Fuel cell system

Abstract

To eliminate the necessity of increasing the amount of air to dilute gas to be exhausted when the amount of power generation is smaller than a predetermined threshold.SOLUTION: A control unit 96 of a fuel cell system 10 acquires the concentration of hydrogen in a merged fluid of an anode discharge fluid flowing through anode discharge channels (42, 46, 48, 80) and a cathode discharge fluid flowing through cathode discharge channels 64 (64A, 64B), and when the amount of power generated by a fuel cell stack 12 is equal to or less than a predetermined power generation threshold, controls opening and closing of anode discharge valves 56, 58, 82 to repeat an opening and closing operation in which the anode discharge valves open by a predetermined opening time based on the hydrogen concentration and then close.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] In a fuel cell system, when discharging anode off-gas containing hydrogen, nitrogen, moisture, etc. to the outside (atmosphere) of the fuel cell system, the gas to be discharged is diluted using air (nitrogen, oxygen, etc.) inhaled from the outside. Therefore, when a large amount of air is required for dilution, the power consumption of a compressor or the like that sends air increases. Thus, a system that achieves both improved fuel efficiency and maintenance of hydrogen concentration has been proposed (see Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the technique described in Patent Document 1, even when the target power generation amount is smaller than a predetermined threshold value, in order to suppress the hydrogen concentration of the gas to be discharged to a predetermined concentration, the amount of air for dilution may be increased (in other words, the rotation amount of a compressor or the like may be increased).

Means for Solving the Problems

[0005] One aspect of the present invention is a fuel cell system comprising: a fuel cell stack that generates electricity using anode gas in an anode channel and cathode gas in a cathode channel; an anode supply channel that supplies anode gas to the anode channel; a cathode supply channel that supplies cathode gas to the cathode channel; an anode discharge channel through which anode discharge fluid discharged from the anode channel flows; a cathode discharge channel through which cathode discharge fluid discharged from the cathode channel flows; a fluid confluence section that combines the anode discharge fluid that has flowed through the anode discharge channel and the cathode discharge fluid that has flowed through the cathode discharge channel; a discharge pipe that guides the combined fluid that has been combined at the fluid confluence section to the outside; an anode discharge valve that controls the flow of anode discharge fluid toward the fluid confluence section; and a control unit that controls the opening and closing of the anode discharge valve, wherein the control unit acquires the hydrogen concentration of the combined fluid, and when the amount of electricity generated by the fuel cell stack is less than or equal to a predetermined power generation threshold, the hydrogen concentration It was determined that it would not reach the predetermined value. The anode discharge valve is controlled to repeatedly open and close for a predetermined period of time. [Effects of the Invention]

[0006] According to the present invention, when the amount of power generated is less than a predetermined threshold, it becomes unnecessary to increase the amount of air used to dilute the gas to be discharged. Reducing the amount of air used contributes to energy efficiency. [Brief explanation of the drawing]

[0007] [Figure 1] A schematic diagram of a fuel cell system according to an embodiment of the present invention. [Figure 2] A schematic diagram showing the relationship between valve opening and closing timing and hydrogen concentration in the combined gas. [Figure 3] A flowchart illustrating an example of valve control processing performed by the control unit based on a program. [Modes for carrying out the invention]

[0008] Embodiments of the invention will be described below with reference to the drawings. <Fuel cell system configuration> Figure 1 is a schematic diagram of the fuel cell system 10 according to the present invention. The fuel cell system 10 is mounted on a vehicle (fuel cell vehicle). Separately, the fuel cell system 10 can also be mounted on, for example, ships, aircraft, robots, etc. The fuel cell system 10 has a fuel cell stack 12, a hydrogen tank 14, an anode system 16, a cathode system 18, and a cooling system 20. The fuel cell system 10 also has a control device 94. The output (power) of the fuel cell stack 12 is supplied to a load (not shown) such as a motor.

[0009] The fuel cell stack 12 has a plurality of power generation cells 22 stacked in one direction. Each power generation cell 22 has an electrolyte membrane / electrode structure 24 (also simply called an electrode structure 24) and a pair of separators 26, 28. The pair of separators 26, 28 sandwich the electrode structure 24.

[0010] The electrode structure 24 includes a solid polymer electrolyte membrane 30 (also simply called 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 have a gas diffusion layer made of carbon paper or the like. An 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.

[0011] An anode channel 36 is formed on the surface of the separator 26 facing 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 discharge channel 42 via the first anode outlet 17B. The anode channel 36 is also connected to the second drain channel 48 via the second anode outlet 17C. The second anode outlet 17C is located lower than the first anode outlet 17B. A cathode channel 38 is formed on the surface of the separator 28 facing 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 discharge channel 64 via the cathode outlet 19B.

[0012] Anode gas (hydrogen) is supplied to the anode electrode 32. At the anode electrode 32, hydrogen ions and electrons are generated from hydrogen molecules by an electrode reaction mediated by a catalyst. The hydrogen ions permeate the electrolyte membrane 30 and move to the cathode electrode 34. The electrons move in the following order: the negative electrode terminal (not shown) of the fuel cell stack 12, a load such as a motor, the positive electrode terminal (not shown) of the fuel cell stack 12, and the cathode electrode 34. At the cathode electrode 34, water is produced by the reaction of hydrogen ions and electrons with oxygen contained in the supplied air through the action of a catalyst.

[0013] The anode system 16 has components for supplying anode gas to the anode electrode 32 and components for discharging anode off-gas from the anode electrode 32. The anode system 16 has an anode supply channel 40, an anode discharge channel 42, a circulation channel 44, a first drain channel 46, and a second drain channel 48. The anode system 16 also has an injector 50, an ejector 52, a gas-liquid separator 54, a first drain valve 56, and a second drain valve 58. The anode discharge channel 42, the first drain channel 46, and the second drain channel 48 are sometimes collectively referred to as the anode discharge channel. Furthermore, the first drain valve 56 and the second drain valve 58 are sometimes collectively referred to as the drain valve.

[0014] The anode supply channel 40 connects the outlet of the hydrogen tank 14 to the anode inlet 17A. The anode supply channel 40 is equipped 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.

[0015] The anode discharge channel 42 connects the first anode outlet 17B to the intake port of the gas-liquid separator 54. The circulation channel 44 connects the exhaust port of the gas-liquid separator 54 to the ejector 52. The first drain channel 46 connects the drain port of the gas-liquid separator 54 to the inlet of the diluent 60. The first drain channel 46 is provided with a first drain valve 56. The second drain channel 48 connects the second anode outlet 17C to the portion of the first drain channel 46 downstream of the first drain valve 56. The second drain channel 48 is provided with a second drain valve 58. The third drain channel 80 connects the circulation channel 44 to the inlet of the diluent 60. The third drain channel 80 is provided with a bleed valve 82.

[0016] The cathode system 18 has components for supplying cathode gas to the cathode electrode 34 and components for discharging cathode off gas from the cathode electrode 34. The cathode system 18 has a cathode supply channel 62, a cathode discharge channel 64, and a bypass channel 66. The cathode system 18 also has a compressor 68, a humidifier 70, a first sealing valve 74, a second sealing valve 76, and a bypass valve 78.

[0017] The cathode supply passage 62 communicates an air intake port (not shown) with the cathode inlet 19A. The compressor 68, the first shutoff valve 74, and the passage 72A of the humidifier 70 are provided in the cathode supply passage 62. The upstream portion of the cathode supply passage 62 from the humidifier 70 is defined as the cathode supply passage 62A. The downstream portion of the cathode supply passage 62 from the humidifier 70 is defined as the cathode supply passage 62B. The pressure sensor 95, the compressor 68, and the first shutoff valve 74 are provided in the cathode supply passage 62A. The first shutoff valve 74 is disposed closer to the humidifier 70 than the compressor 68. The pressure sensor 95 is disposed on the air intake port (not shown) side of the compressor 68. The pressure sensor 95 detects the pressure of the intake air (atmospheric air). The pressure sensor 95 also functions as an atmospheric pressure sensor outside the vehicle.

[0018] The cathode discharge passage 64 communicates the cathode outlet 19B with the inlet of the diluter 60. The passage 72B of the humidifier 70 and the second shutoff valve 76 are provided in the cathode discharge passage 64. The upstream portion of the cathode discharge passage 64 from the humidifier 70 is defined as the cathode discharge passage 64A. The downstream portion of the cathode supply passage 62 from the humidifier 70 is defined as the cathode discharge passage 64B. The second shutoff valve 76 is provided in the cathode discharge passage 64B.

[0019] The discharge pipe 100 is composed of, for example, a hollow pipe with a length of about 1 m. The inlet 100A of the discharge pipe 100 is connected to the outlet of the diluter 60. The outlet 100C of the discharge pipe 100 is located, for example, under the floor at approximately the center of the vehicle. By providing the discharge pipe 100, the gas diluted by the diluter 60 (the combined gas of the cathode off-gas flowing through the cathode discharge passage 64B and the anode off-gas flowing through the anode discharge passage 42, the first drain passage 46, the second drain passage 48, and the third drain passage 80) is discharged to the outside (into the atmosphere) in a space away from the vehicle user.

[0020] The bypass passage 66 communicates the cathode supply passage 62A and the cathode discharge passage 64B. For example, the bypass passage 66 communicates a portion between the compressor 68 and the first shutoff valve 74 in the cathode supply passage 62A and a portion downstream of the second shutoff valve 76 in the cathode discharge passage 64B. A bypass valve 78 is provided in the bypass passage 66.

[0021] The cooling system 20 has each component for supplying a refrigerant to the fuel cell stack 12 and each component for discharging the refrigerant from the fuel cell stack 12. The cooling system 20 has a refrigerant supply passage 84 and a refrigerant discharge passage 86. Further, the cooling system 20 has a refrigerant pump 88, a radiator 90, and a temperature sensor 92.

[0022] Inside the fuel cell stack 12, a refrigerant passage (not shown) for cooling the fuel cell stack 12 is formed. The refrigerant supply passage 84 communicates the outlet of the radiator 90 and the inlet of the refrigerant passage. A refrigerant pump 88 is provided in the refrigerant supply passage 84. The refrigerant discharge passage 86 communicates the outlet of the refrigerant passage and the inlet of the radiator 90. A temperature sensor 92 is provided in the refrigerant discharge passage 86. The temperature sensor 92 detects the temperature of the refrigerant discharged from the fuel cell stack 12.

[0023] The control device 94 is a computer (e.g., the vehicle's ECU). The control device 94 has a control unit 96 and a storage unit 98. The control unit 96 has a processing circuit. The processing circuit may be a processor such as a CPU. The processing circuit may be an integrated circuit such as an ASIC or an FPGA. The processor can execute various processes by executing a program stored in the storage unit 98. At least a part of the plurality of processes may be executed by an electronic circuit including discrete devices.

[0024] The control unit 96 controls the operation of the fuel cell system 10. For example, the control unit 96 receives detection signals from various sensors provided in the fuel cell system 10. Based on each detection signal, the control unit 96 outputs control signals to control each of the valves, injectors 50, compressors 68, refrigerant pumps 88, etc. Each of the valves, injectors 50, compressors 68, refrigerant pumps 88, etc. operates according to the control signals.

[0025] The storage unit 98 includes volatile memory and non-volatile memory. Examples of volatile memory include RAM. The volatile memory is used as the processor's working memory. The volatile memory temporarily stores data necessary for processing or calculations. Examples of non-volatile memory include ROM and flash memory. The non-volatile memory is used as storage memory. The non-volatile memory stores programs, tables, maps, etc. At least a part of the storage unit 98 may be provided in a processor, integrated circuit, etc., as described above.

[0026] The non-volatile memory further stores a first threshold and a second threshold. The first threshold is used to determine whether or not to perform a nitrogen purge to reduce the nitrogen in the anode channel 36. The second threshold is used to determine whether the fuel cell stack 12 is under low load or under medium to high load. In this embodiment, the first threshold is the amount of hydrogen (hydrogen concentration) estimated relatively based on the amount of nitrogen in the anode channel 36. The second threshold is the amount of power generated by the fuel cell stack 12. Furthermore, the non-volatile memory stores information indicating the opening time tx and closing time ty of the second drain valve 58, which periodically opens and closes in the second state described later. Information indicating the first threshold, second threshold, open time tx, and closed time ty is pre-set by the technician and recorded in the storage unit 98.

[0027] <Fluid flow> 1. Anode System The fluid flow in the anode system 16 will be described. The injector 50 injects anode gas (hydrogen) from the hydrogen tank 14 downstream of the anode supply channel 40. The anode gas injected from the injector 50 flows through the anode supply channel 40 and is supplied to the anode channel 36. The anode gas flows through the anode channel 36 and is discharged as anode off gas from the first anode outlet 17B. The anode off gas contains hydrogen that did not react with oxygen, nitrogen in the cathode gas that has permeated the electrolyte membrane 30, and water produced by the reaction between oxygen and hydrogen.

[0028] The anode off gas flows through the anode discharge channel 42 and is supplied to the gas-liquid separator 54. The gas-liquid separator 54 separates the anode off gas into a gaseous component (anode off gas) and a liquid component (water). The anode off gas discharged from the gas-liquid separator 54 flows through the circulation channel 44 and is supplied to the ejector 52. In the ejector 52, the anode off gas and the anode gas injected from the injector 50 merge. Furthermore, when the bleed valve 82 of the third drain channel 80 opens, a portion of the anode off gas flowing through the circulation channel 44 flows through the third drain channel 80 and is discharged to the diluent 60. However, the bleed valve 82 is opened only at low loads when the target power generation amount, described later, falls below the second threshold.

[0029] The water separated in 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 through the first drain channel 46 and is discharged to the diluent 60. When the first drain valve 56 is opened after the water in the gas-liquid separator 54 has been depleted, the anode-off gas from the gas-liquid separator 54 flows through the first drain channel 46 and is discharged to the diluent 60.

[0030] When the inside of the fuel cell stack 12 is humid, water is stored at the bottom of the anode channel 36. With the second drain valve 58 open, the water stored in the anode channel 36 flows through the second drain channel 48 and the first drain channel 46 and is discharged to the diluent 60. When the second drain valve 58 opens after the water in the anode channel 36 has been depleted, the anode off-gas in the anode channel 36 flows through the second drain channel 48 and the first drain channel 46 and is discharged to the diluent 60.

[0031] 2. Cathode System The fluid flow in the cathode system 18 will be described. The compressor 68 discharges cathode gas (air) drawn in from outside the vehicle downstream of the cathode supply passage 62. With the first sealing valve 74 open, the cathode gas discharged from the compressor 68 flows through the cathode supply passage 62 and is supplied to the cathode passage 38. The cathode gas flows through the cathode passage 38 and is discharged from the cathode outlet 19B as cathode-off gas. The cathode-off gas contains the various components contained in the air, as well as water produced by the reaction of oxygen and hydrogen.

[0032] With the second sealing valve 76 open, the cathode-off gas flows through the cathode discharge channel 64 and is discharged to the diluent 60. The cathode-off gas contains moisture. In the humidifier 70, the moisture in the cathode-off gas is used to humidify the cathode gas.

[0033] With the bypass valve 78 open, the cathode gas flows through the bypass channel 66 and the cathode discharge channel 64 and is discharged to the diluent 60. The bypass channel 66 is used to reduce the amount of cathode gas supplied to the fuel cell stack 12.

[0034] <Status of the second drain valve and bleed valve> 1. First state The state in which either the second drain valve 58 or the bleed valve 82 is open will be referred to as the first state. The reason why the control unit 96 controls the second drain valve 58 and the bleed valve 82 to the first state will be explained. The control unit 96 suppresses the decrease in hydrogen concentration in the anode channel 36 and controls it to maintain the hydrogen concentration above a certain level. The following (a) and (b) are possible factors that cause the hydrogen concentration in the anode channel 36 to decrease. (a) The hydrogen in the anode channel 36 is consumed by the power generated by the fuel cell stack 12. (b) Nitrogen contained in the cathode gas permeates through the electrolyte membrane 30 and enters the anode channel 36, thereby relatively increasing the nitrogen concentration in the anode channel 36.

[0035] In response to factor (a) above, the control unit 96 controls the injector 50. This increases the amount of hydrogen in the anode channel 36, and the hydrogen concentration in the anode channel 36 increases. In response to factor (b) above, the control unit 96 opens the second drain valve 58 or the bleed valve 82. This causes the anode off-gas containing nitrogen to be discharged from the anode channel 36. Hydrogen is supplied to the anode channel 36 as anode gas as appropriate, so the hydrogen concentration in the anode channel 36 increases relatively.

[0036] When the target power generation is greater than the second threshold under medium to high load conditions, for the following reasons, it is preferable to open the second drain valve 58 rather than the bleed valve 82 in order to suppress the decrease in hydrogen concentration (in other words, the increase in nitrogen concentration) in the anode flow path 36. In this embodiment, as an example, the nitrogen flow rate discharged through the bleed valve 82 is configured to be smaller than the nitrogen flow rate discharged through the second drain valve 58. Also, the nitrogen flow rate discharged through the second drain valve 58 is configured to be larger than the maximum nitrogen flow rate that permeates from the cathode flow path 38 to the anode flow path 36. Generally, when the fuel cell stack 12 becomes hot under medium to high load conditions, the nitrogen flow rate permeating from the cathode channel 38 to the anode channel 36 increases (in other words, the rate of nitrogen increase increases). As a result, the nitrogen flow rate permeating from the cathode channel 38 to the anode channel 36 may become greater than the nitrogen flow rate discharged through the bleed valve 82. Therefore, under medium to high load conditions, when the nitrogen flow rate permeating from the cathode channel 38 to the anode channel 36 may be greater than the nitrogen flow rate discharged through the bleed valve 82, the second drain valve 58 is opened instead of the bleed valve 82 to discharge nitrogen, preventing insufficient nitrogen discharge. As a result, the hydrogen concentration in the fuel cell stack 12 can be maintained, making it possible to continue driving the vehicle.

[0037] The rate of nitrogen increase in the anode channel 36 depends on the cathode pressure, the refrigerant temperature of the cooling system 20, 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 amount used by the control unit 96. In other words, there is a correlation between the rate of nitrogen increase in the anode channel 36 and the target power generation amount. Therefore, the control unit 96 decides whether to open the second drain valve 58 or the bleed valve 82 based on the target power generation amount. For example, if the target power generation amount is above the second threshold, the bleed valve 82 is closed and the second drain valve 58 is opened, and if the target power generation amount is below the second threshold, the bleed valve 82 is opened and the second drain valve 58 is closed.

[0038] 2. Second state The state in which the bleed valve 82 is opened and closed periodically will be referred to as the second state. The reason why the control unit 96 controls the bleed valve 82 to the second state will be explained. In the diluter 60 described above, the reason for diluting the hydrogen in the anode-off gas is to prevent the hydrogen contained in the gas discharged to the outside (into the atmosphere) from igniting. If the target power generation is below the second threshold, the amount of cathode-off gas is sufficiently greater than the amount of anode-off gas. Therefore, the amount of combined gas flowing from the diluent 60 to the discharge pipe 100 is effectively determined by the amount of cathode-off gas.

[0039] When the control unit 96 opens the bleed valve 82, the anode-off gas containing nitrogen and hydrogen merges with the cathode-off gas in the diluent 60. Upon merging, the hydrogen concentration of the merged gas flowing through the discharge pipe 100 increases. Here, the rate at which the hydrogen concentration of the merged gas increases slows down at the outlet 100C of the discharge pipe 100 compared to the vicinity of the inlet 100A of the discharge pipe 100. Therefore, in this embodiment, the control unit 96 opens the bleed valve 82 to discharge the anode-off gas from the anode flow path 36, and closes the bleed valve 82 before the hydrogen concentration at the outlet 100C of the discharge pipe 100 (corresponding to the second hydrogen concentration described later), which increases due to the opening of the bleed valve 82, reaches a specified value (an upper limit that does not pose a risk of ignition). When the hydrogen concentration at outlet 100C decreases due to the control unit 96 closing the bleed valve 82, the control unit 96 opens the bleed valve 82 again. By repeatedly opening and closing the bleed valve 82 in this manner, the control unit 96 suppresses the decrease in the hydrogen concentration in the anode flow path 36 (in other words, the increase in the nitrogen concentration in the anode flow path 36).

[0040] Figure 2 is a schematic diagram showing the relationship between the opening and closing timing of the bleed valve 82 and the hydrogen concentration of the combined gas. The horizontal axis represents time, and the upper vertical axis represents the open / closed state of the bleed valve 82. 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 on the lower vertical axis corresponds to the specified value mentioned above.

[0041] When the bleed valve 82 opens at time t0, the hydrogen concentration at outlet 100C of the discharge pipe 100 (the second hydrogen concentration) rises at a slower rate than the rate at which the hydrogen concentration at inlet 100A of the discharge pipe 100 rises. When the bleed valve 82 closes at time t1, the hydrogen concentration at outlet 100C of the discharge pipe 100 (the second hydrogen concentration) decreases at a slower rate than the rate at which the hydrogen concentration at inlet 100A of the discharge pipe 100 falls. Note that the rate of decrease is faster than the rate of increase. Therefore, the closing time ty of the bleed valve 82 may be shorter than the opening time tx. Thereafter, the bleed valve 82 repeatedly opens and closes with a period T. In the second state, while the bleed valve 82 is repeatedly opening and closing, the control unit 96 controls the opening and closing timing of the bleed valve 82 so that the hydrogen concentration at the outlet 100C of the discharge pipe 100 (second hydrogen concentration) does not reach a threshold (specified value). As a result, there is no need to increase the amount of dilution air sent to the diluter 60, and the power consumption of the compressor 68 is suppressed.

[0042] In the second state, the opening time tx of the bleed valve 82, which repeatedly opens and closes, is determined for each hydrogen concentration by, for example, an engineer, taking into account the flow velocity of the combined gas at the outlet 100C of the discharge pipe 100, and is recorded in the memory unit 98 in advance. More specifically, the flow velocity at the outlet 100C of the discharge pipe 100 can be determined from information indicating the shape of the discharge pipe 100 and the amount of combined gas flowing through the discharge pipe 100. The amount of combined gas is calculated, for example, by subtracting the amount of oxygen consumed in power generation from the amount of cathode gas supplied to the cathode supply channel 62. As an example, an engineer conducts an ignition test at the outlet 100C of the discharge pipe 100 while changing the conditions of the combination of hydrogen concentration and flow velocity of the combined gas, and based on the test results, the opening time tx of the bleed valve 82 and a shorter closing time ty are determined for each hydrogen concentration (second hydrogen concentration). Then, while the fuel cell system 10 is in operation, the open time tx and closed time ty corresponding to the hydrogen concentration (second hydrogen concentration) are recorded in the storage unit 98 so that the control unit 96 can read them out. Furthermore, if the control unit 96 calculates the required open time tx and closed time ty in real time using the hydrogen concentration (second hydrogen concentration), information indicating the shape of the discharge pipe 100, and the amount of cathode gas supplied to the cathode supply channel 62 while the fuel cell system 10 is in operation, then the information indicating the shape of the discharge pipe 100 and the calculation formulas for deriving the open time tx and closed time ty should be stored in the storage unit 98 in advance.

[0043] 3. Third state The state in which the second drain valve 58 and the bleed valve 82 are closed will be referred to as the third state. The reason why the control unit 96 controls the second drain valve 58 and the bleed valve 82 to the third state will be explained. As described above, the control unit 96 suppresses the decrease in the hydrogen concentration (first hydrogen concentration) in the anode flow path 36 and controls it to maintain the hydrogen concentration (first hydrogen concentration) above a certain level. If the hydrogen concentration (first hydrogen concentration) in the anode flow path 36 does not decrease, there is no reason to set the second drain valve 58 and the bleed valve 82 to the first and second states in order to discharge anode off gas containing nitrogen from the anode flow path 36 (nitrogen purging). Therefore, the control unit 96 closes the second drain valve 58 and the bleed valve 82 (i.e., sets them to the third state). As a result, the anode off gas flows in the order of anode discharge flow path 42, gas-liquid separator 54, circulation flow path 44, and ejector 52, and is returned to the anode supply flow path 40.

[0044] <Explanation of the flowchart> Figure 3 is a flowchart showing an example of valve control processing performed by the control unit 96 based on a predetermined program. The control unit 96 repeatedly performs the valve control processing shown in Figure 2 while the fuel cell system 10 is in operation.

[0045] In step S1, the control unit 96 estimates a first hydrogen concentration. More specifically, the control unit 96 estimates the amount of nitrogen in the anode channel 36. The amount of nitrogen that has permeated from the cathode channel 38 to the anode channel 36 (permeated nitrogen amount) can be calculated by multiplying the nitrogen partial pressure difference between the anode channel 36 and the cathode channel 38 by the nitrogen permeation coefficient. There is a correlation between the internal temperature of the fuel cell stack 12 and the nitrogen permeation coefficient. There is also a correlation between the internal humidity of the fuel cell stack 12 and the nitrogen permeation coefficient. For example, the control unit 96 controls each component of the fuel cell system 10 so that the internal humidity of the fuel cell stack 12 becomes 100%. In this case, the nitrogen permeation 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 channel 36 based on the internal temperature of the fuel cell stack 12. The control unit 96 estimates the amount of hydrogen (first hydrogen concentration) relative to the estimated amount of nitrogen. Various estimation methods are stored in the memory unit 98. The internal temperature of the fuel cell stack 12 can also be calculated from the temperature of the cathode off-gas flowing through the cathode discharge channel 64 or the temperature of the anode off-gas flowing through the anode discharge channel 42. Furthermore, the internal temperature of the fuel cell stack 12 can also be directly detected by a temperature sensor or the like. After completing step S1, the control unit 96 proceeds to step S2.

[0046] In step S2, the control unit 96 determines whether the condition "first hydrogen concentration > first threshold" is met. If the first hydrogen concentration, which is estimated relatively from the amount of nitrogen estimated in step S1, exceeds the first threshold, the control unit 96 affirms step S2 and proceeds to step S3. If the first hydrogen concentration is less than or equal to the first threshold, the control unit 96 negates step S2 and proceeds to step S4. Alternatively, the system may proceed to step S4 if the first hydrogen concentration is equal to the second threshold.

[0047] The process proceeds to step S3 if there is no reason to discharge nitrogen from the anode flow path 36. In step S3, the control unit 96 controls both the bleed valve 82 and the second drain valve 58 to a third state, thereby ending the process shown in Figure 3. If the bleed valve 82 is already closed, the control unit 96 maintains the state of the bleed valve 82. On the other hand, if the bleed valve 82 is open, the control unit 96 closes the bleed valve 82. If the second drain valve 58 is already closed, the control unit 96 maintains the state of the second drain valve 58. On the other hand, if the second drain valve 58 is open, the control unit 96 closes the second drain valve 58. The anode off-gas flows through the anode discharge flow path 42, the gas-liquid separator 54, the circulation flow path 44, and the ejector 52 in that order, and is returned to the anode supply flow path 40.

[0048] In step 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 under medium to high load. During the operation of the fuel cell system 10, the control unit 96 calculates the target power generation amount and controls each component so that the power generation amount of the fuel cell stack 12 becomes 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 step S4, the control unit 96 proceeds to step S5.

[0049] In step S5, the control unit 96 determines whether the condition < second threshold is met. If the target power generation is below the second threshold (low load), the control unit 96 affirms step S3 and proceeds to step S6. If the target power generation is equal to or greater than the second threshold (medium to high load), the control unit 96 negates step S5 and proceeds to step S8. Alternatively, the system may proceed to step S8 if the target power generation is equal to the second threshold.

[0050] In step S6, the control unit 96 estimates the second hydrogen concentration. More specifically, the amount of hydrogen in the combined gas flowing from the diluent 60 to the discharge pipe 100 is the sum of the amount of hydrogen discharged from the first drain channel 46, the second drain channel 48, and the third drain channel 80, and the amount of hydrogen that permeates from the anode channel 36 to the cathode channel 38 (permeated hydrogen amount). The amount of hydrogen discharged from the first drain channel 46, the second drain channel 48, and the third drain channel 80 can be calculated based on the anode gas pressure (detected by the pressure sensor 93), atmospheric pressure (detected by the pressure sensor 95), and the gas density of the anode channel 36. The gas density of the anode channel 36 can be calculated based on the anode gas pressure, the internal temperature of the fuel cell stack 12 (calculated based on the temperature sensor 92), and the average molecular weight. The average molecular weight is calculated from the anode gas pressure and the hydrogen partial pressure. The hydrogen partial pressure is calculated, for example, by assuming that only water vapor and hydrogen are present. Furthermore, the amount of hydrogen permeated 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). When the control unit 96 completes step S6, it proceeds to step S7.

[0051] In step S7, the control unit 96 periodically opens and closes at least one of the bleed valve 82 and the second drain valve 58 (here, the bleed valve 82, which is exemplified as the second state) to terminate the process shown in Figure 3. A portion of the anode off gas flows through the third drain channel 80 and is discharged directly to the diluent 60. Furthermore, the control unit 96 reads information from the storage unit 98 indicating the opening time tx and closing time ty corresponding to the second hydrogen concentration estimated in step S6, and controls the opening and closing of the second drain valve 58.

[0052] In step S8, which proceeds after determining that step S5 is negative, the control unit 96 controls the system to a third state in which at least one of the bleed valve 82 and the second drain valve 58 is open. For example, the process shown in Figure 3 is terminated by closing the bleed valve 82 and opening the second drain valve 58. If the bleed valve 82 is already closed, the control unit 96 maintains the state of the bleed valve 82. On the other hand, if the bleed valve 82 is open, the control unit 96 closes the bleed valve 82. If the second drain valve 58 is already open, the control unit 96 maintains the state of the second drain valve 58. On the other hand, if the second drain valve 58 is closed, the control unit 96 opens the second drain valve 58. A portion of the anode off gas flows through the second drain channel 48 and is discharged directly to the diluent 60.

[0053] According to the embodiments described above, the following effects and advantages are achieved. (1) The fuel cell system 10 includes a fuel cell stack 12 that generates electricity using anode gas in an anode channel 36 and cathode gas in a cathode channel 38, an anode supply channel 40 that supplies anode gas to the anode channel 36, a cathode supply channel 62 that supplies cathode gas to the cathode channel 38, an anode discharge channel (anode discharge channel 42, first drain channel 46, second drain channel 48, third drain channel 80) through which anode off gas, which is discharged from the anode channel 36 as anode discharge fluid, flows, and a cathode discharge channel 64 (64A, 64B) through which cathode off gas, which is discharged from the cathode channel 38 as a cathode discharge fluid, flows. The system includes a diluent 60 as a fluid confluence section that combines the anode off-gas that has flowed through the anode discharge channels (anode discharge channel 42, first drain channel 46, second drain channel 48, third drain channel 80) and the cathode off-gas that has flowed through the cathode discharge channels 64 (64A, 64B); a discharge pipe 100 that guides the combined gas, which is the combined fluid formed in the diluent 60, to the outside; a bleed valve 82, a second drain valve 58, and a first drain valve 56 as anode discharge valves that control the flow of anode off-gas toward the diluent 60; and a control unit 96 that controls the opening and closing of the anode discharge valves (bleed valve 82, second drain valve 58, and first drain valve 56). The control unit 96 obtains a second hydrogen concentration as the hydrogen concentration of the combined gas, and controls the opening and closing of the anode discharge valve (for example, a bleed valve 82) so that it repeatedly opens and closes for a predetermined opening time tx based on the second hydrogen concentration when the amount of power generated by the fuel cell stack 12 is less than or equal to a second threshold as a predetermined power generation threshold. With this configuration, for example, when the fuel cell stack 12 is under low load, the bleed valve 82 can be opened and closed repeatedly based on the opening and closing time of the second hydrogen concentration of the combined gas, thereby keeping the second hydrogen concentration of the combined gas below a specified value without increasing the amount of dilution air.

[0054] (2) In the fuel cell system 10 described in (1) above, the control unit 96 further obtains a first hydrogen concentration as the hydrogen concentration of the anode off-gas, and when the first hydrogen concentration is less than or equal to a first threshold as a predetermined concentration threshold, and the amount of power generated by the fuel cell stack 12 is less than or equal to a second threshold, the control unit 96 controls the opening and closing of the anode discharge valve (for example, a bleed valve 82) so as to repeatedly open it for an open time based on the second hydrogen concentration of the combined gas and close it for a shorter closing time than the open time. With this configuration, when the hydrogen concentration of the anode off-gas decreases and the fuel cell stack 12 is under low load, the bleed valve 82 can be opened or closed based on the second hydrogen concentration of the combined gas, thereby keeping the second hydrogen concentration of the combined gas below a specified value without increasing the amount of dilution air.

[0055] (3) In the fuel cell system 10 described in (1) above, the control unit 96 further obtains a first hydrogen concentration as the hydrogen concentration of the anode off-gas, and opens the anode discharge valve (second drain valve 58) when the hydrogen concentration of the anode off-gas is less than or equal to a first threshold as a predetermined concentration threshold, and the amount of power generated by the fuel cell stack 12 exceeds a second threshold. With this configuration, when the hydrogen concentration of the anode off-gas decreases and the fuel cell stack 12 is under medium to high load, the second drain valve 58 is opened to perform nitrogen purging, thereby suppressing the decrease in the hydrogen concentration in the anode flow path 36.

[0056] (4) In the fuel cell system 10 described in (1) to (3) above, the control unit 96 acquires the hydrogen concentration at the outlet 100C of the discharge pipe 100 as the hydrogen concentration of the combined gas (second hydrogen concentration), and further acquires the flow velocity of the combined gas at the outlet 100C of the discharge pipe 100, and determines the opening time tx of the bleed valve 82 based on the flow velocity at the outlet 100C and the second hydrogen concentration. With this configuration, when the fuel cell stack 12 is under low load, it becomes possible to determine an appropriate opening time tx for each combination of the second hydrogen concentration and flow velocity at the outlet 100C of the discharge pipe 100. This allows the bleed valve 82 to be opened and closed repeatedly for an opening time tx. As a result, it becomes possible to keep the hydrogen concentration of the combined gas (second hydrogen concentration) below a specified value without increasing the amount of dilution air.

[0057] (5) In the fuel cell system 10 described in (1) to (3) above, the control unit 96 obtains a second hydrogen concentration as the hydrogen concentration of the combined gas at the outlet 100C of the discharge pipe 100, and determines the opening time tx of the bleed valve 82 based on the shape of the discharge pipe 100 and the second hydrogen concentration at the outlet 100C. With this configuration, it becomes possible to determine an appropriate opening time tx for each second hydrogen concentration of the combined gas at the outlet 100C of the discharge pipe 100, taking into account the shape of the discharge pipe 100 (in other words, reflecting the flow velocity at the outlet 100C estimated based on the shape). This allows the bleed valve 82 to be opened and closed repeatedly for an opening time tx. As a result, it becomes possible to keep the hydrogen concentration of the combined gas (second hydrogen concentration) below a specified value without increasing the amount of dilution air.

[0058] (6) In the fuel cell system 10 described in (1) to (3) above, the control unit 96 obtains a second hydrogen concentration as the hydrogen concentration of the combined gas at the outlet 100C of the discharge pipe 100, and determines the opening time tx to be shorter than the time it takes for the second hydrogen concentration at the outlet 100C to rise to a predetermined value after the bleed valve 82 is opened. With this configuration, it becomes possible to determine an opening time tx shorter than the time it takes for the second hydrogen concentration at the outlet 100C of the discharge pipe 100 to rise to a specified value corresponding to the ignition concentration after opening the bleed valve 82. This allows the bleed valve 82 to be opened and closed repeatedly for an opening time tx. As a result, it becomes possible to keep the hydrogen concentration of the combined gas (second hydrogen concentration) below a specified value without increasing the amount of dilution air.

[0059] The above embodiment can be modified into various forms. Modifications will be described below. (Variation 1) In the above embodiment, an example was described in which, in the second state, only the bleed valve 82 among the first drain valve 56, the second drain valve 58, and the bleed valve 82 is periodically opened and closed. Alternatively, in the second state, it is also possible to configure the system to periodically open and close at least one of the first drain valve 56, the second drain valve 58, and the bleed valve 82, not limited to the bleed valve 82. In the case of the second state, it is advisable to determine appropriate opening time tx and closing time ty for each valve depending on which valve is to be subject to periodic opening and closing control. Furthermore, an example was described in which, in the first state, only the second drain valve 58 among the first drain valve 56, the second drain valve 58, and the bleed valve 82 is opened. Alternatively, in the first state, the system may be configured to open at least one of the valves among the first drain valve 56, the second drain valve 58, and the bleed valve 82, not limited to the second drain valve 58.

[0060] (Modification 2) In the above embodiment, a fuel cell system 10 is exemplified that includes a first drain channel 46, a second drain channel 48, and a third drain channel 80 as anode discharge channels, and a first drain valve 56, a second drain valve 58, and a bleed valve 82 as anode discharge valves for controlling the flow of anode off-gas through the anode discharge channels. However, the present invention may also be applied to a fuel cell system that has a connecting channel that connects the anode discharge channels to a cathode supply channel and an on / off valve that opens and closes the connecting channel. Furthermore, the present invention may also be applied to fuel cell systems that do not have either the first drain valve 56 or the second drain valve 58 in the above embodiment.

[0061] The above description is merely an example, and the present invention is not limited by the embodiments and modifications described above, as long as they do not impair the features of the present invention. [Explanation of Symbols]

[0062] 10 Fuel cell system, 12 Fuel cell stack, 36 Anode channel, 38 Cathode channel, 40 Anode supply channel, 42 Anode discharge channel, 46 First drain channel, 48 Second drain channel, 56 First drain valve, 58 Second drain valve, 60 Diluter, 62 Cathode supply channel, 64, 64A, 64B Cathode discharge channels, 80 Third drain channel, 82 Bleed valve, 96 Control unit, 100 Discharge pipe, 100A Inlet, 100C Outlet

Claims

1. A fuel cell stack that generates electricity using the anode gas in the anode channel and the cathode gas in the cathode channel, an anode supply channel for supplying the anode gas to the anode channel, A cathode supply channel for supplying the cathode gas to the cathode channel, An anode discharge channel through which the anode discharge fluid discharged from the anode channel flows, A cathode discharge channel through which the cathode discharge fluid discharged from the cathode channel flows, A fluid confluence section that combines the anode discharge fluid that has flowed through the anode discharge channel and the cathode discharge fluid that has flowed through the cathode discharge channel, A discharge pipe that guides the combined fluids that merged at the aforementioned fluid confluence to the outside, an anode discharge valve that controls the flow of the anode discharge fluid toward the fluid confluence, A fuel cell system comprising a control unit that controls the opening and closing of the anode discharge valve, The control unit, The hydrogen concentration of the aforementioned combined fluid is obtained, When the amount of power generated by the fuel cell stack is below a predetermined power generation threshold, the anode discharge valve is controlled to repeatedly open and close for a predetermined opening time determined so that the hydrogen concentration does not reach a predetermined specified value. A fuel cell system characterized by the following features.

2. In the fuel cell system according to claim 1, The control unit, The hydrogen concentration of the anode discharge fluid is further obtained, and when the hydrogen concentration of the anode discharge fluid is below a predetermined concentration threshold and the amount of power generated by the fuel cell stack is below the power generation threshold, the opening and closing operation of the anode discharge valve is controlled so as to repeat the opening operation, which is performed by opening for a certain opening time based on the hydrogen concentration of the combined fluid and closing for a shorter closing time than the opening time. A fuel cell system characterized by the following features.

3. In the fuel cell system according to claim 1, The control unit, The hydrogen concentration of the anode discharge fluid is further obtained, and if the hydrogen concentration of the anode discharge fluid is below a predetermined concentration threshold and the amount of power generated by the fuel cell stack exceeds the power generation threshold, the anode discharge valve is opened. A fuel cell system characterized by the following features.

4. In the fuel cell system according to any one of claims 1 to 3, The control unit, The hydrogen concentration at the outlet of the discharge pipe is obtained as the hydrogen concentration of the combined fluid, and the flow velocity of the combined fluid at the outlet of the discharge pipe is further obtained. The opening time of the anode discharge valve is determined based on the flow velocity and hydrogen concentration at the outlet. A fuel cell system characterized by the following features.

5. In the fuel cell system according to any one of claims 1 to 3, The control unit, The hydrogen concentration of the combined fluid at the outlet of the discharge pipe is obtained. The opening time of the anode discharge valve is determined based on the shape of the discharge pipe and the hydrogen concentration at the outlet. A fuel cell system characterized by the following features.

6. In the fuel cell system according to any one of claims 1 to 3, The control unit, The hydrogen concentration of the combined fluid at the outlet of the discharge pipe is obtained. The opening time is determined to be shorter than the time it takes for the hydrogen concentration at the outlet to rise to the specified value after the anode discharge valve is opened. A fuel cell system characterized by the following features.

Images