Method of controlling fuel cell system

A method of controlling a fuel cell system includes circulating a coolant through a fuel cell circulation passage in which a fuel cell and a gas liquid separator are provided. A valve is controlled selectively to connect or disconnect the fuel cell circulation passage and an air conditioning equipment circulation passage in which an air conditioning mechanism is provided. The valve is maintained to connect the fuel cell circulation passage and the air conditioning equipment circulation passage to circulate the coolant through the air conditioning equipment circulation passage when it is determined that the coolant includes air bubbles more than or equal to the threshold amount, when the valve connects the fuel cell circulation passage and the air conditioning equipment circulation passage, and when a temperature of the fuel cell is higher than or equal to a threshold temperature even if the air conditioning mechanism stops.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2016-147929, filed Jul. 28, 2016, entitled “Method of Controlling Fuel Cell System.” The contents of this application are incorporated herein by reference in their entirety.

BACKGROUND

The present disclosure relates to a method of controlling a fuel cell system.

2. Description of the Related Art

A fuel cell includes an electrolyte electrode structure where an anode is provided on one face of an electrolyte and a cathode is provided on the other face. A power generation cell (a unit cell) is configured by the electrolyte electrode structure being sandwiched between separators. Typically, a stack obtained by stacking a predetermined number of such power generation cells is used. This kind of fuel cell is installed in an automobile for example. In such vehicle-installed applications, a solid polymer electrolyte fuel cell where a polymer ion-exchange membrane constitutes an electrolyte is often employed.

Most automobiles are each equipped with an air conditioning mechanism (a so-called car air conditioner). In this case, the fuel cell and the car air conditioner share coolant. Thus, as described in Japanese Unexamined Patent Application Publication No. 2012-23795 and Japanese Unexamined Patent Application Publication No. 2005-166497, a first circulation supply passage through which the coolant circulates and is supplied to the fuel cell and a second circulation supply passage through which the coolant is supplied to the car air conditioner are provided. Further, the fuel cell is provided with a fuel gas supply flow passage for supplying fuel gas, such as hydrogen gas, to the anode or an oxidizer gas supply flow passage for supplying oxidizer gas, such as oxygen containing gas like compressed air, to the cathode. Thus, a fuel cell system is structured.

The first circulation supply passage and the second circulation supply passage are each provided with a coolant pump. Each coolant pump serves to boost the pressure of the coolant so as to increase the transfer speed of the coolant. That is, the coolant is forcibly transferred under the action of each coolant pump.

SUMMARY

According to one aspect of the present invention, a method of controlling a fuel cell system including a fuel cell, includes circulating a coolant through a fuel cell circulation passage in which the fuel cell and a gas liquid separator are provided. A valve is controlled selectively to connect or disconnect the fuel cell circulation passage and an air conditioning equipment circulation passage in which an air conditioning mechanism is provided. It is determined whether the coolant includes air bubbles more than or equal to a threshold amount. The valve is maintained to connect the fuel cell circulation passage and the air conditioning equipment circulation passage to circulate the coolant through the air conditioning equipment circulation passage when it is determined that the coolant includes air bubbles more than or equal to the threshold amount, when the valve connects the fuel cell circulation passage and the air conditioning equipment circulation passage, and when a temperature of the fuel cell is higher than or equal to a threshold temperature even if the air conditioning mechanism stops.

According to another aspect of the present invention, a method of controlling a fuel cell system including a fuel cell, includes circulating a coolant through a fuel cell circulation passage in which the fuel cell and a gas liquid separator are provided. A valve is controlled selectively to connect or disconnect the fuel cell circulation passage and an air conditioning equipment circulation passage in which an air conditioning mechanism is provided. It is determined whether the coolant includes air bubbles more than or equal to a threshold amount. The valve is maintained to connect the fuel cell circulation passage and the air conditioning equipment circulation passage to circulate the coolant through the air conditioning equipment circulation passage when it is determined that the coolant includes air bubbles more than or equal to the threshold amount, when the valve connects the fuel cell circulation passage and the air conditioning equipment circulation passage, and when a temperature of the fuel cell is higher than or equal to a threshold temperature even if a condition for issuing a command to disconnect the fuel cell circulation passage and the air conditioning equipment circulation passage is satisfied.

DESCRIPTION OF THE EMBODIMENTS

A method of controlling a fuel cell system according to the present application is described in detail below in preferred embodiments by referring to the drawings.

A fuel cell system10is described first by referring toFIG. 1, which is an explanatory schematic diagram of a configuration of the fuel cell system10. The fuel cell system10includes a fuel cell stack (a fuel cell)12.

The fuel cell stack12includes a fuel gas supply unit14, which supplies fuel gas, an oxidizer gas supply unit16, which supplies oxidizer gas, and a coolant supply unit18, which supplies coolant. In the present embodiment, hydrogen gas is used as the fuel gas and compressed air is used as the oxidizer gas. The fuel cell system10further includes a control unit20, which is a system controller.

The fuel cell stack12is made up of a plurality of power generation cells24stacked in a direction indicated by arrow A inFIG. 1, which is a horizontal direction or a vertical direction. The power generation cell24is obtained by sandwiching an electrolyte membrane electrode structure26between a first separator28and a second separator30. The first separator28and the second separator30are formed from metal or carbon.

The electrolyte membrane electrode structure26includes for example, a solid polyelectrolyte membrane32, which is a perfluorosulfonic acid membrane containing moisture, and an anode34and a cathode36between which the solid polyelectrolyte membrane32is sandwiched. As the solid polyelectrolyte membrane32, hydrocarbon (HC)-based electrolyte may be employed instead of the above-described fluorine-based electrolyte.

Between the first separator28and the electrolyte membrane electrode structure26, the first separator28is provided with a hydrogen gas flow passage (a fuel gas flow passage)38for supplying hydrogen gas to the anode34. Between the second separator30and the electrolyte membrane electrode structure26, the second separator30is provided with an air flow passage40for supplying air to the cathode36. Between the first separator28and the second separator30that are adjacent to each other, a coolant flow passage42through which the coolant is allowed to flow is provided.

The fuel cell stack12are provided with a hydrogen gas inlet44a,a hydrogen gas outlet44b,an air inlet46a,an air outlet46b,a coolant inlet48a,and a coolant outlet48b. The hydrogen gas inlet44apenetrates in the direction in which the power generation cells24are stacked and is linked with the supply side of the hydrogen gas flow passage38. Similarly, the hydrogen gas outlet44balso penetrates in the direction in which the power generation cells24are stacked, and is linked with the discharge side of the hydrogen gas flow passage38. The hydrogen gas flow passage38, the hydrogen gas inlet44a,and the hydrogen gas outlet44bconstitute an anode flow passage.

Similarly, the air inlet46apenetrates in the direction in which the power generation cells24are stacked and is linked with the supply side of the air flow passage40. The air outlet46bpenetrates in the direction in which the power generation cells24are stacked and is linked with the discharge side of the air flow passage40. The air flow passage40, the air inlet46a,and the air outlet46bconstitute a cathode flow passage.

Further, the coolant inlet48apenetrates in the direction in which the power generation cells24are stacked and is linked with the supply side of the coolant flow passage42. The coolant outlet48bpenetrates in the direction in which the power generation cells24are stacked and is linked with the discharge side of the coolant flow passage42.

The fuel gas supply unit14includes a hydrogen tank50for storing high-pressure hydrogen gas and the hydrogen tank50is connected to the hydrogen gas inlet44aof the fuel cell stack12through a hydrogen gas supply passage (a fuel gas supply flow passage)51. The hydrogen gas supply passage51supplies hydrogen gas to the fuel cell stack12.

In the hydrogen gas supply passage51, an injector54and an ejector56are provided in series. When the pressure in the ejector56becomes negative, hydrogen from a hydrogen circulation flow passage66is sucked.

A hydrogen gas discharge passage (an anode off-gas discharge passage)62is connected to the hydrogen gas outlet44bof the fuel cell stack12. The hydrogen gas discharge passage62derives discharge hydrogen gas (anode off-gas), which is hydrogen gas used at least partially at the anode34, from the fuel cell stack12.

The hydrogen gas discharge passage62is provided with a gas liquid separator64. The hydrogen circulation flow passage66branches from the downstream of the gas liquid separator64and the downstream side of the hydrogen circulation flow passage66is connected to the ejector56. The hydrogen circulation flow passage66is provided with a hydrogen pump68. Particularly on activation, the hydrogen pump68causes the anode off-gas discharged to the hydrogen gas discharge passage62to circulate to the hydrogen gas supply passage51through the hydrogen circulation flow passage66and the ejector56.

An end of a purge flow passage70is linked with the hydrogen gas discharge passage62and on the purge flow passage70, the purge valve72is provided. An end of a drain flow passage74from which fluid mainly containing a liquid constituent is discharged is connected to a bottom portion of the gas liquid separator64. On the drain flow passage74, a drain valve76is provided.

The oxidizer gas supply unit16includes an air pump78as an oxidizer gas supply mechanism provided on an air supply passage (an oxidizer gas supply flow passage)80. The air pump78is configured as a compressor including a motor that is a rotation driving unit so as to compress and supply atmosphere (air). That is, the air pump78compresses atmosphere with the rotation of a motor and supplies the resultant compressed air from the air supply passage80to the fuel cell stack12.

Since the air pump78having the above-described configuration is known, detailed description of the air pump78is omitted.

The air supply passage80is positioned on the downstream side of the air pump78and is connected to the air inlet46aof the fuel cell stack12, and between the air pump78and the fuel cell stack12, a supply-side open/close valve (an inlet sealing valve)82aand a humidifier84are interposed. A bypass supply passage86for detouring around the humidifier84is connected to the air supply passage80. The bypass supply passage86is provided with an open/close valve88.

An air discharge passage (a cathode off-gas discharge passage)90is connected to the air outlet46bof the fuel cell stack12. The air discharge passage90allows discharge compressed air (cathode off-gas), which is compressed air used at least partially at the cathode36, to be discharged from the fuel cell stack12.

Downstream of the air discharge passage90, the humidifier84is provided and thus, the humidifier84exchanges moisture and heat between the compressed air supplied from the air pump78and the cathode off-gas. Further, on the downstream side of the humidifier84, a discharge-side open/close valve (an outlet sealing valve)82band a back pressure valve92are provided on the air discharge passage90. Downstream of the air discharge passage90, the other end of the purge flow passage70and the other end of the drain flow passage74are connected and join to constitute a dilution unit, accordingly.

The back pressure valve92is a pressure adjustment valve provided so as to control the pressure of the compressed air that is supplied to the cathode flow passage. That is, the back pressure valve92regulates the internal pressure of the cathode36.

Both ends of a bypass flow passage94are linked with the air supply passage80and the air discharge passage90, positioned on the upstream side of the supply-side open/close valve82a,on the downstream side of the discharge-side open/close valve82b,and on the downstream side of the back pressure valve92. The bypass flow passage94is provided with a bypass (BP) flow rate adjustment valve96for adjusting the flow rate of air flowing through the bypass flow passage94.

The coolant supply unit18includes a coolant supply passage102, which is connected to the coolant inlet48aof the fuel cell stack12, and a first water pump (a first coolant pump)104is placed on the coolant supply passage102. The coolant supply passage102is connected to a radiator106and a coolant discharge passage108linked with the coolant outlet48bis connected to the radiator106.

The coolant discharge passage108and the coolant supply passage102constitute a first coolant circulation passage109as a fuel cell-side coolant circulation flow passage. That is, the coolant supply passage102is a return passage of the first coolant circulation passage109directed to the fuel cell stack12from the radiator106, and the coolant discharge passage108is an outward passage directed to the radiator106from the fuel cell stack12.

The coolant supply passage102is provided with the thermostatic valve110. The thermostatic valve110automatically enters the open state when the temperature of the coolant flowing through the coolant supply passage102is high, or automatically enters the closed state when the temperature is low. Since such operating principles of the thermostatic valve110are known as described in for example, Japanese Unexamined Patent Application Publication No. 2014-232684, detailed description thereof is omitted.

Between the coolant discharge passage108and the coolant supply passage102, a bypass passage111for detouring around the radiator106and the thermostatic valve110is provided. That is, the coolant discharge passage108and the coolant supply passage102are linked through the bypass passage111. Accordingly, when the thermostatic valve110is in the closed state, the coolant that flows through the coolant discharge passage108travels to the coolant supply passage102through the bypass passage111.

On the downstream side of the thermostatic valve110, an expansion tank (a gas liquid separator)112is provided and a reserve tank (a coolant reservoir)114, which allows the coolant to travel between the reserve tank114and the expansion tank112is arranged. That is, when necessary, the coolant is supplied to the reserve tank114from the expansion tank112or coolant is supplied to the expansion tank112from the reserve tank114.

The thermostatic valve110and the expansion tank112described above are interposed between the radiator106and the first water pump104. On the downstream side of the first water pump104, a branch pipe115, which branches from the coolant supply passage102to join the coolant discharge passage108, and on the branch pipe115, an ion exchanger116is interposed. The ion exchanger116prevents liquid junction of the fuel cell stack12by removing ions contained in the coolant.

In the coolant discharge passage108, a second coolant circulation passage (an air conditioning equipment-side coolant circulation flow passage)121, which branches from the coolant discharge passage108and extends through a car air conditioner (an air conditioning mechanism)120, and then joins the coolant discharge passage108, is provided. The second coolant circulation passage121includes an outward passage122directed to the car air conditioner120from the coolant discharge passage108, and a return passage123directed to the coolant discharge passage108from the car air conditioner120.

On the outward passage122, a three-way valve124, a second water pump (a second coolant pump)126, and an air conditioner heater128are interposed in this order from the upstream side. The side more downstream in the outward passage122than the air conditioner heater128is connected to a compressor130included in the car air conditioner120.

The car air conditioner120includes a duct casing131and in the duct casing131, an air blower132, an evaporator134, an air mixing door, not illustrated, and the compressor130are placed in this order from the upstream side. The air blower132takes in air inside or outside a vehicle by rotating under the action of a motor and conducts a function of sending the air taken in into the duct casing131.

When air is conditioned, the evaporator134functions as a vaporizer and cools the air supplied from the air blower132. The compressor130compresses the coolant. The coolant supplied to the compressor130is heated and its temperature is raised when the air conditioner heater128is urged.

An air mixing door is constituted of for example, a rotatable door and by rotating in accordance with a command from the control unit20, the degree of opening is adjusted. Thus, the flow rate ratio between the air that flows into the compressor130and the air that detours around the compressor130, that is, the temperature of the conditioned air supplied to vehicle room is adjusted.

The return passage123directed to the coolant discharge passage108from the compressor130is provided with a bypass branch pipe140extending to the three-way valve124. Accordingly, the outward passage122and the compressor130are selectively switched to the connected state or the disconnected state by the three-way valve124being operated.

The method of controlling the fuel cell system10according to the present embodiment is described next in relation to the operation of the fuel cell system10referring to a flowchart inFIG. 8.

The fuel cell system10configured as described above is installed in for example, a fuel cell vehicle, not illustrated, such as a fuel cell electric automobile. Described below is the case in which the fuel cell system10is installed in a fuel cell vehicle.

When the operation of the fuel cell vehicle is started, the fuel cell stack12is activated with the ignition turned on. At the time, the control unit20transmits a command signal for opening the injector54so as to supply hydrogen gas from the fuel gas supply unit14to the anode flow passage. Thus, a predetermined amount of the hydrogen gas supplied from the hydrogen tank50to the hydrogen gas supply passage51passes through the injector54and the ejector56to be supplied to the hydrogen gas inlet44aof the fuel cell stack12.

The hydrogen gas is further guided from the hydrogen gas inlet44ato the hydrogen gas flow passage38and travels along the hydrogen gas flow passage38. Thus, the hydrogen gas is supplied to the anode34of the electrolyte membrane electrode structure26.

The control unit20issues a command signal for urging the air pump78so that compressed air is supplied from the oxidizer gas supply unit16. Accordingly, atmosphere is compressed under the action of the rotation of the air pump78and sent to the air supply passage80as the compressed air. The compressed air is humidified when passing through the humidifier84and after that, is supplied to the air inlet46aof the fuel cell stack12. The compressed air is guided from the air inlet46ato the air flow passage40and after that, travels along the air flow passage40and is thus supplied to the cathode36of the electrolyte membrane electrode structure26.

Accordingly, in each electrolyte membrane electrode structure26, the hydrogen gas supplied to the anode34and the oxygen in the compressed air supplied to the cathode36are consumed by electrochemical reaction in an electrodecatalytic layer and electricity is generated (step S1inFIG. 8). Part of the moisture added to the compressed air in the humidifier84enters the solid polyelectrolyte membrane32from the cathode36and reaches the anode34.

The hydrogen gas supplied to the anode34and partially consumed is discharged as the anode off-gas from the hydrogen gas outlet44bto the hydrogen gas discharge passage62. At the time, the anode off-gas accompanies the moisture that reaches the anode34as described above. That is, the anode off-gas guided to the gas liquid separator64is damp gas that contains moisture.

In the gas liquid separator64, most part of the moisture in the anode off-gas is separated. The liquid part (water) is discharged from the drain flow passage74by the drain valve76being opened. While containing mist that slightly remains, the anode off-gas from which the moisture is separated is guided from the hydrogen gas discharge passage62to the hydrogen circulation flow passage66under the action of the hydrogen pump68. Further, the anode off-gas is sucked from the hydrogen circulation flow passage66to the ejector56, passes through the injector54, joins new hydrogen gas, and is supplied from the hydrogen gas supply passage51to the anode flow passage. Thus, the anode off-gas circulates and is supplied to the fuel cell stack12.

The anode off-gas discharged to the hydrogen gas discharge passage62is discharged (purged) outside under the action of the opening of the purge valve72when necessary.

Similarly, the compressed air supplied to the cathode36and partially consumed is discharged as the cathode off-gas from the air outlet46bto the air discharge passage90. The cathode off-gas humidifies new compressed air supplied from the air supply passage80through the humidifier84and is then adjusted to a set pressure of the back pressure valve92to be discharged to a dilution unit.

While the fuel cell stack12is being operated as described above, in the coolant supply unit18, the coolant, such as pure water, ethylene glycol, or oil, is supplied from the coolant supply passage102to the coolant inlet48aof the fuel cell stack12under the action of the first water pump104. The coolant flows along the coolant flow passage42to cool the power generation cells24and is then discharged from the coolant outlet48bto the coolant discharge passage108.

Immediately after the activation of the fuel cell stack12, the coolant derived from the fuel cell stack12has a low temperature and thus the thermostatic valve110is closed. At the time, the coolant passes through the bypass passage111to travel through the coolant supply passage102, and further passes through the expansion tank112and the first water pump104to be returned from the coolant inlet48ato the coolant flow passage42in the fuel cell stack12.

In contrast, when the temperature of the fuel cell stack12sufficiently rises and the temperature of the coolant becomes sufficiently high, the thermostatic valve110is switched to the open state. Accordingly, the coolant is guided to the radiator106to be cooled and after that, passes through the thermostatic valve110, the expansion tank112, and the first water pump104to be returned from the coolant inlet48ato the coolant flow passage42in the fuel cell stack12. In the manner described above, the coolant circulates and flows through the first coolant circulation passage109(step S2inFIG. 8). That is, the coolant circulates and is supplied to the fuel cell stack12.

When the three-way valve124is operated in the direction where the outward passage122becomes connected to the second water pump126and the second water pump126is urged, part of the coolant that flows through the coolant discharge passage108is distributed to the outward passage122, which constitutes the second coolant circulation passage121, by being sucked by the second water pump126(step S3inFIG. 8). The coolant distributed to the outward passage122is forcibly transferred to the compressor130through the air conditioner heater128under the action of the second water pump126.

The air blower132is rotated by the motor urged under the control action of the control unit20. Thus, air is supplied into the duct casing131. The control unit20further adjusts the degree of the opening of the air mixing door and regulates the amount of the air in contact with the compressor130. At the time, heat transfer is performed between the coolant flowing in the compressor130and the air in contact with the compressor130. That is, the heat of the coolant is seized by the air.

As a result, the air is heated and the coolant is cooled. The heated air is supplied to the inside of the fuel cell vehicle as a heat source for room heating. Since the three-way valve124is in a state of allowing the return passage123and the coolant discharge passage108to be linked with each other, the cooled coolant is returned to the coolant discharge passage108through the return passage123and after that, flows to the side of the radiator106.

Into the outward passage122, new coolant flows from the coolant discharge passage108. Accordingly, the coolant circulates and flows in the second coolant circulation passage121. That is, the coolant circulates and is supplied to the car air conditioner120.

While the coolant flows through the first coolant circulation passage109as described above, the expansion tank112stores the coolant temporarily. During this storage, the coolant is separated into a vapor phase and a liquid phase. The vapor phase is released outside the expansion tank112through a pressure release valve with which the expansion tank112is provided. To compensate for the reduced amount of the released coolant, a predetermined amount is supplied to the expansion tank112from the coolant stored in the reserve tank114.

The volume of the coolant changes with a rise or drop in temperature. When for example, the temperature rises and the coolant expands, the coolant is transferred from the expansion tank112to the reserve tank114. Conversely, when the coolant shrinks, the coolant is replenished from the reserve tank114to the expansion tank112.

When the coolant is supplied to the expansion tank112by an amount beyond the gas liquid separation performance of the expansion tank112, it is assumed that air remains in the coolant. Under such circumstances, the first water pump104can cause idle running and performance in forcible transfer of the coolant can decrease and, as a result, cooling performance can decrease. In the present embodiment, therefore, when air is recognized as being caught in the coolant, control for removing the air is performed. Thus, an air bubble determination process is performed first (step S4inFIG. 8).

Whether air is caught in the coolant, in other words, at least one air bubble is trapped in the coolant can be determined on the basis of for example, the number of revolutions of the first water pump104. That is, the number of revolutions of the first water pump104rises as air bubbles trapped in the coolant increase. Accordingly, when the number of revolutions of the first water pump104is larger than or equal to a predetermined value, the control unit20recognizes that “air bubbles more than or equal to a predetermined amount are trapped in the coolant.”

For another example, the determination may be performed on the basis of the liquid level in the reserve tank114. As the liquid level in the reserve tank114becomes lower, more air bubbles are likely to be trapped in the coolant. That is, when the liquid level in the reserve tank114is lower than or equal to a predetermined lower-limit threshold, the control unit20may also be caused to recognize that “air bubbles more than or equal to a predetermined amount are trapped in the coolant.”

Not to mention, both of these may be used concurrently. That is, when a timing at which the number of revolutions of the first water pump104becomes larger than or equal to the predetermined value or a timing at which the liquid level in the reserve tank114becomes lower than or equal to the predetermined lower-limit threshold arrives first, the control unit20may also be caused to recognize that “air bubbles more than or equal to a predetermined amount are trapped in the coolant.”

When the control unit20recognizes that “air bubbles more than or equal to a predetermined amount are trapped in the coolant,” the control unit20determines whether the three-way valve124is operated in the direction where the outward passage122and the compressor130become connected or is operated in the direction where the outward passage122and the compressor130become disconnected (a state determination process (step S5inFIG. 8)).

When for example, the user has started using the car air conditioner120before the air bubble determination process is performed, the three-way valve124is operated in the direction where the outward passage122and the compressor130become connected. That is, the outward passage122and the compressor130are in the connected state. Thus, the coolant that flows into the outward passage122from the coolant discharge passage108is directed to the compressor130.

In this case, the control unit20recognizes that “air bubbles are also trapped in the coolant flowing through the second coolant circulation passage121.” When the control unit20makes such determination and the temperature of the fuel cell stack12is larger than or equal to a predetermined temperature, even if the user stops using the car air conditioner120while air bubbles are being removed, the three-way valve124is not operated as illustrated inFIG. 2(step S6inFIG. 8). That is, the outward passage122and the compressor130are not disconnected and the connected state is maintained. InFIG. 2, the state of the three-way valve124in a case where the outward passage122and the compressor130are in the connected state is referred to as the “open valve” and that in a case in the disconnected state is referred to as the “closed valve.”

On the other hand, as illustrated inFIG. 3, the number of revolutions of the first water pump104is reduced so as to be lower than or equal to the predetermined value. As a result, the coolant supplied to the expansion tank112has an amount that suits the gas liquid separation performance of the expansion tank112. That is, the vapor phase included in the coolant is separated from the liquid phase. Thus, as illustrated inFIGS. 3 and 4, removal of the air bubbles trapped in the coolant is started. Since the outward passage122and the compressor130are not connected yet, air bubbles are removed from both the coolant in the first coolant circulation passage109and the coolant in the second coolant circulation passage121. The “amount of trapped air bubbles” inFIG. 3indicates a value in the first coolant circulation passage109and the similar applies to the other drawings.

When the number of revolutions of the first water pump104is reduced and predetermined time elapses, the control unit20determines that air bubbles are sufficiently discharged and the amount of the air bubbles is within tolerance (step S7inFIG. 8). On the basis of this determination, the control unit20causes the three-way valve124to be operated in the direction where the outward passage122and the compressor130become disconnected (step S8inFIG. 8).

Thus, the coolant with the amount of air bubbles that is reduced within tolerance is filled in the second coolant circulation passage121. Accordingly, it can be avoided that when the operation of the fuel cell vehicle is resumed after the operation of the fuel cell vehicle had been stopped and predetermined time has elapsed, the second water pump126senses air bubbles and causes incorrect stopping.

The above-described air bubble removal control may be performed while stopping the fuel cell vehicle and the fuel cell system10. That is, when a command to “stop the operation of the fuel cell system10” is issued during the operation of the fuel cell vehicle and as described above, the control unit20determines that “air is trapped in the coolant,” the air bubbles are removed while the operation stopping process is being performed.

By switching the outward passage122and the compressor130between the connected state and the disconnected state using the three-way valve124in this manner, it is enabled to remove air bubbles from both the coolant in the first coolant circulation passage109and the coolant in the second coolant circulation passage121, and to fill the coolant with the amount of air bubbles that is reduced within tolerance in the second coolant circulation passage121. Thus, at a subsequent start of the operation of the fuel cell vehicle, the second water pump126operates normally and the coolant can flow in the second coolant circulation passage121.

In addition, simply by providing a single expansion tank,112, it is enabled to remove air bubbles from the coolant in the first coolant circulation passage109and the coolant in the second coolant circulation passage121. Accordingly, increase in the size of the fuel cell system10can be avoided. As a result, flexibility in placement layout in the fuel cell vehicle increases.

The above-described air bubble removal control is performed when the fuel cell stack12is maintained at the predetermined temperature or higher after the state determination process. That is, when the temperature of the fuel cell stack12is higher than or equal to the predetermined temperature, for example, even if the user stops using the car air conditioner120after the state determination process, the three-way valve124is not operated in the direction where the outward passage122and the compressor130become disconnected.

In contrast, even when it is determined in the air bubble determination process that air bubbles more than or equal to the predetermined amount are trapped in the coolant, if after that, the temperature of the fuel cell stack12falls below the predetermined temperature, it is desirable to disconnect the outward passage122and the compressor130. This is because further decrease in the temperature of the fuel cell stack12can be avoided accordingly.

That is, when for example, even after the state determination process, the temperature of the fuel cell stack12falls below a predetermined value, it is desirable to issue a command to operate the three-way valve124in the direction where the outward passage122and the compressor130are disconnected. Not to mention, the command is issued by the control unit20.

When after this operation is performed, the temperature of the fuel cell stack12becomes higher than or equal to the predetermined temperature, the three-way valve124may be operated under the control action of the control unit20to connect the outward passage122and the compressor130.

In the description above, a timing to execute a command for operating the three-way valve124in the direction where the outward passage122and the compressor130become disconnected may be set as desired according to conditions of the fuel cell system10. For example, immediately after the command is issued from the control unit20, the three-way valve124may be operated. When the air bubble removal operation is performed as described above, the three-way valve124may be operated after the removal operation ends.

The present application is not particularly limited to the above-described embodiments and various changes may be made within the scope not departing from the spirit of the present application.

When for example, the outward passage122and the compressor130are in the disconnected state, the control unit20may be caused to determine that “the coolant in the second coolant circulation passage121includes no air bubble trapped.” That is, in this case, when it is determined in the air bubble determination process that air bubbles are trapped in the coolant, as illustrated inFIGS. 5 to 7, the control unit20determines that “ air bubbles are trapped in the coolant in the second coolant circulation passage121” only in the case where the outward passage122and the compressor130are in the connected state.

The present disclosure describes a method of controlling a fuel cell system including a fuel cell that generates electricity by electrochemical reaction between fuel gas supplied to an anode through a fuel gas supply flow passage and oxidizer gas supplied to a cathode through an oxidizer gas supply flow passage, a fuel cell-side coolant circulation flow passage through which coolant circulates and is supplied to a coolant flow passage provided in the fuel cell, a heat exchanger that is provided on the fuel cell-side coolant circulation flow passage and cools the coolant by heat exchange with air, a gas liquid separator that is provided on a downstream side of the heat exchanger on the fuel cell-side coolant circulation flow passage and separates the coolant into a vapor phase and a liquid phase, a first coolant pump provided on the fuel cell-side coolant circulation flow passage, an air conditioning mechanism that shares the coolant with the fuel cell, an air conditioning equipment-side coolant circulation flow passage that branches from the fuel cell-side coolant circulation flow passage and returns to the fuel cell-side coolant circulation flow passage through the air conditioning mechanism and through which the coolant circulates and is supplied to the air conditioning mechanism, a second coolant pump provided on the air conditioning equipment-side coolant circulation flow passage, and a three-way valve that selectively connects or disconnects the air conditioning equipment-side coolant circulation flow passage and the air conditioning mechanism, the method including: an air bubble determination process in which whether air bubbles more than or equal to a predetermined amount are trapped in the coolant flowing through the air conditioning equipment-side coolant circulation flow passage is determined; and a state determination process in which when it is determined in the air bubble determination process that air bubbles more than or equal to the predetermined amount are trapped in the coolant, whether the air conditioning equipment-side coolant circulation flow passage and the air conditioning mechanism are in a connected state or a disconnected state is determined, wherein when it is determined in the state determination process that the air conditioning equipment-side coolant circulation flow passage and the air conditioning mechanism are in the connected state and when a temperature of the fuel cell is higher than or equal to a predetermined temperature, connection is maintained even if a command to cause the disconnected state by operating the three-way valve is issued.

That is, when air bubbles are removed while the air conditioning equipment-side coolant circulation flow passage and the air conditioning mechanism are in the connected state, even if a command to disconnect the air conditioning equipment-side coolant circulation flow passage and the air conditioning mechanism (i.e. to interrupt the connection therebetween) is issued, the connected state is maintained. Accordingly, air bubbles can be removed from both the coolant in the fuel cell-side coolant circulation flow passage and the coolant in the air conditioning equipment-side coolant circulation flow passage.

Then, after the amount of air bubbles is reduced within tolerance, the air conditioning equipment-side coolant circulation flow passage and the air conditioning mechanism are disconnected in accordance with the command. Thus, coolant with the amount of air bubbles that is reduced within tolerance is filled in the air conditioning equipment-side coolant circulation flow passage. Accordingly, it can be avoided that on subsequent activation of the fuel cell system, determination indicating that “air bubbles are trapped in the second coolant pump” is made and incorrect stopping is caused.

Additionally, in this case, even when the number of gas liquid separators is only one, while air bubbles are being removed, the connected state between the air conditioning equipment-side coolant circulation flow passage and the air conditioning mechanism is maintained. Thus, air bubbles can be removed from both the coolant in the fuel cell-side coolant circulation flow passage and the coolant in the air conditioning equipment-side coolant circulation flow passage. Accordingly, increase in the size of the fuel cell system can be avoided. Since increase in space for placing the fuel cell system, which the increase in size involves, can be avoided, flexibility in placement layout increases when the fuel cell system is installed in an automobile for example.

This control method can be performed in stopping the operation of the fuel cell. That is, when it is determined that “air is trapped in the coolant” at the time of issue of a command indicating “stop the operation” while the fuel cell is operating, the above-described air bubble removal control is performed during the operation stopping process.

Whether any air bubble is trapped in the coolant can be determined on the basis of for example, the number of revolutions of the first coolant pump. This is because the number of revolutions of the first coolant pump rises as the amount of the trapped air bubbles increases. Accordingly, when the number of revolutions of the first coolant pump is larger than or equal to a predetermined value, it can be determined in the air bubble determination process that air bubbles more than or equal to the predetermined amount are trapped.

When a coolant reservoir for adjusting the amount of the coolant is provided on the fuel cell-side coolant circulation flow passage, the liquid level in the coolant reservoir is likely to become lower as the amount of the trapped air bubbles increases. On the basis of this, it can be determined in the air bubble determination process that, when the liquid level in the coolant reservoir is lower than or equal to a predetermined lower-limit threshold, air bubbles more than or equal to the predetermined amount are trapped. Not to mention, the above-described two determination methods may be used concurrently.

To remove air bubbles from the coolant, for example, the first coolant pump may be operated at a predetermined number of revolutions or less. This is because in such a case, the amount of the coolant guided to the gas liquid separator is within the range of gas liquid separation performance of the gas liquid separator. That is, the coolant is sufficiently separated into a vapor phase and a liquid phase in the gas liquid separator and as a result, the amount of the air bubbles in the coolant is reduced within tolerance. Accordingly, when a state in which the number of revolutions of the first coolant pump is lower than or equal to a predetermined value continues for predetermined time, it can be determined that air bubbles are removed.

A command to bring the air conditioning equipment-side coolant circulation flow passage and the air conditioning mechanism into the disconnected state is issued when for example, the air conditioning mechanism is stopped. Even in such a case, while air bubbles are being removed, no disconnection is caused between the air conditioning equipment-side coolant circulation flow passage and the air conditioning mechanism as described above.

Even if it is determined in the air bubble determination process that air bubbles more than or equal to the predetermined amount are trapped in the coolant, when the temperature of the fuel cell falls below a predetermined temperature, it is desirable to bring the air conditioning equipment-side coolant circulation flow passage and the air conditioning mechanism into the disconnected state by operating the three-way valve. That is, even if it is determined in the state determination process that the air conditioning equipment-side coolant circulation flow passage and the air conditioning mechanism are in the connected state, when the temperature of the fuel cell falls below the predetermined temperature after that, it is desirable to issue a command to bring the air conditioning equipment-side coolant circulation flow passage and the air conditioning mechanism into the disconnected state by operating the three-way valve. This is because by executing the command, further decrease in the temperature of the fuel cell can be avoided.

It can be assumed that a command to bring the air conditioning equipment-side coolant circulation flow passage and the air conditioning mechanism into the disconnected state (a command to operate the three-way valve) is issued while an air bubble removal operation is being performed. In such a case, the disconnected state may be caused by operating the three-way valve after the removal operation ends.