Method for controlling internal reforming fuel cell system and device for controlling internal reforming fuel cell system

Abstract

In an internal reforming fuel cell system having an internal reforming fuel cell, fuel supplied to a main body is reformed to fuel gas with a reforming catalyst inside the main body, and the fuel gas is reacted with cathode gas inside the main body to generate power. In a method for controlling this system, when a system operation stop command for stopping the power generation of the fuel cell is detected, measurement of inter-electrode resistance between the anode electrode and the cathode electrode of the fuel cell is started. While the inter-electrode resistance is less than a predetermined value, the supply of the fuel to the reforming catalyst is continued and, when the inter-electrode resistance rises to or more than the predetermined value, the supply of the fuel to the reforming catalyst is stopped.

Description

Control Method for Internally Reforming Fuel Cell System and Control Device for Internally Reforming Fuel Cell System

[0001] The present invention relates to a control method for an internally reforming fuel cell system and a control device for an internally reforming fuel cell system.

[0002] Patent Document 1 describes an operation stop method for a reforming system using a fuel cell structure in which a fuel cell and a reformer are provided in a heat-insulating housing. In Patent Document 1, a method is proposed for rapidly reducing the temperature of the reformer while effectively preventing oxidation of the reforming catalyst that leads to a decrease in reforming ability after power generation of the fuel cell stops. In the reforming system of Patent Document 1, a mixed gas of hydrocarbon gas and water vapor is supplied to the reformer to be reformed into a hydrogen-rich fuel gas. In the operation stop method of Patent Document 1, after power generation of the fuel cell stops, the supply amount of hydrocarbon gas to the reformer is reduced until the temperature of the reforming catalyst in the reformer drops within the range of ±150 degrees of the oxidation generation temperature. When the temperature of the reforming catalyst drops within this range, the supply of the mixed gas to the reformer is stopped, and the reformer and the fuel cell are purged with a purge gas of air or hydrocarbon gas.

[0003] Japanese Patent No. 4906242

[0004] In the method of Patent Document 1, since the reformer in the housing is heated to a temperature equivalent to the operating temperature of the fuel cell, after power generation of the fuel cell stops, for some time, it is necessary to measure the temperature of the reforming catalyst in a high-temperature environment. It is difficult to accurately measure the temperature of the reforming catalyst in a high-temperature environment. Considering the low measurement accuracy of the temperature, if the supply of the mixed gas of hydrocarbon gas and water vapor is continuously supplied to a lower temperature, the amount of hydrocarbon gas that is consumed uselessly without contributing to power generation will increase.

[0005] The present invention has been made in view of the above circumstances, and an object of the present invention is to suppress the consumption of fuel gas for maintaining the reforming catalyst in a reduced state after power generation stops in an internally reforming fuel cell system.

[0006] A method according to one aspect of the present invention, which solves the above-mentioned problems, provides a control method for an internally reforming fuel cell system having an internally reforming fuel cell. In an internally reforming fuel cell system, fuel supplied to the main body is reformed into fuel gas by a reforming catalyst inside the main body, and electricity is generated by reacting the fuel gas with cathode gas inside the main body. In a control method according to one aspect of the present invention, a system shutdown command to stop the power generation of the fuel cell is detected. After the detection of the shutdown command, the timing for stopping the fuel supply to the reforming catalyst, which has been continuing since before the detection of the shutdown command, is determined based on the inter-electrode resistance between the anode and cathode electrodes of the fuel cell.

[0007] According to the present invention, it is possible to suppress the consumption of fuel gas required to maintain the reforming catalyst in a reduced state after power generation has stopped.

[0008] Figure 1 is a diagram of the fuel cell system according to the first embodiment. Figure 2 is a diagram showing an example of the stack configuration. Figure 3 is a graph showing an example of the change in inter-electrode resistance of the fuel cell. Figure 4 is a flowchart showing an example of the startup process. Figure 5 is a graph showing an example of the change in inter-electrode resistance of the fuel cell. Figure 6 is a flowchart showing an example of the shutdown process. Figure 7 is a flowchart showing the procedure for changing the shutdown process. Figure 8 is a diagram of the fuel cell system according to the second embodiment.

[0009] Embodiments and modifications thereof of the present invention will be described below with reference to the drawings. In the drawings, the same parts are denoted by the same reference numerals and their descriptions are omitted. The present invention can be applied, for example, to an internally reformed fuel cell system mounted on an electric vehicle. The internally reformed fuel cell system to which the present invention is applied is not limited to one mounted on an electric vehicle. In the following description, an internally reformed fuel cell system will be abbreviated as a fuel cell system.

[0010] [First Embodiment] As shown in Figure 1, the fuel cell system 1 of the first embodiment includes a fuel cell 10, a fuel tank 20, a blower 30, a heat exchanger 40, a combustor 50, and a controller 60.

[0011] As shown in Figure 2, the fuel cell 10 is a metal-supported solid oxide fuel cell (metal-supported SOFC), and the stack 11, which is the main body of the fuel cell 10, is composed of metal support cells. The metal support cells that make up the stack 11 are constructed by sequentially stacking an anode electrode 13, an electrolyte layer 14, a cathode electrode 15, and a cathode-side metal support 16 on the anode-side metal support 12. The anode electrode 13 has an anode catalyst. The anode catalyst may contain at least one of the following, for example, nickel (Ni), ruthenium (Ru), platinum (Pt), rhodium (Rh), and cobalt (Co). The surfaces of the metal supports 12 and 16 are covered with separators 17 and 18. The separators 17 and 18 have a plurality of passages 171 and 181 formed by grooves on the back surface that contacts the surface of the metal supports 12 and 16. Passage 171 communicates with the space of the metal support 12, and passage 181 communicates with the space of the metal support 16. The metal support 12 on the anode side incorporates a reformer 19 of the direct internal reforming method. For example, nickel, platinum, etc., can be used for the reforming catalyst 191 of the reformer 19. At low temperatures, the reforming catalyst 191 does not contribute to the reforming of the fuel into fuel gas. When the reforming catalyst 191 is heated to a certain temperature, it begins to contribute to the reforming of the fuel into fuel gas. As shown in Figure 1, the reformer 19 incorporated in the metal support 12 is located in the anode side portion 111 of the stack 11, excluding the cathode electrode 15, together with the anode electrode 13 and the electrolyte layer 14. The reformer of the fuel cell 10 may also be of the indirect internal reforming method. The reformer of the indirect internal reforming method is located outside the metal support 12 and reforms the fuel in passage 171 into fuel gas and supplies it to the metal support 12.

[0012] The fuel tank 20 supplies fuel to the anode portion 111 of the stack 11 when the on-off valve 21 is open. In the following embodiment, the case where the fuel is methane (CH4) will be described. The fuel may be a hydrocarbon other than methane. The methane supplied to the anode portion 111 is introduced into the passage 171 of the anode-side separator 17 in Figure 2. The fuel tank 20 in Figure 1 supplies methane to the combustor 50 when the on-off valve 22 is open.

[0013] The blower 30 supplies air to the cathode-side portion 112 of the stack 11, which is separated from the anode-side portion 111, via the heat exchanger 40. The cathode-side portion 112 has a cathode electrode 15. The air supplied to the cathode-side portion 112 is introduced into the passage 181 of the cathode-side separator 18 in Figure 2. The blower 30 in Figure 1 supplies air to the anode-side portion 111 of the stack 11 without going through the heat exchanger 40 when the on / off valve 31 is open. The air supplied to the anode-side portion 111 is introduced into the passage 171 of the anode-side separator 17 in Figure 2. The heat exchanger 40 heats the air supplied by the blower 30 to the cathode-side portion 112 by heat exchange. The combustor 50 has a heater (not shown). The heater burns methane supplied from the fuel tank 20 to generate high-temperature combustion gas. The combustion gas is supplied to the heat exchanger 40.

[0014] During the startup operation of the fuel cell system 1, a heater (not shown) of the combustor 50 is turned on. When the heater is turned on, the on-off valve 22 is opened, and methane from the fuel tank 20 is supplied to the combustor 50. Combustion gas is generated in the heater of the combustor 50 by the combustion of methane. The methane in the fuel tank 20 may be supplied to the combustor 50 in predetermined amounts, for example, when the temperature of the combustion gas in the combustor 50 is below a first temperature. The first temperature can be, for example, 350°C. The combustion gas from the combustor 50 is supplied to the heat exchanger 40. The heat exchanger 40 heats the air from the blower 30 with the heat of the combustion gas. The stack 11 is heated by the air from the blower 30 that is heated in the heat exchanger 40 and supplied to the cathode side portion 112.

[0015] During the startup operation of the fuel cell system 1, when the temperature of the combustion gas in the combustor 50 rises to a second temperature higher than the first temperature, the on-off valves 21 and 31 are opened while the combustion of methane in the combustor 50 continues. The second temperature is lower than the operating temperature of the fuel cell 10. The operating temperature can be, for example, the lower limit of the temperature range in which the fuel cell 10 can generate electricity using hydrogen (H2) reformed from methane by the reformer 19. The second temperature can be, for example, the starting temperature of the partial oxidation reaction (POX). When the reformer 19 rises to the starting temperature of the partial oxidation reaction, it can partially oxidize the mixed gas of methane and air. For example, if the operating temperature is 1000°C and the reforming catalyst 191 is nickel, the second temperature can be 800°C.

[0016] When the on-off valves 21 and 31 are opened, methane from the fuel tank 20 and air from the blower 30 are introduced into the passage 171 of the anode-side portion 111 of the stack 11, and the methane-air mixture in the passage 171 is partially combusted in the reformer 19 by a partial oxidation reaction. The synthesis gas of hydrogen and carbon monoxide generated in the metal support 12 by this combustion is supplied to the anode electrode 13. Air from the blower 30 is introduced into the passage 181 of the cathode-side portion 112 of the stack 11 via the heat exchanger 40 and supplied to the cathode electrode 15 through the metal support 16. The fuel cell 10 generates electricity through the reaction of hydrogen in the synthesis gas at the anode electrode 13 with oxygen (O2) in the air at the cathode electrode 15. The fuel cell 10 discharges high-temperature combustion gas due to the heat of combustion from the partial oxidation reaction. The heat exchanger 40 uses the heat from the combustion gas discharged by the fuel cell 10 to heat the air in the blower 30.

[0017] During the startup operation of the fuel cell system 1, when the stack 11 is heated to the operating temperature by the combustion gas of the combustor 50, the on-off valve 31 is closed, and the introduction of air from the blower 30 into the passage 171 for the partial oxidation reaction is stopped. Even after closing the on-off valve 31, the on-off valve 21 of the fuel tank 20 remains open, and the introduction of methane into the passage 171 continues. While the on-off valve 31 is open, the reforming catalyst 191 is oxidized by the air in the passage 171, and the power generation efficiency of the fuel cell 10 decreases. After closing the on-off valve 31, the fuel cell system 1 ends the startup operation and transitions to steady-state operation when the stack 11 is heated to the operating temperature and the reforming catalyst 191 is sufficiently reduced. At this point, the reforming catalyst 191 is heated to a temperature that contributes to reforming. In steady-state operation, the temperature of the stack 11 is maintained above the operating temperature by the air heated by the heat exchanger 40. In the stack 11, which is maintained above the operating temperature, methane introduced into the passage 171 of the anode portion 111 is reformed into hydrogen by the reforming catalyst 191. During steady-state operation, the fuel cell 10 generates electricity through the reaction between the hydrogen reformed by the reforming catalyst 191 and oxygen in the air introduced into the passage 181. The remaining methane discharged from the stack 11 without being reformed by the reforming catalyst 191 is burned in the heater of the combustor 50.

[0018] The oxidation and reduction state of the reforming catalyst 191 can be estimated by using the interelectrode resistance between the anode 13 and the cathode 15 as a guide. The oxidation and reduction rates of the reforming catalyst 191 are faster when the temperature of the reforming catalyst 191 is high and slower when the temperature of the reforming catalyst 191 is low. If the reforming catalyst 191 is not sufficiently reduced, it will not contribute to the reforming, and methane will not be easily reformed into hydrogen. The current flowing between the anode 13 and the cathode 15 due to power generation by the fuel cell 10 will be lower as the amount of hydrogen reformed by the reforming catalyst 191 decreases. The interelectrode resistance between the anode 13 and the cathode 15 will be higher as the amount of hydrogen reformed by the reforming catalyst 191 decreases. From the above, it can be seen that the interelectrode resistance between the anode 13 and the cathode 15 reflects the oxidation and reduction state of the reforming catalyst 191 and the temperature of the reforming catalyst 191. For example, when a fuel cell 10, with its stack 11 heated to operating temperature, is generating electricity through a reaction between fuel gas and cathode gas, the inter-electrode resistance between the anode 13 and the cathode 15 reflects the oxidation and reduction state of the reforming catalyst 191.

[0019] Figure 3 shows an example of the change in inter-electrode resistance between the anode electrode 13 and the cathode electrode 15 when the oxidation and reduction state of the reforming catalyst 191 changes, at different temperatures of the reforming catalyst 191. In the example in Figure 3, the fluid inside the metal support 12, which also serves as the anode gas flow path, changes over time in the order of air, hydrogen, and air. The state in which air is present in the metal support 12 simulates the state in which the reforming catalyst 191 is oxidizing. The state in which hydrogen is present in the metal support 12 simulates the state in which the reforming catalyst 191 has reformed methane into hydrogen, or in other words, simulates the state in which the reforming catalyst 191 is being reduced by methane. The temperature of the reforming catalyst 191 is the temperature inside the stack 11 and is difficult to measure. The temperature of the reforming catalyst 191 can be simulated, for example, by measuring the temperature at the outlet portion of the anode gas flow path of the stack 11. The interelectrode resistance between the anode 13 and the cathode 15 does not change significantly even if the oxidation-reduction state of the reforming catalyst 191 changes, when the temperature at the outlet of the stack 11 is around 300°C or 400°C. When the temperature at the outlet of the stack 11 rises to around 500°C, the change in the oxidation-reduction state of the reforming catalyst 191 causes a change in the interelectrode resistance between the anode 13 and the cathode 15. Specifically, in the reduced state of the reforming catalyst 191, the interelectrode resistance is lower than in the oxidized state. When the temperature at the outlet of the stack 11 is 600°C, the interelectrode resistance in the reduced state of the reforming catalyst 191 is lower than when the temperature at the outlet of the stack 11 is 500°C. The changes in interelectrode resistance due to changes in the oxidation-reduction state of the reforming catalyst 191 vary depending on the degree of oxidation of the reforming catalyst 191 at the point when the fluid present in the metal support 12 changes from air to hydrogen. The changes in interelectrode resistance shown in Figure 3 are merely examples. During the startup operation of the fuel cell system 1, at the time the on-off valve 31 is closed, the temperature of the combustion gas in the combustor 50 has risen to the operating temperature of the fuel cell 10, and the temperature of the stack 11, which has been heated by the heat of the combustion gas, has risen to near the operating temperature. At the time the on-off valve 31 is closed, the reforming catalyst 191 has been heated to a temperature that contributes to reforming. If the reforming catalyst 191 is sufficiently reduced, the fuel cell 10 generates electricity through the reaction between the hydrogen reformed by the reforming catalyst 191 and the oxygen in the air at the cathode electrode 15, and an electric current flows between the anode electrode 13 and the cathode electrode 15.At the time the valve 31 is closed, the interelectrode resistance between the anode 13 and the cathode 15 should have decreased to a value corresponding to the temperature of the reforming catalyst 191. Based on the change in interelectrode resistance after closing the valve 31, the oxidation and reduction state of the reforming catalyst 191 can be estimated. The interelectrode resistance between the anode 13 and the cathode 15 can be measured, for example, by applying a constant voltage or constant current between the anode 13 and the cathode 15. For measuring the interelectrode resistance, electrochemical impedance spectroscopy can be used, for example.

[0020] The control method for the fuel cell system 1 can be performed by the controller 60 shown in Figure 1, which functions as a control device for the fuel cell system 1. The controller 60 has a general-purpose microcontroller equipped with an arithmetic unit and an input / output unit. The arithmetic unit includes a CPU (Central Processing Unit) and memory. The memory includes a read-only ROM (Read Only Memory) and a read and writeable RAM (Random Access Memory). The arithmetic unit of the controller 60 can virtually construct multiple information processing circuits by having the CPU execute a program stored in the ROM. The information processing circuits constructed in the arithmetic unit of the controller 60 can, for example, constitute the detection unit 61 and the determination unit 62 of the controller 60. The information processing circuits of the controller 60 may also be configured with dedicated hardware. Dedicated hardware includes devices such as application-specific integrated circuits (ASICs) arranged to perform the necessary information processing functions, and conventional circuit components.

[0021] The CPU of the controller 60 executes a program (not shown) stored in memory during the startup operation of the fuel cell system 1, causing the fuel cell system 1 to perform the startup process shown in Figure 4. The controller 60 starts blowing air from the blower 30 (BLW ON), introducing the air from the blower 30 into the passage 181 of the cathode side portion 112 of the stack 11 via the heat exchanger 40 (step S11). The controller 60 turns on the heater of the combustor 50, heating the air from the blower 30 introduced into the passage 181 by the combustion gas of the combustor 50 as it passes through the heat exchanger 40 (step S13). The controller 60 checks whether the temperature of the combustion gas has reached or exceeded the first temperature (step S15). If the combustion gas is below the first temperature (NO in step S15), the controller 60 opens the closed on-off valve 22 for a certain period of time, allowing a predetermined amount of methane from the fuel tank 20 to be supplied to the heater of the combustor 50 (step S17), and returns to step S15. If the combustion gas is at or above the first temperature (YES in step S15), the controller 60 checks whether the temperature of the combustion gas has reached or exceeded the second temperature (step S19). If the combustion gas is below the second temperature (NO in step S19), the controller 60 repeats step S19. If the combustion gas is at or above the second temperature (YES in step S19), the controller 60 applies a constant voltage or constant current from a power supply (not shown) between the anode 13 and the cathode 15 to start measuring the inter-electrode resistance (step S21). The controller 60 opens the closed on-off valve 31 (POX valve) for the partial oxidation reaction, introducing air from the blower 30 into the passage 171 of the anode side portion 111 of the stack 11 (step S23). The controller 60 opens the closed on-off valve 21, introducing methane from the fuel tank 20 into the passage 171 of the stack 11 (step S25). The order of steps S21 to S25 may be changed. The controller 60 determines whether the inter-electrode resistance between the anode electrode 13 and the cathode electrode 15 has decreased to the resistance value when the reforming catalyst 191, heated to the operating temperature of the fuel cell 10, has been sufficiently reduced (step S27).If the inter-electrode resistance has not decreased to the resistance value of the reduced state (NO in step S27), the controller 60 repeats step S27. If the inter-electrode resistance has decreased to the resistance value of the reduced state (YES in step S27), the controller 60 terminates the measurement of the inter-electrode resistance (step S29). The controller 60 closes the open on / off valve 31 (POX valve) to terminate the introduction of air into the passage 171 of the anode side portion 111 (step S31). The order of steps S29 and S31 may be reversed. The controller 60 then completes the startup process and transitions the fuel cell system 1 from startup operation to steady-state operation.

[0022] During steady-state operation of the fuel cell system 1, the controller 60 heats the stack 11 to the operating temperature of the fuel cell 10. During steady-state operation, the controller 60 controls the amount of methane and air supplied to the stack 11, as well as the temperature of the supplied air, so that the fuel cell 10 generates the required amount of electricity. During steady-state operation, the combustor 50 is supplied with residual methane discharged from the stack 11 without being reformed by the reforming catalyst 191. The combustor 50 burns the methane supplied from the fuel tank 20 and the methane from the stack 11 via the on-off valve 22, and supplies the combustion gas to the heat exchanger 40. During steady-state operation, the controller 60 controls the amount of methane supplied from the fuel tank 20 and the temperature of the air heated by the heat exchanger 40 and supplied to the stack 11, taking into account that residual methane is supplied from the stack 11 to the combustor 50.

[0023] The interelectrode resistance between the anode 13 and the cathode 15 changes depending on the operating state of the fuel cell system 1, for example, as shown in Figure 5. During startup, as methane is introduced into the passage 171 of the anode portion 111, the reduction of the reforming catalyst 191 progresses, and the interelectrode resistance Ri between the anode 13 and the cathode 15 gradually decreases. When the outlet temperature Ts of the stack 11, heated by the combustion gas of the combustor 50, reaches the operating temperature and the reforming catalyst 191 is heated to a temperature that contributes to reforming, the reforming catalyst 191 begins to reform methane into hydrogen. By the time the outlet temperature Ts of the stack 11 reaches the operating temperature, the reforming catalyst 191 is sufficiently reduced, and the reforming catalyst 191 efficiently reforms methane into hydrogen. As the operating state of the fuel cell system 1 transitions from startup to steady-state operation, the fuel cell 10 generates electricity and current flows between the anode electrode 13 and the cathode electrode 15, causing the inter-electrode resistance Ri to decrease to a value corresponding to the temperature of the reforming catalyst 191.

[0024] When shutting down the fuel cell system 1, the supply of methane to the anode portion 111 of the stack 11 is generally continued, thereby continuing to introduce methane into the reforming catalyst 191. By supplying methane to the reforming catalyst 191 even when the fuel cell system 1 is shut down, oxidation of the reforming catalyst 191 can be prevented from progressing during shutdown, thus avoiding an increase in the time required to reduce the reforming catalyst 191 when the fuel cell system 1 is next started. In the example in Figure 5, the supply of methane FS from the fuel tank 20 to the anode portion 111, which was turned on during the startup operation of the fuel cell system 1, is not turned off when the fuel cell system 1 transitions from steady-state operation to shutdown, but continues even after the transition to shutdown. When the fuel cell system 1 is shut down, the controller 60 stops the supply of methane to the combustor 50, ending the heating of the stack 11 by the combustion gas of the combustor 50. Once the heating of the stack 11 ends, the temperature of the stack 11 begins to decrease. When the temperature of the reforming catalyst 191 drops to a level where oxidation and reduction are less likely to occur, the reforming reaction, which is an endothermic reaction, becomes less likely to occur, and the reforming catalyst 191 becomes less likely to contribute to the reforming. When the reforming catalyst 191 stops reforming methane into hydrogen, the fuel cell 10 stops generating electricity, and no current flows between the anode electrode 13 and the cathode electrode 15, so the inter-electrode resistance Ri increases. For example, if the reforming catalyst 191 is nickel, after the fuel cell system 1 is shut down, if the temperature of the reforming catalyst 191 drops to 400°C, the value of the inter-electrode resistance Ri increases even if the reforming catalyst 191 is reduced, and it becomes the same as when the reforming catalyst 191 is oxidized. When methane is supplied to the anode side portion 111 to keep the reforming catalyst 191 in a reduced state after the fuel cell system 1 is shut down, the controller 60 can determine the timing to stop the supply of methane to the reforming catalyst 191 based on the inter-electrode resistance Ri after the shutdown.

[0025] When the CPU of the controller 60 shuts down the fuel cell system 1, it executes a program (not shown) stored in memory to cause the fuel cell system 1 to perform the shutdown process shown in Figure 6. This shutdown process allows the controller 60 to use the detection unit 61 and the determination unit 62 to determine the timing for stopping the supply of methane, which continues even after the fuel cell system 1 has shut down. The detection unit 61 detects a shutdown command for the fuel cell system 1 to stop the power generation of the fuel cell 10. This shutdown command may be generated internally by the controller 60 or input externally. The determination unit 62, after the detection unit 61 detects the shutdown command, determines the timing for stopping the supply of methane to the reforming catalyst 191 based on the inter-electrode resistance Ri between the anode 13 and the cathode 15.

[0026] In the shutdown process, when the detection unit 61 detects a shutdown command, the controller 60 stops power generation by the fuel cell 10 (step S41). The controller 60 applies a constant voltage or constant current from an unshown power source between the anode 13 and the cathode 15 to measure the inter-electrode resistance Ri (step S43). Based on the inter-electrode resistance Ri measured in step S43, the controller 60 determines whether or not to supply methane to the reforming catalyst 191 to maintain the reforming catalyst 191 in a reduced state (step S45). The controller 60 can make the determination in step S45 based on the amount of increase in the value of the inter-electrode resistance Ri measured in step S43 from the value of the inter-electrode resistance Ri during steady-state operation. If the value of the inter-electrode resistance Ri measured in step S43 has increased significantly from the value of the inter-electrode resistance Ri during steady-state operation, it is considered that the reforming catalyst 191 has oxidized after power generation of the fuel cell 10 was stopped. If the value of the interelectrode resistance Ri measured in step S43 has not risen significantly from the value of the interelectrode resistance Ri during steady-state operation, it is considered that the reforming catalyst 191 has been gradually lowering its temperature while maintaining the reduced state after the fuel cell 10 has stopped generating power. In step S45, the controller 60 determines whether the increase in the interelectrode resistance Ri measured in step S43 is above a threshold. The threshold can be predetermined, for example, based on experimental or estimated values ​​of the increase in interelectrode resistance Ri when the reforming catalyst 191 is oxidized after the fuel cell 10 has stopped generating power. If the increase in the interelectrode resistance Ri measured in step S43 is above the threshold, the controller 60 decides not to supply methane to maintain the reforming catalyst 191 in the reduced state (NO in step S45) and performs the reduction treatment on the reforming catalyst 191 (step S47). In the reduction process, the controller 60 increases the valve opening of the on-off valve 21 of the fuel tank 20, supplying a large amount of methane from the fuel tank 20 to the reforming catalyst 191, which is necessary to reduce the oxidized reforming catalyst 191. After the reduction process in step S47, the process returns to step S43. If the increase in the interelectrode resistance Ri measured in step S43 is below a threshold, the controller 60 determines that it will supply methane to maintain the reforming catalyst 191 in a reduced state (YES in step S45).The controller 60 continuously supplies a smaller amount of methane to the reforming catalyst 191 than the reduction treatment to maintain the reforming catalyst 191 in a reduced state (step S49), and measures the interelectrode resistance Ri again (step S51). The controller 60 determines whether the interelectrode resistance Ri measured in step S51 has risen to or above a predetermined value Rth, which corresponds to the interelectrode resistance Ri when the reforming catalyst 191 has dropped to a temperature at which oxidation and reduction are less likely to proceed (step S53). When the reforming catalyst 191 drops to a temperature at which oxidation and reduction are less likely to proceed, the reforming catalyst 191 becomes less involved in reforming, and changes in the interelectrode resistance Ri due to power generation by the fuel cell 10 become less likely. Based on whether the interelectrode resistance Ri measured in step S51 has risen to or above a predetermined value Rth, the controller 60 determines whether the reforming catalyst 191 has dropped to a temperature at which oxidation and reduction are less likely to proceed.

[0027] The predetermined value Rth may be set, for example, based on the value of the interelectrode resistance Ri at a first time point before the reforming of methane to hydrogen by the reforming catalyst 191 begins upon startup of the fuel cell system 1. The first time point may be, for example, the time from the start of the startup process in Figure 4 to the opening of the on-off valve 21 in step S25, before methane is introduced into the passage 171 of the stack 11. At the first time point, methane from the fuel tank 20 has not been introduced into the passage 171, and methane is not supplied to the reformer 19 of the metal support 12, so the reforming catalyst 191 has not started reforming. The interelectrode resistance Ri at this time will be similar to the value when the reforming catalyst 191 is at a temperature where oxidation and reduction do not proceed easily and is not contributing to reforming. By setting the predetermined value Rth based on the value of the interelectrode resistance Ri at the first time point, the predetermined value Rth can be easily set to a value corresponding to the interelectrode resistance Ri when the temperature of the reforming catalyst 191 has dropped to a temperature where oxidation and reduction do not proceed easily.

[0028] The predetermined value Rth may be, for example, a value that is a predetermined percentage lower than the value of the inter-electrode resistance Ri at the first time point. If the predetermined value Rth is set to a value that is a predetermined percentage lower than the value of the inter-electrode resistance Ri at the first time point, the timing for stopping the supply of methane to the reforming catalyst 191 after detection of the operation stop command will arrive earlier than when the value of the inter-electrode resistance Ri at the first time point is set to the predetermined value Rth. The timing for stopping the supply of methane to the reforming catalyst 191 can be set to an appropriate timing that arrives earlier than when the temperature of the reforming catalyst 191 drops to a temperature at which oxidation and reduction do not proceed easily.

[0029] The first time point may be, for example, the time immediately after the fuel cell system 1 is started up. Immediately after the fuel cell system 1 is started up, the heating of the stack 11 by the combustion gas of the combustor 50 in the start-up process shown in Figure 4 has just begun, so it is considered that the reforming catalyst 191 has not yet been heated to a temperature that contributes to reforming. By setting the time immediately after the start-up of the fuel cell system 1, when the reforming catalyst 191 is at a temperature that does not contribute to reforming, as the first time point, the predetermined value Rth can be set to an appropriate value.

[0030] The first time point may be, for example, after the combustor 50, which heats the stack 11 to the operating temperature at which the fuel cell 10 generates electricity, is started up in conjunction with the startup of the fuel cell system 1, but before the fuel cell system 1 transitions to steady-state operation. When the fuel cell system 1 is started up, the combustor 50 starts up and heats the stack 11 to the operating temperature of the fuel cell 10. Once the stack 11 is heated to the operating temperature, the started fuel cell system 1 transitions to steady-state operation. During steady-state operation of the fuel cell system 1, methane is supplied to the reforming catalyst 191, and the fuel cell 10 generates electricity using the hydrogen reformed from methane by the reforming catalyst 191. Even after the fuel cell system 1 is started up, until the fuel cell system 1 transitions to steady-state operation, methane to be reformed into hydrogen is not supplied to the reforming catalyst 191, and the power generation of the fuel cell 10 is suppressed. During the period from the start-up of the combustor 50 to the transition to steady-state operation of the fuel cell system 1, the reforming catalyst 191 does not reform the methane, similar to when the temperature is such that oxidation and reduction do not proceed easily, and the power generation of the fuel cell 10 is suppressed, causing the inter-electrode resistance Ri of the fuel cell 10 to rise. By designating a point in time during this period as the first point in time, the predetermined value Rth can be set to an appropriate value.

[0031] The first time point may be, for example, the time immediately before the temperature of the reforming catalyst 191, which has been heated in conjunction with the startup of the fuel cell system 1, reaches a temperature that contributes to reforming, and reforming by the reforming catalyst 191 begins. When the fuel cell system 1 is started up, the stack 11 is heated to the operating temperature of the fuel cell 10. In the process of the stack 11 being heated to the operating temperature, the reforming catalyst 191 is heated from a temperature that does not contribute to reforming to a temperature that does contribute to reforming. As long as the temperature of the heated reforming catalyst 191 is at a temperature that does not contribute to reforming, the reforming catalyst 191 has not started reforming methane to hydrogen, so the power generation of the fuel cell 10 is suppressed and the inter-electrode resistance Ri of the fuel cell 10 increases. By setting the time immediately before the reforming by the reforming catalyst 191 begins as the first time point, the predetermined value Rth can be set to an appropriate value.

[0032] In the shutdown process, if the inter-electrode resistance Ri in step S51 is less than a predetermined value Rth (NO in step S53), the controller 60 returns to step S49 and continues supplying methane to the reforming catalyst 191. If the inter-electrode resistance Ri in step S51 is greater than or equal to a predetermined value Rth (YES in step S53), the controller 60 stops supplying methane to the reforming catalyst 191, which has been continuing since before the detection of the shutdown command (step S55). The controller 60 confirms that the temperature of the stack 11 has dropped to or below the temperature at which operation was stopped, and shuts down the operation of the fuel cell system 1 (step S57).

[0033] In the fuel cell system 1 of the first embodiment, when the controller 60 detects a command to stop the operation of the fuel cell system 1, it stops heating the stack 11 with the combustion gas from the combustor 50. After detection of the operation stop designation, the temperature of the reforming catalyst 191 built into the stack 11 begins to decrease due to the end of heating of the stack 11. Even after detection of the operation stop command, the controller 60 continues to supply methane to the reforming catalyst 191 as long as the measured inter-electrode resistance Ri is less than a predetermined value Rth. When the inter-electrode resistance Ri rises to or above the predetermined value Rth after detection of the operation stop command, the controller 60 assumes that the reforming catalyst 191 has fallen to a temperature at which oxidation and reduction are less likely to proceed, and stops supplying methane to the reforming catalyst 191. By determining the timing to stop supplying methane to the reforming catalyst 191 based on the inter-electrode resistance Ri after detection of the operation stop command, it becomes easier to stop supplying methane to the reforming catalyst 191 at an appropriate timing when the reforming catalyst 191 has fallen to a temperature at which oxidation and reduction are less likely to proceed. If the temperature of the reforming catalyst 191 drops to a level where oxidation and reduction are less likely to occur, the oxidation of the reforming catalyst 191 will not progress significantly even if methane is continuously supplied to the reforming catalyst 191. When it is thought that the temperature of the reforming catalyst 191 has dropped to a level where oxidation and reduction are less likely to occur, stopping the supply of methane to the reforming catalyst 191 can suppress the wasteful consumption of methane that would otherwise be used to suppress the oxidation of the reforming catalyst 191. Even after detecting a stop command, as long as it is thought that the temperature of the reforming catalyst 191 has not dropped to a level where oxidation and reduction are less likely to occur, continuing the supply of methane to the reforming catalyst 191 can suppress the rapid oxidation of the reforming catalyst 191 through the supply of methane.

[0034] In this embodiment of the fuel cell 10, which is a metal-supported solid oxide fuel cell, the anode electrode 13, electrolyte layer 14, and cathode electrode 15 of the metal support cells constituting the stack 11 are reinforced by metal supports 12 and 16. This reinforcement allows the anode electrode 13, electrolyte layer 14, and cathode electrode 15 to be made thinner, thereby enabling miniaturization of the stack 11. After detecting a command to stop the operation of the fuel cell system 1, the supply of methane to the reforming catalyst 191 is stopped at a timing determined based on the inter-electrode resistance Ri of the fuel cell 10, thereby achieving both the suppression of wasteful methane consumption and miniaturization of the stack 11. Since the anode catalyst of the anode electrode 13 contains at least one of nickel, ruthenium, platinum, rhodium, and cobalt, when platinum is used for the reforming catalyst 191, the reforming of methane by the reforming catalyst 191 can be performed in the anode-side portion 111 of the stack 11. In a fuel cell 10 that can be constructed at a lower cost than a configuration in which methane reforming is performed in the cathode-side portion 112 of the stack 11, it is possible to achieve both the suppression of wasteful methane consumption and miniaturization of the stack 11.

[0035] [Modification of the First Embodiment] In the stop process shown in Figure 6, the determination in step S53 may be changed to a determination of whether the stabilization time T for which the interelectrode resistance Ri measured in step S51 does not change continuously has reached a predetermined time Tth, as shown in step S54 in Figure 7. If the controller 60 determines that the stabilization time T for the interelectrode resistance Ri measured in step S51 is less than the predetermined time Tth (NO in step S54), it returns to step S49. If the controller 60 determines that the stabilization time T has reached the predetermined time Tth (YES in step S54), it proceeds to step S55. After detection of the stop command for the fuel cell system 1, when the heating of the stack 11 ends and the temperature of the reforming catalyst 191 drops to a temperature at which oxidation and reduction are less likely to proceed, the rise in the interelectrode resistance Ri of the fuel cell 10 saturates, and the interelectrode resistance Ri stops changing continuously. When the controller 60 detects a stop command and the inter-electrode resistance Ri remains unchanged for a predetermined time Tth, it assumes that the temperature of the reforming catalyst 191 has dropped to a level where oxidation and reduction are less likely to occur, and stops supplying methane to the reforming catalyst 191. By monitoring the stable time T during which the inter-electrode resistance Ri remains unchanged after the detection of a stop command, it is possible to stop the supply of methane to the reforming catalyst 191 at the timing when the temperature of the reforming catalyst 191 has dropped to a level where oxidation and reduction are less likely to occur, thereby suppressing unnecessary consumption of methane.

[0036] [Second Embodiment] As shown in Figure 8, the fuel cell system 1 of the second embodiment has two stacks, a first and a second, 11 and 81, each composed of a metal support cell and connected in series, as a stack. The first stack 11 is composed of the stack 11 of the first embodiment. As shown in Figure 2, the second stack 81, like the first stack 11, has a metal support 82 on the anode side, an anode electrode 83, an electrolyte layer 84, a cathode electrode 85, a metal support 86 on the cathode side, and separators 87 and 88. The separators 87 and 88 have multiple passages 871 and 881 formed by grooves on the back surface that contact the surfaces of the metal supports 82 and 86. The metal support 82 on the anode side incorporates a reformer 89 of the direct internal reforming type and is arranged together with the anode electrode 83 and the electrolyte layer 84 in the anode side portion 811 of the second stack 81, excluding the cathode electrode 85. The reforming catalyst 891 in reformer 89 is the same as the reforming catalyst 191 in the first stack 11. In the second embodiment, air from the blower 30 heated in the heat exchanger 40 in Figure 8 is supplied to the passage 881 of the cathode-side portion 812 of the second stack 81, which is separated from the anode-side portion 811. The air discharged from the second stack 81 is supplied to the passage 181 of the cathode-side portion 112 of the first stack 11. In the second embodiment, the partial oxidation reaction of the methane-air mixed gas during startup of the fuel cell system 1 is carried out in the reformers 19 and 89 of the first and second stacks 11 and 81. In the second embodiment, during steady-state operation of the fuel cell system 1, the remaining methane discharged from the first stack 11 without being reformed by the reforming catalyst 191 of the first stack 11 is supplied to the passage 871 of the anode-side portion 811 of the second stack 81. The methane in passage 871 is reformed into hydrogen by the reforming catalyst 891 of the reformer 89, similar to the first stack 11. The remaining methane discharged from the second stack 81 without being reformed by the reforming catalyst 891 is burned by the heater of the combustor 50. In the second embodiment as well, the controller 60 continues to supply methane to the reforming catalysts 191 and 891 after detecting a stop command during the shutdown process.The controller 60 stops supplying methane to the reforming catalysts 191 and 891 when the inter-electrode resistance Ri of the fuel cell 10 rises to or above a predetermined value Rth after detecting a stop command. The inter-electrode resistance Ri used for the stop process can be at least one of the measured value of the first stack 11 and the measured value of the second stack 81. The predetermined value Rth, stabilization time T, and predetermined time Tth used for the stop process can be set, for example, to correspond to the stack from which the inter-electrode resistance Ri used for the stop process was measured among the first and second stacks 11 and 81. In the second embodiment, in a fuel cell system 1 in which the power generation efficiency is increased by utilizing the residual methane discharged from the first stack 11 for reforming by the reforming catalyst 891 in the second stack 81, the wasteful consumption of methane to suppress oxidation of the reforming catalysts 191 and 891 can be suppressed.

[0037] The amount of reforming catalyst 891 in the second stack 81 may be the same as the amount of reforming catalyst 191 in the first stack 11. Since the reforming catalyst 891 in the second stack 81 reforms the remaining methane that was not reformed by the reforming catalyst 191 in the first stack 11, the amount of reforming catalyst 891 may be less than the amount of reforming catalyst 191. If the amount of reforming catalyst 891 is reduced, the costs incurred for the reforming catalysts 191 and 891 can be suppressed. In a metal-supported solid oxide fuel cell, it is possible to suppress the wasteful consumption of methane and to miniaturize the first and second stacks 11 and 81 at the same time.

[0038] The control method for the fuel cell system 1 according to the above embodiments is also applicable, for example, when precious metals other than nickel and platinum are used for the reforming catalysts 191 and 891. It may also be performed while the electric vehicle is stopped or parked. The power supply system to be controlled by the control method or control device according to the present invention is not limited to a power supply system mounted on an electric vehicle. The present invention is broadly applicable to a power supply system in which a first energy storage device connected to a solar cell is charged with the power of the solar cell, and a second energy storage device is charged with the converted power obtained by converting the power of the first energy storage device. The fuel cell only needs to have a reformer that is integrally formed with the anode electrode and is located on the anode side of the stack. An example of a fuel cell in which the reformer is integrally formed with the anode electrode is a medium-temperature fuel cell such as a proton-conducting ceramic fuel cell (PCFC).

[0039] The embodiments described above are merely examples of the present invention. Therefore, the present invention is not limited to the embodiments described above, and various modifications are possible in forms other than those described above, as long as they do not depart from the technical spirit of the present invention, depending on the design and other factors.

[0040] 1 Internal reforming fuel cell system, 10, 80 Fuel cell, 11 (First) stack (main body), 12, 16, 82, 86 Metal support, 13, 83 Anode electrode, 14, 84 Electrolyte layer, 15, 85 Cathode electrode, 61 Detection unit, 62 Determination unit, 81 Second stack (main body), 191, 891 Reforming catalyst, Ri Inter-electrode resistance, Rth Determined value, T Stabilization time, Tth Determined time.

Claims

1. A control method for an internally reforming fuel cell system having an internally reforming fuel cell, which reforms fuel supplied to the main body into fuel gas using a reforming catalyst inside the main body, and generates electricity by reacting the fuel gas with a cathode gas inside the main body, the method comprising: detecting a system operation stop command to stop the power generation of the fuel cell; and determining the timing to stop the supply of the fuel to the reforming catalyst, which has been continuing since before the detection of the operation stop command, based on the inter-electrode resistance between the anode and cathode electrodes of the fuel cell after the detection of the operation stop command.

2. A control method for an internal reforming fuel cell system according to claim 1, wherein, after detection of the operation stop command, the supply of the fuel to the reforming catalyst is continued as long as the inter-electrode resistance is below a predetermined value, and the supply of the fuel to the reforming catalyst is stopped when the inter-electrode resistance rises to or above the predetermined value.

3. The control method for an internal reforming fuel cell system according to claim 2, wherein the predetermined value is set based on the value of the inter-electrode resistance at a first time point before the reforming of the fuel into the fuel gas by the reforming catalyst begins upon startup of the system.

4. The control method for an internal reforming fuel cell system according to claim 3, wherein the predetermined value is a value that is a predetermined percentage lower than the value of the inter-electrode resistance at the first time point.

5. The control method for an internal reforming fuel cell system according to claim 3 or 4, wherein the first time point is the time immediately after the system is started up.

6. The control method for an internal reforming fuel cell system according to claim 3 or 4, wherein the first time point is after the combustor that heats the main body to an operating temperature at which the fuel cell generates electricity is started in conjunction with the startup of the system, and before the system transitions to steady-state operation in which the main body is heated to the operating temperature and the fuel cell generates electricity.

7. The control method for an internal reforming fuel cell system according to claim 3 or 4, wherein the first time point is the time immediately before the temperature of the reforming catalyst, which has been heated in conjunction with the startup of the system, reaches a temperature that contributes to the reforming of the fuel into the fuel gas, and the reforming of the fuel into the fuel gas by the reforming catalyst begins.

8. A control method for an internal reforming fuel cell system according to claim 1, wherein, after the heating of the main body is stopped in response to the detection of the operation stop command, the supply of the fuel to the reforming catalyst is continued as long as the stabilization time during which the inter-electrode resistance does not change continuously is less than a predetermined time, and the supply of the fuel to the reforming catalyst is stopped when the stabilization time reaches the predetermined time.

9. A control method for an internal reforming fuel cell system according to any one of claims 2 to 8, wherein the fuel is continuously supplied to the reforming catalyst during the period in which the supply of the fuel to the reforming catalyst is continued after the detection of the operation stop command.

10. A control method for an internal reforming fuel cell system according to any one of claims 1 to 9, wherein the fuel cell is a metal-supported solid oxide fuel cell (metal-supported SOFC), the stack being composed of metal support cells in which the anode electrode, electrolyte layer, and cathode electrode are sequentially stacked on a metal support.

11. A control method for an internal reforming fuel cell system according to claim 10, wherein the anode electrode has an anode catalyst comprising at least one of nickel, ruthenium, platinum, rhodium, and cobalt, and the stack has platinum as the reforming catalyst in the anode-side portion of the stack excluding the cathode electrode.

12. A control method for an internal reforming fuel cell system according to claim 10 or 11, wherein the stack comprises a first stack and a second stack, each composed of the metal support cells and connected in series, the fuel supplied to the main body is supplied to the first stack, and the second stack is supplied with the remaining fuel that has been discharged from the first stack from the fuel supplied to the first stack.

13. The control method for an internal reforming fuel cell system according to claim 12, wherein the amount of reforming catalyst in the second stack is less than the amount of reforming catalyst in the first stack.

14. A control device for an internally reforming fuel cell system having an internally reforming fuel cell, which reforms fuel supplied to the main body into fuel gas using a reforming catalyst in the main body, and generates electricity by reacting the fuel gas with a cathode gas in the main body, comprising: a detection unit for detecting a system operation stop command to stop the power generation of the fuel cell; and a determination unit that, after detection of the operation stop command, determines the timing for stopping the supply of the fuel to the reforming catalyst, which has been continuing since before detection of the operation stop command, based on the inter-electrode resistance between the anode and cathode electrodes of the fuel cell.

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