Fuel cell system and fuel cell control method

In a fuel cell system, a preceding-stage fuel cell and a following-stage fuel cell are connected via a fuel flow path. The fuel cell system includes a reformer that supplies reformed gas to the preceding-stage fuel cell; an acquisition unit that acquires the amount of heat generation and the amount of heat absorption of the preceding-stage fuel cell; and a control unit that controls at least one of the amount of current of the preceding-stage fuel cell, the flow rate of air to be supplied to the reformer, and the temperature of the preceding-stage fuel cell if the amount of heat absorption acquired by the acquisition unit is larger than the amount of heat generation acquired by the acquisition unit.

TECHNICAL FIELD

The present invention relates to a fuel cell system and a fuel cell control method.

BACKGROUND ART

Typical solid electrolyte fuel cells require a time to raise their temperatures at cold start. Conventional fuel cell systems have therefore been mainly used as a stationary type, which is not frequently cold-started. There are, however, many demands for applying fuel cell systems to mobile objects such as vehicles. Patent Literature 1 proposes a multi-stage fuel cell stack including a small fuel cell stack and a large fuel cell stack to achieve both fast start-up and large output required by a mobile object.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

Meanwhile, in Patent Literature 1, fuel pipes of the small fuel cell stack and the large fuel cell stack are cascaded. Thus, the small fuel cell stack, situated at the preceding stage, is supplied with whole reformed gas to be used in the entire fuel cell system. Consequently, a large amount of methane contained in the reformed gas causes a large endothermic reaction during power generation of the small fuel cell stack, making it difficult to maintain the temperature of the small fuel cell stack. It is therefore necessary to heat the air to be used for power generation to or above the operating temperature of the small fuel cell stack. The problem of this is that the power generation efficiency of the whole fuel cell system decreases.

The present invention has been made in view of the above problem, and an object thereof is to provide a fuel cell system and a fuel cell control method capable of achieving enhanced power generation efficiency by reducing introduction of fuel for heating air to be used for power generation.

Solution to Problem

A fuel cell system according to one aspect of the present invention: reforms fuel and supplies reformed gas to a preceding-stage fuel cell; acquires an amount of heat generation and an amount of heat absorption of the preceding-stage fuel cell; and controls at least one of an amount of current of the preceding-stage fuel cell, a flow rate of air to be supplied to a reformer, and temperature of the preceding-stage fuel cell if the acquired amount of heat absorption is larger than the acquired amount of heat generation.

Advantageous Effects of Invention

According to the present invention, it is possible to achieve enhanced power generation efficiency by reducing introduction of fuel for heating air to be used for power generation.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described below with reference to the drawings. In the illustration of the drawings, identical parts will be denoted by identical reference signs, and description thereof will be omitted.

The configuration of a fuel cell system1according to a first embodiment will be described with reference toFIG. 1. As illustrated inFIG. 1, the fuel cell system1includes a preceding-stage fuel cell10including an anode10aand a cathode10b, a following-stage fuel cell11including an anode11aand a cathode11b, and a reformer12. Reformed gas discharged from the reformer12is supplied to the anode10aand further supplied to the anode11a via a fuel flow path16as well.

The fuel cell system1also includes a fuel pump13that supplies new raw fuel to the reformer12, an air blower14that supplies air to the reformer12, the cathode10b, and the cathode11b, a load15connected to the preceding-stage fuel cell10and the following-stage fuel cell11, and a control unit20. The fuel cell system1further includes a temperature sensor S1that detects temperature Trefof the reformer12, a sensor S2(acquisition unit) that detects temperature T1of the preceding-stage fuel cell10, and a temperature sensor S3that detects temperature T2of the following-stage fuel cell11.

The preceding-stage fuel cell10and the following-stage fuel cell11are each a solid oxide fuel cell (SOFC), for example. The preceding-stage fuel cell10and the following-stage fuel cell11generate electric power by reacting the reformed gas supplied to the anode10aand the anode11awith the air supplied to the cathode10band the cathode11b, and supply this electric power to the load15.

The reformer12reforms steam supplied from an evaporator (not illustrated), the new raw fuel supplied from the fuel pump13, and the air supplied from the air blower14by using a catalytic reaction, and supplies the fuel thus reformed (reformed gas containing hydrogen gas) to the anode10a. In doing so, the reformer12also supplies the reformed gas to be used at the anode11aof the following-stage fuel cell11to the anode10aat the same time.

The control unit20is a device that performs overall control on fuel cell system1and is, for example, a computer including a CPU, an ROM, an RAM, a data bus connecting them to each other, and input-output interfaces. The control unit20is connected to the fuel pump13, the air blower14, the load15, and the temperature sensors S1to S3. Upon acquiring required output of the load15, the control unit20acquires the detection signals of the temperature sensors S1to S3. Using the acquired detection signals, the control unit20outputs control signals to the fuel pump13and the air blower14to control the amount of fuel to be discharged from the fuel pump13and the amount of air to be discharged from the air blower14. The control unit20then supplies electric power satisfying the required output of the load15from the preceding-stage fuel cell10and the following-stage fuel cell11.

Moreover, the control unit20stores maps to be used to control the fuel cell system1. Specifically, the control unit20stores maps illustrated inFIGS. 2 to 5. The maps illustrated inFIGS. 2 to 5will be described along with a flowchart inFIG. 7mentioned below.

Next, the operation of the fuel cell system1according to the first embodiment of the present invention with the above configuration will be described with reference to the flowcharts illustrated inFIGS. 6 and 7. First, the operation of the fuel cell system1will be schematically described with reference toFIG. 6.

In step S101, the control unit20acquires required output of the load15.

In step S102, the control20sets an operation target for satisfying the required output of the load15.

In step S103, the control unit20executes operation of the preceding-stage fuel cell10and the following-stage fuel cell11based on the operation target set in step S102.

Next, details of the operation in step S102, illustrated inFIG. 6, will be described with reference toFIG. 7.

In step S11, the control unit20detects the temperature Trefof the reformer12, the temperature T1of the preceding-stage fuel cell10, and the temperature T2of the following-stage fuel cell11.

In step S12, the control unit20sets O2/C. O2/C is the ratio of the molar flow rate of oxygen in the air to be introduced into the reformer12to the molar flow rate of carbon atoms in the new raw fuel to be introduced into the reformer12.

In step S13, the control unit20predicts reforming efficiency ηrefand the temperature Trefof the reformer12which are obtainable by introducing O2/C set in step S12into the reformer12, by referring to the map illustrated inFIG. 2. As illustrated inFIG. 2, the reforming efficiency ηrefdecreases as O2/C increases. The reforming efficiency ηrefis expressed by equation (1).

where ΔHrefis the amount of enthalpy change after reforming, and ΔH is the amount of enthalpy change at introduction. For both of them, the definition of lower heating value is employed here as a scale for the enthalpy change. The present case is not limited to this definition.

In step S14, the control unit20sets the temperature T1of the preceding-stage fuel cell10.

In step S15, the control unit20sets current I1of the preceding-stage fuel cell10.

In step S16, the control unit20predicts an amount of power generation P1and an amount of heat generation Q1of the preceding-stage fuel cell10by using the temperature T1and the current I1set in step S14and step S15.

The amount of heat generation Q1will now be described.

In the first embodiment, the reformed gas to be used at the preceding-stage fuel cell10and the following-stage fuel cell11is supplied to the anode10a. In other words, the reformed gas necessary for the power generation of the two fuel cells is supplied to the anode10a. The reformed gas contains methane, so that a large amount of methane is supplied to the anode10a.

When the preceding-stage fuel cell10generates electric power, oxide ions move through the electrolyte in the preceding-stage fuel cell10in accordance with the amount of current generated. Then, the reformed gas, the oxide ions, and electrons react with each other, thus resulting in an amount of heat generation Q1equivalent to the difference obtained by subtracting the electric power from the heat of formation by the chemical reaction. The amount of heat generation Q1is expressed by equation (2).

where ΔH1is the amount of enthalpy change of the fuel used in the power generation of the preceding-stage fuel cell10.

On the other hand, the reformed gas, which is determined by the operating temperature of the reformer12and the new raw fuel supplied, is subjected to internal reforming inside the preceding-stage fuel cell10by the operating temperature of the preceding-stage fuel cell10and the movement of the oxide ions. More specifically, a large amount of methane and water introduced into the preceding-stage fuel cell10cause a steam reforming reaction, which produces hydrogen and CO. This reaction is an endothermic reaction and results in an amount of heat absorption QDR. If this amount of heat absorption QDRexceeds the amount of heat generation Q1, the operating point of the preceding-stage fuel cell10is shifted, thereby lowering the efficiency of the whole fuel cell system1. The amount of heat absorption QDRis expressed by equation (3).

where ΔHout1is the amount of enthalpy change of the unused fuel at the exit of the preceding-stage fuel cell10.

As illustrated inFIG. 3, there is a correlation between the amount of heat absorption QDR, and the ratio of the molar flow rate of oxygen in the air to be introduced into the reformer12to the molar flow rate of carbon atoms in the new raw fuel to be introduced into the reformer12and the amount of oxygen I1/4F (F: Faraday constant) of the oxide ions that move with the current I1, which is generated during power generation of the preceding-stage fuel cell10. Specifically, the amount of heat absorption QDRdecreases as an amount of air O2to be introduced into the reformer12or an amount of current I1N1of the preceding-stage fuel cell10increases. Thus, the control unit20can decrease the amount of heat absorption QDRby controlling the amount of air O2or the amount of current I1N1of the preceding-stage fuel cell10with reference to the map illustrated inFIG. 3. Note that N1is the number of preceding-stage fuel cells10piled in the stack.

In step S17, the control unit20predicts an amount of power generation P2and current I2of the following-stage fuel cell11. Specifically, the control unit20predicts the amount of power generation P2by subtracting the amount of power generation P1of the preceding-stage fuel cell10from an amount of power generation P of the whole fuel cell system1. Moreover, the control unit20predicts the current I2of the following-stage fuel cell11by using the predicted amount of power generation P2.

In step S18, the control unit20determines the flow rate of the new raw fuel to be supplied to the reformer12by using the current I1, the current I2, and a fuel usage ratio ηFU. The fuel usage ratio ηFUis the ratio of the new raw fuel used for power generation to the fuel introduced into the fuel cell system1. The fuel usage ratio ηFUis expressed by equation (4).

where ΔHinis the total amount of enthalpy change of the fuel used for the power generation. With ΔH1as the amount of enthalpy change of the fuel used for the power generation of the preceding-stage fuel cell10and ΔH2as the amount of enthalpy change of the fuel used for the power generation of the following-stage fuel cell11, ΔHinis expressed by equation (5).

In step S19, the control unit20predicts the amount of heat absorption QDRof the preceding-stage fuel cell10.

In step S20, the control unit20determines whether or not the amount of heat generation Q1is larger than the amount of heat absorption QDR. If the amount of heat generation Q1is larger than the amount of heat absorption QDR(Yes in step S20), the process proceeds to step S21. On the other hand, if the amount of heat generation Q1is smaller than or equal to the amount of heat absorption QDR(No in step S20), the process proceeds to step S24.

In step S21, the control unit20predicts system efficiency ηS. The system efficiency ηSis an index indicating the efficiency of the whole fuel cell system1, and a larger value indicates better efficiency. The system efficiency ηSis expressed by equation (6).

where ηFCis the power generation efficiency of the preceding-stage fuel cell10and the following-stage fuel cell11. This power generation efficiency ηFCwill be described later. As described in the above equation (6), the system efficiency ηSis represented as the product of the reforming efficiency ηref, the power generation efficiency ηFC, and the fuel usage ratio ηFU.

In step S22, the control unit20determines whether or not the system efficiency ηSis highest within a trial range. If the system efficiency ηSis highest (Yes in step S22), the process proceeds to step S23. On the other hand, if the system efficiency ηSis not highest (No in step S22), the process proceeds to step S24.

In step S23, the control unit20stores the trial result. Specifically, the control unit20records the set O2/C, current I1, and temperature T1.

In step S24, the control unit20determines whether or not the current I1has been checked within an entire predetermined range. If the current I1has been checked within the entire predetermined range (Yes in step S24), the process proceeds to step S25. On the other hand, if the current I1has not been checked within the entire predetermined range (No in step S24), the process returns to step S15.

The predetermined range for the current I1will now be described.

As illustrated inFIG. 4, with the power generation efficiency ηFCalong a vertical axis and with the ratio of the amount of current I1N1of the preceding-stage fuel cell10to the total amount of current of the preceding-stage fuel cell10and the following-stage fuel cell11(I1N1+I2N2) along a horizontal axis, the correlation between the vertical axis and the horizontal axis represents a parabolic curve with a given peak. Note that N2is the number of following-stage fuel cells11piled in the stack. The power generation efficiency ηFCis expressed by equation (7).

The power generation efficiency ηFCrises up to the peak illustrated inFIG. 4as the current I1increases. Also, there is a point before the peak at and above which the amount of heat generation Q1exceeds the amount of heat absorption QDR. More specifically, as illustrated inFIG. 5, there is a correlation in which the amount of heat generation Q1rises as the amount of current I1N1increases, and there is a point at and above which the amount of heat generation Q1exceeds the amount of heat absorption QDR. Thus, the control unit20searches for the current I1within a range within which the amount of heat generation Q1exceeds the amount of heat absorption QDRand the power generation efficiency ηFCis high, by referring to the maps illustrated inFIGS. 4 and 5.

Note that the amount of heat absorption QDRdecreases as the amount of current I1N1increases, as described with the map illustrated inFIG. 3, because the amount of heat generation Q1rises as the amount of current I1N1increases, as illustrated in the map ofFIG. 5.

In step S25, the control unit20determines whether or not the temperature T1of the preceding-stage fuel cell10has been checked within an entire predetermined range. If the temperature T1has been checked within the entire predetermined range (Yes in step S25), the process proceeds to step S26. If the temperature T1has not been checked within the entire predetermined range (No in step S25), the process returns to step S14.

It has been mentioned that in the map ofFIG. 4there is a point before the peak at and above which the amount of heat generation Q1exceeds the amount of heat absorption QDR. In other words, this means that the amount of heat generation Q1falls below the amount of heat absorption QDRin a range where the amount of current I1N1is small. In addition, the amount of heat generation Q1falls below the amount of heat absorption QDRwhen the amount of current I1N1is small and the exit temperature of the reformer12is lower than the temperature T1of the preceding-stage fuel cell10. Thus, the control unit20sets a predetermined range for the temperature T1of the preceding-stage fuel cell10in which the temperature T1is lower than the exit temperature of the reformer12, and searches for a temperature T1at which the amount of heat generation Q1exceeds the amount of heat absorption QDR.

In step S26, the control unit20determines whether or not O2/C has been checked within an entire predetermined range. If O2/C has been checked within the entire predetermined range (Yes in step S26), the process proceeds to step S27. On the other hand, if O2/C has not been checked within the entire predetermined range (No in step S26), the process returns to step S12. The reforming efficiency ηrefdecreases as O2/C increases, as illustrated in Fig,2. On the other hand, the amount of heat absorption QDRdecreases as the O2/C increases, as illustrated inFIG. 3. Thus, the control unit20searches for O2/C at which the amount of heat generation Q1exceeds the amount of heat absorption QDRand high efficiency is obtained.

In step S27, the control unit20sets the O2/C, current I1, and temperature T1stored in step S23as an operating condition.

As described above, the fuel cell system1according to the first embodiment can offer the following advantageous effects.

The fuel cell system1predicts the amount of heat generation Q1and the amount of heat absorption QDRof the preceding-stage fuel cell10and searches for an operating point at which the amount of heat absorption QDRfalls below the amount of heat generation Q1, by controlling at least one of the amount of current I1N1of the preceding-stage fuel cell10, the flow rate of air to be supplied to the reformer12, and the temperature T1of the preceding-stage fuel cell10. In this way, it is not necessary to heat the air to be used for power generation to make the amount of heat absorption QDRfall below the amount of heat generation Q1. The fuel cell system1can therefore be operated at high efficiency.

Also, the fuel cell system1sets the temperature T1of the preceding-stage fuel cell10, the amount of current I1N1of the preceding-stage fuel cell10, and the flow rate of air to be supplied to the reformer12based on the system efficiency ηS. In this way, the fuel cell system1can set an operating point with high efficiency and therefore be operated at high efficiency.

Also, the fuel cell system1increases the amount of heat generation Q1by increasing the amount of current I1N1, to thereby decrease the amount of heat absorption QDR. Specifically, the fuel cell system1searches for and sets a current I1with which the amount of heat generation Q1exceeds the amount of heat absorption QDR. In this way, it is not necessary to heat the air to be used for power generation to make the amount of heat absorption QDRfall below the amount of heat generation Q1. The fuel cell system1can therefore be operated at high efficiency.

Next, a second embodiment of the present invention will be described. A fuel cell system1according to the second embodiment is identical to the above-described first embodiment in the configuration but differs in the method of calculating the operating condition. Specifically, while the first embodiment involves setting the operating condition by referring the maps illustrated inFIGS. 3 to 6, the second embodiment involves setting the operating condition by referring to maps illustrated inFIGS. 8 to 11in addition to those inFIGS. 3 to 6, as will be discussed below. The operation of the fuel cell system according to the second embodiment will be described below in detail with reference flowcharts illustrated inFIGS. 12 and 13.

In step S51, the control unit20detects the temperature Trefof the reformer12, the temperature T1of the preceding-stage fuel cell10, and the temperature T2of the following-stage fuel cell11.

In step S52, the control unit20sets O2/C to 0.

In step S53, the control unit20sets the temperature T1of the preceding-stage fuel cell10to the largest possible value.

In step S54, the control unit20predicts the reforming efficiency ηrefand the temperature Trefof the reformer12which are obtainable by introducing O2/C set in step S12into the reformer12, by referring to the map illustrated inFIG. 2.

In step S55, the control unit20sets the current I1of the preceding-stage fuel cell10to0.

In step S56, the control unit20predicts the amount of power generation P1and the amount of heat generation Q1of the preceding-stage fuel cell10.

In step S57, the control unit20predicts the amount of power generation P2and the current I2of the following-stage fuel cell11. Specifically, the control unit20predicts the amount of power generation P2by subtracting the amount of power generation P1of the preceding-stage fuel cell10from the amount of power generation P of the whole fuel cell system1. Moreover, the control unit20predicts the current I2of the following-stage fuel cell11by using the predicted amount of power generation P2.

In step S58, the control unit20determines the flow rate of the new raw fuel to be supplied to the reformer12by using the current I1, the current I2, and the fuel usage ratio ηFU.

In step S59, the control unit20predicts the amount of heat absorption QDRof the preceding-stage fuel cell10.

In step S60, the control unit20determines whether or not the amount of heat generation Q1is larger than the amount of heat absorption QDR. If the amount of heat generation Q1is larger than the amount of heat absorption QDR(Yes in step S60), the process proceeds to step S61. On the other hand, if the amount of heat generation Q1is smaller than or equal to the amount of heat absorption QDR(No in step S60), the process proceeds to step S63.

In step S61, the control unit20determines whether or not a gradient dηFC/dI1is 0. If the gradient dηFC/dI1is 0 (Yes in step S61), the process proceeds to step S67. On the other hand, if the gradient dηFC/dI1is not 0 (No in step S61), the process proceeds to step S62. As illustrated inFIG. 8, the gradient dηFC/dI1is the gradient of the power generation efficiency ηFCwith respect to the current I1of the preceding-stage fuel cell10.

In step S62, the control unit20determines whether or not the gradient dηFC/dI1is larger than 0. If the gradient dηFC/dI1is larger than 0 (Yes in step S62), the process proceeds to step S63. On the other hand, if the gradient dηFC/dI1is smaller than or equal to 0 (No in step S62), the process proceeds to step S67.

In step S63, the control unit20determines whether or not the current I1has been checked within an entire predetermined range. If the current I1has been checked within the entire predetermined range (Yes in step S63), the process proceeds to step S65. On the other hand, if the current I1has not been checked within the entire predetermined range (No in step S63), the process proceeds to step S64.

In step S64, the control unit20increases the current I1within the predetermined range, and the process returns to step S56. When the gradient dηFC/dI1is not 0 but larger than 0 in step S62, it means that the power generation efficiency ηFCgets closer to a peak as the amount of current I1N1increases, as illustrated inFIG. 8. Thus, if the current I1has not been checked within the predetermined range, the control unit20searches for a current I1which provides the peak illustrated inFIG. 8. Meanwhile, the range to the right of (1) presented inFIG. 8represents a range in which the amount of heat generation Q1exceeds the amount of heat absorption QDR.

In step S65, the control unit20determines whether or not O2/C has been checked within an entire predetermined range. If O2/C has been checked within the entire predetermined range (Yes in step S65), the process proceeds to step S66. On the other hand, if O2/C has not been checked within the entire predetermined range (No in step S65), the process proceeds to step S73.

In step S66, the control unit20determines whether or not the temperature T1of the preceding-stage fuel cell10has been checked within an entire predetermined range. If the temperature T1has been checked within the entire predetermined range (Yes in step S66), the process proceeds to step S76. If the temperature T1has not been checked within the entire predetermined range (No in step S66), the process proceeds to step S75.

In step S67, the control unit20predicts the system efficiency ηS. When the gradient dηFC/dI1is 0 in step S61, it means that the power generation efficiency ηFCis highest, as illustrated inFIG. 8, so that the system efficiency ηSis high. Thus, the control unit20predicts the system efficiency ηS. Also, when the gradient dηFC/dI1is smaller than or equal to 0, it means that the power generation efficiency ηFCis to the right of the peak, as illustrated inFIG. 8. In this case, the system efficiency ηSdecreases as the amount of current I1N1increases, but there is still a range in which the system efficiency ηSis high, depending on the amount of current I1N1. Thus, the control unit20predicts the system efficiency ηS.

In step S68, the control unit20determines whether or not the system efficiency ηSis highest within a trial range. If the system efficiency ηSis highest (Yes in step S68), the process proceeds to step S69. On the other hand, if the system efficiency ηSis not highest (No in step S68), the process proceeds to step S70.

In step S69, the control unit20stores the resultant operating condition and system efficiency ηS.

In step S70, the control unit20determines whether or not the gradient dηFC/dI1is 0. If the gradient dηFC/dI1is 0 (Yes in step S70), the process proceeds to step S76. On the other hand, if the gradient dηFC/dI1is not 0 (No in step S70), the process proceeds to step S71.

In step S71, the control unit20determines whether or not the power generation efficiency ηFCis higher than power generation efficiency ηFC2. The power generation efficiency ηFC2(second power generation efficiency) is power generation efficiency which is obtainable by using only the following-stage fuel cell11to generate electric power corresponding to the requested output of the load15at the detected the temperature T2. If the power generation efficiency ηFCis higher than the power generation efficiency ηFC2(Yes in step S71), the process proceeds to step S72. On the other hand, if the power generation efficiency ηFCis lower than or equal to the power generation efficiency ηFC2(No in step S71), the process proceeds to step S74.

The control unit20determines whether or not the power generation efficiency ηFCis higher than the power generation efficiency ηFC2in step S71in order to determine whether the operating condition falls in a range (2) or range (3) illustrated inFIG. 8. The range (2) illustrated inFIG. 8is a range in which the gradient dηFC/dI1is smaller than 0 and the power generation efficiency ηFCis higher than the power generation efficiency ηFC2. On the other hand, the range (3) illustrated inFIG. 8is a range in which the gradient dηFC/dI1is smaller than 0 and the power generation efficiency ηFCis lower than the power generation efficiency ηFC2.

If the operating condition falls in the range (2) illustrated inFIG. 8, the control unit20increases the flow rate of air to be supplied to the reformer12, as will be described later. This is because increasing the flow rate of air to be supplied to the reformer12decreases the amount of heat absorption QDR, as illustrated inFIG. 9.

On the other hand, if the operating condition falls in the range (3) illustrated inFIG. 8, the power generation efficiency ηFCis lower than the power generation efficiency ηFC2, and the efficiency of the whole fuel cell system1will therefore be better if operated only with the following-stage fuel cell11. Thus, the control unit20lowers the temperature T1of the preceding-stage fuel cell10to decrease the amount of power generation P1and the amount of heat absorption QDRof the preceding-stage fuel cell10.

In step S72, the control unit20determines whether or not the amount of heat absorption QDRis larger than 0. If the amount of heat absorption QDRis larger than 0 (Yes in step S72), the process proceeds to step S73. On the other hand, if the amount of heat absorption Qua is smaller than or equal to 0 (No in step S72), the process proceeds to step S76.

In step S73, the control unit20increases O2/C within a predetermined range, and the process returns to step S54. More specifically, in step S73, since the operating condition falls in the range (2) illustrated inFIG. 8, the control unit20increases O2/C to decrease the amount of heat absorption QDRto search for an operating condition with better efficiency.

In step S74, the control unit20determines whether or not the temperature T1of the preceding-stage fuel cell10is higher than the temperature Trefof the reformer12. If the temperature T1is higher than the temperature Tref(Yes in step S74), the process proceeds to step S75. On the other hand, if the temperature T1is lower than or equal to the temperature Tref(No in step S74), the process proceeds to step S76.

In step S75, the control unit20lowers the temperature T1within a predetermined range, and the process returns to step S54. As illustrated inFIG. 9, the amount of heat absorption QDRdecreases as the temperature T1decreases. More specifically, the amount of heat absorption QDRdecreases the further the temperature T1falls below the temperature Tref. Meanwhile, inFIG. 9, the temperature T1is highest when being equal to the temperature T2, and the temperature T1becomes lower the further it shifts toward the bottom of the map illustrated inFIG. 9.

Also, as illustrated inFIG. 10, the peak of the power generation efficiency ηFCshifts such that the further the temperature T1falls below the temperature Tref, the smaller the amount of current I1N1is required for the power generation efficiency ηFCto peak. Also, as illustrated inFIG. 10, the peak of the power generation efficiency ηFCis higher than the power generation efficiency ηFC2, and the range to the right of any of the arrows is a range in which the amount of heat generation Q1exceeds the amount of heat absorption QDR. Specifically, as illustrated inFIG. 10, by lowering the temperature T1, the control unit20can search for an operating condition with which the power generation efficiency ηFCis high and the amount of heat generation Q1is larger than the amount of heat absorption QDR, with a small amount of current I1N1.

Also, as illustrated inFIG. 11, the amount of heat generation Q1decreases as the temperature T1decreases, and the range to the right of any of the arrows is a range in which the amount of heat generation Q1exceeds the amount of heat absorption QDR. Specifically, as illustrated inFIG. 11, although the amount of heat generation Q1decreases as the temperature T1decreases, a range in which the amount of heat generation Q1exceeds the amount of heat absorption QDRcan be obtained with a small amount of current I1N1. Thus, by lowering the temperature T1, the control unit20searches for an operating condition with which the amount of heat generation Q1exceeds the amount of heat absorption QDRwith a small amount of current I1N1.

In step S76, the control unit20sets the O2/C, the current I1, and the temperature T1stored in step S69as the operating condition.

As described above, the fuel cell system1according to the second embodiment can offer the following advantageous effects.

When the gradient dηFC/dI1of the power generation efficiency ηFCwith respect to the current I1of the preceding-stage fuel cell10is positive, the fuel cell system1further increases the current I1of the preceding-stage fuel cell10to search for an operating point with high power generation efficiency ηFC. In this way, the fuel cell system1can set an operating point with high efficiency and therefore be operated at high efficiency.

Also, when determining the operating condition, the fuel cell system1sets the flow rate of air to be supplied to the reformer12to 0 and checks the possible operating temperature of the preceding-stage fuel cell10for operation from its highest temperature. Thus, the fuel cell system1sets the operating condition by determining that the point at which the gradient dηFC/dI1is 0 is the point at which the power generation efficiency ηFCis highest. In this way, the fuel cell system1can set an operating point with high efficiency and therefore be operated at high efficiency.

Also, when the gradient dηFC/dI1is negative, further increasing the current I1will lower the system efficiency ηS, and the fuel cell system1therefore predicts the system efficiency ηSunder the operating condition at that point. If the predicted system efficiency ηSis highest within a trial range, the fuel cell system1sets this operating condition. In this way, the fuel cell system1can set an operating point with high efficiency and therefore be operated at high efficiency.

Also, when the gradient dηFC/dI1is negative and the power generation efficiency ηFCis higher than the power generation efficiency ηFC2, the fuel cell system1increases the flow rate of air to be supplied to the reformer12to decrease the amount of heat absorption QDR. By searching for an operating point at which the amount of heat absorption QDRfalls below the amount of heat generation Q1in this manner, it is not necessary to heat the air to be used for power generation to make the amount of heat absorption QDRfall below the amount of heat generation Q1. The fuel cell system1can therefore be operated at high efficiency.

Also, when the gradient dηFC/dI1is negative and the power generation efficiency ηFCis lower than the power generation efficiency ηFC2, the fuel cell system1lowers the temperature T1of the preceding-stage fuel cell10. This is because the system efficiency ηSwill be higher if power generation is performed not by just using only the following-stage fuel cell11instead of supplying air to the reformer12to decrease the amount of heat absorption QDRand decrease the reforming efficiency ηref. By lowering the temperature T1, the fuel cell system1searches for an operating point at which the amount of heat absorption QDRfalls below the amount of heat generation Q1. Thus, the fuel cell system1does not set an operating condition that decreases the system efficiency ηSto maintain the temperature T1, but lowers the temperature T1to search for an operating point at which the amount of heat absorption QDRfalls below the amount of heat generation Q1. In this way, the fuel cell system1can set an operating point with higher efficiency than the power generation efficiency ηFC2and therefore be operated at high efficiency.

While embodiments of the present invention have been described above, it should not be understood that the statement and the drawings constituting part of this disclosure limit the present invention. Various alternative embodiments, examples, and operation techniques will become apparent to those skilled in the art from this disclosure.

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