Fuel cell system

A fuel cell system repeats first processing and second processing when the system is started. In the first processing, a control section controls an electric power distribution section so that electric power generated by a fuel cell stack is supplied to accessories and a secondary battery. In the second processing, the control section controls the electric power distribution section so that electric power generated by the fuel cell stack and electric power discharged from the secondary battery are supplied to the accessories. An electric power calculation means of the control section calculates electric power generation by the fuel cell stack and inputs an output command, representing electric power, into air compressor drive/control means. The electric power calculation means gradually changes the magnitude of electric power generation by the fuel cell stack, represented by the output command, at the time of transition between the first processing and the second processing.

This is a 371 national phase application of PCT/JP2007/070760 filed 18 Oct. 2007, claiming priority to Japanese Patent Application No. JP 2006-284724 filed 19 Oct. 2006, the contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a fuel cell system including a fuel cell that generates electric power via an electrochemical reaction between a fuel gas and an oxidizing gas, as well as an oxidizing gas supply section, a secondary battery, and an electric power distribution section.

BACKGROUND ART

A fuel cell system includes a fuel cell that generates electric power via an electrochemical reaction between a fuel gas such as a gas containing hydrogen and an oxidizing gas such as air, an oxidizing gas supply section (i.e., an air compressor or the like) for supplying the oxidizing gas to the fuel cell, and a secondary battery capable of being charged and discharged. When a fuel cell system is to be started in a low-temperature environment, power-generating performance of the fuel cell and output characteristic of the secondary battery may be degraded.

In consideration of this situation, the fuel cell system described in JP 2004-281219 A is devised to include a power distribution section that supplies electric power generated by a fuel cell to accessory devices required for power generation by the fuel cell and to a secondary battery for its charging, or that supplies electric power discharged by the secondary battery to the accessory devices; and a control section that performs warm-up control processing for warming up the fuel cell and the secondary battery by alternately performing first processing and second processing at system startup. In the first processing, the power distribution section is controlled to supply generated power of the fuel cell to the accessory devices and the secondary battery, and, in the second processing, the power distribution section is controlled to supply generated power of the fuel cell and discharged power from the secondary battery to the accessory devices.

In the fuel cell system described in JP 2004-281219 A, although it may be possible to warm up the fuel cell and the secondary battery quickly by alternately performing the first processing and the second processing, the possibility that overcharge and over-discharge may occur in the secondary battery as a result cannot be denied. For example, while the drive state of an oxidizing gas supply section composed of an air compressor or the like must be controlled in order to cause the fuel cell to generate power, at the time of transition between the first processing and the second processing, there are possibilities that a change in a target value of the rotational frequency of the oxidizing gas supply section cannot be accurately followed by a change in the actual rotational frequency. The reason for this is that a time lag is generated in a change in the actual rotational frequency of the oxidizing gas supply section with respect to a command signal from the control section regarding a change of rotational frequency of the oxidizing gas supply section. In other words, the change is effected with an error. A change of rotational frequency of the oxidizing gas supply section corresponds to a change in the power generation level of the fuel cell. Accordingly, when the power generation level of the fuel cell is lowered, an undershoot may occur, resulting in an excessive decrease in the actual power generation level with respect to the target power generation value. On the other hand, when the power generation level of the fuel cell is increased, an overshoot may occur, resulting in an excessive increase in the actual power generation level with respect to the target power generation value.

When an undershoot of the power generation level of the fuel cell occurs, during an increase in power discharge from the secondary battery for compensating the decrease in power generation of the fuel cell, an over-discharge exceeding an upper threshold of power discharge tends to occur. On the other hand, when an overshoot of the power generation level of the fuel cell occurs, during an increase in power charge of the secondary battery which is charged by receiving a supply of generated power, an overcharge exceeding an upper threshold of power charge tends to occur. Such occurrences of over-discharge and overcharge of the secondary battery cause early deterioration of the secondary battery, and are therefore undesirable. Over-discharge and overcharge tend to occur particularly when the secondary battery is discharged to a level close to an upper threshold of power discharge and when the secondary battery is charged to a level close to an upper threshold of power charge.

Under a low-temperature environment, performance of the secondary battery may be degraded not only at the time of system start-up but also during continuous power generation by the fuel cell. Accordingly, in such a case, it is desirable to similarly perform the warm-up control processing to warm up the secondary battery.

An object of the present invention is to reduce possibilities of occurrence of overcharge and over-discharge of a secondary battery in a fuel cell system when the secondary battery is warmed up by alternately performing the operation of supplying generated power from a fuel cell to the secondary battery and the operation of discharging the secondary battery.

DISCLOSURE OF THE INVENTION

A fuel cell system according to the present invention includes a fuel cell that generates electric power via an electrochemical reaction between a fuel gas and an oxidizing gas; an oxidizing gas supply section for supplying the oxidizing gas to the fuel cell; a secondary battery that performs charging and discharging of electric power; an electric power distribution section that supplies electric power generated by the fuel cell to an accessory device required for power generation by the fuel cell and to the secondary battery for its charging, or supplies electric power discharged by the secondary battery to at least one of the accessory device and a load; and a control section that performs warm-up control processing for warming up the secondary battery by alternately performing first processing and second processing, the first processing for controlling the electric power distribution section to supply generated power of the fuel cell to the accessory device and the secondary battery, and the second processing for controlling the electric power distribution section to supply at least discharged power from the secondary battery to at least one of the accessory device and the load. The control section includes a power generation calculation unit that calculates, when performing the warm-up control processing, a power generation level of the fuel cell based on a chargeable/dischargeable power level of the secondary battery; and a drive control unit for the oxidizing gas supply section, that controls a drive state of the oxidizing gas supply section based on the calculated value of power generation level of the fuel cell indicated in an output command from the power generation calculation unit. The power generation calculation unit gradually changes the magnitude of the power generation level of the fuel cell at a time of transition between the first processing and the second processing, the power generation level being indicated in the output command input into the drive control unit for the oxidizing gas supply section.

Preferably, the oxidizing gas supply section is an air compressor, and the drive control unit for the oxidizing gas supply section controls a rotational frequency of the air compressor based on the calculated value of power generation level of the fuel cell indicated in the output command from the power generation calculation unit.

In a fuel cell system according to the present invention, the power generation calculation unit gradually changes the magnitude of the power generation level of the fuel cell at a time of transition between the first processing and the second processing, the power generation level at the time of transition being indicated in the output command input into the drive control unit for the oxidizing gas supply section. Accordingly, at a time of transition between the first processing and the second processing, a change in a target value of the drive state of the oxidizing gas supply section can be accurately followed by a change in the actual drive state. Therefore, it is possible to reduce the likelihood of occurrence of an excessive decrease of the power generation level with respect to a target value when the power generation level of the fuel cell is decreased, and to reduce the likelihood of occurrence of an excessive increase of the power generation level with respect to a target value when the power generation level of the fuel cell is increased. As a result, possibilities of occurrence of over-discharge and overcharge of the secondary battery can be reduced even when the secondary battery is discharged at a level close to an upper threshold of power discharge and when the secondary battery is charged at a level close to an upper threshold of power charge.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

First Embodiment of the Invention

Embodiments related to the present invention are explained in detail below with reference to the drawings.FIGS. 1-4and7show a first embodiment of the present invention.FIG. 1is a block diagram showing a basic configuration of a fuel cell system10according to the present embodiment, andFIG. 2is a configuration diagram showing a detailed configuration of the same embodiment.

The fuel cell system10according to the present embodiment is for use by being mounted on a fuel cell vehicle, and includes a fuel cell stack12. This fuel cell stack12includes a fuel cell lamination structure formed by laminating a plurality of fuel cells, and further includes a current collector plate and an endplate provided at each of the two end portions of the fuel cell lamination structure located along the lamination direction. The fuel cell lamination structure, current collector plates, and endplates are clamped together by use of components such as tie rods and nuts. An insulator plate may be provided between the current collector plate and the endplate.

Although a detailed view of each fuel cell is not given herein, each fuel cell may include a membrane assembly formed by holding an electrolyte membrane between an anode electrode and a cathode electrode, and separators provided on both sides of the assembly. Hydrogen gas serving as the fuel gas can be supplied to the anode electrode, while air serving as the oxidizing gas can be supplied to the cathode electrode. Hydrogen ions generated at the anode electrode are moved through the electrolyte membrane to the cathode electrode, and subjected to electrochemical reaction with oxygen at the cathode electrode, thereby generating water. Further, electrons are moved from the anode electrode to the cathode electrode via an external circuit, thereby generating electromotive force.

Inside the fuel cell stack12, an internal coolant path (not shown) is provided near the separators. By allowing cooling water (i.e., a coolant) to flow in this internal coolant path, even when the temperature is increased due to heat generation accompanying power generation by the fuel cell stack12, excessive temperature increase is prevented.

In order to supply air (i.e., the oxidizing gas) to the fuel cell stack12, an oxidizing gas supply channel14is provided, and in addition, an air compressor16serving as the oxidizing gas supply section is provided at the upstream of gas of the oxidizing gas supply channel14. Air introduced from outside air into the air compressor16is pressurized by the air compressor16and subsequently humidified by means of a humidifier (not shown). The air compressor16is driven by a drive motor (not shown). The rotational frequency of the drive motor is controlled by an air compressor drive control unit20(FIG. 2) provided in a control section18implemented as an electronic control unit (ECU) or the like. The humidified air is supplied to a channel located on the cathode electrode side within the fuel cell stack12.

Air off-gas released after the air is supplied to the fuel cell stack12and subjected to the electrochemical reaction in each fuel cell is discharged from the fuel cell stack12through an oxidizing-gas-related discharge gas channel22. At some intermediate point in the oxidizing-gas-related discharge gas channel22, a pressure adjustment valve (not shown) is provided, and is controlled such that the supply pressure of the air delivered to the fuel cell stack12conforms to an appropriate pressure value in accordance with the operating state of the fuel cell stack12. To enable this control, a pressure detection value detected at the pressure adjustment valve is input into the control section18.

Further, in order to supply hydrogen gas (i.e., the fuel gas) to the fuel cell stack12, a fuel gas supply channel24is provided. In addition, upstream of gas of the fuel gas supply channel24, there is provided a hydrogen gas supply apparatus26serving as the fuel gas supply apparatus, which includes devices such as a high-pressure hydrogen tank and a reforming device for generating hydrogen via reforming reaction. Hydrogen gas delivered from the hydrogen gas supply apparatus26to the fuel gas supply channel24is supplied, via a pressure-reducing valve28(i.e., a pressure adjustment valve), to a channel located on the anode electrode side within the fuel cell stack12. The degree of opening of the pressure-reducing valve28is adjusted by controlling an actuator30via the control section18.

Hydrogen-related discharge gas released after the hydrogen gas is supplied to the channel on the anode electrode side in the fuel cell stack12and subjected to the electrochemical reaction is discharged from the fuel cell stack12and recirculated via a fuel gas circulation path32back to the fuel cell stack12. The fuel gas circulation path32includes a hydrogen gas circulation pump (not shown). Hydrogen-related discharge gas is subjected to pressure increase using the hydrogen gas circulation pump, subsequently combined with hydrogen gas introduced from the hydrogen gas supply apparatus26, and then again introduced into the fuel cell stack12.

A gas-liquid separator (not shown) is provided within the fuel gas circulation path32, and an upstream end of a fuel-gas-related discharge path34is connected to the gas-liquid separator. In other words, the fuel-gas-related discharge path34is branched off from the fuel gas circulation path32. A part of the hydrogen-related discharge gas delivered to the gas-liquid separator is introduced, together with moisture separated in the gas-liquid separator, into a diluter (not shown) through the fuel-gas-related discharge path34including a purge valve35, combined with the air off-gas delivered through the oxidizing-gas-related discharge gas channel22, and exhausted after the hydrogen concentration is lowered.

At some intermediate point in the fuel gas circulation path32, the fuel-gas-related discharge path34may be branched off at a point other than the gas-liquid separator, such as at a point between the fuel cell stack12and the gas-liquid separator, and hydrogen-related discharge gas delivered through this fuel-gas-related discharge path34may introduced into the diluter.

For state detection of the fuel cell stack12, there are provided a current sensor36for detecting the output current of the fuel cell stack12, a voltage sensor38for detecting the output voltage of the fuel cell stack12, and a temperature sensor40for detecting the temperature of the fuel cell stack12. When power is generated in the fuel cell stack12, detection signals from the respective sensors36,38,40are read by the control section18. Based on these detection signals, the control section18controls the rotational frequency of the rotational axis (i.e., the drive state) of the air compressor16, the pressure adjustment valve provided at an intermediate point of the oxidizing-gas-related discharge gas channel22, the pressure-reducing valve28, and the like which serve as the accessory devices, so as to attain hydrogen pressure and flow rate and air pressure and flow rate in accordance with a target power generation level.

Power generated in the fuel cell stack12is supplied to a power distribution section42. The power distribution section42is controlled by the control section18to supply the generated power of the fuel cell stack12to a load44and the accessory devices for their consumption, and to cause a secondary battery46that performs power charging and discharging to discharge power as necessary and to supply this discharged power to the load44and the accessory devices for their consumption. Further, the power distribution section42is controlled by the control section18to supply the generated power of the fuel cell stack12to the secondary battery46for its charging. State of charge (SOC) and temperature of the secondary battery46are detected by means of a secondary battery sensor48.

The load44shown inFIGS. 1 and 2is a travel motor used for vehicle travel. Further, an actual load in the fuel cell system10includes a heater50(FIG. 1) that can heat the fuel cell stack12and the secondary battery46. Accessory devices include, in addition to the air compressor16, devices necessary for causing the fuel cell stack12to generate power, such as a cooling water pump for circulating the cooling water in a cooling water path for cooling the fuel cell stack12, a radiator cooling fan provided in the cooling water path, an inverter provided in the power distribution section42, and the control section18.

The control section18executes a control program stored in a memory (not shown) in order to control the power generation state of the fuel cell stack12. Further, the control section18controls the power distribution section42so as to control power supplied to the load44and the accessory devices. The control section18also executes the warm-up control processing at the time of start-up of the fuel cell system10. In the warm-up control processing, the control section18executes a control program such that the fuel cell stack12generates power to heat itself, and the secondary battery46repeats charging and discharging operations a plurality of times to heat itself, thereby warming up the fuel cell stack12and the secondary battery46. As a result of this processing, stable power can be supplied from the fuel cell stack12and the secondary battery46to the load44.

More specifically, in the warm-up control processing, at the time of start-up of the fuel cell system10, the fuel cell stack12and the secondary battery46are warmed up by alternately performing, in the control section18, first processing that controls the power distribution section42to supply generated power of the fuel cell stack12to the accessory devices and the secondary battery46, and second processing that controls the power distribution section42to supply generated power of the fuel cell stack12and discharged power from the secondary battery46to the accessory devices. In order to perform this warm-up control processing, the control section18includes a power generation calculation unit52that calculates a power generation level of the fuel cell stack12based on a chargeable power level and a dischargeable power level (i.e., chargeable/dischargeable power level) of the secondary battery46, and the air compressor drive control unit20that controls the drive state of the air compressor based on the calculated value of power generation level of the fuel cell indicated in an output command from the power generation calculation unit52. The above-noted warm-up control processing is explained in further detail below by reference toFIG. 3.

FIG. 3is a flowchart showing the warm-up control processing for warming up the fuel cell stack12and the secondary battery46at the time of start-up of the fuel cell system10by causing the fuel cell stack12to generate power and by repeating charging and discharging of the secondary battery46. First, at the time of start-up of the fuel cell system10, in step S1, a judgment is made every elapse of a given time interval as to whether or not the temperature of the fuel cell stack12is equal to or higher than a given temperature. In other words, in step S1, the control section18(FIGS. 1 and 2) reads a detection signal from the temperature sensor40(FIG. 2) to determine the temperature of the fuel cell stack12, and judges whether or not the temperature of the fuel cell stack12is equal to or higher than the predetermined given temperature. Further, in step S1, in a case where sufficient time has elapsed from a stop of power generation in the fuel cell stack12, the control section18may read, instead of the detection signal from the temperature sensor40, a detection signal from an outside temperature sensor (not shown) to determine the outside temperature, and judge whether or not the outside temperature is equal to or higher than a predetermined given temperature. Alternatively, it is also possible to judge whether or not the temperature of the fuel cell stack12is equal to or higher than a given temperature by detecting the temperature of the cooling water for cooling the fuel cell stack12.

When it is judged in step S1ofFIG. 3that the temperature of the fuel cell stack12or the outside temperature is equal to or higher than a given temperature, warming up the fuel cell stack12and the secondary battery46is unnecessary. Accordingly, in that case, a shift is made to the normal operation mode in step S2, and the warm-up control processing is ended. On the other hand, when it is judged that the temperature of the fuel cell stack12or the outside temperature is lower than the given temperature, the processing proceeds to step S3.

In step S3, the control section18reads detection signals from the secondary battery sensor48(FIG. 2) to detect the temperature and SOC (i.e., the state) of the secondary battery46. Subsequently, in step S4ofFIG. 3, the power generation calculation unit52(FIG. 2) of the control section18calculates the chargeable power level and the dischargeable power level (i.e., the chargeable/dischargeable power level) of the secondary battery46based on the detected temperature and SOC values of the secondary battery46. The detected temperature and SOC values of the secondary battery46are used because the chargeable/dischargeable power level of the secondary battery46is influenced by the temperature and the SOC.

Subsequently, in step S5ofFIG. 3, the power generation calculation unit52(FIG. 2) calculates power consumption A1of start-up accessory devices (refer toFIG. 4). Power consumption A1of start-up accessory devices is the power consumption by the accessory devices required for causing the fuel cell stack12to generate power having a level that corresponds to the chargeable power level of the secondary battery46obtained in step S4. For example, power consumption A1of start-up accessory devices is calculated using data of a map showing the relationship between power level corresponding to the chargeable power level and power consumption by the accessory devices.

Next, in step S6ofFIG. 3, the power generation calculation unit52calculates, based on the power consumption A1of start-up accessory devices calculated in step S5and the chargeable power level of the secondary battery46calculated in step S4, a value of maximum power taken out A2from the fuel cell stack12(refer toFIG. 4), duration t1of continuous taking out at the maximum power taken out value A2(refer toFIG. 4), a value of minimum power taken out A3from the fuel cell stack12which is no greater than the power consumption A1of start-up accessory devices (refer toFIG. 4), and duration t2of continuous taking out at the minimum power taken out value A3(refer toFIG. 4). Specifically, a sum of the power consumption A1of start-up accessory devices calculated in step S5ofFIG. 3and the chargeable power level of the secondary battery46calculated in step S4equals the value of maximum power taken out A2from the fuel cell stack12.

In particular, the power generation calculation unit52(FIG. 2) calculates, as shown inFIG. 4(a), transition period t3which is a period after taking out of power from the fuel cell stack12at the maximum value A2for duration t1and before start of taking out of power from the fuel cell stack12at the minimum value A3, and transition period t4which is the period after taking out of power from the fuel cell stack12at the minimum value A3for duration t2and before start of taking out of power from the fuel cell stack12at the maximum value A2. As shown inFIG. 4(a), the power generation calculation unit52outputs a command for, subsequent to power taken out from the fuel cell stack12at the maximum value A2, gradually reducing, at a constant rate, the power taking out level to the minimum value A3within transition period t3. Further, the power generation calculation unit52outputs a command for, subsequent to power retrieval from the fuel cell stack12at the minimum value A3, gradually increasing, at a constant rate, the power retrieval level to the maximum value A2within transition period t4. Output commands indicating the values A2, A3of power taken out from the fuel cell stack12, durations t1, t2, and transition periods t3, t4calculated in the power generation calculation unit52are input into the air compressor drive control unit20(FIG. 2)of the control section18.

In accordance with the output commands, the air compressor drive control unit20first calculates the rotational frequency of the air compressor16necessary for attaining the maximum power taken out value A2, and controls the air compressor16to rotate at the calculated rotational frequency for duration t1. In other words, when it is judged in step S7ofFIG. 3that the warm-up control processing is at its initial point, the air compressor drive control unit20controls the rotational frequency of the air compressor16so that, in step S8, power is taken out from the fuel cell stack12at the maximum power retrieval value A2. In addition, hydrogen gas flow rate and pressure that correspond to the air flow rate and pressure attained at this rotational frequency are calculated in a pressure-reducing valve control unit of the control section18, and the degree of opening of the pressure-reducing valve28(FIG. 2) is controlled accordingly to a predetermined constant degree. Further, accompanying the above operations, the first processing is performed by the control section18to control the power distribution section42to supply, from among the generated power, power corresponding to the chargeable power level of the secondary battery46calculated in step S4, from the fuel cell stack12(FIG. 1) to the secondary battery46so as to charge the secondary battery46, and also to supply part of the generated power to the accessory devices.

Next, in step S9ofFIG. 3, the control section18judges whether or not the time during which power is taken out from the fuel cell stack12at the maximum value has lasted for duration t1(FIG. 4). When it is judged that this power taking out time has lasted for duration t1, subsequently, in step S10ofFIG. 3, the air compressor drive control unit20performs control for reducing the rotational frequency of the air compressor such that power taken out from the fuel cell stack12decreases at a constant rate within transition period t3, from the maximum value A2to the minimum value A3, as shown inFIG. 4(a). In accordance with this operation, the pressure-reducing valve control unit performs control for reducing the degree of opening of the pressure-reducing valve28within transition period t3. Accompanying this operation, as shown inFIG. 4(b), after the charging level of the secondary battery46is gradually decreased to reach zero, the discharging level of the secondary battery46is gradually increased, and the second processing is performed by the control section18to control the power distribution section42to supply the generated power of the fuel cell stack12and discharged power from the secondary battery46to the accessory devices such as the air compressor.

Next, in step S11ofFIG. 3, the air compressor drive control unit20controls the rotational frequency of the air compressor16such that power is taken out from the fuel cell stack12at the minimum power taken out value A3. In addition, the pressure-reducing valve control unit in the control section18controls the degree of opening of the pressure-reducing valve28to a predetermined constant degree corresponding to the rotational frequency of the air compressor16. Further, accompanying this operation, the control section18controls the power distribution section42to cause the secondary battery46to discharge power at the dischargeable power level calculated in step S4and to supply the discharged power to the accessory devices.

Next, in step S12ofFIG. 3, the control section18judges whether or not the time during which power is taken out from the fuel cell stack12at the minimum value A3has lasted for duration t2. When it is judged that this power taking out time has lasted for duration t2, the processing returns to step S1. At that point, a judgment is made as to whether or not the temperature of the fuel cell stack12or the outside temperature is equal to or higher than a given temperature, and, until it is judged that the given temperature has been reached and a shift is made to the normal operation mode in step S2, the processing from step S3to step S12is repeated. During the repetition, because the processing has already undergone durations t1, t2and transition period t3, the processing proceeds from step S7to S7a. In step S7a, the air compressor drive control unit20performs control for increasing the rotational frequency of the air compressor16such that power taken out from the fuel cell stack12increases at a constant rate within transition period t4, from the minimum value A3to the maximum value A2, as shown inFIG. 4(a). In accordance with this operation, the pressure-reducing valve control unit performs control for increasing the degree of opening of the pressure-reducing valve28within transition period t4. Further, accompanying this operation, as shown inFIG. 4(b), after the discharging level of the secondary battery46is gradually decreased to reach zero, the control section18controls the power distribution section42so as to gradually increase the charging level of the secondary battery46.

In the warm-up control processing as described above, by causing the fuel cell stack12to generate power and repeating the charging and discharging operations of the secondary battery46, temperatures of the fuel cell stack12and the secondary battery46are gradually increased, resulting in warming up the fuel cell stack12and the secondary battery46. Along with the increase of temperature of the secondary battery46, because the chargeable power level and the dischargeable power level of the secondary battery46become greater as shown inFIG. 4(b), the value A2of maximum power taken out from the fuel cell stack12that is indicated in an output command from the power generation calculation unit52becomes increased as shown inFIG. 4(a).

In contrast to the above-described warm-up control processing of the fuel cell system10according to the present embodiment, conventionally, warm-up control processing as shown inFIGS. 5 and 6has been devised. That is, conventionally, the power generation level indicated in an output command input from the power generation calculation unit52(refer toFIG. 2) to the air compressor drive control unit20has changed been over time between the maximum value A2and the minimum value A3in a stepwise manner as shown inFIG. 5(a). More specifically, conventionally, in a step corresponding to step S6of the present embodiment shown inFIG. 3, calculations of transition periods t3, t4are not conducted, and steps S7, S7a, and S10are omitted. When the power generation level of the fuel cell stack12(refer toFIGS. 1 and 2)indicated in the output command is changed over time in a stepwise manner, the charging and discharging power level of the secondary battery46is also changed over time in a substantially stepwise manner as shown inFIG. 5(b).

However, when the power generation level of the fuel cell stack12indicated in the output command is changed in a stepwise manner, the rotational frequency of the air compressor cannot be made to change in a manner that accurately follows the change in the output command.FIG. 6(b) shows a change over time of the rotational frequency of the air compressor16(refer toFIGS. 1 and 2)that occurs when the power level indicated in the output command from the power generation calculation unit52(refer toFIG. 2) is decreased abruptly in a stepwise manner from the maximum value A2to the minimum value A3. InFIG. 6(b), the dot-and-dash line indicates the target rotational frequency value of the air compressor16, while the solid line indicates the actual measured rotational frequency value of the air compressor16. As shown inFIG. 6(b), even when the power generation level indicated in the output command is changed in a stepwise manner, a time delay occurs in the change of the rotational frequency of the air compressor16, resulting in a gradual decrease in the rotational frequency. However, the rate of decrease in rotational frequency (i.e., the rate by which the rotational frequency decreases) is high, and an undershoot in which the actual measured rotational frequency value of the air compressor16(depicted in a solid line) becomes much lower than the target value (depicted in a dot-and-dash line) tends to occur, as shown at portion X inFIG. 6(b).

Meanwhile,FIG. 6(a) shows a change over time, corresponding toFIG. 6(b), of the power charging and discharging level when the state of the secondary battery46(refer toFIGS. 1 and 2)is shifted from a charging state to a discharging state. InFIG. 6(a), the dot-and-dash line indicates the target charging and discharging value of the secondary battery46, while the solid line indicates the actual measured charging and discharging value of the secondary battery46. As shown inFIG. 6(a), when the state of the secondary battery46is shifted from a charging state to a discharging state, accompanying the occurrence of the undershoot of the rotational frequency of the air compressor16, there may occur an overshoot (portion Y inFIG. 6(a)) in which the actual measured discharging value of the secondary battery46(depicted in a solid line) becomes greatly deviated from the target value (depicted in a dot-and-dash line) and exceeds an upper threshold of discharging level. In other words, an over-discharge may occur in the secondary battery46.

Further, although not shown in the drawings, when the rotational frequency of the air compressor16is to be increased from a predetermined low value to a predetermined high value, there may occur an overshoot in which the actual measured rotational frequency value of the air compressor16greatly exceeds the target value, and the actual measured charging value of the secondary battery46may become greatly deviated from the target value and exceed an upper threshold of charging level, possibly resulting in an overcharge of the secondary battery46. Over-discharge and overcharge of the secondary battery46tend to occur particularly when the secondary battery is discharged to a level close to the upper threshold of discharging level and when the secondary battery is charged to a level close to an upper threshold of charging level.

In a case where the amount of change in the power generation level of the fuel cell stack12indicated in an output command from the power generation calculation unit52is small, it may be possible to reduce the likelihood of occurrence of undershoot and overshoot when the rotational frequency of the air compressor16is changed during execution of a warm-up control processing in a conventional fuel cell system. However, as the amount of change in the power generation level of the fuel cell stack12indicated in the output command becomes larger, undershoot and overshoot tend to occur when the rotational frequency of the air compressor16is changed during execution of warm-up control processing of a conventional fuel cell system.

The present embodiment is devised for eliminating inconveniences that occur during execution of warm-up control processing in such a conventional fuel cell system. In the present embodiment as described above, the power generation calculation unit52gradually changes the calculated power generation value of the fuel cell stack12at the time of transition between the first processing and the second processing; i.e., at the time of transition in power retrieval from the fuel cell stack12between the maximum power retrieval level A2and the minimum power retrieval level A3as shown inFIG. 4(a) noted above, the calculated power generation value being indicated in an output command. In other words, the magnitude of the calculated value is either gradually decreased at a constant rate or gradually increased at a constant rate. Accordingly, even at the time of reducing the rotational frequency of the air compressor16from a predetermined high value to a predetermined low value during a transition between the first processing and the second processing, a change in a target value of rotational frequency (i.e., drive state) of the air compressor16can be accurately followed by a change in the actual rotational frequency.

FIGS. 7(a) and7(b) are diagrams for explaining the advantages of the present embodiment in further detail, and show a change over time of the power discharging level of the secondary battery46and the rotational frequency of the air compressor16by indicating target values and actual measured values. InFIG. 7(a), the dot-and-dash line indicates the target discharging and charging value of the secondary battery46, while the solid line indicates the actual measured discharging and charging value of the secondary battery46. InFIG. 7(b), the dot-and-dash line indicates the target rotational frequency value of the air compressor16, while the solid line indicates the actual measured rotational frequency value of the air compressor16. As is clear from a comparison between the measured results inFIGS. 7(b) and6(b), in the case of the present embodiment shown inFIG. 7(b), the rate of change over time of the rotational frequency of the air compressor16becomes reduced. In other words, the change in the rotational frequency can be made gentle. Further, in the present embodiment shown inFIG. 7(b), a change in the target rotational frequency value (depicted in a dot-and-dash line) of the air compressor16can be accurately followed by a change in the actual rotational frequency (depicted in a solid line). Therefore, in contrast to the case shown inFIG. 6(b), it is possible to prevent occurrence of an undershoot in which the actual measured rotational frequency value (depicted in a solid line inFIG. 7(b)) of the air compressor16becomes much lower than the target value (depicted in a dot-and-dash line inFIG. 7(b)). Accordingly, it can be understood that, at the time of reducing the power generation level of the fuel cell stack12and causing the secondary battery46to discharge power, it is possible to reduce the likelihood of occurrence of an undershoot in which the actual power generation level of the fuel cell stack12becomes excessively lower than the target value.

As a result, as shown inFIG. 7(a), when the state of the secondary battery46is being shifted from a charging state to a discharging state, it is possible to prevent the actual discharging value (depicted in a solid line inFIG. 7(a)) of the secondary battery46from greatly deviating from the target value (depicted in a dot-and-dash line inFIG. 7(a)) and exceeding an upper threshold of discharging level, such that occurrence of an over-discharge in the secondary battery46can be prevented.

Further, although not shown in the drawings, according to the present embodiment, when the rotational frequency of the air compressor16is to be increased from a predetermined low value to a predetermined high value, it is also possible to prevent occurrence of an overshoot in which the actual measured rotational frequency value of the air compressor16greatly exceeds the target value, such that the actual measured charging value of the secondary battery46can be prevented from greatly deviating from the target value and exceeding an upper threshold of charging level, thereby preventing occurrence of an overcharge of the secondary battery46. As a result, even at the time of discharging the secondary battery to a level close to the upper threshold of discharging level and at the time of charging the secondary battery to a level close to an upper threshold of charging level, possibilities of occurrence of over-discharge and overcharge of the secondary battery46can be reduced.

Moreover, according to the present embodiment, even when the amount of change in the power generation level of the fuel cell stack12indicated in an output command from the power generation calculation unit52is large, it is possible to reduce the likelihood of occurrence of undershoot and overshoot at the time of changing the rotational frequency of the air compressor16. Accordingly, possibilities of occurrence of over-discharge and overcharge of the secondary battery46can be reduced.

In the present embodiment shown inFIG. 3noted above, in step S5, the calculated power consumption A1of start-up accessory devices may be employed as the basic power consumption value of start-up accessory devices, and a value obtained by adding power consumption of the accessory devices to the basic power consumption value of start-up accessory devices may be calculated as the corrected power consumption value of start-up accessory devices. Further, in step S6, a sum of the chargeable power level of the secondary battery46calculated in step S4and the corrected power consumption value of start-up accessory devices calculated in step S5can be calculated as the value of maximum power taken out A2from the fuel cell stack12.

When the value of maximum power taken out A2from the fuel cell stack12is calculated in this manner using the corrected power consumption value of start-up accessory devices, because the corrected power consumption value of start-up accessory devices is higher than the minimum required power consumption for causing the fuel cell stack12to generate power, the maximum power output value of the fuel cell stack12can be set to a value higher than in the normal operation mode to thereby increase the amount of power generated by the fuel cell stack12, thereby enabling an increase in the amount of heat generated by the fuel cell stack12.

In the control section18, in addition to the control for repeating the charging of the secondary battery46and the discharging from the secondary battery46, it is possible to also perform control for supplying one or both of the generated power of the fuel cell stack12and the discharged power from the secondary battery46to the heater50(FIG. 1) and, by means of heat generation by the heater50, enhancing warm-up of the fuel cell stack12and the secondary battery46.

Second Embodiment of the Invention

FIG. 8is a flowchart that corresponds toFIG. 3, and shows warm-up control processing for warming up a secondary battery at the time of start-up of a fuel cell system according to a second embodiment of the present invention. In the case of the above-described first embodiment, in step S1ofFIG. 3, the control section18(FIGS. 1 and 2) judges whether or not the temperature of the fuel cell stack12(FIGS. 1 and 2) is equal to or higher than a given temperature, and, depending on the judged result, the processing proceeds to either a shift to normal operation in step S2(FIG. 3) or to the warm-up control processing from step S3(FIG. 3) through step S12(FIG. 3). In contrast, according to the present embodiment, whether to shift to normal operation or to proceed with the warm-up control processing is selected depending on whether the detected temperature of the secondary battery46(FIGS. 1 and 2) is equal to or higher than a given temperature. This procedure is explained below in detail. In the following explanation, elements constituting the portions corresponding to the labeled elements inFIGS. 1 and 2are denoted by the same reference numerals.

As shown inFIG. 8, in the present embodiment, at the start-up of the fuel cell system10, in step S1a, the control section18reads a detection signal from the secondary battery sensor48every elapse of a given time interval, to thereby determine the temperature of the secondary battery46. Subsequently, in step S1b, the control section18judges whether or not the temperature of the secondary battery46is equal to or higher than a given temperature arbitrarily determined in advance. In step S1a, in a case where sufficient time has elapsed from a stop of operation of the fuel cell system10, the control section18may read, instead of the detection signal from the secondary battery sensor48, a detection signal from an outside temperature sensor (not shown) to determine the outside temperature, and judge whether or not the outside temperature is equal to or higher than a predetermined given temperature.

When it is judged in step S1bofFIG. 8that the temperature of the secondary battery46or the outside temperature is equal to or higher than a given temperature, warming up the secondary battery46is unnecessary. Accordingly, in that case, a shift is made to the normal operation mode in step S2, and the warm-up control processing is ended. On the other hand, when it is judged that the temperature of the secondary battery46or the outside temperature is lower than the given temperature, the processing proceeds to step S3′.

In step S3′, the control section18reads a detection signal from the secondary battery sensor48to detect not only the temperature of the secondary battery46but also the SOC of the secondary battery46. Subsequently, in step S4through step S12ofFIG. 3, the warm-up control processing is executed similarly to that in the above-described first embodiment, and processing returns to step S1a.

In the present embodiment, as in the above-described first embodiment, the power generation calculation unit52gradually changes the calculated power generation value of the fuel cell stack12at the time of transition between the first processing and the second processing; i.e., at the time of transition in power taken out from the fuel cell stack12between the maximum power taken out level A2and the minimum power taken out level A3as shown inFIG. 4(a) noted above, the calculated power generation value being indicated in an output command. In other words, the magnitude of the calculated value is either gradually decreased at a constant rate or gradually increased at a constant rate. Accordingly, even at the time of reducing the rotational frequency of the air compressor16from a predetermined high value to a predetermined low value, which is during a transition between the first processing (for controlling the power distribution section42to supply generated power of the fuel cell stack12to the accessory devices and the secondary battery46) and the second processing (for controlling the power distribution section42to supply generated power of the fuel cell stack12and discharged power of the secondary battery46to the accessory devices), a change in a target value of rotational frequency (i.e., drive state) of the air compressor16can be accurately followed by a change in the actual rotational frequency. Further, in turn, when the rotational frequency of the air compressor16is to be increased from a predetermined low value to a predetermined high value, a change in a target value of rotational frequency (i.e., drive state) of the air compressor16can similarly be accurately followed by a change in the actual rotational frequency. As a result, even at the time of discharging the secondary battery to a level close to the upper threshold of discharging level and at the time of charging the secondary battery to a level close to an upper threshold of charging level, possibilities of occurrence of over-discharge and overcharge of the secondary battery46can be reduced. In view that other configurations and achieved effects are similar to those of the first embodiment, explanations and drawings thereof will not be repeated.

Although the above embodiments are explained by reference to the case of performing warm-up control processing at the time of start-up of a fuel cell system10, the present invention is not limited to such a case. For example, performance of the secondary battery46may become degraded when the fuel cell system10is under a low-temperature environment in situations other than the start-up, such as when the temperature of the fuel cell stack12or the secondary battery46becomes lower than a predetermined temperature during traveling or idling of a fuel cell vehicle incorporating the fuel cell system10. The present invention may be applied to a configuration for executing warm-up control processing to warm up the secondary battery46in such instances. Further, it is also possible to apply the present invention to a configuration in which, when performing the second processing of the warm-up control processing, the fuel cell stack12is not caused to generate power, and only the power obtained by the discharging operation of the secondary battery46is supplied to at least one of the accessory devices and the load44(refer toFIGS. 1 and 2) in order to warm up the secondary battery46.

INDUSTRIAL APPLICABILITY

The present invention is employed in a fuel cell system. For example, the present invention is employed in a fuel cell system that is used by being mounted on a fuel cell vehicle.