Fuel cell system

Abstract

To provide a fuel cell system that can optimize power generation efficiency.SOLUTION: A fuel cell system 100 includes: a hot module 1 having a cell stack 14 formed by stacking a plurality of fuel cell cells C that generates electricity in an internal space, and a combustion portion 15 that combusts off-gas discharged from the cell stack 14 to raise the temperature of the internal space; and a control portion 10 that controls the operation of the hot module 1. The control portion 10 is configured to be able to switch between a normal operation mode in which the hot module 1 is operated while maintaining the fuel utilization rate at a predetermined value, and a fuel utilization rate confirmation mode in which the value of the fuel utilization rate is gradually changed, the value of the fuel utilization rate is calculated from the power generation efficiency that shows the highest value, and the acquired value of the fuel utilization rate is determined as the value of the fuel utilization rate in the normal operation mode.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a fuel cell system. [Background technology]

[0002] Patent Document 1 discloses a fuel cell system that performs high-efficiency power generation operation with a higher fuel utilization rate than normal operation. In the fuel cell system disclosed in Patent Document 1, the flow rate of fuel gas supplied to the fuel cell is reduced in stages to a lower limit fuel flow rate each time a state in which it is determined that the power generated by the fuel cell is within a predetermined power range and the temperature of the combustion section is within a predetermined temperature range continues for a predetermined period of time, thereby performing high-efficiency power generation operation. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-121466 Summary of the Invention [Problem to be solved by the invention]

[0004] The fuel cell system disclosed in Patent Document 1 is considered to be an effective means for fuel cells whose power generation efficiency has little dependency on temperature and fuel utilization rate. However, not all fuel cells have little dependency on temperature and fuel utilization rate, and the technology disclosed in Patent Document 1 may not be able to optimize the power generation efficiency of the fuel cell system.

[0005] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a fuel cell system that can optimize power generation efficiency. [Means for solving the problem]

[0006] The fuel cell system according to the present invention, which achieves the above object, has the following characteristic configuration: a cell stack formed by stacking a plurality of fuel cell units in an internal space, the fuel cell units generating electricity by reacting a fuel gas with an oxidant gas; and a hot module having a combustion section that combusts off-gas discharged from the cell stack to increase the temperature of the internal space; a fuel supply unit that supplies the fuel gas to the cell stack; an oxidant gas supply unit that supplies the oxidant gas to the cell stack; a control unit that controls operations of the fuel supply unit and the oxidant gas supply unit, the control unit is configured to be able to switch between a normal operation mode and a fuel utilization rate confirmation mode as an operation state, the normal operation mode is an operation mode in which the fuel utilization rate, which is calculated by dividing the amount of the fuel gas used by the cell stack for power generation by the amount of the fuel gas supplied to the cell stack by the fuel supply unit, is maintained at a predetermined value; the fuel utilization rate confirmation mode is an operation mode in which the value of the fuel utilization rate is changed stepwise while the engine is operated, In the fuel utilization rate confirmation mode, the control unit acquires the fuel utilization rate value corresponding to the highest power generation efficiency from the power generation efficiency values ​​of the fuel cell calculated from the fuel utilization rate whose value is gradually changed, and determines the acquired fuel utilization rate value as the fuel utilization rate value in the normal operation mode.

[0007] According to the above characteristic configuration, the power generation efficiency value when the fuel utilization rate value is actually changed in the actual device can be obtained, and the fuel utilization rate calculated from the power generation efficiency that shows the highest value among the obtained power generation efficiencies (i.e., the optimal fuel utilization rate) can be determined as the fuel utilization rate value for the next normal operation mode, thereby optimizing the power generation efficiency of the fuel cell system.

[0008] Another characteristic feature of the fuel cell system according to the present invention is that the control unit executes the fuel utilization rate confirmation mode at each execution interval time.

[0009] According to the above characteristic configuration, the fuel cell system can be operated at a fuel utilization rate that corresponds to, for example, aging deterioration of the fuel cell units.

[0010] Another characteristic feature of the fuel cell system of the present invention is that in the fuel utilization rate confirmation mode, the control unit gradually changes the value of the fuel utilization rate within a predetermined allowable change range relative to the initial value of the fuel utilization rate or the value of the fuel utilization rate in the most recently executed normal operation mode.

[0011] According to the above characteristic configuration, the optimum value of the fuel utilization rate can be efficiently found.

[0012] Another characteristic feature of the fuel cell system according to the present invention is that in the fuel utilization rate confirmation mode, the control unit increases the value of the fuel utilization rate stepwise by a predetermined specified change value.

[0013] According to the above characteristic configuration, the optimum value of the fuel utilization rate can be efficiently found.

[0014] Another characteristic configuration of the fuel cell system of the present invention is that in the fuel utilization rate confirmation mode, the control unit increases the value of the fuel utilization rate by the specified change value every time a predetermined time period has elapsed.

[0015] According to the above characteristic configuration, it is possible to measure the value of the power generation efficiency when the hot module is in a stable state, and it is possible to more accurately find the value of the optimum fuel utilization rate.

[0016] Another characteristic configuration of the fuel cell system of the present invention is that in the fuel utilization rate confirmation mode, the control unit increases the value of the fuel utilization rate by reducing the amount of fuel gas supplied to the cell stack by the fuel supply unit.

[0017] According to the above characteristic configuration, it is possible to utilize an existing configuration of the fuel cell system, and to avoid making the configuration of the fuel cell system more complicated. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a schematic diagram showing the configuration of a fuel cell system according to an embodiment; [Figure 2] 4 is a flowchart illustrating a fuel utilization rate confirmation process according to the embodiment. [Figure 3] 10 is a graph showing the relationship between the output voltage and the fuel utilization rate of the cell stack according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0019] A fuel cell system 100 according to an embodiment of the present invention will be described below with reference to the drawings. Fig. 1 is a schematic diagram showing the configuration of a fuel cell system 100 according to an embodiment.

[0020] [Fuel cell system] As shown in FIG. 1, the fuel cell system 100 includes a hot module 1, a reforming water supply unit 2, a raw fuel supply unit 3 (an example of a fuel supply unit), an oxidizing gas supply unit 4, a power measurement unit 5, and a control unit .

[0021] [Reformed Water Supply Department] The reforming water supply unit 2 supplies reforming water to the hot module 1. The reforming water is, for example, tap water from which impurities have been removed.

[0022] The reforming water supply unit 2 has a reforming water supply passage L1, a water tank 21, and a pump 22. The water tank 21 stores reforming water. The pump 22 pumps the reforming water stored in the water tank 21. As a result, the reforming water is supplied to the hot module 1 via the reforming water supply passage L1. The amount of reforming water supplied to the hot module 1 per unit time is adjusted by controlling the operation of the pump 22.

[0023] [Raw and fuel supply department] The raw fuel supply unit 3 supplies a raw fuel (raw fuel gas) containing hydrocarbons to the hot module 1. The raw fuel is, for example, city gas, LP gas, or the like.

[0024] The raw fuel supply unit 3 includes a raw fuel supply line L2, a solenoid valve 31, a fuel flow meter 32, a gas blower 33, and a desulfurizer 34. The solenoid valve 31 can adjust the flow rate of the raw fuel flowing through the raw fuel supply line L2. The fuel flow meter 32 measures the flow rate of the raw fuel flowing through the raw fuel supply line L2. The gas blower 33 supplies the raw fuel to the hot module 1 via the raw fuel supply line L2 and the desulfurizer 34. In other words, the amount of raw fuel supplied to the hot module 1 per unit time can be adjusted by controlling the operation of the solenoid valve 31 and the gas blower 33. The desulfurizer 34 is disposed upstream of the hot module 1 in the flow direction of the raw fuel and removes sulfur from the raw fuel. As a result, the raw fuel from which the sulfur has been removed is supplied to the hot module 1.

[0025] [Oxidant gas supply unit] The oxidant gas supply unit 4 supplies an oxidant gas containing oxygen to the hot module 1. The oxidant gas is, for example, air. The oxidant gas supply unit 4 has an oxidant gas supply channel L3, an oxidant gas blower 41, and an oxidant gas flow meter 42. The oxidant gas blower 41 supplies the oxidant gas to the hot module 1 via the oxidant gas supply channel L3. The oxidant gas flow meter 42 measures the flow rate of the oxidant gas flowing through the oxidant gas supply channel L3. The amount of oxidant gas supplied to the hot module 1 per unit time is adjusted by controlling the operation of the oxidant gas blower 41.

[0026] [Hot Module] The hot module 1 is a fuel cell module that generates electricity by reacting hydrogen and oxygen. The hot module 1 includes a container 11, a vaporization section 12, a reforming section 13, a cell stack 14, a combustion section 15, a fuel gas flow path L11, an off-gas flow path L12, and a combustion exhaust gas path L13.

[0027] The container 11 has thermal insulation properties and houses the vaporization section 12, the reforming section 13, the cell stack 14, the combustion section 15, the fuel gas flow path L11, the off-gas flow path L12, and the combustion exhaust gas path L13.

[0028] A reforming water supply unit 2, a raw fuel supply unit 3, and an oxidant gas supply unit 4 are connected to the hot module 1, and the reforming water, raw fuel, and oxidant gas are supplied to the hot module 1. In this embodiment, the reforming water and raw fuel are supplied to the vaporization unit 12, and the oxidant gas is supplied to the cell stack 14.

[0029] [Vaporization section] In addition to the reforming water and raw fuel, combustion heat generated in the combustion section 15 is supplied to the vaporization section 12. The vaporization section 12 vaporizes the reforming water by utilizing the combustion heat to generate steam. In this embodiment, the vaporization section 12 supplies a mixed gas obtained by mixing the raw fuel with steam vaporized from the reforming water to the reforming section 13.

[0030] [Modification section] The reforming unit 13 is supplied with the mixed gas from the vaporization unit 12. In addition to the mixed gas, combustion heat generated in the combustion unit 15 is also supplied to the reforming unit 13. The reforming unit 13 generates fuel gas by steam reforming the mixed gas (raw fuel) by utilizing the combustion heat. The reforming unit 13 also generates fuel gas by utilizing the combustion heat. The fuel gas generated in the reforming unit 13 is supplied to the cell stack 14 via the fuel gas flow path L11. When the reforming unit 13 does not generate fuel gas, the raw fuel flows through the fuel gas flow path L11.

[0031] [Cell stack] The cell stack 14 is formed by stacking a plurality of cells C (an example of a fuel cell). In this embodiment, the cells C are solid oxide fuel cells, and are composed of an anode, a cathode, and an electrolyte. Note that the cells C may be fuel cells other than solid oxide fuel cells.

[0032] Fuel gas and oxidant gas are supplied to the cell C. The cell C generates power based on the fuel gas and the oxidant gas. Specifically, the cell C generates power by chemically reacting hydrogen contained in the fuel gas with oxygen contained in the oxidant gas. The fuel gas (anode off-gas) and oxidant gas (cathode off-gas) that are not used for power generation are supplied to the combustion section 15 as off-gas (excess gas) via the off-gas flow path L12.

[0033] [Combustion section] The combustion unit 15 is disposed between the vaporization unit 12 and the reforming unit 13 and the cell stack 14, and is connected to an off-gas flow path L12. The combustion unit 15 generates combustion heat by burning the off-gas. The combustion heat generated in the combustion unit 15 increases the temperature of the internal space of the container 11. The combustion unit 15 also discharges combustion exhaust gas along with the off-gas fuel. The combustion exhaust gas is discharged to the outside of the container 11 via a combustion exhaust gas path L13.

[0034] [Power measurement section] The power measurement unit 5 has a current sensor 51 and a voltage sensor 52. The current sensor 51 measures the output current of the cell stack 14 (cell C) and transmits information indicating the output current to the control unit 10. The voltage sensor 52 measures the output voltage V of the cell stack 14 (cell C) and transmits information indicating the output voltage V to the control unit 10.

[0035] [Control Unit] The control unit 10 is configured with a microcontroller including a processor, a semiconductor memory, etc. The control unit 10 controls the operations of the hot module 1, the reforming water supply unit 2, the raw fuel supply unit 3, the oxidant gas supply unit 4, and the power measurement unit 5.

[0036] The control unit 10 is also configured to be able to switch the operation mode M (an example of an operation state) of the fuel cell system 100 (hot module 1). In this embodiment, the operation mode M of the fuel cell system 100 includes a normal operation mode M1 and a fuel utilization rate confirmation mode M2.

[0037] The control unit 10 periodically (at every execution interval) executes the fuel utilization rate confirmation mode M2. That is, the control unit 10 periodically executes a mode switching process for switching between the normal operation mode M1 and the fuel utilization rate confirmation mode M2. The execution interval is arbitrarily set in advance by the designer of the fuel cell system 100 or the like. The execution interval is set to several thousand hours, for example, 2,000 hours.

[0038] When the fuel cell system 100 is started up and begins operation, the control unit 10 first executes the normal operation mode M1 as the operation mode M of the fuel cell system 100. After that, when an execution interval period has elapsed, the control unit 10 switches the operation mode M from the normal operation mode M1 to the fuel utilization rate confirmation mode M2, and when the fuel utilization rate confirmation mode M2 ​​ends, the control unit 10 switches the operation mode M back to the normal operation mode M1, and when the execution interval time has further elapsed, the control unit 10 repeats the process of switching the operation mode M from the normal operation mode M1 to the fuel utilization rate confirmation mode M2.

[0039] In the normal operation mode M1, the fuel cell system 100 is operated in a state (rated operation state) where the fuel utilization rate Uf is maintained at a predetermined value (constant value). In the normal operation mode M1, the control unit 10 controls the operation of each part of the fuel cell system 100 so that the fuel utilization rate Uf is the determined value. The fuel utilization rate Uf can be calculated by dividing the amount (flow rate) used in the combustion gas reaction by the amount (flow rate) supplied to the cell stack 14.

[0040] In the fuel utilization rate confirmation mode M2, the value of the fuel utilization rate Uf is changed in stages to operate the fuel cell system 100. In the fuel utilization rate confirmation mode M2, the control unit 10 controls the operation of each unit of the fuel cell system 100 to search for the optimum value of the fuel utilization rate Uf. More specifically, in the fuel utilization rate confirmation mode M2, the control unit 10 calculates the value of the power generation efficiency Rf when the value of the fuel utilization rate Uf is actually changed in the actual device. The value of the power generation efficiency Rf (power generation efficiency η of a single cell) is calculated by the following (Equation 1). In (Equation 1), W is the generated power, and q × ΔJ is the calorific value of q moles of fuel gas. The number of moles of fuel gas is calculated based on the flow rate measured by the fuel flow meter 32 of the raw fuel supply unit 3. η=W / (q×ΔJ) (Equation 1) The generated power W can be calculated by multiplying the output voltage V measured by the voltage sensor 52 by the output current I measured by the current sensor 51. Therefore, (Equation 1) can be expressed as follows: This can be expressed as η=V×I / (q×ΔJ) (Equation 2), The fuel utilization rate can be calculated by Uf = n × F × I / q, so (Equation 1) becomes: It can be expressed as η=Uf×n×F×V / ΔJ (Equation 3). As shown in (Equation 3), the power generation efficiency η (power generation efficiency Rf) of a single cell is proportional to the fuel utilization rate Uf and the output voltage V. Here, n is the number of electrons when q moles of fuel gas are used in an electrochemical reaction, and F is the Faraday constant.

[0041] The control unit 10 acquires the value of the fuel utilization factor Uf corresponding to the highest power generation efficiency Rf from among the acquired values ​​of power generation efficiency Rf as the optimal fuel utilization factor Uf, and determines the value of the optimal fuel utilization factor Uf as the value of the fuel utilization factor Uf for the next normal operation mode M1. Hereinafter, the process of actually changing the value of the fuel utilization factor Uf with the actual device to calculate (acquire) the value of the power generation efficiency Rf, and determining the value of the fuel utilization factor Uf corresponding to the highest power generation efficiency Rf from among the calculated (acquired) values ​​of power generation efficiency Rf as the value of the fuel utilization factor Uf for the next normal operation mode M1 to be executed will be referred to as the fuel utilization factor confirmation process.

[0042] [Fuel utilization rate confirmation process] FIG. 2 is a flowchart showing the fuel utilization rate confirmation process. As shown in FIG. 2, in the fuel utilization rate confirmation process, the control unit 10 gradually changes the value of the fuel utilization rate Uf and calculates the value of the power generation efficiency Rf of the hot module 1 from each of the gradually changed fuel utilization rates Uf (step S1). After changing the value of the fuel utilization rate Uf, the control unit 10 measures the amount of power generation after a stabilization time has elapsed. The stabilization time is set arbitrarily (e.g., several hours) by a designer or the like as the time it takes for the state of the hot module 1 to stabilize after changing the value of the fuel utilization rate Uf. In other words, the control unit 10 changes the value of the fuel utilization rate Uf each time the stabilization time elapses. Specifically, the control unit 10 gradually changes the value of the fuel utilization rate Uf in a rated operation state in which the amount of power generation (output) of the fuel cell system 100 (hot module 1) is maintained constant, and calculates the value of the power generation efficiency Rf. In this embodiment, the control unit 10 changes the value of the fuel utilization rate Uf while maintaining the power generation amount of the fuel cell system 100 (hot module 1) at a predetermined value (for example, a constant value within a range of 50 W or more and 700 W or less).

[0043] The control unit 10 changes the value of the fuel utilization factor Uf within a preset allowable change range relative to the reference value of the fuel utilization factor Uf. When the fuel utilization factor confirmation process is executed for the first time (immediately after the hot module 1 starts operation), the reference value of the fuel utilization factor Uf is the initial value of the fuel utilization factor Uf. When the fuel utilization factor confirmation process is executed for the second time or later, the reference value is the value indicated by the fuel utilization factor Uf in the normal operation mode M1 executed immediately before (most recently after) the fuel utilization factor confirmation process (fuel utilization factor confirmation mode M2) is executed. The initial value of the fuel utilization factor Uf (the fuel utilization factor Uf at the start of operation of the hot module 1) is arbitrarily set in advance by a designer or the like. In this embodiment, the initial value is set to, for example, 80%. The control unit 10 changes the value of the fuel utilization factor Uf by controlling the operation of the raw fuel supply unit 3 (the solenoid valve 31 and / or the gas blower 33) to change (increase or decrease) the amount (flow rate) of raw fuel supplied to the hot module 1. In this embodiment, the control unit 10 increases the value of the fuel utilization factor Uf by reducing the amount of raw fuel supplied to the hot module 1.

[0044] The control unit 10 increases the value of the fuel utilization factor Uf stepwise by a specified change value (a fixed amount) starting from a value obtained by subtracting the lower limit of the allowable change range from the reference value (hereinafter referred to as the start value) until it reaches a value obtained by adding the upper limit of the allowable change range to the reference value (hereinafter referred to as the end value). The allowable change range and the specified change value are each arbitrarily set in advance by a designer or the like. The allowable change range is set to, for example, ±5% of the reference value, and the specified change value is set to, for example, 1%.

[0045] For example, in the initial fuel utilization rate confirmation mode M2, the control unit 10 increases the value of the fuel utilization rate Uf in steps of 1% within a range of ±5% of 80%, which is set as the initial value of the fuel utilization rate Uf, i.e., with 75% as the starting value and 85% as the ending value.

[0046] After changing the value of the fuel utilization factor Uf, the control unit 10 measures the amount of power generated after the stabilization time has elapsed. That is, the control unit 10 increases the value of the fuel utilization factor Uf by a specified change value every time the stabilization time has elapsed.

[0047] Every time the value of the fuel utilization factor Uf is increased, the control unit 10 associates information indicating the value of the power generation efficiency Rf calculated in a state where the value of the fuel utilization factor Uf is changed with information indicating the value of the fuel utilization factor Uf in that state, and stores these information in the storage unit 101 (see FIG. 1). The storage unit 101 is configured by a semiconductor memory included in the control unit 10.

[0048] The control unit 10 associates and stores in the memory unit 101 the values ​​of the power generation efficiency Rf when the value of the fuel utilization factor Uf is changed in stages from the starting value to the end value and the corresponding values ​​of the fuel utilization factor Uf at each stage, obtains the power generation efficiency Rf that shows the highest value from the information showing the values ​​of the power generation efficiency Rf stored in the memory unit 101 (step S2), and obtains information showing the value of the fuel utilization factor Uf associated with the power generation efficiency Rf that shows the highest value (step S3).

[0049] Next, the control unit 10 determines the value of the fuel utilization factor Uf indicated by the acquired information as the value of the fuel utilization factor Uf in the next normal operation mode M1 (step S4), and ends the fuel utilization factor confirmation process.

[0050] When the control unit 10 finishes the fuel utilization rate confirmation process (determines the value of the optimal fuel utilization rate Uf), it controls the operation of each unit of the fuel cell system 100 to execute the normal operation mode M1 so that the system operates at the determined value of the fuel utilization rate Uf. Thereafter, the fuel utilization rate confirmation mode M2 ​​is executed at each execution interval, the value of the fuel utilization rate Uf corresponding to the power generation efficiency Rf showing the highest value is acquired, and the next normal operation mode M1 is executed at the acquired value of the fuel utilization rate Uf, and this control is repeated.

[0051] As described above, according to this embodiment, the optimal value of the fuel utilization factor Uf is found at each execution interval, and the value of the fuel utilization factor Uf in the normal operation mode M1 is adjusted to the optimal value of the fuel utilization factor Uf. That is, according to this embodiment, the power generation efficiency Rf can be maintained high compared to a fuel cell system that controls the value of the fuel utilization factor Uf to a constant value, and the power generation efficiency Rf of the fuel cell system 100 can be optimized. Furthermore, because the fuel utilization factor confirmation mode M2 ​​is executed at each execution interval, the value of the fuel utilization factor Uf of the fuel cell system 100 can be adjusted in accordance with the heat generation amount that changes due to deterioration of the cell stack 14.

[0052] In the fuel cell system 100, as the value of the fuel utilization rate Uf increases (off-gas is reduced), the combustion heat generated by the combustion unit 15 decreases. This causes the temperature of the cell stack 14 to decrease, and the output voltage V of the cell stack 14 to decrease. The degree of temperature decrease of the cell stack 14 when the value of the fuel utilization rate Uf increases varies depending on the heat insulating performance of the container 11, etc.

[0053] Furthermore, as deterioration of the cell stack 14 progresses due to aging or other reasons, the output voltage V of the cell stack 14 decreases, and the amount of heat generated by the cell stack 14 increases. In the case of constant output control, which keeps the output value of the fuel cell system 100 constant, the current increases as the output voltage V of the cell stack 14 decreases, and therefore the amount of heat generated also increases as the current increases. In other words, the amount of heat in the internal space of the hot module 1 increases due to deterioration of the cell stack 14 (cell C), so the temperature in the internal space of the hot module 1 can be kept constant even if the heat of combustion of the off-gas fuel is reduced. In other words, by increasing the value of the fuel utilization rate Uf, the power generation efficiency Rf of the fuel cell system 100 can be improved.

[0054] Furthermore, as shown by curve L in Fig. 3, in the fuel cell system 100, when the temperature of the cell stack 14 is constant, the amount of decrease in the output voltage V of the cell stack 14 when the value of the fuel utilization factor Uf is increased (i.e., the slope of curve L) differs between a low region RL where the fuel utilization factor Uf is lower than a predetermined value (e.g., 85%) and a high region RH where the fuel utilization factor Uf is higher. More specifically, when the value of the fuel utilization factor Uf is increased, the amount of decrease ΔV1 in the output voltage V of the cell stack 14 in the low region RL becomes smaller than the amount of decrease ΔV2 in the output voltage V of the cell stack 14 in the high region RH. Note that Fig. 3 is a graph showing the relationship between the output voltage V of the cell stack 14 and the fuel utilization factor Uf, with the vertical axis representing the output voltage V of the cell stack 14 and the horizontal axis representing the fuel utilization factor Uf of the fuel cell system 100.

[0055] In other words, as shown in the graph of Figure 3, when the value of the fuel utilization rate Uf indicates the low region RL, it is advantageous to increase the fuel utilization rate Uf in order to optimize the power generation efficiency Rf of the fuel cell system 100, whereas when the value of the fuel utilization rate Uf indicates the high region RH, it is advantageous to decrease the fuel utilization rate Uf in order to optimize the power generation efficiency Rf of the fuel cell system 100.

[0056] Since the fuel cell system 100 has the above-described characteristics, as in this embodiment, the value of the fuel utilization rate Uf of the fuel cell system 100 is actually changed in stages using an actual device to obtain the value of the fuel utilization rate Uf with the highest power generation efficiency Rf (optimal fuel utilization rate Uf), and the fuel cell system 100 is operated in normal operation mode M1 at the obtained value of the fuel utilization rate Uf, thereby optimizing the power generation efficiency Rf of the fuel cell system 100.

[0057] <Another embodiment> In the above embodiment, a specific example of the configuration of the fuel cell system 100 has been described, but the configuration can be changed as appropriate.

[0058] The configurations disclosed in the above embodiments (including other embodiments, the same applies below) can be applied in combination with configurations disclosed in other embodiments, as long as no contradiction arises. Furthermore, the embodiments disclosed in this specification are examples, and the embodiments of the present invention are not limited to these, and can be modified as appropriate within the scope that does not deviate from the purpose of the present invention.

[0059] The reforming section 13 may be supplied with the raw fuel directly as fuel gas from the raw fuel supply section 3 without passing through the vaporizing section 12 . [Industrial Applicability]

[0060] The present invention can be used in a fuel cell system. [Explanation of symbols]

[0061] 1: Hot module 3: Raw fuel supply section (fuel supply section) 4: Oxidant gas supply section 10: Control section 14: Cell stack 15: Combustion section 100: Fuel cell system C: Cell (fuel cell) M: Operation mode (operating state) M1: Normal operation mode M2: Fuel utilization rate check mode Rf: Power generation efficiency Uf:Fuel utilization rate

Claims

1. a cell stack formed by stacking a plurality of fuel cell units in an internal space, the fuel cell units generating electricity by reacting a fuel gas with an oxidant gas; and a hot module having a combustion section that combusts off-gas discharged from the cell stack to increase the temperature of the internal space; a fuel supply unit that supplies the fuel gas to the cell stack; an oxidant gas supply unit that supplies the oxidant gas to the cell stack; a control unit that controls operations of the fuel supply unit and the oxidant gas supply unit, the control unit is configured to be able to switch between a normal operation mode and a fuel utilization rate confirmation mode as an operation state, the normal operation mode is an operation mode in which the fuel utilization rate, which is calculated by dividing the amount of the fuel gas used by the cell stack for power generation by the amount of the fuel gas supplied to the cell stack by the fuel supply unit, is maintained at a predetermined value; the fuel utilization rate confirmation mode is an operation mode in which the value of the fuel utilization rate is changed stepwise while the engine is operated, In the fuel utilization rate confirmation mode, the control unit acquires the value of the fuel utilization rate corresponding to the highest power generation efficiency from the power generation efficiency values ​​of the fuel cell calculated from the fuel utilization rate whose value is gradually changed, and determines the acquired value of the fuel utilization rate as the value of the fuel utilization rate in the normal operation mode.

2. 2. The fuel cell system according to claim 1, wherein the control unit executes the fuel utilization rate confirmation mode at each execution interval time.

3. 3. The fuel cell system according to claim 1, wherein in the fuel utilization ratio confirmation mode, the control unit gradually changes the value of the fuel utilization ratio within a preset allowable change range relative to an initial value of the fuel utilization ratio or the value of the fuel utilization ratio in the most recently executed normal operation mode.

4. 4. The fuel cell system according to claim 3, wherein in the fuel utilization ratio confirmation mode, the control unit increases the value of the fuel utilization ratio stepwise by a predetermined specified change value.

5. 5. The fuel cell system according to claim 4, wherein in the fuel utilization ratio confirmation mode, the control unit increases the value of the fuel utilization ratio by the specified change value every time a preset time period elapses.

6. 6. The fuel cell system according to claim 5, wherein in the fuel utilization rate confirmation mode, the control unit increases the value of the fuel utilization rate by reducing the amount of the fuel gas supplied to the cell stack by the fuel supply unit.

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