplied to the anode flow path 36 as anode gas. As a result, the hydrogen concentration in the anode flow path 36 increases relatively.

[0042] For the following reasons, it is preferable to open the bleed valve 82 rather than the second drain valve 58. When the second drain valve 58 is opened, the anode offgas discharged from the anode flow path 36 flows through the second drain flow path 48 and is directly discharged to the diluter 60. In this case, a large amount of air is required in the diluter 60 to dilute the hydrogen in the anode offgas. This increases the power consumption of the compressor 68. On the other hand, when the bleed valve 82 is opened, the anode offgas discharged from the anode flow path 36 flows through the anode discharge flow path 42, the gas-liquid separator 54, the circulation flow path 44, the connection flow path 80, the cathode supply flow path 62B, the cathode flow path 38, and the cathode discharge flow path 64 in this order, before being discharged to the diluter 60. In this case, the hydrogen in the anode offgas is consumed by catalytic action inside the fuel cell stack 12. Therefore, a large amount of air is not required in the diluter 60 to dilute the hydrogen in the anode offgas. This reduces the power consumption of the compressor 68. From the viewpoint of improving fuel economy, it is preferable that the anode off-gas be supplied to the cathode supply flow path 62B via the connection flow path 80.

[0043] [4. Valve control process of bleed valve 82] 2 is a flowchart of the valve control process. The control unit 96 repeatedly performs the valve control process shown in FIG.

[0044] In step S1, the control unit 96 estimates the amount of nitrogen in the anode flow channel 36. The amount of nitrogen permeated from the cathode flow channel 38 to the anode flow channel 36 (permeated nitrogen amount) can be calculated by multiplying the nitrogen partial pressure difference between the anode flow channel 36 and the cathode flow channel 38 by the nitrogen permeability coefficient. There is a correlation between the temperature inside the fuel cell stack 12 and the nitrogen permeability coefficient. There is also a correlation between the humidity inside the fuel cell stack 12 and the nitrogen permeability coefficient. The control unit 96 controls each component of the fuel cell system 10 so that the humidity inside the fuel cell stack 12 is 100%, for example. In this case, the nitrogen permeability coefficient can be estimated based on the temperature inside the fuel cell stack 12. In this embodiment, the control unit 96 calculates the temperature inside the fuel cell stack 12 based on the temperature of the refrigerant detected by the temperature sensor 92. Furthermore, the control unit 96 estimates the amount of nitrogen in the anode flow channel 36 based on the temperature inside the fuel cell stack 12. Various estimation methods are stored in the memory unit 98. The temperature inside the fuel cell stack 12 can also be calculated from the temperature of the cathode offgas flowing through the cathode exhaust flow path 64 or the temperature of the anode offgas flowing through the anode exhaust flow path 42. The temperature inside the fuel cell stack 12 can also be detected directly using a temperature sensor or the like. Instead of estimating the amount of nitrogen, the nitrogen may be measured directly, for example, using a sensor or the like provided in the anode exhaust flow path 42. When step S1 is completed, the process proceeds to step S2.

[0045] In step S2, the control unit 96 determines whether or not it is necessary to discharge nitrogen from the anode flow channel 36 based on the estimated amount of nitrogen. Specifically, the control unit 96 compares the estimated amount of nitrogen with a first threshold value stored in the memory unit 98 (first comparison). If the amount of nitrogen exceeds the first threshold value (step S2: YES), the process proceeds to step S3. In this case, the control unit 96 determines that it is necessary to discharge nitrogen from the anode flow channel 36. On the other hand, if the amount of nitrogen is equal to or less than the first threshold value (step S2: NO), the process proceeds to step S6. In this case, the control unit 96 determines that it is not necessary to discharge nitrogen from the anode flow channel 36. Note that if the amount of nitrogen is equal to the first threshold value, the process may proceed to step S3.

[0046] When the process proceeds from step S2 to step S3, the control unit 96 acquires each pressure value. The pressure values ​​are the pressure value of the anode flow path 36 (anode pressure value) and the pressure value of the cathode flow path 38 (cathode pressure value). The control unit 96 uses the pressure value acquired from the anode pressure sensor 59 as the anode pressure value. The control unit 96 uses the pressure value acquired from the cathode pressure sensor 79 as the cathode pressure value. When step S3 ends, the process proceeds to step S4.

[0047] In step S4, the control unit 96 determines whether to open the bleed valve 82. Specifically, the control unit 96 calculates the differential pressure between the anode pressure value and the cathode pressure value by subtracting the cathode pressure value from the anode pressure value. The control unit 96 compares the calculated differential pressure with a second threshold value stored in the memory unit 98 (second comparison). If the differential pressure exceeds the second threshold value (step S4: YES), the process proceeds to step S5. On the other hand, if the differential pressure is equal to or less than the second threshold value (step S4: NO), the process proceeds to step S6. Note that the process may also proceed to step S5 if the target power generation amount is equal to the second threshold value.

[0048] The second threshold value may be zero or a predetermined positive value. If multiple anode pressure sensors 59 are provided in the anode system 16, the control unit 96 uses the smallest value among the multiple sensor values. Cathode System 18 When multiple cathode pressure sensors 79 are provided, the control unit 96 uses the maximum value of the multiple sensor values.

[0049] When the process moves from step S4 to step S5, the pressure in the anode flow path 36 is higher than the pressure in the cathode flow path 38. When the bleed valve 82 is opened in this state, the anode off-gas flows through the connecting flow path 80 from the circulation flow path 44 toward the cathode supply flow path 62. Therefore, the control unit 96 opens the bleed valve 82. If the bleed valve 82 is already open, the control unit 96 maintains the state of the bleed valve 82. On the other hand, if the bleed valve 82 is closed, the control unit 96 opens the bleed valve 82. A portion of the anode off-gas flows through the anode discharge flow path 42, the gas-liquid separator 54, the circulation flow path 44, the connecting flow path 80, the cathode supply flow path 62B, the cathode flow path 38, and the cathode discharge flow path 64, in this order, and is then discharged to the diluter 60.

[0050] When the process moves from step S2 or step S4 to step S6, the pressure in the anode flow path 36 is lower than the pressure in the cathode flow path 38. When the bleed valve 82 is opened in this state, the cathode gas flows through the connecting flow path 80 from the cathode supply flow path 62B toward the circulation flow path 44. Therefore, the control unit 96 closes the bleed valve 82. If the bleed valve 82 is already closed, the control unit 96 maintains the state of the bleed valve 82. On the other hand, if the bleed valve 82 is open, the control unit 96 closes the bleed valve 82. The anode off-gas flows through the anode discharge flow path 42, the gas-liquid separator 54, the circulation flow path 44, and the ejector 52, in that order, and is returned to the anode supply flow path 40.

[0051] [5 Inventions Obtained from the Embodiments] The invention that can be understood from the above embodiment will be described below.

[0052] A first aspect of the present invention is a fuel cell system (10) including: a fuel cell stack (12) that generates power using an anode gas in an anode flow path (36) and a cathode gas in a cathode flow path (38), an anode supply flow path (40) that supplies the anode gas to the anode flow path, a cathode supply flow path (62) that supplies the cathode gas to the cathode flow path, a circulation flow path (44) that supplies an exhaust fluid discharged from the anode flow path to the anode supply flow path, a connection flow path (80) that supplies the exhaust fluid discharged from the anode flow path to the cathode supply flow path, a valve (82) that opens and closes the connection flow path, and a control unit (96) that controls opening and closing of the valve. The fuel cell system further includes a memory unit (98) that stores a first threshold value of the amount of nitrogen used to determine whether to perform control to reduce nitrogen in the anode flow path, and a second threshold value of the pressure value used to determine whether to open or close the valve. The control unit measures or estimates the amount of nitrogen in the anode flow path, performs a first comparison in which the amount of nitrogen is compared with the first threshold value, and if the amount of nitrogen exceeds the first threshold value in the first comparison, performs a second comparison in which the differential pressure obtained by subtracting a cathode pressure value indicative of the pressure in the cathode flow path from an anode pressure value indicative of the pressure in the anode flow path is compared with the second threshold value, and controls opening and closing of the valve based on the results of the first comparison and the second comparison.

[0053] In the first aspect, a first comparison (step S2) and a second comparison (step S4) are performed to open and close the valve (bleed valve 82) at appropriate timing. According to the first aspect, the cathode gas is prevented from flowing through the connection flow path 80, and a decrease in hydrogen concentration due to air mixing into the anode gas is prevented. Therefore, according to the first aspect, power generation in the fuel cell stack 12 is stabilized. Furthermore, according to the first aspect, deterioration of the anode electrode 32 due to hydrogen deficiency can be prevented.

[0054] According to the first aspect, the anode off-gas is appropriately supplied to the cathode supply flow path 62B. A portion of the hydrogen supplied from the cathode supply flow path 62B to the cathode flow path 38 is consumed by catalytic action in the fuel cell stack 12. This reduces the amount of hydrogen discharged and the operation of the compressor 68. Therefore, according to the first aspect, the fuel economy of the fuel cell system 10 is improved. The first aspect ultimately contributes to energy efficiency.

[0055] In the above aspect, the control unit may open the valve when the differential pressure exceeds the second threshold value in the second comparison, and close the valve when the differential pressure falls below the second threshold value.

[0056] In the above aspect, the control unit may measure or estimate a temperature of the fuel cell stack, and estimate the amount of nitrogen based on the temperature of the fuel cell stack.

[0057] A second aspect of the present invention is a valve control method for a fuel cell system including: a fuel cell stack that generates electricity using anode gas in an anode flow path and cathode gas in a cathode flow path; an anode supply flow path that supplies the anode gas to the anode flow path; a cathode supply flow path that supplies the cathode gas to the cathode flow path; a circulation flow path that supplies a discharge fluid discharged from the anode flow path to the anode supply flow path; a connecting flow path that supplies the discharge fluid discharged from the anode flow path to the cathode supply flow path; a valve that opens and closes the connecting flow path; and a computer (94) that controls the opening and closing of the valve. The computer stores a first threshold value of the amount of nitrogen used to determine whether to perform control to reduce nitrogen in the anode flow path, and a second threshold value of the pressure value used to determine whether to open or close the valve, measures or estimates the amount of nitrogen in the anode flow path, and performs a first comparison in which the estimated amount of nitrogen is compared with the first threshold value, and if the amount of nitrogen exceeds the first threshold value in the first comparison, performs a second comparison in which the differential pressure obtained by subtracting a cathode pressure value indicating the pressure in the cathode flow path from an anode pressure value indicating the pressure in the anode flow path is compared with the second threshold value, and controls the opening and closing of the valve based on the results of the first comparison and the second comparison. [Explanation of symbols]

[0058] 10...Fuel cell system 12...Fuel cell stack 36...Anode flow path 38...Cathode flow path 62: Cathode supply flow path 80: Connection flow path 82... Bleed valve (valve) 94... Control device (computer) 96...control unit 98...storage unit

Claims

1. a fuel cell stack that generates electricity using an anode gas in the anode flow channel and a cathode gas in the cathode flow channel; an anode supply flow path that supplies the anode gas to the anode flow path; a cathode supply flow path that supplies the cathode gas to the cathode flow path; a circulation flow path that supplies a fluid discharged from the anode flow path to the anode supply flow path; a connecting flow path that supplies the discharged fluid discharged from the anode flow path to the cathode supply flow path; a valve that opens and closes the connecting flow path; a control unit that controls opening and closing of the valve; A fuel cell system comprising: a storage unit configured to store a first threshold value of the nitrogen amount for determining whether to perform control to reduce nitrogen in the anode flow path and a second threshold value of the pressure value for determining whether to open or close the valve, The control unit measuring or estimating the amount of nitrogen in the anode flow channel; performing a first comparison of comparing the amount of nitrogen with the first threshold value; if the amount of nitrogen exceeds the first threshold value in the first comparison, a second comparison is performed in which a differential pressure obtained by subtracting a cathode pressure value indicating a pressure in the cathode flow channel from an anode pressure value indicating a pressure in the anode flow channel is compared with the second threshold value; In the second comparison, if the differential pressure exceeds the second threshold, the valve is opened, and if the differential pressure falls below the second threshold, the valve is closed.

2. 2. The fuel cell system according to claim 1, The control unit measures or estimates a temperature of the fuel cell stack, and estimates the amount of nitrogen based on the temperature of the fuel cell stack.

3. a fuel cell stack that generates electricity using an anode gas in the anode flow channel and a cathode gas in the cathode flow channel; an anode supply flow path that supplies the anode gas to the anode flow path; a cathode supply flow path that supplies the cathode gas to the cathode flow path; a circulation flow path that supplies a fluid discharged from the anode flow path to the anode supply flow path; a connecting flow path that supplies the discharged fluid discharged from the anode flow path to the cathode supply flow path; a valve that opens and closes the connecting flow path; a computer that controls the opening and closing of the valve; A valve control method for a fuel cell system comprising: The computer a first threshold value of the nitrogen amount for determining whether or not to perform control to reduce nitrogen in the anode flow path, and a second threshold value of the pressure value for determining whether or not to open or close the valve; measuring or estimating the amount of nitrogen in the anode flow channel; performing a first comparison of comparing the estimated nitrogen amount with the first threshold value; if the amount of nitrogen exceeds the first threshold value in the first comparison, a second comparison is performed in which a differential pressure obtained by subtracting a cathode pressure value indicating a pressure in the cathode flow channel from an anode pressure value indicating a pressure in the anode flow channel is compared with the second threshold value; The valve control method for a fuel cell system, wherein in the second comparison, when the differential pressure exceeds the second threshold value, the valve is opened, and when the differential pressure falls below the second threshold value, the valve is closed.

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