Fuel battery system

Abstract

To provide an economically advantageous fuel battery system capable of reducing a fuel usage amount while avoiding damages to a fuel battery cell due to a differential pressure between a fuel chamber side and an oxidant gas chamber side of a fuel battery in an operation stop step by effectively utilizing a fuel off-gas.SOLUTION: A fuel battery system includes: a solid oxide fuel battery part 1; a combustion part 4 for combusting a fuel off-gas and an oxidant off-gas from the fuel battery part 1; an operation control part 9 for performing an operation stop step for stopping an operation of the fuel battery part 1 by lowering a temperature of the fuel battery part 1 to a target temperature; and fuel off-gas recirculation means 40 of being capable of recirculating the fuel off-gas discharged from the fuel chamber of the fuel battery part 1 to a fuel gas supply part 20. In the operation stop step, the operation control part 9 increases a flow rate of an oxygen-containing gas supplied from an oxygen-containing gas supply part 30 to an oxygen-containing gas chamber of the fuel battery part 1, and causes the fuel off-gas recirculation means 40 to recirculate the fuel off-gas discharged from the fuel chamber to the fuel gas supply part 20.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a fuel cell system comprising: a solid oxide fuel cell section that generates electricity through an electrochemical reaction between fuel gas supplied from a fuel gas supply section to a fuel chamber and oxidant gas supplied from an oxidant gas supply section to an oxidant gas chamber; a combustion section that combusts fuel off-gas discharged from the fuel chamber and oxidant off-gas discharged from the oxidant gas chamber; and an operation control section that can freely perform an operation shutdown process that controls the operation of each section to lower the temperature of the fuel cell section to a target temperature and stop operation. [Background technology]

[0002] Background of the present invention is, for example, a solid oxide fuel cell system described in Patent Document 1. This solid oxide fuel cell system includes a reformer for steam reforming fuel gas, a cell stack for generating electricity by oxidation and reduction of the reformed fuel gas reformed in the reformer and the oxidant gas, an oxidant gas supply means for supplying the oxidant gas to the cell stack, a fuel gas supply means for supplying the fuel gas to the reformer, a combustor for burning the fuel off-gas and air off-gas from the cell stack, and a water vapor condenser for condensing the water vapor contained in the fuel off-gas.

[0003] In this solid oxide fuel cell system, a fuel off-gas delivery passage that delivers the fuel off-gas from which water vapor has been removed in the water vapor condenser to the combustor is provided with a recycle passage that returns a portion of the fuel off-gas to the fuel gas supply passage that supplies the fuel gas to the reformer.

[0004] As a result, in this solid oxide fuel cell system, a portion of the fuel off-gas after the water vapor has been condensed and removed in the water vapor condenser is returned from the fuel off-gas delivery passage through the recycle passage to the fuel gas supply passage, mixed with the fuel gas in the fuel gas supply passage, reformed in the reformer, and delivered to the cell stack. By recycling a portion of the fuel off-gas in this way, the power generation efficiency of the cell stack can be improved. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2021-125290 Summary of the Invention [Problem to be solved by the invention]

[0006] In this way, in the solid oxide fuel cell system described in Patent Document 1, fuel off-gas is utilized to increase the power generation efficiency of the cell stack, but there is a demand for more effective utilization of fuel off-gas.

[0007] However, a fuel cell system that operates at high temperatures, such as a solid oxide fuel cell system, has a problem in that the shutdown process, which involves lowering the temperature of the fuel cell to a target temperature before shutting down the system, takes several hours or more to shut down (cool down). Moreover, during the shutdown process, it is necessary to continue supplying a small amount of fuel to the fuel electrode of the fuel cell to prevent oxidation of the fuel electrode. Therefore, from an economical point of view, it is desirable to shorten the time required for cooling during the shutdown process as much as possible.

[0008] The cooling rate during the shutdown process can be increased by increasing the flow rate of oxidant gas supplied to the fuel cell or its surroundings. However, in a system in which the oxidant gas chamber and the fuel chamber are closed, such as a flat-type cell stack, increasing the flow rate of oxidant gas supplied to the oxidant gas chamber increases the pressure in the oxidant gas chamber. This creates a pressure difference between the oxidant gas chamber and the fuel chamber, exerting force on the fuel cell located between them. If this force becomes excessive, the fuel cell may be damaged. Therefore, the flow rate of oxidant gas supplied to the oxidant gas chamber is limited, leaving room for improvement in shortening the time required for cooling during the shutdown process.

[0009] In view of this situation, the main objective of the present invention is to provide an economically advantageous fuel cell system that can effectively utilize fuel off-gas to avoid the risk of damage to the fuel cell cells due to an excessive pressure difference between the fuel chamber side and the oxidizer gas chamber side of the fuel cell during the shutdown process, while shortening the cooling time by increasing the flow rate of oxidizer gas supplied to the oxidizer gas chamber, thereby reducing the amount of fuel used during the shutdown process. [Means for solving the problem]

[0010] The first characteristic configuration of the present invention is a solid oxide fuel cell unit that generates electricity by an electrochemical reaction between a fuel gas supplied from a fuel gas supply unit to a fuel chamber and an oxidant gas supplied from an oxidant gas supply unit to an oxidant gas chamber; a combustion section that combusts the fuel off-gas discharged from the fuel chamber and the oxidant off-gas discharged from the oxidant gas chamber; an operation control unit capable of controlling the operation of each unit to lower the temperature of the fuel cell unit to a target temperature and stop the operation, a fuel off-gas recirculation means for recirculating at least a portion of the fuel off-gas discharged from the fuel chamber to the fuel gas supply section, During the operation shutdown process, the operation control unit increases the flow rate of the oxidant gas supplied from the oxidant gas supply unit to the oxidant gas chamber, and returns at least a portion of the fuel off-gas discharged from the fuel chamber to the fuel gas supply unit using the fuel off-gas return means.

[0011] According to this configuration, during the shutdown process, the flow rate of the oxidant gas supplied from the oxidant gas supply unit to the oxidant gas chamber of the fuel cell unit is increased, thereby making it possible to more effectively cool the fuel cell unit and shorten the cooling time required to lower the temperature of the fuel cell unit to the target temperature. As a result, even during cooling of the fuel cell unit, the amount of fuel gas that needs to be continuously supplied to the anode of the fuel cell unit to prevent oxidation of the anode can be reduced.

[0012] Furthermore, by simultaneously increasing the flow rate of the oxidant gas supplied to the oxidant gas chamber of the fuel cell unit and returning at least a portion of the fuel off-gas discharged from the fuel chamber of the fuel cell unit to the fuel gas supply unit by the fuel off-gas return means, the fuel off-gas can be effectively used as part of the fuel gas supplied to the fuel chamber of the fuel cell unit. This allows the flow rate of the fuel gas supplied to the fuel chamber of the fuel cell unit to be increased in accordance with the increase in the flow rate of the oxidant gas supplied to the oxidant gas chamber, without increasing the amount of fuel gas used during the shutdown process. As a result, the pressure on the oxidant gas chamber side of the fuel cell unit increases with the increase in the flow rate of the oxidant gas. As a result, the risk of damage to the fuel cells located between the fuel chamber and the oxidant gas chamber of the fuel cell unit due to an excessive pressure difference between the fuel chamber side and the oxidant gas chamber side of the fuel cell unit can be avoided.

[0013] Furthermore, by returning at least a portion of the fuel off-gas to the fuel gas supply section, the flow rate of the fuel off-gas flowing from the fuel chamber of the fuel cell section to the combustion section is reduced, weakening the combustion power in the combustion section, thereby further shortening the cooling time mentioned above.

[0014] Therefore, by effectively utilizing the fuel off-gas, it is possible to avoid the risk of damage to the fuel cell cells due to an excessive pressure difference between the fuel chamber side and the oxidizer gas chamber side of the fuel cell section during the shutdown process, while also shortening the cooling time by increasing the amount of oxidizer gas supplied to the oxidizer gas chamber of the fuel cell section and reducing the amount of fuel off-gas flowing to the combustion section.As a result, it is possible to provide an economically advantageous fuel cell system that can reduce the amount of fuel used during the shutdown process.

[0015] A second characteristic configuration of the present invention is such that the fuel off-gas circulation means includes a fuel off-gas flow rate adjusting unit that can freely adjust the flow rate of the fuel off-gas to be refluxed to the fuel gas supply unit, During the operation shutdown process, the operation control unit monitors the pressure on the fuel chamber side and the pressure on the oxidant gas chamber side, and adjusts the flow rate of the fuel off-gas returned to the fuel gas supply unit by the fuel off-gas flow rate adjustment unit so that the differential pressure between the fuel chamber side and the oxidant gas chamber side is maintained within an appropriate range.

[0016] According to this configuration, during the shutdown process, for example, if the flow rate of the oxidant gas supplied from the oxidant gas supply unit to the oxidant gas chamber of the fuel cell unit is increased to shorten the cooling time of the fuel cell unit, causing the pressure on the oxidant gas chamber side to become higher than the pressure on the fuel chamber side in the fuel cell unit, and the differential pressure between the fuel chamber side and the oxidant gas chamber side of the fuel cell unit is about to deviate from the appropriate range, the fuel off-gas flow rate adjuster correspondingly increases the flow rate of the fuel off-gas returned to the fuel gas supply unit. As a result, the flow rate of the fuel gas supplied from the fuel gas supply unit to the fuel chamber of the fuel cell unit increases, and the pressure on the fuel chamber side of the fuel cell unit increases, maintaining the differential pressure between the fuel chamber side and the oxidant gas chamber side of the fuel cell unit within the appropriate range.

[0017] Furthermore, when the flow rate of the fuel off-gas returned to the fuel gas supply unit is increased in this manner, the pressure on the fuel chamber side of the fuel cell unit rises to exceed the pressure on the oxidant gas chamber side, and the pressure difference between the fuel chamber side and the oxidant gas chamber side of the fuel cell unit approaches an appropriate range, the fuel off-gas flow rate adjuster accordingly reduces the flow rate of the fuel off-gas returned to the fuel gas supply unit, thereby reducing the flow rate of the fuel gas supplied from the fuel gas supply unit to the fuel chamber of the fuel cell unit and lowering the pressure on the fuel chamber side of the fuel cell unit, thereby maintaining the pressure difference between the fuel chamber side and the oxidant gas chamber side of the fuel cell unit within an appropriate range.

[0018] As a result, the risk of damage to the fuel cell cells located between the fuel chamber and oxidant gas chamber of the fuel cell section due to an excessively large pressure difference between the fuel chamber side and the oxidant gas chamber side of the fuel cell section can be more reliably avoided.

[0019] A third characteristic configuration of the present invention is provided with a fuel off-gas supply passage that guides fuel off-gas discharged from the fuel chamber to the combustion section, the fuel off-gas circulation means includes a fuel off-gas circulation path that guides the fuel off-gas discharged from the fuel chamber to the fuel gas supply part, and a flow path switching part that can freely switch a flow path through which the fuel off-gas discharged from the fuel chamber flows between the fuel off-gas supply path and the fuel off-gas circulation path, In the operation stopping step, the operation control unit switches the flow path of the fuel off-gas discharged from the fuel chamber from the fuel off-gas supply path to the fuel off-gas return path by the flow path switching unit.

[0020] According to this configuration, during the operation shutdown process, the flow rate of the oxidant gas supplied to the oxidant gas chamber of the fuel cell section is increased to shorten the cooling time of the fuel cell section, and at the same time, the flow path of the fuel off-gas discharged from the fuel chamber of the fuel cell section is switched from the fuel off-gas supply path to the fuel off-gas return path by the flow path switching section, so that the entire amount of fuel off-gas discharged from the fuel chamber is returned to the fuel gas supply section, and fuel gas containing the entire amount of fuel off-gas is supplied to the fuel chamber of the fuel cell section.

[0021] This allows the flow rate of the oxidant gas supplied to the oxidant gas chamber of the fuel cell unit to be increased to shorten the cooling time of the fuel cell unit, while minimizing the amount of fuel gas that needs to be continuously supplied to the anode to prevent oxidation of the anode as described above. As a result, the pressure on the oxidant gas chamber side of the fuel cell unit can be increased in an appropriate manner along with the increase in the pressure on the oxidant gas chamber side that accompanies the increase in the flow rate of the oxidant gas supplied to the oxidant gas chamber.

[0022] Furthermore, since the entire amount of fuel off-gas is returned to the fuel gas supply section, fuel off-gas does not flow from the fuel chamber of the fuel cell section to the combustion section, which significantly weakens the combustion power in the combustion section, thereby making it possible to further reduce the cooling time mentioned above.

[0023] Therefore, during the shutdown process, the amount of fuel used can be more effectively reduced while avoiding the risk of damage to the fuel cell cells due to an excessively large differential pressure between the fuel chamber side and the oxidizer gas chamber side of the fuel cell section.

[0024] A fourth characteristic configuration of the present invention is such that the flow path switching unit includes a supply path opening / closing valve that opens and closes the fuel off-gas supply path and a return path opening / closing valve that opens and closes the fuel off-gas return path, During the operation stopping process, the operation control unit switches the flow path of the fuel off-gas discharged from the fuel chamber from the fuel off-gas supply path to the fuel off-gas return path by closing the supply path opening / closing valve and opening the return path opening / closing valve, and monitors the pressure on the fuel chamber side and the pressure on the oxidizer gas chamber side, and opens the supply path opening / closing valve when the pressure on the fuel chamber side becomes higher than the pressure on the oxidizer gas chamber side, and closes the supply path opening / closing valve when the pressure on the fuel chamber side becomes lower than the pressure on the oxidizer gas chamber side, thereby adjusting the flow rate of the fuel off-gas returned to the fuel gas supply unit, so that the differential pressure between the fuel chamber side and the oxidizer gas chamber side is maintained within an appropriate range.

[0025] According to this configuration, during the operation shutdown process, when the flow rate of the oxidant gas supplied to the oxidant gas chamber of the fuel cell section is increased to shorten the cooling time of the fuel cell section, at the same time, the supply path opening / closing valve is closed and the return path opening / closing valve is opened, thereby switching the flow path of the fuel off-gas discharged from the fuel chamber of the fuel cell section from the fuel off-gas supply path to the fuel off-gas return path, and the entire amount of fuel off-gas discharged from the fuel chamber of the fuel cell section is returned to the fuel gas supply section.

[0026] This allows the flow rate of the fuel gas supplied to the fuel electrode of the fuel cell section to be suitably increased as the flow rate of the oxidant gas supplied to the oxidant gas chamber of the fuel cell section increases, and the pressure on the fuel chamber side of the fuel cell section to be suitably increased as the pressure on the oxidant gas chamber side increases due to the increase in the flow rate of the oxidant gas supplied to the oxidant gas chamber.

[0027] When the pressure on the fuel chamber side of the fuel cell unit becomes higher than the pressure on the oxidant gas chamber side due to the entire fuel off-gas being returned to the fuel gas supply unit in this way, and the pressure difference between the fuel chamber side and the oxidant gas chamber side of the fuel cell unit is about to deviate from the appropriate range, the supply path on-off valve opens in response, and the fuel off-gas discharged from the fuel chamber of the fuel cell unit is distributed to the combustion unit. As a result, the flow rate of the fuel off-gas returned to the fuel gas supply unit decreases, the flow rate of the fuel gas supplied from the fuel gas supply unit to the fuel chamber of the fuel cell unit decreases, and the pressure on the fuel chamber side of the fuel cell unit drops, so that the pressure difference between the fuel chamber side and the oxidant gas chamber side of the fuel cell unit is maintained within the appropriate range.

[0028] Furthermore, when the pressure on the fuel chamber side of the fuel cell unit becomes lower than the pressure on the oxidant gas chamber side due to the fuel off-gas being distributed to the combustion unit in this manner, and the pressure difference between the fuel chamber side and the oxidant gas chamber side of the fuel cell unit is about to deviate from the appropriate range, the supply path on-off valve closes in response, and all of the fuel off-gas discharged from the fuel chamber of the fuel cell unit is returned to the fuel gas supply unit. This increases the flow rate of fuel off-gas returned to the fuel gas supply unit, increases the flow rate of fuel gas supplied from the fuel gas supply unit to the fuel chamber of the fuel cell unit, and increases the pressure on the fuel chamber side of the fuel cell unit, thereby maintaining the pressure difference between the fuel chamber side and the oxidant gas chamber side of the fuel cell unit within the appropriate range.

[0029] Therefore, with a simple control configuration that simply opens and closes the supply path opening / closing valve based on the pressure on the fuel chamber side and the pressure on the oxidizer gas chamber side in the fuel cell section, the differential pressure between the fuel chamber side and the oxidizer gas chamber side of the fuel cell section can be maintained within an appropriate range, and the risk of damage to the fuel cell cells located between the fuel chamber and the oxidizer gas chamber of the fuel cell section due to the differential pressure between the fuel chamber side and the oxidizer gas chamber side of the fuel cell section becoming excessive can be more reliably avoided. [Brief explanation of the drawings]

[0030] [Figure 1] FIG. 1 is a block diagram showing the configuration of a fuel cell system according to a first embodiment; [Figure 2] Vertical cross-sectional view showing the configuration of a fuel cell [Figure 3] Flowchart showing the operation shutdown process in the first embodiment [Figure 4] FIG. 10 is a block diagram showing the configuration of a fuel cell system according to a second embodiment. [Figure 5] Flowchart showing the operation shutdown process in the second embodiment DETAILED DESCRIPTION OF THE INVENTION

[0031] [First embodiment] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of a fuel cell system according to the present invention will now be described with reference to the accompanying drawings.

[0032] As shown in FIG. 1, the fuel cell system exemplified in this first embodiment includes a solid oxide fuel cell section 1, a fuel gas supply section 20 that supplies fuel gas to the fuel cell section 1, and an oxidant gas supply section 30 that supplies oxidant gas (air containing oxygen gas) to the fuel cell section 1.

[0033] As shown in FIG. 1, the fuel cell section 1 includes a flat-plate cell stack 10 configured by stacking a plurality of fuel cell units 11 (see FIG. 2). As shown in FIG. 2, the fuel cell unit 11 includes a plate-shaped electrolyte 11A made of solid oxide, a fuel electrode 11B and an oxidant gas electrode 11C arranged with the electrolyte 11A sandwiched therebetween, a fuel electrode-side separator 11D forming a fuel chamber 12 to which fuel gas is supplied from a fuel gas supply unit 20 between the fuel electrode 11B and the separator 11D, and an oxidant gas electrode-side separator 11E forming an oxidant gas chamber 13 to which oxidant gas is supplied from an oxidant gas supply unit 30 between the fuel cell unit 11 and the separator 11C. The fuel cell unit 11 generates electricity through an electrochemical reaction between the fuel gas supplied from the fuel gas supply unit 20 to the fuel chamber 12 and the oxidant gas supplied from the oxidant gas supply unit 30 to the oxidant gas chamber 13. The plurality of fuel cell units 11 are stacked while electrically connected in series.

[0034] As shown in Fig. 1, the fuel gas supply unit 20 includes a fuel gas supply blower 22 that supplies hydrocarbon fuel gas such as city gas through a fuel gas supply path 21 to the fuel chamber 12 (see Fig. 2) of the fuel cell unit 1, a desulfurization unit 23 that desulfurizes the fuel gas from the fuel gas supply blower 22, a vaporization unit 24 that generates steam by heating reforming water supplied from a reforming water tank (not shown), and a reforming unit 25 that steam-reforms the fuel gas desulfurized in the desulfurization unit 23 using the steam generated in the vaporization unit 24. Thus, the fuel gas supply unit 20 is configured to supply the fuel gas (reformed fuel gas) reformed in the reforming unit 25 to the fuel chamber 12 of the fuel cell unit 1. The rotation speed of the fuel gas supply blower 22 is controlled to adjust the flow rate of the fuel gas supplied to the fuel chamber 12 of the fuel cell unit 1.

[0035] Fuel gas is supplied to the desulfurization unit 23 through the fuel gas supply path 21 by the operation of the fuel gas supply blower 22. The reforming water in the reforming water tank is supplied through the reforming water supply path 3 by the operation of the reforming water pump 2 to the junction 21A located downstream of the desulfurization unit 23 in the fuel gas supply path 21. After being merged with the fuel gas from the desulfurization unit 23 at the junction 21A, the fuel gas is supplied together with the fuel gas through the fuel gas supply path 21 to the vaporization unit 24. The fuel gas containing steam produced in the vaporization unit 24 is then supplied to the reforming unit 25, where the fuel gas is steam reformed to produce a reformed fuel gas mainly composed of hydrogen gas. The reformed fuel gas produced in the reforming unit 25 is then supplied to the fuel chamber 12 (see FIG. 2 ) of the fuel cell unit 1 through the fuel gas supply path 21. The reforming water pump 2 adjusts the flow rate of the reforming water supplied to the vaporization unit 24 by controlling its rotation speed.

[0036] The oxidant gas supply unit 30 is configured to take in oxidant gas using an oxidant gas supply blower 31 and supply it to the oxidant gas chamber 13 (see FIG. 2) of the fuel cell unit 1 through an oxidant gas supply path 32. The rotation speed of the oxidant gas supply blower 31 is controlled to adjust the flow rate of the oxidant gas supplied to the oxidant gas chamber 13 of the fuel cell unit 1.

[0037] The fuel cell system is provided with a combustion unit 4 that combusts fuel off-gas discharged from a fuel chamber 12 of the fuel cell unit 1 and oxidant off-gas discharged from an oxidant gas chamber 13. The fuel off-gas is led from the fuel chamber 12 of the fuel cell unit 1 to the combustion unit 4 through a fuel off-gas flow path 5. The oxidant off-gas is led from the oxidant gas chamber 13 of the fuel cell unit 1 to the combustion unit 4 through an oxidant off-gas flow path 6. The combustion exhaust gas from the combustion unit 4 is subjected to heat recovery, for example, in a heat exchange unit not shown, and then discharged to the atmosphere through an exhaust gas discharge path 7.

[0038] The combustion section 4 is housed in the internal space of a highly insulating storage container 8 together with the fuel cell section 1, vaporization section 24, and reforming section 25 described above. The internal space of the storage container 8 is kept at a high temperature by the combustion heat generated by combustion in the combustion section 4. The vaporization section 24 performs a process of generating steam using the combustion heat generated by combustion in the combustion section 4. The reforming section 25 performs a process of reforming the fuel gas using the combustion heat generated by combustion in the combustion section 4.

[0039] The fuel cell system is provided with a fuel off-gas reflux means 40 that can reflux at least a portion of the fuel off-gas discharged from the fuel chamber 12 of the fuel cell unit 1 to the fuel gas supply unit. The fuel off-gas reflux means 40 includes a fuel off-gas reflux path 41 that guides the fuel off-gas flowing through the fuel off-gas flow path 5 to the vicinity of the confluence 21A of the fuel gas supply path 21, a fuel off-gas reflux blower 42 that refluxes the fuel off-gas through the fuel off-gas reflux path 41 to the fuel gas supply path 21 at an adjustable flow rate, an electromagnetic reflux path on-off valve 43 that opens and closes the fuel off-gas reflux path 41, and a check valve 44 that prevents backflow of the fuel off-gas that has passed through the reflux path on-off valve 43. The fuel off-gas reflux means 40 also includes a water vapor condenser 45 that condenses and removes water vapor contained in the fuel off-gas, and a heat exchanger 46 that cools the fuel off-gas.

[0040] The rotation speed of the fuel off-gas reflux blower 42 is controlled to adjust the flow rate of the fuel off-gas that is refluxed to the fuel gas supply unit 20. As a result, the fuel off-gas reflux blower 42 is configured to function as a fuel off-gas flow rate adjusting unit that can freely adjust the flow rate of the fuel off-gas that is refluxed to the fuel gas supply unit 20.

[0041] When the return path on-off valve 43 is closed, all of the fuel off-gas discharged from the fuel chamber 12 of the fuel cell unit 1 is guided to the combustion unit 4 through the fuel off-gas flow path 5. When the return path on-off valve 43 is open, the fuel off-gas return blower 42 is operated to supply a portion of the fuel off-gas to the water vapor condenser 45 through the fuel off-gas return path 41, and the remaining fuel off-gas is guided to the combustion unit 4 through the fuel off-gas flow path 5. The fuel off-gas supplied to the water vapor condenser 45 has water vapor contained in the fuel off-gas condensed and removed therefrom, and is then supplied to the heat exchanger 46, where heat is recovered and cooled, and the fuel off-gas is then returned to the fuel gas supply unit 20 near the confluence 21A of the fuel gas supply path 21.

[0042] The fuel cell system is equipped with a temperature detector 51 that detects the temperature of the fuel cell section 1, a fuel chamber-side pressure gauge 52 that detects the pressure outside the storage container 8 in the fuel gas supply section 20 downstream of the junction 21A as the pressure on the fuel chamber 12 side, and an oxidant gas chamber-side pressure gauge 53 that detects the pressure outside the storage container 8 in the oxidant gas supply section 30 as the pressure on the oxidant gas chamber 13 side. The system is also equipped with an operation control section 9 that controls the operation of each section, such as the fuel gas supply blower 22 and the oxidant gas supply blower 31, based on various information from the temperature detector 51 and the pressure gauges 52, 53, thereby managing the operation of the fuel cell system.

[0043] The operation control unit 9 controls the operation of each unit, such as the fuel gas supply blower 22, the oxidant gas supply blower 31, and the reforming water pump 2, to adjust the flow rate of the fuel gas and reforming water supplied to the vaporization unit 24, the flow rate of the oxidant gas supplied to the oxidant gas chamber 13 of the fuel cell unit 1, and the combustion amount in the combustion unit 4. By making these adjustments, the operation control unit 9 performs a start-up process in which the temperature of the fuel cell unit 1 is raised to a power generation temperature to start up the fuel cell unit 1 in a high-temperature state, an operation process in which the fuel cell unit 1 in a high-temperature state generates electricity after start-up, and an operation shutdown process in which the temperature of the fuel cell unit 1 in a high-temperature state is lowered to a target temperature close to room temperature to stop operation.

[0044] During the startup process, the operation control unit 9 closes the return path opening / closing valve 43 and stops the fuel off-gas return blower 42, while operating and controlling the operation of the fuel gas supply blower 22, the oxidant gas supply blower 31, the reforming water pump 2, etc., to gradually increase the flow rate of the fuel gas and reforming water supplied to the vaporization unit 24, the flow rate of the oxidant gas supplied to the oxidant gas chamber 13 of the fuel cell unit 1, and the amount of combustion in the combustion unit 4, etc., to set amounts, thereby raising the temperature of the fuel cell unit 1 to a high-temperature state where it is at the power generation temperature and starting it up.

[0045] During the operation process, the operation control unit 9 closes the return flow path opening / closing valve 43, maintains the fuel off-gas return blower 42 stopped, and controls the operation of the fuel gas supply blower 22, the oxidant gas supply blower 31, the reforming water pump 2, etc., to sequentially adjust the flow rate of the fuel gas and reforming water supplied to the vaporization unit 24, the flow rate of the oxidant gas supplied to the oxidant gas chamber 13 of the fuel cell unit 1, and the amount of combustion in the combustion unit 4, etc., thereby causing the fuel cell unit 1 in a high-temperature state to output generated power according to the power load.

[0046] Here, in the operation shutdown process, it is common to gradually reduce the flow rates of the fuel gas and reforming water supplied to the vaporization section 24, the flow rate of the oxidant gas supplied to the oxidant gas chamber 13 of the fuel cell section 1, and the combustion amount in the combustion section 4, thereby lowering the temperature of the fuel cell section 1 from a high temperature to a target temperature and stopping operation. However, in this case, there is a problem that the time required for the operation shutdown (cooling down) can be several hours or more. Moreover, while the operation shutdown process is being carried out, it is necessary to continue supplying a small amount of fuel to the fuel electrode 11B of the fuel cell section 1 to prevent oxidation of the fuel electrode 11B. Therefore, from an economical point of view, it is desirable to shorten the time required for cooling down in the operation shutdown process as much as possible.

[0047] The cooling rate during the shutdown process can be increased by increasing the flow rate of the oxidant gas supplied to the fuel cell section 1. However, in a fuel cell section 1 including a flat-plate cell stack 10 in which the oxidant gas chamber 13 and the fuel chamber 12 form a closed system, increasing the flow rate of the oxidant gas supplied to the oxidant gas chamber 13 increases the pressure in the oxidant gas chamber 13, which creates a pressure difference between the oxidant gas chamber 13 and the fuel chamber 12, applying force to the fuel cell 11 located between them. If this force becomes excessive, there is a risk that the fuel cell 11 may be damaged, and therefore the flow rate of the oxidant gas supplied to the oxidant gas chamber 13 is restricted, leaving room for improvement in terms of shortening the time required for cooling during the shutdown process.

[0048] Therefore, in the operation shutdown step, the operation control unit 9 illustrated in the first embodiment increases the flow rate of the oxidant gas supplied from the oxidant gas supply unit 30 to the oxidant gas chamber 13 of the fuel cell unit 1, opens the return path on-off valve 43, and operates the fuel off-gas return blower 42 to return a portion of the fuel off-gas discharged from the fuel chamber 12 of the fuel cell unit 1 to the fuel gas supply unit 20 through the fuel off-gas return path 41. The operation control unit 9 also monitors the pressure on the fuel chamber 12 side and the pressure on the oxidant gas chamber 13 side of the fuel cell unit 1, and adjusts the flow rate of the fuel off-gas returned to the fuel gas supply unit 20 by the fuel off-gas return blower 42 so that the differential pressure therebetween is maintained within a predetermined appropriate range to prevent damage to the fuel cell 11 due to the differential pressure. The operation is then stopped when the temperature of the fuel cell unit 1 detected by the temperature detector 51 drops to a target temperature for operation shutdown.

[0049] The control operation of the operation control unit 9 in the operation shutdown process will be described below with reference to the flowchart of FIG.

[0050] The operation control unit 9 first performs an oxidant gas flow rate increase process (step #1) in which the rotation speed of the oxidant gas supply blower 31 is increased to increase the flow rate of the oxidant gas supplied from the oxidant gas supply unit 30 to the oxidant gas chamber 13 of the fuel cell unit 1 to the maximum flow rate for cooling, while monitoring the temperature of the fuel cell unit 1 detected by the temperature detector 51.

[0051] This allows the fuel cell section 1 to be cooled more effectively, shortening the cooling time required to lower the temperature of the fuel cell section 1 to the target temperature. Accordingly, even when the fuel cell section 1 is being cooled, the amount of fuel gas that needs to be continuously supplied to the fuel electrode 11B of the fuel cell section 1 to prevent oxidation of the fuel electrode 11B can be reduced.

[0052] At the same time as the oxidant gas flow rate increase process of step #1, the operation control unit 9 opens the return path opening / closing valve 43 and operates the fuel off-gas return blower 42, thereby performing a fuel off-gas return process in which a portion of the fuel off-gas discharged from the fuel chamber 12 of the fuel cell unit 1 is returned to the fuel gas supply unit 20 through the fuel off-gas return path 41 (step #2).

[0053] This makes it possible to effectively utilize a portion of the fuel off-gas as fuel gas to be supplied to the fuel chamber 12 of the fuel cell unit 1. This effective utilization makes it possible to increase the flow rate of the oxidant gas supplied to the oxidant gas chamber 13 by the oxidant gas flow rate increasing process described above, as well as the flow rate of the fuel gas supplied from the fuel gas supply unit 20 to the fuel chamber 12 of the fuel cell unit 1, without increasing the amount of fuel gas used in the operation shutdown process. This increase in the fuel gas flow rate increases the pressure on the oxidant gas chamber 13 side of the fuel cell unit 1 due to the increase in the oxidant gas flow rate, and also increases the pressure on the fuel chamber 12 side of the fuel cell unit 1. Furthermore, by returning a portion of the fuel off-gas to the fuel gas supply section 20, the flow rate of the fuel off-gas flowing from the fuel chamber 12 of the fuel cell section 1 to the combustion section 4 is reduced, weakening the combustion power in the combustion section 4, thereby further shortening the cooling time mentioned above.

[0054] After performing the fuel off-gas reflux process of step #2, the operation control unit 9 monitors the pressure on the fuel chamber 12 side and the pressure on the oxidant gas chamber 13 side of the fuel cell unit 1 based on the detection information from the fuel chamber side pressure gauge 52 and the oxidant gas chamber side pressure gauge 53, and performs a first pressure determination process to determine whether the pressure on the oxidant gas chamber 13 side of the fuel cell unit 1 is higher than the pressure on the fuel chamber 12 side (step #3).If the first pressure determination process shows that the pressure on the oxidant gas chamber 13 side is higher than the pressure on the fuel chamber 12 side, the operation control unit 9 performs a fuel off-gas reflux flow rate increase process to increase the rotation speed of the fuel off-gas reflux blower 42 and increase the flow rate of the fuel off-gas refluxed by the fuel off-gas reflux blower 42 through the fuel off-gas reflux path 41 to the fuel gas supply unit 20 (step #4). As a result, the flow rate of the fuel gas supplied from the fuel gas supply unit 20 to the fuel chamber 12 of the fuel cell unit 1 increases, and the pressure on the fuel chamber 12 side of the fuel cell unit 1 rises.

[0055] If the first pressure determination process of step #3 determines that the pressure on the oxidant gas chamber 13 side is not higher than the pressure on the fuel chamber 12 side, the operation control unit 9 performs a second pressure determination process (step #5) to determine whether the pressure on the oxidant gas chamber 13 side is lower than the pressure on the fuel chamber 12 side. If the second pressure determination process determines that the pressure on the oxidant gas chamber 13 side is lower than the pressure on the fuel chamber 12 side, the operation control unit 9 performs a fuel off-gas recirculation flow rate reduction process (step #6) to reduce the rotation speed of the fuel off-gas recirculation blower 42 and reduce the flow rate of the fuel off-gas recirculated to the fuel gas supply unit 20 through the fuel off-gas recirculation path 41. As a result, the flow rate of the fuel gas supplied from the fuel gas supply unit 20 to the fuel chamber 12 of the fuel cell unit 1 decreases, and the pressure on the fuel chamber 12 side of the fuel cell unit 1 decreases.

[0056] If the pressure on the oxidant gas chamber 13 side is not lower than the pressure on the fuel chamber 12 side in the second pressure determination process of step #5, the operation control unit 9 performs a fuel off-gas reflux state continuation process to continue the current fuel off-gas reflux state (step #7), since the pressure on the fuel chamber 12 side of the fuel cell unit 1 and the pressure on the oxidant gas chamber 13 side are the same.

[0057] After performing any one of the fuel off-gas reflux flow rate increase process of step #4, the fuel off-gas reflux flow rate decrease process of step #6, and the fuel off-gas reflux state continuation process of step #7, the operation control unit 9 performs a fuel cell unit temperature determination process to determine whether the temperature of the fuel cell unit 1 has dropped to the target temperature for operation shutdown (step #8).If the temperature of the fuel cell unit 1 has not dropped to the target temperature for operation shutdown in the fuel cell unit temperature determination process, the operation control unit 9 returns to the first pressure determination process of step #3, and performs an operation shutdown process to stop the operation of the fuel cell unit 1 if the temperature has dropped to the target temperature for operation shutdown (step #9).

[0058] In other words, in the operation shutdown process performed by the operation control unit 9 exemplified in this first embodiment, for example, the flow rate of the oxidant gas supplied from the oxidant gas supply unit 30 to the oxidant gas chamber 13 of the fuel cell unit 1 is increased to shorten the cooling time of the fuel cell unit, causing the pressure on the oxidant gas chamber 13 side to rise, or the flow rate of the fuel off-gas returned to the fuel gas supply unit 20 by the fuel off-gas return blower 42 is reduced, causing the pressure on the fuel chamber 12 side in the fuel cell unit 1 to become higher than the pressure on the fuel chamber 12 side, and the pressure difference between the fuel chamber 12 side and the oxidant gas chamber 13 side of the fuel cell unit is about to fall outside the appropriate range, and accordingly, the flow rate of the fuel off-gas returned to the fuel gas supply unit by the fuel off-gas return blower 42 is increased. This increases the flow rate of fuel gas supplied from the fuel gas supply unit 20 to the fuel chamber 12 of the fuel cell unit 1, and increases the pressure on the fuel chamber 12 side relative to the fuel cell unit 1, thereby maintaining the differential pressure between the fuel chamber 12 side and the oxidizer gas chamber 13 side of the fuel cell unit 1 within an appropriate range.

[0059] Furthermore, when the flow rate of fuel off-gas returned to the fuel gas supply unit 20 is increased in this manner, the pressure on the fuel chamber 12 side of the fuel cell unit 1 rises to become higher than the pressure on the oxidant gas chamber 13 side, and the pressure difference between the fuel chamber 12 side and the oxidant gas chamber 13 side of the fuel cell unit 1 approaches an appropriate range, the fuel off-gas return blower 42 correspondingly reduces the flow rate of fuel off-gas returned to the fuel gas supply unit 20. As a result, the flow rate of fuel gas supplied from the fuel gas supply unit 20 to the fuel chamber 12 of the fuel cell unit 1 decreases, and the pressure on the fuel chamber 12 side of the fuel cell unit 1 drops, so that the pressure difference between the fuel chamber 12 side and the oxidant gas chamber 13 side of the fuel cell unit 1 is maintained within an appropriate range.

[0060] Therefore, in the operation shutdown process performed by the operation control unit 9 exemplified in this first embodiment, the amount of fuel used in the operation shutdown process can be reduced by shortening the cooling time by increasing the amount of oxidant gas supplied to the oxidant gas chamber 13 of the fuel cell section 1 and by effectively utilizing the fuel off-gas, while maintaining the pressure difference between the fuel chamber 12 side and the oxidant gas chamber 13 side of the fuel cell section 1 within an appropriate range, thereby avoiding the risk of damage to the fuel cell 11 located between the fuel chamber 12 and the oxidant gas chamber 13 of the fuel cell section 1 due to the pressure difference between the oxidant gas chamber 13 side and the fuel chamber 12 side of the fuel cell section 1 becoming excessive.

[0061] Second Embodiment A second embodiment, which is an example of a mode for carrying out a fuel cell system according to the present invention, will be described below with reference to the drawings. The fuel cell system illustrated in this second embodiment differs from the fuel cell system illustrated in the first embodiment above in the configuration for directing the fuel off-gas discharged from the fuel chamber of the fuel cell section to the combustion section, the configuration of the fuel off-gas reflux means that can freely reflux the fuel off-gas discharged from the fuel chamber of the fuel cell section to the fuel gas supply section, and the operation shutdown process performed by the operation control section based on these configurations.Therefore, only these configurations and the operation shutdown process will be described below.

[0062] 4, the fuel cell system exemplified in the second embodiment is provided with a fuel off-gas flow path 61 that guides the fuel off-gas discharged from the fuel chamber 12 (see FIG. 2) of the fuel cell unit 1 to the combustion unit 4, and the fuel off-gas supply path 62 that guides the fuel off-gas guided to the outside of the storage container 8 by the fuel off-gas flow path 61 to the combustion unit 4. A check valve 63 that prevents backflow of the fuel off-gas discharged from the fuel chamber 12 of the fuel cell unit 1 is provided on the outside of the storage container 8 of the fuel off-gas flow path 61. A water vapor condenser 74 that condenses and removes water vapor contained in the fuel off-gas that is guided to the combustion unit 4 of the fuel off-gas supply path 62 is provided on the outside of the storage container 8 of the fuel off-gas flow path 61.

[0063] The fuel off-gas reflux means 70 of the fuel cell system exemplified in the second embodiment is provided with a fuel off-gas reflux path 71 that guides fuel off-gas, which has been guided to the outside of the storage container 8 via the fuel off-gas path 61, to the vicinity of the confluence 21A of the fuel gas supply path 21. Also provided is a path switching unit 72 that can freely switch the path through which fuel off-gas, which is discharged from the fuel chamber 12 of the fuel cell unit 1 through the fuel off-gas path 61, flows between the fuel off-gas supply path 62 and the fuel off-gas reflux path 71. Furthermore, the fuel off-gas reflux blower 73 that refluxes the fuel off-gas to the fuel gas supply path 21 through the fuel off-gas reflux path 71, and a water vapor condenser 74 that condenses and removes water vapor contained in the fuel off-gas refluxed to the fuel gas supply path 21. The path switching unit 72 is provided with a supply path opening / closing valve 75 that opens and closes the fuel off-gas path 61, and a reflux path opening / closing valve 76 that opens and closes the fuel off-gas reflux path 71.

[0064] During the startup process, the operation control unit 9 switches the flow path through which the fuel off-gas flows to the fuel off-gas supply path 62 by opening the supply path opening / closing valve 75 of the flow path switching unit 72 and closing the return path opening / closing valve 76, and then operates and controls the operation of the fuel gas supply blower 22, the oxidant gas supply blower 31, the reforming water pump 2, etc., to gradually increase the flow rate of the fuel gas and reforming water supplied to the evaporation unit 24, the flow rate of the oxidant gas supplied to the oxidant gas chamber 13 (see Figure 2) of the fuel cell unit 1, and the amount of combustion in the combustion unit 4, etc., to set amounts, thereby raising the temperature of the fuel cell unit 1 to a high-temperature state where it is at the power generation temperature and starting it up.

[0065] During the operation process, the operation control unit 9 maintains the state in which the flow path through which the fuel off-gas flows switched to the fuel off-gas supply path 62, and controls the operation of the fuel gas supply blower 22, the oxidant gas supply blower 31, the reforming water pump 2, etc., to sequentially adjust the flow rate of the fuel gas and reforming water supplied to the vaporization unit 24, the flow rate of the oxidant gas supplied to the oxidant gas chamber 13 of the fuel cell unit 1, and the amount of combustion in the combustion unit 4, etc., thereby causing the fuel cell unit 1 in a high-temperature state to output generated power according to the power load.

[0066] During the operation shutdown process, the operation control unit 9 increases the flow rate of the oxidant gas supplied from the oxidant gas supply unit 30 to the oxidant gas chamber 13 of the fuel cell unit 1, closes the supply path opening / closing valve 75 of the flow path switching unit 72 and opens the return path opening / closing valve 76 to switch the flow path through which the fuel off-gas flows from the fuel off-gas supply path 62 to the fuel off-gas return path 71, and operates the fuel off-gas return blower 73, thereby returning the entire amount of fuel off-gas discharged from the fuel chamber 12 of the fuel cell unit 1 to the fuel gas supply unit 20 through the fuel off-gas return path 71. The pressure on the fuel chamber 12 side and the pressure on the oxidant gas chamber 13 side of the fuel cell section 1 are monitored, and the differential pressure between them is maintained within a preset appropriate range to prevent damage to the fuel cell 11 (see FIG. 2) due to the differential pressure by opening the supply path on-off valve 75 when the pressure on the fuel chamber 12 side becomes higher than the pressure on the oxidant gas chamber 13 side, and closing the supply path on-off valve 75 when the pressure on the fuel chamber 12 side becomes lower than the pressure on the oxidant gas chamber 13 side, thereby adjusting the flow rate of the fuel off-gas returned to the fuel gas supply section 20. Then, operation is stopped when the temperature of the fuel cell section 1 detected by the temperature detector 51 drops to a target temperature for stopping operation.

[0067] In this way, in the fuel cell system exemplified in this second embodiment, the flow path switching unit 72 is configured to function as a fuel off-gas flow rate adjusting unit that can freely adjust the flow rate of the fuel off-gas that is returned to the fuel gas supply unit 20.

[0068] The control operation of the operation control unit 9 in the operation shutdown process will be described below with reference to the flowchart of FIG.

[0069] The operation control unit 9 first performs an oxidant gas flow rate increase process (step #11) in which the rotation speed of the oxidant gas supply blower 31 is increased to increase the flow rate of the oxidant gas supplied from the oxidant gas supply unit 30 to the oxidant gas chamber 13 of the fuel cell unit 1 to the maximum flow rate for cooling, while monitoring the temperature of the fuel cell unit 1 detected by the temperature detector 51.

[0070] This allows the fuel cell section 1 to be cooled more effectively, shortening the cooling time required to lower the temperature of the fuel cell section 1 to the target temperature. Accordingly, even when the fuel cell section 1 is being cooled, the amount of fuel gas that needs to be continuously supplied to the fuel electrode 11B of the fuel cell section 1 to prevent oxidation of the fuel electrode 11B can be reduced.

[0071] At the same time as performing the oxidant gas flow rate increase process of step #11, the operation control unit 9 closes the supply path opening / closing valve 75 of the path switching unit 72 and opens the return path opening / closing valve 76 to switch the path through which the fuel off-gas flows from the fuel off-gas supply path 62 to the fuel off-gas return path 71, and then operates the fuel off-gas return blower 73 to perform a total fuel off-gas return process in which the entire amount of fuel off-gas discharged from the fuel chamber 12 of the fuel cell unit 1 is returned to the fuel gas supply unit 20 through the fuel off-gas return path 71 (step #12).

[0072] As a result, the entire amount of fuel off-gas can be effectively utilized as fuel gas to be supplied to fuel chamber 12 of fuel cell unit 1. This effective utilization minimizes the amount of fuel gas that needs to be continuously supplied to fuel electrode 11B to prevent oxidation of fuel electrode 11B, while also suitably increasing the flow rate of oxidant gas supplied to oxidant gas chamber 13 of fuel cell unit 1 to shorten the cooling time of fuel cell unit 1. This increase in the flow rate of fuel gas increases the pressure on the oxidant gas chamber 13 side of fuel cell unit 1 in accordance with the increase in the flow rate of oxidant gas, and also suitably increases the pressure on the fuel chamber 12 side of fuel cell unit 1.

[0073] After performing the total fuel off-gas reflux process of step #12, the operation control unit 9 monitors the pressure on the fuel chamber 12 side and the pressure on the oxidant gas chamber 13 side of the fuel cell unit 1 based on the detection information from the fuel chamber side pressure gauge 52 and the oxidant gas chamber side pressure gauge 53, and performs a first pressure determination process (step #13) to determine whether the pressure on the oxidant gas chamber 13 side of the fuel cell unit 1 is higher than the pressure on the fuel chamber 12 side. If the first pressure determination process determines that the pressure on the oxidant gas chamber 13 side is higher than the pressure on the fuel chamber 12 side, the total fuel off-gas reflux state performed by the total fuel off-gas reflux process of step #12 is continued. As a result, the flow rate of the fuel gas supplied from the fuel gas supply unit 20 to the fuel chamber 12 of the fuel cell unit 1 increases, and the pressure on the fuel chamber 12 side of the fuel cell unit 1 rises.

[0074] If the first pressure determination process of step #13 determines that the pressure on the oxidant gas chamber 13 side is not higher than the pressure on the fuel chamber 12 side, the operation control unit 9 performs a second pressure determination process (step #14) to determine whether the pressure on the oxidant gas chamber 13 side is lower than the pressure on the fuel chamber 12 side. If the second pressure determination process determines that the pressure on the oxidant gas chamber 13 side is lower than the pressure on the fuel chamber 12 side, the operation control unit 9 performs a supply path opening / closing valve opening process to open the supply path opening / closing valve 75 of the flow path switching unit 72 (step #15).

[0075] As a result, a portion of the fuel off-gas discharged from the fuel chamber 12 of the fuel cell section 1 is supplied to the combustion section 4 through the fuel off-gas supply path 62, and this supply reduces the flow rate of fuel off-gas returned from the fuel chamber 12 of the fuel cell section 1 to the fuel gas supply section 20 through the fuel off-gas return path 71, and also reduces the flow rate of fuel gas supplied from the fuel gas supply section 20 to the fuel chamber 12 of the fuel cell section 1, thereby reducing the pressure on the fuel chamber 12 side in the fuel cell section 1.

[0076] If the pressure on the fuel chamber 12 side is not lower than the pressure on the oxidant gas chamber 13 side in the second pressure determination process of step #14, the operation control unit 9 continues the full fuel off-gas reflux state by the full fuel off-gas reflux process of step #12, since the pressure on the fuel chamber 12 side of the fuel cell unit 1 and the pressure on the oxidant gas chamber 13 side are the same.

[0077] After performing the supply path valve opening process of step #15, the operation control unit 9 performs a third pressure determination process (step #16) to determine whether the pressure on the fuel chamber 12 side in the fuel cell unit 1 is lower than the pressure on the oxidant gas chamber 13 side. If the third pressure determination process determines that the pressure on the fuel chamber 12 side is lower than the pressure on the oxidant gas chamber 13 side, the operation control unit 9 performs a supply path valve closing process to close the supply path valve 75 of the flow path switching unit 72 (step #17).

[0078] As a result, the entire amount of fuel off-gas discharged from the fuel chamber 12 of the fuel cell section 1 is returned to the fuel gas supply section 20 through the fuel off-gas return path 71, and this return increases the flow rate of fuel off-gas returned to the fuel gas supply section 20, thereby increasing the pressure on the fuel chamber 12 side of the fuel cell section 1.

[0079] If the pressure on the fuel chamber 12 side is not lower than the pressure on the oxidant gas chamber 13 side in the third pressure determination process of step #16, the operation control unit 9 maintains the open state of the supply path opening / closing valve 75 in the supply path opening / closing valve opening process of step #15, since the pressure on the fuel chamber 12 side of the fuel cell unit 1 and the pressure on the oxidant gas chamber 13 side are the same.

[0080] The operation control unit 9 performs a fuel cell unit temperature determination process (step #18) to determine whether the temperature of the fuel cell unit 1 has dropped to a target temperature for shutting down the fuel cell unit 1 in any of the following cases: if the pressure on the oxidant gas chamber 13 side is higher than the pressure on the fuel chamber 12 side in the first pressure determination process of step #13; if the pressure on the oxidant gas chamber 13 side is not lower than the pressure on the fuel chamber 12 side in the second pressure determination process of step #14; if the pressure on the fuel chamber 12 side is not lower than the pressure on the oxidant gas chamber 13 side in the third pressure determination process of step #16; or if the supply path on-off valve closing process of step #17 has been performed. If the temperature of the fuel cell unit 1 has not dropped to the target temperature for shutting down the fuel cell unit 1 in the fuel cell unit temperature determination process, the process returns to the third determination process of step #13, and if the temperature has dropped to the target temperature for shutting down the fuel cell unit 1, the process performs an operation shutdown process (step #19).

[0081] In other words, in the operation shutdown process performed by the operation control unit 9 exemplified in this second embodiment, when the flow rate of the oxidant gas supplied to the oxidant gas chamber 13 of the fuel cell unit 1 is increased to shorten the cooling time of the fuel cell unit 1, the supply path opening / closing valve 75 is closed and the return path opening / closing valve 76 is opened, thereby switching the flow path of the fuel off-gas discharged from the fuel chamber 12 of the fuel cell unit 1 from the fuel off-gas supply path 62 to the fuel off-gas return path 71, and the entire amount of fuel off-gas discharged from the fuel chamber 12 of the fuel cell unit 1 is returned to the fuel gas supply unit 20.

[0082] When the pressure on the fuel chamber 12 side of the fuel cell unit 1 becomes higher than the pressure on the oxidant gas chamber 13 side due to the entire amount of fuel off-gas being returned to the fuel gas supply unit 20 in this manner, and the pressure difference between the fuel chamber 12 side and the oxidant gas chamber 13 side of the fuel cell unit 1 approaches an appropriate range, the supply path on-off valve 75 opens in response, and the fuel off-gas discharged from the fuel chamber 12 of the fuel cell unit 1 is distributed to the combustion unit 4. This reduces the flow rate of the fuel off-gas returned to the fuel gas supply unit 20, reduces the flow rate of the fuel gas supplied from the fuel gas supply unit 20 to the fuel chamber 12 of the fuel cell unit 1, and lowers the pressure on the fuel chamber 12 side of the fuel cell unit 1, thereby maintaining the pressure difference between the fuel chamber 12 side and the oxidant gas chamber 13 side of the fuel cell unit 1 within an appropriate range.

[0083] Furthermore, when the pressure on the fuel chamber 12 side of the fuel cell unit 1 becomes lower than the pressure on the oxidant gas chamber 13 side due to the fuel off-gas being distributed to the combustion unit 4 in this manner, and the pressure difference between the fuel chamber 12 side and the oxidant gas chamber 13 side of the fuel cell unit 1 approaches an appropriate range, the supply path on-off valve 75 closes in response, and all of the fuel off-gas discharged from the fuel chamber 12 of the fuel cell unit 1 is returned to the fuel gas supply unit 20. This increases the flow rate of the fuel off-gas returned to the fuel gas supply unit 20, increases the flow rate of the fuel gas supplied from the fuel gas supply unit 20 to the fuel chamber 12 of the fuel cell unit 1, and increases the pressure on the fuel chamber 12 side of the fuel cell unit 1, thereby maintaining the pressure difference between the fuel chamber 12 side and the oxidant gas chamber 13 side of the fuel cell unit 1 within an appropriate range.

[0084] Therefore, with a simple control configuration that simply opens and closes the supply path opening / closing valve 75 based on the pressure on the fuel chamber 12 side and the pressure on the oxidant gas chamber 13 side in the fuel cell section 1, the pressure difference between the fuel chamber 12 side and the oxidant gas chamber 13 side of the fuel cell section 1 can be maintained within an appropriate range, and the risk of damage to the fuel cell 11 located between the fuel chamber 12 and the oxidant gas chamber 13 of the fuel cell section 1 due to the pressure difference between the fuel chamber 12 side and the oxidant gas chamber 13 side of the fuel cell section 1 becoming excessive can be avoided.

[0085] [Another embodiment] Another embodiment of the present invention will now be described. The configurations of the other embodiments described below are not limited to being applied alone, but can also be applied in combination with the configurations of the above-described embodiment or other other embodiments.

[0086] (1) In the first embodiment described above, as a control operation during the operation stop process of the operation control unit 9, when the pressure on the oxidant gas chamber 13 side is lower than the pressure on the fuel chamber 12 side, the fuel off-gas reflux flow rate reduction process is performed to reduce the rotation speed of the fuel off-gas reflux blower 42, thereby reducing the flow rate of the fuel off-gas refluxed to the fuel gas supply unit 20 through the fuel off-gas reflux path 41. However, this is not limited to this, and for example, the rotation speed of the fuel gas supply blower 22 may be reduced to reduce the flow rate of the fuel gas (raw fuel gas) supplied by the fuel gas supply blower 22 from a fuel gas supply source (not shown), thereby reducing the flow rate of the fuel off-gas refluxed to the fuel gas supply unit 20 through the fuel off-gas reflux path 41.

[0087] (2) In the second embodiment described above, as a control operation in the operation stop process of the operation control unit 9, the total fuel off-gas reflux process is performed to reflux the total amount of fuel off-gas discharged from the fuel chamber 12 of the fuel cell unit 1 to the fuel gas supply unit 20, thereby increasing the flow rate of the fuel off-gas refluxed to the fuel gas supply unit 20 through the fuel off-gas reflux path 71. However, this is not limited to this, and for example, the flow rate of the fuel off-gas refluxed to the fuel gas supply unit 20 through the fuel off-gas reflux path 71 may be increased by increasing the rotation speed of the fuel gas supply blower 22 to increase the flow rate of the fuel gas (raw fuel gas) supplied by the fuel gas supply blower 22 from a fuel gas supply source (not shown).

[0088] (3) In the second embodiment described above, as a control operation during the operation stop process of the operation control unit 9, a portion of the fuel off-gas discharged from the fuel chamber 12 of the fuel cell unit 1 is supplied to the combustion unit 4 by the supply path on-off valve opening process, thereby reducing the flow rate of the fuel off-gas returned to the fuel gas supply unit 20 through the fuel off-gas return path 71. However, this is not limited to this. For example, in addition to the supply path on-off valve opening process, a fuel off-gas return flow rate adjustment process may be performed in which the rotation speed of the fuel off-gas return blower 73 is controlled to adjust the flow rate of the fuel off-gas returned to the fuel gas supply unit 20 through the fuel off-gas return path 71. [Explanation of symbols]

[0089] 1 Fuel cell section 4 Combustion section 9 Operation control unit 12 Fuel chamber 13 Oxidizer gas chamber 20 Fuel gas supply unit 30 Oxidant gas supply unit 40 Fuel off-gas recirculation means 42 Fuel off-gas recirculation blower (fuel off-gas flow rate adjustment unit) 62 Fuel off-gas supply line 70 Fuel off-gas recirculation means 71 Fuel off-gas return path 72 Flow path switching unit (fuel off-gas flow rate adjusting unit) 73 Fuel off gas flow rate adjustment section 75 Supply channel on-off valve 76 Circulation path on-off valve

Claims

1. a solid oxide fuel cell unit that generates electricity through an electrochemical reaction between a fuel gas supplied from a fuel gas supply unit to a fuel chamber and an oxidant gas supplied from an oxidant gas supply unit to an oxidant gas chamber; a combustion section that combusts the fuel off-gas discharged from the fuel chamber and the oxidant off-gas discharged from the oxidant gas chamber; an operation control unit capable of controlling the operation of each unit to lower the temperature of the fuel cell unit to a target temperature and stop the operation, a fuel off-gas recirculation means for recirculating at least a portion of the fuel off-gas discharged from the fuel chamber to the fuel gas supply section, In the operation stopping process, the operation control unit increases the flow rate of the oxidant gas supplied from the oxidant gas supply unit to the oxidant gas chamber, and causes the fuel off-gas reflux means to reflux at least a portion of the fuel off-gas discharged from the fuel chamber to the fuel gas supply unit.

2. the fuel off-gas recirculation means is provided with a fuel off-gas flow rate adjustment unit that can freely adjust the flow rate of the fuel off-gas to be recirculated to the fuel gas supply unit, 2. The fuel cell system according to claim 1, wherein, during the operation stopping process, the operation control unit monitors the pressure on the fuel chamber side and the pressure on the oxidant gas chamber side, and adjusts the flow rate of the fuel off-gas returned to the fuel gas supply unit by the fuel off-gas flow rate adjustment unit so that the differential pressure between the fuel chamber side and the oxidant gas chamber side is maintained within an appropriate range.

3. a fuel off-gas supply passage for guiding fuel off-gas discharged from the fuel chamber to the combustion section; the fuel off-gas circulation means includes a fuel off-gas circulation path that guides the fuel off-gas discharged from the fuel chamber to the fuel gas supply part, and a flow path switching part that can freely switch a flow path through which the fuel off-gas discharged from the fuel chamber flows between the fuel off-gas supply path and the fuel off-gas circulation path, 3. The fuel cell system according to claim 1, wherein the operation control unit switches the flow path of the fuel off-gas discharged from the fuel chamber from the fuel off-gas supply path to the fuel off-gas return path using the flow path switching unit during the operation stop process.

4. the flow path switching unit includes a supply path opening / closing valve that opens and closes the fuel off-gas supply path, and a return path opening / closing valve that opens and closes the fuel off-gas return path, 4. The fuel cell system of claim 3, wherein, during the operation stopping step, the operation control unit closes the supply path on-off valve and opens the return path on-off valve to switch the flow path of the fuel off-gas discharged from the fuel chamber from the fuel off-gas supply path to the fuel off-gas return path, monitors the pressure on the fuel chamber side and the pressure on the oxidant gas chamber side, and opens the supply path on-off valve when the pressure on the fuel chamber side becomes higher than the pressure on the oxidant gas chamber side, and closes the supply path on-off valve when the pressure on the fuel chamber side becomes lower than the pressure on the oxidant gas chamber side, thereby adjusting the flow rate of the fuel off-gas returned to the fuel gas supply unit, so that the differential pressure between the fuel chamber side and the oxidant gas chamber side is maintained within an appropriate range.

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