Solid oxide cell system

Abstract

Provided is a solid oxide cell system (1) that includes a solid oxide cell module (100), to which a plurality of solid oxide cell stacks (10) are connected, and that is capable of switching between an SOFC mode and a SOEC mode; The voltages of the plurality of solid oxide cell stacks (10) in operation are acquired. The difference between the voltage Vs of each acquired solid oxide cell stack (10) and a preset value Vex is calculated. The degree of deterioration of each solid oxide cell stack (10) is estimated on the basis of the calculated difference value. In a subsequent operation mode, control is performed to preferentially operate a solid oxide cell stack (10) that has a small estimated degree of deterioration.

Description

Solid Oxide Cell System

[0001] The present disclosure relates to solid oxide cell systems.

[0002] A solid oxide fuel cell (SOFC) can output electricity and heat using hydrogen, hydrocarbon, or carbon monoxide as a raw material. It can also function as a solid oxide electrolysis cell (SOEC), which performs the reverse reaction of electrolysis, electrolyzing water vapor or carbon dioxide into hydrogen, carbon monoxide, or oxygen. An electrolysis device using a reversible SOEC (solid oxide cell) capable of both power generation and electrolysis has been proposed (see, for example, Patent Document 1).

[0003] The electrolyzer disclosed in Patent Document 1 produces hydrogen by operating in electrolysis mode when the supplied power is equal to or greater than a threshold, and generates electricity by operating in power generation mode when the supplied power falls below the threshold. This allows the electrolyzer to operate stably even when the power supply source is subject to power fluctuations, such as renewable energy. Furthermore, because stopping the electrolysis mode due to a power shortage requires the SOC to be heated again, switching the operating mode and continuing operation allows for more efficient operation of the electrolyzer.

[0004] JP 2018-131647 A

[0005] In recent years, the development of reversible SOCs has progressed, and in solid oxide cell systems equipped with reversible SOCs, a stack structure consisting of multiple SOCs has been proposed, with large-capacity SOC modules in which multiple SOC stacks are connected. However, SOCs are exposed to reactant gases at high temperatures in both power generation mode and electrolysis mode, which causes degradation. Therefore, in large-capacity SOC modules in which multiple SOC stacks are connected, degradation variations occur, with the degree of degradation varying from SOC stack to SOC stack, which may prevent the SOC module from operating stably. Prior art has not taken into account degradation of the SOC or SOC stacks.

[0006] The present disclosure discloses a technology for solving the above-mentioned problems, and aims to provide a solid oxide cell system capable of stable operation by detecting deterioration of an SOC stack equipped in an SOC module and operating the SOC module based on the detected degree of deterioration.

[0007] The solid oxide cell system of the present disclosure comprises: a solid oxide cell module having a solid oxide cell stack switchable between an SOFC mode in which a first gas is used to generate and output electric power, and an SOEC mode in which electric power is input and a second gas is electrolyzed, the solid oxide cell module comprising a plurality of connected solid oxide cell stacks; and a control device for controlling the operation of the plurality of solid oxide cell stacks, the control device having: a stack voltage acquisition unit that acquires the voltages of the plurality of solid oxide cell stacks operating in the SOFC mode or the SOEC mode; and a deterioration level estimation unit that estimates the degree of deterioration of the solid oxide cell stacks based on the acquired voltages of the solid oxide cell stacks, and in the next SOFC mode or the SOEC mode, the control device performs control to prioritize operation of the solid oxide cell stack with the least estimated degree of deterioration.

[0008] According to the present disclosure, it is possible to provide a solid oxide cell system capable of stable operation.

[0009] 1 is a diagram showing the configuration of a solid oxide cell system according to an embodiment. FIG. 2 is a diagram showing the configuration of an SOC stack according to an embodiment. FIG. 3 is a diagram for explaining operation in SOFC mode in the solid oxide cell system according to an embodiment. FIG. 4 is a diagram for explaining operation in SOEC mode in the solid oxide cell system according to an embodiment. FIG. 5 is a functional block diagram showing an example of a system control unit according to an embodiment. FIG. 6 is a flowchart showing the operation of the solid oxide cell system according to an embodiment. FIG. 7 is a diagram for explaining an example of a method for determining the priority of an SOC stack to be operated in the next operation. FIG. 8 is a diagram for explaining another example of a method for determining the priority of an SOC stack to be operated in the next operation. FIG. 9 is a diagram for explaining yet another example of a method for determining the priority of an SOC stack to be operated in the next operation. FIG. 10 is another flowchart showing the operation of the solid oxide cell system according to an embodiment. FIG. 11 is a diagram showing an example of the hardware configuration of a control device of the solid oxide cell system according to an embodiment.

[0010] An embodiment of a solid oxide cell system according to the present disclosure will be described below with reference to the drawings. This disclosure focuses on a solid oxide cell system using a reversible SOC that has both SOFC and SOEC functions. Note that the same reference numerals in each drawing indicate the same or corresponding parts. Therefore, detailed descriptions of these parts may be omitted to avoid duplication. In the following, a solid oxide cell will be referred to as an SOC, a solid oxide cell stack will be referred to as an SOC stack, and a solid oxide cell module will be referred to as an SOC module. The operating mode in which the SOC stack operates as an SOFC will be referred to as the SOFC mode, and the operating mode in which it operates as an SOEC will be referred to as the SOEC mode.

[0011] <Configuration of Solid Oxide Cell System> Fig. 1 is a diagram showing the configuration of a solid oxide cell system according to an embodiment, and Fig. 2 is a diagram showing the configuration of an SOC stack. In Fig. 1, the solid oxide cell system 1 includes a gas supply unit 21 that supplies raw material gases required in the SOFC mode, and a gas supply unit 22 that supplies water vapor (H ２The solid oxide cell system 1 includes a water vapor supply unit 22 that supplies a gas (water vapor) containing oxygen (O), a temperature adjustment device 20 that adjusts the temperature of the supplied gas and water vapor, an SOC module 100 to which gas adjusted to a preset temperature by the temperature adjustment device 20 is supplied, and a control device 200 that switches the operation modes of the temperature adjustment device 20 and the SOC module 100 and controls the opening and closing of valves V1-V6 provided in the gas piping. These constitute the main components 1A of the solid oxide cell system 1 shown by the dotted line area in Fig. 1. An external device 300 is connected to the SOC module 100. The external device 300 is a load to which power generated by the SOC module 100 in the SOFC mode is supplied, or a current source that supplies power to the SOC module 100 in the SOEC mode.

[0012] The SOC module 100 is an assembly of multiple SOC stacks 10_1-10_n (n is a natural number equal to or greater than 2) connected together. Each SOC stack 10 is provided with an SOC stack temperature regulator 18. The temperature regulator 18 is, for example, a heater or other heating device. Note that the SOC stacks are collectively referred to as the SOC stack 10.

[0013] <Configuration of SOC Stack> Here, the structure of the multiple SOC stacks 10 that make up the SOC module 100 will be described with reference to Fig. 2. The SOC module 100 is an assembly of multiple SOC stacks 10_1-10_n (n is a natural number of 2 or more) connected together. Each SOC stack 10 is configured by stacking multiple SOCs 11_1-11_m (m is a natural number of 2 or more).

[0014] Taking the kth (1<k≦m; k is a natural number) SOC11_k as an example, each SOC11 has a structure in which an electrolyte 13 is sandwiched between an air electrode 12 and an anode 14, and is connected to adjacent SOC11s by an interconnector 15. A diffusion prevention layer to suppress material degradation may be inserted between the air electrode 12 and the electrolyte 13. Note that SOCs are collectively referred to as SOC11.

[0015] The control device 200 includes a system control unit 201 and a temperature control unit 202. The system control unit 201 switches the operation mode of the SOC module 100 and controls the opening and closing of valves V1-V6 provided in the gas piping. The system control unit 201 also controls each SOC stack 10 of the SOC module 100 so that the output power becomes the required output power in the SOFC mode and so that electrolysis corresponding to the input power (input current) can be performed in the SOEC mode. The temperature control unit 202 controls the temperature adjustment device 20 so that the supplied gas and steam reach a preset temperature, and controls the SOC stack temperature regulator 18 so that each SOC stack 10 reaches a preset temperature.

[0016] As described above, the external device 300 is a load or a current source. The load to which the power generated by the SOC module 100 in the SOFC mode is supplied is, for example, a power converter or a motor connected via a power converter. The current source to which the SOC module 100 is supplied in the SOEC mode is, for example, a power converter or a current source such as a grid. The control device 200 controls the external device 300 to control the voltage of each SOC stack.

[0017] The gas supply unit 21 supplies hydrogen (H ２ ), etc., and in SOEC mode, hydrogen, carbon dioxide (CO ２ ) or the like. A gas cylinder may be provided externally and supplied to the gas supply unit 21. In addition, the gas supplied to the air electrode in the SOFC mode may be, for example, oxygen (O ２ ), air, etc. These are also used in the SOEC mode, and since the gas species are theoretically the same during supply (upstream) and after reaction (downstream), the gas may be circulated.

[0018] The temperature adjustment device 20 adjusts the temperature of the (upstream) gas and steam pipes supplied to the SOC module 100, and is, for example, a heater or a heating device provided around the pipes. In addition, the downstream gas generated from the SOC module 100 may be used as a heat source to heat the upstream gas.

[0019] Optional components of the solid oxide cell system 1 may include a reformer 30, a steam generator 31, a water storage unit 32, a condenser 33, a gas storage unit 34, and a storage gas adjustment unit 35. The reformer 30 is used when using city gas (gas mainly composed of methane) in the SOFC mode. Downstream gas generated from the SOC module 100 may be supplied as fuel to a burner, which is a heat source for reforming.

[0020] The water vapor generator 31 heats water supplied from the water storage unit 32 to generate water vapor, and supplies the water vapor to the SOC module 100 via the water vapor supply unit 22. Water vapor may be supplied directly from the outside without using the water vapor generator 31.

[0021] The water reservoir 32 stores water to be supplied to the steam generator 31. Steam in the downstream gas on the fuel electrode side discharged from the SOC module 100 may be recovered and reused as supply gas or fuel for the reformer 30.

[0022] The condenser 33 dehydrates the downstream gas on the fuel electrode side discharged from the SOC module 100. For example, in the SOFC mode, the condenser 33 separates the water vapor generated in the SOC module 100 from unreacted hydrogen and the like by dehydration, and in the SOEC mode, the condenser 33 separates the hydrogen and the like generated in the SOC module 100 from unreacted water vapor by dehydration.

[0023] The gas storage unit 34 recovers the downstream gas (hydrogen, etc.) on the fuel electrode side discharged from the SOC module 100 in the SOEC mode and stores it as the upstream gas on the fuel electrode side required in the SOFC mode. The gas discharged from the SOC module 100 may be directly supplied to the outside of the system.

[0024] The storage gas adjusting unit 35 adjusts the gas by appropriately processing it in order to store it in the gas storage unit 34. For example, the form of the gas is adjusted by compressing the gas using a compressor, separating a mixed gas of, for example, hydrogen and carbon monoxide (CO), which is the gas downstream on the fuel electrode side discharged from the SOC module 100 in the SOEC mode, using a separator, or synthesizing methane using hydrogen and carbon monoxide (CO) in a methanation reactor. The gas adjusted by the storage gas adjusting unit 35 is stored in the gas storage unit 34.

[0025] In Figure 1 and the following figures showing the configuration of a solid oxide cell system, each line indicates the following: Solid line: Pre-reaction gas flow path (upstream) Dash-dotted line: Post-reaction gas flow path (downstream) Dashed line: Flow path for gases used other than fuel cells and steam electrolysis reactions Black arrow: Gases supplied to or discharged from the fuel electrode (hydrogen, nitrogen, carbon monoxide, hydrocarbons, water vapor, carbon dioxide, etc.) Open arrow: Gases supplied to or discharged from the air electrode (oxygen, air, etc.)

[0026] <Operation in SOFC Mode> Next, the operation of the solid oxide cell system 1 in the SOFC mode will be described. Figure 3 shows the gas flow in the SOFC mode in Figure 1, indicated by the bold line. In the SOFC mode, gases described below are supplied to the anode 14 and the cathode 12, respectively, and the generated electric power is output.

[0027] (1) Gas Supply to SOC Module 100 (Upstream) A gas containing hydrogen as its main component is supplied to the fuel electrode 14 side. The gas containing hydrogen as its main component is supplied from the gas supply unit 21 to the SOC module 100 via the temperature adjustment device 20. A gas containing hydrogen as its main component may be supplied from the gas storage unit 34. A gas containing methane as its main component may be supplied to the fuel electrode 14 side. The gas containing methane as its main component may be supplied from the gas storage unit 34 to the SOC module 100 via the temperature adjustment device 20. Alternatively, the gas may be supplied from the outside via the reformer 30. Alternatively, the gas may be supplied from the gas storage unit 34 via the reformer 30. In the reformer 30, steam reforming is carried out using steam supplied from the water storage unit 32 through the following reaction, and the reformed gas is supplied to the fuel electrode 14 side. CH ４ +H ２ O → CO + 3H ２ As will be explained below, when the SOC module 100 is not operating in SOFC mode and is undergoing temperature adjustment or is at rest, a purge gas or reducing gas containing hydrogen and nitrogen as its main components is supplied from the gas supply unit 21 or the gas storage unit 34 to the fuel electrode 14 side of the SOC stack 10.

[0028] Oxygen or air is supplied to the air electrode 12 side from a gas supply unit 21. Oxygen or air is also supplied to the air electrode 12 side of the SOC stack 10 of the SOC module 100 that is not operating in SOFC mode and is undergoing temperature adjustment or is at rest. In gas supply to the SOC module 100, the gas supply source to the anode 14 side is determined by the opening and closing direction of valve V1. When the gas supplied to the anode 14 side is a gas mainly composed of methane, the opening and closing direction of valve V2 is controlled to supply the gas mainly composed of methane from a gas storage unit 34 to the reformer 30. The opening and closing direction of valve V3 is determined by whether or not gas is supplied from the reformer 30. These valves V1 to V3 are three-way valves, and their opening and closing directions are controlled by a system control unit 201.

[0029] (2) Gas Discharge from the SOC Module 100 (Downstream) When a gas containing hydrogen as a main component is supplied from the fuel electrode 14 side, water vapor and unreacted hydrogen, etc. are discharged, and when a gas containing methane as a main component is supplied, water vapor and carbon dioxide, etc. are discharged. The discharged gas may be discharged directly to the outside, or, as shown in FIG. 3 , the water vapor and other gases may be separated in a condenser 33, and then the hydrogen and carbon dioxide, etc. may be separated in a storage gas adjustment unit 35. The water vapor may be stored in a water reservoir 32. Unreacted oxygen or air is discharged from the air electrode 12 side. The discharged gas may be returned to the gas supply unit 21 to circulate through the system.

[0030] When gas is exhausted from the SOC module 100, water can be supplied from the water reservoir 32 when the reformer 30 is operating by opening and closing the valve V5. Also, part of the exhaust gas can be supplied as fuel for the heat supply burner of the reformer 30 by opening and closing the valve V4. The valve V4 is a three-way valve, and the valve V5 is an on-off valve, and the opening and closing directions and opening and closing operations are controlled by the system control unit 201.

[0031] The temperature adjustment device 20 may be provided with a heat exchange function so that the supply gas is heated by heat exchange with the exhaust gas. During operation, the temperatures of the temperature adjustment device 20, the SOC stack temperature regulator 18, and the reformer 30 are controlled to the respective set temperatures by a temperature control unit 202.

[0032] <Operation in SOEC Mode> Next, the operation of the solid oxide cell system 1 in the SOEC mode will be described. Figure 4 shows the gas flow in the SOEC mode in Figure 1, indicated by the bold line. In the SOEC mode, gases described below are supplied to the anode 14 and cathode 12, respectively, and an electrolysis reaction occurs due to the input power.

[0033] (1) Gas Supply to SOC Module 100 (Upstream) A gas containing water vapor as a main component is supplied to the anode 14 side from a water vapor supply unit 22. Water vapor may be introduced directly from the outside, or water supplied from a water reservoir 32 may be heated by a water vapor generator 31 to generate water vapor, as shown in FIG. 4 . Alternatively, hydrogen may be supplied from a gas storage unit 34 or a gas supply unit 21, mixed with water vapor by a temperature adjustment device 20, and supplied to the anode 14 side. Alternatively, a gas containing carbon dioxide as a main component may be supplied to the anode 14 side from the gas supply unit 21. At this time, a mixed gas of water vapor and carbon dioxide may be supplied to the anode 14 side. As will be described below, a purge gas or reducing gas containing hydrogen and nitrogen as main components is supplied from the gas supply unit 21 or a gas storage unit 34 to the anode 14 side of the SOC stack 10 of the SOC module 100 that is not operating in the SOEC mode and is undergoing temperature adjustment or is at rest.

[0034] Oxygen or air is supplied to the air electrode 12 side from a gas supply unit 21. Oxygen or air is also supplied to the air electrode 12 side of the SOC stack 10 of the SOC module 100 that is not operating in the SOEC mode and is undergoing temperature adjustment or is at rest. In supplying gas to the SOC module 100, the gas supply source to the fuel electrode 14 side is determined by the opening and closing direction of valve V1 and the opening and closing of valve V6. Valve V1 is a three-way valve, and valve V6 is an on-off valve, and the opening and closing direction and opening and closing operation are controlled by a system control unit 201.

[0035] (2) Gas Discharge (Downstream) from the SOC Module 100 When a gas primarily composed of water vapor is supplied, hydrogen and unreacted water vapor are discharged from the fuel electrode 14 side. When a gas primarily composed of carbon dioxide is supplied, carbon monoxide and unreacted carbon dioxide are discharged. The discharged gas may be discharged directly to the outside of the system. However, as shown in FIG. 4 , the water vapor and other gases may be separated in a condenser 33, and then the hydrogen and carbon monoxide may be separated in a storage gas adjustment unit 35. Alternatively, the mixed gas containing hydrogen and carbon monoxide may be introduced as is into the methanation reactor to produce methane. Gases such as hydrogen, carbon monoxide, and methane processed in the storage gas adjustment unit 35 are stored in a gas storage unit 34. Unreacted water vapor may be stored in a water reservoir 32 and then supplied to the fuel electrode 14 again. Generated oxygen or air with a high oxygen concentration is discharged from the air electrode 12 side. The discharged gas may be returned to the gas supply unit 21 to circulate through the system.

[0036] The temperature adjustment device 20 may be provided with a heat exchange function so that the supply gas is heated by heat exchange with the exhaust gas. Also, the steam generator 31 may be provided with a heat exchange function so that steam is generated by heat exchange with the exhaust gas. As in the SOFC mode, the temperature of the temperature adjustment device 20 during operation and the temperature of the SOC stack temperature regulator 18 are controlled to respective set temperatures by the temperature control unit 202.

[0037] <Configuration of operating SOC stack selection unit 203> Fig. 5 is a diagram showing the configuration of the control device 200 according to this embodiment. The control device 200 according to this embodiment 1 includes an operating SOC stack selection unit 203 in addition to a system control unit 201 and a temperature control unit 202 that control the basic operations described above. In Fig. 5, the operating SOC stack selection unit 203 includes a stack voltage acquisition unit 231 that acquires the voltage of each SOC stack 10 included in the SOC module 100, and a stack voltage V ｓ and a preset value V ｅｘand a deterioration level estimation unit 232 that calculates the difference between the calculated value and the SOC stack 10 and estimates the deterioration level of each SOC stack 10 based on the calculated value, and an operation priority determination unit 233 that determines the priority of the SOC stack 10 to be operated next based on the estimated deterioration level.

[0038] The stack voltage acquisition unit 231 acquires the voltage of each SOC stack 10, for example, from a voltage sensor (not shown) provided in each SOC stack 10. Alternatively, the voltage of each SOC stack 10 may be acquired by measuring the input / output power of each SOC stack 10 during operation from the relationship between the input / output power of each SOC stack 10 and the stack voltage during operation.

[0039] The deterioration level estimation unit 232 estimates the stack voltage V of the SOC stack 10 obtained. ｓ and a preset value V ｅｘ The difference between the value V and the value V is calculated, and the larger the calculated difference value, the greater the degree of deterioration is estimated to be. ｅｘ The setting method will be described later.

[0040] The operation priority determination unit 233 compares the estimated deterioration levels of the SOC stacks 10 included in the SOC module 100 and determines to give a higher priority to the SOC stack 10 with the least deterioration level as the SOC stack 10 to be operated next. The determined priority is transmitted to the system control unit 201.

[0041] <Operation Example 1 of Solid Oxide Cell System 1> Fig. 6 is a flowchart showing the operation of the solid oxide cell system 1 according to this embodiment. First, the SOC module 100 starts operation in the SOFC mode or the SOEC mode under the control of the system control unit 201 (step S100). The stack voltage acquisition unit 231 acquires the stack voltage V of each SOC stack 10 during operation. ｓ is acquired (step S101).

[0042] The deterioration level estimation unit 232 estimates the acquired stack voltage V ｓ and a preset value V ｅｘThe operation priority determination unit 233 calculates the difference between the estimated deterioration levels and estimates the deterioration level (step S102). The operation priority determination unit 233 compares the estimated deterioration levels of the SOC stacks 10 included in the SOC module 100, determines to assign a higher priority to the SOC stack 10 with the smaller deterioration level as the SOC stack 10 to be operated next, and transmits the determined priority to the system control unit 201 (step S103). The system control unit 201 stops the operation of the SOC module 100 (step S104).

[0043] In the next operation, the system control unit 201 starts operation of the SOC stack 10 with the highest priority among the SOC stacks 10 included in the SOC module 100 (step S105). If there is an SOC stack 10 for which input / output is to be stopped (YES in step S106), the system control unit 201 stores the deterioration level of the SOC stack 10 to be stopped (step S107). In the next operation, the stack voltage V of the SOC stack 10 that was stopped in step S101 is ｓ However, since the previous degradation level is stored, the stored degradation level can be used to determine the operation priority of the SOC stack 10 in step S103. By repeating the operations from step S101 to step S107, the load on the SOC stack 10 with a large degradation level is reduced, and the variation in degradation among the SOC stacks 10 can be suppressed.

[0044] The system control unit 201 operates the SOC stack 10 of the SOC module 100 in response to the required output power in the SOFC mode and the required input power in the SOEC mode. Since the input / output of the SOC module 100 is usually designed with a margin for the required input / output, in steps S106 and S107, the deterioration level of the SOC stack 10 for which the input / output is stopped is stored. At the next operation, the deterioration level stored in step S103 is taken into consideration and an operation priority is determined based on the deterioration level. By stopping or adjusting the input / output of the SOC stack 10 in this manner, the load on the SOC stack 10 with a high degree of deterioration can be reduced.

[0045] The flow shown in FIG. 6 has been described as an example in which either the SOFC mode or the SOEC mode is repeated, but the operation mode may be switched.

[0046] <Value V ｅｘ Setting method of stack voltage V ｓ The value V ｅｘ The method for setting V is explained below with an example. (1) The chemical reaction occurring in the SOC 11 is an oxidation reaction in the SOFC mode, and a reduction reaction in the SOEC mode. The theoretical voltage V of these chemical reactions is called the theoretical electromotive force or theoretical decomposition voltage, and its value is determined by the temperature. Therefore, the theoretical voltage V corresponding to the operating temperature in the SOFC mode and the SOEC mode is set to the value V ｅｘ can be used for comparison.

[0047] (2) The theoretical voltage V0 of the chemical reaction occurring in the SOC 11 is determined by its operating temperature, but the theoretical operating voltage of the SOC 11 varies depending on the gas pressure at the air electrode 12 and the fuel electrode 14 of the SOC 11. For example, when generating electricity using hydrogen in the SOFC mode or when electrolyzing water vapor in the SOEC mode, the theoretical voltage is V0 and the theoretical operating voltage is V ｒ The relationship between the two is expressed by the following equation (1), which is generally called the Nernst equation.

[0048] Here, the temperature T can be obtained by measuring the temperature of each SOC stack 10, or from the operating temperature preset for each SOC stack 10 in the temperature control unit 202. The temperature of the SOC stack 10 may be the average value of the temperatures of a plurality of SOC stacks 10. If the temperature varies depending on the position of the SOC stack 10 (if there is a temperature distribution within the SOC module 100), it is desirable to obtain the temperature near the SOC stack 10 whose stack voltage is to be measured. The pressure of each gas can be obtained using, for example, the supply amount (gas flow rate) of each gas controlled by the system control unit 201. Specifically, the value of a mass flow controller that controls the gas flow rate may be used.

[0049] Therefore, based on the formula (1), the theoretical voltage V, the stack temperature T, the pressure P of each gas, and the theoretical operating voltage V ｒ The relationship between the theoretical operating voltage V and the SOC stack selection unit 203 is stored in a storage device in the control device 200. ｒ to the value V ｅｘ The power generation using hydrogen in the SOFC mode and the electrolysis of water vapor in the SOEC mode have been explained using equation (1), but the pressure P term in equation (1) can be similarly determined by using the supply gas, product gas, and reaction formula in each mode.

[0050] (3) Stack voltage V at the beginning of operation of the solid oxide cell system 1 ｓ to the value V ｅｘ For example, when the solid oxide cell system 1 is newly operated, the value V of each mode may be calculated from the average of the stack voltage during operation for a certain period of time in the SOEC mode and the SOFC mode. ｅｘ In this case, the operation before step S100 may be performed by calculating the value V ｅｘ is not set, and the value V ｅｘ (4) As the SOC stack deteriorates, the stack voltage V ｓ In the SOEC mode, the stack voltage V ｓ Therefore, the above value V ｅｘ The setting methods of each are examples, and the stack voltage V ｓ A constant value V can be used as a reference within a range that can show a significant difference and determine the priority between each SOC stack. ｅｘ It may be set in advance as

[0051] <Method of Determining Priority> In step S103, the degree of deterioration estimated for each SOC stack 10 is compared to determine the priority of the SOC stack 10 to be operated next. An example of the method of determining the priority will be described below.

[0052] (1) Method of assigning priorities to all SOC stacks In step S102, the degree of deterioration of each SOC stack 10 is estimated, and the priority is assigned in ascending order of the estimated degree of deterioration. That is, the n SOC stacks 10 included in the SOC module 100 are assigned a higher priority in descending order of the degree of deterioration.

[0053] (2) Method of determining priority by grouping SOC stacks into preset ranges of estimated deterioration levels Figures 7A, 7B, and 7C are diagrams for explaining a method of determining the priority of SOC stacks to be operated the next time based on the deterioration level. In each figure, black circles (●) indicate the deterioration level of each SOC stack, and the overall distribution of the deterioration levels can be seen.

[0054] 7A, a high priority is assigned to an SOC stack group A1 whose degradation level is lower than a preset threshold value α, and individual SOC stacks within the SOC stack group A1 are not assigned a priority and are operated in any order. SOC stacks whose degradation level is equal to or higher than the threshold value are assigned a high priority next to the priority of the SOC stack group A1, in order from the least to the most deteriorated.

[0055] (B) In Figure 7B, the average value of the degradation levels of all SOC stacks is calculated, and a preset range of deviation from the average value, for example, a range from the average value +X% to the average value -Y%, is set. The degradation level of the average value +X% is set as threshold β2, and the degradation level of the average value -Y% is set as threshold β1. SOC stacks equal to or greater than threshold β2 are set as SOC stack group B2, and SOC stacks less than threshold β1 are set as SOC stack group B1. Here, X may be equal to Y.

[0056] For the grouping shown in Fig. 7B, the priority is determined, for example, as follows: (B-1) High priority is assigned to SOC stacks other than those belonging to SOC stack group B2 in order of decreasing degree of deterioration. (B-2) The highest priority is assigned to SOC stack group B1, and no priority is assigned to the individual SOC stacks within SOC stack group B1, and they are operated in an arbitrary order. High priority is assigned to SOC stacks with a threshold value β1 or higher in order of decreasing degree of deterioration. (B-3) SOC stacks with a degree of deterioration below the average value are assigned a high priority, but in an arbitrary order, and SOC stacks with a degree of deterioration above the average value are assigned a high priority in order of decreasing degree of deterioration.

[0057] (C) In FIG. 7C , the degradation levels of all SOC stacks are divided into three groups. SOC stack group C1 has a low degradation level, SOC stack group C2 has an average degradation level, and SOC stack group C3 has a high degradation level. Priorities are assigned to each group in the order of SOC stack group C1, SOC stack group C2, and SOC stack group C3. The SOC stacks within each SOC stack group may be assigned a higher priority in descending order of degradation level, or the priority of each SOC stack may be set arbitrarily. While an example of dividing into three groups is shown, the number of groups is not limited to three and any number of groups may be used.

[0058] As shown in the above example, the priority may be determined by mixing grouped and ungrouped SOCs. In this case, an arbitrary value within a set range of degradation levels is used as the representative degradation level for the grouped SOCs, and the degradation levels of ungrouped SOC stacks and stack groups are compared to give priority to the SOC stack or stack group with the lower degradation level.

[0059] The priorities determined by the method shown in the above example are transmitted from the operation priority determination unit 233 to the system control unit. The system control unit 201 selects and operates the SOC stacks in descending order of priority so as to correspond to the power input / output during the next operation. The stack to be preferentially operated among the SOC stacks 10 in the next operation mode and its load may be calculated during the pause period between operation modes.

[0060] <Operation Example 2 of Solid Oxide Cell System 1> It is known that a solid oxide cell system equipped with a reversible SOC generally contributes more to degradation when operated in SOEC mode than in SOFC mode. That is, degradation is more likely when operated in SOEC mode. It has also been confirmed that alternating operation in SOFC mode and SOEC mode suppresses degradation of the SOC stack compared to continuous operation in one operation mode.

[0061] In the operation example 1 of the solid oxide cell system 1 shown in Fig. 6, the operation mode is not taken into consideration. Below, an example of operating the SOC stack while taking into consideration both the operation mode and the deterioration degree of the SOC stack will be described with reference to Fig. 8.

[0062] 8A and 8B are flowcharts showing another operation of the solid oxide cell system 1 according to this embodiment. Steps S200 to S204 are similar to steps S100 to S104 in FIG. 6, and therefore a description thereof will be omitted.

[0063] In step S205, the system control unit 201 determines whether the next operating mode is SOEC mode or SOFC mode, and if it is SOFC mode, in step S206, it determines whether the previous operating mode, i.e., the operating mode when the priority was determined, was SOEC mode or SOFC mode.

[0064] If the next operation mode is determined to be the SOEC mode in step S205, the system control unit 201 operates the SOC stacks 10 with the highest priority based on the priorities determined in step S203 and transmitted from the operation priority determination unit 233, and operates them in the SOEC mode to accommodate the input power (step S210). If the next operation mode is determined to be the SOFC mode in step S205, the system control unit 201 proceeds to step S206. If the previous operation mode is the SOFC mode in step S206, the next operation mode is also the SOFC mode, which is continuous operation, and therefore operation based on the deterioration level of the SOC stacks 10 is required, and the system control unit 201 proceeds to step S210. The next operation is started in the SOEC mode starting with the SOC stack 10 with the highest priority so as to achieve the input power requested in step S210. In step S211, the system control unit 201 determines whether there is any SOC stack 10 for which input / output is to be stopped. If there is any SOC stack 10 for which input / output is to be stopped (YES in step S211), the system control unit 201 proceeds to step S212 and stores the deterioration level of the SOC stack 10 to be stopped. In step S211, if there is no SOC stack 10 for which input / output is to be stopped (NO in step S211), the priority information of the SOC stack 10 determined in step S203 and transmitted from the operation priority determination unit 233 is reset (step S213).

[0065] In step S206, if the previous operating mode was the SOEC mode, the next operating mode has been switched to the SOFC mode, and the process proceeds to step S207. In step S207, it is determined whether there is an SOC stack 10 for which input / output is to be stopped. If there is an SOC stack 10 for which input / output is to be stopped (YES in step S207), the process proceeds to step S208, where the deterioration level of the SOC stack 10 to be stopped is stored, and the process returns to step S200, where operation in the SOFC mode is started using an SOC stack 10 that is not stopped so as to achieve the requested output power. If there is no SOC stack 10 for which input / output is to be stopped (NO in step S207), the process is determined in step S203, and the priority information of the SOC stack 10 transmitted from the operation priority determination unit 233 is reset (step S209). Thereafter, the process returns to step S200, where an arbitrary SOC stack 10 is used, regardless of the deterioration level of the SOC stack 10, and operation in the SOFC mode is started so as to achieve the requested output power.

[0066] As described above, when the next operation mode is the SOEC mode, or when the next operation mode is the SOFC mode but the SOFC mode continues, the SOC stacks 10 with a lower degree of degradation are operated preferentially, and the input / output of each SOC stack 10 is adjusted or stopped, thereby reducing the load on the SOC stacks 10 with a higher degree of degradation. That is, for the SOC stacks 10 with a higher degree of degradation and for which the input / output is stopped, the degradation level is stored and taken into consideration in determining the next operation priority. Therefore, by repeating the operations from step S203 onward, the load on the SOC stacks 10 with a higher degree of degradation is reduced, and the variation in degradation among the SOC stacks 10 can be suppressed. Furthermore, when there is no SOC stack 10 for which the input / output is stopped, the priority information is reset, and the priority is determined based on the newly estimated degradation level. This makes it possible to grasp the variation in degradation among the SOC stacks 10 and suppress the variation in degradation among the SOC stacks 10.

[0067] 8A and 8B, the example of FIG. 6 shows that the SOC stack 10 with the least degree of degradation is preferentially operated regardless of the operation mode. Operating each SOC stack according to the priority and the input / output adjusts the degree of degradation of each SOC stack 10 and reduces the load. On the other hand, operating each SOC stack 10 according to the priority and the input / output requires the system control unit 201 to calculate an operation plan for each SOC stack 10. Therefore, when alternate operation is determined in step S206, as in operation example 2 shown in FIGS. 8A and 8B, calculation is not required, and the calculation load on the system control unit 201 is reduced.

[0068] As mentioned above, the deterioration is more likely to occur when operating in the SOEC mode. However, the deterioration also differs depending on the type of gas supplied to the SOC stack 10 and the SOC 11. This is because the theoretical operating voltage V ｒ This is because the voltages are different. In SOEC mode electrolysis, it is known that for the same input power, co-electrolysis, in which water vapor and carbon dioxide are used to simultaneously decompose both, results in a stack voltage that is equal to or higher than that of water vapor electrolysis, and that the stack voltage of carbon dioxide electrolysis is even higher than that of water vapor electrolysis or co-electrolysis. Therefore, in SOEC mode electrolysis, if carbon dioxide is used, applying the operation of this embodiment will achieve the effect of further suppressing degradation and suppressing the variation in degradation.

[0069] 9 is a diagram showing an example of the hardware configuration of a control device 200 included in a solid oxide cell system according to an embodiment. The control device 200 includes a processor 1000 and a storage device 2000. The processor 1000 includes, for example, a CPU (Central Processing Unit), an ASIC (Application Specific Integrated Circuit), a DSP (Digital Signal Processor), an FPGA (Field Programmable Gate Array), etc.

[0070] The main memory of the storage device 2000 is composed of a volatile memory device such as a random access memory, and the auxiliary memory device is a nonvolatile memory device such as a flash memory or a hard disk. The auxiliary memory device stores a predetermined program to be executed by the processor 1000, which reads and executes the program as appropriate to perform various arithmetic processing. At this time, the predetermined program is temporarily stored in the volatile memory device from the auxiliary memory device, and the processor 1000 reads the program from the volatile memory device. The processor 1000 may output data such as arithmetic results to the volatile memory device of the storage device 2000, or may store the data in the auxiliary memory device via the volatile memory device.

[0071] In order to enable remote control, a transmitter and a receiver (neither of which are shown) may be provided as a communication module for communicating with the outside.

[0072] As described above, according to this embodiment, the solid oxide cell system includes an SOC module having an SOC stack switchable between an SOFC mode, which generates electricity using a first gas and outputs electric power, and an SOEC mode, which receives electric power and electrolyzes a second gas, and a control device for controlling the operation of the multiple SOC stacks. The control device includes a stack voltage acquisition unit that acquires the voltages of the multiple SOC stacks operating in the SOFC mode or the SOEC mode, and a degradation level estimation unit that estimates the degradation level of the SOC stacks based on the acquired solid oxide cell stack voltages. In the next operating mode, the control device prioritizes operation of the SOC stack with the least estimated degradation level. This configuration prioritizes operation of the SOC stack with the least degradation level, thereby suppressing degradation variations between SOC stacks and enabling stable operation of the solid oxide cell system. Furthermore, the solid oxide cell system can be extended in life and its maintenance and upgrades can be simplified. The first gas is, for example, a gas whose main component is hydrogen or a gas whose main component is methane supplied to the fuel electrode side, or oxygen or air supplied to the air electrode side, and the second gas is, for example, a gas whose main component is water vapor or a gas whose main component is carbon dioxide supplied to the fuel electrode side, or oxygen or air supplied to the air electrode side.

[0073] Furthermore, when the control device next operates the SOC stack in the SOEC mode, it controls the operation of the SOC stack with the least estimated degree of degradation. Since SOC stacks deteriorate more rapidly in the SOEC mode than in the SOFC mode, by operating the SOC stack with the least estimated degree of degradation in the SOEC mode with priority, it is possible to suppress the variation in deterioration between SOC stacks and enable stable operation of the solid oxide cell system.

[0074] Furthermore, since continuous operation in the same mode causes more deterioration of the SOC stack than alternating operation, when the SOC stacks are operated continuously in the same mode, by controlling the operation of the SOC stack with the least estimated degree of deterioration as a priority, it is possible to suppress the variation in deterioration between the SOC stacks and enable stable operation of the solid oxide cell system.

[0075] Furthermore, the degradation level is estimated based on the difference between the voltage of the SOC stack and a preset value. The preset value is the theoretical voltage required for the chemical reaction of the supply gas in each operating mode, or a theoretical operating voltage calculated from the temperature of the SOC stack and the pressure ratio of the gas before and after the chemical reaction. Therefore, if the preset value is stored in advance in a storage device within the control device, calculations can be easily performed. Note that the degradation level may be estimated without calculating the difference between the voltage of the SOC stack and the preset value. For example, consider a case where all SOC stacks are operated under the same conditions. In this case, an estimation method may be applied in which a stack with a higher voltage in SOEC mode is determined to have a higher degradation level, and a SOC stack with a lower voltage in SOFC mode is determined to have a higher degradation level.

[0076] Furthermore, for SOC stacks having a degradation level estimated by the degradation level estimation unit, multiple SOC stacks within a predetermined range of degradation levels are grouped and their operation is controlled with the same priority, thereby reducing the amount of calculation required for priority.

[0077] Although exemplary embodiments are described in the present disclosure, the various features, aspects, and functions described in the embodiments are not limited to the application of a particular embodiment, but may be applied to the embodiments alone or in various combinations. Therefore, countless variations not illustrated are anticipated within the scope of the technology disclosed in this specification. For example, variations in, addition to, or omission of at least one component are included.

[0078] 1: Solid oxide cell system, 10: SOC stack, 11: SOC, 12: Air electrode, 13: Electrolyte, 14: Anode, 15: Interconnector, 18: Temperature regulator for SOC stack, 20: Temperature regulator, 21: Gas supply unit, 22: Steam supply unit, 30: Reformer, 31: Steam generator, 32: Water storage unit, 33: Condenser, 34: Gas storage unit, 35: Storage gas adjustment unit, 100: SOC module, 200: Control device, 201: System control unit, 202: Temperature control unit, 203: Operating SOC stack selection unit, 231: Stack voltage acquisition unit, 232: Deterioration degree estimation unit, 233: Operation priority determination unit, 1000: Processor, 2000: Storage device.

Claims

1. A solid oxide cell system comprising: a solid oxide cell module having a solid oxide cell stack switchable between an SOFC mode in which a first gas is used to generate electricity and output electric power, and an SOEC mode in which electric power is input and a second gas is electrolyzed, the solid oxide cell module being formed by connecting a plurality of the solid oxide cell stacks; and a control device for controlling the operation of the plurality of solid oxide cell stacks, the control device having: a stack voltage acquisition unit for acquiring the voltages of the plurality of solid oxide cell stacks operating in the SOFC mode or the SOEC mode; and a deterioration degree estimation unit for estimating the degree of deterioration of the solid oxide cell stack based on the acquired voltages of the solid oxide cell stacks, the control device controlling the operation of the solid oxide cell stack having a smaller estimated degree of deterioration in the next SOFC mode or SOEC mode by giving priority to the solid oxide cell stack having a smaller estimated degree of deterioration.

2. The solid oxide cell system according to claim 1, wherein the control device performs control so that the next time the solid oxide cell stack is operated in the SOEC mode, the control device gives priority to the operation of the solid oxide cell stack having the smaller estimated degree of deterioration.

3. The solid oxide cell system according to claim 1, wherein the control device performs control to give priority to the operation of a solid oxide cell stack having a smaller estimated degree of deterioration when the solid oxide cell stacks are operated continuously in the same mode.

4. A solid oxide cell system according to any one of claims 1 to 3, wherein the deterioration level estimation unit calculates the difference between the acquired voltage of the solid oxide cell stack and a preset value, and estimates the deterioration level of the solid oxide cell stack based on the calculated difference value.

5. The solid oxide cell system according to claim 4, wherein the control device uses a theoretical voltage required for a power generation reaction by the first gas as the preset value when the solid oxide cell stack is operating in the SOFC mode, and uses a theoretical voltage required for an electrolysis reaction by the second gas as the preset value when the solid oxide cell stack is operating in the SOEC mode.

6. The solid oxide cell system of claim 4, wherein the control device, when operating the solid oxide cell stack in the SOFC mode, uses as the preset value a theoretical operating voltage calculated using a theoretical voltage required for a power generation reaction with the first gas, the temperature of the solid oxide cell stack, and the gas pressure ratio before and after the power generation reaction with the first gas, and, when operating the solid oxide cell stack in the SOEC mode, uses as the preset value a theoretical operating voltage calculated using a theoretical voltage required for an electrolysis reaction with the second gas, the temperature of the solid oxide cell stack, and the gas pressure ratio before and after the electrolysis reaction with the second gas.

7. A solid oxide cell system according to any one of claims 1 to 6, wherein in the SOFC mode, electric power is output by a power generation reaction using hydrogen or a mixed gas of hydrogen and carbon monoxide as the first gas, and in the SOEC mode, at least one of water vapor and carbon dioxide as the second gas is electrolyzed by input electric power.

8. The solid oxide cell system according to any one of claims 1 to 7, wherein the control device provides at least one stack group having a plurality of the solid oxide cell stacks within a preset range of deterioration levels for the solid oxide cell stack having a deterioration level estimated by the deterioration level estimation unit, and in the next SOFC mode or SOEC mode, compares the deterioration levels of the stack group and the solid oxide cell stacks not belonging to the stack group with those of the stack group and the solid oxide cell stacks not belonging to the stack group, and performs control to preferentially operate the solid oxide cell stack or the stack group having the smaller estimated deterioration level.

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