Fuel cell system

In a fuel cell system, a circulation pump provided in a circulation flow path is a two-lobe roots pump configured such that two rotors rotate in opposite directions. An exhaust/drainage portion is provided at a bottom in an internal space of a gas liquid separator provided in the circulation flow path and is configured to collect a liquid separated by the gas liquid separator and discharge the collected liquid via a discharge valve. The gas liquid separator includes an opening that is connected with the circulation pump and that is configured to face in a direction of the exhaust/drainage portion. At a system stop time, when a predetermined condition is satisfied, a controller configured to control operation of the circulation pump causes the rotors of the circulation pump to be rotated in reverse directions from rotations at a system operation time.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the priority based on Japanese Patent Application No. 2017-081705 filed on Apr. 18, 2017, the disclosure of which is hereby incorporated by reference in its entirety.

BACKGROUND

Field

The present disclosure relates to a fuel cell system.

Related Art

JP 2007-59333A and JP 2007-5289A disclose fuel cell systems in which a roots pump configured such that two rotors rotate in opposite directions is used as a hydrogen pump provided to supply hydrogen gas to a fuel cell. In the fuel cell system described in JP 2007-5289A, the hydrogen pump is reversely rotated at a system stop time to feed the hydrogen gas in a direction toward a gas liquid separator. This causes water in a flow path including the hydrogen pump and a piping connected with the hydrogen pump to be discharged via the gas liquid separator and an exhaust/drainage valve and thereby accelerates discharge of water to the outside. In the fuel cell system described in JP 2007-59333A, the rotors are reciprocated at a system stop time to remove the water content adhering to the rotors and thereby prevent the hydrogen pump from being frozen.

The fuel cell system including the gas liquid separator as described in JP 2007-5289A, however, has the following problem. The exhaust/drainage valve is connected with the gas liquid separator at a position that is generally in a bottom of an internal space of the gas liquid separator (hereinafter called “exhaust/drainage portion”), for the efficient water drainage. Accordingly, liquid water remains in the exhaust/drainage portion, for example, when the water drainage fails to completely remove the liquid water or when dew condensation occurs in the internal space with a decrease in temperature. When this remaining water is frozen, the water in the flow path including the hydrogen pump cannot be discharged to the outside. The fuel cell system is thus likely to be inoperable.

In order to address the problems described above, the disclosure may be implemented by aspects described below.

SUMMARY

According to one aspect of the present disclosure, there is provided a fuel cell system. This fuel cell system comprises a fuel cell; a supply flow path configured to supply a fuel gas to a supply port of an anode of the fuel cell; a circulation flow path configured to resupply a discharge gas that is discharged from a discharge port of the anode, as the fuel gas to the supply port; a gas liquid separator provided in the circulation flow path and configured to separate the discharged gas into a gas and a liquid; a circulation pump provided in the circulation flow path and configured to supply the gas separated by the gas liquid separator to the supply flow path; a discharge valve connected with the gas liquid separator and configured to discharge the separated liquid from the gas liquid separator; and a controller configured to control operation of the circulation pump. The circulation pump is a two-lobe roots pump configured such that two rotors rotate in opposite directions. An exhaust/drainage portion is provided at a bottom in an internal space of the gas liquid separator and is configured to collect the separated liquid and discharge the collected liquid via the discharge valve. The gas liquid separator includes an opening that is connected with the circulation pump and that is configured to face in a direction of the exhaust/drainage portion. At a system stop time, when a predetermined condition is satisfied, the controller causes the rotors of the circulation pump to be rotated in reverse directions from rotations at a system operation time.

DETAILED DESCRIPTION

A. First Embodiment

FIG. 1is a diagram illustrating the schematic configuration of a fuel cell system10according to a first embodiment of the present disclosure. The fuel cell system10is mounted on, for example, a vehicle to output electric power as a power source of the vehicle in response to the driver's request. The fuel cell system10includes a fuel cell20having a plurality of cells, a hydrogen supply discharge mechanism50, an air supply discharge mechanism30, a cooling water circulation mechanism80and a controller90. The fuel cell system10is started in response to an ON operation of a power switch (not shown) and is stopped in response to an OFF operation of the power switch. The power switch corresponds to an ignition switch of an engine vehicle and serves as an input interface to switch over between the stop state and the operating state of the fuel cell system10.

The hydrogen supply discharge mechanism50is under control of the controller90to supply hydrogen (fuel gas) to an anode of the fuel cell20and discharge a gas including unreacted hydrogen from the anode. The hydrogen supply discharge mechanism50includes a hydrogen tank40, a shutoff valve41, a hydrogen supply flow path60, a regulator51, an injector54, a hydrogen circulation flow path61, a hydrogen pump55, a gas liquid separator56, a drainage shutoff valve57and a discharge flow path58.

The hydrogen tank40is configured to store hydrogen. The high-pressure hydrogen gas having several tens of MPa is stored in the hydrogen tank40. The hydrogen supply flow path60is a piping arranged to connect the hydrogen tank40with a supply port28of the anode of the fuel cell20. The shutoff valve41serves as a valve to shut off the supply of hydrogen from the hydrogen tank40to the hydrogen supply flow path60and is also called main stop valve. The shutoff valve41is controlled to be opened and closed by the controller90. When the shutoff valve41is controlled by the controller90to be opened, the hydrogen gas is supplied from the hydrogen tank40through the hydrogen supply flow path60to the supply port28of the anode of the fuel cell20. When the shutoff valve41is controlled by the controller90to be closed, the supply of the hydrogen gas to the supply port28of the anode is shut off.

The regulator51is controlled by the controller90to regulate the pressure of the hydrogen stored in the hydrogen tank40. The injector54is controlled by the controller90to inject the hydrogen of the pressure regulated by the regulator51toward the anode.

The hydrogen circulation flow path61is a piping connected with the hydrogen supply flow path60at a position on the fuel cell20-side of the injector54. The hydrogen circulation flow path61and the hydrogen supply flow path60connect a discharge port29of the anode of the fuel cell20with the supply port28of the anode. The hydrogen circulation flow path61is provided with the gas liquid separator56and the hydrogen pump55. The gas liquid separator56serves to separate a discharged gas from the discharge port29of the anode into a gas and a liquid. The hydrogen pump55is configured to resupply the gas separated by the gas liquid separator56to the fuel cell20. The gas separated by the gas liquid separator56mainly includes hydrogen that is not consumed but that is discharged from the anode, nitrogen that is transmitted from a cathode side through a membrane electrode assembly of the fuel cell and that is discharged from the anode, and moisture that is not separated by the gas liquid separator56. The discharge flow path58is a piping arranged to connect the gas liquid separator56with an air discharge flow path39(described later) included in the air supply discharge mechanism30. The drainage shutoff valve57is provided on the discharge flow path58. The drainage shutoff valve57is opened to discharge the liquid separated by the gas liquid separator56and nitrogen and the like. The injector54and the drainage shutoff valve57are controlled to regulate the supply amount of hydrogen to the fuel cell20. The hydrogen pump55is also called “circulation pump”. The drainage shutoff valve57is also called “discharge valve”. The shutoff valve41, the regulator51, the injector54, the hydrogen pump55and the drainage shutoff valve57are controlled by the controller90.

The air supply discharge mechanism30is under control of the controller90to supply and discharge the air to and from a cathode of the fuel cell20. The air supply discharge mechanism30includes a compressor31, an air supply flow path32, a flow dividing valve33, a pressure regulator36, a bypass flow path38, and an air discharge flow path39.

The air supply flow path32is a piping connected with a supply port of the cathode of the fuel cell20. The air discharge flow path39is a piping connected with a discharge port of the cathode of the fuel cell20. The bypass flow path38is a piping branched off from the air supply flow path32at a position on an upstream side of the fuel cell20and connected with the air discharge flow path39. The compressor31is provided in the middle of the air supply flow path32to take in the air from an open air port of the air supply flow path32and compress the intake air. The compressor31is provided at a position that is closer to the open air port than a connecting position of the air supply flow path32with the bypass flow path38.

The flow dividing valve33is provided on a downstream side of the compressor31in the air supply flow path32, or more specifically provided between the compressor31and the fuel cell20to be located at the connecting position of the air supply flow path32with the bypass flow path38. The flow dividing valve33serves to change over the flow direction of the air flowing from the compressor31between the fuel cell20-side and the bypass flow path38-side. This flow dividing valve33is also called three-way valve. The bypass flow path38is a piping arranged to connect the flow dividing valve33with the air discharge flow path39. The pressure regulator36is provided in the air discharge flow path39to be located at a position on the fuel cell20-side of a connecting position of the air discharge flow path39with the bypass flow path38. The pressure regulator36serves to regulate a flow passage area of the air discharge flow path39according to its opening position. The air passing through the pressure regulator36goes through the connecting position of the air discharge flow path39with the bypass flow path38and is discharged from an open air port to the open air. The compressor31, the flow dividing valve33and the pressure regulator36are controlled by the controller90.

The cooling water circulation mechanism80is under control of the controller90to cool down the fuel cell20. The cooling water circulation mechanism80includes a radiator81, a cooling water pump82, a cooling water discharge flow path83and a cooling water supply flow path84.

The cooling water supply flow path84is a flow path arranged to connect the radiator81with the fuel cell20and is a piping provided to supply cooling water to the fuel cell20. The cooling water discharge flow path83is a flow path arranged to connect the fuel cell20with the radiator81and is a piping provided to discharge cooling water from the fuel cell20. The cooling water pump82is provided in the cooling water supply flow path84between the radiator81and the fuel cell20and serves to circulate the cooling water. The operations of the radiator81and the cooling water pump82are controlled by the controller90according to temperatures measured by thermometers85and86provided in the cooling water discharge flow path83and in the cooling water supply flow path84.

The controller90is configured as a computer including a CPU, a RAM and a ROM and is more specifically configured by an ECU (electronic control unit). The controller90outputs signals to control the operations of the fuel cell system10. In response to a power generation request, the controller90controls the respective parts of the fuel cell system10to cause the fuel cell20to generate electric power. The controller90also performs an exhaust/drainage process described later at a stop of the system.

The fuel cell system10mounted on the vehicle further includes a secondary battery and a DC/DC converter configured to control the output voltage of the fuel cell20and control charging and discharging of the secondary battery, although not being specifically illustrated or described in detail. The secondary battery is configured to store the electric power output from the fuel cell20and the regenerative power and serves as the power source along with the fuel cell20.

FIG. 2is a schematic configuration diagram illustrating a positional relationship between the hydrogen pump55, the gas liquid separator56, and the drainage shutoff valve57.FIG. 2illustrates the hydrogen pump55and the gas liquid separator56in a perspective view to show the inside thereof with respective casings551and561hatched. A direction Dv indicates a vertical direction, and a direction Dh indicates a horizontal direction.

The gas liquid separator56includes an internal space562which the discharged gas from the fuel cell20flows in. The internal space562includes an approximately funnel shape part. An exhaust/drainage portion563is provided at a bottom in the middle of the approximately funnel shape part. A liquid receiving surface566is provided on an outer circumferential side of the exhaust/drainage portion563to be located above the exhaust/drainage portion563. The drainage shutoff valve57is connected with the exhaust/drainage portion563via a communication path564that is provided in a side wall of the exhaust/drainage portion563. The liquid separated from the discharged gas that is discharged from the discharge port29of the anode (i.e., liquid water) is accumulated in the internal space562. The exhaust/drainage portion563is configured to collect the liquid water accumulated in the internal space562and discharge the liquid water via the communication path564and the drainage shutoff valve57. The liquid receiving surface566serves to receive and hold the liquid water blown off by a sprayed gas as described later.

The hydrogen pump55is placed on an upper face of the casing561of the gas liquid separator56. An opening565is provided in the casing561of the gas liquid separator56to be located in a region where the hydrogen pump55is placed. More specifically, the opening565is provided at a position above the exhaust/drainage portion563in the vertical direction Dv to be opposed to the exhaust/drainage portion563of the gas liquid separator56. The opening565is connected with an intake port554of the hydrogen pump55. The opening565is also called “opening connected with the circulation pump”.

The hydrogen pump55used is a two-lobe roots pump including two rotors552aand552binside of the casing551. The roots pump is a pump configured to rotate the two rotors552aand552bin opposite directions simultaneously with rotation of a motor (not shown) and thereby generate a gas stream according to the directions of rotations. Rotating shafts of the two rotors552aand552bare arranged along a horizontal direction that is perpendicular to the horizontal direction Dh shown inFIG. 2(i.e., direction perpendicular to the sheet surface). The gas stream generated by the rotations of the rotors552aand552bmostly flows in a direction along the vertical direction Dv.

At a system operation time when power generating operation is performed, the hydrogen pump55discharges the gas that is taken in from the intake port554, from a discharge port556provided above the intake port554in the vertical direction Dv. At the system operation time, the hydrogen pump55rotates the two rotors552aand552bsuch as to generate a gas stream from the gas liquid separator56toward the hydrogen pump55. The rotating directions are shown by one-dot chain line arrows inFIG. 2. The rotations of the pair of rotors552aand552bof the hydrogen pump55in this state are called “HP normal rotation”, and the gas stream generated by this rotation is called “gas stream in a normal direction”. The discharged gas that is discharged from the discharge port29of the anode of the fuel cell20accordingly flows into the internal space562of the gas liquid separator56and is separated into the gas and the liquid (liquid water). The separated gas is taken in through the opening565and the intake port554by the hydrogen pump55and is discharged from the discharge port556to be resupplied to the fuel cell20. Solid line arrows shown inFIG. 2indicate the outline of the gas stream in the gas liquid separator56and the hydrogen pump55.

The hydrogen pump55is also configured such that the two rotors552aand552bare rotated in reverse directions from those at the system operation time by reverse rotation of the motor (not shown), so as to generate a downward gas stream from the hydrogen pump55toward the gas liquid separator56. The rotations of the pair of rotors552aand552bof the hydrogen pump55in this state are called “HP reverse rotation”, and the gas stream generated by this rotation is called “gas stream in a reverse direction”. An operation of reversely rotating the hydrogen pump55to generate the gas stream in the reverse direction” is performed in a remaining water removal process in a system stop time as described later.

A procedure of terminating the system operation to stop the system first performs a purging operation. During the purging operation, the rotors552aand552bare rotated in the HP normal rotation continuously from the system operation time. This results in removing the water content accumulated in the fuel cell20and in the flow paths. After the purging operation, the drainage shutoff valve57is opened to discharge the liquid water accumulated in the internal space562. After the rotation of the hydrogen pump55is stopped, the drainage shutoff valve57is closed, and the system stops operation. At the system stop time, a remaining water removal process described later is also performed.

FIG. 3is a flowchart showing a procedure of remaining water removal process performed at the system stop time. The remaining water removal process is performed at the system stop time by the controller90. In the remaining water removal process, the hydrogen pump55at a stop is started to reversely rotate at a predetermined rotating speed (rate of rotation) (step S110). After elapse of a predetermined time period, the controller90stops the reverse rotation of the hydrogen pump55(step S120) and terminates this process.

FIG. 4is diagram illustrating the state of the gas liquid separator56during reverse rotation of the hydrogen pump55. When the hydrogen pump55is reversely rotated in directions shown by two-dot chain line arrows (HP reverse rotation) inFIG. 4, a gas stream in a reverse direction is generated from the opening565toward the exhaust/drainage portion563located below the opening565in the vertical direction Dv as shown by broken line arrows inFIG. 4. The liquid water accumulated in the hydrogen pump55is discharged from the hydrogen pump55to the gas liquid separator56by this gas stream in the reverse direction and the gravitational force. The gas from the opening565is sprayed onto the liquid water discharged from the hydrogen pump55and the remaining water accumulated in the exhaust/drainage portion563, so as to blow off the remaining water along the funnel-shape inclined surface from the exhaust/drainage portion563as shown by solid line arrows.

The liquid receiving surface566configured to receive and hold the blown-off liquid water is provided on the outer circumferential side of the exhaust/drainage portion563to be located above the exhaust/drainage portion563as described above. The liquid receiving surface566is a horizontal plane (horizontal surface) along the horizontal direction Dh. The blown-off liquid water (liquid droplet) is thus unlikely to flow down to the exhaust/drainage portion563after adhering to the liquid receiving surface566. The gas liquid separator56of the embodiment accordingly enables the blown-off liquid water to be received and held by the liquid receiving surface566.

The rotating speed and the time duration of the HP reverse rotation may be set, for example, as described below. The rotating speed is set to be equal to or higher than a rotation speed that allows for generation of a gas stream that at least enables the gas to be sprayed from the opening565toward the remaining water accumulated in the exhaust/drainage portion563and to thereby blow off the remaining water. The rotating speed and the time duration of the HP reverse rotation are also set, such that the drainage shutoff valve57is not inoperable even when water remains in the exhaust/drainage portion563and is frozen. For example, when water remains in the exhaust/drainage portion563at a water level that does not cause the liquid water to enter the communication path564, the maximum amount of water is determinable from the volume of the exhaust/drainage portion563located below the communication path564. The amount of water after the blow-off of the gas by the HP reverse rotation may thus be set to a level that does not cause the drainage shutoff valve57to be inoperable even when the water is frozen. A relationship between the rotating speed and the time duration of the hydrogen pump55(i.e., a combination of the rotating speed and the time duration) may be determined in advance. The rotating speed and the time duration of the HP reverse rotation (S110and S120inFIG. 3) are set according to the determined relationship between the rotating speed and the time duration.

As described above, the hydrogen pump55and the gas liquid separator56of the embodiment are configured such that the hydrogen pump55is reversely rotated to generate the gas stream in the reverse direction and thereby blow off the remaining water in the exhaust/drainage portion563and that the blown-off remaining water is received and held by the liquid receiving surface566. This configuration suppresses the liquid water from remaining in the exhaust/drainage portion563. This accordingly suppresses the drainage shutoff valve57from being inoperable due to freezing of the remaining water in the exhaust/drainage portion563and thereby suppresses the fuel cell system10from being inoperable.

In the above description, the remaining water removal process shown inFIG. 3is performed at the system operation time, and its start timing is not specified. The start timing of the remaining water removal process may, however, be specified as a timing when a predetermined condition, for example, one of conditions given below, is satisfied:

Condition 1: The system stops operation. In this case, the remaining water removal process is performed continuously after the stop of operation of the system; and

Condition 2: An elapsed time or a system temperature exceeds a corresponding reference value after the stop of operation of the system. In this case, the remaining water removal process is performed some time after the stop of operation of the system. For example, after the stop of operation of the system, the system is naturally cooled down from the high temperature condition during the system operation. The remaining water removal process may be performed at a timing after the moisture vapor becomes liquid water due to dew condensation.

The remaining water removal process may be performed at any of various timings after some waiting, for example, after lapse of a predetermined time period or after the temperature of the system becomes lower than a predetermined temperature.

The temperature of the system may be any of various environmental temperatures, for example, the temperature of cooling water in the cooling water discharge flow path83measured by the thermometer85, the temperature of cooling water in the cooling water supply flow path84measured by the thermometer86, the temperature of the fuel cell20, the temperature of the hydrogen pump55, the temperature of the gas liquid separator56, or the atmosphere temperature of the fuel cell system10.

The predetermined condition is not limited to the above examples but may be any condition that determines the start timing of the remaining water removal process described above to remove the remaining water in the exhaust/drainage portion563at the system stop time.

FIG. 5andFIG. 6are schematic configuration diagrams respectively illustrating a liquid receiving surface566aand a liquid receiving surface566baccording to modifications. The liquid receiving surface566described above is configured to be a planar surface along the horizontal direction Dh. This configuration is, however, not restrictive.

The liquid receiving surface566aofFIG. 5is configured by a surface including an inclined surface that is inclined more downward toward the outer circumferential side to be lower in the vertical direction Dv on the outer circumferential side. This configuration of the liquid receiving surface566acauses the liquid water to be accumulated on the outer circumferential side and makes it unlikely that the liquid water moves toward the exhaust/drainage portion563. The inclined surface is not necessarily a planar surface but may be a curved surface inclined outward.

The liquid receiving surface566bofFIG. 6is configured to include a concave that is provided at the center in the horizontal direction and that is inclined downward in the vertical direction Dv. The liquid receiving surface566bis accordingly configured such that liquid water is accumulated in the concave. The inclination of the concave may be set in a range of 0 degree to 90 degrees relative to the horizontal direction. The position of the concave is not limited to the center in the horizontal direction of the liquid receiving surface566bbut may be any position of the liquid receiving surface566b. The liquid receiving surface566bmay be configured to be recessed as a whole. Accordingly, the liquid receiving surface may have any configuration that allows the blown-off liquid water to be received and held.

B. Second Embodiment

FIG. 7is a flowchart showing a procedure of the remaining water removal process performed at the system stop time according to a second embodiment. The configuration of a fuel cell system that performs the remaining water removal process of the second embodiment is identical with the configuration of the fuel cell system10of the first embodiment (shown inFIG. 1).

When the remaining water removal process is triggered, the controller90starts the reverse rotation of the hydrogen pump55(step S110) like the remaining water removal process of the first embodiment (shown inFIG. 3) and opens the drainage shutoff valve57to start gas exhaust and water drainage (step S115). After elapse of a predetermined time period, the controller90closes the drainage shutoff valve57to stop the gas exhaust and water drainage (step S116). This causes the liquid water discharged from the hydrogen pump55and the remaining water accumulated in the exhaust/drainage portion563to be discharged to outside. The process of step S110and the process of step S115may be started simultaneously or may be started in a reverse sequence to that of the sequence shown inFIG. 7. The reverse rotation of the hydrogen pump55continues in the closed position of the drainage shutoff valve57(i.e., the remaining water removal process of the first embodiment shown inFIG. 3is performed). After elapse of a predetermined time period, the controller90stops the reverse rotation of the hydrogen pump55(step S120) and terminates this process. This causes the liquid water remaining in the exhaust/drainage portion563after the gas exhaust and water drainage to the outside (step S116) to be blown off from the exhaust/drainage portion563.

As described above, the configuration of the second embodiment opens the drainage shutoff valve57during the HP reverse rotation, so as to discharge the liquid water discharged from the hydrogen pump55and the remaining water accumulated in the exhaust/drainage portion563, to the outside and additionally blow of the liquid water remaining in the exhaust/drainage portion563. This configuration more effectively suppresses the liquid water from remaining in the exhaust/drainage portion563, compared with the configuration of the first embodiment. The configuration of the second embodiment more effectively suppresses the drainage shutoff valve57from being inoperable due to freezing of the remaining water in the exhaust/drainage portion563and thereby more effectively suppresses the fuel cell system from being inoperable.

The start timing of the remaining water removal process described in the first embodiment may be similarly applied to the remaining water removal process of the second embodiment.

FIG. 8is a flowchart showing a procedure of the remaining water removal process performed at the system stop time according to a third embodiment. The configuration of a fuel cell system that performs the remaining water removal process of the third embodiment is identical with the configuration of the fuel cell system10of the first embodiment (shown inFIG. 1).

The remaining water removal process of the third embodiment additionally includes a process of waiting until a temperature Tm measured to determine the likelihood of freezing (measured temperature Tm) becomes lower than a freezing reference value Th (step S105), prior to starting the HP reverse rotation (step S110) in the remaining water removal process of the first embodiment (shown inFIG. 3). This additional process may be regarded as a process of determining whether a predetermined condition is satisfied, in order to detect the start timing of the remaining water removal process.

The remaining water removal process of the third embodiment additionally includes the process of step S105. When the remaining water in the exhaust/drainage portion563is likely to be frozen, the remaining water removal process of the third embodiment causes the liquid water accumulated in the hydrogen pump55to be discharged from the hydrogen pump55and blows off the remaining water in the exhaust/drainage portion563. When the remaining water in the exhaust/drainage portion563is unlikely to be frozen, on the other hand, the remaining water removal process of the third embodiment does not blow off the remaining water in the exhaust/drainage portion563. Unless the remaining water is frozen, the drainage shutoff valve57does not become inoperable and the fuel cell system10does not become inoperable. There is accordingly no need to perform the remaining water removal process. When the remaining water is likely to be frozen, on the other hand, the remaining water is to be removed, in order to suppress the drainage shutoff valve57from being inoperable.

The temperature Tm measured to determine the likelihood of freezing may be any of various environmental temperatures in the fuel cell system, for example, the temperature of cooling water in the cooling water discharge flow path83measured by the thermometer85, the temperature of cooling water in the cooling water supply flow path84measured by the thermometer86(as shown inFIG. 1), the temperature of the fuel cell20, the temperature of the hydrogen pump55, the temperature of the gas liquid separator56, or the atmosphere temperature of the fuel cell system. The freezing reference value Th may be set to a temperature below which water is likely to be frozen and is set to, for example, a temperature in a 0° C. to 10° C.

As described above, the configuration of the third embodiment blows off the liquid water remaining in the exhaust/drainage portion563when the remaining water in the exhaust/drainage portion563is likely to be frozen. This configuration effectively suppresses the liquid from remaining in the exhaust/drainage portion563. This configuration more effectively suppresses the drainage shutoff valve57from being inoperable due to freezing of the remaining water in the exhaust/drainage portion563and thereby more effectively suppresses the fuel cell system from being inoperable. When the remaining water is unlikely to be frozen, on the other hand, this configuration prevents the remaining water removal process from being unnecessarily performed.

The foregoing describes the remaining water removal process of the third embodiment (shown inFIG. 8) additionally including the process of waiting until the remaining water is likely to be frozen (step S105) comparing to the remaining water removal process of the first embodiment. The process of waiting until the remaining water is likely to be frozen may similarly be applicable to the remaining water removal process of the second embodiment (shown inFIG. 7).

The foregoing describes the remaining water removal process of the third embodiment (shown inFIG. 8) that is performed in place of the remaining water removal process of the first embodiment (shown inFIG. 3) or the remaining water removal process of the second embodiment (shown inFIG. 7). The remaining water removal process of the third embodiment (shown inFIG. 8) may, however, be performed along with the remaining water removal process of the first embodiment and/or the remaining water removal process of the second embodiment.

FIG. 9is a schematic configuration diagram illustrating a positional relationship between the hydrogen pump55, a gas liquid separator56B and the drainage shutoff valve57according to a fourth embodiment. The diagram ofFIG. 9corresponds to the schematic configuration diagram (ofFIG. 2) illustrating the positional relationship between the hydrogen pump55, the gas liquid separator56, and the drainage shutoff valve57according to the first embodiment. A fuel cell system of the fourth embodiment has a similar configuration to that of the fuel cell system10of the first embodiment (shown inFIG. 1), except the positional relationship between the hydrogen pump55, the gas liquid separator56B and the drainage shutoff valve57shown inFIG. 9.

The positional relationship between the hydrogen pump55, the gas liquid separator56B and the drainage shutoff valve57according to the fourth embodiment is similar to the positional relationship between the hydrogen pump55, the gas liquid separator56, and the drainage shutoff valve57according to the first embodiment, except differences 1 and 2 given below.

Difference 1: In the gas liquid separator56of the first embodiment (shown inFIG. 2), the opening565is located above the exhaust/drainage portion563in the vertical direction Dv. In the fourth embodiment, on the other hand, an opening565B of the gas liquid separator56B is located at a position deviated along the horizontal direction Dh from a position above the exhaust/drainage portion563in the vertical direction Dv.

Difference 2: In the fourth embodiment, the opening565B of the gas liquid separator56B is configured such that a gas stream is generated from the opening565B connected with the intake port554of the hydrogen pump55toward the exhaust/drainage portion563located at a position different from a position vertically below the opening565B during the HP reverse rotation. In other words, the opening565B is configured to face in the direction of the exhaust/drainage portion563.

In the above configuration, when the hydrogen pump55is reversely rotated (HP reverse rotation) in directions shown by two-dot chain line arrows inFIG. 9, the gas stream in the reverse direction is generated from the opening565B toward the exhaust/drainage portion563as shown by broken line arrows inFIG. 9. The liquid water accumulated in the hydrogen pump55is discharged from the hydrogen pump55to the gas liquid separator56B by this gas stream in the reverse direction and the gravitational force. The gas from the opening565B is sprayed onto the liquid water discharged from the hydrogen pump55and the remaining water accumulated in the exhaust/drainage portion563, so as to blow off the remaining water from the exhaust/drainage portion563as shown by solid line arrows. This configuration suppresses the drainage shutoff valve57from being inoperable due to freezing of the remaining water in the exhaust/drainage portion563and thereby suppresses the fuel cell system from being inoperable.

The modified configurations of the liquid receiving surface566described in the first embodiment (shown inFIGS. 5 and 6) may be applied to the fourth embodiment. The respective remaining water removal processes described in the first to the third embodiments (shown inFIGS. 3, 7 and 8) and their modifications may be applied to the fourth embodiment.

The present disclosure is not limited to the embodiments and their modifications described above but may be implemented in various aspects without departing from the scope of the disclosure. Some of possible modifications are given below.

The first embodiment and the fourth embodiment describe the configurations including the liquid receiving surface that receives and holds the liquid water blown off into the internal space of the gas liquid separator. The gas liquid separator may, however, be configured without such a liquid receiving surface. When the blown-off liquid water is in a small mass, this modified configuration still enables the blown-off liquid water to adhere to and to be held on a wall surface of the internal space and achieves the effect of removing the remaining water from the exhaust/drainage portion.

In the above description, the remaining water removal processes of the first to the third embodiments are performed independently. For example, the remaining water removal process of the first embodiment may be performed in combination with the remaining water removal process of the third embodiment. The remaining water removal process of the second embodiment may be performed in combination with the remaining water removal process of the third embodiment. All the remaining water removal processes of the first to the third embodiments may be performed in combination. For example, after a stop of the system, the process of the first embodiment may be preformed on satisfaction of a first condition, and the process of the second embodiment may be performed on satisfaction of a second condition. Subsequently, when the liquid water is likely to be frozen in the third embodiment, the remaining water removal process may be performed.

The disclosure is not limited to any of the embodiment and its modifications described above but may be implemented by a diversity of configurations without departing from the scope of the disclosure. For example, the technical features of any of the above embodiments and their modifications may be replaced or combined appropriately, in order to solve part or all of the problems described above or in order to achieve part or all of the advantageous effects described above. Any of the technical features may be omitted appropriately unless the technical feature is described as essential in the description hereof. The present disclosure may be implemented by aspects described below.

(1) According to one aspect of the present disclosure, there is provided a fuel cell system. This fuel cell system comprises a fuel cell; a supply flow path configured to supply a fuel gas to a supply port of an anode of the fuel cell; a circulation flow path configured to resupply a discharge gas that is discharged from a discharge port of the anode, as the fuel gas to the supply port; a gas liquid separator provided in the circulation flow path and configured to separate the discharged gas into a gas and a liquid; a circulation pump provided in the circulation flow path and configured to supply the gas separated by the gas liquid separator to the supply flow path; a discharge valve connected with the gas liquid separator and configured to discharge the separated liquid from the gas liquid separator; and a controller configured to control operation of the circulation pump. The circulation pump is a two-lobe roots pump configured such that two rotors rotate in opposite directions. An exhaust/drainage portion is provided at a bottom in an internal space of the gas liquid separator and is configured to collect the separated liquid and discharge the collected liquid via the discharge valve. The gas liquid separator includes an opening that is connected with the circulation pump and that is configured to face in a direction of the exhaust/drainage portion. At a system stop time, when a predetermined condition is satisfied, the controller causes the rotors of the circulation pump to be rotated in reverse directions from rotations at a system operation time.

The fuel cell system of this aspect enables the remaining water in the exhaust/drainage portion to be blown off by rotation of the rotors of the circulation pump at the system stop time. This configuration reduces the remaining water in the exhaust/drainage portion. As a result, this configuration reduces the possibility that the system is inoperable due to freezing of the remaining water.

(2) In the fuel cell system of the above aspect, the opening of the gas liquid separator may be placed above the exhaust/drainage portion in a vertical direction and may be configured to face downward in the vertical direction.

The fuel cell system of this aspect enables the remaining water in the exhaust/drainage portion to be blown off efficiently.

(3) In the fuel cell system of the above aspect, the gas liquid separator may include a liquid receiving surface that is provided above the exhaust/drainage portion in the internal space. The liquid receiving surface may be configured by a horizontal surface or a by a surface including an inclined surface that is inclined more downward on an outer circumferential side.

In the fuel cell system of this aspect, the blown-off liquid water is received and held by the liquid receiving surface. This configuration suppresses the blown-off liquid water from flowing down to the exhaust/drainage portion.

(4) In the fuel cell system of the above aspect, the controller may open the discharge valve when rotating the rotors in the reverse directions.

The fuel cell system of this aspect causes the remaining water to be discharged via the discharge valve, while blowing off the remaining water in the exhaust/drainage portion. This configuration enables the remaining water to be reduced more efficiently.

(5) In the fuel cell system of the above aspect, the controller may rotate the rotors in the reverse directions when a measured temperature is lower than a freezing reference value at the system stop time.

When the remaining water is likely to be frozen, the fuel cell system of this aspect blows off the remaining water in the exhaust/drainage portion to reduce the remaining water in the exhaust/drainage portion. This configuration effectively reduces the possibility that the system is inoperable due to freezing of the remaining water.

The present disclosure may be implemented by various aspects other than the aspects of the fuel cell system described above, for example, an exhaust/drainage mechanism for the fuel cell system and an exhaust/drainage control method of the fuel cell system.