WO2010123144A1

WO2010123144A1 - Method of controlling fuel cell system

Info

Publication number: WO2010123144A1
Application number: PCT/JP2010/057526
Authority: WO
Inventors: Yukihiko Kiyohiro
Original assignee: Honda Motor Co., Ltd.
Priority date: 2009-04-24
Filing date: 2010-04-21
Publication date: 2010-10-28
Also published as: JP2010257751A

Abstract

A method of controlling a fuel cell system (10) comprises the first step of setting a target output level for a fuel cell module (12), the second step of supplying a fuel gas depending on fuel gas data, the third step of producing a current from the module (12) depending on current data, the fourth step of detecting a present output level of the module (12), the fifth step of comparing the present output level with the target output level, the sixth step of adjusting a fuel gas flow rate supplied to the module (12), the seventh step of adjusting the current from the module (12), the eighth step of updating the fuel gas data corresponding to the target output level, and the ninth step of updating the current data corresponding to the target output level. At least one of the sixth through ninth steps is carried out based on the comparison of the fifth step.

Description

DESCRIPTION

Title of Invention

METHOD OF CONTROLLING FUEL CELL SYSTEM

Technical Field

The present invention relates to a method of controlling a fuel cell system including a fuel cell module which has a fuel cell stack formed by stacking a plurality of fuel cells, each fuel cell for generating electrical energy by electrochemical reactions of a fuel gas and an oxygen-containing gas, and a control device for controlling the amount of electrical energy generated in the fuel cell module.

Background Art

Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (hereinafter also referred to as "MEA" ) . The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, normally, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.

The fuel cell stack may suffer output reduction because of degradation of its power generating performance. For example, a fuel cell power generating apparatus for reducing a power generating performance degradation is known from Japanese Laid-Open Patent Publication No. 08-096825.

The known fuel cell power generating apparatus measures a load current and voltage of a fuel cell, records the measured load current and voltage as a chronological change corresponding to an operating time, evaluates the state of the fuel cell by comparing the chronological change with a normal chronological output change of the fuel cell, and controls the flow rate of either a fuel or an oxygen-containing gas depending on the evaluation. In other words, a deteriorated state or an operating state of the fuel cell is detected by measuring the load current and voltage of the fuel cell, and the amount of a reactant gas such as the fuel or the oxygen-containing gas supplied to the fuel cell is adjusted to a level appropriate for the state of the fuel cell, thereby stabilizing the characteristics of the fuel cell.

A fuel cell system disclosed in Japanese Laid-Open Patent Publication No. 2007-234347 includes a solid polymer electrolyte fuel cell, a fuel supply means, an oxygen-containing gas supply means, and a control means for controlling the solid polymer electrolyte fuel cell, the fuel supply means, and the oxygen-containing gas supply means.

The fuel cell system also includes an open circuit voltage measuring means for measuring an open circuit voltage of the solid polymer electrolyte fuel cell. The control means controls at least one of the fuel supply means and the oxygen-containing gas supply means depending on the measured open circuit voltage . Specifically, the fuel cell system has a first operation mode in which the solid polymer electrolyte fuel cell is supplied with a fuel and an oxygen-containing gas in a preset first amount, and a second operation mode in which the solid polymer electrolyte fuel cell is supplied with the fuel and the oxygen-containing gas in a second amount wherein the supplied amount of at least one of the fuel and the oxygen-containing gas is reduced.

Summary of Invention

According to Japanese Laid-Open Patent Publication No. 08-096825, however, since the load current and voltage is measured as a chronological change, time measurement is required. In addition, the fuel utilization ratio is greatly changed because the deteriorated state of the fuel cell is detected by measuring the voltage thereof.

Furthermore , inasmuch as chronological changes are recorded before the fuel cell is halted and after it is restarted, the deterioration of the fuel cell can be judged only when the fuel cell is halted. If the fuel cell is halted infrequently, then the frequency of judgments of the deterioration is reduced, and the deterioration cannot be judged at an appropriate time. Moreover, the fuel cell power generating apparatus disclosed in Japanese Laid-Open Patent Publication No. 08-096825 is aimed at operating the fuel cell at an optimum reactant gas utilization ratio, and does not provide an output desired by the user and does not operate the fuel cell with high efficiency.

According to Japanese Laid-Open Patent Publication No. 2007-234347, the control means controls at least one of the fuel supply means and the oxygen-containing gas supply means depending on the measured open circuit voltage, and cannot detect deterioration of the fuel cell unless the fuel cell is under no load, i.e., it is in an open circuit state.

Since the control means controls at least one of the fuel supply means and the oxygen-containing gas supply means depending on the measured open circuit voltage, it is unable to control the fuel supply means or the oxygen- containing gas supply means in an optimum fashion when the fuel cell is in a closed circuit state, i.e., in actual use.

The open circuit state in which deterioration of the fuel cell is detected and the closed circuit state in which the fuel cell is actually operated depending on the deterioration, are different from each other. Therefore, the fuel cell may fail to be actually operated accurately depending on the deterioration.

It is an object of the present invention to provide a method of controlling a fuel cell system in a simple manner to keep a target fuel cell output level even if a fuel cell voltage drops.

The present invention relates to a method of controlling a fuel cell system including a fuel cell module which has a fuel cell stack formed by stacking a plurality of fuel cells for generating electrical energy by electrochemical reactions of a fuel gas and an oxygen- containing gas , and a control device for controlling an amount of electrical energy generated in the fuel cell module .

In the method, fuel gas data and current data corresponding to respective target output levels for the fuel cell module are preliminarily set in the fuel cell system. The method comprises the first step of setting one of the target output levels for the fuel cell module, the second step of supplying a fuel gas depending on the fuel gas data based on the target output level set in the first step, the third step of producing a current in the fuel cell module depending on the current data based on the target output level set in the first step, the fourth step of detecting a present output level of the fuel cell module, the fifth step of comparing the present output level with the target output level or a prescribed range of the target output level, the sixth step of adjusting a flow rate of the fuel gas supplied to the fuel cell module, the seventh step of adjusting the current produced in the fuel cell module, the eighth step of updating the fuel gas data corresponding to the target output level, and the ninth step of updating the current data corresponding to the target output level. At least one of the sixth step, the seventh step, the eighth step, and the ninth step is carried out based on the comparison result of the fifth step.

According to the present invention, either one of the step of adjusting the flow rate of the fuel gas supplied to the fuel cell module, the step of adjusting the current produced by the fuel cell module, the step of updating the fuel gas data corresponding to the target output level, and the step of updating the current data corresponding to the target output level is carried out based on the result of comparison between the target output level and the present output level.

Consequently, even when the fuel cells of the fuel cell module fail to achieve the desired target output level due to a voltage drop, the fuel cells can achieve reliably the target output level by adjusting the fuel gas flow rate or the current.

Then, at least one of the fuel gas data corresponding to the target output level and the current data corresponding to the target output level is updated. For achieving the same target output level in subsequent cycles, therefore, the fuel gas can be supplied and the current can be produced quickly and accurately depending on the target output level.

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

Brief Description of Drawings

FIG. 1 is a diagram schematically showing a mechanical circuit of a fuel cell system to which a control method according to an embodiment of the present invention is applied; FIG. 2 is a circuit diagram of the fuel cell system; FIG. 3 is a flowchart of a control sequence of the control method;

FIG. 4 is a graph showing the power generating performance of a fuel cell module,- FIG. 5 is a diagram showing current values and fuel gas flow rates which correspond to target output levels for the fuel cell module;

FIG. 6 is a diagram showing ranges of one of the target output levels ; FIG. 7 is a graph showing the manner in which the output level of the fuel cell module is increased from a lowered state of the power generating performance; and

FIG. 8 is a diagram showing the manner in which one of the current values and one of the fuel gas flow rates which correspond to one of the target output levels are rewritten.

Description of Embodiments

A fuel cell system 10 shown in FIGS. 1 and 2, to which a control method according to an embodiment of the present invention is applied, is used in various applications, e.g., used as a stationary fuel cell system, a vehicle-mounted fuel cell system, or the like. The fuel cell system 10 comprises a fuel cell module (SOFC module) 12 for generating electrical energy in power generation by electrochemical reactions of a fuel gas (hydrogen gas) and an oxygen-containing gas (air), a raw fuel supply apparatus (including a fuel gas pump) 16 for supplying a raw fuel (e.g., city gas) to the fuel cell module 12, an oxygen-containing gas supply apparatus (including an air pump) 18 for supplying the oxygen- containing gas to the fuel cell module 12, a water supply apparatus (including a water pump) 20 for supplying water to the fuel cell module 12, a power converter 22 for converting the direct current electrical energy generated in the fuel cell module 12 into electrical energy according to the requirement specifications , and a control device 24 for controlling the amount of electrical energy generated in the fuel cell module 12. The fuel cell module 12 comprises a solid oxide fuel cell stack 28 formed by stacking a plurality of solid oxide fuel cells 26 in a vertical direction. Each of the fuel cells 26 has a plurality of electrolyte electrode assemblies (membrane electrode assemblies: MEA) each including an electrolyte, and an anode and a cathode that are disposed one on each side of the electrolyte. The electrolyte is made of ion-conductive solid oxide such as stabilized zirconia, for example. The electrolyte electrode assemblies each have a circular disc shape, and form a sealless-type fuel cell.

As shown in FIG. 2, the fuel cell stack 28 has a fuel gas supply passage 30 defined centrally therein and extending in the stacked direction (indicated by the arrow A) of the fuel cells 26. The fuel gas supply passage 30 supplies the fuel gas to the anodes of the fuel cells 26.

The fuel cell stack 28 also has a plurality of oxygen-containing gas supply channels 32 defined in a central edge area on a circle around the fuel gas supply passage 30. The oxygen-containing gas supply channels 32 supply the oxygen-containing gas to the cathodes of the fuel cells 26. The oxygen-containing gas supply channels 32 double as exhaust gas passages 34 for discharging the fuel gas used in the anodes and the oxygen-containing gas used in the cathodes.

As shown in FIGS. 1 and 2, on the upper end side (or lower end side) of the fuel cell stack 28 in the stacking direction, a heat exchanger 36 for heating the oxygen- containing gas before it is supplied to the fuel cell stack 28, an evaporator 38 for evaporating water to generate a mixed fuel of raw fuel and water vapor, and a reformer 40 for reforming the mixed fuel to produce a reformed gas are provided.

On the lower end side (or upper end side) of the fuel cell stack 28 in the staking direction, a load applying mechanism 42 for applying a tightening load to the fuel cells 26 of the fuel cell stack 28 in the stacking direction indicated by the arrow A is provided (see FIG. 2) . The reformer 40 is a preliminary reformer for reforming high hydrocarbon (C₂₊) such as ethane (C₂H₆), propane (C₃H₈), and butane (C₄Hi₀) in the city gas (raw fuel) by steam reforming to produce a fuel gas chiefly containing methane (CH₄) , hydrogen, and CO. The operating temperature of the reformer 40 is several hundred "C.

The operating temperature of the fuel cell 26 is as high as several hundred "C. In the electrolyte electrode assembly, the methane in the fuel gas is reformed to produce hydrogen and CO, and the hydrogen and CO are supplied to the anodes. The heat exchanger 36 has an exhaust gas channel 44 for passing therethrough a consumed reactant gas (hereinafter referred to as the exhaust gas) discharged from the fuel cell stack 28, and an air channel 46 for passing therethrough air (heated fluid) , such that the air and the exhaust gas flow in a counterflow manner. The upstream side of the air channel 46 is connected to an air supply pipe 48, and the downstream side of the air channel 46 is connected to the oxygen-containing gas supply channels 32 of the fuel cell stack 28.

The evaporator 38 has a raw fuel channel 50 and a water channel 52. The raw fuel channel 50 is connected to the raw fuel supply apparatus 16. The reformer 40 is held in fluid communication with the fuel gas supply passage 30. The water channel 52 is connected to the water supply apparatus 20. The oxygen-containing gas supply apparatus 18 is connected to the air supply pipe 48.

The raw fuel supply apparatus 16, the oxygen- containing gas supply apparatus 18, and the water supply apparatus 20 are controlled by the control device 24. A voltage and current monitor 56 for monitoring the voltage and current of the fuel cell stack 28 when it generates electrical energy is electrically connected to the control device 24. A commercial power source 58 (or a load or a secondary battery), for example, is connected to the power converter 22 (see FIG. 2).

As shown in FIGS. 1 and 2, the fuel cell system 10 also includes a first flow rate sensor 62a for detecting the flow rate of the raw fuel (fuel gas) supplied from - li ¬

the raw fuel supply apparatus 16 to the evaporator 38, a second flow rate sensor 62b for detecting the flow rate of the air (oxygen-containing gas) supplied from the oxygen-containing gas supply apparatus 18 to the heat exchanger 36, and a third flow rate sensor 62c for detecting the flow rate of the water supplied from the water supply apparatus 20 to the evaporator 38.

The first to third flow rate sensors 62a, 62b, 62c are electrically connected to the control device 24. The control device 24 has a function to control the amount of the fuel gas supplied from the raw fuel supply apparatus 16, the amount of the air supplied from the oxygen- containing gas supply apparatus 18, and the amount of the water supplied from the water supply apparatus 20, and also has a function to adjust the electric current of the fuel cell stack 28.

Operation of the fuel cell system 10 will be described below.

As shown in FIGS. 1 and 2, by operation of the raw fuel supply apparatus 16, for example, a raw fuel such as the city gas (including CH₄, C₂H₆, C₃H₈, C₄Hi₀) is supplied to the raw fuel channel 50. Further, by operation of the water supply apparatus 20, water is supplied to the water channel 52, and the oxygen-containing gas such as the air is supplied to the air supply pipe 48 through the oxygen- containing gas supply apparatus 18.

In the evaporator 38, the raw fuel is mixed with the water vapor, and a mixed fuel is obtained. The mixed fuel is supplied to the reformer 40. The mixed fuel undergoes steam reforming in the reformer 40. Thus, hydrocarbon of C₂₊ is removed (reformed), and a fuel gas (reformed gas) chiefly containing methane is obtained. The fuel gas is supplied to the fuel gas supply passage 30 in the fuel cell stack 28. The air supplied from the air supply pipe 48 to the heat exchanger 36 moves along the air channel 46 of the heat exchanger 36, and is heated to a predetermined temperature by heat exchange with the below-mentioned exhaust gas which moves along the exhaust gas channel 44. The air heated by the heat exchanger 36 is supplied to the oxygen-containing gas supply channels 32 of the fuel cell stack 28.

In the electrolyte electrode assemblies, therefore, the fuel gas is supplied to the anodes of the fuel cells 26, and the air is supplied to the cathodes of the fuel cells 26. The fuel cells 26 generate electricity by chemical reaction. The exhaust gas, which includes the fuel gas and the air used in the chemical reaction, is discharged from the fuel cell stack 28 as an off gas through the exhaust gas channel 44.

The control method according to the present embodiment will be described below with reference to a flowchart shown in FIG. 3.

As shown in FIG. 4, the power generating performance (the relationship between the output and the power generation efficiency) of the fuel cell module 12 is established depending on the percentage of the fuel gas flow rate. As shown in FIG. 5, the control device 24 stores a map of electric current values and fuel gas flow rates with respect to target output levels established for the fuel cell module 12.

Next , the control method according to the present embodiment will be explained below.

In the control method, fuel gas data and current data corresponding to respective target output levels for the fuel cell module are preliminarily set in the fuel cell system. The control method comprises the first step of setting one of the target output levels for the fuel cell module 12, the second step of supplying a fuel gas depending on the fuel gas data based on the target output level set in the first step, the third step of producing a current in the fuel cell module 12 depending on the current data based on the target output level set in the first step, the fourth step of detecting a present output level of the fuel cell module 12, the fifth step of comparing the present output level with the target output level or a prescribed range of the target output level, the sixth step of adjusting a flow rate of the fuel gas supplied to the fuel cell module 12, the seventh step of adjusting the current produced in the fuel cell module 12, the eighth step of updating the fuel gas data corresponding to the target output level, and the ninth step of updating the current data corresponding to the target output level. At least one of the sixth step, the seventh step, the eighth step, and the ninth step is carried out based on the comparison result of the fifth step.

Specifically, as indicated by the flowchart shown in FIG. 3, the control device 24 of the fuel cell system 10 establishes a target output level (voltage x current) for the fuel cell module 12 (step Sl).

As shown in FIG. 5, the control device 24 sets a fuel gas flow rate (Qf8) corresponding to the target output level (required output level) (Pr8) from the stored map (step S2), and then sets a current value (18) corresponding to the target output level (Pr8) (step S3). The fuel cell module 12 is supplied with a fuel gas at the fuel gas flow rate set in step S2 , and produces a current which has the current value set in step S3. Step 2 and step 3 may be switched around.

Then, control goes to step S4 in which the control device 24 detects a present output level (voltage x current) (FC output level) of the fuel cell module 12. The control device 24 compares the detected present output level of the fuel cell module 12 with a prescribed range of the target output level or the target output level (step S5) .

If the target output level for the fuel cell module 12 is 1000 W, for example, then a coefficient α for establishing the prescribed range of the target output level is set to a value shown in FIG. 6. The coefficient α of 0.05 serves to give priority to responsiveness. The coefficient α of 0.01 serves to give priority to accuracy. Otherwise, the maximum value (MAX) of the present output level is set as the target output level in order to prevent an excessive output level (including a reverse power flow) .

If the control device 24 judges that the present output level is smaller than the prescribed range of the target output level or in excess of the prescribed range of the target output level (NO in step S5), then control goes to step S6 in which the control device 24 determines whether the present output level is smaller than the target output level or not . If the control device 24 judges that the present output level is smaller than the target output level (YES in step S6), then control goes to step S7 in which the flow rate of the fuel gas is increased. After the flow rate of the fuel gas is increased, control goes to step S8 in which the current produced by the fuel cell module 12 is increased.

Specifically, as shown in FIG. 7, the required output level is satisfied at an initial operation point Pl. When the output level and efficiency of the fuel cell module 12 are lowered due to a power generating performance degradation, a degraded operation point P2 is reached. Then, while a fuel utilization ratio Uf is being kept constant, the flow rate of the fuel gas supplied to the fuel cell module 12 is increased and the current produced by the fuel cell module 12 is increased, reaching a degradation suppressing operation point P3 at which the desired required output level (target output level) is achieved.

After the flow rate of the fuel gas is adjusted in step S7 and the current is adjusted in step S8, control goes to step S9. Since the fuel gas flow rate and the current have been adjusted, an adjustment flag is turned on in step S9. After step S9, control goes back to step S4. If the control device 24 judges that the present output level is greater than the target output level (NO in step S6), then control goes to step SlO in which the current produced by the fuel cell module 12 is reduced. Thereafter, the flow rate of the fuel gas supplied to the fuel cell module 12 is reduced (step SIl). Then, control goes to step S9.

If the control device 24 judges that the present output level falls within the prescribed range of the target output level (YES in step S5), then control goes to step S12 in which the control device 24 determines whether the adjustment flag is on or not. If the control device 24 judges that the adjustment flag is on (YES in step S12), i.e., if the control device 24 judges that the fuel gas flow rate has been adjusted in step S7 and the current has been adjusted in step S8 or the current has been adjusted in step SlO and the fuel gas flow rate has been adjusted in step SIl, then control goes to step S13 in which the fuel gas data corresponding to the target output level is updated. Control then goes to step S14 in which the current data corresponding to the target output level is updated. Specifically, as shown in FIG. 8, with respect to the required output level Pr8 , the fuel gas flow rate Qf8 is rewritten into Qf8 + Qfα, and the current value 18 is rewritten into 18 + Ia. After the rewriting process in step S14, control goes to step S15 in which the adjustment flag is turned off.

According to the present embodiment, at least one of the step of adjusting the flow rate of the fuel gas supplied to the fuel cell module 12, the step of adjusting the current produced by the fuel cell module 12, the step of updating the fuel gas data corresponding to the target output level, and the step of updating the current data corresponding to the target output level is carried out based on the result of comparison between the target output level and the present output level.

Consequently, even when the fuel cells 26 fail to achieve the desired target output level due to a voltage drop, the fuel cells 26 can achieve the target output level by adjusting the fuel gas flow rate or the current.

Then, at least one of the fuel gas data corresponding to the target output level and the current data corresponding to the target output level is updated. For achieving the same target output level in subsequent cycles, therefore, the fuel gas corresponding to the target output level can be supplied and the current corresponding to the target output level can be produced quickly and accurately.

If it is judged that the present output level is smaller than the target output level (YES in step S6), then the flow rate of the fuel gas supplied to the fuel cell module 12 is increased (step S7) and then the current produced by the fuel cell module 12 is increased (step S8), after which control returns from step S9 to step S4.

Accordingly, when the fuel cells 26 fail to achieve a desired output level due to a voltage drop though the fuel cells 26 are supplied with the fuel gas and produce the current depending on the target output level, the flow rate of the fuel gas supplied to the fuel cell module 12 is first increased and then the current produced by the fuel cell module 12 is increased. As a result, the fuel cell module 12 is prevented from undergoing an undue fuel shortage due to a quick change in the fuel utilization ratio, particularly an excessive rise in the fuel utilization ratio, and hence the electrolyte electrode assemblies are prevented from being unduly deteriorated. The fuel cell module 12 is thus made more reliable and durable . If it is judged that the present output level is in excess of the target output level (NO in step S6), then the current produced by the fuel cell module 12 is reduced (step SlO) and then the flow rate of the fuel gas supplied to the fuel cell module 12 is reduced (step SIl), after which control returns from step S9 to step S4.

Accordingly, when the present output level of the fuel cells 26 exceeds the target output level though the fuel cells 26 are supplied with the fuel gas and produce the current depending on the target output level, the current produced by the fuel cell module 12 is first reduced and then the flow rate of the fuel gas supplied to the fuel cell module 12 is reduced. As a result, the fuel cell module 12 is prevented from undergoing an undue fuel shortage due to a quick change in the fuel utilization ratio, particularly an excessive rise in the fuel utilization ratio, and hence the electrolyte electrode assemblies are prevented from being unduly deteriorated. The fuel cell module 12 is thus made more reliable and durable. The step of adjusting the flow rate of the fuel gas supplied to the fuel cell module 12 and the step of adjusting the current produced by the fuel cell module 12 is carried out while the fuel utilization ratio is kept within a preset prescribed range. Specifically, the fuel utilization ratio (Uf) of the general fuel cell system 10 is in the range from 10% to 80%.

According to the present embodiment, in the sixth and seventh steps , the prescribed range of the fuel utilization ratio is set to ± 5% if priority is to be given to responsiveness, and to ± 1% if priority is to be given to accuracy. As a consequence, in the step of adjusting the fuel gas flow rate and the step of adjusting the current, the fuel cell module 12 is prevented from undergoing an undue fuel shortage due to a quick change in the fuel utilization ratio, particularly an excessive rise in the fuel utilization ratio, and hence the electrolyte electrode assemblies are prevented from being unduly deteriorated. The fuel cell module 12 is thus made more reliable and durable.

If it is judged that the present output level is equal to the target output level (YES in step S5), then it is determined whether at least one of the sixth step of adjusting the fuel gas flow rate and the seventh step of adjusting the current has been carried out or not (step S12). If the fuel gas flow rate has not been adjusted and the current has not been adjusted (NO in step S12), then both do not need to be adjusted, and the fuel gas data corresponding to the target output level and the current data corresponding to the target output level are not updated. Therefore, the fuel cell module 12 undergoes no output drop and can generate electricity efficiently.

If it is judged that at least one of the sixth step of adjusting the fuel gas flow rate and the seventh step of adjusting the current has been carried out (YES in step S12), then at least one of the eighth step of updating the fuel gas data and the ninth step of updating the current data is carried out . For achieving the same target output level in subsequent cycles, therefore, the fuel gas can be supplied and the current can be produced quickly and accurately depending on the target output level .

The fuel cell module 12 comprises a solid oxide fuel cell module. Therefore, the fuel cells 26 are high- temperature fuel cells which generate a large amount of heat, and the fuel cell system 10 has increased durability and longevity. The solid oxide fuel cell module is prevented from undergoing temperature drops at the time the output level thereof is changed, and hence can continuously operate at a high temperature.

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

Claims

Claim 1. A method of controlling a fuel cell system (10) including a fuel cell module (12) which has a fuel cell stack (28) formed by stacking a plurality of fuel cells (26) for generating electrical energy by electrochemical reactions of a fuel gas and an oxygen- containing gas, and a control device (24) for controlling an amount of electrical energy generated in the fuel cell module (12), fuel gas data and current data corresponding to respective target output levels for the fuel cell module (12) being preliminarily set in the fuel cell system (10), comprising: the first step of setting one of the target output levels for the fuel cell module (12); the second step of supplying a fuel gas depending on the fuel gas data based on the target output level set in the first step; the third step of producing a current in the fuel cell module (12) depending on the current data based on the target output level set in the first step; the fourth step of detecting a present output level of the fuel cell module (12); the fifth step of comparing the present output level with the target output level or a prescribed range of the target output level; the sixth step of adjusting a flow rate of the fuel gas supplied to the fuel cell module (12); the seventh step of adjusting the current produced in the fuel cell module (12); the eighth step of updating the fuel gas data corresponding to the target output level; and the ninth step of updating the current data corresponding to the target output level; wherein at least one of the sixth step, the seventh step, the eighth step, and the ninth step is carried out based on the comparison result of the fifth step.

Claim 2. A method according to claim 1 , wherein if it is judged in the fifth step that the present output level is smaller than the target output level or the prescribed range of the target output level, then the flow rate of the fuel gas supplied to the fuel cell module (12) is increased in the sixth step, and then the current produced in the fuel cell module (12) is increased in the seventh step, after which control returns to the fourth step.

Claim 3. A method according to claim 1 , wherein if it is judged in the fifth step that the present output level is greater than the target output level or the prescribed range of the target output level, then the current produced in the fuel cell module (12) is reduced in the seventh step, and then the flow rate of the fuel gas supplied to the fuel cell module (12) is reduced, after which control returns to the fourth step.

Claim 4. A method according to claim 1 , wherein the sixth step and the seventh step are carried out while a fuel utilization ratio of the fuel cell module (12) is kept within a preset prescribed range of the fuel utilization ratio.

Claim 5. A method according to claim 1 , wherein if it is judged in the fifth step that the present output level is equal to the target output level or falls within the prescribed range of the target output level , then it is determined whether at least one of the sixth step and the seventh step has been carried out or not .

Claim 6. A method according to claim 5 , wherein if it is judged that at least one of the sixth step and the seventh step has been carried out , then at least one of the eighth step and the ninth step is carried out.

Claim 7. A method according to claim 1 , wherein the fuel cell module (12) comprises a solid oxide fuel cell module .