Fuel cell system, control method for the fuel cell system, and electric vehicle equipped with the fuel cell system

A fuel cell system that includes a fuel cell that generates electricity through an electrochemical reaction between a fuel gas and an oxidant gas, and a control portion that determines whether there is leakage of the fuel gas. The control portion has start means for starting the fuel cell by raising the voltage of the fuel cell from a starting voltage to an operation voltage that is lower than an open-circuit voltage, and leakage determination means for determining whether there is leakage of the fuel gas before the voltage of the fuel cell reaches the operation voltage when the fuel cell is started.

This is a 371 national phase application of PCT/IB2010/000558 filed 18 Mar. 2010, claiming priority to Japanese Patent Application No. 2009-084637 filed Mar. 31 2009, the contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to fuel cell system, a control method for the fuel cell system, and a control that is performed on an electric vehicle equipped with the fuel cell system, at the time of starting the electric vehicle.

BACKGROUND OF THE INVENTION

Practical application of a fuel cell that supplies hydrogen as a fuel gas to a fuel electrode, and that supplies air as an oxidant gas to an oxidant electrode, and that generates electricity through an electrochemical reaction between hydrogen and oxygen in the air while producing water on an oxidant electrode is now being considered.

In such a fuel cell, if at the time of start of operation, the pressure of hydrogen supplied to the fuel electrode and the pressure of air supplied to the oxidant electrode are about equal to the respective pressures occurring during ordinary operation, it sometimes happens that hydrogen gas and air are unevenly distributed in the fuel electrode and the oxidant electrode, respectively, and the electrodes are degraded by electrochemical reaction caused by the uneven distribution of these gases. Japanese Patent Application Publication No. 2007-26891 (JP-A-2007-26891) discloses a method of preventing the degradation of the electrodes of a fuel cell by causing the pressures of hydrogen and air supplied to the fuel electrode and the oxidant electrode, respectively, at the time of start of operation of the fuel cell to be higher than the ordinary supplied pressures of these gases.

However, if hydrogen gas and air are supplied at high pressure to a fuel cell when the fuel cell starts operation, it sometimes happen that the rate of rise of the voltage of the fuel cell becomes large so that the voltage of the fuel cell overshoots its upper-limit voltage. In conjunction with this problem, Japanese Patent Application Publication No. 2007-26891 (JP-A-2007-26891) discloses a method in which when hydrogen gas and air are supplied, at the time of starting a fuel cell, at pressures that are higher than their pressures given during ordinary power generation, output electric power is extracted from the fuel cell, and is put out to a vehicle driving motor, resistors, etc., provided that the voltage of the fuel cell reaches a predetermined voltage that is lower than the upper-limit voltage.

By the way, since the fuel cell uses hydrogen as a fuel gas, it is necessary to check that there is no leakage of hydrogen when the fuel cell is started. To this end, a method in which the presence/absence of hydrogen leakage from the system is determined by sealing the hydrogen system and then checking whether or not the pressure in the system becomes low. However, during the state where hydrogen and oxygen in air are undergoing an electrochemical reaction within the fuel cell, the hydrogen supplied to the fuel cell is consumed by the electrochemical reaction. Therefore, the pressure of the hydrogen system that is sealed decreases even when there is no leakage of hydrogen, and the hydrogen leakage sometimes cannot be accurately determined. Therefore, in a related-art technology as shown inFIG. 8, after an ignition key is turned on at time t0′, the control value of the output voltage of the fuel cell shown by a line a′ is set at an open-circuit voltage OCV. At time t1′, the voltage of the fuel cell starts to be raised as shown by a line b′ by supplying hydrogen and oxygen are supplied to the fuel cell, and therefore pressurizing a hydrogen system and an oxygen system. As a result, the voltage of the fuel cell is temporarily raised to the open-circuit voltage OCV. Then, in the related-art method, it is detected whether or not there is hydrogen leakage, during a period from time t2′ to time t3′ during which the voltage of the fuel cell remains at the open-circuit voltage OCV. When the voltage of the fuel cell reaches the open-circuit voltage OCV, the electrochemical reaction between hydrogen and oxygen within the fuel cell does not progress any longer, so that the hydrogen in the sealed hydrogen system is not consumed. Therefore, a state in which the pressure of the sealed hydrogen system undergoes hardly any decrease can be created if there is no leakage of hydrogen. Then, it can be determined whether or not there is hydrogen leakage by detecting the degree of pressure decrease of the hydrogen system during the foregoing state. However, when the voltage of the fuel cell reaches the open-circuit voltage OCV, the durability of the fuel cell can be adversely affected.

SUMMARY OF THE INVENTION

The invention provides a fuel cell system that determines whether there is hydrogen leakage, while restraining the adverse influence on the durability of the fuel cell when the fuel cell is started, and also provides a control method for the fuel cell system, and an electric vehicle that is equipped with the fuel cell system.

A first aspect of the invention relates to a fuel cell system. This fuel cell system includes: a fuel cell that generates electricity through an electrochemical reaction between a fuel gas and an oxidant gas; and a control portion that determines whether there is leakage of the fuel gas. The control portion has: start means for starting the fuel cell by raising voltage of the fuel cell from a starting voltage to an operation voltage that is lower than an open-circuit voltage; and leakage determination means for determining whether there is leakage of the fuel gas, before the voltage of the fuel cell reaches the operation voltage when the fuel cell is started.

The foregoing fuel cell system may further include: fuel gas supply means for supplying the fuel gas to a fuel electrode of the fuel cell; and oxidant gas supply means for supplying the oxidant gas to an oxidant electrode of the fuel cell, and the start means may raise the voltage of the fuel cell by supplying the fuel gas to the fuel electrode of the fuel cell by the fuel gas supply means and then supplying the oxidant gas to the oxidant electrode by the oxidant gas supply means, and the leakage determination means may determine whether there is leakage of the fuel gas during a period from when the fuel gas is supplied to when the oxidant gas starts to be supplied.

In the fuel cell system, the fuel gas supply means may include a fuel gas supply channel, and a fuel supply valve provided in the fuel gas supply channel, and may further include a gas discharge channel that discharges a post-reaction fuel gas from the fuel electrode of the fuel cell, a gas discharge valve provided in the gas discharge channel, and a pressure sensor that detects pressure in the fuel gas channel that is on a fuel electrode side of the fuel supply valve, and that is on a fuel electrode side of the gas discharge valve, and the leakage determination means may close the fuel supply valve and the gas discharge valve, and may determine whether there is leakage of the fuel gas based on a rate of pressure decrease that is detected by the pressure sensor.

A second aspect of the invention relates to a fuel cell system. This fuel cell system includes: a fuel cell that generates electricity through an electrochemical reaction between a fuel gas and an oxidant gas; fuel gas supply means for supplying the fuel gas to a fuel electrode of the fuel cell; oxidant gas supply means for supplying the oxidant gas to an oxidant electrode of the fuel cell; and a control portion that determines whether there is leakage of the fuel gas. The control portion includes: start means for starting the fuel cell by lowering voltage of the fuel cell from a starting voltage to an operation voltage that is lower than an open-circuit voltage at a time of starting the fuel cell if the starting voltage of the fuel cell is lower than the open-circuit voltage, but is higher than the operation voltage, and by supplying the fuel gas to the fuel electrode of the fuel cell by the fuel gas supply means, and then by supplying the oxidant gas to the oxidant electrode by the oxidant gas supply means; and leakage determination means for determining whether there is leakage of the fuel gas during a period from when the fuel gas is supplied to when the oxidant gas starts to be supplied.

In this fuel cell system, the fuel gas supply means may include a fuel gas supply channel, and a fuel supply valve provided in the fuel gas supply channel, and may further include a gas discharge channel that discharges a post-reaction fuel gas from the fuel electrode of the fuel cell, a gas discharge valve provided in the gas discharge channel, and a pressure sensor that detects pressure in the fuel gas channel that is on a fuel electrode side of the fuel supply valve, and that is on a fuel electrode side of the gas discharge valve, and the leakage determination means may close the fuel supply valve and the gas discharge valve at a time of starting the fuel cell, and may determine whether there is leakage of the fuel gas from a first rate of pressure decrease detected via the pressure sensor, and a second rate of pressure decrease based on an amount of consumption of the fuel gas estimated from an output current of the fuel cell.

In this fuel cell system, the leakage determination means may calculate a third rate of pressure decrease by subtracting the second rate of pressure decrease from the first rate of pressure decrease, and may determine that there is leakage of the fuel gas if the third rate of pressure decrease is greater than or equal to a first threshold value. Besides, the leakage determination means may determine that there is leakage of the fuel gas if the first rate of pressure decrease is greater than or equal to a second threshold value that is greater than the first threshold value.

A third aspect of the invention relates to an electric vehicle. This electric vehicle is equipped with the foregoing fuel cell system.

A fourth aspect of the invention relates to a control method for a fuel cell system. This method is a control method for a fuel cell system that includes a fuel cell that generates electricity through an electrochemical reaction between a fuel gas and an oxidant gas, and includes: starting the fuel cell by raising voltage of the fuel cell from a starting voltage to an, operation voltage that is lower than an open-circuit voltage; and determining whether there is leakage of the fuel gas, before the voltage of the fuel cell reaches the operation voltage when the fuel cell is started.

A fifth aspect of the invention relates to a control method for a fuel cell system. This method is a control method for a fuel cell system that includes a fuel cell that generates electricity through an electrochemical reaction between a fuel gas and an oxidant gas, a fuel gas supply portion that supplies the fuel gas to a fuel electrode of the fuel cell, and an oxidant gas supply portion that supplies the oxidant gas to an oxidant electrode of the fuel cell. The control method includes: starting the fuel cell by lowering voltage of the fuel cell from a starting voltage to an operation voltage that is lower than an open-circuit voltage at a time of starting the fuel cell if the starting voltage of the fuel cell is lower than the open-circuit voltage, but is higher than the operation voltage, and by supplying the fuel gas to the fuel electrode of the fuel cell by the fuel gas supply portion, and then by supplying the oxidant gas to the oxidant electrode by the oxidant gas supply portion; and determining whether there is leakage of the fuel gas during a period from when the fuel gas is supplied to when the oxidant gas starts to be supplied.

The invention achieves an effect of being able to determine whether there is hydrogen leakage, without impairing the durability of the fuel cell, when the fuel cell is started.

DETAILED DESCRIPTION OF THE EMBODIMENTS

As shown inFIG. 1, a fuel cell system100mounted in an electric vehicle200includes a chargeable and dischargeable secondary cell12, a step-up/down voltage converter13that raises or lowers the voltage of the secondary cell12, an inverter14that converts direct-current electric power of the step-up/down voltage converter13into alternating-current electric power, and supplies the electric power to a traction motor15, and a fuel cell11.

The secondary cell12is constructed of a chargeable and dischargeable lithium-ion battery, or the like. The voltage of the secondary cell12in this embodiment is lower than the drive voltage of the traction motor15. However, the voltage of the secondary cell is not limited so, but may also be a voltage that is equivalent to or higher than the drive voltage of the traction motor. The step-up/down voltage converter13includes a plurality of switching elements, and converts a primary-side voltage supplied from the secondary cell12to a secondary-side voltage for driving the traction motor, by the on/off operations of the switching elements. The step-up/down voltage converter13is a non-insulated bidirectional DC/DC converter whose reference electrical path32is connected to both a minus-side electrical path34of the secondary cell12and a minus-side electrical path39of the inverter14, and whose primary-side electrical path31is connected to a plus-side electrical path33of the secondary cell12, and whose secondary-side electrical path35is connected to a plus-side electrical path38of the inverter14. Besides, the plus-side electrical path33and the minus-side electrical path34of the secondary cell12are each provided with a system relay25that turns on and off the connection between the secondary cell12and a load system.

The fuel cell11is supplied with a hydrogen gas, which is a fuel gas, and with air, which is an oxidant gas, and generates electric power though an electrochemical reaction between the hydrogen gas and the oxygen in the air. In the fuel cell11, hydrogen gas is supplied from a high-pressure hydrogen tank17to a fuel electrode (anode) through a hydrogen supply pipe27that is provided with a hydrogen supply valve18, and air is supplied to an oxidant electrode (cathode) by an air compressor19. Herein, the hydrogen supply valve18is a fuel supply valve, and the hydrogen supply pipe27is a fuel gas supply channel. A pressure sensor47that detects the pressure in a hydrogen system is attached to the hydrogen supply pipe27. The hydrogen supplied, and the oxygen in the air supplied undergo an electrochemical reaction within the fuel cell11, thus outputting electricity, and producing water at the oxidant electrode. The produced water is discharged out of the fuel cell, together with the air that has been used for the reaction. On the other hand, the hydrogen supplied to the fuel electrode changes to a reaction gas whose hydrogen concentration has decreased due to the reaction, and then is discharged through a hydrogen gas discharge pipe28. The discharged reaction gas is pressurized by a hydrogen circulation pump26that is provided on a recirculation pipe29, so as to be circulated into the hydrogen supply pipe27and therefore to the fuel electrode. The amount of hydrogen consumed by the reaction is supplied from the hydrogen tank17into the hydrogen supply pipe27by adjusting the degree of opening of the hydrogen supply valve18. Besides, the gas that resides at the fuel electrode, including nitrogen gas, etc., is discharged, together with the post-reaction gas, to the outside through a gas discharge pipe45. A gas discharge valve22that adjusts the amount of gas discharged is attached to the gas discharge pipe45. As described above, the hydrogen system is provided as a circulation system. Therefore, when the hydrogen supply valve18and the gas discharge valve22are closed, a region46that includes the hydrogen supply pipe27on the fuel electrode side of the hydrogen supply valve18, a fuel-side portion of the fuel cell11, the hydrogen gas discharge pipe28, the hydrogen circulation pump26, the recirculation pipe29, and the gas discharge pipe45on the fuel electrode side of the gas discharge valve22assumes a sealed state.

A plus-side electrical path36of the fuel cell11is connected to the secondary-side electrical path35of the step-up/down voltage converter13via an FC relay24and a blocking diode23. A minus-side electrical path37of the fuel cell11is connected to the reference electrical path32of the step-up/down voltage converter13via another FC relay24. The secondary-side electrical path35of the step-up/down voltage converter13is connected to the plus-side electrical path38of the inverter14, and the reference electrical path32of the step-up/down voltage converter13is connected to the minus-side electrical path39of the inverter14. The plus-side electrical path36and the minus-side electrical path37of the fuel cell11are connected to the plus-side electrical path38and the minus-side electrical path39, respectively, of the inverter14, via the FC relays24. The FC relays24turn on and off the connection between the load system and the fuel cell11. When the FC relays24are closed, the fuel cell11is connected to the secondary side of the step-up/down voltage converter13, so that the electric power generated by the fuel cell11is supplied together with the secondary-side electric power of the secondary cell12obtained by raising the voltage of the primary-side electric power of the secondary cell12, to the inverter, which thereby drives the traction motor15that rotates wheels60. At this time, the voltage of the fuel cell11becomes equal to the output voltage of the step-up/down voltage converter13and to the input voltage of the inverter14. Besides, the air compressor19, and accessories16of the fuel cell11that include a cooling water pump, a hydrogen circulation pump26, etc., are supplied with drive electric power from the secondary cell12.

A primary-side capacitor20that smoothes the primary-side voltage is connected between the plus-side electrical path33and the minus-side electrical path34of the secondary cell12. The primary-side capacitor20is provided with a voltage sensor41that detects the voltage between the two ends of the primary-side capacitor20. Besides, a secondary-side capacitor21that smoothes the secondary-side voltage is provided between the plus-side electrical path38and the minus-side electrical path39of the inverter14. The secondary-side capacitor21is provided with a voltage sensor42that detects the voltage between the two ends of the secondary-side capacitor21. The voltage across the primary-side capacitor20is a primary-side voltage VLthat is the input voltage of the step-up/down voltage converter13, and the voltage across the secondary-side capacitor21is a secondary-side voltage VHthat is the output voltage of the step-up/down voltage converter13. Besides, a voltage sensor43that detects the voltage of the fuel cell11is provided between the plus-side electrical path36and the minus-side electrical path37of the fuel cell11, and an electric current sensor44that detects the output current of the fuel cell11is provide on the plus-side electrical path36of the fuel cell11.

A control portion50is a computer that contains a CPU that performs signal processing, and a storage portion that stores programs and control data. The fuel cell11, the air compressor19, the step-up/down voltage converter13, the inverter14, the traction motor15, the accessories16, the hydrogen supply valve18, the gas discharge valve22, the FC relays24, and the system relays25are connected to the control portion50, and are constructed so as to operate according to commands from the control portion50. Besides, the secondary cell12, the voltage sensors41to43, the electric current sensor44, and the pressure sensor47are each connected to the control portion50, and are constructed so that the state of the secondary cell12, and detection signals of the voltage sensors41to43, the electrical current sensor44, and the pressure sensor47are input to the control portion50. The electric vehicle200is provided with an ignition key30that is a switch for starting and stopping the fuel cell system100. The ignition key30is connected to the control portion50, and is constructed so that an on/off-signal of the ignition key30is input to the control portion50.

Operations of the fuel cell system100constructed as described above will be described with reference toFIG. 2toFIG. 4. InFIG. 2, a line a shows the secondary-side voltage VHthat is the output voltage of the step-up/down voltage converter13, and a line b shows the FC voltage VFthat is the voltage of the fuel cell11. The fuel cell11is started from a state of zero voltage as shown inFIG. 2.

When a driver, that is, an operating person, turns on the ignition key30at time t0shown inFIG. 2, the on-signal from the ignition key30is input to the control portion50, whereby the control portion50recognizes the on-state of the ignition key30as shown in step S101inFIG. 4. When the on-signal of the ignition key30is input, the control portion50closes the system relays25to connect the secondary cell12to the system, so that the primary-side capacitor20is charged by the electric power supplied from the secondary cell12. After that, the control portion50starts the voltage raising operation of the step-up/down voltage converter13to start the charging of the secondary-side capacitor21, as shown in steps S102and S103inFIG. 4. The control portion50raises the secondary-side voltage VHwhile detecting the secondary-side voltage VHby the voltage sensor42. When the secondary-side voltage VHreaches the open-circuit voltage OCV, the charging of the secondary-side capacitor21is completed, and the supply of electric power from the secondary cell12becomes possible. Therefore, at time t1shown inFIG. 2, the control portion50lights a READY lamp to indicate that the preparation for supplying electric power to the traction motor15has been completed. When the driver depresses an accelerator pedal after the READY lamp is lighted, the electric power from the secondary cell12is supplied to the traction motor15that rotates the wheels60, so that the electric vehicle200can start to move. Although electric power is supplied from the secondary cell12to the traction motor15, electric power does not flow into the fuel cell11since the FC relays24are open and therefore the fuel cell11is cut off from the system.

The control portion50acquires the value of the starting voltage VF0of the fuel cell11from the voltage sensor43, and compares the value with an operation voltage V0, as shown in step S104inFIG. 4. The operation voltage V0is lower than the open-circuit voltage OCV. Then, for example, if the starting voltage VF0of the fuel cell11is lower than the operation voltage V0that is lower than the open-circuit voltage OCV as shown inFIG. 2, the control portion50outputs a command to pressurize the hydrogen system as shown in step S105inFIG. 4, at time t1shown inFIG. 2. Due to this command, the hydrogen supply valve18is opened, so that hydrogen starts to be supplied from the hydrogen tank17to the fuel cell11. When hydrogen is supplied, the pressure at the fuel electrode of the fuel cell11rises. However, since the oxidant electrode has not been supplied with air, the electrochemical reaction does not occur within the fuel cell11, and therefore the fuel cell11does not generate electricity. Thus, at this time, the FC voltage VFof the fuel cell11is zero, as is the case with the starting voltage VF0of the fuel cell11.

Besides, if the starting voltage VF0of the fuel cell11is higher than the operation voltage V0, the control portion50jumps to step S205shown inFIG. 7(described later), in which the control portion50closes the FC relays24.

When the control portion50determines that the pressure of the hydrogen system detected by the pressure sensor47has reached a certain pressure, for example, the pressure occurring during ordinary operation, as shown in step S106inFIG. 4, the control portion50outputs a command to seal the hydrogen system as shown in step S107inFIG. 4. Due to this command, the hydrogen supply valve18and the gas discharge valve22are closed at time t2shown inFIG. 2. Due to this operation, the region46that includes the hydrogen supply pipe27on the fuel electrode side of the hydrogen supply valve18, the fuel-side portion of the fuel cell11, the hydrogen gas discharge pipe28, the hydrogen circulation pump26, the recirculation pipe29, and the gas discharge pipe45on the fuel electrode side of the gas discharge valve22assumes a sealed state. At this time, since the air compressor19has not been started, the oxidant electrode has not been supplied with air, that is, the oxidant gas. Therefore, the hydrogen in the sealed region46does not react with oxygen, so that the amount of hydrogen in the region46hardly decreases.

As shown inFIG. 3, although the region46shown inFIG. 1is sealed, the pressure slightly decreases from a pressure P0as shown by a dashed one-dotted line c inFIG. 3due to cross leak between the fuel electrode and the oxidant electrode of the fuel cell11. That is, as shown inFIG. 3, over the time interval Δt1between time t2and time t21, the pressure decreases from the initial pressure P0to an end pressure P0′ that occurs at the end of the interval, by ΔP0.

On the other hand, if there is leakage of hydrogen gas from the sealed hydrogen system, the pressure in the sealed region46shown inFIG. 1decreases by an amount ΔP1from the initial pressure P0at time t2to an end pressure P1at time t21. The pressure decrease ΔP1in the time interval Δt1from time t2to time t21is considerably larger than the pressure decrease ΔP0that occurs in the case where there is no leakage of hydrogen. The control portion50calculates a rate of pressure decrease that occurs in the case where there is no hydrogen leakage, from the time interval Δt1and the pressure decrease ΔP0, and stores the result of the calculation in a memory. Then, the control portion50determines the presence/absence of hydrogen leakage by comparing the rate of pressure decrease calculated from the pressure decrease ΔP1detected during the time interval Δt1with the stored rate of pressure decreased.

When the hydrogen system becomes sealed, the control portion50acquires the value of the initial pressure P0in the region46shown inFIG. 1which has been sealed, via the pressure sensor47as shown in step S108inFIG. 4, and then waits for the time interval Δt1that is a certain time shown inFIG. 3, as shown in step S109inFIG. 4. After that, the control portion50acquires the value of the pressure P1that occurs at the elapse of the time interval Δt1via the pressure sensor47, as the end pressure occurring at the end of the time interval, as shown in step S110inFIG. 4. Then, the control portion50calculates a rate of pressure decrease in the time interval Δt1as shown in step S111inFIG. 4, and then determines the presence/absence of hydrogen leakage by comparing the calculated rate of pressure decrease with the rate of pressure decrease that occurs in the case where there is no hydrogen leakage, as shown in step S112inFIG. 4.

If the control portion50determines that there is hydrogen leakage in step S112inFIG. 4, the control portion50then determines whether or not the determination of the presence of hydrogen leakage has been made for the first time, as shown in step S113inFIG. 4, in order to avoid the stop of the fuel cell system100caused by a false determination. Then, if the determination of the presence of hydrogen leakage is the first determination, the control portion50returns to step S108inFIG. 4, in which the control portion50acquires the value of the initial pressure again.

Since as shown inFIG. 3, it is after the first determination of the presence/absence of hydrogen that the value of the initial pressure is acquired again, the pressure P2of time t22after time t21shown inFIG. 3is acquired as the initial pressure. Then, the pressure P3occurring at a certain time interval Δt2following time t22is acquired as the end-interval pressure, and a rate of pressure decrease is calculated from a pressure difference ΔP2between the pressure P2and the pressure P3, and from the certain time Δt2. The presence/absence of hydrogen leakage is determined by comparing the calculated rate of pressure decrease with the rate of pressure decrease that occurs when there is no hydrogen leakage.

If it is determined in the second presence/absence determination regarding hydrogen leakage that there is hydrogen leakage, the control portion50stops the fuel cell system100as shown in step S114inFIG. 4.

On the other hand, if it is determined in the first or second determination of the presence/absence of hydrogen leakage that there is no hydrogen leakage, the control portion50closes the FC relays24at time t3inFIG. 2to connect the fuel cell11and a load system in step S115inFIG. 4, and then starts the air compressor19as shown in step S116inFIG. 4. As the air compressor19is started, the supply of air to the fuel cell11starts. As air begins to be supplied to the fuel cell11, the electrochemical reaction between the hydrogen and the oxygen begins in the air within the fuel cell11. Therefore, the FC voltage VFof the fuel cell11detected by the voltage sensor43gradually increases from the starting voltage, that is, zero, as shown by the line b inFIG. 2, and reaches the operation voltage V0at time t4shown inFIG. 2.

The control portion50, after determining that the FC voltage VFof the fuel cell11has reached the operation voltage V0as shown in step S117inFIG. 4, holds the state of the fuel cell system100for a stabilization time Δt from time t4to time t5shown inFIG. 2as shown in step S118inFIG. 4. Then, the control portion50completes the starting of the fuel cell system100at time t5shown inFIG. 2as shown in step S119inFIG. 4, and shifts to the ordinary operation.

In this embodiment, the presence/absence of leakage of hydrogen gas at the time of starting can be determined without a need to raise the FC voltage VFof the fuel cell11to the open-circuit voltage OCV. Therefore, the presence/absence of hydrogen leakage can be determined without impairing the durability of the fuel cell11.

Next, another example of the starting of the fuel cell system100of this embodiment will be described with reference toFIGS. 5 to 7. Portions shown inFIGS. 5 to 7that are substantially the same as those described above with reference toFIGS. 2 to 4are represented by the same reference characters, and descriptions thereof are omitted below. In this embodiment, the starting voltage VF0of the fuel cell11is equal to the open-circuit voltage OCV that is higher than the operation voltage V0.

As in the foregoing embodiment, the control portion50, after recognizing the turning-on of the ignition key30as shown in step S201inFIG. 7, closes the system relays25and then starts an operation of the step-up/down voltage converter13. Then, as shown in steps S202and S203inFIG. 7, the control portion50charges the secondary-side capacitor21to raise the secondary-side voltage VH, which is the output voltage of the step-up/down voltage converter13, to the open-circuit voltage OCV of the fuel cell11. Then, at time t12inFIG. 5, the secondary-side voltage VHreaches the open-circuit voltage OCV. After the secondary-side voltage VHreaches the open-circuit voltage OCV, it is possible to supply electric power from the secondary cell12to the traction motor15, and therefore the control portion50turns on the READY lamp at time t11. After that, the electric vehicle200can start moving as the driver depresses the accelerator pedal. However, at this time point, the FC relays24are open, and therefore the fuel cell11is disconnected from the system, so that electric power does not flow into the fuel cell11.

The control portion50acquires the value of the starting voltage VF0of the fuel cell11from the voltage sensor43, and compares it with the operation voltage V0, as shown in step S204inFIG. 7. As in the foregoing embodiment, the operation voltage V0is lower than the open-circuit voltage OCV. Then, if the starting voltage VF0is higher than the operation voltage V0, the control portion50closes the FC relays24as shown in step S205inFIG. 7. In this embodiment, the starting voltage VF0of the fuel cell11is equal to the open-circuit voltage OCV as shown inFIG. 5. After that, the control portion50lowers the secondary-side voltage VH, which is the output voltage of the step-up/down voltage converter13, from the open-circuit voltage OCV to the operation voltage V0, as shown by a line e inFIG. 5, in step S206inFIG. 7. Then, as the secondary-side voltage VHdecreases, the voltage VFof the fuel cell11decreases from the open-circuit voltage OCV, and an electric current AFis output from the fuel cell11as shown by a line f inFIG. 5.

Besides, if the starting voltage VF0of the fuel cell11is lower than the operation voltage V0, the control portion50jumps to step S105inFIG. 4described above, in which the control portion40starts the pressurization of the hydrogen system.

The control portion50outputs a command to pressurize the hydrogen system at time t12immediately following time t11inFIG. 5after the secondary-side voltage VHreaches the open-circuit voltage OCV. Due to this command, the hydrogen supply valve18is opened, so that hydrogen starts to be supplied from the hydrogen tank17to the fuel cell11. The FC voltage VFof the fuel cell11is kept at the operation voltage V0as is the case with the secondary-side voltage VH. Therefore, the fuel cell11continues outputting current after the voltage of the fuel cell11is lowered from the starting voltage VF0, which is equal to the open-circuit voltage OCV, to the operation voltage V0.

After determining that the pressure of the hydrogen system has reached a certain pressure, for example, an ordinary operation pressure, as shown in step S208inFIG. 7, the control portion50outputs a command to seal the hydrogen system as shown in step S209inFIG. 7. Due to this command, the hydrogen supply valve18and the gas discharge valve22shown inFIG. 1are closed at time t13shown inFIG. 5. Due to this, the region46, which includes the hydrogen supply pipe27on the fuel electrode side of the hydrogen supply valve18, the fuel-side portion of the fuel cell11, the hydrogen gas discharge pipe28, the hydrogen circulation pump26, the recirculation pipe29, and the gas discharge pipe45on the fuel electrode side of the gas discharge valve22, assumes a sealed state. At this time, since the air compressor19has not been started, the oxidant electrode has not been supplied with air, that is, the oxidant gas. However, since the fuel cell11is in a state in which the electric current produced by the electricity generation is output due to the voltage of the fuel cell11having been lowered from the open-circuit voltage OCV to the operation voltage V0, the hydrogen at the fuel electrode is consumed in the reaction with the oxygen contained in the air that remains at the oxidant electrode. Therefore, although the region46is sealed, the pressure of the hydrogen system decreases by an amount that corresponds to the output current.

The pressure in the sealed region46slightly decreases from the pressure P0to the pressure P0′as shown by a dashed one-dotted line g inFIG. 6, due to the cross leak between the fuel electrode and the oxidant electrode even in the case where no hydrogen is consumed within the fuel cell11. As shown inFIG. 6, during a time interval Δt3from time t13to time t13′ the pressure decreases by an amount ΔP10from the initial pressure P0to the end pressure P0′. The pressure decrease ΔP10can be estimated from the side of the fuel cell11, or the like. Therefore, the control portion50estimates the pressure decrease ΔP10, and stores the estimated value thereof beforehand in a memory.

Besides, in the case where the fuel cell11is outputting electric power as shown by a line f inFIG. 6although the hydrogen system is in the sealed state, the hydrogen in the sealed region46is consumed by the electricity generation in addition to the consumption of hydrogen caused by cross leak, and therefore the pressure in the region46decreases by an amount ΔP11from the initial pressure P0at time t13to the end pressure P11at time t13′ as shown by a dashed two-dotted line h inFIG. 6. However, the pressure decrease ΔP11′ caused by the consumption of hydrogen by power generation can be estimated by a computation performed within the control portion50through the use of the FC voltage VFof the fuel cell11detected by the voltage sensor43, and the output current AFof the fuel cell11detected by the electric current sensor44. The control portion50stores into the memory a pressure decrease ΔP11′ that is estimated from the FC voltage VFand the output current AF. The control portion50adds the stored pressure decrease ΔP11′ and the pressure decrease ΔP10caused by the cross leak that has been stored in the memory to calculate a pressure decrease ΔP11. And then, using the pressure decrease ΔP11and the time interval Δt3, the control portion50calculates a rate of pressure decrease (second rate of pressure decrease) occurring in the case where hydrogen is consumed due to both the cross leak and the power generation, and stores the calculated rate in the memory.

In the case where there is leakage of hydrogen gas from the sealed hydrogen system while the fuel cell11is outputting current, the pressure in the sealed region46shown inFIG. 1decreases by an amount ΔP12from the initial pressure P0at time t13to an end pressure P12at time t13′, as shown by a solid line j inFIG. 6. The pressure decrease ΔP12in the time interval Δt3from time t13to time t13′ is considerably larger than the pressure decrease ΔP11that occurs in the same time interval in the case where hydrogen is consumed due to cross leak and electricity generation. Then, the control portion50calculates a rate of pressure decrease (third rate of pressure decrease) for use for the determination of the presence/absence of hydrogen leakage by subtracting the rate of pressure decrease (second rate of pressure decrease) stored earlier in the memory which occurs when there is no hydrogen leakage but there is consumption of hydrogen due to cross leak and electricity generation, from the rate of pressure decrease (first rate of pressure decrease) that is calculated from the pressure decrease ΔP12detected in the time interval Δt3. Then, the control portion50compares the rate of pressure decrease (third rate of pressure decrease) for the determination regarding the presence/absence of hydrogen leakage, with a prescribed threshold value, to determine whether there is hydrogen leakage.

The control portion50, after sealing the hydrogen system as shown in step S209inFIG. 7, acquires the values of the initial pressure P0in the region46, the FC voltage VF, and the FC current AFas shown in step S210inFIG. 7. Then, the control portion50calculates the above-described first rate of pressure decrease as shown in step S213inFIG. 7, and calculates the second rate of pressure decrease and then calculates the third rate of pressure decrease as shown in step S214inFIG. 7. Then, the control portion50determines whether or not there is hydrogen leakage as shown in step S215inFIG. 7.

If it is also determined that there is hydrogen leakage, in the hydrogen leakage presence/absence determination in step S215inFIG. 7, the control portion50stops the fuel cell system100as shown in step S216inFIG. 7.

On other hand, if it is determined that there is no hydrogen leakage in the hydrogen leakage determination in step S215inFIG. 7, the control portion50starts the air compressor19at time t14inFIG. 5, as shown in step S217inFIG. 7. As the air compressor19is started, the supply of air to the fuel cell11starts. As air begins to be supplied to the fuel cell11, the electrochemical reaction between the hydrogen and the oxygen in the air begins within the fuel cell11. Therefore, the FC current AFof the fuel cell11detected by the electric current sensor44gradually rises as shown by the line f inFIG. 5.

After the FC current AFof the fuel cell11has increased, the control portion50holds the state of the fuel cell system100during the stabilization time from time t14to time t15shown inFIG. 5, as shown in step S218inFIG. 7, and then completes the starting of the fuel cell system100at time t15inFIG. 5, as shown in step S219inFIG. 7.

In this embodiment, at the time of starting the fuel cell11, the determination regarding the presence/absence of leakage of hydrogen gas can be performed after the FC voltage VFof the fuel cell11is lowered from the open-circuit voltage OCV to the operation voltage V0. Therefore, it is possible to determine whether there is hydrogen leakage, without impairing the durability of the fuel cell11.

In the foregoing embodiment, the rate of pressure decrease (third rate of pressure decrease) for use for the hydrogen leakage determination is calculated by subtracting the rate of pressure decrease (second rate of pressure decrease) stored earlier in the memory which occurs in the case where there is no hydrogen leakage but there is consumption of hydrogen due to electricity generation from the rate of pressure decrease (first rate of pressure decrease) that is calculated from the pressure decrease ΔP12that is detected in the time interval Δt3. Then, the rate of pressure decrease (third rate of pressure decrease) for the leakage determination is compared with the threshold value to determine whether or not there is hydrogen leakage. However, the determination regarding the presence/absence of hydrogen leakage may also be performed by comparing the rate of pressure decrease (first rate of pressure decrease) calculated from the pressure decrease ΔP12detected in the time interval Δt3with a second threshold value that is greater than the prescribed threshold value. In this case, the second threshold value may be a sum of the prescribed threshold value and the rate of pressure decrease (second rate of pressure decrease) that occurs when there is no hydrogen leakage but there is consumption of hydrogen due to electricity generation.

While the invention has been described with reference to example embodiments thereof, it is to be understood that the invention is not limited to the described embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the disclosed invention are shown in various example combinations and configurations, other combinations and configurations, including more, less or only a single element, are also within the scope of the appended claims.