Fuel cell system

Abstract

To provide a fuel cell system that reduces the amount of fuel off-gas discharged from a drain valve by precisely controlling the discharge of liquid water from the drain valve, thereby improving energy efficiency.SOLUTION: A fuel cell system includes a drain valve 164 provided in a gas-liquid separator 36 in a fuel off-gas flow path 74 that discharges liquid water within the gas-liquid separator 36, a pressure sensor 73 provided in a fuel gas supply flow path 72 or the fuel off-gas flow path 74 to detect the gas pressure Ph within the flow path, a power generation state acquisition unit 115 that acquires the power generation state of a fuel cell stack 18, and a control device 15. The control device 15 controls the opening and closing of the drain valve 164 on the basis of the power generation state and the gas pressure Ph.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a fuel cell system capable of accurately controlling drainage from a drain valve provided in a fuel off-gas flow path without providing a water level sensor.

Background Art

[0002] In recent years, research and development have been conducted on fuel cells (FCs) that contribute to energy efficiency in order to enable more people to access affordable, reliable, sustainable, and advanced energy.

[0003] For example, in Patent Document 1, a gas-liquid separator is provided in the fuel off-gas flow path of a fuel cell stack. A water level sensor for detecting the water level in the gas-liquid separator is provided in the gas-liquid separator.

[0004] The control unit of the fuel cell system switches the opening and closing of the drain valve based on the detection signal of the water level sensor.

[0005] It is disclosed that, thereby, while suppressing the discharge of fuel gas together with water from the gas-liquid separator, water can be discharged at an appropriate timing.

[0006] Patent Document 2 discloses a technique for detecting pressure fluctuations in a gas-liquid separator and determining the opening of the drain valve without providing a water level sensor.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, if there are fluctuations in the power generation state of the fuel cell stack, pressure fluctuations occur in the anode path due to the consumption of fuel gas during power generation, and there is a problem that it is difficult to accurately determine the change in the water level in the gas-liquid separator.

[0009] An object of the present invention is to solve the above-described problems.

Means for Solving the Problems

[0010] A fuel cell system according to an aspect of the present invention includes a fuel cell stack that generates power by an electrochemical reaction between a fuel gas and an oxidant gas, a fuel gas supply flow path that supplies the fuel gas to the fuel cell stack, a fuel off-gas flow path through which fuel off-gas discharged from the fuel cell stack flows, a gas-liquid separator provided in the fuel off-gas flow path, a drain valve provided in the gas-liquid separator for discharging liquid water in the gas-liquid separator, a pressure sensor provided in the fuel gas supply flow path or the fuel off-gas flow path for detecting the gas pressure in the flow path, a power generation state acquisition unit that acquires the power generation state of the fuel cell stack, and a control device. The control device controls the opening and closing of the drain valve based on the power generation state and the gas pressure. Further, the control device estimates the water level in the gas-liquid separator when the drain valve is closed based on the power generation state, and when the estimated water level becomes equal to or higher than a threshold water level, opens the drain valve to discharge the liquid water. During the opening of the drain valve, it is determined whether the fluctuation range of the power generation current in the power generation state is within a predetermined current fluctuation range. When the power generation current exceeds the upper limit power generation current of the predetermined current fluctuation range, the drain valve is maintained open without performing the determination of the pressure drop rate of the gas pressure. When the fluctuation range of the power generation current is within the predetermined current fluctuation range, when the pressure drop rate of the gas pressure detected by the pressure sensor becomes equal to or higher than a threshold drop rate during the opening of the drain valve, the drain valve is closed.

Advantages of the Invention

[0011] According to the present invention, by controlling the opening and closing of the drain valve in consideration of the power generation state of the fuel cell stack and the gas pressure in the fuel gas flow path or the gas pressure in the fuel off-gas flow path, it is possible to accurately control the discharge of liquid water considering the water level state in the gas-liquid separator without using a water level sensor. Since the discharge of liquid water from the drain valve can be accurately controlled, the amount of fuel off-gas discharged from the drain valve can be suppressed, which contributes to the improvement of energy efficiency.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0013] [Embodiment] [Configuration] FIG. 1 is a schematic configuration diagram of a fuel cell vehicle 12 incorporating a fuel cell system 10 according to an embodiment of the present invention.

[0014] The fuel cell system 10 can also be incorporated into other moving bodies such as ships, aircraft, robots, etc. other than the fuel cell vehicle 12.

[0015] The fuel cell vehicle 12 is composed of a control device 15 that controls the entire fuel cell vehicle 12, a fuel cell system 10, and an output unit 16 that is electrically connected to the fuel cell system 10.

[0016] The control device 15 may be divided into two or more control devices, for example, for the fuel cell system 10 and for the output unit 16, instead of being a single one.

[0017] The fuel cell system 10 is composed of a fuel cell stack (simply referred to as a fuel cell) 18, a hydrogen tank 20, an oxidant gas supply device 22, a fuel gas supply device 24, and a refrigerant supply device 26.

[0018] The oxidant gas supply device 22 includes a compressor (CP) 28 and a humidifier (HUM) 30.

[0019] The fuel gas supply device 24 includes an injector (INJ) 32, an ejector 34, and a gas-liquid separator 36. The injector 32 may be replaced with a pressure reducing valve. The refrigerant supply device 26 includes a refrigerant pump (WP) 38 and a radiator 40.

[0020] The output unit 16 includes a drive unit 42, a high-voltage power storage device (battery) 44, and a motor (electric motor) 46.

[0021] The load of the drive unit 42 includes, in addition to the motor 46 which is the main vehicle body, vehicle auxiliary machines such as the compressor 28, the refrigerant pump 38, and other air conditioners. The fuel cell vehicle 12 travels by the driving force generated by the motor 46.

[0022] The fuel cell stack 18 has a plurality of power generation cells 50 stacked. The power generation cell 50 includes an electrolyte membrane - electrode structure 52, and separators 53, 54 that sandwich the electrolyte membrane - electrode structure 52.

[0023] The electrolyte membrane - electrode structure 52 includes, for example, a solid polymer electrolyte membrane 55 which is a thin film of perfluorosulfonic acid containing moisture, and a cathode electrode 56 and an anode electrode 57 sandwiching the solid polymer electrolyte membrane 55.

[0024] The cathode electrode 56 and the anode electrode 57 have a gas diffusion layer (not shown) made of carbon paper or the like. A porous carbon particle with a platinum alloy supported on its surface is uniformly coated on the surface of the gas diffusion layer, thereby forming an electrode catalyst layer (not shown). The electrode catalyst layer is formed on both sides of the solid polymer electrolyte membrane 55.

[0025] On the surface of one separator 53 facing the electrolyte membrane - electrode structure 52, a cathode flow path (oxidant gas flow path) 58 that communicates an oxidant gas inlet communication port 101 and an oxidant gas outlet communication port 102 is formed.

[0026] On the surface of the other separator 54 facing the electrolyte membrane - electrode structure 52, an anode flow path (fuel gas flow path) 59 that communicates a fuel gas inlet communication port 103 and a fuel gas outlet communication port 104 is formed.

[0027] At the anode electrode 57, when a fuel gas (hydrogen) is supplied, hydrogen ions are generated from hydrogen molecules by an electrode reaction with a catalyst, and the hydrogen ions permeate through the solid polymer electrolyte membrane 55 and move to the cathode electrode 56. At the same time, electrons are released from the hydrogen molecules.

[0028] The electrons released from the hydrogen molecules move from the negative terminal 106 through loads such as the drive unit 42 and the motor 46 and reach the cathode electrode 56 via the positive terminal 108.

[0029] At the cathode electrode 56, the hydrogen ions, the electrons, and oxygen contained in the supplied oxidant gas react under the action of the catalyst to generate water.

[0030] A voltage sensor 110 for detecting the generated voltage Vfc is provided between the positive electrode terminal 108 and the negative electrode terminal 106 and the wiring connecting the drive unit 42. Further, a current sensor 112 for detecting the generated current Ifc is provided in the wiring connecting the positive electrode terminal 108 and the drive unit 42.

[0031] A power generation state acquisition unit 115 for detecting the generated power as the power generation state is formed by the voltage sensor 110 and the current sensor 112. The power generation state acquisition unit 115 may be formed only by the current sensor 112.

[0032] The compressor 28 is composed of a mechanical supercharger or the like driven by a compressor motor (not shown) to which the stored power of the power storage device 44 is supplied through the drive unit 42, and has functions such as sucking and pressurizing outside air (atmospheric air, air) from the outside air intake 113 and supplying it to the fuel cell stack 18 through the humidifier 30.

[0033] The humidifier 30 has a flow path 31A and a flow path 31B. Compressed, heated, and dried air (oxidant gas) by the compressor 28 flows through the flow path 31A. Exhaust gas discharged from the oxidant gas outlet communication port 102 of the fuel cell stack 18 flows through the flow path 31B.

[0034] Here, when the bleed valve 70 described later is closed, the exhaust gas is a wet oxidant off-gas (wet oxidant exhaust gas), and when the bleed valve 70 is opened, it is a wet exhaust gas in which the wet oxidant off-gas and the fuel off-gas are mixed.

[0035] The humidifier 30 has a function of humidifying the oxidant gas supplied from the compressor 28. That is, the humidifier 30 moves the moisture contained in the exhaust gas (off-gas) from the flow path 31B to the supply gas (oxidant gas) flowing through the flow path 31A through the internal porous membrane for humidification, and supplies the humidified oxidant gas to the fuel cell stack 18.

[0036] In the oxidant gas supply passage 60 (including the oxidant gas supply passages 60A and 60B) from the outside air intake port 113 to the oxidant gas inlet communication port 101, a compressor 28, a supply side shutoff valve 118, and a humidifier 30 are provided in order from the outside air intake port 113. Note that the passages such as the oxidant gas supply passage 60 drawn with double lines are formed by piping (the same applies hereinafter). The supply side shutoff valve 118 opens and closes the oxidant gas supply passage 60.

[0037] In the oxidant off-gas passage 62 communicating with the oxidant gas outlet communication port 102, a humidifier 30 and a discharge side shutoff valve 120 that also functions as a back pressure valve are provided in order from the oxidant gas outlet communication port 102.

[0038] A bypass passage 64 that communicates the oxidant gas supply passage 60 and the oxidant off-gas passage 62 is provided between the suction port of the supply side shutoff valve 118 and the discharge port of the discharge side shutoff valve 120. A bypass valve 122 that opens and closes the bypass passage 64 is provided in the bypass passage 64. The bypass valve 122 adjusts the flow rate of the oxidant gas that bypasses the fuel cell stack 18. The confluence passage of the bypass passage 64 and the oxidant off-gas passage 62 communicates with the discharge passage 62A.

[0039] The hydrogen tank 20 is a container that includes an electromagnetic actuated shutoff valve (not shown) and stores highly pure hydrogen compressed at a high pressure.

[0040] The fuel gas discharged from the hydrogen tank 20 is supplied to the inlet of the anode passage 59 of the fuel cell stack 18 through an injector 32 and an ejector 34 provided in the fuel gas supply passage 72 via the fuel gas inlet communication port 103.

[0041] In this case, a pressure sensor 73 that detects (measures) the gas pressure Ph of the fuel gas in the fuel gas supply passage 72 is provided in the fuel gas supply passage 72.

[0042] The outlet of the anode flow path 59 communicates with the fuel gas outlet communication port 104 and the inlet 151 of the gas-liquid separator 36 through the fuel off-gas flow path 74 of the fuel gas, and the fuel off-gas, which is a hydrogen-containing gas from the anode flow path 59, is supplied to the gas-liquid separator 36.

[0043] Also, at the lower end (bottom) of the case that houses the fuel cell stack 18, a case lower end side discharge flow path 172, which is a case side discharge flow path communicating with the fuel gas outlet communication port 104, is provided. The case lower end side discharge flow path 172 communicates with the discharge flow path 99 through a case lower end side on-off valve 174, which is a case side on-off valve.

[0044] When the case lower end side on-off valve 174 is opened, the liquid water accumulated at the bottom inside the case is discharged into the case lower end side discharge flow path 172. If the case lower end side on-off valve 174 remains open even after the discharge of the liquid water is completed, similar to the drain valve 164, the fuel off-gas is discharged.

[0045] In practice, a part of the water generated by the power generation of the fuel cell stack 18 moves by reverse diffusion (permeation) from the cathode flow path 58 through the electrolyte membrane / electrode structure 52 into the anode flow path 59.

[0046] If this reverse diffused water cannot be properly drained from the case lower end side discharge flow path 172, the fuel off-gas flow path 74, or the circulation flow path 77, water will penetrate into the anode electrode 57 of the fuel cell stack 18, blocking the anode flow path (fuel gas flow path) 59. In this case, electrode deterioration due to local hydrogen deficiency (fuel gas deficiency) in the power generation cell 50 and deterioration of the power generation stability of the fuel cell stack 18 due to gas diffusion inhibition will occur.

[0047] To prevent these inconveniences, the gas-liquid separator 36 that temporarily stores water separates the fuel off-gas into a gas component and a liquid component (liquid water).

[0048] The gaseous component of the fuel off-gas (fuel off-gas) is discharged from the gas outlet 152 of the gas-liquid separator 36 and supplied to the suction port of the ejector 34 through the circulation flow path 77. On the other hand, when the bleed valve 70 is opened, the fuel off-gas is also supplied to the oxidant gas supply flow path 60B through the connection flow path (communication flow path) 78 and the bleed valve 70.

[0049] The liquid component (liquid water) of the fuel off-gas, which consists of reverse diffusion water, passes through the drain flow path 162 provided with the drain valve 164 from the liquid outlet 160 of the gas-liquid separator 36, is mixed with the exhaust gas discharged from the exhaust flow path 62A, and is discharged to the outside air through the exhaust flow path 99 and the exhaust gas outlet 168.

[0050] In practice, a part of the fuel off-gas (hydrogen-containing gas) is discharged into the drain flow path 162 together with the liquid water. Also, only the fuel off-gas (hydrogen-containing gas) is discharged into the drain flow path 162 after the discharge of the liquid water is completed.

[0051] In order to dilute the hydrogen gas in the fuel off-gas and discharge it to the outside, a part of the oxidant gas discharged from the compressor 28 is supplied to the exhaust flow path 62A through the bypass flow path 64.

[0052] If the drain valve 164 is continuously opened even after the water has drained from the drain flow path 162, hydrogen will be wasted. Therefore, it is necessary to properly close the drain valve 164 after the water has been drained from the gas-liquid separator 36.

[0053] The bleed valve 70 provided in the connection flow path 78 that connects the circulation flow path 77 of the fuel off-gas and the oxidant gas supply flow path 60B is opened to prevent deterioration of the anode electrode 57 caused by nitrogen gas present in the cathode flow path 58 permeating through the electrolyte membrane / electrode structure 52 and reducing the hydrogen concentration in the anode flow path 59 during the running of the fuel cell vehicle 12.

[0054] When the bleed valve 70 is opened, the fuel off-gas discharged from the fuel cell stack 18 through the fuel off-gas flow path 74 and via the gas-liquid separator 36 is caused to flow into the cathode flow path 58 through the connection flow path 78, the oxidant gas supply flow path 60B, and the oxidant gas inlet communication port 101.

[0055] The fuel gas in the fuel off-gas that has flowed into the cathode flow path 58 is hydrogen-ionized by the catalytic reaction at the cathode electrode 56, and the hydrogen ions react with the oxidant gas to produce water. The remaining unreacted fuel off-gas (consisting of nitrogen gas and a small amount of unreacted hydrogen gas) is discharged from the fuel cell stack 18 as oxidant off-gas and flows into the oxidant off-gas flow path 62.

[0056] The oxidant gas supplied through the bypass flow path 64 of the oxidant gas is mixed with the oxidant off-gas flowing in the oxidant off-gas flow path 62 (including the remaining unreacted fuel off-gas), and the oxidant off-gas in which the concentration of the fuel off-gas (including fuel gas) in the oxidant off-gas is diluted flows into the discharge flow path 62A.

[0057] The discharge flow path 62A communicates with the drain flow path 162 and the case lower-end side discharge flow path 172, and the merged flow communicates with the discharge flow path 99.

[0058] In the discharge flow path 99, the fuel gas in the mixed fluid of the liquid water and the fuel off-gas discharged from the case lower-end side discharge flow path 172 and the drain flow path 162 is diluted by the oxidant off-gas from the discharge flow path 62A, and is discharged to the outside (atmosphere) of the fuel cell vehicle 12 through the exhaust gas exhaust port 168.

[0059] The refrigerant supply device 26 of the fuel cell system 10 has a refrigerant flow path 138 through which the refrigerant flows. The refrigerant flow path 138 has a refrigerant supply flow path 140 and a refrigerant discharge flow path 142. The refrigerant supply flow path 140 supplies the refrigerant to the fuel cell stack 18, and the refrigerant discharge flow path 142 discharges the refrigerant from the fuel cell stack 18. A radiator 40 is connected to the refrigerant supply flow path 140 and the refrigerant discharge flow path 142.

[0060] The radiator 40 cools the refrigerant. A refrigerant pump 38 is provided in the refrigerant supply passage 140. The refrigerant pump 38 circulates the refrigerant within the refrigerant circulation circuit. The refrigerant circulation circuit includes the refrigerant supply passage 140, the internal refrigerant passage of the fuel cell stack 18, the refrigerant discharge passage 142, and the radiator 40. A temperature sensor 76 is provided in the refrigerant discharge passage 142. The temperature (refrigerant outlet temperature) Ts of the cooling medium detected by the temperature sensor 76 is detected (measured) as the (internal) temperature of the fuel cell stack 18.

[0061] Each component of the fuel cell system 10 described above is comprehensively controlled by the control device 15. Note that the supply-side shutoff valve 118, the discharge-side shutoff valve 120, the bleed valve 70, the drain valve 164, and the case lower-end side on-off valve 174 are flow control valves whose opening degrees are controlled by the control device 15, but electromagnetic control on-off valves may be used for duty control.

[0062] The control device 15 is constituted by an ECU (Electronic Control Unit). The ECU is constituted by a computer having one or more processors (CPUs), a memory, an input / output interface, and electronic circuits. The one or more processors (CPUs) execute a program (not shown) stored in the memory.

[0063] The processor (CPU) of the control device 15 performs operation control of the fuel cell vehicle 12 and the fuel cell system 10 by executing calculations according to the program.

[0064] A power switch (power SW) 71 of the fuel cell vehicle 12 is connected to the control device 15. The power switch 71 starts or continues (turns ON) or ends (turns OFF) the power generation operation of the fuel cell stack 18 of the fuel cell system 10. Also connected to the control device 15 are an accelerator opening sensor, a vehicle speed sensor, and an SOC sensor of the power storage device 44 (each not shown).

[0065] [Operation] The fuel cell system 10 according to this embodiment is basically configured as described above. Hereinafter, the operation of the fuel cell system 10 will be described with reference to the flowchart of FIG. 2.

[0066] The processing according to the flowchart of FIG. 2 is repeatedly executed by the control device 15 at a predetermined cycle during the power generation of the fuel cell stack 18 when the power switch 71 is in the ON state.

[0067] For the convenience of understanding the processing according to the flowchart, an example of the operation will be described based on the timing charts of FIGS. 3A to 3D.

[0068] FIG. 3A shows a state in which, under the control of the control device 15, the injector 32 supplies (ejects) fuel gas into the anode flow path 59 through the fuel gas supply flow path 72 and through the fuel gas inlet communication port 103 during the ejection period Ton at a constant cycle (constant interval) Tint (Tint = Ton + Toff).

[0069] The ejection period Ton is the operation ON period of the injector 32. The non-ejection period Toff is the operation OFF period of the injector 32.

[0070] FIG. 3B shows the open / closed state of the drain valve 164 whose opening / closing operation is controlled by the control device 15. The drain valve 164 is in the open state between time point t3 and time point t9, and is in the closed state during the period up to time point t3 and after time point t9.

[0071] FIG. 3C shows the variation (change with time) of the pressure (gas pressure) Ph [Pa] of the fuel gas in the fuel gas supply flow path 72 continuously detected by the pressure sensor 73 and acquired by the control device 15.

[0072] As can be seen from FIGS. 3A and 3C, during the ejection period Ton when the injector 32 ejects fuel gas into the anode flow path 59, the gas pressure Ph of the fuel gas increases, and during the non-ejection period Toff, the gas pressure Ph of the fuel gas decreases in proportion to time due to the power generation of the fuel cell stack 18.

[0073] Figure 3D shows the calculated value of the pressure drop rate Vp (Vp = ΔP [Pa] / [sec]) of the gas pressure Ph within the non-discharge period Toff in which the drain valve 164 is in the closed state and the gas pressure Ph is decreasing proportionally, calculated by the control device 15, and the calculated value of the average pressure drop rate Vpmean {Vpmean = (ΣVp) ÷ n (n is the number of acquisitions of the pressure drop rate Vp)} within a predetermined period (between time point t1 and time point t2 in Figure 3D) of the pressure drop rate Vp. ΔP is the amount of decrease (drop gradient) of the gas pressure Ph within a minute unit of time calculated from Figure 3C.

[0074] In the example of Figure 3D, when the drain valve 164 opens at time point t3, the average pressure drop rate Vpmean = (ΔP / sec) for a predetermined number of times within the non-discharge period Toff when the drain valve 164 was closed between time point t1 and time point t2 before time point t3 is calculated immediately after time point t4 (immediately after the start of the non-discharge period Toffa).

[0075] At the time of calculating the average pressure drop rate Vpmean, the control device 15 further calculates a threshold drop rate Vth [Pa / sec] as a reference value. The threshold drop rate Vth is obtained by adding a margin (margin drop rate amount) ΔVp [Pa / sec] that takes into account the variation in the measured value and the noise intrusion to the average pressure drop rate Vpmean.

[0076] That is, the control device 15 calculates the threshold drop rate Vth as Vth = Vpmean + ΔVp (see Figure 3D).

[0077] The control device 15 uses the threshold drop rate Vth calculated in this way to determine the time point t9 of the closing timing of the drain valve 164 as described below. The drainage control of the liquid water stored in the gas-liquid separator 36, which is the opening and closing control of the drain valve 164 including the determination of the opening timing of the drain valve 164 at time point t3, will be described below.

[0078] In step S1 of FIG. 2, the control device 15 obtains the generated current Ifc [A] by the current sensor 112 and advances the process to step S2.

[0079] In step S2, the control device 15 obtains the generated energy amount {∫(Ifc)dt} (unit: Asec = ampere × time), which is the integrated value of the generated current Ifc, calculates the estimated value (water level estimated value) Hest of the water level of the liquid water stored in the gas-liquid separator 36, and advances the process to step S3. Note that the generated energy amount may be the amount of electric energy [Wh] considering the generated voltage Vfc.

[0080] FIG. 4 shows an estimated water level calculation map 250, which is a map (characteristic) for calculating the water level estimated value Hest. During power generation, the amount of water that diffuses backward from the cathode flow path 58 to the electrolyte membrane / electrode structure 52 into the anode flow path 59 is proportional to the generated energy amount {∫(Ifc)dt} and can be uniquely calculated. Since the box structure (volume structure) of the gas-liquid separator 36 is known, by referring to the estimated water level calculation map 250 with the generated energy amount {∫(Ifc)dt} as an argument, the water level estimated value Hest, which is the water level corresponding to the current generated energy amount {∫(Ifc)dt}, can be calculated. In FIG. 4, it is assumed that the horizontal cross-sectional area of the water storage part of the gas-liquid separator 36 is constant.

[0081] Further, FIG. 4 shows a threshold water level Hth for determining the valve opening timing of the drain valve 164. The threshold water level Hth is set in advance to a water level slightly lower than the water level at which the liquid water starts to flow backward from the inlet 151 of the gas-liquid separator 36 in the gas-liquid separator 36 or a water level slightly lower than the water level at which the liquid water flows out from the gas outlet 152 of the gas-liquid separator 36 into the circulation flow path 77, with a slight margin less than the lower one of them, and is recorded as a predetermined value (see FIG. 4) on the estimated water level calculation map 250. The generated energy amount threshold corresponding to the threshold water level Hth for determining the valve opening timing is called Ahth [A×sec].

[0082] In step S3, the control device 15 acquires the gas pressure Ph detected by the pressure sensor 73 when the drain valve 164 is closed, records it in the storage unit, and calculates the pressure drop rate Vp (Vp = ΔP [Pa] / [sec]) described with reference to FIG. 3D using the difference from the gas pressure Ph recorded in step S3 a certain processing time before, records it in the storage unit, and advances the process to step S4.

[0083] In step S4, the control device 15 determines whether the estimated water level Hest calculated from the power generation amount {∫(Ifc)dt} exceeds the threshold water level Hth (power generation amount threshold Ahth). If it does not exceed (step S4: NO), the processes after step S1 are repeated. If it exceeds (step S4: YES), the process advances to step S5.

[0084] That is, when the power generation amount {∫(Ifc)dt} of the fuel cell stack 18 is integrated up to the power generation amount threshold Ahth and the estimated water level Hest reaches the threshold water level Hth corresponding to the power generation amount threshold Ahth, the control device 15 advances the process to step S5.

[0085] In step S5, the control device 15 opens the drain valve 164 so that the liquid water does not flow out from the gas-liquid separator 36 to the fuel off-gas flow path 74 side or the circulation flow path 77 side. Thereby, the discharge of the liquid water is started through the drain valve 164 (see the valve opening time t3 in FIG. 3B), and the process advances to step S6.

[0086] In step S6 (near the time point t4 in FIG. 3D), the control device 15 calculates the average pressure drop rate Vpmean of the gas pressure Ph when the drain valve 164 is open, further adds a margin ΔP to calculate the threshold drop rate Vth [Pa / sec], and advances the process to step S7.

[0087] In step S7, the control device 15 checks whether both of the other valves excluding the drain valve 164, specifically the bleed valve 70 and the case lower end side on-off valve 174 whose valve opening affects the gas pressure Ph acquired by the pressure sensor 73 in the fuel gas supply passage 72, are closed, and waits until both are closed (step S7: YES) (step S7: NO), and proceeds with the process to step S8.

[0088] In step S8, the control device 15 determines whether the fluctuation range of the generated current Ifc is within a predetermined current fluctuation range, waits until it is within the predetermined current fluctuation range (step S8: NO), and proceeds with the process to step S9.

[0089] The reason for determining whether the fluctuation range of the generated current Ifc is within the predetermined current fluctuation range is to determine the pressure drop rate Vp under the situation where no extreme fluctuation of the gas pressure Ph occurs, as will be described with reference to FIGS. 5A to 5C below.

[0090] Note that the fluctuation of the generated current Ifc during the period when no fuel gas is supplied from the fuel gas supply passage 72 into the fuel cell stack 18 (non-discharge period Toff in FIG. 3A) is directly related to the ionization reaction of hydrogen at the anode electrode 57, and causes a fluctuation in the gas pressure Ph in the anode passage 59 (fuel gas supply passage 72).

[0091] FIGS. 5A to 5C are timing charts of an example for operation explanation.

[0092] In order to proceed to the process of step S9, as shown between time point t10 and time point t11 in FIG. 5A, it is a prerequisite that the generated current Ifc is a value within the predetermined current fluctuation range between the upper limit generated current (Ifci + ΔIfcu) obtained by adding the upper specified value ΔIfcu to a generated current Ifci of a certain initial value and the lower limit generated current (Ifci - ΔIfcd) obtained by subtracting the lower specified value Δifcd.

[0093] As shown between time points t11 and t14, when the generated current Ifc exceeds the upper limit generated current (Ifci + ΔIfcu), for example, in response to a sudden change in the accelerator opening (step S8: NO), the process does not proceed to steps S9 and S10 for determining the rate of pressure drop.

[0094] When the generated current Ifc takes a value outside the predetermined current fluctuation range, there is a risk that the detection reliability of whether the change in the pressure drop rate Vp of the gas pressure Ph is caused by the opening of the drain valve 164 will decrease.

[0095] When the generated current Ifc is within the predetermined current fluctuation range (step S8: YES), the control device 15 proceeds with the process to step S9.

[0096] In step S9, the control device 15 calculates the pressure drop rate Vp within the non-discharge period Toff while the drain valve 164 is open (opened in step S5, corresponding to after time point t3 in Fig. 3B), and proceeds with the process to step S10.

[0097] In step S10, the control device 15 determines whether the calculated pressure drop rate Vp is equal to or greater than the threshold pressure drop rate Vth. In the case of a negative determination, the process returns to step S7. In the case of an affirmative determination (after time point t7), the process proceeds to step S11.

[0098] Referring to Figs. 3A to 3D, the valve closing timing will be described. Between the valve opening time points t3 and t5 of the drain valve 164, as shown in Fig. 3B, only liquid water is discharged from the drain valve 164, so there is almost no fluctuation in the gas pressure Ph, and the pressure drop rate Vp does not become equal to or greater than the threshold pressure drop rate Vth.

[0099] At time point t6 during the non-discharge period Toffb, when the liquid water in the gas-liquid separator 36 flowing through the drain valve 164 decreases and fuel off-gas is discharged from the drain valve 164, the pressure drop rate Vp increases, but there is still little fluctuation in the gas pressure Ph, and the pressure drop rate Vp does not become equal to or greater than the threshold pressure drop rate Vth (Vp = Vp2 < Vth).

[0100] As shown at time t7 to time t8, when only the fuel off-gas flows through the drain valve 164, the gas pressure Ph drops rapidly, the pressure drop rate Vp rises sharply, and it reaches or exceeds the threshold drop rate Vth (Vp = Vp3 ≥ Vth, step S10: YES) during the non-discharge period Toffc.

[0101] In step S11, the control device 15 closes the drain valve 164 (instruction to close the drain valve 164) assuming that the discharge of the liquid water from the gas-liquid separator 36 is completed (time t9) (pressure drop rate Vp4).

[0102] Note that the determination process in step S10 may be a process that becomes affirmative when the pressure drop rate Vp immediately after opening the drain valve 164 becomes larger by a predetermined drop rate.

[0103] Thus, in the above embodiment, a current fluctuation monitoring function is provided for the condition of detecting the completion of drainage by the pressure drop rate Vp (determination in step S10). That is, the drainage detection by the pressure drop is performed only when the fluctuation range of the generated current Ifc is within the predetermined current fluctuation range (step S8: YES), thereby enhancing the detection reliability.

[0104] [Invention that can be grasped from the embodiment] Here, the invention that can be grasped from the above embodiment will be described below. For the convenience of understanding, some of the components are labeled with the reference numerals used in the above embodiment, but the components are not limited to those with the labeled reference numerals.

[0105] (1) The fuel cell system 10 according to the present invention includes a fuel cell stack 18 that generates electricity through an electrochemical reaction between a fuel gas and an oxidant gas, a fuel gas supply passage 72 that supplies the fuel gas to the fuel cell stack, a fuel off-gas passage 74 through which fuel off-gas discharged from the fuel cell stack flows, a gas-liquid separator 36 provided in the fuel off-gas passage, a drain valve 164 provided in the gas-liquid separator for discharging liquid water in the gas-liquid separator, a pressure sensor 73 provided in the fuel gas supply passage or the fuel off-gas passage for detecting the gas pressure Ph in the passage, a power generation state acquisition unit 115 for acquiring the power generation state of the fuel cell stack, and a control device 15. The control device controls the opening and closing of the drain valve based on the power generation state and the gas pressure.

[0106] With this configuration, by controlling the opening and closing of the drain valve in consideration of the power generation state of the fuel cell stack and the gas pressure in the fuel gas passage or the fuel off-gas passage, it is possible to accurately control the discharge of liquid water considering the water level state in the gas-liquid separator without using a water level sensor. Since the discharge of liquid water from the drain valve can be accurately controlled, the amount of fuel off-gas discharged from the drain valve can be suppressed, which in turn contributes to improving energy efficiency.

[0107] (2) Further, in the fuel cell system, the control device estimates the water level in the gas-liquid separator when the drain valve is closed based on the power generation state, and when the estimated water level (water level estimated value Hest) becomes equal to or higher than a threshold water level Hth, the drain valve is opened to discharge the liquid water. During the opening of the drain valve, when the pressure drop rate Vp of the gas pressure detected by the pressure sensor becomes equal to or higher than a threshold drop rate Vth in a state where the fluctuation of the power generation state is within a predetermined range, the drain valve is closed.

[0108] According to this, the water level is estimated based on the power generation state, and when the estimated water level becomes equal to or higher than the threshold water level, the drain valve is opened to discharge the liquid water. During the discharge, when the pressure drop rate of the gas pressure in the fuel gas flow path or the gas pressure in the fuel off-gas flow path becomes equal to or higher than the threshold drop rate, it can be detected that the discharge of the off-gas is started and the drainage is completed.

[0109] In this way, since the drain valve can be opened and closed in consideration of the power generation state, the liquid water can be accurately discharged from the drain valve without using a water level sensor. As a result, it is possible to suppress the drain valve from being opened for an unnecessary period of time and the fuel off-gas from being exhausted from the drain valve. Since power generation can be performed by the fuel gas contained in the fuel off-gas, it contributes to the improvement of energy efficiency.

[0110] (3) Further, in the fuel cell system, a connection flow path 78 connected from the fuel off-gas flow path to the oxidant gas supply flow path, and an on-off valve for opening and closing the communication state of the connection flow path are further provided. The control device may control the opening and closing of the drain valve based on the power generation state and the gas pressure when the on-off valve is closed.

[0111] Thereby, when the gas pressure fluctuates due to the opening of the on-off valve of the connection flow path, the opening and closing control of the drain valve is not performed, so that it is possible to prevent the drain valve from being accidentally opened.

[0112] (4) Furthermore, in the fuel cell system, a case-side on-off valve provided in a case that houses the fuel cell stack and discharges the liquid water in the fuel cell stack to the outside is further provided. The control device may control the opening and closing of the drain valve based on the power generation state and the gas pressure when the case-side on-off valve is closed.

[0113] Thereby, when the gas pressure fluctuates due to the opening of the case-side on-off valve, the opening and closing control of the drain valve is not performed, so that it is possible to prevent the drain valve from being accidentally opened.

[0114] Note that the present invention is not limited to the above-described embodiments, and various configurations can be adopted without departing from the gist of the present invention.

Explanation of Reference Numerals

[0115] 10…Fuel cell system 12…Fuel cell vehicle 15…Control device 18…Fuel cell stack 22…Oxidant gas supply device 24…Fuel gas supply device 26…Refrigerant supply device 36…Gas-liquid separator 70…Bleed valve 72…Fuel gas supply flow path 73…Pressure sensor 74…Fuel off-gas flow path 110…Voltage sensor 112…Current sensor 115…Power generation state acquisition unit 164…Drain valve 172…Case lower end side discharge flow path 174…Case lower end side on-off valve Hest…Estimated water level Hth…Threshold water level Ifc, Ifci…Power generation current Ph…Gas pressure Vfc…Power generation voltage Vp…Pressure drop rate Vth…Threshold drop rate

Claims

1. a fuel cell stack that generates electricity through an electrochemical reaction between a fuel gas and an oxidant gas; a fuel gas supply passage that supplies the fuel gas to the fuel cell stack; a fuel off-gas flow path through which a fuel off-gas discharged from the fuel cell stack flows; a gas-liquid separator provided in the fuel off-gas flow path; a drain valve provided in the gas-liquid separator for discharging liquid water within the gas-liquid separator; a pressure sensor provided in the fuel gas supply passage or the fuel off-gas passage for detecting a gas pressure in the passage; a power generation state acquisition unit for acquiring a power generation state of the fuel cell stack; A control device, the control device controls opening and closing of the drain valve based on the power generation state and the gas pressure; Furthermore, the control device a water level in the gas-liquid separator when the drain valve is closed is estimated based on the power generation state, and when the estimated water level becomes equal to or higher than a threshold water level, the drain valve is opened to discharge the liquid water; While the drain valve is open, it is determined whether or not a fluctuation range of a generated current in the power generating state is within a predetermined current fluctuation range; When the generated current exceeds the upper limit generated current of the predetermined current fluctuation range, the pressure drop rate of the gas pressure is not determined, and the drain valve is kept open; When the fluctuation range of the generated current is within the predetermined current fluctuation range, the drain valve is closed when the pressure drop rate of the gas pressure detected by the pressure sensor becomes equal to or greater than a threshold drop rate while the drain valve is open. Fuel cell system.

2. 2. The fuel cell system according to claim 1, a connection flow path that is connected from the fuel off-gas flow path to an oxidant gas supply flow path; An on-off valve that opens and closes the communication state of the connection flow path, The control device includes: When the on-off valve is closed, the drain valve is controlled to be opened or closed based on the power generation state and the gas pressure. Fuel cell system.

3. 3. The fuel cell system according to claim 1, a case-side opening / closing valve provided in a case that houses the fuel cell stack and that discharges liquid water in the fuel cell stack to the outside; The control device includes: When the case side opening / closing valve is closed, the drain valve is controlled to open or close based on the power generation state and the gas pressure. Fuel cell system.

Images