Fuel cell power supply device

Disclosed is a fuel cell power supply device in which an I-V characteristics estimating unit estimates output current/voltage characteristics (I-V characteristics) of a fuel cell by substituting a linear function (V=F(I)) in which a gradient indicates an internal resistance of the fuel cell and an interception of an axis representing a voltage (V) indicates an open-circuit voltage of the fuel cell calculated by a fuel cell open-circuit voltage calculator for the I-V characteristics. On the basis of the I-V characteristics of the fuel cell estimated by the I-V characteristics estimating unit, a requested output voltage determining unit and a requested output current determining unit determine a requested output voltage and a requested output current, respectively, which are needed to obtain a target total electric energy.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fuel cell power supply device for controlling an amount of reactive gases supplied to a fuel cell depending on an electric energy requested by a load.

2. Description of the Related Art

There have heretofore been employed fuel cell power supply devices which use fuel cells as a power supply device for vehicles such as electric vehicles. An amount of reactive gases (a reducing gas such as hydrogen and an oxidizing gas for extracting electrons by reacting with the reducing gas) to be consumed varies depending on an amount of an output current of the fuel cell. Therefore, it is necessary to control a flow rate of reactive gases so that the amount of reactive gases supplied to the fuel cell is not excessive or insufficient with respect to an electric energy requested by a load such as an electric motor.

One conventional fuel cell power supply device incorporates a control arrangement shown inFIG. 5for determining a target output current (Ifc_CMD) for a fuel cell on a vehicle. First, a fuel cell vehicle control unit100calculates a requested electric energy (PD_CAL) of an electric motor101depending on an amount of depression (Ap) of an accelerator pedal and a vehicle speed (Nm).

An output limiting unit102calculates a target output (PD_REQ) to limit an upper limit of the requested electric energy (PD_CAL) depending on a capacity of a fuel cell103to generate an electric energy. A torque command calculator104calculates a torque command (TRQ_CMD) depending on the target output (PD_REQ) and then outputs the torque command to a motor driving unit105.

On the other hand, a P/I converter106applies the target output (PD_REQ) to map data107, stored in a memory, representing output current/voltage characteristics (I-V characteristics) of the fuel cell103to determine the target output current (Ifc_CMD) of the fuel cell103. A reactive gas supply unit108supplies the reactive gases depending on the target output current (Ifc_CMD) to the fuel cell103so that the fuel cell103outputs a current appropriate to the target output (PD_REQ).

By the above processing, a driving electric energy needed to obtain the target output (PD_REQ) is supplied from the motor driving unit105to the electric motor101, and the reactive gases needed to obtain the target output current (Ifc_CMD) depending on the target output (PD_REQ) are supplied from the reactive gas supply unit108to the fuel cell103.

However, in some cases, the I-V characteristics of the fuel cell103vary (shown by a line B inFIG. 5) from an initial state (shown by a line A inFIG. 5) depending on a change in temperature, supply pressure, and humidity of the reactive gases and a change of the fuel cell103with time. When the I-V characteristics of the fuel cell103vary, the target output current (Ifc_CMD) set for the target output (PD_REQ) is improper, so that the current is excessively or insufficiently outputted from the fuel cell103.

Therefore, the following processing is performed: A current (Ifc) and a voltage (Vfc) actually outputted from the fuel cell103are detected, a difference (ΔI) between the actual current (Ifc) and the target output current (Ifc_CMD) and a difference (ΔV) between the actual voltage (Vfc) and a target output voltage (Vfc_CMD) corresponding to the target output current (Ifc_CMD) in the I-V characteristics are calculated, and the I-V characteristics are corrected depending on the differences (ΔI, ΔV).

However, in the correction of the I-V characteristics as mentioned above, for example, as in the case of abrupt acceleration of the fuel cell vehicle, when a power of the electric motor101increases so as to exceed a response speed of the reactive gas supply unit108and the actual current (Ifc) and the actual voltage (Vfc) are insufficient with respect to the target output current (Ifc_CMD) and the target output voltage (Vfc_CMD) (in this case, the insufficient current is backed up by discharging an electric energy from a capacitor109), the calculated differences (ΔI, ΔV) are larger than differences based on the actual initial values of the I-V characteristics of the fuel cell due to a delayed response from the reactive gas supply unit108.

Accordingly, there are the following disadvantages: The I-V characteristics of the fuel cell103are corrected so as to be larger than the actual I-V characteristics. When the target output (PD_REQ) is applied to the corrected I-V characteristics, the target output current (Ifc_CMD) cannot be determined accurately.

SUMMARY OF THE INVENTION

The present invention is made in order to overcome the above disadvantages. It is an object of the present invention to provide a fuel cell power supply device which can accurately determine a requested output current of a fuel cell depending on an electric energy requested by a load even when output current/voltage characteristics of the fuel cell change.

According to the present invention, there is provided a fuel cell power supply device comprising a fuel cell, reactive gas supply means for supplying reactive gases to the fuel cell, supplied-amount regulating means for regulating an amount of reactive gases supplied from the reactive gas supply means to the fuel cell, requested output current determining means for determining a requested output current of the fuel cell depending on a requested electric energy of a load when the load is connected to the fuel cell and is then supplied with an electric energy, and gas supply control means for permitting the supplied-amount regulating means to control the amount of reactive gases supplied to the fuel cell so as to obtain the requested output current.

As a result of various investigations for the purpose of accomplishing the above object, the present inventors discovered that the output current/voltage characteristics of the fuel cell can relatively accurately be approximate to a linear function, in which a gradient indicates an internal resistance of the fuel cell and an intercept of an output voltage axis indicates an open-circuit voltage of the fuel cell, in a range where the fuel cell is ordinarily used.

According to the present invention, the fuel cell power supply device further comprises first storage means for storing data of an internal resistance of the fuel cell, fuel cell open-circuit voltage recognizing means for recognizing an open-circuit voltage of the fuel cell, and output characteristics estimating means for estimating the output current/voltage characteristics of the fuel cell by substituting a linear function in which a gradient indicates the internal resistance of the fuel cell and an intercept of the output voltage axis indicates the open-circuit voltage of the fuel cell for the output current/voltage characteristics. The requested output current determining means determines an output current of the fuel cell obtained by applying the requested electric energy of the load to the linear function as the requested output current.

According to the present invention, on the basis of the open-circuit voltage of the fuel cell recognized by the fuel cell open-circuit voltage recognizing means and data of the internal resistance of the fuel cell stored in the first storage means, the output characteristics estimating means estimates the output current/voltage characteristics of the fuel cell by substituting the linear function in which the gradient indicates the internal resistance of the fuel cell and the intercept of the output voltage axis indicates the open-circuit voltage of the fuel cell for the output current/voltage characteristics. Since the open-circuit voltage of the fuel cell changes depending on a change in actual output current/voltage characteristics of the fuel cell, the linear function, which is estimated as the output current/voltage characteristics of the fuel cell by the output characteristics estimating means, reflects the actual output current/voltage characteristics of the fuel cell. Therefore, even when the output current/voltage characteristics of the fuel cell change, the requested output current determining means can accurately determine the requested output current depending on the requested electric energy by applying the requested electric energy to the linear function.

The fuel cell power supply device further comprises fuel cell current detecting means for detecting an output current of the fuel cell. When the output current of the fuel cell obtained by applying the requested electric energy of the load to the linear function is smaller than a current detected by the fuel cell current detecting means, the requested output current determining means determines the detected current as the requested output current.

According to the present invention, when the requested electric energy of the load increases quickly, due to a time lag until the requested output current determined by the requested output current determining means changes depending on the increase, a current actually outputted from the fuel cell may become larger than the requested output current.

When the current (the actual output current of the fuel cell) detected by the fuel cell current detecting means is larger than the requested output current, the gas supply control means determines the detected current as the requested output current. The amount of reactive gases supplied to the fuel cell is thus increased quickly, preventing the fuel cell from running short of reactive gases.

The fuel cell power supply device further comprises an electric double layer capacitor connected parallel to the fuel cell, second storage means for storing data of an internal resistance of the electric double layer capacitor, capacitor open-circuit voltage recognizing means for recognizing an open-circuit voltage of the electric double layer capacitor, capacitor charged/discharged current recognizing means for dividing a difference between a requested output voltage corresponding to the requested output current in the linear function and the open-circuit voltage of the electric double layer capacitor by the internal resistance of the electric double layer capacitor to recognize a current charged into or discharged from the electric double layer capacitor when an output voltage of the electric double layer capacitor is equivalent to the requested output voltage, and requested output current correcting means for performing at least one of first correction to subtract a discharged current from the requested output current when the capacitor charged/discharged current recognizing means recognizes the current discharged from the electric double layer capacitor and second correction to add a charged current to the requested output current when the capacitor charged/discharged current recognizing means recognizes the current charged into the electric double layer capacitor.

According to the present invention, due to the first correction, the current discharged from the electric double layer capacitor is subtracted from the requested output current to reduce the amount of reactive gases supplied from the gas supply means as much as the discharged current, preventing the reactive gases from being supplied to the fuel cell excessively. Due to the second correction, the current charged into the electric double layer capacitor is added to the requested output current to increase the amount of reactive gases supplied from the gas supply means as much as the charged current, thus preventing the reactive gases from being supplied to the fuel cell insufficiently.

The fuel cell power supply device further comprises fuel cell current detecting means for detecting an output current of the fuel cell. When the requested output current subjected to the first correction or the second correction by the requested output current correcting means is smaller than a current detected by the fuel cell current detecting means, the requested output current determining means determines the detected current as the requested output current.

According to the present invention, in the case where the first correction or the second correction is performed, when the requested electric energy of the load increases quickly, it is possible to prevent the fuel cell from running short of reactive gases.

The fuel cell open-circuit voltage recognizing means recognizes the open-circuit voltage of the fuel cell every predetermined cycle. The output characteristics estimating means estimates the output current/voltage characteristics of the fuel cell every predetermined cycle by substituting the linear function for the characteristics.

According to the present invention, the output characteristics estimating means estimates the output current/voltage characteristics of the fuel cell every predetermined cycle using the linear function based on the open-circuit voltage of the fuel cell recognized every predetermined cycle by the fuel cell open-circuit voltage recognizing means. Therefore, the requested current determining means can accurately determine the requested output current using the linear function updated so as to reflect the latest actual output current/voltage characteristics of the fuel cell.

The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate a preferred embodiment of the present invention by way of example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will now be described with reference toFIGS. 1 to 4.FIG. 1is a block diagram showing a constitution of a fuel cell power supply device according to the present invention,FIG. 2is a block diagram showing a control arrangement of electric energy management means shown inFIG. 1,FIG. 3is an equivalent circuit diagram of the fuel cell power supply device, and FIG.4is a graph showing output current/voltage characteristics (hereinbelow, referred to as I-V characteristics) of a fuel cell stack.

Referring toFIG. 1, a fuel cell power supply device1according to the present invention is mounted on a vehicle and functions as a power supply for driving the vehicle. The fuel cell power supply device1is a hybrid fuel cell power supply device comprising a fuel cell stack2for outputting an electric current based on an electrochemical reaction between reactive gases of hydrogen and air, and an electric double layer capacitor3(hereinbelow, referred to as a capacitor3) connected parallel to the fuel cell stack2. An output electric energy produced by the fuel cell power supply device1is controlled by a controller4which comprises a microcomputer, a memory, and other components.

The output electric energy produced by the fuel cell power supply device1is supplied to a motor driver5, an air-conditioning unit6, and a 12-V load8through a DC/DC converter7. The motor driver5controls currents flowing through armatures of an electric motor10depending on a torque command (TRQ_CMD) outputted from the driver control unit9provided for the controller4. A drive power generated by the electric motor10is transferred to drive wheels12through a transmission11.

The driver control unit9outputs a signal indicative of a motor-requested electric energy (PD_REQ), which is required by the motor driver5, based on an amount of depression (Ap) of an accelerator pedal13and a rotational speed (Nm) of the electric motor10to the power supply management control unit14.

The power supply management control unit14is supplied with detected signals of a load current (Iload) and a load voltage (Vload) which are detected by a load sensor15. Based on the detected signals, the power supply management control unit14recognizes the electric energy consumed by electric accessories other than the electric motor10.

The power supply management control unit14takes into account an allowable output current (Ifc_LMT) outputted from the fuel cell control unit16and indicative of an upper limit for the current that can be supplied from the fuel cell stack2and a current (Icap) charged into or discharged from the capacitor3and a voltage (Vcap) across the capacitor3which are detected by a capacitor sensor31, determines a target output current (Ifc_CMD) which is a target value for a current outputted from the fuel cell stack2depending on the sum of the motor-requested electric energy (PD_REQ) and the electric energy consumed by the electric accessories other than the electric motor10, and outputs a signal indicative of the target output current (Ifc_CMD) to the fuel cell control unit16. The power supply management control unit14also outputs a signal indicative of an output limit electric energy (PLD) to the driver control unit9, the output limit electric energy (PLD) representing an upper limit for the electric energy that can be supplied from the fuel cell stack2.

The fuel cell control unit16is supplied with detected signals outputted from a reactive gas sensor20and indicating a pressure (Pgas), a flow rate (Qgas), and a temperature (Tgas) of reactive gases (hydrogen and air) supplied to the fuel cell stack2, and detected signals indicative of states (Vcell_indiv) of individual fuel cells (not shown) that make up the fuel cell stack2. The fuel cell control unit16determines the allowable output current (Ifc_LMT) in consideration of the state of the fuel cell stack2as recognized from these detected signals.

The driver control unit9outputs the torque command (TRQ_CMD) to the motor driver5so as not to exceed the output limit electric energy (PLD) indicated by the power supply management control unit14. The motor driver5controls the armature currents of the electric motor10to cause the electric motor10to generate torque depending on the torque command (TRQ_CMD).

The fuel cell control unit16(including a function as gas supply control means according to the present invention) outputs a signal indicative of a target amount of reactive gases (CMP_CMD) supplied to the fuel cell stack2to a reactive gas supply device21(corresponding to reactive gas supply means according to the present invention) so that the fuel cell stack2will output a current corresponding to the target output current (Ifc_CMD: corresponding to a requested output current according to the present invention) outputted from the power supply management control unit14.

Consequently, the fuel cell stack2is supplied with air and hydrogen at a flow rate according to the target output current (Ifc_CMD). The reactive gas supply device21has a mechanism (not shown) for regulating the flow rate at which the reactive gases are supplied. The regulating mechanism such as an air compressor corresponds to supplied-amount regulating means according to the present invention.

Hydrogen supplied from the reactive gas supply device21is supplied to hydrogen electrodes of the fuel cell stack2through an ejector (not shown) and a humidifier (not shown), and reacts electrically and chemically with oxygen in air supplied to air electrodes of the fuel cell stack2, producing water which is discharged through a discharge valve22. The opening of the discharge valve22is controlled by a control signal (VLV_CMD) supplied from the fuel cell control unit16in order to keep the pressure in the fuel cell stack2at a constant gradient depending on the pressures of the supplied air and hydrogen.

The fuel cell stack2has a water-cooled cooling unit (not shown). The fuel cell control unit16controls the flow rate and the temperature of cooling water supplied to the water-cooled cooling unit depending on the temperature of the cooling water supplied to the water-cooled cooling unit and the temperature of the cooling water discharged from the water-cooled cooling unit.

The fuel cell power supply device1also has a fuel cell sensor30(including a function as fuel cell current detecting means according to the present invention) for detecting an output current (Ifc) and an output voltage (Vfc) from the fuel cell stack2. Signals detected by the fuel cell sensor30are also supplied to the power supply management control unit14.

The fuel cell stack2and the capacitor3are fundamentally held in a directly coupling state except the start time and the stop time of the fuel cell stack2. In the directly coupling state, when the total electric energy consumed by the electric motor10and the electric accessories other than the electric motor10is increased to decrease the output voltage of the fuel cell stack2, a discharge current corresponding to a difference between an open-circuit voltage of the capacitor3and the output voltage of the fuel cell stack2is supplied to the electric motor10and the electric accessories other than the electric motor10. On the other hand, when the total electric energy consumed is decreased to increase the output voltage of the fuel cell stack2, a charge current corresponding to the difference between the open-circuit voltage of the capacitor3and the output voltage of the fuel cell stack2is supplied from the fuel cell stack2to the capacitor3.

Consequently, in both the above cases, the open-circuit voltage of the capacitor3becomes equivalent to the output voltage of the fuel cell stack2. Therefore, it is unnecessary to cause the output voltage of the fuel cell stack2to always match an open-circuit voltage of a battery by a large DC/DC converter which can switch a heavy current in a manner similar to a case where the battery, whose open-circuit voltage does not change very much even if the remaining amount of charging electric energy changes, is connected parallel to the fuel cell stack2.

Accordingly, a small switching device (not shown) may be provided in order to limit the passage of the electric current between the capacitor3and the fuel cell stack2at the start time and the stop time of the fuel cell stack2, at which the output current of the fuel cell stack2is small.

The constitution and the operation of the power supply management control unit14will now be described with reference toFIG. 2. The power supply management control unit14comprises a target total electric energy calculator50, a fuel cell open-circuit voltage calculator51(corresponding to fuel cell open-circuit voltage recognizing means according to the present invention), an I-V characteristics estimating unit52(corresponding to output characteristics estimating means according to the present invention), a requested output voltage determining unit53, a requested output current determining unit54(constituting requested output current determining means according to the present invention), a capacitor open-circuit voltage calculator55(corresponding to capacitor open-circuit voltage recognizing means according to the present invention), a capacitor assist current calculator56(corresponding to capacitor charged/discharged current recognizing means according to the present invention), a requested output current correcting unit57(corresponding to requested output current correcting means according to the present invention), and a target output current determining unit58(constituting requested output current determining means according to the present invention).

The target total electric energy calculator50adds a motor-requested electric energy (PD_REQ) to an electric energy consumed by the electric accessories, the electric energy being calculated by multiplying the load current (Iload) by the load voltage (Vload), to calculate a target total electric energy (Psys) serving as a total electric energy required for the operation of the fuel cell vehicle.

The capacitor open-circuit voltage calculator55deals with the capacitor3so as to replace the capacitor3with an equivalent circuit in which reference symbol Vcap_o denotes an open-circuit voltage and reference symbol Rcap denotes an internal resistance as shown inFIG. 3. The capacitor open-circuit voltage calculator55calculates the open-circuit voltage (Vcap_o) of the capacitor3on the basis of the output current (Icap) and an output voltage (Vout) of the capacitor3and data60indicating the internal resistance (Rcap) of the capacitor3stored in a memory (corresponding to second storage means according to the present invention) using the following equation (1).
Vcap_o=Vout+Icap×Rcap  (1)

The capacitor assist current calculator56calculates a capacitor assist current (Icap_AST) serving as a current charged into or discharged from the capacitor3when the output voltage (Vout) of the fuel cell stack2is equivalent to a requested output voltage (Vfc_REQ) depending on the target total electric energy (Psys) using the following equation (2).
Icap_AST=(Vcap_o=Vfc−REQ)/Rcap  (2)

The requested output current correcting unit57corrects a requested output current (Ifc_REQ) by subtracting the capacitor assist current (Icap_AST) from the requested output current (Ifc_REQ) depending on the target total electric energy (Psys). Accordingly, when the capacitor assist current (Icap_AST) denotes a positive value, namely, when a current is discharged from the capacitor3, the requested output current (Ifc_REQ) is reduced as much as the discharged current (the reduction corresponds to first correction according to the present invention), thereby preventing the reactive gases from being supplied to the fuel cell stack2excessively.

When the capacitor assist current (Icap_AST) denotes a negative value, namely, when a current is charged into the capacitor3, the requested output current (Ifc_REQ) is increased as much as the charged current (the increase corresponds to second correction according to the present invention), thereby preventing the reactive gases from being supplied to the fuel cell stack2insufficiently.

When a requested output current (Ifc_REQ′) corrected by the requested output current correcting unit57is equal to or smaller than the actual output current (Ifc) of the fuel cell stack2detected by the fuel cell sensor30, the target output current determining unit58outputs the corrected requested output current (Ifc_REQ′) as it is as the target output current (Ifc_CMD: corresponding to the requested output current according to the present invention).

On the other hand, when the actual output current (Ifc) of the fuel cell stack2detected by the fuel cell current sensor30is larger than the requested output current (Ifc_REQ′) corrected by the requested output current correcting unit57, the target output current determining unit58outputs the actual output current (Ifc) as the target output current (Ifc_CMD).

Accordingly, the target output current determining unit58prevents such a state that the target output current (Ifc_CMD) is smaller than the actual output current (Ifc) of the fuel cell stack2to cause a shortage of the reactive gases supplied to the fuel cell stack2.

In the initial state, the fuel cell stack2has I-V characteristics as shown by a line (1) inFIG. 4. In a graph ofFIG. 4, the axis of ordinate (V) denotes the output voltage of the fuel cell stack2and the axis of abscissa (I) denotes the output current thereof. If the I-V characteristics of the fuel cell stack2are always held to the line (1), data of the line (1) is previously stored to the memory and the target total electric energy (Psys) is applied to the I-V characteristics of the line (1), so that the requested output current (Ifc_REQ) and the requested output voltage (Vfc_REQ) of the fuel cell stack2which are needed to obtain the target total electric energy (Psys) can be determined.

However, in some cases, the I-V characteristics of the fuel cell stack2are actually deviated from those in the initial state due to a change in temperature, pressure, or humidity of the reactive gases supplied to the fuel cell stack2or a change of the fuel cell stack2with time. When the I-V characteristics of the fuel cell stack2are deviated from the initial values, there is a disadvantage in that the amount of reactive gases is excessive or insufficient with respect to the target total electric energy (Psys).

The following case is considered. For example, the line (1) represents the I-V characteristics of the fuel cell stack2. At a point P1, the fuel cell stack2is equilibrated with the capacitor3(in this state, the charge/discharge current of the capacitor3is equal to 0) with respect to the requested output current (Ifc_REQ) and the requested output voltage (Vfc_REQ) of the fuel cell stack2determined depending on the target total electric energy (Psys). The I-V characteristics of the fuel cell stack2are changed from the line (1) indicating the above state to a line (2).

In this case, the output voltage of the fuel cell stack2is momentarily equivalent to the requested output voltage (Vfc_REQ) by the output voltage of the capacitor3connected parallel to the fuel cell stack2, the output current of the fuel cell stack2is reduced to a value shown by reference symbol Ifc_1, and a current (Icap_1) corresponding to the amount of reduction with respect to the requested output (Ifc_REQ) is discharged from the capacitor3.

After that, the output voltage of the capacitor3(=the output voltage of the fuel cell stack2) is reduced to the requested output current (Ifc_REQ) by discharging and the fuel cell stack2is then equilibrated with the capacitor3at a point P3. An output voltage (V3) of the fuel cell stack2in the equilibrating state at the point P3is lower than an output voltage (V1) in the equilibrating state at the point P1. In this case, since the output current (Icap) of the capacitor3is equal to 0 in the equilibrating state, the open-circuit voltage (Vcap_o) of the capacitor3calculated by the capacitor open-circuit voltage calculator55using the foregoing equation (1) is equivalent to the output voltage (Vout) of the fuel cell stack2.

Accordingly, in the foregoing equation (2), Vcap_o<Vfc_REQ. The calculated capacitor assist current (Icap_AST) indicates a negative value. Therefore, the requested output current correcting unit57performs the correction to increase the requested output current (Ifc_REQ→Ifc_REQ′).

However, when the correction is performed as mentioned above, a difference between the requested output voltage (Vfc_REQ) and the output voltage (Vfc) of the fuel cell stack2increases from ΔVfc_1to ΔVfc_2. As a result, the correction to increase the requested output current (Ifc_REQ) is further performed, so that the reactive gases are excessively supplied to the fuel cell stack2. When the reactive gases are excessively supplied to the fuel cell stack2, the electric energy is wasted by the reactive gas supply device21(refer toFIG. 1) and the humidity in the fuel cell stack2decreases to dry an electrolytic membrane of the fuel cell stack2, resulting in a deterioration in performance of the fuel cell stack2.

On the other hand, when the I-V characteristics of the fuel cell stack2are deviated from the line (1) in the normal direction of the axis of voltage (V) (upward inFIG. 4), Vcap_o>Vfc_REQ, so that the calculated capacitor assist current (Icap_AST) denotes a positive value in the foregoing equation (2).

Therefore, the requested output current correcting unit57performs the correction to decrease the requested output current (Ifc_REQ). Actually, the charging current is supplied to the capacitor3in order to increase the output voltage of the capacitor3. Accordingly, the requested output current (Ifc_REQ) is insufficient with respect to the target total electric energy (Psys), resulting in a shortage of the reactive gases supplied to the fuel cell stack2.

In order to inhibit an effect of a change of the I-V characteristics of the fuel cell stack2and then determine the requested output current (Ifc_REQ) and the requested output voltage (Vfc_REQ), the I-V characteristics estimating unit52deals with the fuel cell stack2so as to replace the fuel cell stack2with an equivalent circuit having an open-circuit voltage (Vfc_o) and an internal resistance (Rfc) as shown inFIG. 3, thereby estimating the I-V characteristics of the fuel cell stack2.

In this instance, as shown inFIG. 4, the I-V characteristics of the fuel cell stack2are approximate to a line whose gradient is substantially fixed in a range where the fuel cell stack2is ordinarily used (I10to I20). Even if the I-V characteristics of the fuel cell stack2change due to a change with time, a change in gradient of the I-V characteristics is relatively small in this range.

The I-V characteristics estimating unit52estimates the I-V characteristics of the fuel cell stack2by substituting a linear function in which a gradient denotes the internal resistance (Rfc) in the initial state and an intercept of the voltage axis (V axis) denotes an open-circuit voltage (Vfc_o) of the fuel cell stack2and which is expressed by the following equation (3) for the I-V characteristics:

V=F⁡(I)=Rfc×I+Vf⁢⁢c_o(3)
where, reference symbol V denotes the output voltage of the fuel cell stack2and reference symbol I denotes the output current of the fuel cell stack2.

In this instance, on the basis of output current (Ifc) and the output voltage (Vout) of the fuel cell stack2and data61indicative of the internal resistance (Rfc) of the fuel cell stack2, stored in a memory (corresponding to first storage means according to the present invention), the fuel cell open-circuit voltage calculator51calculates the open-circuit voltage (Vfc_o) of the fuel cell stack2using the following equation (4).
Vfc_o=Vout+Ifc×Rfc  (4)

The fuel cell open-circuit voltage calculator51calculates the open-circuit voltage (Vfc_o) of the fuel cell stack2every predetermined control cycle (corresponding to a predetermined cycle according to the present invention) using the above equation (4). Based on the calculated open-circuit voltage (Vfc_o), the I-V characteristics estimating unit52estimates the I-V characteristics of the fuel cell stack2using the foregoing equation (3).

Accordingly, the I-V characteristics estimating unit52updates the linear function represented by the equation (3) by reflecting the actual change if the I-V characteristics of the fuel cell stack2every control cycle. Consequently, the I-V characteristics estimating unit52can accurately estimate the I-V characteristics of the fuel cell stack2.

On the basis of the I-V characteristics of the fuel cell stack2estimated by the I-V characteristics estimating unit52, the requested output voltage determining unit53can accurately determine the requested output voltage (Vfc_REQ) depending on the target total electric energy (Psys). On the basis of the I-V characteristics of the fuel cell stack2estimated by the I-V characteristics estimating unit52, the requested output current determining unit54can accurately determine the requested output voltage (Ifc_REQ) depending on the target total electric energy (Psys).

Accordingly, it is possible to inhibit the foregoing excessive or insufficient supply of reactive gases to the fuel cell stack2caused by the deviation of the actual I-V characteristics of the fuel cell stack2from the I-V characteristics of the fuel cell stack2used for the calculation of the requested output current (Ifc_REQ).

The present embodiment has described the fuel cell power supply device having the fuel cell stack2and the capacitor3connected parallel to the fuel cell stack2. According to the present invention, even if the device does not have the capacitor3, the I-V characteristics of the fuel cell stack2are estimated by substituting the linear function represented by the foregoing equation (3) for the I-V characteristics, so that the requested output current (Ifc_REQ) depending on the target total electric energy (Psys) can be determined accurately.

According to the present embodiment, the requested output current correcting unit57performs the correction to increase or decrease the requested output current (Ifc_REQ) using the capacitor assist current (Icap_AST). The advantages of the present invention are effective even when only the correction to increase the requested output current is performed, when only the correction to decrease the current is performed, or when the correction is not performed.

According to the present embodiment, the target output current determining unit58sets the target output current (Ifc_CMD) so as not to be equal to or lower than the actual output current (Ifc) of the fuel cell stack2. The advantages of the present invention are effective even when the present device does not have the target output current determining unit58.

Although a certain preferred embodiment of the present invention has been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.