Fuel cell system and method for controlling the fuel cell system

Abstract

Fuel cell system (100) comprising: a fuel cell stack (10) configured to receive reaction gas to generate electrical power; a gas-liquid separator (32) connected to the fuel cell stack (10) and configured to separate exhaust gas from the fuel cell stack (10) into a liquid component and a gas component and to store liquid water of the liquid component; a circulation line (33) which is connected to the gas-liquid separator (32) and which forms a circulation path which is set up to circulate the gas component in the gas-liquid separator (32) to the fuel cell stack (10); an outlet line (35) which is connected to the gas-liquid separator (32) and which is designed to drain the liquid water from the gas-liquid separator; an outlet valve (36) configured to open and close the outlet line (35); and a control unit (50) which is configured to control the supply of the reaction gas to the fuel cell stack (10) and to perform a final purging process, wherein the final purging process is a process for circulating the gas component of the exhaust gas as purge gas in order to perform purging when a process of the fuel cell system (100) is completed, wherein the control unit (50) is configured to perform an outlet valve control, wherein the outlet valve control is a control during which, when a valve opening condition is met which is determined in advance in conjunction with a quantity of liquid water stored in the gas-liquid separator (32), the outlet valve (36) is opened to discharge the liquid water from the gas-liquid separator (32), and the control unit (50) is configured such that the control unit (50) performs the outlet valve control in the final purge process using the valve opening condition which is set such that the amount of liquid water stored in the gas-liquid separator (32) at the time when the outlet valve (36) is opened in the final purge process is greater than the amount of liquid water stored in the gas-liquid separator (32) at the time when the outlet valve (36) is opened during normal operation of the fuel cell system (100).

Description

Background of the invention 1. Field of the invention

[0001] The invention relates to fuel cell systems and methods for controlling a fuel cell system. 2. Description of the state of the art

[0002] For example, JP 2009-104966A discloses a fuel cell system that uses a gas-liquid separator to separate exhaust gas discharged from a fuel cell stack into a gas component and a liquid component, circulates the gas component to the fuel cell stack, and stores the liquid component as liquid water in the gas-liquid separator and then discharges the liquid water from the gas-liquid separator. US 2010 / 055523A1 discloses a fuel cell system that discharges water in a gas-liquid separator, wherein the fuel cell system comprises: a fuel cell stack; a hydrogen circulation line; a gas-liquid separator; a discharge unit with a discharge valve for discharging the water stored in the gas-liquid separator; and a discharge control unit for controlling the discharge unit.a determination unit for determining whether the drain control unit decides that the drain unit should be opened or not; a freezing state determination unit that determines whether the drain unit is frozen or not; and a thawing state determination unit that determines whether the drain unit is thawed or not, wherein the drain control unit controls the drain unit to discharge a larger quantity of water than a quantity of water that would be discharged in an ordinary state process, assuming that the drain unit is not frozen, after the freezing state assumption unit assumes that the drain unit is frozen, and when the thawing state determination unit determines that the drain unit is thawed. Summary of the invention

[0003] In such a fuel cell system, where exhaust gas is circulated to and reused by a fuel cell stack that uses a gas-liquid separator as described above, water droplets formed on the inner wall surface of the gas-liquid separator can remain there even after the fuel cell system has ceased operation. When the fuel cell system stops operating and such water droplets remain in the gas-liquid separator, these water droplets can either freeze within the separator or migrate to an outlet valve formed in an outlet path from the gas-liquid separator and freeze in the outlet valve in a low-temperature environment, such as below freezing.The frozen water droplets in the gas-liquid separator make it difficult to determine the amount of liquid water stored within. Furthermore, the frozen outlet valve prevents the drainage of liquid water from the separator. Conventionally, such water droplets remaining on the inner surface of the separator cannot be sufficiently reduced, even by performing a purge after the fuel cell system has shut down.

[0004] The technology of the present invention can be implemented in the following embodiments.

[0005] A first embodiment is designed as a fuel cell system. This embodiment's fuel cell system comprises: a fuel cell stack configured to receive reaction gas to generate electrical power; a gas-liquid separator connected to the fuel cell stack, configured to separate exhaust gas from the fuel cell stack into a liquid component and a gas component, and to store liquid water from the liquid component; a circulation line connected to the gas-liquid separator, forming a circulation path configured to circulate the gas component in the gas-liquid separator to the fuel cell stack; an outlet line connected to the gas-liquid separator, configured to discharge the liquid water from the gas-liquid separator; and an outlet valve configured toto open and close the outlet line; and a control unit configured to control the supply of reaction gas to the fuel cell stack and to perform a final purge process, wherein the final purge process is a process for circulating the gas component of the exhaust gas as purge gas to perform purging when a process of the fuel cell system is completed. The control unit is configured to perform an outlet valve control, wherein the outlet valve control is a control during which, when a valve opening condition is met, which is predetermined in conjunction with a quantity of liquid water stored in the gas-liquid separator, the outlet valve is opened to discharge the liquid water from the gas-liquid separator. The control unit is configured such that the control unit performs the outlet valve control in the final purge process using the valve opening condition.which is adjusted such that the amount of liquid water stored in the gas-liquid separator at the time the outlet valve is opened during the final purge process is greater than the amount of liquid water stored in the gas-liquid separator at the time the outlet valve is opened during normal operation of the fuel cell system. According to this fuel cell system configuration, the level of liquid water in the gas-liquid separator is higher during the final purge process at the time the outlet valve is opened, and therefore the area in which water droplets remain on an inner wall surface of the gas-liquid separator is reduced. This configuration thus reduces the amount of water droplets remaining on the inner wall surface of the gas-liquid separator.when the fuel cell system is shut down. Accordingly, a fault due to freezing of moisture remaining in the gas-liquid separator is less likely to occur when the fuel cell system is restarted in a low-temperature environment.

[0006] In the fuel cell system of the above configuration, the control unit can be configured to measure the amount of liquid water stored in the gas-liquid separator. The control unit can be configured such that, during the outlet valve control, which is performed during normal operation of the fuel cell system, the control unit determines that the valve opening condition is met and opens the outlet valve when the amount of liquid water stored in the gas-liquid separator exceeds a predetermined first threshold.The control unit can be configured such that, during the outlet valve control performed in the final purge process, the control unit determines that the valve opening condition is met and the outlet valve opens when the amount of liquid water stored in the gas-liquid separator exceeds a second threshold, which is set to a higher value than the first threshold. According to the fuel cell system of this configuration, the liquid water can be discharged at a suitable time, based on the amount of liquid water stored in the gas-liquid separator.

[0007] In the fuel cell system of the above configuration, the control unit can be configured such that it determines a purge gas flow rate during the final purge process and sets the valve opening condition according to this determined flow rate before purging is initiated. According to this fuel cell system configuration, because the valve opening condition is set, the level of liquid water in the gas-liquid separator at the time the outlet valve opens can be altered according to the purge gas flow rate during the final purge process. Consequently, even if the purge gas flow rate is altered, it is less likely that liquid water will be disturbed in the gas-liquid separator due to pump suction.

[0008] In the fuel cell system of the above embodiment, a filter can be incorporated into the gas-liquid separator, configured such that the liquid water passes through the filter. The filter can be arranged to divide the interior of the gas-liquid separator into a first region above the filter and a second region below the filter. The control unit can be configured to control the outlet valve during normal operation of the fuel cell system using a first valve opening condition, wherein the first valve opening condition is determined such that the outlet valve opens when the liquid water level in the gas-liquid separator is in the second region.The control unit can be configured such that, during the final purge process, it controls the outlet valve using a second valve opening condition. This second condition is determined such that the outlet valve opens when the liquid water level in the gas-liquid separator is in the first region. According to this configuration of the fuel cell system, because of the filter design, it is less likely that foreign particles contained in the liquid water will reach the outlet valve, and therefore, it is less likely that the outlet valve will fail due to these particles. Furthermore, it is less likely that moisture will remain in the filter after the liquid water has been drained from the gas-liquid separator during the final purge process.Filter clogging due to frozen moisture remaining on it is less likely when the fuel cell system is restarted in a low-temperature environment.

[0009] Another embodiment is designed as a method for controlling a fuel cell system. The fuel cell system comprises a fuel cell stack configured to receive reaction gas to generate electrical power, a gas-liquid separator configured to separate exhaust gas from the fuel cell stack into a liquid component and a gas component and to store liquid water from the liquid component, a circulation line forming a circulation path configured to circulate the gas component in the gas-liquid separator to the fuel cell stack, an outlet line configured to discharge the liquid water from the gas-liquid separator, and an outlet valve configured to open and close the outlet line.The method for controlling the fuel cell system according to this embodiment comprises: performing a final purging process, wherein the final purging process is a process for circulating the gas component of the exhaust gas as purge gas to perform purging when a process of the fuel cell system is completed; and performing an outlet valve control, wherein the outlet valve control is a control during which, when a valve opening condition is met, which is determined in advance in conjunction with an amount of liquid water stored in the gas-liquid separator, the outlet valve is opened to discharge the liquid water from the gas-liquid separator.In the final purge process, the outlet valve control is performed using the valve opening condition, which is set such that the amount of liquid water stored in the gas-liquid separator at the time the outlet valve is opened in the final purge process is greater than the amount of liquid water stored in the gas-liquid separator at the time the outlet valve is opened during normal operation of the fuel cell system.

[0010] The technology of the present invention can be implemented in various embodiments other than the fuel cell system. For example, the technology of the present invention can be implemented in embodiments such as a vehicle equipped with a fuel cell system, a method for controlling a fuel cell vehicle, a method for performing drainage in a fuel cell system or a fuel cell vehicle, a method for performing a flushing procedure, a method for performing drainage in a fuel cell system or a fuel cell vehicle, a control device or a computer program implementing these methods, and a non-volatile storage medium on which the computer program is stored. Brief description of the drawing

[0011] Features, advantages and technical and industrial significance of exemplary embodiments of the invention are described below with reference to the accompanying drawing, in which the same reference numerals denote the same components and wherein: Fig. 1 is a schematic diagram that represents a configuration of a fuel cell system; Fig. 2 is a schematic sectional view showing a configuration of a gas-liquid separator; Fig. 3 a flow diagram of a final rinsing process of a first embodiment; Fig. 4 is a schematic representation depicting a configuration of a gas-liquid separator of a second embodiment; Fig. 5 is a flow diagram of a final rinsing process of a third embodiment; Fig. 6 is a flowchart of a process condition determination process of the third embodiment; Fig. 7 represents an example of a characteristic map used to determine a valve opening condition in the third embodiment; and Fig. Figure 8 is an example of a characteristic map used to determine a valve opening condition in a fourth embodiment. Detailed description of embodiments 1. First embodiment

[0012] Fig. Figure 1 is a schematic diagram representing a configuration of a fuel cell system 100 according to a first embodiment. The fuel cell system 100 is, for example, mounted in a vehicle and supplies electrical power to a drive motor that generates motive power and to auxiliary equipment used in the vehicle. In other embodiments, the fuel cell system 100 can be mounted in other movable bodies besides vehicles, such as a ship, or it can be stationary as a power generation system for buildings, etc.

[0013] The fuel cell system 100 comprises a fuel cell stack 10 to which reaction gas, namely fuel gas and oxidation gas, is supplied to generate electrical power. The fuel cell stack 10 is, for example, a solid polymer electrolyte fuel cell stack and generates electrical power through an electrochemical reaction of hydrogen as fuel gas and oxygen as oxidation gas. The fuel cell stack 10 has a stacked structure in which a plurality of individual cells 11 are stacked. Each individual cell 11 is a power generating element capable of generating electrical power independently and has a membrane electrode assembly and two separators that sandwich the membrane electrode assembly between them. The membrane electrode assembly is a power generator formed from an electrolyte membrane and electrodes arranged on both sides of the electrolyte membrane.The electrolyte membrane is a thin solid polymer membrane that exhibits sufficient proton conductivity in a wet state, namely when it contains moisture.

[0014] The fuel cell system 100 further comprises a gas supply unit 20, a gas circulation unit 30, and a gas supply / drain unit 40 as components for supplying reaction gas to the fuel cell stack 10 and for exhaust gas from the fuel cell stack 10. The gas supply unit 20 and the gas circulation unit 30 control a fuel gas that is supplied to an anode of the fuel cell stack 10, and the gas supply / drain unit 40 controls an oxidation gas that is supplied to a cathode of the fuel cell stack 10.

[0015] The gas supply unit 20 supplies fuel gas to the anode of the fuel cell stack 10. The gas supply unit 20 comprises a tank 21, a fuel gas line 22, a main shut-off valve 23, a regulator 24, and a feed device 25. The tank 21 stores high-pressure fuel gas. The fuel gas line 22 connects the tank 21 to an anode inlet of the fuel cell stack 10. The main shut-off valve 23, the regulator 24, and the feed device 25 are arranged in the fuel gas line 22 in this order, starting from the upstream side, i.e., the side of the tank 21. The main shut-off valve 23 is an electromagnetic valve. The main shut-off valve 23 is controlled by a control unit 50 to open and close the fuel gas line 22, thus controlling the outflow of fuel gas from the tank 21. The regulator 24 is a pressure reducing valve and adjusts the pressure in the fuel gas line 22 on the upstream side of the supply device 25.The supply device 25 is actuated at regular intervals so that it opens or closes to deliver the fuel gas to the fuel cell stack 10. The supply device 25 is, for example, an injection device, which is an electromagnetic on / off valve that is driven to open and close at set intervals. The intervals at which the supply device 25 is actuated are controlled by the control unit 50.

[0016] The gas circulation unit 30 has the function of circulating fuel gas contained in the exhaust gas discharged from the anode of the fuel cell stack 10 to the fuel cell stack 10, and of venting inert gas and wastewater contained in the exhaust gas from the fuel cell system 100 to the outside. The gas circulation unit 30 comprises an exhaust gas line 31, a gas-liquid separator 32, a circulation line 33, a pump 34, an outlet line 35, and an outlet valve 36.

[0017] The exhaust line 31 is connected to an anode outlet of the fuel cell stack 10 and the gas-liquid separator 32. The exhaust line 31 carries exhaust gas on the anode side, containing a gas component such as fuel gas not used for power generation at the anode and inert gas, as well as a liquid component such as water vapor and liquid water, into the gas-liquid separator 32. As used here, the "liquid component" of the exhaust gas is a concept that is not limited to the liquid component in the form of liquid water, but includes a component in a gaseous state, such as water vapor, which transforms into liquid water through a change of state. This is because water vapor in the exhaust gas partially condenses into liquid water in the gas-liquid separator 32.Other embodiments may use a configuration in which the exhaust line 31 is omitted and the gas-liquid separator 32 is directly connected to the anode outlet of the fuel cell stack 10.

[0018] The gas-liquid separator 32 separates the liquid and gas components contained in the exhaust gas flowing through the exhaust pipe 31 and stores the liquid component as liquid water. The liquid component of the exhaust gas includes both moisture in a gaseous state and moisture in the form of liquid water. The liquid water stored in the gas-liquid separator 32 comprises liquid water that has condensed from the moisture in a gaseous state within the separator and the liquid water that flows into the separator. The configuration of the gas-liquid separator 32 and the mechanism of gas-liquid separation of the exhaust gas within the separator 32 are described later.

[0019] The circulation line 33 is connected to the gas-liquid separator 32 and forms a circulation path for the circulation of the gas component separated by the gas-liquid separator 32 to the anode of the fuel cell stack 10. In the present embodiment, the circulation line 33 connects the gas-liquid separator 32 and a portion of the fuel gas line 22 located downstream of the supply device 25. The pump 34 is integrated into the circulation line 33. The pump 34 is controlled by the control unit 50, generating a pressure that delivers the gas component from the gas-liquid separator 32 to the fuel cell stack 10.

[0020] The outlet line 35 is connected to the gas-liquid separator 32 and conveys the liquid water stored in the gas-liquid separator 32, as well as the inert gas contained in the exhaust gas, from the gas-liquid separator 32 to the outside. The outlet valve 36 is located in the outlet line 35. The outlet valve 36 is controlled by the control unit 50, so that it opens and closes the outlet line 35. In the fuel cell system 100, liquid water is stored in the gas-liquid separator 32 while the outlet valve 36 is closed, and the liquid water stored in the gas-liquid separator 32 is discharged as wastewater from the fuel cell system 100 to the outside through the outlet line 35 when the outlet valve 36 is opened. The downstream end of the outlet line 35 can be connected to a drain line 46 of the gas supply / drain unit 40.

[0021] The gas supply / drain unit 40 has a function to supply oxidation gas to the cathode of the fuel cell stack 10 and a function to discharge exhaust gas discharged from the cathode of the fuel cell stack 10 from the fuel cell system 100 to the outside. In the first embodiment, oxygen contained in the ambient air is supplied to the fuel cell stack 10 as oxidation gas. The gas supply / drain unit 40 comprises a supply line 41, a compressor 42, an on / off valve 43, the drain line 46, and a pressure regulating valve 48.

[0022] One end of the supply line 41 is connected to ambient air, and the other end is connected to a cathode inlet of the fuel cell stack 10. The compressor 42 compresses the ambient air supplied from one end of the supply line 41 and delivers the compressed air towards the on / off valve 43, which is located in the supply line 41 at a position closer to the other end. The on / off valve 43 is normally closed and is opened by the pressure of the compressed gas supplied by the compressor 42 to allow the compressed gas to flow into the cathode of the fuel cell stack 10.

[0023] The drain line 46 is connected to a cathode outlet of the fuel cell stack 10 and directs the exhaust gas that has been drained from the cathode of the fuel cell stack 10 out of the fuel cell system 100. The pressure regulating valve 48 is integrated into the drain line 46 and adjusts the back pressure on the cathode side of the fuel cell stack 10.

[0024] The fuel cell system 100 further comprises the control unit 50, which controls the operation of the fuel cell system 100. The control unit 50 is an electronic control unit (ECU) comprising at least one processor and a main memory device. The control unit 50 executes programs and instructions read into the main memory device via the processor and performs various functions to control the operation of the fuel cell system 100. At least some of the functions of the control unit 50 can be provided by a hardware circuit.

[0025] The control unit 50 controls the power generation of the fuel cell stack 10. The control unit 50 controls the gas supply unit 20, the gas circulation unit 30, and the gas supply / drain unit 40 to control the supply of reaction gas to the fuel cell stack 10 and the discharge of exhaust gas from the fuel cell system 100. The control unit 50 controls the intervals at which the supply device 25 is driven and the rotational speed of the pump 34 to control the quantity and pressure of fuel gas supplied to the fuel cell stack 10. The control unit 50 controls the rotational speed of the compressor 42 to control the quantity of oxidation gas to be supplied to the fuel cell stack 10 and controls the opening of the pressure regulating valve 48 to adjust the pressure on the cathode side of the fuel cell stack 10.

[0026] While the fuel cell system 100 is powered, the control unit 50 controls the opening and closing of the outlet valve 36 in the gas circulation unit 30 to perform outlet valve control. Outlet valve control is a control mechanism during which the outlet valve 36 is normally kept closed, and when a specific valve opening condition is met, the outlet valve 36 opens to discharge liquid water from the gas-liquid separator 32. The valve opening condition is determined in relation to the amount of liquid water stored in the gas-liquid separator 32. In the first embodiment, the valve opening condition for the outlet valve 36 is met when the amount of liquid water stored in the gas-liquid separator 32 exceeds a predetermined threshold.In the first embodiment, the control unit 50 determines a measured quantity of liquid water stored in the gas-liquid separator 32 and uses this measured value to determine whether the valve opening condition is met. The quantity of liquid water stored in the gas-liquid separator 32 can be determined using a known functional expression and a known characteristic map, and using parameters such as the amount of power generated by the fuel cell stack 10, the water content of the fuel cell stack 10, the water temperature, and the partial pressure of water vapor in a reaction gas.

[0027] The control unit 50 performs the exhaust valve control during normal operation of the fuel cell system 100 or during a purge process, which will be described later. As used here, “normal operation of the fuel cell system 100” means operation in which reaction gas is supplied from the gas supply unit 20, the gas circulation unit 30, and the gas supply / drain unit 40 to the fuel cell stack 10 to cause the fuel cell stack 10 to generate a set amount of power. The period of normal operation of the fuel cell system 100 includes a period during which the fuel cell stack 10 ceases to generate the set amount of power so that a predetermined temporary process can be carried out without stopping the fuel cell system 100, such as a warm-up period or a purge process described later.During normal operation of the fuel cell system 100, the control unit 50 opens the outlet valve 36 via the outlet valve control and then closes the outlet valve 36 at the time when the discharge of liquid water from the gas-liquid separator 32 and the discharge of a predetermined quantity of inert gas from the gas-liquid separator 32 are assumed to be complete. The time at which the outlet valve 36 is closed can be determined based on, for example, the quantity of liquid water in the gas-liquid separator 32 at the time the outlet valve 36 opens.

[0028] The control unit 50 performs a purging process when a predefined execution condition is met during the normal operation of the fuel cell system 100. The purging process is a process in which the fuel cell system 100 is purged. For example, the control unit 50 performs the purging process when a detected water content of the fuel cell stack 10 exceeds a predefined threshold, when a blockage of a gas flow path by produced water is detected, when a purging command is detected that is given by the user by performing a switching operation, etc.

[0029] During the purging process, the control unit 50 stops the feed device 25 and drives the pump 34 to circulate the gas component of the exhaust gas, separated by the gas-liquid separator 32, between the gas circulation unit 30 and the fuel cell stack 10. During the purging process, this gas component is used as purge gas. The fuel gas flow path, which includes the flow path in the fuel cell stack 10, is purged by this circulation of the gas component of the exhaust gas. During the purging process, the control unit 50 can also drive the compressor 42 to purge the oxidation gas flow path, which includes the flow path in the fuel cell stack 10, using ambient air, in addition to purging the gas flow path on the anode side of the fuel cell stack 10.

[0030] The control unit 50 also performs the outlet valve control during the purging process. Accordingly, when the valve opening condition is met, liquid water that has been directed to and is stored in the gas-liquid separator 32 is discharged from the gas-liquid separator 32 by the purge gas. During the outlet valve control, which is also performed during the purging process, the control unit 50 determines whether the outlet valve 36 should be opened or not, using the determined quantity of water stored in the gas-liquid separator 32, which is calculated by a similar procedure to the one described above. The amount of power generated by the fuel cell stack 10 using the reaction gas component contained in the purge gas is used to calculate the determined quantity of liquid water stored in the gas-liquid separator 32 during the purging process.

[0031] In the fuel cell system 100, the control unit 50, in addition to the purging process performed during normal operation of the fuel cell system 100, executes the final purging process when the operation of the fuel cell system 100 is terminated. The phrase "when the operation of the fuel cell system 100 is terminated" means when preparations to terminate the operation of the fuel cell system 100 are initiated in response to a command from the user or the control unit 50, and signifies the phase before the operation of the fuel cell system 100 is completely finished. The phrase "when the operation of the fuel cell system 100 is complete" includes, for example,, if the control unit 50 terminates the operation of the fuel cell system 100 again after it has automatically restarted the fuel cell system 100 according to a program during an interruption period after the operation of the fuel cell system 100 had been terminated.

[0032] In the final purging process, the fuel gas flow path is purged in a similar manner to the purging process performed during the normal operation of the fuel cell stack 10, as described above. Specifically, the pump 34 is driven to circulate the gas component of the exhaust gas as purge gas, thereby purging the fuel gas flow path. Also in the final purging process, the compressor 42 can be driven to purge the oxidation gas flow path. The outlet valve control described above is also performed in the final purging process. However, the outlet valve control used in the final purging process employs a valve opening condition that differs from the valve opening condition used during the normal operation of the fuel cell system 100, in order to reduce the amount of liquid water remaining in the gas-liquid separator 32.The final rinsing process will be explained in detail later.

[0033] Fig. Figure 2 is a schematic sectional view illustrating an example of the configuration of the gas-liquid separator 32. The body of the gas-liquid separator 32 is formed by a container 60, which has an interior 61 into which the exhaust gas can be introduced. The gas-liquid separator 32 has an exhaust gas inlet 62, which is connected to the exhaust gas line 31, a circulation gas outlet 63, which is connected to the circulation line 33, and a discharge outlet 64, which is connected to the discharge line 35.

[0034] The gas-liquid separator 32 is normally arranged in the fuel cell system 100 such that the exhaust gas inlet 62 and the circulation gas outlet 63 are located in an upper region of the interior 61 in the direction of gravity, and the drain outlet 64 is located in a lower region of the interior 61 in the direction of gravity. In the following description of the gas-liquid separator 32, the terms "upper", "lower", "horizontal direction", and "vertical direction" refer to the directions based on the direction of gravity when the gas-liquid separator 32 is arranged in this manner or in this position.

[0035] The interior 61 of the gas-liquid separator 32 comprises a main region 61M, an upper region 61U, and a lower region 61L. The main region 61M is the widest region in the horizontal direction and has the largest volume. The upper region 61U projects locally upwards from the main region 61M in the direction of gravity. The lower region 61L projects locally downwards from the main region 61M in the direction of gravity. The exhaust gas inlet 62 opens into the upper region 61U, the recirculation gas outlet 63 opens into the upper end of the main region 61M, and the discharge outlet 64 opens into the lower region 61L. The exhaust gas inlet 62 and the recirculation gas outlet 63 are located approximately on opposite sides of the interior 61 in the horizontal direction. A structure such as a lamella for simplified gas-liquid separation can be formed in the gas-liquid separator 32.

[0036] The gas-liquid separator 32 has an opposing inner wall surface 65 opposite the exhaust gas inlet 62 in the interior space 61. The opposing inner wall surface 65 forms part of an inner wall surface that defines the upper region 61U of the interior space 61. Exhaust gas, which is directed from the exhaust line 31 through the exhaust gas inlet 62 into the gas-liquid separator 32, flows towards and impacts the opposing inner wall surface 65 as indicated by a dashed arrow. As soon as the exhaust gas impacts the opposing inner wall surface 65, the gas component of the exhaust gas disperses, flows from the upper region 61U into the main region 61M, which has lower flow path resistance, and flows into the circulation line 33 through the circulation gas outlet 63.A large portion of the liquid component contained in the exhaust gas strikes the opposite inner wall surface 65 and forms droplets WD of liquid water on this surface. The liquid component also forms water droplets WD on the inner wall surfaces of the interior 61, except for the opposite inner wall surface 65. These water droplets WD are formed from splashes of liquid water generated when the liquid component contained in the exhaust gas strikes the opposite inner wall surface 65, and from liquid water that condenses when the liquid component contained in the exhaust gas comes into contact with the inner wall surface of the gas-liquid separator 32.As soon as the water droplets WF on the opposite inner wall surface 65 and the inner wall surfaces of the interior space 61 (excluding the opposite inner wall surface 65) merge, the water droplets WD become large enough to be drawn by gravity into the lower region of the interior space 61, which includes the lower region 61L, and are stored there. However, many water droplets WD remain on the inner wall surfaces.

[0037] To prevent liquid water LS stored in the interior 61 from remaining after the outlet valve 36 is opened, the gas-liquid separator 32 is designed such that liquid water LS tends to collect in the lower region 61L at the bottom of the interior 61. The lower region 61L is narrower in the horizontal direction than the main region 61M. Accordingly, the surface area of ​​the liquid water LS stored in the lower region 61L is less likely to fluctuate when the gas-liquid separator 32 is tilted or vibrated. Therefore, the liquid water LS stored in the lower region 61L is less likely to splash through the circulation gas outlet 63 into the circulation line 33.

[0038] Fig. Figure 3 is a flowchart of the final purging process of the first embodiment. The control unit 50 starts the final purging process in response to a command from the user to terminate the operation of the fuel cell system 100. The control unit 50 also starts the final purging process at a predetermined time to terminate the operation of the fuel cell system 100.

[0039] In step S10, the control unit 50 determines whether a condition for initiating purging is met. In the first embodiment, the condition for initiating purging is met when the ambient air temperature, measured by a temperature reference unit such as a temperature sensor (not shown), is lower than a predetermined threshold temperature. The threshold temperature is, for example, 10 °C or less. If the ambient air temperature is lower than the threshold temperature, the control unit 50 executes the processes from step S20 and the subsequent steps to perform purging. This serves to reduce difficulties in starting the fuel cell system 100 the next time it is used, which are caused when moisture remaining in the gas flow path of the fuel cell system 100 freezes in a low-temperature environment where the ambient temperature is lower than the threshold temperature.If the outside air temperature is higher than the threshold temperature, the control unit 50 terminates the final purging process without performing the purge. Similarly, if it is less likely that any remaining moisture in the gas flow path of the fuel cell system 100 will freeze, no purge is performed, allowing the fuel cell system 100 to be shut down more quickly.

[0040] The condition for initiating purging in step S10 is not limited to the above condition based on the outside air temperature. For example, the condition for initiating purging in step S10 can be met if the water content of fuel cell stack 10 exceeds a predefined threshold. The condition for initiating purging in step S10 can also be met if purging is scheduled in advance by the user. Alternatively, the condition for initiating purging in step S10 can be met if it is determined, based on calendar information, that it is winter.

[0041] In step S20, the control unit 50 changes the valve opening condition for the exhaust valve control from the valve opening condition used during the normal operation of the fuel cell system 100. As described above, the exhaust valve control is performed in the final purge process. Steps S40 and S70 described below correspond to the exhaust valve control process. As described above, in the exhaust valve control of the first embodiment, the control unit 50 opens the exhaust valve 36 when the amount of liquid water stored in the gas-liquid separator 32 is greater than the predetermined threshold.In step S20, the valve opening condition is changed so that the amount of liquid water stored in the gas-liquid separator 32 at the time the outlet valve 36 opens, as performed during the final purge process, is greater than the amount of liquid water stored in the gas-liquid separator 32 at the time the outlet valve 36 opens, as performed during the normal operation of the fuel cell system 100. Specifically, in step S20, the threshold for the valve opening condition is changed from a first threshold Tha to a second threshold Thb, which is greater than the first threshold Tha. The first threshold Tha and the second threshold Thb are values ​​representing the amount of liquid water LS stored in the gas-liquid separator 32.

[0042] The first threshold Tha is a threshold that is preset as a valve opening condition so that the exhaust valve control is carried out during the normal operation of the fuel cell system 100.

[0043] Fig. Figure 2 shows a first water level LV1 as an example of the water level at the time when the quantity of liquid water LS corresponding to the first threshold Tha is stored in the gas-liquid separator 32, and shows a second water level LV2 as an example of the water level at the time when the quantity of liquid water LS corresponding to the second threshold Thb is stored in the gas-liquid separator 32.

[0044] In the outlet valve control, which is performed during the normal operation of the fuel cell system 100, the control unit 50 opens the outlet valve 36 when the amount of liquid water stored in the gas-liquid separator 32 exceeds the first threshold value Tha. For example, the first threshold value Tha is set to a value corresponding to an amount of liquid water such that the surface of the liquid water LS is at or nearer the upper end of the lower region 61L when the gas-liquid separator 32 is in a predetermined reference position, namely when the gas-liquid separator 32 is in an upright position. The expression "is nearer the upper end of the lower region 61L" means that the surface of the liquid water LS is closer to the upper end of the lower region 61L than to its lower end in the vertical direction.The first threshold value Tha can be set to a value corresponding to the amount of liquid water that is 5% to 20% of the capacity of the gas-liquid separator 32. Fig. Figure 2 shows the water level at the time when the amount of liquid water LS, corresponding to the first threshold Tha, is stored in the gas-liquid separator 32, as the first water level LV1. Because the first threshold Tha is set to such a low value, the outlet valve 36 opens to discharge the liquid water LS from the gas-liquid separator 32 before the liquid water level LS in the gas-liquid separator 32 becomes too high during normal operation of the fuel cell system 100. Accordingly, it is less likely that the liquid water LS stored in the gas-liquid separator 32 will enter the circulation line 33 through the circulation gas outlet 63, even if the angle of the gas-liquid separator 32 varies during normal operation of the fuel cell system 100.

[0045] In the final purge process, the second threshold Thb is used instead of the first threshold Tha. For example, the second threshold Thb is set to a value corresponding to a quantity of liquid water such that the surface of the liquid water LS is located at or near the center of the main region 61M in the vertical direction when the gas-liquid separator 32 is in the reference position, that is, when the gas-liquid separator 32 is in an upright position. The phrase "located near the center of the main region 61M in the vertical direction" means that the surface of the liquid water LS is located nearer the center of the main region 61M in the vertical direction than at its upper and lower ends in the vertical direction.The second threshold Thb can be set to a value corresponding to the amount of liquid water that represents 30% to 70% of the capacity of the gas-liquid separator 32. It is desirable to set the second threshold Thb such that the amount of liquid water stored in the gas-liquid separator 32 is low enough that the liquid water LS will not enter the circulation gas outlet 63 due to the suction force of the pump 34.

[0046] The reason why the valve opening condition is changed from the first threshold Tha to the second threshold Thb in step S20 is described later. In the outlet valve control of the first embodiment, as described above, the valve opening time for the gas-liquid separator 32 is determined by the valve opening condition, which uses the threshold representing the amount of liquid water stored in the gas-liquid separator 32, such as the first threshold Tha and the second threshold Thb. The liquid water LS is thus discharged from the gas-liquid separator 32 at a suitable time, when the amount of liquid water stored in the gas-liquid separator 32 reaches a level large enough to be discharged.

[0047] In Fig. In step S30, the control unit 50 starts the purging process. The control unit 50 stops the feed device 25 and drives the pump 34 at a predetermined rotational speed. In step S40, the control unit 50 determines whether the valve opening condition for the outlet valve 36 is met. If the amount of liquid water stored in the gas-liquid separator 32 is less than the second threshold Thb, the control unit 50 determines that the valve opening condition is not met and, in step S50, determines whether a condition for completing the purging process is met. In the first embodiment, the condition for completing the purging process is met if the purging continues for a predetermined threshold time interval. If the condition for completing the purging process is not met, the control unit 50 continues the purging process and repeatedly makes the determinations in steps S40 and S50.The gas flow path on the anode side is purged until the valve opening condition for the outlet valve 36 is met or until the condition to complete the purging is met.

[0048] If the valve opening condition for the outlet valve 36 is met in step S40, that is, if the amount of liquid water LS is greater than the amount corresponding to the second threshold Thb stored in the gas-liquid separator 32, the control unit 50 completes the purging in step S60. This is because it is assumed that a sufficient amount of liquid water has been drained from the gas flow path on the anode side of the fuel cell system 100.

[0049] If the condition for completing the purge in step S50 is met, that is, if the purge continues beyond the predetermined threshold time interval, the control unit 50 completes the purge in step S60. This is because it is assumed that a sufficient amount of moisture has been removed from the fuel gas flow path by the purge. The condition for completing the purge in step S50 is not limited to the condition based on the duration of the purge. In another embodiment, for example, the control unit 50 can determine in step S50 that the condition for completing the purge is met when the amount of water in the fuel cell stack 10 or the wetness of the purge gas discharged from the fuel cell stack 10 falls below a predetermined threshold, and can then terminate the purge in step S60.

[0050] In step S70, the control unit 50 opens the outlet valve 36 to initiate the draining of liquid water from the gas-liquid separator 32. Also in step S70, the control unit 50 can actuate the feed device 25 to increase the pressure in the gas-liquid separator 32 to facilitate the draining of the liquid water. In step S80, the control unit 50 determines whether the draining of liquid water from the gas-liquid separator 32 is complete.For example, the control unit 50 calculates the rate at which the liquid water is drained from the gas-liquid separator 32, using the pressure in the gas-liquid separator 32, and determines that the draining of the liquid water from the gas-liquid separator 32 is complete when a determined quantity of liquid water drained from the gas-liquid separator 32, calculated using this rate, becomes greater than or equal to the quantity of liquid water that was stored in the gas-liquid separator 32 before the outlet valve 36 was opened. Alternatively, the control unit 50 can determine that the draining of the liquid water from the gas-liquid separator 32 is complete when a predetermined time has elapsed after the outlet valve 36 was opened.The control unit 50 keeps the outlet valve 36 open until step S80 determines that the draining of the liquid water from the gas-liquid separator 32 is complete.

[0051] When the control unit 50 determines in step S80 that the draining of liquid water from the gas-liquid separator 32 is complete, the control unit 50 executes a drain valve discharge process in step S90. The drain valve discharge process is a process in which moisture in the drain valve 36 is drained through the discharge line 35. In the drain valve discharge process, the control unit 50 repeats the operation of opening and closing the drain valve 36 a predetermined number of times. As the operation of opening and closing the drain valve 36 is repeated, the differential pressure between the pressure in the gas-liquid separator 32 and the ambient atmospheric pressure serves as the driving force to discharge moisture in the drain valve 36. Because the drain valve discharge process is performed, it is less likely that moisture will remain in the drain valve 36.Accordingly, it is less likely that the outlet valve 36 will become stuck due to frozen moisture in a low-temperature environment.

[0052] The final purging process is therefore terminated. The control unit 50 then performs various preparatory processes to terminate the operation of the fuel cell system 100 and terminates the operation of the fuel cell system 100. The control unit 50 performs a process to change the valve opening condition for the exhaust valve control from the valve opening condition for the final purging process back to its original valve opening condition when the operation of the fuel cell system 100 has been terminated or the fuel cell system 100 is restarted.

[0053] According to the fuel cell system 100 of the first embodiment, the control unit 50 uses, in the outlet valve control performed in the final purging process, the valve opening condition that has been determined such that the amount of liquid water stored in the gas-liquid separator 32 at the time the outlet valve 36 is opened is greater than in the outlet valve control performed during the normal operation of the fuel cell system 100. As in Fig. As shown in Figure 2, the level of liquid water LS stored in the gas-liquid separator 32 is raised during the final purge process than during normal operation of the fuel cell system 100. When the liquid water LS is drained from the gas-liquid separator 32, less moisture is likely to remain on the inner wall surface that was in contact with the stored liquid water LS. This is because, due to the viscosity of water, the moisture on the inner wall surface of the gas-liquid separator 32 follows the total flow of liquid water LS generated when the stored liquid water LS is drained. Accordingly, in the final rinsing process, the area of ​​the region in which the water droplets WD, which have formed on the inner wall surface of the gas-liquid separator 32 through the gas-liquid separation in the gas-liquid separator 32, remain is reduced.This configuration reduces the amount of water droplets remaining in the gas-liquid separator 32 when the fuel cell system 100 is shut down. A reduction in the space within the gas-liquid separator 32 that can store liquid water LS, caused by the freezing of such water droplets in a low-temperature environment, is therefore less likely. Consequently, a reduction in the accuracy of determining the amount of liquid water LS stored in the gas-liquid separator 32, caused by such a reduction in the space within the gas-liquid separator 32, is less likely, thus reducing difficulties in ascertaining the amount of liquid water LS stored in the gas-liquid separator 32.Furthermore, a blockage of the circulation path for the circulation of the gas component of the exhaust gas, caused by the freezing of the water droplets remaining in the gas-liquid separator 32 when the fuel cell system 100 is restarted, becomes less likely, and a jamming of the outlet valve 36, caused by such water droplets moving to the outlet valve 36 and freezing therein, becomes less likely. 2. Second embodiment

[0054] Fig. Figure 4 is a schematic representation depicting an internal configuration of a gas-liquid separator 32A used in a fuel cell system of a second embodiment. The configuration of the fuel cell system of the second embodiment is essentially the same as that of the fuel cell system 100 of the first embodiment, except that the fuel cell system of the second embodiment includes a gas-liquid separator 32A instead of the gas-liquid separator 32 described in the first embodiment.

[0055] The gas-liquid separator 32A of the second embodiment comprises a filter 68 with pores through which the liquid water LS passes. The filter 68 traps foreign particles contained in the liquid water LS that move with the liquid water LS towards the outlet 64. Because the filter 68 is present, it is less likely that foreign particles will reach the outlet valve 36, and therefore a failure of the outlet valve 36, which might be stuck open due to foreign particles, is less likely.

[0056] The filter 68 is arranged such that it divides the interior 61 of the gas-liquid separator 32A into a first region AF on the upstream side of the filter 68 and a second region AS on the downstream side of the filter 68. As used here, the terms "upstream" and "downstream" are based on the direction in which the liquid water LS flows toward the drain outlet 64 in the interior 61. In the second embodiment, the filter 68 is located at the boundary between the main region 61M and the lower region 61L such that it closes the opening at the upper end of the lower region 61L, and the filter 68 divides the interior 61 into the first region AF and the second region AS. The first region AF consists of the upper region 61U and the main region 61M, and the second region AS consists of the lower region 61L. The position of filter 68 is not limited to that.In other embodiments, the filter 68 can be arranged in the main region 61M.

[0057] In the fuel cell system of the second embodiment, the control unit 50 performs the final purging process according to the sequence. Fig. 3 in a similar manner to that described in the first embodiment. In the second embodiment, however, the control unit 50 sets the valve opening condition for the outlet valve 36 for the outlet valve control performed during normal operation of the fuel cell system 100, and the valve opening condition for the outlet valve 36 for the outlet valve control performed during the final purge process, as follows. In the second embodiment, the control unit 50 uses the first threshold Tha and the second threshold Thb, which represents the amount of liquid water stored in the gas-liquid separator 32A, as the valve opening condition, as in the first embodiment.

[0058] The control unit 50 uses a first valve opening condition as the valve opening condition during the normal operation of the fuel cell system 100. The first valve opening condition is determined such that the outlet valve 36 opens when the level of liquid water LS in the gas-liquid separator 32A is in the second region AS. The control unit 50 sets the first threshold Tha, which is the first valve opening condition, to a value corresponding to such a quantity of liquid water LS that the surface of the liquid water LS is located in the second region AS when the gas-liquid separator 32A is in the reference position, namely when the gas-liquid separator 32A is in an upright position.This means that the control unit 50 sets the first threshold Tha to a value less than or equal to the amount of liquid water LS corresponding to the capacity of the second region AS. The level of liquid water LS stored in the gas-liquid separator 32A will therefore be less likely to rise higher than the position of the filter 68 during normal operation of the fuel cell system 100.

[0059] The control unit 50 uses a second valve opening condition in the final purge process. This second valve opening condition is determined such that the outlet valve 36 opens when the liquid water level LS in the gas-liquid separator 32A is in the first region AF. The control unit 50 sets the second threshold Thb, which is the second valve opening condition, to a value corresponding to a quantity of liquid water LS such that the surface of the liquid water LS is in the first region AF when the gas-liquid separator 32A is in its reference position, i.e., when the gas-liquid separator 32A is in an upright position. This means that the control unit 50 sets the second threshold Thb to a value that is greater than the amount of liquid water LS that corresponds to the capacity of the second region AS.Accordingly, in the final rinsing process, the outlet valve 36 is opened when the level of the liquid water LS stored in the gas-liquid separator 32A is higher than the position of the filter 68.

[0060] As described above, in the final purge process according to the fuel cell system of the second embodiment, the outlet valve 36 is opened and the liquid water LS is drained from the gas-liquid separator 32A when the level of the liquid water LS stored in the gas-liquid separator 32A is higher than the level of the filter 68. Moisture is therefore less likely to remain in the filter 68, and clogging of the filter 68 caused by freezing of moisture remaining in the filter 68 while the operation of the fuel cell system 100 is stopped is less likely. During normal operation of the fuel cell system 100, the outlet valve 36 is opened and the liquid water LS is drained from the gas-liquid separator 32A when the level of the liquid water LS stored in the gas-liquid separator 32A is below the filter 68.Accordingly, the liquid water LS is forced out or retained by the filter 68, even if the position of the gas-liquid separator 32A varies during normal operation of the fuel cell system 100 and the surface area of ​​the liquid water LS fluctuates. The liquid water LS is therefore less likely to enter the circulation line 33 through the circulation gas outlet 63. Furthermore, the fuel cell system of the second embodiment has various functions and effects similar to those described in the first embodiment. 3. Third embodiment

[0061] Fig. Figure 5 is a flowchart of a final purging process carried out in a fuel cell system of a third embodiment. The configuration of the fuel cell system of the third embodiment is essentially the same as that of the fuel cell system 100 of the first embodiment, which is shown in Fig. Figure 1 shows the final rinsing process of the third embodiment. This process is essentially the same as that described in the first embodiment, except that the final rinsing process of the third embodiment includes step S25 instead of step S20. In step S25, the control unit 50 performs a process condition determination process described below.

[0062] Fig. Figure 6 is a flowchart of the process condition determination process. In this process condition determination process, a condition for performing the rinsing and a valve opening condition for controlling the outlet valve are determined.

[0063] In step S100, the control unit 50 determines the purge gas flow rate as a condition for performing the purge. In the third embodiment, the control unit 50 determines the purge gas flow rate based on the tilt angle of the gas-liquid separator 32. The control unit 50 reduces the purge gas flow rate as the angle by which the gas-liquid separator 32 is tilted from the predetermined reference position in such a direction of rotation that the exhaust gas inlet 62 side of the gas-liquid separator 32 is moved upwards and the circulation gas outlet 63 side of the gas-liquid separator 32 is moved downwards.This is because, when the gas-liquid separator 32 is tilted in such a direction, the surface of the liquid water LS stored in the gas-liquid separator 32 approaches the circulation gas outlet 63, and the liquid water LS enters the circulation gas outlet 63 more easily due to the suction force of the pump 34.

[0064] The control unit 50 determines the tilt angle of the gas-liquid separator 32 using, for example, an accelerometer (not shown). The control unit 50 also determines the purge gas flow rate for the tilt angle of the gas-liquid separator 32 using a pre-defined characteristic map or equation that defines the relationship between the tilt angle of the gas-liquid separator 32 and the purge gas flow rate. The control unit 50 can increase the purge duration, which is necessary to complete the purge process, while the purge gas flow rate decreases.

[0065] In step S110, the control unit 50 determines the valve opening condition for the outlet valve 36 in the outlet valve control by using the purge gas flow rate determined in step S100. The control unit 50 derives the valve opening condition for the purge gas flow velocity by using a pre-prepared characteristic map. In the third embodiment, the control unit 50 uses as the valve opening condition the first threshold Tha and the second threshold Thb, which represents the amount of liquid water stored in the gas-liquid separator 32, as described in the first embodiment.

[0066] Fig. Figure 7 presents an example of a characteristic map MP used to determine the valve opening condition in step S110. The characteristic map MP defines the relationship between the purge gas flow rate and the amount of liquid water. Specifically, according to the characteristic map MP, the amount of liquid water decreases as the purge gas flow rate decreases. Using the characteristic map MP, the control unit 50 procures the amount V of liquid water for the purge gas flow rate Q determined in step S110. The control unit 50 sets the second threshold Thb to the procured amount V of liquid water. The characteristic map MP is set such that the amount V of liquid water procured for the purge gas flow rate Q is greater than the first threshold Tha used during normal operation of the fuel cell system 100.

[0067] In Fig. In step S30, the control unit 50 starts the purging process using the purge gas flow rate determined in the process condition determination process as the target flow rate. The control unit 50 drives the pump 34 at a rotational speed corresponding to the target flow rate. In step S40, the control unit 50 determines that the valve opening condition is met when the amount of liquid water LS stored in the gas-liquid separator 32 is greater than the second threshold Thb determined in the process condition determination process. The control unit 50 completes the purging process in step S60 and then opens the outlet valve 36 to begin draining the liquid water LS from the gas-liquid separator 32 in step S70.

[0068] As described above, in the third embodiment, the second threshold Thb is smaller, and the amount of liquid water LS stored in the gas-liquid separator 32 at the time the outlet valve 36 is opened is smaller, the lower the purge gas flow rate. The purge gas flow rate is reduced when the gas-liquid separator 32 is tilted and the surface of the liquid water LS stored therein approaches the circulation gas outlet 63. Since the amount of liquid water stored in the gas-liquid separator 32 is reduced, and the purge gas flow rate decreases, it is even less likely that the liquid water LS will enter the circulation gas outlet 63.

[0069] As described above, in the final purging process of the fuel cell system of the third embodiment, the purge gas flow rate is reduced when the gas-liquid separator 32 is tilted, bringing the surface of the liquid water LS closer to the circulation gas outlet 63. Therefore, it is less likely that the liquid water LS will be disturbed by the suction force of the pump 34 during purging. As the purge gas flow rate decreases, the second threshold Thb, which is the valve opening condition for the outlet valve 36, is also reduced, and the amount of liquid water LS stored in the gas-liquid separator 32 is decreased. Therefore, it is even less likely that the liquid water LS stored in the gas-liquid separator 32 will be disturbed by the suction force of the pump 34.Furthermore, the fuel cell system of the third embodiment has various functions and effects similar to those described in the first embodiment. 4. Fourth embodiment

[0070] Fig. Figure 8 presents an example of a characteristic map MPf used in a fuel cell system of a fourth embodiment. The configuration of the fuel cell system of the fourth embodiment is essentially the same as that of the fuel cell system of the third embodiment. In the fuel cell system of the fourth embodiment, the control unit 50 performs the final purging process and the process condition determination process according to the parameters set out in the figures. Fig. 5 and Fig. The processes shown in Figure 6 are as described in the third embodiment. However, the process condition determination process of the fourth embodiment differs from that of the third embodiment with regard to the method for determining the flow rate of the purge gas and the method for determining the valve opening condition. The characteristic curve MPf in Fig. 8 is used to determine the valve opening condition in the process condition determination process.

[0071] In step S100 of the process condition determination process of the fourth embodiment, the control unit 50 adjusts the purge gas flow rate to a higher value as the exhaust gas temperature decreases. When the exhaust gas temperature is lower, the amount of saturated water vapor in the purge gas is less, and therefore the amount of moisture removed by the purge gas is reduced. Since the purge gas flow rate is increased as the exhaust gas temperature decreases, the amount of moisture removed by the purge gas is less likely to be reduced, and therefore, the purge efficiency is less likely to be reduced.

[0072] In step S110, the control unit 50 procures the quantity V of liquid water for the purge gas flow rate Q determined in step S100 using the MPf characteristic map. The MPf characteristic map defines the relationship between the purge gas flow rate and the quantity of liquid water. Specifically, according to the MPf characteristic map, the quantity of liquid water decreases as the purge gas flow rate increases. The control unit 50 sets the second threshold Thb to the quantity V of liquid water obtained from the MPf characteristic map. The MPf characteristic map is set such that the quantity V of liquid water obtained for the purge gas flow rate Q is greater than the first threshold Tha, which is used during normal operation of the fuel cell system 100.

[0073] In step S30 of the final purging process, the control unit 50 starts the purging by driving the pump 34 at a rotational speed corresponding to the purge gas flow rate determined in the process condition determination process. In step S40, the control unit 50 determines that the valve opening condition is met when the amount of liquid water LS stored in the gas-liquid separator 32 is greater than the second threshold Thb determined in the process condition determination process. The control unit 50 completes the purging in step S60 and then opens the outlet valve 36 in step S70.

[0074] According to the final purging process of the fuel cell system of the fourth embodiment, if the purge gas flow rate is increased to limit a reduction in purging efficiency, the second threshold Thb, which is the valve opening condition for the outlet valve 36, is set to a lower value. This reduces the amount of liquid water LS stored in the gas-liquid separator 32 during the final purging process. Accordingly, even if the suction power of the pump 34 is increased as a result of an increase in the purge gas flow rate, it is less likely that the liquid water LS in the gas-liquid separator 32 will be stirred up into the circulation line 33. Furthermore, the fuel cell system of the fourth embodiment has various functions and effects similar to those described in the first and third embodiments. 5. Other embodiments

[0075] The various configurations described in the embodiments above can be modified, for example, as follows. Like the embodiments above, other embodiments described below are each considered an example of how the technology of the present invention can be implemented. Other embodiment 1

[0076] In the embodiments described above, the first threshold Tha and the second threshold Thb, representing the amount of liquid water LS stored in the gas-liquid separator 32, are used as valve opening conditions in the outlet valve control. However, other valve opening conditions can be used. For example, the duration of purging can be used as a valve opening condition for the outlet valve control. In this case, the control unit 50 can determine that the valve opening condition is met and open the outlet valve 36 if purging has continued for longer than a predetermined threshold time interval after the purging process was initiated. In this case, the threshold time interval used in the final purging process is set to a value greater than the threshold time interval used in the outlet valve control during normal operation of the fuel cell system.Even when this condition is used, the amount of liquid water LS stored in the gas-liquid separator 32 at the time the outlet valve 36 opens during the final purge process is greater than during normal operation of the fuel cell system. Various other conditions can be used as the valve opening condition for the outlet valve control. In the embodiments described above, the valve opening condition for the outlet valve control is changed during the final purge process. However, the valve opening condition for the final purge process can be set as the default condition, and the valve opening condition can be changed from the valve opening condition for the final purge process to the valve opening condition for normal operation of the fuel cell system 100 when normal operation of the fuel cell system 100 is initiated. Other embodiment 2

[0077] The gas-liquid separators 32, 32A are not limited to the configuration described in the embodiments above. For example, the lower region 61L can be omitted from the gas-liquid separators 32, 32A, or the gas-liquid separators 32, 32A can be configured such that the lower surface is formed by an inner surface that is conical downwards. In the embodiments above, the process for determining in step S10 whether or not purging can be performed, as well as the outlet valve discharge process in step S90 of the final purging process, can be omitted. In the final purging process of the embodiments above, the purging and valve opening control can be repeated after the valve opening condition in step S40 is met, and the liquid water is discharged from the gas-liquid separator 32, 32A in steps S70 and S80.In the above embodiments, the control unit 50 can obtain the quantity of liquid water stored in the gas-liquid separator 32, 32A using a water level sensor, etc., which is formed in the gas-liquid separators 32, 32A, instead of calculating the determined quantity of liquid water stored in the gas-liquid separators 32, 32A. Other embodiment 3

[0078] The gas-liquid separator 32A with the filter 68 according to the second embodiment can be used in the fuel cell system of the third or the fourth embodiment. In this case, during normal operation of the fuel cell system, the outlet valve 36 can be opened when the surface of the liquid water LS is in the second region AS, and in the final purge process, the outlet valve 36 can be opened when the surface of the liquid water LS is in the first region AF. Other embodiment 4

[0079] In the third and fourth embodiments, the purge gas flow rate can be changed according to conditions other than the tilt angle of the gas-liquid separator 32 and the exhaust gas temperature. For example, the purge gas flow rate can be determined according to the exhaust gas density, the actual drive efficiency of the pump 34, or the actual water content of the fuel cell stack 10. Other embodiment 5

[0080] In the embodiments described above, the configuration for circulating fuel gas to the fuel cell stack 10, which is used in the fuel gas supply system, can also be used in the oxidation gas supply system for the fuel cell stack 10. In this case, the outlet valve control and the final purging process described above can be carried out in the oxidation gas supply system for the fuel cell stack 10. 6. More

[0081] In the above embodiments, some or all of the functions and processes implemented by software can be implemented by hardware. Various circuits can be used as hardware, such as an integrated circuit, a discrete circuit, or a circuit module formed from a combination of the two.

[0082] The technology of the present invention is not limited to the above embodiments and can be implemented by various configurations without deviating from the scope of the present invention as defined by the claims.

Claims

[1] comprising fuel cell system (100): a fuel cell stack (10) configured to receive reaction gas to generate electrical power; a gas-liquid separator (32) connected to the fuel cell stack (10) and configured to separate exhaust gas from the fuel cell stack (10) into a liquid component and a gas component and to store liquid water of the liquid component; a circulation line (33) which is connected to the gas-liquid separator (32) and which forms a circulation path which is set up to circulate the gas component in the gas-liquid separator (32) to the fuel cell stack (10); an outlet line (35) which is connected to the gas-liquid separator (32) and which is designed to drain the liquid water from the gas-liquid separator; an outlet valve (36) configured to open and close the outlet line (35); and a control unit (50) which is configured to control the supply of the reaction gas to the fuel cell stack (10) and to perform a final purging process, wherein the final purging process is a process for circulating the gas component of the exhaust gas as purge gas in order to perform purging when a process of the fuel cell system (100) is completed, wherein the control unit (50) is configured to perform an outlet valve control, wherein the outlet valve control is a control during which, when a valve opening condition is met which is determined in advance in conjunction with a quantity of liquid water stored in the gas-liquid separator (32), the outlet valve (36) is opened to discharge the liquid water from the gas-liquid separator (32), and the control unit (50) is configured such that the control unit (50) performs the outlet valve control in the final purge process using the valve opening condition which is set such that the amount of liquid water stored in the gas-liquid separator (32) at the time when the outlet valve (36) is opened in the final purge process is greater than the amount of liquid water stored in the gas-liquid separator (32) at the time when the outlet valve (36) is opened during normal operation of the fuel cell system (100). [2] Fuel cell system (100) according to claim 1, wherein the control unit (50) is set up to obtain the quantity of liquid water stored in the gas-liquid separator (32), the control unit (50) is configured such that during the outlet valve control, which is carried out during the normal operation of the fuel cell system (100), the control unit (50) determines that the valve opening condition is met and the outlet valve (36) opens when the amount of liquid water stored in the gas-liquid separator (32) becomes greater than a predetermined first threshold value, and the control unit (50) is configured such that during the outlet valve control carried out in the final rinsing process, the control unit (50) determines that the valve opening condition is met and the outlet valve (36) opens when the amount of liquid water stored in the gas-liquid separator (32) becomes greater than a second threshold value, which is set to a higher value than the first threshold value. [3] Fuel cell system (100) according to claim 1 or 2, wherein the control unit (50) is configured such that the control unit (50) determines a flow rate of the purge gas during the final purge process and sets the valve opening condition according to the determined flow rate before the purge is started. [4] Fuel cell system (100) according to one of claims 1 to 3, wherein a filter (68) which is arranged such that the liquid water passes through the filter (68) in which a gas-liquid separator (32A) is formed, the filter (68) is arranged such that it divides an interior of the gas-liquid separator (32A) into a first region above the filter (68) and a second region below the filter (68), the control unit (50) is configured such that, during normal operation of the fuel cell system (100), the control unit (50) performs the outlet valve control using a first valve opening condition as the valve opening condition, wherein the first valve opening condition is determined such that the outlet valve (36) is opened when a level of liquid water in the gas-liquid separator (32A) is in the second region, and the control unit (50) is configured such that during the final rinsing process the control unit (50) performs the outlet valve control using a second valve opening condition as the valve opening condition, wherein the second valve opening condition is determined such that the outlet valve (36) is opened when the level of liquid water in the gas-liquid separator (32) is in the first region. [5] A method for controlling a fuel cell system (100), wherein the fuel cell system (100) comprises a fuel cell stack (10) configured to receive reaction gas to generate electrical power, a gas-liquid separator (32) configured to separate exhaust gas from the fuel cell stack (10) into a liquid component and a gas component and to store liquid water of the liquid component, a circulation line (33) forming a circulation path configured to circulate the gas component in the gas-liquid separator (32) to the fuel cell stack (10), an outlet line (35) configured to discharge the liquid water from the gas-liquid separator (32), and an outlet valve (36) configured to open and close the outlet line (35), wherein the method comprises: Performing a final purging process, wherein the final purging process is a process for circulating the gas component of the exhaust gas as purge gas to perform purging when a process of the fuel cell system (100) is completed; and Performing an outlet valve control, wherein the outlet valve control is a control during which, when a valve opening condition is met which is determined in advance in conjunction with a quantity of liquid water stored in the gas-liquid separator (32), the outlet valve (36) is opened to discharge the liquid water from the gas-liquid separator (32), wherein The outlet valve control in the final purging process is carried out using the valve opening condition, which is set such that the amount of liquid water stored in the gas-liquid separator (32) at the time when the outlet valve (36) is opened during the final purging process is greater than the amount of liquid water stored in the gas-liquid separator (32) at the time when the outlet valve (36) is opened during normal operation of the fuel cell system (100).

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