FUEL CELL SYSTEM AND CONTROL METHOD FOR FUEL CELL SYSTEM

A control unit performs water discharge in the first water discharge mode during electrical power generation at the time of stoppage in the case that a determination unit determines that the fuel cell stack is in moist condition, and the control unit performs water discharge in the second water discharge mode during the electrical power generation at the time of stoppage in the case that the determination unit determines that the fuel cell stack is not in moist condition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2023-137999 filed on Aug. 28, 2023, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a fuel cell system and a method for controlling the fuel cell system.

Description of the Related Art

In recent years, research and development have been conducted on fuel cells that contribute to energy efficiency in order to ensure that more people have access to affordable, reliable, sustainable and modern energy.

A power generation system including a fuel cell stack is referred to as a fuel cell system. The fuel cell stack includes a plurality of power generation cells. The power generation cell generates power by electrochemical reactions between a fuel gas (hydrogen-containing gas) and an oxygen-containing gas. Water is produced during power generation by the power generation cells. The water that is produced remains in the fuel cell stack or the like. Then, if the operation of the fuel cell system is stopped in a low temperature environment, the water remaining in the fuel cell stack or the like may freeze. If water freezes in a fuel cell system or the like, because flow of gas is restrained or blocked, the fuel cell stack cannot generate electrical power.

JP 2021-180076 A discloses that scavenging is performed at the end of operation of a fuel cell system. The scavenging is a process of blowing off impurities such as water and nitrogen from the fuel cell stack.

SUMMARY OF THE INVENTION

It is desirable to properly discharge water from the fuel cell stack.

An object of the present invention is to solve the aforementioned problem.

According to a first aspect of the present invention, there is provided a fuel cell system including: a fuel cell stack having a plurality of power generation cells configured to generate electrical power using an oxygen-containing gas and a fuel gas; an oxygen-containing gas supplier configured to supply the oxygen-containing gas to the fuel cell stack; a supply path through which the oxygen-containing gas to be supplied to the fuel cell stack flows; a discharge path through which the oxygen-containing gas discharged from the fuel cell stack flows; a temperature sensor configured to detect a temperature of the fuel cell stack; and a control device configured to control the oxygen-containing gas supplier, wherein the control device includes: a control unit configured to execute a stoppage operation in which power generation is continued until a predetermined condition is met after receipt of a system shutdown command; an acquisition unit configured to acquire a temperature of the fuel cell stack before the stoppage operation; a determination unit configured to determine whether the fuel cell stack is in moist condition on a basis of at least the temperature of the fuel cell stack acquired by the acquisition unit, and the control unit is configured to enable execution of water discharge during execution of electrical power generation under the stoppage operation by selectively discharging water remaining in the fuel cell stack in either a first mode in which an amount of oxygen-containing gas supplied from the oxygen-containing gas supplier to the fuel cell stack is a first amount, or a second mode in which the amount is a second amount less than the first amount, the control unit is configured to execute the water discharge in the first mode during execution of electrical power generation under the stoppage operation, in a case that the determination unit determines that the fuel cell stack is in moist condition, and the control unit is configured to execute the water discharge in the second mode during execution of the electrical power generation under the stoppage operation, in a case that the determination unit determines that the fuel cell stack is not in moist condition.

According to a second aspect of the present invention, there is provided a method of controlling a fuel cell system including: a fuel cell stack having a plurality of power generation cells configured to generate electrical power using an oxygen-containing gas and a fuel gas; an oxygen-containing gas supplier configured to supply the oxygen-containing gas to the fuel cell stack; a supply path through which the oxygen-containing gas to be supplied to the fuel cell stack flows; a discharge path through which the oxygen-containing gas discharged from the fuel cell stack flows; a temperature sensor configured to detect a temperature of the fuel cell stack; and a control device configured to control the oxygen-containing gas supplier, wherein the control device executes a stoppage operation in which power generation is continued until a predetermined condition is met after receipt of a system shutdown command; acquires the temperature of the fuel cell stack before the stoppage operation; determines whether the fuel cell stack is in moist condition on a basis of at least the temperature of the fuel cell stack acquired; enables execution of water discharge during execution of electrical power generation under the stoppage operation by selectively discharging water remaining in the fuel cell stack in either a first mode in which an amount of oxygen-containing gas supplied from the oxygen-containing gas supplier to the fuel cell stack is a first amount, or a second mode in which the amount is a second amount less than the first amount; executes the water discharge in the first mode during execution of electrical power generation under the stoppage operation, in a case that the fuel cell stack is determined to be in moist condition; and executes the water discharge in the second mode during execution of the electrical power generation under the stoppage operation, in a case that the fuel cell stack is determined not to be in moist condition.

According to the present invention, water can be appropriately discharged from the fuel cell stack.

DETAILED DESCRIPTION OF THE INVENTION

1. Configuration of Fuel Cell System10

FIG.1is a schematic diagram of a fuel cell system10. The fuel cell system10may be mounted on, for example, a vehicle (fuel cell vehicle). Apart from a vehicle, the fuel cell system10can be mounted on, a ship, an aircraft, a robot, or the like. The fuel cell system10can also be used as a power source in facilities, homes, and the like.

In the fuel cell system10, a fuel gas and an oxygen-containing gas are used as reactant gases. The fuel gas is a hydrogen-containing gas. The oxygen-containing gas is a gas containing oxygen. The fuel gas and the oxygen-containing gas are individually supplied to a fuel cell stack12and subjected to electrochemical reactions. In the present specification, the fuel gas discharged from the fuel cell stack12without being consumed in the electrochemical reactions is also referred to as a fuel off-gas. Further, in the present specification, the oxygen-containing gas discharged from the fuel cell stack12without being consumed in the electrochemical reactions is also referred to as an oxygen-containing off-gas.

The fuel cell system10includes the fuel cell stack12, a tank14, an anode system16, a cathode system18, and a cooling system20. The fuel cell system10includes a control device22. The electrical power generated by the fuel cell stack12is supplied to a load21. The tank14is filled with a high-pressure fuel gas.

The fuel cell stack12includes a fuel gas inlet22athrough which the fuel gas is supplied into the fuel cell stack12, and a fuel gas outlet22bthrough which the fuel off-gas is discharged from the fuel cell stack12. The fuel cell stack12includes an oxygen-containing gas inlet22cthrough which the oxygen-containing gas is supplied into the fuel cell stack12, and an oxygen-containing gas outlet22dthrough which the oxygen-containing off-gas is discharged from the fuel cell stack12. The fuel cell stack12includes a coolant inlet22ethrough which a coolant is supplied into the fuel cell stack12, and a coolant outlet22fthrough which the coolant is discharged from the fuel cell stack12. The fuel cell stack12includes a water outlet22gthrough which residual water is discharged from the fuel cell stack12. The water outlet22gis arranged in a lower portion of the fuel cell stack12.

Here, the configuration of the fuel cell stack12will be described with reference toFIGS.2to4.FIG.2is an exploded perspective view of a power generation cell24of the fuel cell stack12. The fuel cell stack12is formed by stacking a plurality of power generation cells24in the arrow A direction. A compressive load is applied to the fuel cell stack12in the stacking direction of the plurality of power generation cells24.

The power generation cell24has a horizontally long rectangular shape. The power generation cell24includes a membrane electrode assembly26, and a pair of separators (first separator28and a second separator30). The front surface28aof the first separator28faces the first surface26aof the membrane electrode assembly26. The front surface30aof the second separator30faces the second surface26bof the membrane electrode assembly26. The membrane electrode assembly26is sandwiched between the first separator28and the second separator30.

The first separator28and the second separator30are formed of metal thin plates each having a corrugated cross section. In two adjacent power generation cells24, the first separator28of one power generation cell24and the second separator30of the other power generation cell24are joined to each other. A coolant flow field (not shown) through which the coolant flows is formed between the first separator28and the second separator30.

The membrane electrode assembly26includes a membrane electrode assembly (MEA)32and a resin frame34. The MEA32has an electrolyte membrane36, a cathode40, and an anode38. The electrolyte membrane36is interposed between the cathode40and the anode38. The resin frame34protrudes outward from the outer periphery of the MEA32.

One end of the longer side (one end on the arrow B1side) of the power generation cell24is provided with an oxygen-containing gas supply passage42a, a coolant supply passage44aand a fuel gas discharge passage46b. The oxygen-containing gas supply passage42ais connected to the oxygen-containing gas inlet22c. The oxygen-containing gas flows through the oxygen-containing gas supply passage42ain the direction indicated by the arrow A2. The coolant supply passage44ais connected to the coolant inlet22e. The coolant flows through the coolant supply passage44ain the direction indicated by the arrow A2. The fuel gas discharge passage46bis connected to the fuel gas outlet22b. The fuel gas flows through the fuel gas discharge passage46bin the direction indicated by the arrow A1.

The other end of the longer side (the other end on the arrow B2side) of the power generation cell24is provided with a fuel gas supply passage46a, a coolant discharge passage44b, and an oxygen-containing gas discharge passage42b. The fuel gas supply passage46ais connected to the fuel gas inlet22a. The fuel gas flows through the fuel gas supply passage46ain the direction indicated by the arrow A2. The coolant discharge passage44bis connected to the coolant outlet22f. The coolant flows through the coolant discharge passage44bin the direction indicated by the arrow A1. The oxygen-containing gas discharge passage42bis connected to the oxygen-containing gas outlet22d. The oxygen-containing gas flows through the oxygen-containing gas discharge passage42bin the direction indicated by the arrow A1.

FIG.3is a schematic view of the first separator28.FIG.3shows the front surface28aof the first separator28. The first separator28is formed in a rectangular shape. An oxygen-containing gas flow field50is formed on the front surface28aof the first separator28. When viewed in the arrow A1direction or the arrow A2direction shown inFIG.2, the oxygen-containing gas flow field50overlaps the cathode40of the membrane electrode assembly26. The oxygen-containing gas flow field50extends in the longitudinal direction of the power generation cell24(arrow B direction).

The oxygen-containing gas flow field50includes a plurality of first flow field ridges52and a plurality of first flow field grooves54. The first flow field ridges52protrude in the arrow A2direction. The first flow field grooves54are recessed in the arrow A1direction. Each of the first flow field ridges52and the first flow field grooves54extends in a wave form in the arrow B direction. In the oxygen-containing gas flow field50, the first flow field ridges52and the first flow field grooves54are alternately arranged in the flow field width direction (the arrow C direction).

Two first guides55a,55bare formed on the front surface28aof the first separator28. The first guide55aincludes a plurality of first guide ridges56aand a plurality of first guide grooves57aextending from the oxygen-containing gas supply passage42atoward the oxygen-containing gas flow field50. The first guide55bincludes a plurality of first guide ridges56band a plurality of first guide grooves57bextending from the oxygen-containing gas flow field50toward the oxygen-containing gas discharge passage42b.

A first seal set58for preventing leakage of any of the reaction gases (the oxygen-containing gas and the fuel gas) and the coolant is provided on the front surface28aof the first separator28. The first seal set58includes a plurality of first passage seals60and a first flow field seal62. The first passage seals60and the first flow field seal62protrude in the arrow A2direction. One first passage seal60is provided for one passage (e.g., the oxygen-containing gas supply passage42a, and the like). The first passage seals60individually surround the passages. The first flow field seal62surrounds a region where the oxygen-containing gas flow field50, the first guides55a,55b, and the passages (the oxygen-containing gas supply passage42a, the oxygen-containing gas discharge passage42b, the fuel gas supply passage46a, and the fuel gas discharge passage46b) through which the reactant gases flow are disposed. Each of the plurality of first passage seals60and the first flow field seal62is pressed against the resin frame34of the membrane electrode assembly26.

The first passage seal60surrounding the oxygen-containing gas supply passage42aincludes a tunnel63a. The tunnel63aconnects the oxygen-containing gas supply passage42ato the first guide55aadjacent to the oxygen-containing gas supply passage42a. Although one tunnel63ais shown inFIG.3, a plurality of tunnels63aare actually provided. Similarly, the first passage seal60surrounding the oxygen-containing gas discharge passage42bincludes a tunnel63b. The tunnel63bconnects the oxygen-containing gas discharge passage42bto the first guide55badjacent to the oxygen-containing gas discharge passage42b. Although one tunnel63bis shown inFIG.3, a plurality of tunnels63bare actually provided.

The oxygen-containing gas flows from the oxygen-containing gas supply passage42ainto the oxygen-containing gas flow field50through the tunnels63aand the first guide55a. The oxygen-containing gas is supplied to the cathode40while flowing through the oxygen-containing gas flow field50. The oxygen-containing gas not consumed in the electrochemical reactions is discharged as the oxygen-containing off-gas from the oxygen-containing gas flow field50to the oxygen-containing gas discharge passage42bthrough the first guide55band the tunnels63b.

First bypass stopping protrusions64are formed on the front surface28aof the first separator28. The first bypass stopping protrusions64are disposed between the edges of the oxygen-containing gas flow field50in the flow field width direction (first outermost flow field ridges52a) and the first flow field seal62. The first bypass stopping protrusions64prevent the oxygen-containing gas supplied from the oxygen-containing gas supply passage42afrom flowing between the first outermost flow field ridges52aand the first flow field seal62toward the oxygen-containing gas discharge passage42b. That is, the first bypass stopping protrusions64prevent the oxygen-containing gas from bypassing the oxygen-containing gas flow field.

The first bypass stopping protrusions64include a plurality of first bypass blockers65extending in the arrow C direction. The plurality of first bypass blockers65are arranged along the arrow B direction. The first bypass blockers65protrude in the arrow A2direction. The height of the first bypass blockers65is slightly lower than the height of the first flow field seal62. The first bypass blockers65do not receive the compressive load applied in the arrow A direction. Thus, a minute gap is formed between the first bypass blockers65and the resin frame34of the membrane electrode assembly26.

FIG.4is a schematic view of the second separator30.FIG.4shows the front surface30aof the second separator30. The second separator30is formed in a rectangular shape. A fuel gas flow field66is formed on the front surface30aof the second separator30. When viewed in the arrow A1direction or the arrow A2direction shown inFIG.2, the fuel gas flow field66overlaps the anode38of the membrane electrode assembly26. The fuel gas flow field66extends in the longitudinal direction of the power generation cell24(arrow B direction).

The fuel gas flow field66includes a plurality of second flow field ridges68and a plurality of second flow field grooves70. The second flow field ridges68protrude in the arrow A1direction. The second flow field grooves70are recessed in the arrow A2direction. Each of the second flow field ridges68and the second flow field grooves70extends in a wave form in the arrow B direction. In the fuel gas flow field66, the second flow field ridges68and the second flow field grooves70are alternately arranged in the flow field width direction (the arrow C direction).

Two second guides71a,71bare formed on the front surface30aof the second separator30. The second guide71aincludes a plurality of second guide ridges72aand a plurality of second guide grooves73aextending from the fuel gas supply passage46atoward the fuel gas flow field66. The second guide71bincludes a plurality of second guide ridges72band a plurality of second guide grooves73bextending from the fuel gas flow field66toward the fuel gas discharge passage46b.

A second seal set74for preventing leakage of any of the reaction gases (the oxygen-containing gas and the fuel gas) and the coolant is provided on the front surface30aof the second separator30. The second seal set74includes a plurality of second passage seals76and a second flow field seal78. The second passage seals76and the second flow field seal78protrude in the arrow A1direction. One second passage seal76is provided for one passage (e.g., the fuel gas supply passage46a, and the like). The second passage seals76individually surround the passages. The second flow field seal78surrounds a region where the fuel gas flow field66, the second guides71a,71b, and the passages (the oxygen-containing gas supply passage42a, the oxygen-containing gas discharge passage42b, the fuel gas supply passage46a, and the fuel gas discharge passage46b) through which the reactant gases flow are disposed. Each of the plurality of second passage seals76and the second flow field seal78is pressed against the resin frame34of the membrane electrode assembly26.

The second passage seal76surrounding the fuel gas supply passage46aincludes a tunnel79a. The tunnel79aconnects the fuel gas supply passage46ato the second guide71aadjacent to the fuel gas supply passage46a. Although one tunnel79ais shown inFIG.4, a plurality of tunnels79aare actually provided. Similarly, the second passage seal76surrounding the fuel gas discharge passage46bincludes a tunnel79b. The tunnel79bconnects the fuel gas discharge passage46bto the second guide71badjacent to the fuel gas discharge passage46b. Although one tunnel79bis shown inFIG.4, a plurality of tunnels79bare actually provided.

The fuel gas flows from the fuel gas supply passage46ainto the fuel gas flow field66through the tunnels79aand the second guide71a. The fuel gas is supplied to the anode38while flowing through the fuel gas flow field66. The fuel gas not consumed in the electrochemical reactions is discharged as the fuel off-gas from the fuel gas flow field66to the fuel gas discharge passage46bthrough the second guide71band the tunnels79b.

Second bypass stopping protrusions80are formed on the front surface30aof the second separator30. The second bypass stopping protrusions80are disposed between the edges of the fuel gas flow field66in the flow field width direction (second outermost flow field ridges68a) and the second flow field seal78. The second bypass stopping protrusions80prevent the fuel gas supplied from the fuel gas supply passage46afrom flowing between the second outermost flow field ridges68aand the second flow field seal78toward the fuel gas discharge passage46b. That is, the second bypass stopping protrusions80prevent the fuel gas from bypassing the fuel gas flow field.

The second bypass stopping protrusions80include a plurality of second bypass blockers82extending in the arrow C direction. The plurality of second bypass blockers82are arranged along the arrow B direction. The second bypass blockers82protrude in the arrow A1direction. The height of the second bypass blockers82is slightly lower than the height of the second flow field seal78. The second bypass blockers82do not receive the compressive load applied in the arrow A direction. Thus, a minute gap is formed between the second bypass blockers82and the resin frame34of the membrane electrode assembly26.

Returning toFIG.1, the configuration of the fuel cell system10will be described. The anode system16includes a fuel gas supply path84, a fuel gas discharge path86, a circulation path88, a first water discharge path90, and a second discharge path92. The anode system16also includes an injector94, an ejector96, a gas-liquid separator98, a first water discharge valve100, and a second water discharge valve102.

The fuel gas supply path84is connected to the outlet of the tank14and the fuel gas inlet22aof the fuel cell stack12. The fuel gas supply path84is equipped with the injector94and the ejector96. The ejector96is disposed closer to the fuel cell stack12than the injector94.

The fuel gas discharge path86is connected to the fuel gas outlet22bof the fuel cell stack12and a supply port of the gas-liquid separator98. The circulation path88is connected to a discharge port of the gas-liquid separator98and the ejector96.

The first water discharge path90is connected to a drainage outlet of the gas-liquid separator98and to an inlet of a diluter121. An outlet of the diluter121is connected to an exhaust orifice of the vehicle. The first water discharge path90is provided with the first water discharge valve100. The second water discharge path92is connected to the water outlet22gof the fuel cell stack12and to the first water discharge path90. The second water discharge path92is provided with the second water discharge valve102.

The cathode system18includes an oxygen-containing gas supply path106, an oxygen-containing gas discharge path108(discharge path), and a bypass path110. The cathode system18includes a compressor112(oxygen-containing gas supplier), a humidifier (HUM)114, a first stop valve116, a second stop valve118, and a bypass valve120.

The oxygen-containing gas supply path106is connected to an air intake of the vehicle and the oxygen-containing gas inlet22cof the fuel cell stack12. The oxygen-containing gas supply path106is provided with the compressor112, the first stop valve116, and a humidifier supply path114A of the humidifier114. A portion of the oxygen-containing gas supply path106on the upstream side of the humidifier114is referred to as an oxygen-containing gas supply path106A. A portion of the oxygen-containing gas supply path106on the downstream side of the humidifier114is referred to as an oxygen-containing gas supply path106B. The oxygen-containing gas supply path106A is provided with the compressor112and the first stop valve116. The first stop valve116is disposed closer to the humidifier114than the compressor112.

The oxygen-containing gas discharge path108is connected to the oxygen-containing gas outlet22dof the fuel cell stack12and an inlet of the diluter121. The oxygen-containing gas discharge path108is provided with a humidifier discharge path114B of the humidifier114and the second stop valve118. A portion of the oxygen-containing gas discharge path108on the upstream side of the humidifier114is referred to as an oxygen-containing gas discharge path108A. A portion of the oxygen-containing gas discharge path108on the downstream side of the humidifier114is referred to as an oxygen-containing gas discharge path108B. The oxygen-containing gas discharge path108B is provided with the second stop valve118.

The bypass path110is connected to the oxygen-containing gas supply path106A between the compressor112and the first stop valve116, and to the oxygen-containing gas discharge path108B on the downstream side of the second stop valve118. The bypass path110is provided with the bypass valve120.

The anode system16and the cathode system18are connected to each other by a connection path132. The connection path132is connected to the circulation path88of the anode system16and the oxygen-containing gas supply path106B of the cathode system18. The connection path132is provided with a bleed valve134.

The cooling system20includes a coolant supply path122and a coolant discharge path124. The cooling system20includes a pump126, a radiator128, and a temperature sensor130.

The coolant supply path122is connected to a fluid outlet of the radiator128and the coolant inlet22eof the fuel cell stack12. The coolant supply path122is provided with the pump126. The coolant discharge path124is connected to the coolant outlet22fof the fuel cell stack12and a fluid inlet of the radiator128. The temperature sensor130is attached to the coolant discharge path124. The temperature sensor130detects the temperature of the coolant flowing through the coolant discharge path124. The temperature of the coolant flowing through the coolant discharge path124corresponds to the temperature inside the fuel cell stack12(stack temperature).

An impedance measuring device148may be attached to the fuel cell stack12. For example, the impedance measuring device148measures the impedance of the fuel cell stack12by superimposing alternating current on the output of the plurality of power generation cells24.

The control device22is constituted by an ECU (Electronic Control Unit). The control device22includes a computation unit136and a storage unit138. The computation unit136is, for example, a processor such as a central processing unit (CPU) or a graphics processing unit (GPU). More specifically, the computation unit136can be configured by a processing circuit (processing circuitry). The computation unit136controls each device by executing a program stored in the storage unit138. At least a portion of the computation unit136may be realized by an integrated circuit such as an ASIC (Application Specific Integrated Circuit), an FPGA (Field-Programmable Gate Array), or the like. Further, at least a portion of the computation unit136may be constituted by an electronic circuit including a discrete device.

The computation unit136includes an acquisition unit140, a control unit142, a time measurement unit144, and a determination unit146. The acquisition unit140acquires information from electronic components (sensors, ECUs, and the like) other than the control device22. The control unit142controls the operations of the injector94, the compressor112, the pump126, the valves, and the like. The time measurement unit144measures by a timer the implementation time of the water discharge control to be described later. The determination unit146determines whether the fuel cell stack12is in a moist-condition-predicted state and whether the fuel cell stack12is in moist condition.

The storage unit138may be made up of a volatile memory (not shown), and a non-volatile memory (not shown), as a computer-readable storage medium. Examples of the volatile memory include, for example, a RAM (Random Access Memory) or the like. As the non-volatile memory, there may be cited, for example, a ROM (Read Only Memory), a flash memory, or the like. Data, etc. may be stored in the volatile memory, for example. Programs, tables, maps, and the like are stored, for example, in the nonvolatile memory. At least a portion of the storage unit138may be provided in the processor, the integrated circuit, or the like, which were described above.

2 Fluid Flow in Fuel Cell System10

2-1 Fluid Flow in Anode System16

The injector94injects the fuel gas supplied from the tank14, toward the downstream side of the fuel gas supply path84. The fuel gas injected by the injector94is supplied to the fuel gas inlet22aof the fuel cell stack12through the fuel gas supply path84. The unreacted fuel gas in the fuel cell stack12is discharged as a fuel off-gas from the fuel gas outlet22bof the fuel cell stack12. The fuel off-gas contains hydrogen that has not reacted with oxygen, nitrogen that had been contained in the oxygen-containing gas and has permeated the electrolyte membrane36, and water produced by reactions between oxygen and hydrogen.

The fuel off-gas is supplied to the gas-liquid separator98via the fuel gas discharge path86. The gas-liquid separator98separates the fuel off-gas into a gas component (fuel off-gas) and a liquid component (water). The fuel off-gas discharged from the gas-liquid separator98flows through the circulation path88and is supplied to the ejector96. The fuel off-gas introduced into the ejector96from the gas-liquid separator98is mixed with the fuel gas that is injected by the injector94.

2-2 Fluid Flow in Cathode System18

The compressor112discharges the oxygen-containing gas (air) taken from the outside of the vehicle, toward the downstream side of the oxygen-containing gas supply path106. The oxygen-containing gas discharged from the compressor112is supplied to the oxygen-containing gas inlet22cof the fuel cell stack12through the oxygen-containing gas supply path106. The unreacted oxygen-containing gas in the fuel cell stack12is discharged as an oxygen-containing off-gas from the oxygen-containing gas outlet22dof the fuel cell stack12. The oxygen-containing off-gas contains components that have been contained in the oxygen-containing gas and water generated by the reactions between oxygen and hydrogen.

The oxygen-containing off-gas is discharged to the diluter121through the oxygen-containing gas discharge path108. The oxygen-containing off-gas contains water. In the humidifier114, a part of the water contained in the oxygen-containing off-gas is used for humidifying the oxygen-containing gas flowing through the humidifier supply path114A.

2-3 Fluid Flow in Cooling System20

The pump126discharges the coolant toward the coolant inlet22eof the fuel cell stack12. The coolant discharged from the pump126is supplied to the coolant inlet22eof the fuel cell stack12through the coolant supply path122. The coolant that has flowed through the fuel cell stack12is discharged from the coolant outlet22fof the fuel cell stack12. The coolant discharged from the coolant outlet22fis supplied to the radiator128via the coolant discharge path124. The coolant that has dissipated heat in the radiator128is sucked into the pump126.

3 Water Discharge during Electrical Power Generation at Stoppage

When the operation of the vehicle is stopped, the control device22stops the operation of the fuel cell system10. The computation unit136(control unit142) of the control device22performs power generation to make the electrolyte membrane36uniformly moistened in an appropriate moist condition before stopping power generation by the fuel cell system10. This is called electrical power generation at the time of stoppage. During the electrical power generation at the time of stoppage, the control unit142performs power generation control so as to control output current of the fuel cell stack12at a predetermined value. During the electrical power generation at the time of stoppage, the fuel gas, the oxygen-containing gas and the coolant are supplied to the fuel cell stack12, and electricity is supplied from the fuel cell stack12to the load21(for example, a battery).

The control unit142also causes water to be discharged from the fuel cell stack12during the electrical power generation at the time of stoppage. This is also referred to as scavenging. For example, the control unit142increases the flow rates of the reactant gases to be higher than those in normal power generation operation, thereby discharging water from the fuel cell stack12and the discharge paths. Specifically, the control unit142controls the flow rate of the oxygen-containing gas discharged from the compressor112(the flow rate of the oxygen-containing gas to be supplied to the fuel cell stack12). Water can be discharged from the fuel cell stack12also by increasing the flow rate of the fuel gas. However, if the flow rate of the fuel gas is increased during the electrical power generation at the time of stoppage, the fuel gas is consumed wastefully. Therefore, it is preferable to discharge water from the fuel cell stack12by increasing the amount of the oxygen-containing gas supplied. In the present specification, control for discharging water from the fuel cell stack12(and the oxygen-containing gas discharge path108) during the electrical power generation at time of stoppage is referred to as water discharge control or scavenging control. The water includes liquid water remaining in the fuel cell stack12and the oxygen-containing gas discharge path108, and highly humidified air containing water vapor.

The control unit142can execute, during the electrical power generation at the time of stoppage, water discharge control in the cathode system18in either the first water discharge mode or the second water discharge mode. When the determination unit146determines that the fuel cell stack12is in moist condition, the control unit142performs water discharge in the first water discharge mode. When the determination unit146determines that the fuel cell stack12is not in the moist condition, the control unit142performs water discharge in the second water discharge mode. The moist condition refers to a state in which the fuel cell stack12contains water in an amount equal to or greater than a predetermined amount, and the humidity within the fuel cell stack12is equal to or greater than a predetermined humidity.

In the first water discharge mode, the oxygen-containing gas is discharged from the compressor112at a first flow rate. In the second water discharge mode, the oxygen-containing gas is discharged from the compressor112at a second flow rate. The first flow rate is greater than the second flow rate. Therefore, water discharge in the first water discharge mode can discharge a larger amount of water from the fuel cell stack12than that in the second water discharge mode.

For example, water tends to be accumulated in and around the first bypass stopping protrusions64, the first guides55a,55b, and the like shown inFIG.3. When the flow rate of the oxygen-containing gas supplied to the fuel cell stack12is increased, the water content of the electrolyte membrane36decreases. Then, the electrolyte membrane36absorbs water accumulated in the first bypass stopping protrusions64, and next time discharged from the electrolyte membrane36, the water is discharged to the oxygen-containing gas flow field50. The water discharged to the oxygen-containing gas flow field50flows out of the fuel cell stack12. That is, execution of water discharge in the first water discharge mode can facilitate removal of water accumulated in the first bypass stopping protrusions64. Further, execution of water discharge in the first water discharge mode can facilitate removal of water accumulated in the first guides55a,55b.

Similarly, water tends to be accumulated in and around the second bypass stopping protrusions80, the second guides71a,71b, and the like shown inFIG.4. When the flow rate of the oxygen-containing gas supplied to the fuel cell stack12is increased, the electrolyte membrane36absorbs water accumulated in the second bypass stopping protrusions80and the like, and next time discharged from the electrolyte membrane36, the water is discharged to the oxygen-containing gas flow field50. The water discharged to the oxygen-containing gas flow field50flows out of the fuel cell stack12. That is, the execution of water discharge in the first water discharge mode can also facilitate removal of water accumulated in the second bypass stopping protrusions80. Further, execution of water discharge in the first water discharge mode can also facilitate removal of water accumulated in the second guides71a,71b.

3-1 Determination of Moist Condition of Fuel Cell Stack12

FIG.5is a flowchart of a process for determining a moist condition of the fuel cell stack12.FIG.6is a diagram showing a map150for determining the moist condition.FIG.7Ais a diagram showing a change in instructed current values over time.FIG.7Bis a diagram showing a change in temperature values of the coolant over time.FIG.7Cis a diagram showing time measured by a timer.FIG.7Dis a diagram showing a change in the moist condition flag over time.

As described above, whether water is discharged in the first water discharge mode or the second water discharge mode is determined depending on whether the fuel cell stack12is in moist condition. Whether the fuel cell stack12is in moist condition is determined during operation of the fuel cell system10(during operation of the vehicle). An example of the determination of the moist condition of the fuel cell stack12will be described with reference toFIG.5.

In the determination process to be described with reference toFIG.5, the fuel cell stack12is determined to be in moist condition when the moist-condition-predicted state continues for a predetermined period of time. The moist-condition-predicted state is a state in which the fuel cell stack12is highly likely to be in moist condition. Here, the moist-condition-predicted state is determined on the basis of the output current of the fuel cell stack12and the temperature of the fuel cell stack12. The output current of the fuel cell stack12is regarded as being substantially equal to the current value instructed from the ECU of the vehicle. The temperature of the fuel cell stack12is regarded as being substantially equal to the temperature of the fluid discharged from the fuel cell stack12. Therefore, in the moist condition determination process to be described below, the instructed current value (output current value) from the ECU of the vehicle and a temperature value (stack temperature) detected by the temperature sensor130are used for judging the moist-condition-predicted state. The temperature of the oxygen-containing off-gas may be used instead of the temperature of the coolant.

In step S1, the determination unit146determines whether the fuel cell stack12is in the moist-condition-predicted state. The determination unit146uses, for example, a moist condition determination map150shown inFIG.6. The moist condition determination map150associates a combination of the instructed current value and the temperature value of the coolant with the state of the fuel cell stack12(whether the fuel cell stack12is in the moist-condition-predicted state). As shown inFIG.6, the moist condition determination map150associates a combination of an instructed current value lower than the current threshold Ith and a temperature value of the coolant lower than the temperature threshold Tth with the moist-condition-predicted state. The reason why the moist condition determination map150shows such association is that the fuel cell stack12is likely to be in moist condition when the output current value of the fuel cell stack12is relatively low and the temperature of the fuel cell stack12is relatively low.

The acquisition unit140acquires an instructed current value of the fuel cell stack12from the ECU of the vehicle. The acquisition unit140acquires the temperature value of the coolant from the temperature sensor130. The determination unit146determines whether the fuel cell stack12is in the moist-condition-predicted state based on the instructed current value, the temperature value of the coolant, and the moist condition determination map150.

If the fuel cell stack12is in the moist-condition-predicted state (step S1: YES), the process proceeds to step S2. For example, as shown inFIG.7A, the instructed current values are lower than the current threshold Ith between the time point t1and the time point t2. As shown inFIG.7A, the instructed current values are lower than the current threshold Ith between the time point t3and the time point t4. As shown inFIG.7B, the temperature of the coolant becomes lower than the temperature threshold Tth after the time point t1. The determination unit146determines that the fuel cell stack12is in the moist-condition-predicted state between the time point t1and the time point t2. The determination unit146determines that the fuel cell stack12is in the moist-condition-predicted state between the time point t3and the time point t4.

On the other hand, when the fuel cell stack12is not in the moist-condition-predicted state (step S1: NO), the process proceeds to step S7. For example, as shown inFIG.7B, the coolant temperature is higher than the temperature threshold Tth between the time point t0and the time point t1. As shown inFIG.7A, the instructed current values are higher than the current threshold Ith between the time point t2and the time point t3. The determination unit146determines that the fuel cell stack12is not in the moist-condition-predicted state between the time point to and the time point t1. The determination unit146determines that the fuel cell stack12is not in the moist-condition-predicted state between the time point t2and the time point t3.

In step S2, the time measurement unit144determines whether or not the time is being measured. In the case that the time is being measured (step S2: YES), the process proceeds to step S4. On the other hand, when the time is not being measured (step S2: NO), the process proceeds to step S3.

Upon transitioning from step S2to step S3, the time measurement unit144starts measuring time by a timer. For example, as shown inFIG.7C, the timer starts clocking at time point t1and time point t3. The timer continues to count time between the time point t1and the time point t2. The timer continues to count time between the time point t3and the time point t4. Upon completion of step S3, the process transitions to step S4.

Upon transitioning from step S2or step S3to step S4, the determination unit146compares the time measured by the timer with the predetermined time Cth. The predetermined time Cth is a time threshold value for determining whether the fuel cell stack12is in moist condition. In the moist condition determination process, the fuel cell stack12is determined to be in moist condition when the moist-condition-predicted state continues for the predetermined time Cth. In the case that the time period measured by the timer is greater than or equal to the predetermined time Cth (step S4: YES), the process proceeds to step S5. For example, as shown inFIG.7C, the time period measured by the timer becomes equal to the predetermined time Cth at the time point t4. On the other hand, in the case that the time period measured by the timer is less than the predetermined time Cth (step S4: NO), the process returns to step S1.

Upon transitioning from step S4to step S5, the determination unit146sets the moist condition flag to “1”. For example, as shown inFIG.7D, the moist condition flag is set to “1” at the time point t4. Upon completion of step S5, the process transitions to step S6.

Upon transitioning from step S5or step S8to be described below to step S6, the control unit142determines whether or not a command to stop the operation of the fuel cell system10has already been given. An operator of the vehicle performs an off operation on an unillustrated ignition switch (also referred to as a power switch) to stop the operation of the vehicle. When the acquisition unit140acquires an off signal associated with the off operation on the ignition switch, the control unit142determines that the command to stop the operation of the fuel cell system10is given. If the command to stop the operation of the fuel cell system10has already been given (step S6: YES), the series of process steps shown inFIG.5is brought to an end. On the other hand, if the command to stop the operation of the fuel cell system10has not been given yet (step S6: NO), the process returns to step S1.

Upon transitioning from step S1to step S7, the time measurement unit144resets the timer. For example, as shown inFIG.7C, the timer is reset during the time between the time point to and the time point t1and the time between the time point t2and the time point t3. Upon completion of step S7, the process transitions to step S8.

In step S8, the control unit142sets the moist condition flag to “0”. For example, as shown inFIG.7D, the moist condition flag is set to “0” during the time between the time to and the time point t4. Upon completion of step S8, the process transitions to step S6.

3-2 Water Discharge Process Executed During Electrical Power Generation at Stoppage Operation

FIG.8is a flowchart of a water discharge process executed during electrical power generation at the time of stoppage. The series of process steps shown inFIG.8is executed after the series of process steps shown inFIG.5is brought to an end. In the water discharge process described below, the first water discharge mode can be switched to the second water discharge mode during the water discharge control.

In step S11, the time measurement unit144sets a water discharge time for which the water discharge control is being executed. Here, the time measurement unit144sets the water discharge time according to a preset discharge time stored in the storage unit138. Upon completion of step S11, the process transitions to step S12.

In step S12, the time measurement unit144starts measuring time by the timer. That is, the time measurement unit144starts measuring the execution time of the water discharge control. Upon completion of step S12, the process transitions to step S13.

In step S13, the determination unit146determines whether the fuel cell stack12is in moist condition on the basis of the moist condition flag set in the process shown inFIG.5. In the case that the moist condition flag is set to 1 (step13: 1), the process proceeds to step S14. That is, when the fuel cell stack12is in moist condition, the process proceeds to step S14. On the other hand, in the case that the moist condition flag is set to 0 (step S13: 0), the process proceeds to step S19. That is, when the fuel cell stack12is not in moist condition, the process proceeds to step S19.

Upon transitioning from step S13or step S16to be described below to step S14, the control unit142executes water discharge in the first water discharge mode. In the first water discharge mode, the control unit142controls the operation of the compressor112such that the flow rate of the oxygen-containing gas discharged from the compressor112becomes the first flow rate. Thus, the oxygen-containing gas is supplied to the fuel cell stack12and the oxygen-containing gas discharge path108at the first flow rate. In this case, a relatively large amount of oxygen-containing gas is supplied to the fuel cell stack12and to the oxygen-containing gas discharge path108. With a large amount of oxygen-containing gas supplied, water remaining in the fuel cell stack12and the oxygen-containing gas discharge path108is sufficiently discharged. In the first water discharge mode, the control unit142controls the rotation speed of the pump126to a first rotation speed. Upon completion of step S14, the process transitions to step S15.

In step S15, the control unit142compares the time period measured by the timer (first execution time) with the water discharge time. In the case that the time period is greater than or equal to the water discharge time (step S15: YES), the water discharge control is brought to an end. In this case, the control unit142stops the compressor112and the pump126. Further, the control unit142sequentially closes each valve. On the other hand, in the case that the time period measured by the timer is less than the water discharge time (step S15: NO), the process proceeds to step S16.

Upon transitioning from step S15to step S16, the determination unit146determines whether the fuel cell stack12is in moist condition. For example, the determination unit146may perform the moist condition determination based on the moist condition determination map150. When the moist condition determination map150is used, the determination unit146determines that the fuel cell stack12is in moist condition when the combination of the instructed current value (predetermined value) and the temperature value of the coolant falls within the region of the moist-condition-predicted state. Alternatively, the determination unit146may perform the moist condition determination based on the impedance of the fuel cell stack12. The impedance of the fuel cell stack12is measured by the impedance measuring device148. In the case that the fuel cell stack12is in moist condition (step S16: YES), the process returns to step S14. On the other hand, in the case that the fuel cell stack12is not in moist condition (step S16: NO), the process proceeds to step S17.

Upon transitioning from step S16to step S17, the time measurement unit144calculates the remaining water discharge time by subtracting the first execution time, which is the execution time of water discharge in the first water discharge mode, from the preset discharge time stored in the storage unit138. Further, the time measurement unit144sets the remaining water discharge time as the water discharge time (second execution time). Upon completion of step S17, the process transitions to step S18.

In step S18, the time measurement unit144resets the timer and then starts measuring time by the timer. That is, the time measurement unit144starts measuring the execution time of the water discharge control in the second water discharge mode. Upon completion of step S18, the process transitions to step S19.

Upon transitioning from step S13, step S18or step S20to be described below to step S19, the control unit142executes water discharge in the second water discharge mode. In the case that the process transitions from step S18to step S19, the control unit142switches the water discharge mode from the first water discharge mode to the second water discharge mode. In the second water discharge mode, the control unit142controls the operation of the compressor112such that the flow rate of the oxygen-containing gas discharged from the compressor112becomes the second flow rate. Thus, the oxygen-containing gas is supplied to the fuel cell stack12and to the oxygen-containing gas discharge path108at the second flow rate. In this case, the oxygen-containing gas is supplied to the fuel cell stack12and the oxygen-containing gas discharge path108at a flow rate lower than that in the first water discharge mode and higher than that in the normal power generation operation. Since the water discharge in the first water discharge mode is switched to the water discharge in the second water discharge mode, the water can be appropriately discharged from the fuel cell stack12, but the fuel cell stack12is prevented from being excessively dried. In the second water discharge mode, the control unit142controls the rotation speed of the pump126to a second rotation speed. The second rotation speed is higher than the first rotation speed. Upon completion of step S19, the process transitions to step S20.

In step S20, the control unit142compares the time period measured by the timer with the water discharge time. In the case that the time period is greater than or equal to the water discharge time (step S20: YES), the water discharge control is brought to an end. In this case, the control unit142stops the compressor112and the pump126. Further, the control unit142sequentially closes each valve. On the other hand, in the case that the time period measured by the timer is less than the water discharge time (step S20: NO), the process returns to step S19.

As described above, in the case that the fuel cell stack12is in moist condition, the control unit142performs water discharge in the first water discharge mode. In the case that the fuel cell stack12is no longer in moist condition before a predetermined water discharge time (preset discharge time) is reached, the control unit142performs water discharge in the second water discharge mode.

In the series of process steps shown inFIG.8, the processes of step S17and step S18can be omitted. That is, the timer144may not necessarily calculate the second execution time for executing the water discharge in the second water discharge mode. In this case, the control unit142performs the water discharge in the second water discharge mode until the time measured by the timer reaches the water discharge time set in step S11.

4. Operations and Effects

If the water content of the fuel cell stack12is too high, the water may freeze. Further, if the water content of the fuel cell stack12is too high, iron is easily dissolved in water from metal components. The water containing iron deteriorates the electrolyte membrane36. Therefore, it is preferable to appropriately discharge water from the fuel cell stack12so that the water content of the fuel cell stack12does not become too high.

When the amount of the oxygen-containing gas supplied to the fuel cell stack12is increased, the water discharge from the fuel cell stack12can be facilitated. On the other hand, if the fuel cell stack12is dried out more than necessary, the fuel cell stack12may deteriorate.

In the above embodiment, the control unit142performs water discharge in the first water discharge mode in the case that the fuel cell stack12is in moist condition. That is, during electrical power generation at the time of stoppage, the control unit142relatively increases the amount of the oxygen-containing gas supplied to the fuel cell stack12. In this manner, it is possible to facilitate discharge of water from the fuel cell stack12, and particularly facilitate removal of water accumulated in or around the first bypass stopping protrusions64, the first guides55a,55b, the second bypass stopping protrusions80, the second guides71a,71b, and the like. On the other hand, the control unit142performs the water discharge control in the second water discharge mode in the case that the fuel cell stack12is not in moist condition. That is, during the electrical power generation at the time of stoppage, the control unit142relatively reduces the amount of the oxygen-containing gas supplied to the fuel cell stack12. According to the embodiment, it is possible to facilitate removal of water from portions of the fuel cell stack12through which water is difficult to flow. Further, according to the embodiment, it is possible to discharge water from the fuel cell stack12while suppressing excessive drying of the fuel cell stack12. That is, according to the present invention, water can be appropriately discharged from the fuel cell stack12.

In the above embodiment, the control unit142switches the water discharge control from the first water discharge mode to the second water discharge mode as the state of the fuel cell stack12changes from the moistened state to the predetermined dried state during the water discharge control. As described above, according to the embodiment, because the water discharge control is switched from the first water discharge mode to the second water discharge mode, the water is appropriately discharged from the fuel cell stack12, while the fuel cell stack12is prevented from being excessively dried out.

In the above embodiment, the control unit142performs the water discharge control for a water discharge time set to a preset discharge time. According to the above embodiment, it is possible to prevent the water discharge control from being performed for a longer time than necessary.

In the water discharge control in the first water discharge mode, power consumption by the compressor112is increased. In the above embodiment, the control unit142makes the number of rotations of the pump126during water discharge in the first water discharge mode smaller than the number of rotations of the pump126during water discharge in the second water discharge mode. In this manner, according to the embodiment, it is possible to suppress an increase in power consumption by the entire fuel cell system10.

5 Supplementary Note

With respect to the above disclosure, the following additional remarks are disclosed.

Supplementary Note 1

The first disclosure is to provide the fuel cell system (10) including: the fuel cell stack (12) having the plurality of power generation cells (24) configured to generate electrical power using the oxygen-containing gas and the fuel gas; the oxygen-containing gas supplier (112) configured to supply the oxygen-containing gas to the fuel cell stack; the supply path (106) through which the oxygen-containing gas to be supplied to the fuel cell stack flows; the discharge path (108) through which the oxygen-containing gas discharged from the fuel cell stack flows; the temperature sensor (130) configured to detect a temperature of the fuel cell stack; and the control device (22) configured to control the oxygen-containing gas supplier, wherein the control device includes: the control unit (142) configured to execute a stoppage operation in which power generation is continued until a predetermined condition is met after receipt of a system shutdown command; the acquisition unit (140) configured to acquire a temperature of the fuel cell stack before the stoppage operation; a determination unit (146) configured to determine whether the fuel cell stack is in moist condition on the basis of at least the temperature of the fuel cell stack acquired by the acquisition unit, and the control unit is configured to enable execution of water discharge during execution of electrical power generation under the stoppage operation by selectively discharging water remaining in the fuel cell stack in either the first mode in which the amount of oxygen-containing gas supplied from the oxygen-containing gas supplier to the fuel cell stack is the first amount, or the second mode in which the amount is the second amount less than the first amount, the control unit is configured to execute the water discharge in the first mode during execution of electrical power generation under the stoppage operation, in the case that the determination unit determines that the fuel cell stack is in moist condition, and the control unit is configured to execute the water discharge in the second mode during execution of the electrical power generation under the stoppage operation, in the case that the determination unit determines that the fuel cell stack is not in moist condition.

According to the configuration described above, it is possible to facilitate removal of water from portions of the fuel cell stack through which water is difficult to flow. Further, according to the configuration described above, it is possible to discharge water from the fuel cell stack while suppressing excessive drying of the fuel cell stack. That is, according to the configuration described above, water can be appropriately discharged from the fuel cell stack.

Supplementary Note 2

In the fuel cell system according to Supplementary Note 1, the determination unit may determine whether the fuel cell stack is in moist condition on the basis of the temperature of the fuel cell stack and the output current value of the fuel cell stack.

Supplementary Note 3

In the fuel cell system according to Supplementary Note 2, the determination unit may acquire the temperature of the fuel cell stack and the output current value before starting the stoppage operation, and determine that the fuel cell stack is in the moist-condition-predicted state in a case that the temperature of the fuel cell stack is lower than a predetermined temperature threshold and the output current value is lower than a predetermined current threshold, and determine that the fuel cell stack is in moist condition in a case that the moist-condition-predicted state continues for a predetermined time (Cth) before starting the stoppage operation.

Supplementary Note 4

In the fuel cell system according to Supplementary Note 1, each of the power generation cells may include the assembly (26) of the electrolyte membrane (36), the anode (38) and the cathode (40), and the pair of separators (28,30) sandwiching the assembly, and each of the separators may include the reactant gas flow field (50,66) through which the oxygen-containing gas or the fuel gas flows, the seal (62,78) surrounding the reactant gas flow field, and the bypass stopping portion (64,80) formed between the edge (52a,68a) of the reactant gas flow field in a flow field width direction and the seal to prevent the oxygen-containing gas or the fuel gas from bypassing the reactant gas flow field.

The water accumulated in the bypass stopping portion is not easily discharged by the oxygen-containing gas flowing at the normal flow rate of the oxygen-containing gas during the stoppage operation (the second flow rate of the oxygen-containing gas in the second water discharge mode). On the other hand, the oxygen-containing gas flowing at the first flow rate of the oxygen-containing gas in the first water discharge mode can facilitate removal of water from the bypass stopping portion.

Supplementary Note 5

In the fuel cell system according to Supplementary Note 1, the determination unit may further determine whether the fuel cell stack is in moist condition during the water discharge in the first water discharge mode, and the control unit may switch from the water discharge in the first water discharge mode to the water discharge in the second water discharge mode in the case that the determination unit determines that the fuel cell stack is no longer in moist condition during the water discharge in the first water discharge mode.

According to the above-described arrangement, because the water discharge control is switched from the first water discharge mode to the second water discharge mode, the water is appropriately discharged from the fuel cell stack, while the fuel cell stack is prevented from being excessively dried out. Further, even when the fuel cell stack is no longer in the moistened state (that is, in the predetermined dried state), the liquid water remaining in the exhaust device (for example, the second stop valve) provided in the discharge path can be discharged by performing the water discharge in the second water discharge mode. Consequently, it is possible to prevent the liquid water inside the exhaust device from freezing while the fuel cell system is stopped, thus preventing the fuel cell system from being unable to generate electrical power when started.

Supplementary Note 6

In the fuel cell system according to Supplementary Note 5, the control unit may measure a first execution time which is an execution time of the water discharge in the first water discharge mode, determine the second execution time which is an execution time of the water discharge in the second water discharge mode based on the first execution time, and execute water discharge in the second water discharge mode during the second execution time.

Supplementary Note 7

In the fuel cell system according to Supplementary Note 5, the control unit may measure the first execution time which is an execution time of the water discharge in the first water discharge mode, determine the second execution time which is an execution time of the water discharge in the second water discharge mode by subtracting the first execution time from the preset water discharge time, and execute water discharge in the second water discharge mode for the second execution time.

Supplementary Note 8

The fuel cell system according to any one of the supplementary notes 1 to 7 may further include the coolant supply pump (126) that supplies the coolant to the fuel cell stack, wherein the control device may set a rotation speed of the coolant supply pump during execution of the water discharge in the first water discharge mode to be lower than a rotation speed of the coolant supply pump during execution of the water discharge in the second water discharge mode.

In this manner, it is possible to suppress an increase in the power consumption by the entire fuel cell system.

Supplementary Note 9

The second disclosure is to provide the method of controlling the fuel cell system including: the fuel cell stack having the plurality of power generation cells configured to generate electrical power using the oxygen-containing gas and the fuel gas; the oxygen-containing gas supplier configured to supply the oxygen-containing gas to the fuel cell stack; the supply path through which the oxygen-containing gas to be supplied to the fuel cell stack flows; the discharge path through which the oxygen-containing gas discharged from the fuel cell stack flows; the temperature sensor configured to detect a temperature of the fuel cell stack; and the control device configured to control the oxygen-containing gas supplier, wherein the control device executes the stoppage operation in which power generation is continued until a predetermined condition is met after receipt of the system shutdown command; acquires the temperature of the fuel cell stack before the stoppage operation; determines whether the fuel cell stack is in moist condition on the basis of at least the temperature of the fuel cell stack acquired; enables execution of water discharge during execution of electrical power generation under the stoppage operation by selectively discharging water remaining in the fuel cell stack in either a first mode in which an amount of oxygen-containing gas supplied from the oxygen-containing gas supplier to the fuel cell stack is a first amount, or a second mode in which the amount is a second amount less than the first amount; executes the water discharge in the first mode during execution of electrical power generation under the stoppage operation, in a case that the fuel cell stack is determined to be in moist condition; and executes the water discharge in the second mode during execution of the electrical power generation under the stoppage operation, in a case that the fuel cell stack is determined not to be in moist condition.

With the arrangement described above, it is possible to facilitate removal of water from portions of the fuel cell stack through which water is difficult to flow. Further, with the arrangement described above, it is possible to discharge water from the fuel cell stack while suppressing excessive drying of the fuel cell stack. That is, with the arrangement described above, water can be appropriately discharged from the fuel cell stack.

The present invention is not limited to the above disclosure, and various modifications are possible without departing from the essence and gist of the present invention.