Fuel cell system and its operating method

Abstract

A fuel cell system (100) is provided with a fuel cell stack (4), a cooling water path (40), a cooling water tank (30), a cooling water pump (10), an oxidant gas supply unit (15), an oxidant gas supply path (11), an oxidant gas humidification path (12), an oxidant gas discharge path (13), and an off-gas path (14). The fuel cell system (100) is configured such that, although the inlet of the oxidant gas humidification path (12) becomes completely submerged during suspension of power generation by the fuel cell stack (4), the temperature of the remaining oxidant gas is controlled such that a cathode (4b) is not filled up with cooling water (30r).

Description

[Technical Field]

[0001] The present disclosure relates to a fuel cell system equipped with a cooling water tank that collects water produced in a fuel cell stack, and to an operating method thereof. [Background technology]

[0002] Patent Document 1 discloses a fuel cell system that suppresses the inflow of air into the cathode by switching an on-off valve when power generation is stopped and supplying part of the cooling water to the cathode to fill the cathode with cooling water.

[0003] This fuel cell system includes a fuel gas supply path, a fuel cell stack, an unreacted fuel gas discharge path, an oxidant gas supply path, an oxidant gas discharge path, a cooling water path, a cooling water pump, a cooling water tank, a cooling water supply line, a first water valve, a cooling water discharge line, and a second water valve. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-26182 Summary of the Invention

[0005] The present disclosure provides a fuel cell system and an operating method thereof that can suppress air inflow into the cathode when power generation in the fuel cell system is stopped without filling the cathode with cooling water.

[0006] The fuel cell system of the present disclosure includes a fuel cell stack, a cooling water path, a cooling water tank, a cooling water pump, an oxidant gas supply unit, an oxidant gas supply path, an oxidant gas humidification path, an oxidant gas discharge path, and an off-gas path.

[0007] The fuel cell stack is a solid polymer type and is configured to generate electricity by reacting a fuel gas supplied to the anode with an oxidant gas supplied to the cathode.

[0008] The cooling water path is configured to circulate cooling water that exchanges heat with the fuel cell stack.

[0009] The cooling water tank is a substantially sealed tank provided at a location in the cooling water path vertically lower than the fuel cell stack. When installed, the cooling water tank has a gas reservoir above a cooling water reservoir that stores cooling water, and the gas reservoir separates the first and second spaces so that the first and second spaces do not communicate with each other regardless of the operating state of the fuel cell stack.

[0010] The cooling water pump is provided in the cooling water path so as to pump up the cooling water in the cooling water tank and supply it to the fuel cell stack.

[0011] The oxidant gas supply unit is configured to supply an oxidant gas, and the oxidant gas supply path is a path configured to supply the oxidant gas into the cooling water in the cooling water reservoir below the first space, and is a path that connects the oxidant gas supply unit and the cooling water tank.

[0012] The oxidant gas humidification path is a path configured so that the oxidant gas discharged from the oxidant gas supply path, humidified, and accumulated in the first space is supplied to the cathode, and has an inlet that opens into the first space and an outlet that is connected to the inlet of the cathode.

[0013] The oxidant gas discharge path is a path configured to supply oxidant gas that is not used in the reaction in the fuel cell stack and is discharged from the cathode outlet into the cooling water in the cooling water reservoir below the second space. The oxidant gas discharge path has an inlet connected to the cathode outlet and an outlet that opens into the cooling water in the cooling water reservoir below the second space.

[0014] The off-gas path is a path configured to allow the oxidant gas discharged from the oxidant gas discharge path, separated into gas and liquid, and accumulated in the second space to be discharged to the outside, and is a path whose inlet opens to the second space and whose outlet opens to the outside.

[0015] The fuel cell system disclosed herein is configured to control the temperature of the remaining oxidant gas so that when power generation by the fuel cell stack stops, the inlet of the oxidant gas humidification path is completely submerged, but the cathode is not filled with cooling water.

[0016] The present disclosure also provides a method for operating a fuel cell system including a fuel cell stack, a cooling water passage, a cooling water tank, a cooling water pump, an oxidant gas supply unit, an oxidant gas supply passage, an oxidant gas humidification passage, an oxidant gas exhaust passage, an off-gas passage, and a heater. The fuel cell stack is a solid polymer type that generates electricity by reacting a fuel gas supplied to an anode with an oxidant gas supplied to a cathode. Coolant water circulates through the cooling water passage, exchanging heat with the fuel cell stack. The cooling water tank is a substantially sealed tank located vertically lower than the fuel cell stack in the cooling water passage. In an installed state, the cooling water tank has a gas reservoir above a cooling water reservoir that stores the coolant, and the gas reservoir separates the first space and the second space so that they do not communicate with each other regardless of the operating state of the fuel cell stack. The cooling water pump is provided in the cooling water passage to pump the coolant water from the cooling water tank and supply it to the fuel cell stack. The oxidant gas supply unit supplies oxidant gas. The oxidant gas supply path connects the oxidant gas supply unit and the cooling water tank so that the oxidant gas is supplied into the cooling water in the cooling water reservoir below the first space. The oxidant gas humidification path has an inlet that opens into the first space and an outlet that is connected to the inlet of the cathode so that the oxidant gas discharged from the oxidant gas supply path, humidified, and accumulated in the first space is supplied to the cathode. The oxidant gas discharge path is a path configured to supply oxidant gas that is not used in the reaction in the fuel cell stack and discharged from the outlet of the cathode into the cooling water in the cooling water reservoir below the second space. The oxidant gas discharge path has an inlet that is connected to the outlet of the cathode and an outlet that opens into the cooling water in the cooling water reservoir below the second space. The off-gas path has an inlet that opens into the second space and an outlet that opens to the outside so that the oxidant gas discharged from the oxidant gas discharge path, separated into gas and liquid, and accumulated in the second space can be discharged to the outside. The heater is provided in the cooling water tank and adjusts the temperature of the cooling water. In the method of operating a fuel cell system according to the present disclosure, the fuel cell system keeps the cooling water pump operating even after the fuel cell stack stops generating electricity.In addition, in the operating method of the fuel cell system disclosed herein, when power generation by the fuel cell stack is stopped, the inlet of the oxidant gas humidification path is completely submerged, but the temperature of the cooling water is adjusted by a heater so that the cathode is not filled with cooling water.

[0017] The fuel cell system and the operating method of the fuel cell system according to the present disclosure can suppress air inflow into the cathode when power generation of the fuel cell stack is stopped without filling the cathode with cooling water. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a block diagram showing the configuration of a fuel cell system according to a first embodiment of the present disclosure. [Figure 2] FIG. 2 is a graph showing the relationship between the pressure and volume of the residual oxidizing gas and the temperature of the residual oxidizing gas according to the first embodiment of the present disclosure. [Figure 3] FIG. 3 is a block diagram showing the configuration of a fuel cell system according to the second embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0019] The fuel cell system according to the present disclosure includes a fuel cell stack, a cooling water path, a cooling water tank, a cooling water pump, an oxidant gas supply unit, an oxidant gas supply path, an oxidant gas humidification path, an oxidant gas exhaust path, and an off-gas path. The fuel cell stack is a solid polymer type that generates electricity by reacting a fuel gas supplied to an anode with an oxidant gas supplied to a cathode. Coolant that exchanges heat with the fuel cell stack circulates through the cooling water path. The cooling water tank is a substantially sealed tank located vertically lower than the fuel cell stack in the cooling water path. When installed, the cooling water tank has a gas reservoir above the cooling water reservoir that stores the coolant, and the gas reservoir separates the first and second spaces so that they do not communicate with each other regardless of the operating state of the fuel cell stack. The cooling water pump is located in the cooling water path to pump the coolant from the cooling water tank and supply it to the fuel cell stack. The oxidant gas supply unit supplies the oxidant gas. The oxidant gas supply path connects the oxidant gas supply unit and the cooling water tank so that the oxidant gas is supplied into the cooling water in the cooling water reservoir below the first space. The oxidant gas humidification path has an inlet that opens into the first space and an outlet that is connected to the inlet of the cathode so that the oxidant gas discharged from the oxidant gas supply path, humidified, and accumulated in the first space is supplied to the cathode. The oxidant gas discharge path is a path configured so that the oxidant gas discharged from the outlet of the cathode without being used in the reaction in the fuel cell stack is supplied into the cooling water in the cooling water reservoir below the second space. The oxidant gas discharge path has an inlet that is connected to the outlet of the cathode and an outlet that opens into the cooling water in the cooling water reservoir below the second space. The off-gas path has an inlet that opens into the second space and an outlet that opens to the outside so that the oxidant gas discharged from the oxidant gas discharge path, separated into gas and liquid, and accumulated in the second space can be discharged to the outside. The fuel cell system of the present disclosure is configured to control the temperature of the remaining oxidant gas so that when power generation by the fuel cell stack is stopped, the inlet of the oxidant gas humidification path is completely submerged, but the cathode is not filled with cooling water.

[0020] The fuel cell system of the present disclosure can seal the cathode path without filling the cathode with cooling water when power generation by the fuel cell stack is stopped.

[0021] Therefore, when the reactor is stopped, the cooling water does not accumulate inside the cathode, suppressing the inflow of air, thereby suppressing water clogging when the reactor is started (when power generation starts), and enabling stable power generation.

[0022] The fuel cell system according to the present disclosure may further include a heater and a control unit. The heater may be provided in the cooling water tank and adjust the temperature of the cooling water. The control unit may operate the cooling water pump even after the fuel cell stack stops generating electricity. When the fuel cell stack stops generating electricity, the control unit may control the temperature of the remaining oxidant gas by adjusting the temperature of the cooling water using the heater so that the inlet of the oxidant gas humidification path is completely submerged but the cathode is not filled with cooling water.

[0023] The fuel cell system according to the present disclosure may further include a temperature measuring device. The temperature measuring device may measure the temperature of the cooling water discharged from the fuel cell stack or the temperature of the oxidant gas discharged from the fuel cell stack. The control unit may estimate the temperature of the oxidant gas in the cathode based on the value of the temperature measuring device.

[0024] The fuel cell system according to the present disclosure may further include a water level meter. The water level meter may be provided in the oxidant gas discharge path and detect the level of the cooling water in the oxidant gas discharge path. The control unit may adjust the amount of heat generated by the heater so that the water level meter detects a predetermined water level when power generation by the fuel cell stack is stopped.

[0025] The present disclosure also provides a method for operating a fuel cell system including a fuel cell stack, a cooling water passage, a cooling water tank, a cooling water pump, an oxidant gas supply unit, an oxidant gas supply passage, an oxidant gas humidification passage, an oxidant gas exhaust passage, an off-gas passage, and a heater. The fuel cell stack is a solid polymer type that generates electricity by reacting a fuel gas supplied to an anode with an oxidant gas supplied to a cathode. Coolant water circulates through the cooling water passage, exchanging heat with the fuel cell stack. The cooling water tank is a substantially sealed tank located vertically lower than the fuel cell stack in the cooling water passage. In an installed state, the cooling water tank has a gas reservoir above a cooling water reservoir that stores the coolant, and the gas reservoir separates the first space and the second space so that they do not communicate with each other regardless of the operating state of the fuel cell stack. The cooling water pump is provided in the cooling water passage to pump the coolant water from the cooling water tank and supply it to the fuel cell stack. The oxidant gas supply unit supplies oxidant gas. The oxidant gas supply path connects the oxidant gas supply unit and the cooling water tank so that the oxidant gas is supplied into the cooling water in the cooling water reservoir below the first space. The oxidant gas humidification path has an inlet that opens into the first space and an outlet that is connected to the inlet of the cathode so that the oxidant gas discharged from the oxidant gas supply path, humidified, and accumulated in the first space is supplied to the cathode. The oxidant gas discharge path is a path configured to supply oxidant gas that is not used in the reaction in the fuel cell stack and discharged from the outlet of the cathode into the cooling water in the cooling water reservoir below the second space. The oxidant gas discharge path has an inlet that is connected to the outlet of the cathode and an outlet that opens into the cooling water in the cooling water reservoir below the second space. The off-gas path has an inlet that opens into the second space and an outlet that opens to the outside so that the oxidant gas discharged from the oxidant gas discharge path, separated into gas and liquid, and accumulated in the second space can be discharged to the outside. The heater is provided in the cooling water tank and adjusts the temperature of the cooling water. In the method of operating a fuel cell system according to the present disclosure, the fuel cell system keeps the cooling water pump operating even after the fuel cell stack stops generating electricity.In addition, in the operating method of the fuel cell system disclosed herein, when power generation by the fuel cell stack is stopped, the inlet of the oxidant gas humidification path is completely submerged, but the temperature of the cooling water is adjusted by a heater so that the cathode is not filled with cooling water.

[0026] (Findings that formed the basis of this disclosure) Since the inventors first conceived of the present disclosure, in fuel cell systems, catalyst deterioration in the stack has been suppressed by blocking the flow of air into the cathode of the fuel cell stack when power generation is stopped.

[0027] Therefore, in this industry, it is common to design products that supply some of the cooling water to the cathode to fill it with cooling water, in order to prevent a decrease in power generation efficiency and an increase in costs while also suppressing the inflow of air into the cathode when power generation is stopped.

[0028] However, it was discovered that filling the cathode with cooling water poses a new problem: the cooling water filling the cathode must be thoroughly drained before power generation can be resumed. The subject matter of the present disclosure was developed to solve this problem.

[0029] Therefore, the present disclosure provides a fuel cell system and an operating method thereof that can suppress air inflow into the cathode when power generation in the fuel cell system is stopped without filling the cathode with cooling water.

[0030] Hereinafter, embodiments will be described in detail with reference to the drawings. However, unnecessary detailed description may be omitted. For example, detailed description of well-known matters or redundant description of substantially the same configuration may be omitted.

[0031] The accompanying drawings and the following description are provided to enable those skilled in the art to fully understand the present disclosure, and are not intended to limit the subject matter described in the claims.

[0032] (Embodiment 1) A fuel cell system 100 according to a first embodiment will be described below with reference to FIGS. 1 and 2 as an example of a fuel cell system according to the present disclosure.

[0033] [1-1.Configuration] Fig. 1 is a block diagram showing the configuration of a fuel cell system 100 according to Embodiment 1. As shown in Fig. 1, the fuel cell system 100 includes a fuel gas supply path 1a, a fuel cell stack 4, an unreacted fuel gas discharge path 5, a control unit 8, a cooling water pump 10, an oxidizing gas supply path 11, an oxidizing gas humidification path 12, an oxidizing gas discharge path 13, an off-gas path 14, an oxidizing gas supply unit 15, a barometer 20, a cooling water tank 30, a cooling water path 40, a heater 60, and a first temperature measuring device 61.

[0034] G in the figure indicates the direction of gravity. That is, elements depicted on the upper side of the figure are located at a relatively high position in the vertical direction, and elements depicted on the lower side of the figure are located at a relatively low position in the vertical direction. In the following, the vertical direction in a state in which the fuel cell system 100 is installed so that the cooling water tank 30 is located below (corresponding to the installation state according to the present disclosure) as shown in FIG. 1 may be described as the up-down direction. This also applies to a fuel cell system 200 in a second embodiment described later.

[0035] The fuel cell stack 4 is, for example, a solid polymer type, and is configured to generate electricity by reacting a fuel gas supplied from the fuel gas supplier 1 to the anode 4a with an oxidant gas supplied to the cathode 4b.

[0036] As described above, the fuel cell stack 4 is a polymer electrolyte fuel cell stack, and has a structure in which a plurality of unit cells are stacked in the thickness direction (horizontally in FIG. 1) and the unit cells are electrically connected in series. The fuel cell stack 4 is configured to generate electricity by reacting a fuel gas supplied to the anode 4a with an oxidant gas (air) supplied to the cathode 4b.

[0037] The single cell has an electrolyte membrane-electrode assembly in which an electrolyte membrane 4c made of a hydrogen ion conductive polymer is sandwiched between an anode electrode and a cathode electrode, and the electrolyte membrane-electrode assembly is sandwiched between an anode separator having a groove-shaped anode flow channel formed on the surface that contacts the anode electrode, and a cathode separator having a groove-shaped cathode flow channel formed on the surface that contacts the cathode electrode.

[0038] The anode 4a of the fuel cell stack 4 shown in FIG. 1 refers to a combination of an anode electrode and an anode separator, and the cathode 4b of the fuel cell stack 4 refers to a combination of a cathode electrode and a cathode separator.

[0039] The fuel cell stack 4 is used with both main surfaces of the electrolyte membrane 4c oriented substantially parallel to the vertical direction.

[0040] The anode 4a is provided with an inlet and an outlet. The inlet of the anode 4a is a hole for supplying a hydrogen-containing gas to the anode 4a. The outlet of the anode 4a is a hole for discharging the hydrogen-containing gas that has been supplied to the anode 4a but not used for power generation (unreacted) from the anode 4a.

[0041] The anode flow channel is formed so that the hydrogen-containing gas that has flowed into the anode flow channel from the inlet flows in a meandering manner toward the outlet without resisting gravity.

[0042] The cathode 4b is provided with an inlet and an outlet. The inlet of the cathode 4b is a hole for supplying oxidant gas to the cathode 4b. The outlet of the cathode 4b is a hole for discharging, from the cathode 4b, the oxidant gas that is not used for power generation (unreacted) and water (water vapor) generated by the electrochemical reaction in the fuel cell stack 4.

[0043] The cathode flow path is formed so that the oxidant gas flowing into the cathode flow path from the inlet and the water (water vapor) generated at the cathode 4b by the electrochemical reaction of the fuel cell stack 4 flow in a serpentine manner toward the outlet without resisting gravity.

[0044] The cooling water path 40 is configured in a circular shape so that the cooling water 30r that exchanges heat with the fuel cell stack 4 (to adjust the operating temperature of the fuel cell stack 4) circulates.

[0045] The cooling water tank 30 is a substantially sealed tank provided at a location in the cooling water path 40 lower in the vertical direction (gravity direction) than the fuel cell stack 4. The cooling water tank 30 has a gas storage section 30e above a cooling water storage section 30d that stores cooling water 30r, and the gas storage section 30e is divided into a first space 30f and a second space 30g by a shielding plate 30s. The shielding plate 30s divides the first space 30f and the second space 30g so that they do not communicate with each other.

[0046] When power generation of the fuel cell stack 4 is stopped (when fuel gas and oxidant gas are not being supplied to the fuel cell stack 4), the height of the liquid surface at the bottom of the first space 30f and the height of the liquid surface at the bottom of the second space 30g are both the same height, the stopped liquid surface 30a.

[0047] When the fuel cell stack 4 is generating electricity (when fuel gas and oxidant gas are being supplied to the fuel cell stack 4), the height of the first operation liquid level 30b, which is the lower surface of the first space 30f, is lower than the second operation liquid level 30c, which is the lower surface of the second space 30g. However, because the shielding plate 30s extends downward from the upper part of the gas storage section 30e (the ceiling surface inside the cooling water tank 30) and extends to a position below the first operation liquid level 30b, the first space 30f and the second space 30g do not communicate with each other.

[0048] In the cooling water tank 30, an inlet for the cooling water 30r through the cooling water path 40 is provided in a portion of the side wall on the first space 30f side that is lower than the first operating liquid level 30b. In addition, in the cooling water tank 30, an outlet for the cooling water 30r through the cooling water path 40 is provided in a portion of the side wall on the second space 30g side that is lower than the stop liquid level 30a. In addition, the cooling water tank 30 is configured so that the inlet for the cooling water 30r through the cooling water path 40 and the outlet for the cooling water 30r through the cooling water path 40 face each other.

[0049] The cooling water pump 10 is provided in the cooling water path 40 at a position vertically higher than the fuel cell stack 4, and is configured to pump up the cooling water 30r in the cooling water tank 30 and supply it to the fuel cell stack 4.

[0050] The fuel gas supply unit 1 is a fuel gas infrastructure having a predetermined supply pressure. In this embodiment, a hydrogen-containing gas containing hydrogen as a main component is used as an example of the fuel gas in this disclosure.

[0051] The fuel gas supply path 1a is a path configured to supply the fuel gas from the fuel gas supplier 1 to the anode 4a of the fuel cell stack 4, and is a path that connects the fuel gas outlet in the fuel gas supplier 1 to the inlet of the anode 4a of the fuel cell stack 4.

[0052] The unreacted fuel gas discharge path 5 is a path for discharging unused fuel gas, which is not used in power generation (electrochemical reaction) and is discharged from the outlet of the anode 4a, out of the fuel cell system 100. The inlet of the unreacted fuel gas discharge path 5 is connected to the outlet of the anode 4a.

[0053] The oxidizing gas supply unit 15 is configured by a pump and is provided at the inlet of the oxidizing gas supply path 11. The oxidizing gas supply unit 15 increases the pressure of outside air and supplies the air for power generation to the cathode 4b.

[0054] The oxidant gas supply unit 15 does not have a function to block the oxidant gas, and when the oxidant gas supply unit 15 is stopped, the oxidant gas can pass through the oxidant gas supply unit 15 due to convection and pressure difference.

[0055] The oxidant gas supply path 11 is a path configured to supply the oxidant gas from the oxidant gas supply unit 15 into the cooling water 30r in the cooling water storage unit 30d below the first space 30f of the cooling water tank 30. The oxidant gas supply path 11 is a path that connects the oxidant gas supply unit 15 and the cooling water tank 30. The oxidant gas supply path 11 extends downward through the ceiling surface of the cooling water tank 30 on the first space 30f side, and has an outlet at a position below the first operation liquid level 30b and away from the shielding plate 30s.

[0056] The oxidant gas supplied into the cooling water 30r of the cooling water tank 30 by the oxidant gas supply path 11 is bubbled and supplied to the first space 30f, so that when the oxidant gas supply unit 15 is operating, the humidity in the first space 30f becomes higher than that of the outside air.

[0057] Furthermore, since the oxidant gas supply path 11 has an outlet located away from the shielding plate 30s, the oxidant gas supplied from the oxidant gas supply path 11 can be prevented from passing over the shielding plate 30s and being supplied to the second space 30g.

[0058] The oxidant gas humidification path 12 is a path configured to supply the oxidant gas that has been discharged from the oxidant gas supply path 11 into the cooling water 30r, humidified, and accumulated in the first space 30f to the cathode 4b. The oxidant gas humidification path 12 has an inlet that opens to the first space 30f and an outlet that is connected to the inlet of the cathode 4b.

[0059] The oxidant gas humidification path 12 extends downward through the ceiling surface of the cooling water tank 30 on the first space 30f side, and has an inlet below the stop liquid level 30a and above the first operation liquid level 30b.

[0060] The oxidant gas discharge path 13 is a path through which unused oxidant gas discharged from the outlet of the cathode 4b and water (water vapor) generated at the cathode 4b by the electrochemical reaction in the fuel cell stack 4 are supplied to the cooling water tank 30. The unused oxidant gas discharged from the outlet of the cathode 4b is oxidant gas that has not been used for power generation (electrochemical reaction) among the oxidant gases supplied to the fuel cell stack 4. The oxidant gas discharge path 13 is a path configured so that the unused oxidant gas from the outlet of the cathode 4b and the water (water vapor) generated at the cathode 4b are supplied into the cooling water 30r in the cooling water reservoir 30d below the second space 30g.

[0061] The oxidant gas discharge path 13 has an inlet connected to the outlet of the cathode 4b and an outlet that opens into the cooling water 30r in the cooling water reservoir 30d below the second space 30g. The oxidant gas discharge path 13 is configured to slope downward from the inlet to the outlet.

[0062] The oxidant gas discharge path 13 extends downward through the ceiling surface of the cooling water tank 30 on the second space 30g side, and has an outlet below the stop liquid level 30a and the second operation liquid level 30c.

[0063] The off-gas path 14 is a path configured to enable the oxidant gas, which has been discharged from the oxidant gas discharge path 13 into the cooling water 30r and separated into gas and liquid and accumulated in the second space 30g, to be discharged to the outside of the fuel cell system 100. The off-gas path 14 is a path whose inlet opens to the ceiling surface inside the cooling water tank 30 on the second space 30g side and whose outlet opens to the outside.

[0064] The control unit 8 may be any device having a control function, and may include a computer system (not shown) having an arithmetic processing unit (not shown) and a storage unit (not shown) that stores a control program. An example of the arithmetic processing unit is a CPU (Central Processing Unit). An example of the storage unit is a memory. The CPU executes the control program stored in the memory, causing the computer system to function as the control unit 8. The control program executed by the CPU is pre-recorded in the memory of the computer system here, but it may also be provided by being recorded on a non-transitory recording medium such as a memory card, or via a telecommunications line such as the Internet.

[0065] The barometer 20 is a pressure measuring instrument that measures atmospheric pressure.

[0066] The cooling water tank 30 is a water tank that stores the water produced by the operation of the fuel cell stack 4 as cooling water 30r. The cooling water tank 30 is disposed below the fuel cell stack 4, and the produced water discharged through the oxidant gas discharge path 13 falls by gravity, allowing the fallen produced water to be collected as cooling water 30r.

[0067] The cooling water tank 30 has a substantially sealed structure in which all parts are sealed except for the connections with the oxidant gas supply path 11, the oxidant gas humidification path 12, the oxidant gas discharge path 13, the off-gas path 14, and the cooling water path 40.

[0068] The oxidant gas flow space inside the cooling water tank 30 is divided by a shielding plate 30s into a first space 30f through which humidified oxidant gas flows and a second space 30g that is open to the atmosphere and through which unused oxidant gas flows.

[0069] An oxidant gas supply path 11 that supplies an oxidant gas to the fuel cell stack 4 and an oxidant gas humidification path 12 are connected to the first space 30f side. An oxidant gas discharge path 13 and an off-gas path 14 are connected to the second space 30g side.

[0070] The cooling water path 40 connects the cooling water tank 30, the fuel cell stack 4, and the cooling water pump 10 in a ring shape. The cooling water path 40 is configured so that the cooling water 30r in the cooling water tank 30 is supplied to the fuel cell stack 4 by the cooling water pump 10 (through the cooling water pump 10), and the cooling water 30r returns to the cooling water tank 30 after exchanging heat with the fuel cell stack 4.

[0071] The heater 60 is located in the cooling water storage section 30d (a portion immersed in the cooling water 30r) that stores the cooling water 30r in the cooling water tank 30, and is a heater that heats the cooling water 30r (adjusts the temperature of the cooling water 30r).

[0072] The first temperature measuring device 61 is a temperature measuring device that is provided in a path in the cooling water path 40 through which the cooling water 30r flows after completing heat exchange with the fuel cell stack 4 and returning to the cooling water tank 30, and measures the temperature of the cooling water 30r passing through the cooling water path 40.

[0073] The fuel cell system 100 is configured such that when power generation by the fuel cell stack 4 is stopped, the inlet of the oxidant gas humidification path 12 is completely submerged, but the cathode 4b is not filled with the cooling water 30r.

[0074] [1-2. Operation] The operation and function of the fuel cell system 100 configured as above will be described below.

[0075] First, the operation of the fuel cell system 100 when power generation is stopped will be described with reference to FIG.

[0076] In the first embodiment, the fuel gas supplied from the fuel gas supply device 1 is a gas containing mainly hydrogen. When the fuel gas supplied from the fuel gas supply device 1 is produced by reforming a hydrocarbon-based raw material gas, the fuel gas may contain gases such as carbon dioxide, carbon monoxide, methane, nitrogen, argon, and water vapor as impurities. The oxidant gas supplied from the oxidant gas supply unit 15 is air, which is mainly composed of nitrogen, oxygen, argon, and carbon dioxide.

[0077] When power generation by the fuel cell stack 4 is stopped, the supply of fuel gas for power generation from the fuel gas supplier 1 to the anode 4a is stopped, and the unreacted fuel gas discharge path 5 is also closed. However, to maintain a positive pressure at the anode 4a, fuel gas is periodically supplied from the fuel gas supplier 1 to the anode 4a even after power generation by the fuel cell stack 4 is stopped, thereby preventing oxidant gas from entering the anode 4a from outside the fuel cell system 100.

[0078] Furthermore, when power generation by the fuel cell stack 4 is stopped, the fuel cell system 100 stops the oxidant gas supply unit 15. When the fuel cell system 100 stops the oxidant gas supply unit 15, the pressures in the first space 30f and the second space 30g of the cooling water tank 30 both become atmospheric pressure. This is because the oxidant gas supply unit 15 does not have a function to block the oxidant gas, and because the off-gas path 14 connects the second space 30g of the cooling water tank 30 with the outside air (atmosphere).

[0079] Therefore, when power generation by the fuel cell stack 4 is stopped, the liquid levels in the cooling water tank 30 in both the first space 30f and the second space 30g are aligned with the liquid level 30a at the time of shutdown. When power generation by the fuel cell stack 4 is stopped, the outlet (downstream end) of the oxidant gas supply path 11, the inlet (upstream end) of the oxidant gas humidification path 12, and the outlet (downstream end) of the oxidant gas discharge path 13 are each located below the liquid level 30a at the time of shutdown, and are submerged in the cooling water 30r.

[0080] In this state, the oxidizing gas remaining in the cathode 4b, the oxidizing gas humidification path 12, and the oxidizing gas discharge path 13 can be sealed off.

[0081] Next, we will define the oxidant gas remaining in the cathode 4b, the oxidant gas humidification path 12, and the oxidant gas discharge path 13 after power generation by the fuel cell stack 4 has stopped as residual oxidant gas, and describe a method for controlling the pressure of this residual oxidant gas.

[0082] As a first condition, the pressure of the residual oxidant gas at the high temperature T0 (K) after the fuel cell stack 4 is stopped is P0 (Pa), and the volume is V0 (m 3 ) Immediately after power generation by the fuel cell stack 4 is stopped, the pressure P0 (kPa) of the residual oxidant gas can be approximated as being the same as atmospheric pressure. In other words, the pressure P0 (Pa) can be rephrased as atmospheric pressure P0 (Pa). The temperature T0 (K) is approximately the same as the temperature when the fuel cell stack 4 is operating.

[0083] The second condition is that the pressure of the residual oxidant gas when all the oxygen in the residual oxidant gas is consumed by the fuel gas at the anode 4a is P1 (Pa) and the volume is V1 (m 3 When the oxidant gas is air, the nitrogen concentration in the air is 79% and the oxygen concentration is 21%, so the pressure P1 (Pa) is expressed by the following (Equation 1).

[0084]

number

[0085] To consider only oxygen consumption, the volume V1 (m 3 ) is the volume V0(m 3 )

[0086] As a third condition, in addition to the consumption of oxygen in the residual oxidant gas, the temperature drop of the fuel cell stack 4 is considered. The pressure of the residual oxidant gas at this time is P2 (Pa) and the temperature is T2 (K). To calculate the degree of drop in pressure P2 (Pa), the volume V2 (m3 ) shall be equal to V0(m 3 ). At this time, the pressure P2 (Pa) of the residual oxidant gas is represented by the following (Equation 2).

[0087]

Equation

[0088] Since T2 (K) < T0 (K), the pressure P2 (Pa) is smaller than P0 (Pa), and a negative pressure is generated. Due to this generated negative pressure, the cooling water 30r in the cooling water tank 30 is sucked through the oxidant gas humidification path 12 and the oxidant gas discharge path 13 until it balances with the atmospheric pressure P0 (Pa). Let the head difference, which is the height of the sucked cooling water 30r at this time, be h (m).

[0089] As the fourth condition, consider the balance between the pressure P2 (Pa) of the residual oxidant gas calculated under the third condition and the atmospheric pressure P0 (Pa). Let the density of the cooling water 30r in the cooling water tank 30 be ρ (kg / m 3 ), the gravitational acceleration be g (m / s 2 ), the head difference from the stop liquid level 30a generated due to the negative pressure be h (m), and the pressure of the residual oxidant gas when it balances with the atmospheric pressure be P3 (Pa). The pressure P3 (Pa) of the residual oxidant gas at this time is represented by the following (Equation 3).

[0090]

Equation

[0091] Let the internal cross-sectional area of the pipes of the oxidant gas humidification path 12 and the oxidant gas discharge path 13 be S (m 2 ). When the volume of the residual oxidant gas balances at the pressure P3 (Pa) through the atmospheric pressure and the head difference h (m), let the volume be V3 (m 3 ). Then, the volume V3 (m 3 ) is represented by the following (Equation 4).

[0092]

Equation

[0093] From the above, the pressure P3 (Pa) of the residual oxidant gas when it is in equilibrium with the atmospheric pressure using the Boyle-Charles equation and (Equation 4) can be expressed by the following (Equation 5).

[0094]

number

[0095] From equation (5), if the temperature T2 (K) of the residual oxidant gas is set high, the pressure P3 (Pa) will be high, and if the temperature T2 (K) is low, the pressure P3 (Pa) will be low.

[0096] Since (Equation 3) and (Equation 5) are equal, if we solve the equation as a quadratic equation for the head difference h (m), we can obtain the head difference h (m) by adjusting the temperature T2 (K) of the residual oxidant gas. Therefore, we can obtain the pressure P3 (Pa) from (Equation 3) and the volume V3 (m 3 ) can be controlled.

[0097] If the temperature measured by the first temperature measuring device 61 is TK (K), then it can be considered that the temperature TK (K) of the cooling water 30r passing through the cooling water passage 40 and the temperature T2 (K) of the residual oxidant gas are substantially the same in the fuel cell stack 4. Therefore, by adjusting the temperature of the cooling water 30r in the cooling water tank 30, the temperature T2 (K) of the residual oxidant gas can be varied, and the pressure P3 (Pa) of the residual oxidant gas and the volume V3 (m 3 ) can be controlled.

[0098] Next, a method for controlling the temperature of the cooling water 30r in the cooling water tank 30 to a predetermined temperature TS (K) will be described. The rated heat output of the heater 60 is Q (W), the operation amount of the heater 60 is H (%), and the heat transfer efficiency from the heater 60 to the cooling water 30r is γ (-). Furthermore, the specific heat of the cooling water 30r is Cp (J / kg K), and its mass is W (kg). In this case, the amount of heat Q1 (W) that can be applied from the heater 60 to the cooling water 30r is expressed by the following (Equation 6).

[0099]

number

[0100] Furthermore, when the cooling water 30r is heated by the amount of heat Q1 (W) for a time t (s), the following relationship (Equation 7) approximately holds.

[0101]

number

[0102] Since the heat quantity Q1 (W) is equal for (Equation 6) and (Equation 7), the following (Equation 8) is derived.

[0103]

number

[0104] Therefore, the heating amount Q1 (W) of the heater 60 can be adjusted by operating the operation amount H (%) of the heater 60. This changes the temperature difference TS-T2 (K) required to raise the temperature of the cooling water 30r in the cooling water tank 30 to the predetermined temperature TS (K), thereby controlling the temperature T2 (K) of the residual oxidant gas to the predetermined temperature TS (K).

[0105] The volume of cathode 4b is VC (m 3 ), in order to prevent the cooling water 30r drawn in through the oxidizing gas humidifying passage 12 and the oxidizing gas exhaust passage 13 from entering the inside of the cathode 4b, VC(m 3 )=V3(m 3 ) Therefore, the volume of the remaining oxidizer gas is VC(m 3 The temperature of the cooling water 30r is controlled to a predetermined temperature TS (K) so that the temperature of the cooling water 30r is equal to the predetermined temperature TS (K).

[0106] FIG. 2 shows the relationship between the pressure P3 (Pa) and the volume V3 (m ) of the residual oxidant gas relative to the temperature T2 (K) of the residual oxidant gas in the first embodiment. 32, the relationship between the pressure P3 (Pa) of the residual oxidizing gas and the volume V3 (m 3 ) will be explained.

[0107] When the temperature T2 (K) of the residual oxidizer gas decreases, the pressure P3 (Pa) and the volume V3 (m 3 ) decreases.

[0108] Residual oxidizer gas volume V3 (m 3 ) is the volume VC (m 3 ), the cooling water 30r drawn into the cathode 4b will infiltrate. Therefore, the volume V3 (m 3 ) is the volume VC (m 3 ) is the target predetermined temperature TS(K) of the cooling water 30r.

[0109] That is, the predetermined temperature TS By manipulating the operation amount H (%) of the heater 60 from (Equation 8) so that the temperature is equal to or higher than the predetermined temperature TS (K), the cooling water pump 10 supplies the cooling water 30r to the fuel cell stack 4 at a temperature equal to or higher than the predetermined temperature TS (K).

[0110] This allows the temperature T2 (K) of the residual oxidant gas to be equal to or higher than the predetermined temperature TS (K), and the volume V3 (m 3 ) is the volume VC (m 3 ) or more, the cathode 4b can be sealed without being filled with the cooling water 30r.

[0111] The above has explained the behavior of the fuel cell stack 4 after power generation is stopped. When power generation by the fuel cell stack 4 is stopped for an extended period of time, fuel gas is supplied to maintain a positive pressure at the anode 4a, but some of the fuel gas supplied to the anode 4a permeates the electrolyte membrane 4c and mixes with the residual oxidant gas at the cathode 4b. The fuel gas that flows from the anode 4a to the cathode 4b via the electrolyte membrane 4c causes the pressure P3 (Pa) of the mixed residual oxidant gas to become equal to atmospheric pressure.

[0112] As a result, the head difference h (m) from the liquid level 30a at the time of shutdown, which occurs due to negative pressure, becomes almost 0, and the cooling water 30r that was sucked in via the oxidant gas humidification path 12 and the oxidant gas discharge path 13 is recovered again into the cooling water tank 30.

[0113] Next, the operation of the fuel cell system 100 during operation will be described with reference to FIG.

[0114] As described above, in the first embodiment, the fuel gas supplied from the fuel gas supply device 1 is a gas containing mainly hydrogen. When the fuel gas supplied from the fuel gas supply device 1 is produced by reforming a hydrocarbon-based raw material gas, the fuel gas may contain gases such as carbon dioxide, carbon monoxide, methane, nitrogen, argon, and water vapor as impurities. The oxidant gas supplied from the oxidant gas supply unit 15 is air, which is mainly composed of nitrogen, oxygen, argon, and carbon dioxide.

[0115] When the fuel cell stack 4 is in operation, the fuel gas supplier 1 supplies fuel gas to the anode 4a of the fuel cell stack 4, and the oxidant gas supplier 15 supplies oxidant gas to the cathode 4b of the fuel cell stack 4.

[0116] When the fuel cell stack 4 is in operation, the oxidant gas passes through the oxidant gas supply path 11, is humidified by the cooling water 30r in the cooling water tank 30, passes through the first space 30f and the oxidant gas humidification path 12, and is supplied to the cathode 4b.

[0117] The unreacted oxidant gas and the produced water that are not used in the cathode 4b are supplied to the cooling water tank 30 via the oxidant gas discharge path 13. The produced water among the unreacted oxidant gas and the produced water supplied to the cooling water tank 30 is recovered as cooling water 30r in the cooling water tank 30. The remaining unreacted oxidant gas is discharged to the outside of the fuel cell system 100 via the second space 30g and the off-gas path 14.

[0118] The inside of the cooling water tank 30 is divided partway by a shielding plate 30s, but the cooling water 30r inside the cooling water tank 30 can pass through the gap between the bottom surface of the cooling water tank 30 and the lower end of the shielding plate 30s and move between the first space 30f side and the second space 30g side.

[0119] The liquid level of the cooling water 30r present on the first space 30f side during operation of the fuel cell stack 4 is referred to as the first operation liquid level 30b, and the liquid level of the cooling water 30r present on the second space 30g side during operation is referred to as the second operation liquid level 30c.

[0120] When the fuel cell stack 4 is in operation, the oxidant gas pressurized by the oxidant gas supply unit 15 is supplied to the first space 30f. On the other hand, the oxidant gas is also supplied to the second space 30g from the oxidant gas supply unit 15, so the pressure is increased, but because the second space 30g is located downstream of the first space 30f in the flow direction of the oxidant gas, the pressure drops mainly by the amount of pressure loss at the cathode 4b.

[0121] Therefore, when the fuel cell stack 4 is operating, the second operation time liquid level 30c is higher than the first operation time liquid level 30b, which is the liquid level when the fuel cell stack 4 is operating. When the fuel cell stack 4 is operating, the outlet (downstream end) of the oxidant gas supply path 11 is below the first operation time liquid level 30b and is submerged in the cooling water 30r.

[0122] When the fuel cell stack 4 is operating, the inlet (upstream end) of the oxidant gas humidification path 12 is located at a position higher than the liquid level 30b during the first operation and is not submerged in the cooling water 30r but is present in the first space 30f. When the fuel cell stack 4 is operating, the outlet (downstream end) of the oxidant gas discharge path 13 is located below the liquid level 30c during the second operation and is submerged in the cooling water 30r.

[0123] [1-3. Effects, etc.] As described above, in this embodiment, the fuel cell system 100 includes a fuel cell stack 4, a cooling water path 40, a cooling water tank 30, a cooling water pump 10, an oxidant gas supply unit 15, an oxidant gas supply path 11, an oxidant gas humidification path 12, an oxidant gas discharge path 13, and an off-gas path 14.

[0124] The fuel cell stack 4 is a solid polymer type and is configured to generate electricity by reacting a fuel gas supplied to the anode 4a with an oxidant gas supplied to the cathode 4b.

[0125] The cooling water path 40 is configured so that the cooling water 30r that exchanges heat with the fuel cell stack 4 circulates.

[0126] The cooling water tank 30 is a substantially sealed tank provided at a location in the cooling water path 40 that is vertically lower than the fuel cell stack 4. In an installed state, the cooling water tank 30 has a gas storage section 30e above a cooling water storage section 30d that stores cooling water 30r. The gas storage section 30e is partitioned into a first space 30f and a second space 30g so that the first space 30f and the second space 30g do not communicate with each other regardless of the operating state of the fuel cell stack 4.

[0127] The cooling water pump 10 is provided in the cooling water path 40 so as to pump up the cooling water 30 r in the cooling water tank 30 and supply it to the fuel cell stack 4.

[0128] The oxidizing gas supply unit 15 is configured to supply an oxidizing gas. The oxidizing gas supply path 11 is a path that connects the oxidizing gas supply unit 15 and the cooling water tank 30 so that the oxidizing gas is supplied into the cooling water 30r in the cooling water reservoir 30d below the first space 30f.

[0129] The oxidant gas humidification path 12 is a path whose inlet opens into the first space 30f and whose outlet is connected to the inlet of the cathode 4b so that the oxidant gas discharged from the oxidant gas supply path 11, humidified, and accumulated in the first space 30f can be supplied to the cathode 4b.

[0130] The oxidant gas discharge path 13 is a path configured so that the oxidant gas discharged from the outlet of the cathode 4b without being used in the reaction in the fuel cell stack 4 is supplied into the cooling water 30r in the cooling water reservoir 30d below the second space 30g. The oxidant gas discharge path 13 has an inlet connected to the outlet of the cathode 4b and an outlet that opens into the cooling water 30r in the cooling water reservoir 30d below the second space 30g.

[0131] The off-gas path 14 is a path configured to allow the oxidant gas discharged from the oxidant gas discharge path 13, separated into gas and liquid, and accumulated in the second space 30g to be discharged to the outside, and is a path whose inlet opens to the second space 30g and whose outlet opens to the outside.

[0132] In the present embodiment, the fuel cell system 100 is configured to control the temperature of the remaining oxidant gas so that when power generation by the fuel cell stack 4 stops, the inlet of the oxidant gas humidification path 12 is completely submerged, but the cathode 4b is not filled with cooling water 30r.

[0133] When power generation by the fuel cell stack 4 is stopped, negative pressure is created in the cathode 4b, the oxidant gas humidification path 12, and the oxidant gas discharge path 13, causing the cooling water 30r in the cooling water tank 30 to be sucked up through the oxidant gas humidification path 12 and the oxidant gas discharge path 13. In response to this, a person skilled in the art can appropriately design the dimensions, etc. of each component in the fuel cell system 100 so that the cathode 4b is not filled with the cooling water 30r. Examples of the dimensions, etc. of each component in the fuel cell system 100 include the dimensions of the interior of the cooling water tank 30 (including the paths), the amount of cooling water 30r in the cooling water tank 30, the inner diameter and length of the oxidant gas humidification path 12 and the oxidant gas discharge path 13, the volume (capacity) of the cathode 4b, and the vertical distance between the fuel cell stack 4 and the cooling water tank 30. A fuel cell system designed in this way can prevent the cathode 4b from being filled with cooling water 30r when power generation by the fuel cell stack 4 is stopped, without circulating the cooling water 30r using the cooling water pump 10 or heating the cooling water 30r using the heater 60.

[0134] In contrast, the fuel cell system 100 of this embodiment is configured to control the temperature of the remaining oxidant gas so that the inlet of the oxidant gas humidification path 12 is completely submerged when power generation by the fuel cell stack 4 is stopped, but the cathode 4b is not filled with the cooling water 30r. Therefore, the fuel cell system 100 of this embodiment can seal the cathode path without filling the cathode 4b with the cooling water 30r when power generation by the fuel cell stack 4 is stopped.

[0135] Therefore, when power generation is stopped, the fuel cell system 100 prevents the cooling water 30r from accumulating inside the cathode 4b and suppresses the inflow of air, thereby suppressing water clogging at startup (when power generation begins) and enabling stable power generation.

[0136] In addition, when power generation by the fuel cell stack 4 is stopped, the inlet of the oxidant gas humidification path 12 is completely submerged, but the fuel cell system 100 is not subject to restrictions on the dimensions of each component to prevent the cathode 4b from being filled with the cooling water 30r. Therefore, the fuel cell system 100 can increase the degree of freedom in designing each component in the fuel cell system 100.

[0137] Furthermore, as in this embodiment, the fuel cell system 100 includes a heater 60 and a control unit 8. The heater 60 is provided in the cooling water tank 30 and adjusts the temperature of the cooling water 30r. The control unit 8 keeps the cooling water pump 10 operating even after the fuel cell stack 4 stops generating electricity, and uses the heater 60 to adjust the temperature of the cooling water 30r so that, when the fuel cell stack 4 stops generating electricity, the inlet of the oxidant gas humidification path 12 is completely submerged but the cathode 4b is not filled with the cooling water 30r.

[0138] If the cooling water 30r heated to a predetermined temperature by the heater 60 is not circulated by the cooling water pump 10 when the fuel cell stack 4 is not generating electricity, there is a risk that the cathode 4b will be filled with the cooling water 30r. In contrast, the fuel cell system 100 circulates the cooling water 30r heated to a predetermined temperature by the heater 60 by the cooling water pump 10 when the fuel cell stack 4 is not generating electricity. As a result, when power generation is stopped, the fuel cell system 100 can suck and introduce the cooling water 30r from the cooling water tank 30 into the oxidant gas humidification path 12 and the oxidant gas discharge path 13 while adjusting the temperature of the residual oxidant gas in the cathode 4b. As a result, the fuel cell system 100 can seal the cathode 4b without filling it with the cooling water 30r. Therefore, when power generation is stopped, the fuel cell system 100 prevents the cooling water 30r from accumulating inside the cathode 4b and suppresses the inflow of air, thereby suppressing water clogging of the cathode 4b at startup (when power generation begins) and enabling stable power generation.

[0139] Furthermore, the fuel cell system 100 according to this embodiment includes a first temperature measuring device 61 that measures the temperature of the cooling water 30r discharged from the fuel cell stack 4, and is an example of a temperature measuring device in the present disclosure.

[0140] More specifically, the first temperature measuring device 61 measures the temperature of the cooling water 30r in the cooling water path 40 that returns from the fuel cell stack 4 to the cooling water tank 30. The control unit 8 estimates the temperature of the cathode 4b (sealed residual oxidant gas) from the value of the first temperature measuring device 61.

[0141] This allows the fuel cell system 100 to measure the temperature inside the fuel cell stack 4 upstream of the first temperature measuring device 61. Therefore, the fuel cell system 100 can estimate the temperature of the cathode 4b (sealed residual oxidant gas) based on the value of the first temperature measuring device 61, and adjust the temperature of the cooling water 30r using the heater 60 based on the estimated value, thereby controlling the pressure of the residual oxidant gas.

[0142] In addition, in this embodiment, the fuel cell system 100 may be modified so that the first temperature measuring device 61 measures the temperature of the oxidant gas in the oxidant gas discharge path 13 instead of measuring the temperature of the cooling water 30r in the cooling water path 40.

[0143] As a result, the fuel cell system in this variant can estimate the temperature of the cathode 4b (sealed residual oxidant gas) based on the measurement value of the first temperature measuring device 61, and adjust the temperature of the cooling water 30r using the heater 60 based on that estimated value, thereby controlling the pressure of the residual oxidant gas.

[0144] (Embodiment 2) A fuel cell system 200 according to the second embodiment will be described below with reference to FIGS. 2 and 3 as an example of a fuel cell system according to the present disclosure.

[0145] [2-1.Configuration] Fig. 3 is a block diagram showing the configuration of a fuel cell system 200 according to Embodiment 2. As shown in Fig. 3, the fuel cell system 200 includes a fuel gas supply path 1a, a fuel cell stack 4, an unreacted fuel gas discharge path 5, a control unit 8, a cooling water pump 10, an oxidizing gas supply path 11, an oxidizing gas humidification path 12, an oxidizing gas discharge path 13, an off-gas path 14, an oxidizing gas supply unit 15, an atmospheric pressure gauge 20, a cooling water tank 30, a cooling water path 40, a heater 60, a first temperature measuring device 61, a second temperature measuring device 62, and a water level gauge 70.

[0146] The letter G in the figure indicates the direction of gravity. That is, elements drawn on the upper side of the figure are located at a relatively high position in the vertical direction, and elements drawn on the lower side of the figure are located at a relatively low position in the vertical direction.

[0147] The fuel cell system 200 of the second embodiment differs from the fuel cell system 100 in that, in addition to the configuration of the fuel cell system 100 of the first embodiment, it is equipped with a second temperature measuring device 62 and a water level gauge 70. The same components as those in the fuel cell system 100 of the first embodiment are given the same reference numerals, and the description thereof will be omitted.

[0148] The second temperature measuring device 62 is a temperature measuring device that measures the temperature of the cooling water 30r flowing into (upstream of) the fuel cell stack 4. The second temperature measuring device 62 is provided in the cooling water path 40 on the upstream side of the fuel cell stack 4 (the path between the fuel cell stack 4 and the cooling water pump 10).

[0149] The water level meter 70 is provided on the path of the oxidant gas discharge path 13, and is configured to measure the water level of the cooling water 30r sucked from the cooling water tank 30 and detect when the cooling water 30r has been sucked up to a certain water level.

[0150] [2-2. Operation] The operation and function of the fuel cell system 200 configured as above will be described below.

[0151] First, the operation of the fuel cell system 200 when power generation is stopped will be described with reference to FIG.

[0152] In the second embodiment as well, the fuel gas supplied from the fuel gas supply device 1 is a gas that mainly contains hydrogen. When the fuel gas supplied from the fuel gas supply device 1 is produced by reforming a hydrocarbon-based raw material gas, the fuel gas may contain gases such as carbon dioxide, carbon monoxide, methane, nitrogen, argon, and water vapor as impurities. The oxidant gas supplied from the oxidant gas supply unit 15 is air, and is mainly composed of nitrogen, oxygen, argon, and carbon dioxide.

[0153] When power generation by the fuel cell stack 4 is stopped, the supply of fuel gas for power generation from the fuel gas supplier 1 to the anode 4a is stopped, and the unreacted fuel gas discharge path 5 is also closed. However, to maintain a positive pressure at the anode 4a, fuel gas is periodically supplied from the fuel gas supplier 1 to the anode 4a even after power generation by the fuel cell stack 4 is stopped, thereby preventing oxidant gas from entering the anode 4a from outside the fuel cell system 100.

[0154] Furthermore, when power generation by the fuel cell stack 4 is stopped, the fuel cell system 200 stops the oxidant gas supply unit 15. When the fuel cell system 200 stops the oxidant gas supply unit 15, the pressures in the first space 30f and the second space 30g of the cooling water tank 30 both become atmospheric pressure. This is because the oxidant gas supply unit 15 does not have a function to block the oxidant gas, and because the off-gas path 14 connects the second space 30g of the cooling water tank 30 with the outside air (atmosphere).

[0155] Therefore, when power generation by the fuel cell stack 4 is stopped, the liquid levels in the cooling water tank 30 in both the first space 30f and the second space 30g are aligned with the liquid level 30a at the time of shutdown. When power generation by the fuel cell stack 4 is stopped, the outlet (downstream end) of the oxidant gas supply path 11, the inlet (upstream end) of the oxidant gas humidification path 12, and the outlet (downstream end) of the oxidant gas discharge path 13 are each located below the liquid level 30a at the time of shutdown, and are submerged in the cooling water 30r.

[0156] In this state, the oxidizing gas remaining in the cathode 4b, the oxidizing gas humidification path 12, and the oxidizing gas discharge path 13 can be sealed off.

[0157] Next, we will define the oxidant gas remaining in the cathode 4b, the oxidant gas humidification path 12, and the oxidant gas discharge path 13 after power generation by the fuel cell stack 4 has stopped as residual oxidant gas, and describe a method for controlling the pressure of this residual oxidant gas.

[0158] As a first condition, the pressure of the residual oxidant gas at the high temperature T0 (K) after the fuel cell stack 4 is stopped is P0 (Pa), and the volume is V0 (m 3 ) Immediately after power generation by the fuel cell stack 4 is stopped, the pressure P0 (kPa) of the residual oxidant gas can be approximated as being the same as atmospheric pressure. In other words, the pressure P0 (Pa) can be rephrased as atmospheric pressure P0 (Pa). The temperature T0 (K) is approximately the same as the temperature when the fuel cell stack 4 is operating.

[0159] The second condition is that the pressure of the residual oxidant gas when all the oxygen in the residual oxidant gas is consumed by the fuel gas at the anode 4a is P1 (Pa) and the volume is V1 (m 3 When the oxidant gas is air, the nitrogen concentration in the air is 79% and the oxygen concentration is 21%, so the pressure P1 (Pa) is expressed by (Equation 1).

[0160] To consider only oxygen consumption, the volume V1 (m 3 ) is the volume V0(m 3 )

[0161] As the third condition, in addition to the consumption of oxygen in the residual oxidant gas, the temperature drop of the fuel cell stack 4 is considered. Let the pressure of the residual oxidant gas at this time be P2 (Pa) and the temperature be T2 (K). To determine the degree of decrease in the pressure P2 (Pa), the volume V2 (m 3 ) is assumed to be equal to V0 (m 3 ). The pressure P2 (Pa) of the residual oxidant gas at this time is represented by Equation (2).

[0162] Since T2 (K) < T0 (K), the pressure P2 (Pa) is smaller than P0 (Pa), and a negative pressure is generated. Due to this generated negative pressure, the cooling water 30r in the cooling water tank 30 is sucked through the oxidant gas humidification path 12 and the oxidant gas discharge path 13 until it balances with the atmospheric pressure P0 (Pa). Let the head difference, which is the height of the cooling water 30r sucked at this time, be h (m).

[0163] As the fourth condition, the balance between the pressure P2 (Pa) of the residual oxidant gas calculated under the third condition and the atmospheric pressure P0 (Pa) is considered. Let the density of the cooling water 30r in the cooling water tank 30 be ρ (kg / m 3 ), the gravitational acceleration be g (m / s 2 ), the head difference from the liquid level 30a at stop generated due to the negative pressure be h (m), and the pressure of the residual oxidant gas when it balances with the atmospheric pressure be P3 (Pa). The pressure P3 (Pa) of the residual oxidant gas at this time is represented by Equation (3).

[0164] Let the internal cross-sectional area of the pipes of the oxidant gas humidification path 12 and the oxidant gas discharge path 13 be S (m 2 ). Let the volume of the residual oxidant gas when it balances at the pressure P3 (Pa) through the atmospheric pressure and the head difference h (m) be V3 (m 3 ). Then, the volume V3 (m 3 ) is represented by Equation (4).

[0165] From the above, using the ideal gas law and Equation (4), the pressure P3 (Pa) of the residual oxidant gas when it balances with the atmospheric pressure is represented by Equation (5).

[0166] From equation (5), if the temperature T2 (K) of the residual oxidant gas is set high, the pressure P3 (Pa) will be high, and if the temperature T2 (K) is low, the pressure P3 (Pa) will be low.

[0167] Since (Equation 3) and (Equation 5) are equal, if we solve the equation as a quadratic equation for the head difference h (m), we can obtain the head difference h (m) by adjusting the temperature T2 (K) of the residual oxidant gas. Therefore, we can obtain the pressure P3 (Pa) from (Equation 3) and the volume V3 (m 3 ) can be controlled.

[0168] If the temperature measured by the first temperature measuring device 61 is TK (K), then it can be considered that the temperature TK (K) of the cooling water 30r passing through the cooling water passage 40 and the temperature T2 (K) of the residual oxidant gas are substantially the same in the fuel cell stack 4. Therefore, by adjusting the temperature of the cooling water 30r in the cooling water tank 30, the temperature T2 (K) of the residual oxidant gas can be varied, and the pressure P3 (Pa) of the residual oxidant gas and the volume V3 (m 3 ) can be controlled.

[0169] Next, a method for controlling the temperature of the cooling water 30r in the cooling water tank 30 to a predetermined temperature TS (K) will be described. The rated heat output of the heater 60 is Q (W), the operation amount of the heater 60 is H (%), and the heat transfer efficiency from the heater 60 to the cooling water 30r is γ (-). Furthermore, the specific heat of the cooling water 30r is Cp (J / kg K), and its mass is W (kg). In this case, the amount of heat Q1 (W) that can be applied from the heater 60 to the cooling water 30r is expressed by Equation 6.

[0170] Furthermore, when the cooling water 30r is heated by the amount of heat Q1 (W) for a time t (s), the relationship shown in (Equation 7) is approximately satisfied.

[0171] Since the amount of heat Q1 (W) is equal for (Equation 6) and (Equation 7), (Equation 8) is derived.

[0172] Therefore, the heating amount Q1 (W) of the heater 60 can be adjusted by operating the operation amount H (%) of the heater 60. This changes the temperature difference TS-T2 (K) required to raise the temperature of the cooling water 30r in the cooling water tank 30 to the predetermined temperature TS (K), thereby controlling the temperature T2 (K) of the residual oxidant gas to the predetermined temperature TS (K).

[0173] The volume of cathode 4b is VC (m 3 ), in order to prevent the cooling water 30r drawn in through the oxidizing gas humidifying passage 12 and the oxidizing gas exhaust passage 13 from entering the inside of the cathode 4b, VC(m 3 )=V3(m 3 ) Therefore, the volume of the remaining oxidizer gas is VC(m 3 The temperature of the cooling water 30r is controlled to a predetermined temperature TS (K) so that the temperature of the cooling water 30r is equal to the predetermined temperature TS (K).

[0174] A water level gauge 70 is provided in the oxidant gas discharge path 13. Here, the distance from the stop-time liquid level 30a to the water level gauge 70 is assumed to be J (m). By adjusting the temperature T2 (K) of the residual oxidant gas so that the head difference h (m) matches J (m), the fuel cell system 200 can prevent the cooling water 30r drawn from the cooling water tank 30 from entering the inside of the cathode 4b.

[0175] Based on Figure 2, the pressure P3 (Pa) and volume V3 (m) of the residual oxidizer gas when the temperature T2 (K) of the residual oxidizer gas was controlled were calculated. 3 ) will be explained.

[0176] When the temperature T2 (K) of the residual oxidizer gas decreases, the pressure P3 (Pa) and the volume V3 (m 3 ) decreases.

[0177] Residual oxidizer gas volume V3 (m 3 ) is the volume VC (m 3 ), the cooling water 30r drawn into the cathode 4b will infiltrate. Therefore, the volume V3 (m 3) is the volume VC (m 3 ) is the target predetermined temperature TS(K) of the cooling water 30r.

[0178] That is, by operating the operation amount H (%) of the heater 60 from (Equation 8) so that the temperature becomes equal to or higher than the predetermined temperature T2 (K), the cooling water pump 10 supplies the cooling water 30r having a temperature equal to or higher than the predetermined temperature TS (K) to the fuel cell stack 4. This makes it possible to make the temperature T2 (K) of the residual oxidant gas equal to or higher than the predetermined temperature TS (K), and the volume V3 (m 3 ) is the volume VC (m 3 ) or more, the cathode 4b can be sealed without being filled with the cooling water 30r.

[0179] The above has explained the behavior of the fuel cell stack 4 after power generation is stopped. When power generation by the fuel cell stack 4 is stopped for an extended period of time, fuel gas is supplied to maintain a positive pressure at the anode 4a, but some of the fuel gas supplied to the anode 4a permeates the electrolyte membrane 4c and mixes with the residual oxidant gas at the cathode 4b. The fuel gas that flows from the anode 4a to the cathode 4b via the electrolyte membrane 4c causes the pressure P3 (Pa) of the mixed residual oxidant gas to become equal to atmospheric pressure.

[0180] As a result, the head difference h (m) from the liquid level 30a at the time of shutdown, which occurs due to negative pressure, becomes almost 0, and the cooling water 30r that was sucked in via the oxidant gas humidification path 12 and the oxidant gas discharge path 13 is recovered again into the cooling water tank 30.

[0181] Next, the operation of the fuel cell system 200 during operation will be described with reference to FIG.

[0182] As described above, in the second embodiment, the fuel gas supplied from the fuel gas supply device 1 is a gas containing mainly hydrogen. When the fuel gas supplied from the fuel gas supply device 1 is produced by reforming a hydrocarbon-based raw material gas, the fuel gas may contain gases such as carbon dioxide, carbon monoxide, methane, nitrogen, argon, and water vapor as impurities. The oxidant gas supplied from the oxidant gas supply unit 15 is air, which is mainly composed of nitrogen, oxygen, argon, and carbon dioxide.

[0183] When the fuel cell stack 4 is in operation, the fuel gas supplier 1 supplies fuel gas to the anode 4a of the fuel cell stack 4, and the oxidant gas supplier 15 supplies oxidant gas to the cathode 4b of the fuel cell stack 4.

[0184] When the fuel cell stack 4 is in operation, the oxidant gas passes through the oxidant gas supply path 11, is humidified by the cooling water 30r in the cooling water tank 30, passes through the first space 30f and the oxidant gas humidification path 12, and is supplied to the cathode 4b.

[0185] The unreacted oxidant gas and the produced water that are not used in the cathode 4b are supplied to the cooling water tank 30 via the oxidant gas discharge path 13. The produced water among the unreacted oxidant gas and the produced water supplied to the cooling water tank 30 is recovered as cooling water 30r in the cooling water tank 30. The remaining unreacted oxidant gas is returned to the fuel cell system via the second space 30g and the off-gas path 14. 200 is discharged outside.

[0186] The inside of the cooling water tank 30 is divided partway by a shielding plate 30s, but the cooling water 30r inside the cooling water tank 30 can pass through the gap between the bottom surface of the cooling water tank 30 and the lower end of the shielding plate 30s and move between the first space 30f side and the second space 30g side.

[0187] The liquid level of the cooling water 30r present on the first space 30f side during operation of the fuel cell stack 4 is referred to as the first operation liquid level 30b, and the liquid level of the cooling water 30r present on the second space 30g side during operation is referred to as the second operation liquid level 30c.

[0188] When the fuel cell stack 4 is in operation, the oxidant gas pressurized by the oxidant gas supply unit 15 is supplied to the first space 30f. On the other hand, the oxidant gas is also supplied to the second space 30g from the oxidant gas supply unit 15, so the pressure is increased, but because the second space 30g is located downstream of the first space 30f in the flow direction of the oxidant gas, the pressure drops mainly by the amount of pressure loss at the cathode 4b.

[0189] Therefore, when the fuel cell stack 4 is operating, the second operation time liquid level 30c is higher than the first operation time liquid level 30b, which is the liquid level when the fuel cell stack 4 is operating. When the fuel cell stack 4 is operating, the outlet (downstream end) of the oxidant gas supply path 11 is below the first operation time liquid level 30b and is submerged in the cooling water 30r.

[0190] When the fuel cell stack 4 is operating, the inlet (upstream end) of the oxidant gas humidification path 12 is located at a position higher than the liquid level 30b during the first operation and is not submerged in the cooling water 30r but is present in the first space 30f. When the fuel cell stack 4 is operating, the outlet (downstream end) of the oxidant gas discharge path 13 is located below the liquid level 30c during the second operation and is submerged in the cooling water 30r.

[0191] [2-3. Effects, etc.] As described above, in this embodiment, the fuel cell system 200 includes a fuel cell stack 4, a cooling water path 40, a cooling water tank 30, a cooling water pump 10, an oxidant gas supply unit 15, an oxidant gas supply path 11, an oxidant gas humidification path 12, an oxidant gas discharge path 13, an off-gas path 14, a heater 60, a water level gauge 70, and a control unit 8.

[0192] The fuel cell stack 4 is a solid polymer type and is configured to generate electricity by reacting a fuel gas supplied to the anode 4a with an oxidant gas supplied to the cathode 4b.

[0193] The cooling water path 40 is configured so that the cooling water 30r that exchanges heat with the fuel cell stack 4 circulates.

[0194] The cooling water tank 30 is a substantially sealed tank provided at a location in the cooling water path 40 that is vertically lower than the fuel cell stack 4. In an installed state, the cooling water tank 30 has a gas storage section 30e above a cooling water storage section 30d that stores cooling water 30r. The gas storage section 30e is divided into a first space 30f and a second space 30g so that the first space 30f and the second space 30g do not communicate with each other regardless of the operating state of the fuel cell stack 4.

[0195] The cooling water pump 10 is provided in the cooling water path 40 so as to pump up the cooling water 30 r in the cooling water tank 30 and supply it to the fuel cell stack 4.

[0196] The oxidizing gas supply unit 15 is configured to supply an oxidizing gas. The oxidizing gas supply path 11 is a path configured to supply the oxidizing gas into the cooling water 30r in the cooling water reservoir 30d below the first space 30f, and connects the oxidizing gas supply unit 15 and the cooling water tank 30.

[0197] The oxidant gas humidification path 12 is a path configured so that the oxidant gas discharged from the oxidant gas supply path 11, humidified, and accumulated in the first space 30f is supplied to the cathode 4b, and has an inlet that opens to the first space 30f and an outlet that is connected to the inlet of the cathode 4b.

[0198] The oxidant gas discharge path 13 is a path configured so that the oxidant gas discharged from the outlet of the cathode 4b without being used in the reaction in the fuel cell stack 4 is supplied into the cooling water 30r in the cooling water reservoir 30d below the second space 30g. The oxidant gas discharge path 13 has an inlet connected to the outlet of the cathode 4b and an outlet that opens into the cooling water 30r in the cooling water reservoir 30d below the second space 30g.

[0199] The off-gas path 14 is a path configured to allow the oxidant gas discharged from the oxidant gas discharge path 13, separated into gas and liquid, and accumulated in the second space 30g to be discharged to the outside, and is a path whose inlet opens to the second space 30g and whose outlet opens to the outside.

[0200] The heater 60 is provided in the cooling water tank 30 and is configured to adjust the temperature of the cooling water 30r.

[0201] The water level meter 70 is provided in the oxidant gas discharge path 13 and is configured to detect the water level of the cooling water 30r in the oxidant gas discharge path 13. More specifically, the water level meter 70 is configured to measure the water level of the cooling water 30r sucked from the cooling water tank 30 and detect that the cooling water 30r has been sucked up to a certain water level.

[0202] The control unit 8 operates the cooling water pump 10 even after the fuel cell stack 4 stops generating electricity. When the fuel cell stack 4 stops generating electricity, the control unit 8 adjusts the temperature of the cooling water 30r using the heater 60 so that the water level gauge 70 detects a predetermined water level at which the inlet of the oxidant gas humidification path 12 is completely submerged but the cathode 4b is not filled with the cooling water 30r. By adjusting the temperature of the cooling water 30r using the heater 60, the control unit 8 controls the temperature of the remaining oxidant gas.

[0203] As a result, when power generation is stopped, the fuel cell system 200 sucks and introduces the cooling water 30r from the cooling water tank 30 into the oxidant gas humidification path 12 and the oxidant gas discharge path 13, and can reliably stop the sucked cooling water 30r at a predetermined height. Therefore, the fuel cell system 200 can seal the cathode 4b without filling it with the cooling water 30r.

[0204] Therefore, the fuel cell system 200 can control the pressure of the residual oxidant gas in the cathode 4b with high precision and reliably prevent the cooling water 30r from entering the cathode 4b when power generation is stopped. Therefore, the fuel cell system 200 can prevent water clogging in the cathode 4b at startup (when power generation starts) and can perform stable power generation.

[0205] As in this embodiment, the fuel cell system 200 includes a first temperature measuring device 61, which is an example of a temperature measuring device in the present disclosure, and a second temperature measuring device 62, which is also an example of a temperature measuring device in the present disclosure. The first temperature measuring device 61 measures the temperature of the coolant 30r (after heat exchange with the fuel cell stack 4) in the coolant path 40 returning from the fuel cell stack 4 to the coolant tank 30. The second temperature measuring device 62 measures the temperature of the coolant 30r (before heat exchange with the fuel cell stack 4) in the path between the coolant pump 10 and the fuel cell stack 4 in the coolant path 40. The control unit 8 then estimates the temperature of the cathode 4b (sealed residual oxidant gas) from the values ​​measured by the first temperature measuring device 61 and the second temperature measuring device 62.

[0206] This allows the fuel cell system 200 to measure the temperature inside the fuel cell stack 4. Therefore, the fuel cell system 200 can accurately estimate the temperature of the cathode 4b (sealed residual oxidant gas) based on the values ​​from the first temperature measuring device 61 and the second temperature measuring device 62, and can adjust the temperature of the coolant 30r using the heater 60 based on the estimated value. Therefore, the fuel cell system 200 can reliably stop the coolant 30r from entering the cathode 4b by controlling the pressure of the residual oxidant gas.

[0207] (Other embodiments) As described above, the first and second embodiments have been described as examples of the technology in the present disclosure. However, the technology in the present disclosure is not limited to these and can be applied to embodiments in which modifications, substitutions, additions, omissions, etc. are made. Furthermore, it is also possible to combine the components described in the first and second embodiments to create new embodiments.

[0208] Therefore, other embodiments will be exemplified below.

[0209] In the first and second embodiments, the fuel cell systems 100 and 200 using the first temperature measuring device 61 have been described.

[0210] The temperature of the coolant 30r can be estimated from the amount of heat generated when an operating variable is applied to the heater 60. Therefore, the method for measuring the temperature of the coolant 30r is not limited to the first temperature measuring device 61. However, if a sheathed thermocouple is used as the first temperature measuring device 61, the temperature of the coolant 30r can be easily measured in real time. Therefore, the fuel cell system in other embodiments using a sheathed thermocouple as the first temperature measuring device 61 can speed up the responsiveness of temperature control in accordance with temperature changes after power generation is stopped. [Industrial Applicability]

[0211] The present disclosure is applicable to fuel cell systems that recover water produced in a fuel cell stack, and specifically to pure hydrogen fuel cell systems that generate electricity using hydrogen as fuel gas. [Explanation of symbols]

[0212] 1 Fuel gas supply 1a Fuel gas supply route 4 Fuel cell stack 4a Anode 4b Cathode 4c electrolyte membrane 5. Unreacted fuel gas discharge route 8 Control Unit 10 Cooling water pump 11 Oxidant gas supply route 12 Oxidant gas humidification route 13 Oxidant gas emission route 14 Off-gas route 15 Oxidant gas supply unit 20 Barometer 30 Cooling water tank 30a Liquid level when stopped 30b Liquid level during first operation 30c Second operation liquid level 30d Cooling water reservoir 30e Gas reservoir 30f first space 30g second space 30r cooling water 30s shielding plate 40 Cooling water path 60 Heater 61 First temperature measuring device 62 Second temperature measuring device 70 Water level gauge 100 Fuel Cell System 200 Fuel Cell System

Claims

1. a polymer electrolyte fuel cell stack that generates electricity by reacting a fuel gas supplied to an anode with an oxidant gas supplied to a cathode; a cooling water path through which cooling water circulates to exchange heat with the fuel cell stack; a substantially sealed cooling water tank that has, in an installed state, a gas storage section above a cooling water storage section that stores the cooling water, and that divides the gas storage section into a first space and a second space so that the first space and the second space do not communicate with each other regardless of the operating state of the fuel cell stack, and that is provided at a position vertically lower than the fuel cell stack in the cooling water path; a cooling water pump provided in the cooling water path to pump up the cooling water in the cooling water tank and supply it to the fuel cell stack; an oxidant gas supply unit that supplies the oxidant gas; an oxidant gas supply path connecting the oxidant gas supply unit and the cooling water tank so that the oxidant gas is supplied into the cooling water in the cooling water reservoir below the first space; an oxidant gas humidification path having an inlet that opens into the first space and an outlet that is connected to an inlet of the cathode so that the oxidant gas discharged from the oxidant gas supply path, humidified, and accumulated in the first space is supplied to the cathode; an oxidant gas discharge path having an inlet connected to the outlet of the cathode and an outlet opening into the cooling water in the cooling water reservoir below the second space, so that the oxidant gas discharged from the outlet of the cathode without being used in the reaction in the fuel cell stack is supplied into the cooling water in the cooling water reservoir below the second space; an off-gas passage having an inlet opening into the second space and an outlet opening to the outside, so that the oxidant gas discharged from the oxidant gas discharge passage, separated into gas and liquid, and accumulated in the second space can be discharged to the outside, When power generation by the fuel cell stack is stopped, the inlet of the oxidant gas humidification path is completely submerged, but the temperature of the remaining oxidant gas is controlled so that the cathode is not filled with the cooling water. A fuel cell system characterized by:

2. a heater provided in the cooling water tank to adjust the temperature of the cooling water; a control unit that operates the cooling water pump even after the fuel cell stack stops generating electricity; Furthermore, the control unit controls the temperature of the remaining oxidant gas by adjusting the temperature of the cooling water using the heater so that the inlet of the oxidant gas humidification path is completely submerged when power generation of the fuel cell stack is stopped, but the cathode is not filled with the cooling water.

2. The fuel cell system according to claim 1.

3. a temperature measuring device that measures the temperature of the cooling water discharged from the fuel cell stack or the temperature of the oxidant gas discharged from the fuel cell stack, the control unit estimates the temperature of the oxidant gas in the cathode based on the value of the temperature measuring device.

3. The fuel cell system according to claim 2.

4. a water level meter provided in the oxidant gas discharge path and configured to detect a water level of the cooling water in the oxidant gas discharge path; the control unit adjusts the amount of heat generated by the heater so that the water level meter detects a predetermined water level when power generation by the fuel cell stack is stopped.

4. The fuel cell system according to claim 2 or 3.

5. a polymer electrolyte fuel cell stack that generates electricity by reacting a fuel gas supplied to an anode with an oxidant gas supplied to a cathode; a cooling water path through which cooling water circulates to exchange heat with the fuel cell stack; a substantially sealed cooling water tank that has, in an installed state, a gas storage section above a cooling water storage section that stores the cooling water, and that divides the gas storage section into a first space and a second space so that the first space and the second space do not communicate with each other regardless of the operating state of the fuel cell stack, and that is provided at a position vertically lower than the fuel cell stack in the cooling water path; a cooling water pump provided in the cooling water path to pump up the cooling water in the cooling water tank and supply it to the fuel cell stack; an oxidant gas supply unit that supplies the oxidant gas; an oxidant gas supply path connecting the oxidant gas supply unit and the cooling water tank so that the oxidant gas is supplied into the cooling water in the cooling water reservoir below the first space; an oxidant gas humidification path having an inlet that opens into the first space and an outlet that is connected to an inlet of the cathode so that the oxidant gas discharged from the oxidant gas supply path, humidified, and accumulated in the first space is supplied to the cathode; an oxidant gas discharge path having an inlet connected to the outlet of the cathode and an outlet opening into the cooling water in the cooling water reservoir below the second space, so that the oxidant gas discharged from the outlet of the cathode without being used in the reaction in the fuel cell stack is supplied into the cooling water in the cooling water reservoir below the second space; an off-gas passage having an inlet opening into the second space and an outlet opening to the outside, so that the oxidant gas discharged from the oxidant gas discharge passage, separated into gas and liquid, and accumulated in the second space can be discharged to the outside; a heater provided in the cooling water tank to adjust the temperature of the cooling water; A method for operating a fuel cell system comprising: The cooling water pump is operated even after the power generation of the fuel cell stack is stopped, When the power generation of the fuel cell stack is stopped, the inlet of the oxidant gas humidification path is completely submerged, but the temperature of the cooling water is adjusted by the heater so that the cathode is not filled with the cooling water. A method for operating a fuel cell system comprising:

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