Fuel cell system

This is a 371 national phase application of PCT/JP2009/061222 filed Jun. 19, 2009, the content of which is incorporated herein by reference in its entirety.

The present invention relates to a fuel cell system which performs, when it is started at a temperature below zero, a warm-up operation by reducing a supply of oxidant gas to a fuel cell.

Among fuel cells generating electric power utilizing an electrochemical reaction between hydrogen and oxygen, polymer electrolyte fuel cells are known. A polymer electrolyte fuel cell of this type has a stack which is constituted by a plurality of stacked cells. Each cell constituting the stack has an anode (fuel electrode) and a cathode (air electrode), and a solid polymer electrolyte membrane having a sulfonic acid group as an ion exchange group is disposed between the anode and the cathode.

A fuel gas (hydrogen-enriched reformed hydrogen obtained by reforming hydrogen gas or hydrocarbon) is supplied to the anode, while an oxidant gas (e.g., air) that contains oxygen as an oxidant is supplied to the cathode. Upon the supply of the fuel gas to the anode, hydrogen contained in the fuel gas reacts with a catalyst in a catalyst layer of the anode, resulting in the generation of hydrogen ions. The generated hydrogen ions pass through the solid polymer electrolyte membrane and electrically react with oxygen in the cathode. Through this electrochemical reaction, electric power is generated.

In a fuel cell system that utilizes a polymer electrolyte fuel cell as a power source, if the system stops the operation, the temperature of the fuel cell decreases, and the water within the fuel cell, which has been in hot and humid conditions until then, would condense to form dew drops, or freeze. In particular, when the temperature of the fuel cell is below zero, the water generated through the power-generation reaction freezes on the surface of the electrode, which would interfere with the supply of oxygen and inhibit the power-generation reaction.

So, when starting the system at a temperature below zero, a warm-up operation is performed in which an amount of oxidant gas supplied to the fuel cell is reduced to increase an amount of heat generation (see Patent Document 1 below). Also, in order to reduce problems that would occur when the fuel cell has a negative voltage with insufficient hydrogen gas during such a warm-up operation, a fuel cell system as described in Patent Document 2 below has been proposed.

In the technique described in Patent Document 2, a fuel cell system is controlled using a flowchart illustrated in FIG. 2 of the document. According to the flowchart in FIG. 2 of Patent Document 2, a warm-up operation is performed if the fuel cell is at a temperature of 0° C. or lower, and stopped if the temperature goes above 0° C.

Patent Documents

Patent Document 1: Japanese laid-open patent publication No. 2004-30979

Patent Document 2: Japanese laid-open patent publication No. 2008-198439

Problem to be Solved by the Invention

A fuel cell (fuel cell stack) has a large heat capacity, and it takes time to warm up all portions of the fuel cell (fuel cell stack). Also, even if the portion for which the temperature was measured has a temperature exceeding 0° C., the possibility that ice remains in a gas flow path and causes clogging there cannot be ignored, and in view of this, it would not always be appropriate to stop a warm-up operation simply based on the fact that the temperature of the fuel cell is above 0° C.

The present invention has been made considering the problems described above, and an object of the invention is to provide a fuel cell system that performs, when it is started at a temperature below zero, a warm-up operation by reducing a supply of oxidant gas to a fuel cell, the system being capable of accurately finding whether a gas flow path is in a clogged state or not so that the start and stop of the warm-up operation can be appropriately judged.

Means for Solving the Problem

In order to achieve the above object, the invention provides a fuel cell system that performs a warm-up operation by reducing an amount of oxidant gas supplied to a fuel cell, the system having: a fuel cell; and a control unit that regulates amounts of oxidant gas and fuel gas supplied to the fuel cell and controls a power-generation state of the fuel cell, wherein, during the warm-up operation with a reduced supply of oxidant gas to the fuel cell, the control unit varies a voltage of the fuel cell for a short period of time to obtain current-voltage characteristics which indicate a relationship of an output voltage and an output current of the fuel cell, and wherein the control unit calculates an effective catalyst area of the fuel cell based on the obtained current-voltage characteristics and determines whether the warm-up operation of the fuel cell can be stopped or not based on the calculated effective catalyst area.

According to the invention, while a warm-up operation is being performed by reducing an amount of oxidant gas supplied to the fuel cell, the voltage of the fuel cell is varied for a short period of time; in other words, a current sweep is performed for the fuel cell for a short period of time. When performing a current sweep for the fuel cell for a short period of time, an output current increases together with a decrease of voltage because an oxide coating peels off the catalyst, but after that, the output current decreases due to the oxidation of the catalyst, and by increasing the voltage thereafter, the fuel cell returns to the voltage-current relationship at the point it originated. Since the thus obtained current-voltage characteristics are almost the same as the cyclic voltammetry (CV) curve of the unit cell constituting the fuel cell, an area corresponding to an area of oxidation current in the CV curve can be obtained from the obtained current-voltage characteristics, and the effective catalyst area can consequently be obtained. Accordingly, in this invention, the effective catalyst area of the fuel cell is calculated by varying the voltage of the fuel cell for a short period of time, and the calculated effective catalyst area is used to accurately determine whether the gas flow path of the fuel cell is clogged or not, thereby determining whether the warm-up operation of the fuel cell can be stopped or not.

In the fuel cell system according to the invention, it is preferable that the control unit determines, based on the calculated effective catalyst area, whether circulation in an anode circulation system is allowed or not, the anode circulation system being a system for supplying a fuel gas to the fuel cell in a circulating manner. Since whether the gas flow path of the fuel cell is clogged or not can accurately be determined by calculating the effective catalyst area, the circulation in the anode circulation system can be stopped when the flow path in the anode circulation system is considered to be frozen. It is considered that the clogging in the gas flow path of the fuel cell is due to freezing, and thus, further clogging can be suppressed by stopping the circulation in the anode circulation system in the above manner.

In the fuel cell system according to the invention, it is also preferable that the control unit determines, based on the calculated effective catalyst area, whether circulation in a cooling system for the fuel cell is allowed or not. Even in the case where the circulation in the cooling system is stopped to perform the warm-up operation more efficiently, since whether the gas flow path of the fuel cell is clogged or not can accurately be determined by calculating the effective catalyst area, the circulation in the cooling system can be started if the effective catalyst area becomes equal to or greater than a predetermined value.

Effect of the Invention

The invention can provide a fuel cell system that can accurately find whether a gas flow path is in a clogged state or not and can thus appropriately judge the start and stop of a warm-up operation.

FIG. 1 is a diagram illustrating the configuration of a fuel cell system to be installed in a fuel cell vehicle according to an embodiment of the invention.

FIG. 2 is a flowchart for determining whether a warm-up operation is allowed to be stopped or not in the fuel cell system shown in FIG. 1 .

FIG. 3 is a chart indicating one example of the current-voltage characteristics of a fuel cell obtained based on the flowchart shown in FIG. 2 .

FIG. 4 is a chart showing one example of the CV curve of a unit cell which constitutes a fuel cell.

FIG. 5 is a flowchart for determining whether the circulation in a fuel gas supply system is allowed or not in the fuel cell system shown in FIG. 1 .

FIG. 6 is a flowchart for determining whether the circulation in a cooling system is allowed or not in the fuel cell system shown in FIG. 1 .

Embodiments of the invention will be described below with reference to the attached drawings. In order that the description can easily be understood, the same components are given the same reference numerals to the extent possible in the respective drawings, and any repetitive description will be omitted.

First, a fuel cell system FCS to be installed in a fuel cell vehicle according to an embodiment of the invention will be described with reference to FIG. 1 . FIG. 1 is a diagram showing the system configuration of a fuel cell system FCS that functions as an on-vehicle power source system of a fuel cell vehicle. The fuel cell system FCS can be installed in vehicles such as fuel cell cars (FCHV), electric cars or hybrid cars.

The fuel cell system FCS has: a fuel cell FC; an oxidant gas supply system ASS; a fuel gas supply system FSS (anode circulation system); an electric power system ES; a cooling system CS; and a controller EC. The fuel cell FC receives the supply of reactant gases (fuel gas, oxidant gas) and generates electric power. The oxidant gas supply system ASS is a system for supplying air as an oxidant gas to the fuel cell FC. The fuel gas supply system FSS is a system for supplying hydrogen gas as a fuel gas to the fuel cell FC. The electric power system ES is a system for controlling electric power charge/discharge. The cooling system CS is a system for cooling the fuel cell FC. The controller EC is a controller for the overall control of the entire fuel cell system FCS.

The fuel cell FC is structured as a solid polymer electrolyte type cell stack formed of a number of cells (each unit cell having (as a power generator) an anode, a cathode and electrolyte) stacked in series. In the fuel cell FC under a normal operation, the oxidation reaction shown by formula (1) occurs in the anode and the reduction reaction shown by formula (2) occurs in the cathode, and in the fuel cell FC as a whole, the electrogenic reaction shown by formula (3) occurs.

H 2 →2H + +2 e −   (1)

(½)O 2 +2H + +2 e − →H 2 O  (2)

H 2 +(½)O 2 →H 2 O  (3)

The oxidant gas supply system ASS has an oxidant gas flow path AS 3 and an oxidant-off gas flow path AS 4 . The oxidant gas flow path AS 3 is a flow path through which the oxidant gas to be supplied to the cathode of the fuel cell FC flows. The oxidant-off gas flow path AS 4 is a flow path through which the oxidant-off gas discharged from the fuel cell FC flows.

The oxidant gas flow path AS 3 is provided with an air compressor AS 2 and a humidifier AS 5 . The air compressor AS 2 is a compressor for introducing the oxidant gas from the atmosphere through a filter AS 1 . The humidifier AS 5 is a device for humidifying the oxidant gas compressed by the air compressor AS 2 .

The oxidant-off gas flow path AS 4 is provided with a pressure sensor S 6 , a back pressure regulating valve A 3 and the humidifier AS 5 . The back pressure regulating valve A 3 is a valve for regulating the oxidant gas supply pressure. The humidifier AS 5 is provided as a device for exchanging moisture between the oxidant gas (dry gas) and the oxidant-off gas (wet gas).

The fuel gas supply system FSS has a fuel gas supply source FS 1 , a fuel gas flow path FS 3 , a circulation flow path FS 4 , a circulation pump FS 5 and an exhaust/drain flow path FS 6 . The fuel gas flow path FS 3 is a flow path through which the fuel gas to be supplied from the fuel gas supply source FS 1 to the anode of the fuel cell FC flows. The circulation flow path FS 4 is a flow path for returning the fuel-off gas discharged from the fuel cell FC back to the fuel gas flow path FS 3 . The circulation pump FS 5 pumps the fuel-off gas within the circulation flow path FS 4 into the fuel gas flow path FS 3 . The exhaust/drain flow path FS 6 is a flow path connected to and branching from the circulation flow path FS 4 .

The fuel gas supply source FS 1 is constituted by, for example, a high-pressure hydrogen tank or a hydrogen absorbing alloy, and stores hydrogen gas at a high pressure (e.g., 35 MPa to 70 MPa). When a cutoff valve H 1 is opened, the fuel gas flows to the fuel gas flow path FS 3 from the fuel gas supply source FS 1 . The pressure of the fuel gas is reduced, for example, to approximately 200 kPa by a regulator H 2 or an injector FS 2 , and the resultant gas is supplied to the fuel cell FC.

The fuel gas flow path FS 3 is provided with the cutoff valve H 1 , the regulator H 2 , the injector FS 2 , a cutoff valve H 3 , and a pressure sensor S 4 . The cutoff valve H 1 is a valve for blocking or allowing the fuel gas supply from the fuel gas supply source FS 1 . The regulator H 2 is provided for regulating the pressure of fuel gas. The injector FS 2 is provided for controlling the amount of fuel gas supplied to the fuel cell FC. The cutoff valve H 3 is a valve for blocking the fuel gas supply to the fuel cell FC.

The regulator H 2 is a device for regulating the pressure on the upstream side (primary pressure) to a preset secondary pressure, and is constituted by, for example, a mechanical pressure reducing valve for reducing the primary pressure. The mechanical pressure reducing valve has a casing in which a backpressure chamber and a pressure regulating chamber are formed while being separated by a diaphragm, and has a configuration in which, with the backpressure in the backpressure chamber, the primary pressure is reduced to a predetermined pressure in the pressure regulating chamber, thereby obtaining the secondary pressure. By arranging the regulator H 2 on the upstream side of the injector FS 2 , the upstream-side pressure of the injector FS 2 can effectively be reduced.

The injector FS 2 is an electromagnetic drive type on-off valve having a configuration in which a valve body is directly driven by an electromagnetic driving force with a predetermined drive period so as to be separated from a valve seat, thereby controlling a gas flow rate or a gas pressure. The injector FS 2 is provided with: a valve seat having an injection hole through which gaseous fuel such as fuel gas is injected; a nozzle body for supplying and guiding the gaseous fuel to the injection hole; and a valve body held so as to be contained in a manner such that the valve body is moveable in an axial direction (gas flow direction) with respect to the nozzle body, for opening and closing the injection hole.

The valve body of the injector FS 2 is driven by a solenoid, which is an electromagnetic drive, and is configured such that control signals that are output from the controller EC can control a gas injection duration and a gas injection time of the injector FS 2 . In order to supply gas to the downstream thereof at a required flow rate, the injector FS 2 changes at least one of the opening area (degree of opening) and the opening period of time of the valve body, which is provided in a gas flow path of the injector FS 2 , thereby adjusting the flow rate (or hydrogen mol concentration) of the gas supplied to the downstream side.

The circulation flow path FS 4 is provided with a cutoff valve H 4 , and is connected to the exhaust/drain flow path FS 6 . The exhaust/drain flow path FS 6 has an exhaust/drain valve H 5 , which is operated under the control of the controller EC to discharge the impurity-containing fuel-off gas and water within the circulation flow path FS 4 to the outside. By opening the exhaust/drain valve H 5 , the concentration of impurities in the fuel-off gas in the circulation flow path FS 4 is reduced, and the hydrogen concentration in the fuel-off gas flowing through the circulation system can consequently be increased.

The fuel-off gas discharged via the exhaust/drain valve H 5 is mixed with the oxidant-off gas flowing in the oxidant-off gas flow path AS 4 , and diluted by a diluter (not shown in the drawing). Upon being driven by a motor, the circulation pump FS 5 supplies the fuel-off gas in the circulation system to the fuel cell FC in a circulating manner.

The electric power system ES has a DC/DC converter ES 1 , a battery ES 2 , a traction inverter ES 3 , a traction motor ES 4 and auxiliary devices ES 5 . The fuel cell system FCS is structured as a parallel hybrid system in which the DC/DC converter ES 1 and the traction inverter ES 3 are each parallel-connected to the fuel cell FC.

The DC/DC converter ES 1 has a function of increasing a direct current voltage supplied from the battery ES 2 and outputting it to the traction inverter ES 3 , and also has a function of decreasing the voltage of a direct current power generated by the fuel cell FC or the voltage of a regenerative power collected by the traction motor ES 4 through regenerative braking and charging the battery ES 2 with the resulting power. With the above functions of the DC/DC converter ES 1 , the charging and discharging of the battery ES 2 is controlled. Also, with the voltage conversion control by the DC/DC converter ES 1 , the operation point (output terminal voltage, output current) of the fuel cell FC is controlled. A voltage sensor S 1 and a current sensor S 2 are attached to the fuel cell FC. The voltage sensor S 1 detects an output terminal voltage of the fuel cell FC. The current sensor S 2 detects an output current of the fuel cell FC.

The battery ES 2 functions as: a source in which surplus electric power is to be stored; a source in which regenerative energy is to be stored during regenerative braking; and an energy buffer to be used when the load varies as a result of acceleration or deceleration of the fuel cell vehicle. A secondary battery, such as a nickel/cadmium battery, a nickel/hydrogen battery, or a lithium secondary battery, is preferably used for the battery ES 2 . An SOC sensor S 3 for detecting an SOC (state of charge) is attached to the battery ES 2 .

The traction inverter ES 3 is, for example, a PWM inverter driven by a pulse-width modulation system. In response to control commands from the controller EC, the traction inverter ES 3 converts a direct current voltage output from the fuel cell FC or from the battery ES 2 into a three-phase alternating current voltage, thereby controlling the rotation torque of the traction motor ES 4 . The traction motor ES 4 is, for example, a three-phase AC motor, and constitutes a power source of the fuel cell vehicle.

The term “auxiliary devices ES 5 ” is used as a generic term referring to various motors disposed in each portion of the fuel cell system FCS (for example, power sources for pumps, etc.), inverters for driving such motors, and various on-board auxiliary units (for example, an air compressor, injector, coolant water circulation pump, radiator, etc.).

The cooling system CS has a radiator CS 1 , a coolant pump CS 2 , a coolant inflow path CS 3 and a coolant outflow path CS 4 . The radiator CS 1 cools a coolant for cooling the fuel cell FC by radiating the heat of the coolant. The coolant pump CS 2 is a pump for flowing the coolant back and forth between the fuel cell FC and the radiator CS 1 . The coolant inflow path CS 3 is a flow path connecting the radiator CS 1 and the fuel cell FC, and is provided with the coolant pump CS 2 . When the coolant pump CS 2 is driven, the coolant flows from the radiator CS 1 into the fuel cell FC through the coolant inflow path CS 3 . The coolant outflow path CS 4 is a flow path connecting the fuel cell FC and the radiator CS 1 , and is provided with a water temperature sensor S 5 . When the coolant pump CS 2 is driven, the coolant that has been used to cool down the fuel cell FC flows back to the radiator CS 1 .

The controller EC (control unit) is a computer system which is provided with a CPU, ROM, RAM and an input/output interface, and controls the respective portions of the fuel cell system FCS. For example, when the controller EC receives an ignition signal IG output from an ignition switch, it starts the operation of the fuel cell system FCS. After that, the controller EC determines the required electric power in the entire fuel cell system FCS based on, for example, an acceleration-opening-degree signal ACC output from an acceleration sensor and a vehicle speed signal VC output from a speed sensor. The required electric power in the entire fuel cell system FCS corresponds to the sum of the electric power for running the vehicle and the electric power for auxiliary devices.

The above-mentioned electric power for auxiliary devices includes: power consumed by on-board auxiliary units (a humidifier, air compressor, hydrogen pump, coolant water circulation pump, etc.); power consumed by devices necessary for running the vehicle (a speed change gear, wheel controller, steering device, suspension, etc.); and power consumed by devices arranged in a passenger space (an air conditioner, lighting device, audio system, etc.).

The controller EC determines what portions of the power are to be output from the fuel cell FC and from the battery ES 2 , respectively. The controller EC controls the oxidant gas supply system ASS and the fuel gas supply system FSS so that the power generated by the fuel cell FC corresponds to a target power, and it also controls the DC/DC converter ES 1 to control the operation point (output terminal voltage, output current) of the fuel cell FC. Furthermore, in order to attain a target torque depending on the degree of opening of the accelerator, the controller EC outputs to the traction inverter ES 3 , alternating current voltage command values for the respective U, V and W phases as switching commands, thereby controlling the output torque and the number of rotations of the traction motor ES 4 . Moreover, the controller EC controls the cooling system CS so that the fuel cell FC is at a suitable temperature.

Next, how the fuel cell system FCS of this embodiment determines whether a rapid warm-up operation should be continued or not will be described with reference to FIG. 2 . FIG. 2 is a flowchart for determining whether a rapid warm-up operation should be continued or not. In this embodiment, the “rapid warm-up operation” means a low-efficiency operation performed with a reduced output terminal voltage by decreasing the amount of air supply through the drive of the air compressor AS 2 .

In step S 01 , the temperature of the fuel cell FC is detected. This detection of the fuel cell FC temperature is carried out by a temperature sensor (not shown in the drawing) provided in the fuel cell FC, and the detected temperature is output to the controller EC.

Subsequently to step S 01 , in step S 02 , an effective catalyst area of the fuel cell FC is calculated by means of a current sweep for the fuel cell FC for a short period of time. More specifically, the voltage of the fuel cell FC is varied for a short period of time (not longer than one second), thereby obtaining current-voltage characteristics, which show the relationship between an output terminal voltage and an output current of the fuel cell FC. FIG. 3 shows one example of the obtained current-voltage characteristics. In FIG. 3 , WP is an operation point where a rapid warm-up operation is carried out. In FIG. 3 , S 1 is a curve obtained by varying the output terminal voltage of the fuel cell FC, and this curve S 1 is almost the same as the cyclic voltammetry (CV) curve of the unit cell constituting the fuel cell FC.

FIG. 4 shows an example of the above-described CV curve of the unit cell. In P 1 in FIG. 4 , a hydrogen reaction proceeds, and the reaction of formula (4) below occurs in the anode while the reaction of formula (5) below occurs in the cathode.

H 2 →2H + +2 e −   (4)

2H + +2 e − →H 2   (5)

In P 2 , a catalyst oxidation reaction as shown by formula (6) below occurs, and in P 3 , a catalyst reduction reaction as shown by formula (7) below occurs.

2Pt+O 2 →2PtO  (6)

2PtO→2Pt+O 2   (7)

In P 4 , a hydrogen reaction proceeds, and the reaction of formula (8) below occurs in the anode while the reaction of formula (9) below occurs in the cathode.

2H + +2 e 31 →H 2   (8)

H 2 →2H + +2 e −   (9)

Accordingly, in the CV curve in FIG. 4 , the effective surface area of the catalyst of platinum is obtained by calculating an area of oxidation current which corresponds to about 0.1-0.3 V (area of the region A 2 ) and dividing the obtained area of oxidation current by an electric quantity arising from hydrogen elimination from the catalyst at about 0.1-0.3 V. In other words, the magnitude of the area of the region A 2 in FIG. 4 is indicative of the magnitude of the effective catalyst area.

As already stated above, since the curve S 1 in FIG. 3 , which is a current-voltage curve obtained through a current sweep for the fuel cell FC for a short period of time, is almost the same as the CV curve S 2 of the unit cell constituting the fuel cell FC, the region A 1 in FIG. 3 corresponds to the region A 2 in FIG. 4 and the magnitude of the area of the region A 1 is indicative of the magnitude of the effective catalyst area. According to the above, the effective catalyst area of the fuel cell FC is obtained.

Subsequently to step S 02 , in step S 03 , whether the fuel cell FC temperature detected in step S 01 is below 0° C. and the effective catalyst area calculated in step S 02 is below a predetermined value is judged. If the fuel cell FC is at a temperature below 0° C. and if the effective catalyst area is below the predetermined value, the procedure goes to step S 04 ; and if the fuel cell FC is at a temperature equal to or higher than 0° C. and if the effective catalyst area is equal to or greater than the predetermined value, the procedure goes to step S 05 .

In step S 04 , the rapid warm-up operation of the fuel cell FC is continued. This is because the fuel cell FC temperature is below 0° C. and the effective catalyst area is below the predetermined value, which means that the gas flow path of the fuel cell FC is clogged, and thus, the fuel cell FC is not prepared to perform normal power generation.

In step S 05 , the rapid warm-up operation of the fuel cell FC is stopped. This is because the fuel cell FC temperature is equal to or higher than 0° C. and the effective catalyst area is equal to or greater than the predetermined value, which means that no clogging occurs in the gas flow path of the fuel cell FC, and thus, the fuel gas flows through each unit cell constituting the fuel cell FC, and the fuel cell FC is prepared to perform normal power generation.

The above determination method of this embodiment can also be utilized in determining whether the circulation in the fuel gas supply system FSS is allowed or not, or in determining whether the circulation in the cooling system CS is allowed or not. FIG. 5 is a flowchart showing an application of the above method to the determination of whether the circulation in the fuel gas supply system FSS is allowed or not, and FIG. 6 is a flowchart showing an application of the above method to the determination of whether the circulation in the cooling system CS is allowed or not.

Referring to FIG. 5 , the procedure for determining whether the circulation in the fuel gas supply system FSS is allowed or not will be described. In step S 11 , the temperature of the fuel cell FC is detected. This detection of the fuel cell FC temperature is carried out by a temperature sensor (not shown in the drawing) provided in the fuel cell FC, and the detected temperature is output to the controller EC.

Subsequently to step S 11 , in step S 12 , an effective catalyst area of the fuel cell FC is calculated by means of a current sweep for the fuel cell FC for a short period of time. The method of calculating the effective catalyst area is the same as described above, so a detailed description will be omitted here.

Subsequently to step S 12 , in step S 13 , whether the fuel cell FC temperature detected in step S 11 is below 0° C. and the effective catalyst area calculated in step S 12 is below a predetermined value is judged. If the fuel cell FC is at a temperature below 0° C. and if the effective catalyst area is below the predetermined value, the procedure goes to step S 14 ; and if the fuel cell FC is at a temperature equal to or higher than 0° C. and if the effective catalyst area is equal to or greater than the predetermined value, the procedure goes to step S 17 .

In step S 14 , whether the effective catalyst area calculated in step S 12 is below a second predetermined value is determined. The second predetermined value is smaller than the predetermined value used in step S 13 . If the effective catalyst area is below the second predetermined value, the procedure goes to step S 15 , and if the effective catalyst area is equal to or greater than the second predetermined value, the procedure goes to step S 16 .

In step S 15 , the circulation pump FS 5 is stopped so as to prohibit the circulation in the fuel gas supply system FSS, which is an anode circulation system. If the effective catalyst area is below the second predetermined value, it is suggested that clogging in the gas flow path of the fuel cell FC further proceeds, so the circulation pump FS 5 is stopped to prohibit the flow of water, thereby preventing still further clogging in the gas flow path. When prohibiting the circulation in the fuel gas supply system FSS, a decrease of hydrogen concentration would be an issue of concern; however, since the fuel cell system is in the rapid warm-up operation for a short period of time, the operation can be performed without critical problems by increasing the pressure of the fuel gas supplied from the fuel gas supply source FS 1 .

In step S 16 , the rapid warm-up operation of the fuel cell FC is continued. This is because the fuel cell FC temperature is below 0° C. and the effective catalyst area is below the predetermined value, which means that the gas flow path of the fuel cell FC is clogged, and thus, the fuel cell FC is not prepared to perform normal power generation.

In step S 17 , the rapid warm-up operation of the fuel cell FC is stopped. This is because the fuel cell FC temperature is equal to or higher than 0° C. and the effective catalyst area is equal to or greater than the predetermined value, which means that no clogging occurs in the gas flow path of the fuel cell FC, and thus, the fuel gas flows through each unit cell constituting the fuel cell FC, and the fuel cell FC is prepared to perform normal power generation.

Referring next to FIG. 6 , the procedure for determining whether the circulation in the cooling system CS is allowed or not will be described. In step S 21 , the temperature of the fuel cell FC is detected. This detection of the fuel cell FC temperature is carried out by a temperature sensor (not shown in the drawing) provided in the fuel cell FC, and the detected temperature is output to the controller EC.

Subsequently to step S 21 , in step S 22 , an effective catalyst area of the fuel cell FC is calculated by means of a current sweep for the fuel cell FC for a short period of time. The method of calculating the effective catalyst area is the same as described above, so a detailed description will be omitted here.

Subsequently to step S 22 , in step S 23 , whether the fuel cell FC temperature detected in step S 21 is below 0° C. and the effective catalyst area calculated in step S 22 is below a predetermined value is judged. If the fuel cell FC is at a temperature below 0° C. and if the effective catalyst area is below the predetermined value, the procedure goes to step S 24 ; and if the fuel cell FC is at a temperature equal to or higher than 0° C. and if the effective catalyst area is equal to or greater than the predetermined value, the procedure goes to step S 27 .

In step S 24 , whether the effective catalyst area calculated in step S 22 is below a second predetermined value is determined. The second predetermined value is smaller than the predetermined value used in step S 23 . If the effective catalyst area is below the second predetermined value, the procedure goes to step S 25 , and if the effective catalyst area is equal to or greater than the second predetermined value, the procedure goes to step S 26 .

In step S 25 , the coolant pump CS 2 is stopped so as to prohibit the circulation in the cooling system CS. If the effective catalyst area is below the second predetermined value, it is suggested that clogging in the gas flow path of the fuel cell FC further proceeds, so the coolant pump CS 2 is stopped so as to reduce a heat capacity, thereby giving priority to the warm-up. When prohibiting the circulation in the cooling system CS, a local increase of temperature would be an issue of concern; however, since it is considered that the reaction in the catalyst does not proceed so much in the case where the effective catalyst area is below the second predetermined value, it is preferable that priority is given to the warm-up.

In step S 26 , the rapid warm-up operation of the fuel cell FC is continued. This is because the fuel cell FC temperature is below 0° C. and the effective catalyst area is below the predetermined value, which means that the gas flow path of the fuel cell FC is clogged, and thus, the fuel cell FC is not prepared to perform normal power generation.

In step S 27 , the rapid warm-up operation of the fuel cell FC is stopped. This is because the fuel cell FC temperature is equal to or higher than 0° C. and the effective catalyst area is equal to or greater than the predetermined value, which means that no clogging occurs in the gas flow path of the fuel cell FC, and thus, the fuel gas flows through each unit cell constituting the fuel cell FC, and the fuel cell FC is prepared to perform normal power generation.

FCS: fuel cell system

FC: fuel cell

ASS: oxidant gas supply system

AS 1 : filter

AS 2 : air compressor

AS 3 : oxidant gas flow path

AS 4 : oxidant-off gas flow path

AS 5 : humidifier

A 3 : back pressure regulating valve

CS: cooling system

CS 1 : radiator

CS 2 : coolant pump

CS 3 : coolant inflow path

CS 4 : coolant outflow path

FSS: fuel gas supply system

FS 1 : fuel gas supply source

FS 2 : injector

FS 3 : fuel gas flow path

FS 4 : circulation flow path

FS 5 : circulation pump

FS 6 : exhaust/drain flow path

H 1 : cutoff valve

H 2 : regulator

H 3 : cutoff valve

H 4 : cutoff valve

H 5 : exhaust/drain valve

ES: electric power system

ES 1 : DC/DC converter

ES 2 : battery

ES 3 : traction inverter

ES 4 : traction motor

ES 5 : auxiliary devices

EC: controller

S 1 : voltage sensor

S 2 : current sensor

S 3 : SOC sensor

S 4 : pressure sensor

S 5 : water temperature sensor

ACC: acceleration-opening-degree signal

IG: ignition signal

VC: vehicle speed signal