Fuel cell system

Abstract

A fuel cell system is provided with: a fuel cell stack that generates electricity using an anode gas in an anode flow path and a cathode gas in a cathode flow path; an anode supply flow path for supplying an anode gas to the anode flow path; a cathode supply flow path for supplying a cathode gas to the cathode flow path; an anode discharge flow path through which the anode discharge fluid discharged from the anode flow path flows; a cathode discharge flow path through which the cathode discharge fluid discharged from the cathode flow path flows; a fluid merging part that merges the anode discharge fluid flowing through the anode discharge flow path and the cathode discharge fluid flowing through the cathode discharge flow path; a discharge pipe that guides the combined fluid, which has been combined in the fluid combining part, to the outside; an anode discharge valve for controlling the flow of the anode discharge fluid toward the fluid merging part; and a control unit that controls the opening and closing of the anode discharge valve. The control unit acquires the hydrogen concentration of the confluent fluid, and if the amount of power generated by the fuel cell stack is equal to or less than a predetermined power generation threshold value, controls the opening and closing of the anode discharge valve so as to repeat an opening / closing operation in which the fuel cell stack is opened for a predetermined opening time based on the hydrogen concentration and then closed.

Description

Technical Field

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

[0002] In a fuel cell system, when the anode exhaust gas containing hydrogen, nitrogen, and water is discharged to the outside of the fuel cell system (into the atmosphere), air (nitrogen, oxygen, etc.) is drawn in from the outside to dilute the gas to be discharged. Therefore, when a large amount of air is required for dilution, the power consumption of the compressor, etc. that transports the air, increases. Therefore, a system that achieves both improved fuel efficiency and maintained hydrogen concentration has been proposed.

[0003] For example, in the technology described in Patent Document 1, even when the target power generation is smaller than a predetermined threshold, the amount of air used for dilution is sometimes increased (in other words, the rotation rate of the compressor, etc. is increased) in order to suppress the hydrogen concentration of the gas to be discharged to a predetermined concentration.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2023-132388 (JP 2023-132388 A). Summary of the Invention

[0007] A fuel cell system according to one embodiment of the present invention includes: a fuel cell stack configured to generate electricity using anode gas in an anode flow channel and cathode gas in a cathode flow channel; an anode supply flow channel configured to supply anode gas to the anode flow channel; a cathode supply flow channel configured to supply cathode gas to the cathode flow channel; an anode exhaust flow channel configured to flow an anode exhaust fluid discharged from the anode flow channel; a cathode exhaust flow channel configured to flow a cathode exhaust fluid discharged from the cathode flow channel; a fluid confluence portion configured to merge the anode exhaust fluid flowing through the anode exhaust flow channel and the cathode exhaust fluid flowing through the cathode exhaust flow channel; a discharge pipe configured to guide the merged fluids merged at the fluid confluence portion to the outside; an anode exhaust valve configured to control the flow of the anode exhaust fluid toward the fluid confluence portion; and a control portion configured to control the opening and closing of the anode exhaust valve. The control portion obtains a hydrogen concentration in the merged fluid and, when the power generation amount of the fuel cell stack is below a predetermined power generation threshold, controls the opening and closing of the anode exhaust valve by repeatedly opening and closing the valve for a predetermined opening time based on the hydrogen concentration and then closing the valve. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The objects, features and advantages of the present invention will be further clarified through the following description of the embodiments in conjunction with the accompanying drawings.

[0009] Figure 1 is a schematic structural diagram of a fuel cell system according to an embodiment of the present invention;

[0010] Figure 2 is a schematic diagram showing the relationship between the opening and closing timing of the valve and the hydrogen concentration of the merged gas;

[0011] Figure 3 This is a flowchart illustrating an example of valve control processing executed by the control unit according to a program. DETAILED DESCRIPTION

[0012] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0013] <Structure of the fuel cell system>

[0014] Figure 1 This is a schematic diagram of the fuel cell system 10 of the present invention. The fuel cell system 10 is mounted on a vehicle (fuel cell vehicle). 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 storage 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 (electricity) of the fuel cell stack 12 is supplied to a load (not shown) such as an electric motor.

[0015] The fuel cell stack 12 includes a plurality of power generation cells 22 stacked in one direction. Each power generation cell 22 includes an electrolyte membrane electrode assembly 24 (also simply referred to as an electrode assembly 24) and a pair of separators 26 and 28. The pair of separators 26 and 28 sandwich the electrode assembly 24.

[0016] The electrode structure 24 includes a solid polymer electrolyte membrane 30 (also referred to as the 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 include a gas diffusion layer made of carbon paper or the like. The electrode catalyst layer is formed by uniformly coating the surface of the gas diffusion layer with porous carbon particles. 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.

[0017] An anode channel 36 is formed on the surface of the separator 26 that faces the electrode structure 24. The anode channel 36 is connected to the anode supply channel 40 via the anode inlet 17A. The anode channel 36 is connected to the anode exhaust channel 42 via the first anode outlet 17B. In addition, the anode channel 36 is connected to the second exhaust channel 48 via the second anode outlet 17C. The second anode outlet 17C is at a lower position than the first anode outlet 17B. A cathode channel 38 is formed on the surface of the separator 28 that faces the electrode structure 24. The cathode channel 38 is connected to the cathode supply channel 62 via the cathode inlet 19A. The cathode channel 38 is connected to the cathode exhaust channel 64 via the cathode outlet 19B.

[0018] Anode gas (hydrogen) is supplied to the anode electrode 32. At the anode electrode 32, a catalyst-induced electrode reaction generates hydrogen ions and electrons from hydrogen molecules. The hydrogen ions permeate the electrolyte membrane 30 and migrate toward the cathode electrode 34. The electrons then migrate sequentially toward the negative terminal (not shown) of the fuel cell stack 12, a load such as an electric motor, the positive terminal (not shown) of the fuel cell stack 12, and finally the cathode electrode 34. At the cathode electrode 34, the catalyst causes the hydrogen ions and electrons to react with oxygen contained in the supplied air to generate water.

[0019] The anode system 16 includes various structures for supplying anode gas to the anode electrode 32 and various structures for discharging anode exhaust gas from the anode electrode 32. The anode system 16 includes an anode supply flow path 40, an anode exhaust flow path 42, a circulation flow path 44, a first exhaust flow path 46, and a second exhaust flow path 48. The anode system 16 also includes an ejector 50, an ejector 52, a gas-liquid separator 54, a first exhaust valve 56, and a second exhaust valve 58.

[0020] It should be noted that the anode exhaust flow path 42, the first exhaust flow path 46, and the second exhaust flow path 48 may be collectively referred to as anode exhaust flow paths. In addition, the first exhaust valve 56 and the second exhaust valve 58 may also be collectively referred to as exhaust valves.

[0021] The anode supply flow path 40 connects the outlet of the hydrogen storage tank 14 to the anode inlet 17A. The anode supply flow path 40 is provided with an injector 50, an ejector 52, and a pressure sensor 93. The ejector 52 is positioned closer to the anode inlet 17A than the injector 50. The pressure sensor 93 is positioned closer to the anode inlet 17A than the ejector 52. The pressure sensor 93 detects the pressure of the anode gas.

[0022] The anode exhaust flow channel 42 connects the first anode outlet 17B and the air inlet of the gas-liquid separator 54. The circulation flow channel 44 connects the exhaust port of the gas-liquid separator 54 and the ejector 52. The first discharge flow channel 46 connects the drain port of the gas-liquid separator 54 and the inlet of the diluter 60. A first discharge valve 56 is provided in the first discharge flow channel 46. The second discharge flow channel 48 connects the second anode outlet 17C and a portion of the first discharge flow channel 46 that is downstream of the first discharge valve 56. A second discharge valve 58 is provided in the second discharge flow channel 48. The third discharge flow channel 80 connects the circulation flow channel 44 and the inlet of the diluter 60. A discharge valve 82 is provided in the third discharge flow channel 80.

[0023] The cathode system 18 includes various structures for supplying cathode gas to the cathode electrode 34 and various structures for discharging cathode exhaust gas from the cathode electrode 34. The cathode system 18 includes a cathode supply flow path 62, a cathode exhaust flow path 64, and a bypass flow path 66. The cathode system 18 also includes a compressor 68, a humidifier 70, a first stop valve 74, a second stop valve 76, and a bypass valve 78.

[0024] The cathode supply flow channel 62 connects the air intake port (not shown) and the cathode inlet 19A. A compressor 68, a first stop valve 74, and a flow channel 72A of the humidifier 70 are provided in the cathode supply flow channel 62. The portion of the cathode supply flow channel 62 upstream of the humidifier 70 is used as the cathode supply flow channel 62A. The portion of the cathode supply flow channel 62 downstream of the humidifier 70 is used as the cathode supply flow channel 62B. A pressure sensor 95, a compressor 68, and a first stop valve 74 are provided in the cathode supply flow channel 62A. The first stop valve 74 is arranged at a position closer to the humidifier 70 than the compressor 68. The pressure sensor 95 is arranged on the side of the air intake port (not shown) closer to the compressor 68. The pressure sensor 95 detects the pressure of the inhaled air (atmosphere). The pressure sensor 95 also functions as an atmospheric pressure sensor outside the vehicle.

[0025] The cathode discharge flow channel 64 connects the cathode outlet 19B and the inlet of the diluter 60. A flow channel 72B for the humidifier 70 and a second shutoff valve 76 are provided in the cathode discharge flow channel 64. The portion of the cathode discharge flow channel 64 upstream of the humidifier 70 is designated as the cathode discharge flow channel 64A. The portion of the cathode supply flow channel 62 downstream of the humidifier 70 is designated as the cathode discharge flow channel 64B. A second shutoff valve 76 is provided in the cathode discharge flow channel 64B.

[0026] The exhaust pipe 100 is formed of a hollow tube, for example, approximately 1 meter in length. The inlet 100A of the exhaust pipe 100 is connected to the outlet of the diluter 60. The outlet 100C of the exhaust pipe 100 is located, for example, under the floor in the approximate center of the vehicle. By providing the exhaust pipe 100, the gas diluted in the diluter 60 (the combined gas resulting from the merging of the cathode exhaust gas flowing through the cathode exhaust flow channel 64B and the anode exhaust gas flowing through the anode exhaust flow channel 42, the first exhaust flow channel 46, the second exhaust flow channel 48, and the third exhaust flow channel 80) is discharged to the outside (the atmosphere) in a space away from the users of the vehicle.

[0027] The bypass flow passage 66 connects the cathode supply flow passage 62A and the cathode discharge flow passage 64B. For example, the bypass flow passage 66 connects the portion of the cathode supply flow passage 62A between the compressor 68 and the first stop valve 74 with the portion of the cathode discharge flow passage 64B downstream of the second stop valve 76. A bypass valve 78 is provided in the bypass flow passage 66.

[0028] The cooling system 20 includes various structures for supplying a coolant to the fuel cell stack 12 and various structures for discharging the coolant from the fuel cell stack 12. The cooling system 20 includes a coolant supply flow path 84 and a coolant discharge flow path 86. The cooling system 20 also includes a coolant pump 88, a radiator 90, and a temperature sensor 92.

[0029] A cooling medium flow channel (not shown) for cooling the fuel cell stack 12 is formed within the fuel cell stack 12. A cooling medium supply channel 84 connects the outlet of a radiator 90 to the inlet of the cooling medium flow channel. A cooling medium pump 88 is provided in the cooling medium supply channel 84. A cooling medium discharge channel 86 connects the outlet of the cooling medium flow channel to the inlet of the radiator 90. A temperature sensor 92 is provided in the cooling medium discharge channel 86. The temperature sensor 92 detects the temperature of the cooling medium discharged from the fuel cell stack 12.

[0030] The control device 94 is a computer (e.g., a vehicle ECU). The control device 94 includes a control unit 96 and a storage unit 98. The control unit 96 includes a processing circuit. The processing circuit may be a processor such as a CPU. The processing circuit may also be an integrated circuit such as an ASIC or FPGA. The processor can perform various processes by executing programs stored in the storage unit 98. At least some of the multiple processes may also be performed by an electronic circuit including discrete components.

[0031] 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 the detection signals, the control unit 96 outputs control signals for controlling the valves, the injector 50, the compressor 68, the coolant pump 88, and the like. The valves, the injector 50, the compressor 68, the coolant pump 88, and the like operate in accordance with the control signals.

[0032] The storage unit 98 includes volatile memory and non-volatile memory. Examples of volatile memory include RAM. Volatile memory serves as the processor's working memory. Volatile memory temporarily stores data required for processing or calculations. Non-volatile memory includes ROM, flash memory, etc. Non-volatile memory serves as storage memory. Non-volatile memory stores programs, tables, maps, etc. At least a portion of the storage unit 98 may be incorporated into the aforementioned processor, integrated circuit, etc.

[0033] The nonvolatile memory further stores a first threshold and a second threshold. The first threshold is used to determine whether to perform a nitrogen purge to reduce nitrogen in the anode flow channel 36. The second threshold is used to determine whether the fuel cell stack 12 is under low load or medium to high load. In an embodiment, the first threshold is a hydrogen content (hydrogen concentration) relatively estimated based on the nitrogen content in the anode flow channel 36. The second threshold is the power generation of the fuel cell stack 12.

[0034] Furthermore, the nonvolatile memory stores information indicating the opening time tx and the closing time ty of the second drain valve 58 which periodically repeats opening and closing in a second state described later.

[0035] Information indicating the first threshold value, the second threshold value, the open time tx, and the closed time ty is set in advance by a technician and recorded in the storage unit 98 .

[0036] Fluid Flow

[0037] 1. Anode system

[0038] The flow of fluid in the anode system 16 will be described.

[0039] The injector 50 injects anode gas (hydrogen) from the hydrogen storage tank 14 downstream of 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 exhaust gas. The anode exhaust gas includes hydrogen that has not reacted with oxygen, nitrogen in the cathode gas that has permeated the electrolyte membrane 30, and water generated by the reaction between oxygen and hydrogen.

[0040] The anode off-gas flows through the anode exhaust flow channel 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 channel 44 and is supplied to the ejector 52. In the ejector 52, the anode off-gas merges with the anode gas injected from the ejector 50.

[0041] When the discharge valve 82 of the third discharge flow path 80 is opened, a portion of the anode off-gas flowing through the circulation flow path 44 flows through the third discharge flow path 80 and is discharged to the diluter 60. However, the discharge valve 82 is opened during low load conditions when the target power generation amount, described later, is lower than a second threshold.

[0042] The water separated by 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 flow channel 46 and is discharged to the diluter 60. When the first drain valve 56 is open and no water is present in the gas-liquid separator 54, the anode off-gas from the gas-liquid separator 54 flows through the first drain flow channel 46 and is discharged to the diluter 60.

[0043] When the humidity inside the fuel cell stack 12 is high, water accumulates at the bottom of the anode flow channel 36. When the second drain valve 58 is open, the water accumulated in the anode flow channel 36 flows through the second drain flow channel 48 and the first drain flow channel 46 to be discharged to the diluter 60. When the second drain valve 58 is open without water in the anode flow channel 36, the anode off-gas in the anode flow channel 36 flows through the second drain flow channel 48 and the first drain flow channel 46 to be discharged to the diluter 60.

[0044] 2. Cathode system

[0045] The flow of fluid in the cathode system 18 will be described.

[0046] Compressor 68 draws cathode gas (air) from outside the vehicle and discharges it downstream of cathode supply flow path 62. With first shutoff valve 74 open, cathode gas discharged from compressor 68 flows through cathode supply flow path 62 and is supplied to cathode flow path 38. The cathode gas flows through cathode flow path 38 and is discharged from cathode outlet 19B as cathode exhaust gas. The cathode exhaust gas includes various components contained in air and moisture generated by the reaction of oxygen and hydrogen.

[0047] When the second shutoff valve 76 is open, the cathode off-gas flows through the cathode off-gas flow passage 64 and is discharged to the diluter 60. The cathode off-gas contains moisture, and the moisture in the cathode off-gas is used to humidify the cathode gas in the humidifier 70.

[0048] When the bypass valve 78 is open, the cathode gas flows through the bypass flow path 66 and the cathode exhaust flow path 64 and is discharged to the diluter 60. The bypass flow path 66 is used to reduce the amount of cathode gas supplied to the fuel cell stack 12.

[0049] <Second drain valve and purge valve status>

[0050] 1. First state

[0051] The state in which one of the second drain valve 58 and the purge valve 82 is open is referred to as a first state. The reason why the control unit 96 controls the second drain valve 58 and the purge valve 82 to be in the first state will be described.

[0052] The control unit 96 suppresses the decrease in the hydrogen concentration in the anode flow channel 36 and controls the hydrogen concentration to be maintained at a constant level or higher. The following (a) and (b) are possible reasons for the decrease in the hydrogen concentration in the anode flow channel 36.

[0053] (a) The hydrogen in the anode flow channel 36 is consumed due to power generation by the fuel cell stack 12 .

[0054] (b) Since nitrogen contained in the cathode gas permeates the electrolyte membrane 30 and penetrates into the anode flow path 36 , the nitrogen concentration in the anode flow path 36 relatively increases.

[0055] For the above reason (a), the control unit 96 controls the injector 50. This increases the hydrogen content in the anode flow channel 36, and the hydrogen concentration in the anode flow channel 36 increases. For the above reason (b), the control unit 96 opens the second purge valve 58 or the purge valve 82. This causes the anode off-gas containing nitrogen to be discharged from the anode flow channel 36. Hydrogen, as the anode gas, is appropriately supplied to the anode flow channel 36, and thus the hydrogen concentration in the anode flow channel 36 increases relatively.

[0056] Under medium to high load conditions where the target power generation amount is greater than the second threshold, opening the second purge valve 58 is preferred to opening the drain valve 82 for the following reasons to suppress a decrease in the hydrogen concentration in the anode flow path 36 (in other words, an increase in the nitrogen concentration).

[0057] In the embodiment, as an example, the flow rate of nitrogen discharged through the purge valve 82 is configured to be smaller than the flow rate of nitrogen discharged through the second purge valve 58. Furthermore, the flow rate of nitrogen discharged through the second purge valve 58 is configured to be larger than the maximum flow rate of nitrogen permeating from the cathode flow path 38 to the anode flow path 36.

[0058] Generally, when the fuel cell stack 12 reaches a high temperature during medium to high load conditions, the flow rate of nitrogen permeating from the cathode flow channel 38 to the anode flow channel 36 increases (in other words, the nitrogen increase rate increases). Therefore, the flow rate of nitrogen permeating from the cathode flow channel 38 to the anode flow channel 36 may be greater than the flow rate of nitrogen discharged through the purge valve 82.

[0059] Therefore, during medium to high load conditions, where the flow rate of nitrogen permeating from the cathode flow channel 38 to the anode flow channel 36 may be greater than the flow rate of nitrogen discharged through the purge valve 82, the second purge valve 58 is opened to discharge the nitrogen rather than the purge valve 82, thereby avoiding insufficient nitrogen discharge. As a result, the hydrogen concentration in the fuel cell stack 12 can be maintained, allowing the vehicle to continue traveling.

[0060] The rate of increase in nitrogen in the anode flow channel 36 depends on factors such as the cathode pressure, the temperature of the coolant in the cooling system 20, and the humidity of the electrolyte membrane 30. These factors are determined by the generated current of the fuel cell stack 12. The generated current of the fuel cell stack 12 is determined by the target power generation used by the control unit 96. In other words, the rate of increase in nitrogen in the anode flow channel 36 is related to the target power generation. Therefore, the control unit 96 determines which of the second drain valve 58 and the purge valve 82 to open based on the target power generation. For example, when the target power generation is above a second threshold, the purge valve 82 is closed and the second drain valve 58 is opened. When the target power generation is below the second threshold, the purge valve 82 is opened and the second drain valve 58 is closed.

[0061] 2. Second state

[0062] The state in which the opening and closing of the discharge valve 82 are repeated periodically is referred to as a second state. The reason why the control unit 96 controls the discharge valve 82 to the second state will be described.

[0063] The reason why the hydrogen in the anode off-gas is diluted in the diluter 60 is to prevent ignition of hydrogen contained in the gas discharged to the outside (into the atmosphere).

[0064] When the target power generation amount is less than the second threshold, the amount of cathode off-gas is sufficiently greater than that of anode off-gas. Therefore, the amount of the combined gas flowing from the diluter 60 to the exhaust pipe 100 is substantially determined by the amount of cathode off-gas.

[0065] When the control unit 96 opens the discharge valve 82, the anode exhaust gas containing nitrogen and hydrogen merges with the cathode exhaust gas in the diluter 60. When the gases merge, the hydrogen concentration of the merged gas flowing through the discharge pipe 100 increases. Here, the rate of increase in the hydrogen concentration of the merged gas is slower near the outlet 100C of the discharge pipe 100 than near the inlet 100A of the discharge pipe 100. Therefore, in the embodiment, the control unit 96 opens the discharge valve 82 to discharge the anode exhaust gas from the anode flow channel 36, and before the hydrogen concentration at the outlet 100C of the discharge pipe 100, which increases due to the opening of the discharge valve 82 (corresponding to the second hydrogen concentration described later), reaches a specified value (an upper limit value that does not cause ignition), the control unit 96 closes the discharge valve 82.

[0066] When the hydrogen concentration at the outlet 100C decreases due to the controller 96 closing the purge valve 82, the controller 96 reopens the purge valve 82. By repeating this opening and closing operation of the purge valve 82, the controller 96 suppresses a decrease in the hydrogen concentration in the anode flow channel 36 (in other words, an increase in the nitrogen concentration in the anode flow channel 36).

[0067] Figure 2 This is a schematic diagram showing the relationship between the opening and closing timing of the purge valve 82 and the hydrogen concentration of the combined gas. The horizontal axis represents time, and the upper vertical axis represents the open and closed state of the purge valve 82. Furthermore, the lower vertical axis represents the hydrogen concentration of the combined gas. The solid line L100A represents the hydrogen concentration at the inlet 100A of the discharge pipe 100, and the dashed line L100C represents the hydrogen concentration (second hydrogen concentration) at the outlet 100C of the discharge pipe 100. The threshold value on the lower vertical axis corresponds to the aforementioned specified value.

[0068] When the discharge valve 82 is opened at time t0, the hydrogen concentration (second hydrogen concentration) at the outlet 100C of the discharge pipe 100 increases at a rate slower than the rate of increase of the hydrogen concentration at the inlet 100A of the discharge pipe 100. When the discharge valve 82 is closed at time t1, the hydrogen concentration (second hydrogen concentration) at the outlet 100C of the discharge pipe 100 decreases at a rate slower than the rate of decrease of the hydrogen concentration at the inlet 100A of the discharge pipe 100. It should be noted that the rate of decrease is faster than the rate of increase. Therefore, the closing time ty of the discharge valve 82 can be shorter than the opening time tx. Similarly, the discharge valve 82 repeats opening and closing with a period T.

[0069] In the second state, while the purge valve 82 is repeatedly opened and closed, the controller 96 controls the opening and closing timing of the purge valve 82 so as to prevent the hydrogen concentration (second hydrogen concentration) at the outlet 100C of the discharge pipe 100 from reaching the threshold value (predetermined value). Therefore, the amount of dilution air supplied to the diluter 60 does not need to increase, thereby reducing the power consumption of the compressor 68.

[0070] The opening time tx of the discharge valve 82, which repeatedly opens and closes in the second state, is determined by a technician for each hydrogen concentration, for example, taking into account the flow rate of the combined gas at the outlet 100C of the discharge pipe 100, and is pre-recorded in the storage unit 98. More specifically, the flow rate at the outlet 100C of the discharge pipe 100 is determined based on information indicating the shape of the discharge pipe 100 and the amount of combined gas flowing through the discharge pipe 100. For example, the amount of combined gas is calculated by subtracting the amount of oxygen consumed during power generation from the amount of cathode gas supplied to the cathode supply flow channel 62. For example, a technician conducts a ignition test at the outlet 100C of the discharge pipe 100 while varying the combination of the hydrogen concentration and flow rate of the combined gas. Based on the test results, the opening time tx and the shorter closing time ty of the discharge valve 82 are determined for each hydrogen concentration (second hydrogen concentration). Furthermore, during operation of the fuel cell system 10, the control unit 96 reads the opening time tx and closing time ty corresponding to the hydrogen concentration (second hydrogen concentration) and records them in the storage unit 98.

[0071] It should be noted that when the control unit 96 uses the hydrogen concentration (second hydrogen concentration), information representing the shape of the exhaust pipe 100, and the amount of cathode gas supplied to the cathode supply flow channel 62 to calculate the necessary opening time tx and closing time ty in real time during the operation of the fuel cell system 10, the information representing the shape of the exhaust pipe 100, the calculation formula for deriving the opening time tx and the closing time ty, etc. can be recorded in the storage unit 98 in advance.

[0072] 3. The third state

[0073] The state in which the second drain valve 58 and the discharge valve 82 are closed is referred to as a third state. The reason why the control unit 96 controls the second drain valve 58 and the discharge valve 82 to be in the third state will be described.

[0074] As described above, the control unit 96 suppresses the decrease in the hydrogen concentration (first hydrogen concentration) in the anode flow channel 36 to maintain the hydrogen concentration (first hydrogen concentration) above a certain level. If the hydrogen concentration (first hydrogen concentration) in the anode flow channel 36 does not decrease, there is no reason to set the second drain valve 58 and the purge valve 82 to the first and second positions described above in order to discharge the anode off-gas containing nitrogen from the anode flow channel 36 (nitrogen purge). Therefore, the control unit 96 closes the second drain valve 58 and the purge valve 82 (i.e., sets them to the third position). As a result, the anode off-gas flows sequentially through the anode exhaust flow channel 42, the gas-liquid separator 54, the circulation flow channel 44, and the ejector 52, returning to the anode supply flow channel 40.

[0075] <Explanation of the flow chart>

[0076] Figure 3This is a flowchart of an example of valve control processing executed by the control unit 96 according to a predetermined program. The control unit 96 repeatedly performs the following operations during the operation of the fuel cell system 10: Figure 2 The valve control process is shown.

[0077] In S1 (S: processing step), the control unit 96 estimates the first hydrogen concentration. More specifically, the control unit 96 estimates the nitrogen content in the anode flow channel 36. By multiplying the nitrogen partial pressure difference between the anode flow channel 36 and the cathode flow channel 38 by the nitrogen permeability coefficient, the nitrogen content (nitrogen permeation rate) transmitted from the cathode flow channel 38 to the anode flow channel 36 can be calculated. The temperature inside the fuel cell stack 12 is correlated with the nitrogen permeability coefficient. In addition, the humidity inside the fuel cell stack 12 is correlated with the nitrogen permeability coefficient. The control unit 96 controls the various components of the fuel cell system 10 so that, for example, the humidity inside the fuel cell stack 12 reaches 100%. In this case, the nitrogen permeability coefficient can be estimated based on the temperature inside the fuel cell stack 12. In an embodiment, the control unit 96 calculates the temperature inside the fuel cell stack 12 based on the temperature of the cooling medium detected by the temperature sensor 92. In addition, the control unit 96 estimates the nitrogen content in the anode flow channel 36 based on the temperature inside the fuel cell stack 12. The control unit 96 relatively estimates the hydrogen content (first hydrogen concentration) based on the estimated nitrogen content. Various estimation methods are stored in the storage unit 98 .

[0078] It should be noted that the temperature inside the fuel cell stack 12 can also be calculated based on the temperature of the cathode exhaust gas flowing through the cathode exhaust flow channel 64 or the temperature of the anode exhaust gas flowing through the anode exhaust flow channel 42. Alternatively, the temperature inside the fuel cell stack 12 can be directly detected by a temperature sensor or the like. After completing step S1, the control unit 96 proceeds to step S2.

[0079] In S2, the control unit 96 determines whether the first hydrogen concentration is greater than the first threshold. If the first hydrogen concentration estimated relatively based on the estimated nitrogen content in S1 is higher than the first threshold, the control unit 96 determines S2 as positive (S2: Yes) and proceeds to S3. If the first hydrogen concentration is below the first threshold, the control unit 96 determines S2 as negative (S2: No) and proceeds to S4. It should be noted that the process may also proceed to S4 if the first hydrogen concentration is equal to the second threshold.

[0080] Entering S3 means that there is no reason to discharge nitrogen from the anode flow channel 36. In S3, the control unit 96 controls the discharge valve 82 and the second discharge valve 58 to the third state of both valves being closed, and ends Figure 3If the purge valve 82 is already closed, the control unit 96 maintains the purge valve 82 in its state. On the other hand, if the purge valve 82 is open, the control unit 96 closes the purge valve 82. If the second purge valve 58 is already closed, the control unit 96 maintains the second purge valve 58 in its state. On the other hand, if the second purge valve 58 is open, the control unit 96 closes the second purge valve 58. The anode exhaust gas flows sequentially through the anode exhaust flow channel 42, the gas-liquid separator 54, the circulation flow channel 44, and the ejector 52, returning to the anode supply flow channel 40.

[0081] In S4, the control unit 96 obtains the target power generation amount. As described above, the target power generation amount is used to determine whether the fuel cell stack 12 is under low load or medium to high load. During operation of the fuel cell system 10, the control unit 96 calculates the target power generation amount and controls various components so that the power generation amount of the fuel cell stack 12 reaches the target power generation amount. The control unit 96 uses the calculated target power generation amount to control the power generation amount of the fuel cell stack 12. After completing S4, the control unit 96 proceeds to S5.

[0082] In S5, the control unit 96 determines whether the target power generation amount is less than the second threshold value. If the target power generation amount is less than the second threshold value (low load), the control unit 96 determines S3 as affirmative (S3: Yes) and proceeds to S6. If the target power generation amount is greater than the second threshold value (medium to high load), the control unit 96 determines S5 as negative (S5: No) and proceeds to S8. It should be noted that the process may also proceed to S8 if the target power generation amount is equal to the second threshold value.

[0083] In S6, the control unit 96 estimates the second hydrogen concentration. More specifically, the hydrogen content of the combined gas flowing from the diluter 60 to the discharge pipe 100 is the sum of the hydrogen content discharged from the first discharge flow channel 46, the second discharge flow channel 48, and the third discharge flow channel 80, and the hydrogen content (permeated hydrogen amount) that permeates from the anode flow channel 36 to the cathode flow channel 38.

[0084] The hydrogen content discharged from the first exhaust flow channel 46, the second exhaust flow channel 48, and the third exhaust flow channel 80 can be calculated based on the anode gas pressure (the value detected by the pressure sensor 93), the atmospheric pressure (the value detected by the pressure sensor 95), the gas density of the anode flow channel 36, and the like. The gas density of the anode flow channel 36 can be calculated based on the anode gas pressure, the temperature inside the fuel cell stack 12 (calculated based on the value detected by the temperature sensor 92), and the average molecular weight. The average molecular weight is calculated based on the anode gas pressure and the hydrogen partial pressure. For example, the hydrogen partial pressure is calculated assuming that only water vapor and hydrogen are present.

[0085] The amount of permeated hydrogen can be calculated based on the pressure of the anode gas and the hydrogen permeation characteristics inside the fuel cell stack 12 (which can be estimated based on the temperature inside the fuel cell stack 12). After completing S6, the control unit 96 proceeds to S7.

[0086] In S7, the control unit 96 periodically controls the opening and closing of at least one of the discharge valve 82 and the second drain valve 58 (here, the discharge valve 82 exemplified as the second state), and ends. Figure 3 A portion of the anode exhaust gas flows through the third exhaust flow channel 80 and is directly discharged to the diluter 60.

[0087] Note that the control unit 96 reads information indicating the opening time tx and the closing time ty corresponding to the second hydrogen concentration estimated in S6 from the storage unit 98 , and controls the opening and closing of the second purge valve 58 .

[0088] In step S8, which is entered after the negative judgment of step S5, the control unit 96 controls the third state in which at least one of the discharge valve 82 and the second discharge valve 58 is opened. For example, the discharge valve 82 is closed and the second discharge valve 58 is opened, and the process ends. Figure 3 If the purge valve 82 is already closed, the controller 96 maintains the purge valve 82 in its state. On the other hand, if the purge valve 82 is open, the controller 96 closes the purge valve 82. If the second purge valve 58 is already open, the controller 96 maintains the second purge valve 58 in its state. On the other hand, if the second purge valve 58 is closed, the controller 96 opens the second purge valve 58. A portion of the anode exhaust gas flows through the second exhaust flow passage 48 and is discharged directly to the diluter 60.

[0089] The above-described embodiment has the following effects.

[0090] (1) The fuel cell system 10 comprises: a fuel cell stack 12, which generates electricity using anode gas from an anode flow channel 36 and cathode gas from a cathode flow channel 38; an anode supply flow channel 40, which supplies anode gas to the anode flow channel 36; a cathode supply flow channel 62, which supplies cathode gas to the cathode flow channel 38; an anode exhaust flow channel (anode exhaust flow channel 42, a first exhaust flow channel 46, a second exhaust flow channel 48, a third exhaust flow channel 80), which supplies anode exhaust gas as an anode exhaust fluid discharged from the anode flow channel 36; a cathode exhaust flow channel 64 (64A, 64B), which supplies cathode exhaust gas as a cathode exhaust fluid discharged from the cathode flow channel 38; a dilution The diluter 60 includes a discharge pipe 100 for directing the combined gas as the combined fluid merged in the diluter 60 to the outside; a discharge valve 82, a second discharge valve 58, and a first discharge valve 56, which serve as anode discharge valves and control the flow of the anode discharge gas toward the diluter 60; and a control unit 96 for controlling the opening and closing of the anode discharge valves (the discharge valve 82, the second discharge valve 58, and the first discharge valve 56). The control unit 96 obtains a second hydrogen concentration as the hydrogen concentration of the merged gas, and when the power generation of the fuel cell stack 12 is below the second threshold value as the prescribed power generation threshold value, controls the opening and closing of the anode exhaust valve (for example, the discharge valve 82) so as to repeat the opening and closing action of opening for a prescribed opening time tx based on the second hydrogen concentration and then closing.

[0091] Because of this structure, for example, when the fuel cell stack 12 is under low load, the second hydrogen concentration of the merged gas can be suppressed below the specified value by repeatedly opening and closing the release valve 82 with an opening and closing time based on the second hydrogen concentration of the merged gas, thereby eliminating the need to increase the amount of air for dilution.

[0092] (2) In the fuel cell system 10 of (1) above, the control unit 96 further obtains a first hydrogen concentration as the hydrogen concentration of the anode exhaust gas, and controls the opening and closing of the anode exhaust valve (for example, the discharge valve 82) when the first hydrogen concentration is below a first threshold value as a prescribed concentration threshold value and the power generation of the fuel cell stack 12 is below a second threshold value, so as to repeat the opening and closing action of opening for an opening time based on the second hydrogen concentration of the confluent gas and then closing for a closing time shorter than the opening time.

[0093] Because of this structure, when the hydrogen concentration of the anode exhaust gas decreases and the fuel cell stack 12 is under low load, the purge valve 82 is opened and closed according to the second hydrogen concentration of the merged gas, thereby eliminating the need to increase the amount of air used for dilution and enabling the second hydrogen concentration of the merged gas to be suppressed below the specified value.

[0094] (3) In the fuel cell system 10 of (1) above, the control unit 96 further obtains a first hydrogen concentration as the hydrogen concentration of the anode exhaust gas, and opens the anode exhaust valve (second exhaust valve 58) when the hydrogen concentration of the anode exhaust gas is below a first threshold value as a prescribed concentration threshold value and the power generation of the fuel cell stack 12 exceeds a second threshold value.

[0095] With this configuration, when the hydrogen concentration of the anode off-gas decreases and the fuel cell stack 12 is at a medium to high load, the second purge valve 58 is opened to perform nitrogen purge, thereby suppressing a decrease in the hydrogen concentration in the anode flow path 36 .

[0096] (4) In the fuel cell system 10 of (1) to (3) above, the control unit 96 obtains the hydrogen concentration at the outlet 100C of the discharge pipe 100 as the hydrogen concentration of the merged gas (the second hydrogen concentration), and also obtains the flow rate of the merged gas at the outlet 100C of the discharge pipe 100, and determines the opening time tx of the discharge valve 82 based on the flow rate at the outlet 100C and the second hydrogen concentration.

[0097] Due to this configuration, when the fuel cell stack 12 is under low load, an appropriate opening time tx can be determined for each combination of the second hydrogen concentration and flow rate at the outlet 100C of the discharge pipe 100. Consequently, the purge valve 82 can be opened and closed by repeating the opening and closing operation of opening for the opening time tx and then closing. As a result, the hydrogen concentration (second hydrogen concentration) of the combined gas can be suppressed to below a predetermined value without increasing the amount of dilution air.

[0098] (5) In the fuel cell system 10 of (1) to (3) above, the control unit 96 obtains the second hydrogen concentration as the hydrogen concentration of the merged gas at the outlet 100C of the discharge pipe 100, and determines the opening time tx of the discharge valve 82 based on the shape of the discharge pipe 100 and the second hydrogen concentration at the outlet 100C.

[0099] This configuration allows for determining an appropriate opening time tx for each second hydrogen concentration of the combined gas at outlet 100C of discharge pipe 100, taking into account the shape of discharge pipe 100 (in other words, reflecting the flow velocity at outlet 100C estimated based on the shape). Consequently, the purge valve 82 can be opened and closed by repeating an opening and closing operation of opening for the opening time tx and then closing. As a result, the hydrogen concentration (second hydrogen concentration) of the combined gas can be suppressed to below a predetermined value without increasing the amount of dilution air.

[0100] (6) In the fuel cell system 10 of (1) to (3) above, the control unit 96 obtains the second hydrogen concentration as the hydrogen concentration of the merged gas at the outlet 100C of the discharge pipe 100, and determines the opening time tx as a time shorter than the time it takes for the second hydrogen concentration at the outlet 100C to rise to a predetermined value after the discharge valve 82 is opened.

[0101] This configuration allows for an opening time tx to be shorter than the time it takes for the second hydrogen concentration at outlet 100C of discharge pipe 100 to rise to, for example, a predetermined value corresponding to ignition after opening discharge valve 82. Consequently, discharge valve 82 can be opened and closed repeatedly, opening for the opening time tx and then closing. As a result, the hydrogen concentration (second hydrogen concentration) of the combined gas can be suppressed to below a predetermined value without increasing the amount of dilution air.

[0102] The above-described embodiment can be modified in various ways. Modifications will be described below.

[0103] (Variation 1)

[0104] In the above embodiment, in the second state, only the discharge valve 82 among the first discharge valve 56, the second discharge valve 58, and the purge valve 82 is periodically opened and closed. Alternatively, in the second state, not only the discharge valve 82 but also at least one of the first discharge valve 56, the second discharge valve 58, and the purge valve 82 may be periodically opened and closed. In the second state, appropriate opening time tx and closing time ty may be determined for each valve depending on which valve is subject to periodic opening and closing control.

[0105] In the first state, an example has been described in which only the second drain valve 58 is opened among the first drain valve 56, the second drain valve 58, and the purge valve 82. Alternatively, in the first state, a configuration may be employed in which at least one of the first drain valve 56, the second drain valve 58, and the purge valve 82 is opened, not limited to the second drain valve 58.

[0106] (Variation 2)

[0107] In the above embodiment, a fuel cell system 10 is illustrated that includes a first exhaust flow channel 46, a second exhaust flow channel 48, a third exhaust flow channel 80 as an anode exhaust flow channel, and a first exhaust valve 56, a second exhaust valve 58, and a purge valve 82 as an anode exhaust valve for controlling the flow of anode exhaust gas flowing through the anode exhaust flow channel. However, the present invention can also be applied to a fuel cell system that has a connecting flow channel connecting the anode exhaust flow channel and a cathode supply flow channel and an on-off valve for opening and closing the connecting flow channel.

[0108] Furthermore, the present invention can also be applied to a fuel cell system that does not include either the first purge valve 56 or the second purge valve 58 in the above-described embodiment.

[0109] One or more of the above-described embodiments and modifications may be arbitrarily combined, and modifications may be combined with each other.

[0110] According to the present invention, when the power generation amount is less than a predetermined threshold, it is not necessary to increase the amount of air used to dilute the gas to be exhausted. Suppressing the increase in the amount of air contributes to improving energy efficiency.

[0111] The present invention has been described above with reference to preferred embodiments. However, it should be understood by those skilled in the art that various modifications and changes can be made without departing from the scope of the claims.

Claims

1. A fuel cell system (10), characterized in that: have: A fuel cell stack (12) that generates electricity using anode gas from an anode flow channel (36) and cathode gas from a cathode flow channel (38); an anode supply flow channel (40) for supplying the anode gas to the anode flow channel (36); a cathode supply channel (62) for supplying the cathode gas to the cathode channel (38); an anode exhaust flow channel (42, 46, 48, 80) for allowing the anode exhaust fluid discharged from the anode flow channel (36) to flow; a cathode exhaust flow channel (64) for allowing cathode exhaust fluid discharged from the cathode flow channel (38) to flow; a fluid confluence portion (60) for confluencing the anode exhaust fluid flowing through the anode exhaust flow channel (42, 46, 48, 80) and the cathode exhaust fluid flowing through the cathode exhaust flow channel (64); a discharge pipe (100) for guiding the merged fluids merged at the fluid merging portion (60) to the outside; an anode exhaust valve (82, 58, 56) that controls the flow of the anode exhaust fluid to the fluid junction (60); and a control unit (96) for controlling the opening and closing of the anode exhaust valve (82, 58, 56), The control unit (96) obtains the hydrogen concentration of the merged fluid, When the power generation amount of the fuel cell stack (12) is below a specified power generation threshold, the control unit (96) controls the opening and closing of the anode exhaust valve (82, 58, 56) so as to repeat the opening and closing action of opening for a specified opening time based on the hydrogen concentration and then closing.

2. The fuel cell system (10) according to claim 1, characterized in that The control unit (96) further obtains the hydrogen concentration of the anode exhaust fluid and, when the hydrogen concentration of the anode exhaust fluid is below a predetermined concentration threshold and the power generation of the fuel cell stack (12) is below the power generation threshold, controls the opening and closing of the anode exhaust valve (82, 58, 56) by repeatedly performing the opening and closing action of opening for the opening time based on the hydrogen concentration of the merged fluid and then closing for a closing time shorter than the opening time.

3. The fuel cell system (10) according to claim 1, characterized in that The control unit (96) further obtains the hydrogen concentration of the anode exhaust fluid and opens the anode exhaust valve (82, 58, 56) when the hydrogen concentration of the anode exhaust fluid is below a predetermined concentration threshold and the power generation of the fuel cell stack (12) exceeds the power generation threshold.

4. The fuel cell system (10) according to any one of claims 1 to 3, characterized in that The control unit (96) obtains the hydrogen concentration at the outlet of the discharge pipe (100) as the hydrogen concentration of the combined fluid, and also obtains the flow rate of the combined fluid at the outlet (100C) of the discharge pipe (100). The control unit (96) determines the opening time of the anode exhaust valve (82, 58, 56) based on the flow rate and the hydrogen concentration at the outlet (100C).

5. The fuel cell system (10) according to any one of claims 1 to 3, characterized in that The control unit (96) obtains the hydrogen concentration of the combined fluid at the outlet (100C) of the discharge pipe (100), The control unit (96) determines the opening time of the anode discharge valve (82, 58, 56) based on the shape of the discharge pipe (100) and the hydrogen concentration at the outlet (100C).

6. The fuel cell system (10) according to any one of claims 1 to 3, characterized in that The control unit (96) obtains the hydrogen concentration of the combined fluid at the outlet (100C) of the discharge pipe (100), The control unit (96) determines, as the opening time, a time shorter than the time it takes for the hydrogen concentration at the outlet (100C) to rise to a predetermined value after the anode discharge valve (82, 58, 56) is opened.

7. The fuel cell system (10) according to any one of claims 1 to 3, characterized in that The control unit (96) obtains the hydrogen concentration of the combined fluid based on the hydrogen content of the anode exhaust fluid flowing through the anode exhaust flow channel (42, 46, 48, 80) and the hydrogen content transmitted from the anode flow channel (36) to the cathode flow channel (38).

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