Fuel cell system and valve control method for fuel cell system

Abstract

This invention relates to a fuel cell system and a valve control method for a fuel cell system. The control unit (96) of the fuel cell system (10) estimates the amount of nitrogen in the anode flow path (36), performs a first comparison comparing the estimated amount of nitrogen with a first threshold. If the amount of nitrogen exceeds the first threshold in the first comparison, a second comparison comparing the target value of the power generation of the fuel cell stack (12), i.e., the target power generation, with a second threshold is performed. The opening and closing of the first valve (82) and the opening and closing of the second valve (58) are controlled based on the results of the first comparison and the second comparison.

Description

Technical Field

[0001] This invention relates to a fuel cell system for preventing a decrease in hydrogen concentration in the anode flow path, and a valve control method for the fuel cell system. Background Technology

[0002] In recent years, in order to ensure that more people have access to appropriate, reliable, sustainable and advanced energy, research and development are underway on fuel cells that contribute to energy efficiency.

[0003] Patent Document 1 discloses a fuel cell system for 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 the anode supply path to the anode flow path within the fuel cell stack. The main component of the anode gas is hydrogen. In the first system, cathode gas is supplied from the cathode supply path to the cathode flow path within the fuel cell stack. The cathode gas is air (oxygen, nitrogen, etc.). The fuel cell stack generates electricity through the reaction of hydrogen in the anode gas and oxygen in the cathode gas. Anode exhaust gas (hydrogen, nitrogen, moisture, etc.) is discharged from the anode flow path. The anode exhaust gas is supplied to a gas-liquid separator. The gas-liquid separator separates the anode exhaust gas into air (hydrogen, nitrogen, etc.) and liquid (water).

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

[0005] A new fuel cell system has now been developed. Hereinafter, this newly developed fuel cell system will also 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. That is, in the second system, the anode exhaust 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 exhaust gas reacts with oxygen on the catalyst at the cathode electrode of the fuel cell stack and is consumed. Therefore, in the second system, less hydrogen is discharged from the anode system to the outside, and the amount of air required to dilute the hydrogen in the diluent is also reduced. Consequently, according to the second system, the speed of the air pump supplying air to the diluent can be reduced, resulting in improved fuel efficiency compared to the first system.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2007-35436 Summary of the Invention

[0009] As the load on the fuel cell stack increases, the power generation of the fuel cell stack increases. When the power generation of the fuel cell stack increases, the pressure in the anode system and the cathode system rises, and the rate of increase in nitrogen in the anode system also increases.

[0010] In the first system, the anode exhaust gas from the gas-liquid separator is discharged to the outside (atmosphere) via a diluter. In this first system, the discharge flow rate of the nitrogen-containing anode exhaust gas is determined by the pressure difference between the anode system and atmospheric pressure. As the load on the fuel cell stack increases, the pressure difference between the anode system and atmospheric pressure increases. Therefore, in the first system, the nitrogen discharge flow rate also increases with the increase in the load on the fuel cell stack.

[0011] On the other hand, in the second system, the anode exhaust gas from the gas-liquid separator is supplied (discharged) to the cathode supply flow path via a connecting flow path. In the second system, the discharge flow rate of the anode exhaust gas containing nitrogen is determined by the pressure difference between the anode system and the cathode system. When the load on the fuel cell stack increases, the pressure difference between the anode system and the cathode system is smaller than the pressure difference between the anode system and atmospheric pressure. Therefore, in the second system, even if the load on the fuel cell stack increases, the nitrogen discharge flow rate will not increase as much.

[0012] Furthermore, to improve the durability of the fuel cell stack, the second system maintains a high humidity level within the stack. As a result, the anode exhaust from the second system contains a significant amount of moisture, and the gas-liquid separator constantly stores water. Therefore, the amount of anode exhaust discharged from the gas-liquid separator to the outside via the drain path is relatively small.

[0013] Based on the above reasons, compared to the first system, the second system is less likely to remove nitrogen from the anode system. When the nitrogen concentration in the anode system increases, the hydrogen concentration in the anode system relatively decreases. To ensure stable power generation from the fuel cell stack, it is necessary to remove nitrogen from the anode system and suppress the decrease in hydrogen concentration.

[0014] The purpose of this invention is to solve the above-mentioned problems.

[0015] A first aspect of the present invention relates to a fuel cell system comprising: a fuel cell stack for generating electricity using anolyte gas in an anode flow path and cathode gas in a cathode flow path; an anode supply flow path for supplying the anolyte gas to the anode flow path; a cathode supply flow path for supplying the cathode gas to the cathode flow path; a circulation flow path for supplying discharge fluid discharged from the anode flow path to the anode supply flow path; a connection flow path for supplying the discharge fluid discharged from the anode flow path to the cathode supply flow path; a discharge flow path for discharging the discharge fluid from the anode flow path to the outside; a first valve for opening and closing the connection flow path; a second valve for opening and closing the discharge flow path; and a control unit for controlling the opening and closing of the first valve and the second valve, in the fuel cell system. The control unit also includes a storage unit for storing a first threshold and a second threshold. The first threshold is a threshold for determining the amount of nitrogen to be reduced in the anode flow path, and the second threshold is a threshold for determining the power generation to be generated by controlling the opening and closing of the first valve and the second valve respectively. The control unit estimates the amount of nitrogen in the anode flow path and performs a first comparison by comparing the estimated amount of nitrogen with the first threshold. If the amount of nitrogen exceeds the first threshold in the first comparison, the control unit performs a second comparison by comparing the target power generation of the fuel cell stack, i.e., the target power generation, with the second threshold. The control unit controls the opening and closing of the first valve and the second valve based on the results of the first comparison and the second comparison.

[0016] A second aspect of the present invention is a valve control method for a fuel cell system, the fuel cell system comprising: 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 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 discharge flow path that discharges the discharge fluid from the anode flow path to the outside; a first valve that opens and closes the connecting flow path; a second valve that opens and closes the discharge flow path; and a computer that controls the opening and closing of the first valve and the second valve, respectively, in the fuel cell stack. In the valve control method of the fuel cell system, the computer stores a first threshold and a second threshold. The first threshold is a threshold for determining the amount of nitrogen to be reduced in the anode flow path, and the second threshold is a threshold for determining the power generation to be generated by controlling the opening and closing of the first and second valves respectively. The computer estimates the amount of nitrogen in the anode flow path and performs a first comparison by comparing the estimated amount of nitrogen with the first threshold. If the amount of nitrogen exceeds the first threshold in the first comparison, the computer performs a second comparison by comparing the target power generation of the fuel cell stack with the second threshold. The computer controls the opening and closing of the first valve and the second valve based on the results of the first and second comparisons.

[0017] This invention involves performing a first comparison and a second comparison, and then opening and closing the first valve and the second valve at appropriate timings. By opening the first valve or the second valve at the appropriate timing, nitrogen-containing anode exhaust gas is discharged from the anode flow path. Furthermore, by closing the first valve and the second valve at appropriate timing, anode exhaust gas is not discharged from the anode flow path unnecessarily. Therefore, according to this invention, both improved fuel efficiency and maintained hydrogen concentration can be achieved. This invention further contributes to energy efficiency.

[0018] The above-described objectives, features, and advantages can be easily understood from the following description of the embodiments with reference to the accompanying drawings. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the fuel cell system involved in the present invention.

[0020] Figure 2 This is a flowchart of the valve control process. Detailed Implementation

[0021] [Structure of fuel cell system 10]

[0022] Figure 1 This is a schematic structural diagram of the fuel cell system 10 according to the present invention. The fuel cell system 10 is installed in a vehicle (fuel cell vehicle). In addition, the fuel cell system 10 can also be installed in, for example, ships, aircraft, and robots. 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. Furthermore, the fuel cell system 10 includes a control device 94. The output (electricity) of the fuel cell stack 12 is supplied to a load (not shown) such as a motor.

[0023] The fuel cell stack 12 has multiple power-generating cells 22 stacked in one direction. Each power-generating cell 22 has an electrolyte membrane-electrode structure 24 (also simply referred to as the electrode structure 24) and a set of spacers 26, 28. The set of spacers 26, 28 holds the electrode structure 24.

[0024] The electrode structure 24 comprises 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 formed of carbon paper or the like. Porous carbon particles are uniformly coated on the surface of the gas diffusion layer to form an electrode catalyst layer. A platinum alloy is supported on the surface of the porous carbon particles. The electrode catalyst layer is formed on both sides of the electrolyte membrane 30.

[0025] An anode flow path 36 is formed on the surface of the electrode-facing structure 24 within the surface of the spacer 26. The anode flow path 36 is connected to the anode supply flow path 40 via an anode inlet 17A. The anode flow path 36 is connected to the anode discharge flow path 42 via a first anode outlet 17B. Furthermore, the anode flow path 36 is connected to the second drain flow path 48 via a second anode outlet 17C. The second anode outlet 17C is located at a lower position than the first anode outlet 17B. A cathode flow path 38 is formed on the surface of the electrode-facing structure 24 within the surface of the spacer 28. The cathode flow path 38 is connected to the cathode supply flow path 62 via a cathode inlet 19A. The cathode flow path 38 is connected to the cathode discharge flow path 64 via a cathode outlet 19B.

[0026] Anode gas (hydrogen) is supplied to the anode electrode 32. In the anode electrode 32, hydrogen ions and electrons are generated from hydrogen molecules due to an electrode reaction produced by the catalyst. Hydrogen ions permeate through the electrolyte membrane 30 and move towards the cathode electrode 34. Electrons move sequentially towards 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 the cathode electrode 34. In the cathode electrode 34, hydrogen ions and electrons react with oxygen contained in the supplied air through the action of a catalyst to produce water.

[0027] The anode system 16 has structures for supplying anode gas to the anode electrode 32 and structures for discharging anode exhaust gas from the anode electrode 32. The anode system 16 includes an anode supply flow path 40, an anode discharge flow path 42, a circulation flow path 44, a first discharge flow path 46, and a second discharge flow path 48. Additionally, the anode system 16 includes an ejector 50, an ejector 52, a gas-liquid separator 54, a first discharge valve 56, and a second discharge valve 58 (second valve).

[0028] The anode supply path 40 connects the outlet of the hydrogen tank 14 to the anode inlet 17A. An ejector 50 and an ejector 52 are provided in the anode supply path 40. The ejector 52 is positioned closer to the anode inlet 17A than the ejector 50.

[0029] Anode discharge path 42 connects the first anode outlet 17B to the inlet of the gas-liquid separator 54. Circulation path 44 connects the exhaust port of the gas-liquid separator 54 to the ejector 52. First drain path 46 connects the drain outlet of the gas-liquid separator 54 to the inlet of the diluter 60. A first drain valve 56 is provided in the first drain path 46. Second drain path 48 connects the second anode outlet 17C to the downstream portion of the first drain path 46, below the first drain valve 56. A second drain valve 58 is provided in the second drain path 48.

[0030] The cathode system 18 has structures for supplying cathode gas to the cathode electrode 34 and structures for discharging cathode gas 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. Additionally, the cathode system 18 includes a compressor 68, a humidifier 70, a first sealing valve 74, a second sealing valve 76, and a bypass valve 78.

[0031] The cathode supply flow path 62 connects the air inlet (not shown) to the cathode inlet 19A. A flow path 72A containing the compressor 68, the first sealing valve 74, and the humidifier 70 is provided in the cathode supply flow path 62. The portion of the cathode supply flow path 62 upstream of the humidifier 70 is designated as cathode supply flow path 62A. The portion of the cathode supply flow path 62 downstream of the humidifier 70 is designated as cathode supply flow path 62B. The compressor 68 and the first sealing valve 74 are provided in the cathode supply flow path 62A. The first sealing valve 74 is positioned closer to the humidifier 70 than the compressor 68.

[0032] The cathode discharge path 64 connects the cathode outlet 19B to the inlet of the diluter 60. A flow path 72B for the humidifier 70 and a second sealing valve 76 are provided in the cathode discharge path 64. The portion of the cathode discharge path 64 upstream of the humidifier 70 is designated as cathode discharge path 64A. The portion of the cathode supply path 62 downstream of the humidifier 70 is designated as cathode discharge path 64B. The second sealing valve 76 is provided in cathode discharge path 64B.

[0033] The bypass flow path 66 connects the cathode supply flow path 62A and the cathode discharge flow path 64B. For example, the bypass flow path 66 connects the portion of the cathode supply flow path 62A between the compressor 68 and the first sealing valve 74 to the portion of the cathode discharge flow path 64B downstream of the second sealing valve 76. A bypass valve 78 is provided in the bypass flow path 66.

[0034] The anode system 16 and the cathode system 18 are connected by a connecting flow path 80. The connecting flow path 80 connects the circulation flow path 44 of the anode system 16 with the cathode supply flow path 62B of the cathode system 18. A discharge valve 82 (first valve) is provided in the connecting flow path 80.

[0035] The cooling system 20 has structures for supplying refrigerant to the fuel cell stack 12 and structures for discharging refrigerant from the fuel cell stack 12. The cooling system 20 has a refrigerant supply path 84 and a refrigerant discharge path 86. Additionally, the cooling system 20 includes a refrigerant pump 88, a radiator 90, and a temperature sensor 92.

[0036] A refrigerant flow path (not shown) for cooling the fuel cell stack 12 is formed inside the fuel cell stack 12. A refrigerant supply flow path 84 connects the outlet of the radiator 90 to the inlet of the refrigerant flow path. A refrigerant pump 88 is provided in the refrigerant supply flow path 84. A refrigerant discharge flow path 86 connects the outlet of the refrigerant flow path to the inlet of the radiator 90. A temperature sensor 92 is provided in the refrigerant discharge flow path 86. The temperature sensor 92 detects the temperature of the refrigerant discharged from the fuel cell stack 12.

[0037] The control device 94 is a computer (e.g., an ECU (Electronic Control Unit) in a vehicle). The control device 94 has a control unit 96 and a storage unit 98. The control unit 96 has processing circuitry. The processing circuitry can also be a processor such as a CPU. The processing circuitry can also be an integrated circuit such as an ASIC (Application-Specific Integrated Circuit) or a FPGA (Field-Programmable Gate Array). The processor can perform various processes by executing a program stored in the storage unit 98. At least some of the various processes can also be performed using circuitry containing discrete components.

[0038] 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 installed in the fuel cell system 10. Based on each detection signal, the control unit 96 outputs control signals for controlling each valve, injector 50, compressor 68, and refrigerant pump 88, etc. Each valve, injector 50, compressor 68, and refrigerant pump 88 operates according to the control signals.

[0039] Storage unit 98 includes volatile memory and non-volatile memory. Examples of volatile memory include RAM (Random Access Memory). Volatile memory is used as the processor's working memory. It temporarily stores data required for processing or computation. Examples of non-volatile memory include ROM (Read Only Memory) and flash memory. Non-volatile memory is used as storage memory. It stores programs, forms, and mapping tables. At least a portion of storage unit 98 may be included in processors and integrated circuits as described above.

[0040] The non-volatile memory stores a first threshold and a second threshold. The first threshold is a threshold for determining the amount of nitrogen needed to control whether nitrogen reduction in the anode flow path 36 is required. Specifically, the first threshold is the allowable amount of nitrogen in the anode flow path 36. The second threshold is a threshold for determining the amount of electricity generated by controlling the opening and closing of the discharge valve 82 and the second drain valve 58. The first and second thresholds are preset by the user.

[0041] [2 Fluid Flow]

[0042] [2-1 Fluid Flow in Anode System 16]

[0043] The ejector 50 injects anode gas (hydrogen) from the hydrogen tank 14 downstream of the anode supply path 40. The anode gas injected from the ejector 50 flows in the anode supply path 40 and is supplied to the anode path 36. The anode gas flows in the anode path 36 and is discharged from the first anode outlet 17B as anode exhaust. The anode exhaust contains unreacted hydrogen, nitrogen from the cathode gas permeating the electrolyte membrane 30, and moisture generated by the reaction of oxygen and hydrogen.

[0044] The anode exhaust gas flows in the anode discharge path 42 and is supplied to the gas-liquid separator 54. The gas-liquid separator 54 separates the anode exhaust gas into a gaseous component (anode exhaust gas) and a liquid component (water). The anode exhaust gas discharged from the gas-liquid separator 54 flows in the circulation path 44 and is supplied to the ejector 52. In the ejector 52, the anode exhaust gas merges with the anode gas ejected from the ejector 50.

[0045] The water separated by the gas-liquid separator 54 is temporarily stored at the bottom of the gas-liquid separator 54. With the first drain valve 56 open, the water stored in the gas-liquid separator 54 flows in the first drain path 46 and is discharged to the diluent 60. When the first drain valve 56 is opened when there is no water in the gas-liquid separator 54, the anode exhaust gas of the gas-liquid separator 54 flows in the first drain path 46 and is discharged to the diluent 60.

[0046] In a high-humidity environment inside the fuel cell stack 12, water is stored at the bottom of the anode flow path 36. With the second drain valve 58 open, the water stored in the anode flow path 36 flows through the second drain flow path 48 and the first drain flow path 46 and is discharged into the diluter 60. When the second drain valve 58 is opened when there is no water in the anode flow path 36, the anode exhaust gas from the anode flow path 36 flows through the second drain flow path 48 and the first drain flow path 46 and is discharged into the diluter 60.

[0047] [2-2 Fluid Flow in Cathode System 18]

[0048] Compressor 68 draws in cathode gas (air) from outside the vehicle and ejects it downstream of cathode supply path 62. With the first sealing valve 74 open, the cathode gas ejected from compressor 68 flows in cathode supply path 62 and is supplied to cathode path 38. The cathode gas flows in cathode path 38 and is discharged from cathode outlet 19B as cathode exhaust. Cathode exhaust contains all components of the air and moisture generated by the reaction of oxygen and hydrogen.

[0049] With the second sealing valve 76 open, the cathode exhaust flows in the cathode discharge path 64 and is discharged to the diluter 60. The cathode exhaust contains moisture. In the humidifier 70, the moisture in the cathode exhaust is used to humidify the cathode gas.

[0050] With bypass valve 78 open, cathode gas flows through bypass path 66 and cathode discharge path 64 and is discharged to diluter 60. Bypass path 66 is used when it is necessary to reduce the supply of cathode gas to fuel cell stack 12.

[0051] [2-3 Fluid flow in connecting flow path 80]

[0052] With the discharge valve 82 open, a portion of the anode exhaust flowing in the circulation path 44 flows in the connecting path 80 and is supplied to the cathode supply path 62B. However, the discharge valve 82 is only opened when the pressure in the anode path 36 is higher than the pressure in the cathode path 38.

[0053] [3. Reasons for opening the discharge valve 82]

[0054] The control unit 96 aims to suppress the decrease in hydrogen concentration in the anode flow path 36 and maintain the hydrogen concentration at a certain level or above. The main reasons for the decrease in hydrogen concentration in the anode flow path 36 are considered to be the following (a) and (b).

[0055] (a) Hydrogen in the anode flow path 36 is consumed due to the power generation of the fuel cell stack 12.

[0056] (b) Nitrogen contained in the cathode gas permeates through the electrolyte membrane 30 and seeps into the anode flow path 36, thereby increasing the nitrogen concentration in the anode flow path 36.

[0057] Regarding the main cause described in (a) above, 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. Regarding the main cause described in (b) above, the control unit 96 opens the second discharge valve 58 or the discharge valve 82. As a result, nitrogen-containing anode exhaust gas is discharged from the anode flow path 36. Hydrogen, as the anode gas, is appropriately supplied to the anode flow path 36. Therefore, the hydrogen concentration in the anode flow path 36 relatively increases.

[0058] For the following reasons, it is preferable to open the discharge valve 82 rather than the second discharge valve 58. When the second discharge valve 58 is open, the anode exhaust gas discharged from the anode flow path 36 flows in the second discharge 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 exhaust gas. Therefore, the power consumption of the compressor 68 increases. On the other hand, when the discharge valve 82 is open, the anode exhaust gas discharged from the anode flow path 36 flows sequentially 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, and is discharged to the diluter 60. In this case, the hydrogen in the anode exhaust gas is consumed inside the fuel cell stack 12 due to the action of the catalyst. Therefore, a large amount of air is not required for the diluter 60 to dilute the hydrogen in the anode exhaust gas. Therefore, the power consumption of the compressor 68 is suppressed. From the viewpoint of improving fuel efficiency, it is preferable to supply anode exhaust to cathode supply flow path 62B via connecting flow path 80.

[0059] However, the connecting flow path 80 is only used when the rate at which nitrogen is discharged from the anode flow path 36 to the cathode supply flow path 62B exceeds the rate at which nitrogen increases in the anode flow path 36. The rate at which nitrogen increases in the anode flow path 36 depends on the cathode pressure, the refrigerant temperature of the cooling system 20, and the humidity of the electrolyte membrane 30, etc. These are determined based on the power generation current of the fuel cell stack 12. The power generation current of the fuel cell stack 12 is determined by the target power generation used in the control unit 96. In other words, the rate at which nitrogen increases in the anode flow path 36 is closely related to the target power generation. Therefore, the control unit 96 determines which of the second drain valve 58 and the discharge valve 82 to open based on the target power generation.

[0060] [4 Valve control processing for the second drain valve 58 and the unloading valve 82]

[0061] Figure 2 This is a flowchart of the valve control process. During the operation of the fuel cell system 10, the control unit 96 repeatedly performs the following steps: Figure 2 The valve control process is shown in the figure.

[0062] In step S1, the control unit 96 estimates the amount of nitrogen in the anode flow path 36. By multiplying the nitrogen permeability coefficient by the nitrogen partial pressure difference between the anode flow path 36 and the cathode flow path 38, the amount of nitrogen permeating from the cathode flow path 38 to the anode flow path 36 (permeable nitrogen amount) can be calculated. The internal temperature of the fuel cell stack 12 is closely related to the nitrogen permeability coefficient. In addition, the internal humidity of the fuel cell stack 12 is closely related to the nitrogen permeability coefficient. For example, the control unit 96 controls each structure of the fuel cell system 10 to make the internal humidity of the fuel cell stack 12 100%. In this case, the nitrogen permeability coefficient can be estimated based on the internal temperature of the fuel cell stack 12. In this embodiment, the control unit 96 calculates the internal temperature of 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 path 36 based on the internal temperature of the fuel cell stack 12. Various estimation methods are stored in the storage unit 98. Moreover, the internal temperature of the fuel cell stack 12 can also be calculated based on the temperature of the cathode exhaust flowing in the cathode discharge flow path 64 or the temperature of the anode exhaust flowing in the anode discharge flow path 42. Alternatively, the internal temperature of the fuel cell stack 12 can be directly detected by temperature sensors or the like. When step S1 ends, the process moves to step S2.

[0063] In step S2, the control unit 96 determines whether nitrogen needs to be discharged from the anolyte flow path 36 based on the estimated nitrogen level. Specifically, the control unit 96 compares the estimated nitrogen level with a first threshold stored in the storage unit 98 (first comparison). If the nitrogen level exceeds the first threshold (step S2: Yes), the process proceeds to step S3. In this case, the control unit 96 determines that nitrogen needs to be discharged from the anolyte flow path 36. On the other hand, if the nitrogen level is below the first threshold (step S2: No), the process proceeds to step S7. In this case, the control unit 96 determines that nitrogen does not need to be discharged from the anolyte flow path 36. Alternatively, the process may proceed to step S3 if the nitrogen level is equal to the first threshold.

[0064] When transitioning from step S2 to step S3, the control unit 96 acquires the target power generation. As described above, the target power generation is a determining factor used to decide which of the second drain valve 58 and the discharge valve 82 should be opened. During the operation of the fuel cell system 10, the control unit 96 calculates the target power generation and controls each structure to ensure that the power generation of the fuel cell stack 12 reaches the target power generation. The control unit 96 uses the calculated target power generation to control the power generation of the fuel cell stack 12. When step S3 ends, the process transitions to step S4.

[0065] In step S4, the control unit 96 determines which of the second drain valve 58 and the discharge valve 82 should be opened. Specifically, the control unit 96 compares the target power generation obtained in step S3 with a second threshold stored in the storage unit 98 (second comparison). If the target power generation is lower than the second threshold (step S4: Yes), the process proceeds to step S5. On the other hand, if the target power generation is higher than the second threshold (step S4: No), the process proceeds to step S6. Moreover, it is also possible that the process proceeds to step S5 if the target power generation is equal to the second threshold.

[0066] When transitioning from step S4 to step S5, the control unit 96 opens the discharge valve 82 and closes the second discharge valve 58. When the discharge valve 82 is already open, the control unit 96 maintains the state of the discharge valve 82. Conversely, when the discharge valve 82 is closed, the control unit 96 opens the discharge valve 82. When the second discharge valve 58 is already closed, the control unit 96 maintains the state of the second discharge valve 58. Conversely, when the second discharge valve 58 is open, the control unit 96 closes the second discharge valve 58. A portion of the anode exhaust flows sequentially through the anode discharge path 42, the gas-liquid separator 54, the circulation path 44, the connecting path 80, the cathode supply path 62B, the cathode path 38, and the cathode discharge path 64, and is discharged to the diluter 60.

[0067] When transitioning from step S4 to step S6, the control unit 96 closes the discharge valve 82 and opens the second discharge valve 58. When the discharge valve 82 is already closed, the control unit 96 maintains the state of the discharge valve 82. Conversely, when the discharge valve 82 is open, the control unit 96 closes the discharge valve 82. When the second discharge valve 58 is already open, the control unit 96 maintains the state of the second discharge valve 58. Conversely, when the second discharge valve 58 is closed, the control unit 96 opens the second discharge valve 58. A portion of the anode exhaust flows in the second discharge path 48 and is directly discharged to the diluter 60.

[0068] When transitioning from step S2 to step S7, the control unit 96 closes both the discharge valve 82 and the second drain valve 58. When the discharge valve 82 is already closed, the control unit 96 maintains the state of the discharge valve 82. Conversely, when the discharge valve 82 is open, the control unit 96 closes the discharge valve 82. When the second drain valve 58 is already closed, the control unit 96 maintains the state of the second drain valve 58. Conversely, when the second drain valve 58 is open, the control unit 96 closes the second drain valve 58. The anode exhaust flows sequentially through the anode discharge path 42, the gas-liquid separator 54, the circulation path 44, and the ejector 52, and returns to the anode supply path 40.

[0069] [5. Invention obtained according to the embodiments]

[0070] The invention that can be mastered based on the above embodiments is described below.

[0071] The first aspect of the present invention relates to a fuel cell system (10) comprising: a fuel cell stack (12) for generating electricity using anode gas from an anode flow path (36) and cathode gas from a cathode flow path (38); an anode supply flow path (40) for supplying the anode gas to the anode flow path; a cathode supply flow path (62) for supplying the cathode gas to the cathode flow path; a circulation flow path (44) for supplying discharge fluid discharged from the anode flow path to the anode supply flow path; a connecting flow path (80) for supplying the discharge fluid discharged from the anode flow path to the cathode supply flow path; a discharge flow path (48) for discharging the discharge fluid from the anode flow path to the outside; a first valve (82) for opening and closing the connecting flow path; a second valve (58) for opening and closing the discharge flow path; and a control unit (96) for controlling the opening and closing of the first valve and the second valve respectively. The fuel cell system (10) further includes a storage unit (98) storing a first threshold and a second threshold. The first threshold is a threshold for determining whether to perform control to reduce nitrogen in the anode flow path, and the second threshold is a threshold for determining the power generation of the first valve and the second valve, respectively. The control unit estimates the amount of nitrogen in the anode flow path and performs a first comparison by comparing the estimated amount of nitrogen with the first threshold. If the amount of nitrogen exceeds the first threshold in the first comparison, the control unit performs a second comparison by comparing a target value of the power generation of the fuel cell stack, i.e., the target power generation, with the second threshold. The control unit controls the opening and closing of the first valve and the second valve based on the results of the first comparison and the second comparison.

[0072] The first method involves performing a first comparison (step S2) and a second comparison (step S4) to open and close the first valve (relief valve 82) and the second valve (second drain valve 58) at appropriate timings. By opening the first or second valve at appropriate timings, nitrogen-containing anode exhaust gas is discharged from the anode flow path 36. Furthermore, by closing the first and second valves at appropriate timings, anode exhaust gas is not discharged from the anode flow path 36 when needed. Moreover, the first method further contributes to energy efficiency.

[0073] According to the first method, the anode exhaust 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 in the fuel cell stack 12 due to the action of the catalyst. Therefore, the amount of hydrogen emitted is reduced, and the operation of the compressor 68 is also reduced. In addition, according to the first method, the anode exhaust gas is appropriately discharged to the outside. Therefore, the nitrogen content in the anode flow path 36 decreases. Thus, according to the first method, it is possible to simultaneously improve the fuel efficiency of the fuel cell system 10 and maintain the hydrogen concentration in the anode flow path 36.

[0074] Furthermore, according to the first method, the power generation of the fuel cell stack 12 is stable. Additionally, according to the first method, the degradation of the anode electrode 32 due to hydrogen deficiency can be prevented.

[0075] In the above-described manner, the control unit may close the first valve and the second valve when the nitrogen level in the first comparison is lower than the first threshold, or open the first valve and close the second valve when the nitrogen level in the first comparison exceeds the first threshold and the target power generation in the second comparison is lower than the second threshold.

[0076] According to the above structure, the anode exhaust is appropriately supplied to the cathode supply flow path 62B, thereby improving the fuel efficiency of the fuel cell system 10.

[0077] In the above-described manner, the control unit may close the first valve and the second valve when the nitrogen level in the first comparison is lower than the first threshold, and open the second valve and close the first valve when the nitrogen level in the first comparison exceeds the first threshold and the target power generation in the second comparison exceeds the second threshold.

[0078] According to the above structure, even when the first valve (discharge valve 82) is not open, the second valve (second drain valve 58) will open to discharge the anode exhaust to the outside. Therefore, according to the above structure, the decrease in hydrogen concentration in the anode flow path 36 is suppressed.

[0079] Alternatively, the control unit may measure or estimate the temperature of the fuel cell stack and estimate the nitrogen content based on the temperature of the fuel cell stack.

[0080] A second aspect of the present invention is a valve control method for a fuel cell system, the fuel cell system comprising: 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 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 discharge flow path that discharges the discharge fluid from the anode flow path to the outside; a first valve that opens and closes the connecting flow path; a second valve that opens and closes the discharge flow path; and a computer (94) that controls the opening and closing of the first valve and the second valve respectively. The computer stores a first threshold and a second threshold. The first threshold is a threshold for determining the amount of nitrogen needed to control whether to reduce nitrogen in the anode flow path. The second threshold is a threshold for determining the power generation needed to control the opening and closing of the first and second valves. The computer estimates the amount of nitrogen in the anode flow path and performs a first comparison by comparing the estimated amount of nitrogen with the first threshold. If the amount of nitrogen exceeds the first threshold in the first comparison, the computer performs a second comparison by comparing the target power generation of the fuel cell stack with the second threshold. The computer controls the opening and closing of the first and second valves based on the results of the first and second comparisons.

[0081] Furthermore, the present invention is not limited to the above disclosure, and various structures can be adopted without departing from the spirit of the present invention.

Claims

1. A fuel cell system comprising: The fuel cell stack (12) generates electricity using the anode gas in the anode flow path (36) and the cathode gas in the cathode flow path (38); Anode supply flow path (40) supplies the anode gas to the anode flow path; A cathode supply flow path (62) supplies the cathode gas to the cathode flow path; A circulation flow path (44) supplies the discharge fluid from the anode flow path to the anode supply flow path; A connecting flow path (80) supplies the discharge fluid from the anode flow path to the cathode supply flow path; Discharge flow path (48) that discharges the discharge fluid from the anode flow path to the outside; The first valve (82) opens and closes the connecting flow path; The second valve (58) opens and closes the discharge path; and The control unit (96) controls the opening and closing of the first valve and the second valve respectively. In the fuel cell system (10), It also includes a storage unit (98) for storing a first threshold and a second threshold. The first threshold is a threshold for determining the amount of nitrogen required to control whether to reduce nitrogen in the anode flow path. The second threshold is a threshold for determining the amount of electricity generated by controlling the opening and closing of the first valve and the second valve, respectively. The control unit estimates the amount of nitrogen in the anode flow path. And a first comparison is performed, comparing the estimated nitrogen level with the first threshold. If the nitrogen content exceeds the first threshold in the first comparison, the control unit performs a second comparison, comparing the target value of the power generation of the fuel cell stack (i.e., the target power generation) with the second threshold. The control unit controls the opening and closing of the first valve and the second valve based on the results of the first comparison and the second comparison.

2. The fuel cell system according to claim 1, characterized in that, If the nitrogen level is lower than the first threshold in the first comparison, the control unit closes the first valve and the second valve. If the nitrogen content exceeds the first threshold in the first comparison and the target power generation is lower than the second threshold in the second comparison, the control unit opens the first valve and closes the second valve.

3. The fuel cell system according to claim 1, characterized in that, If the nitrogen level is lower than the first threshold in the first comparison, the control unit closes the first valve and the second valve. If the nitrogen content exceeds the first threshold in the first comparison and the target power generation exceeds the second threshold in the second comparison, the control unit opens the second valve and closes the first valve.

4. The fuel cell system according to claim 2, characterized in that, If the nitrogen level is lower than the first threshold in the first comparison, the control unit closes the first valve and the second valve. If the nitrogen content exceeds the first threshold in the first comparison and the target power generation exceeds the second threshold in the second comparison, the control unit opens the second valve and closes the first valve.

5. The fuel cell system according to any one of claims 1 to 4, characterized in that, The control unit measures or estimates the temperature of the fuel cell stack and estimates the nitrogen content based on the temperature of the fuel cell stack.

6. A valve control method for a fuel cell system, the fuel cell system comprising: A fuel cell stack generates electricity using anolyte gas in the anode flow path and cathode gas in the cathode flow path. An anode supply flow path 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 the discharge fluid from the anode flow path to the anode supply flow path; A connecting flow path is provided to supply the discharge fluid from the anode flow path to the cathode supply flow path; A discharge flow path that discharges the discharge fluid from the anode flow path to the outside; The first valve opens and closes the connecting flow path; A second valve, which opens and closes the discharge path; and Computer (94), which controls the opening and closing of the first valve and the second valve respectively. In the valve control method of the fuel cell system, The computer stores a first threshold and a second threshold. The first threshold is a nitrogen level threshold used to determine whether control to reduce nitrogen in the anode flow path is needed. The second threshold is a power generation threshold used to determine the opening and closing of the first and second valves, respectively. The computer estimates the amount of nitrogen in the anolyte flow path. And a first comparison is performed, comparing the estimated nitrogen level with the first threshold. If the nitrogen content exceeds the first threshold in the first comparison, the computer performs a second comparison, comparing the target power generation of the fuel cell stack (i.e., the target power generation) with the second threshold. The computer controls the opening and closing of the first valve and the second valve based on the results of the first comparison and the second comparison.

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