Fuel cell system and operation method therefor

Abstract

The present invention comprises: a stack (1) obtained by stacking a plurality of cells (11) that generate hydrogen and power by switching between an electrolysis mode in which hydrogen and oxygen are generated by water electrolysis and a fuel cell mode in which hydrogen is oxidized to generate power; and a gas transfer controller (7) that performs fuel cell mode gas transfer control for transferring a hydrogen-containing gas (22) and an oxygen-containing gas (32) from the high-temperature side toward the low-temperature side of the stack in the fuel cell mode and / or electrolysis mode gas transfer control for transferring water vapor (52) and an oxygen electrode gas (62) from the low-temperature side toward the high-temperature side of the stack or from the high-temperature side toward the low-temperature side in accordance with the operation of the stack in the electrolysis mode.

Description

Fuel cell system and its operation method

[0001] The present disclosure relates to a fuel cell system and its operation method.

[0002] Development of a fuel cell system capable of reversible operation to obtain hydrogen as fuel by combining a fuel cell such as a solid oxide fuel cell (SOFC) and an electrolysis cell such as a solid oxide electrolysis cell (SOEC) is underway. Further, for example, in Patent Document 1, in a reversible fuel cell system that switches between a fuel cell mode and an electrolysis mode operating at high temperature, a heat exchanger is incorporated into an interconnector used in both the fuel cell and the electrolyzer, and a heat transfer fluid different from the reaction gas is circulated for heat exchange, and a technique thereof is disclosed.

[0003] Japanese Patent Application Laid-Open No. 2017-532445

[0004] However, a fuel cell system that reversibly alternates between a fuel cell mode and an electrolysis mode uses an electric furnace or the like to heat the stack from the outside to the operating temperature. Therefore, when the stack becomes high temperature, a temperature distribution is likely to occur due to the exothermic reaction and endothermic reaction in the stack. When a temperature distribution occurs in the stack, there is a risk that deterioration of constituent members such as electrode materials and electrolyte materials constituting the cell is accelerated. Therefore, it is preferable to maintain as uniform a temperature distribution as possible. Although it is possible to lower the temperature of the high-temperature portion by circulating a heat transfer fluid different from the reaction gas for heat exchange, there is a problem that the device configuration becomes complicated.

[0005] The present disclosure has been made to solve the above problems, and an object thereof is to provide a fuel cell system capable of uniformly controlling the temperature in the stack without complicating the device configuration and reversibly implementing a fuel cell mode and an electrolysis mode. Another object is to provide an operation method of the fuel cell system.

[0006] The fuel cell system according to this disclosure comprises a stack of multiple cells having fuel electrodes and oxygen electrodes that switch between an electrolysis mode, which generates hydrogen and oxygen by electrolysis of water, and a fuel cell mode, which generates electricity by oxidizing hydrogen, to perform hydrogen generation and electricity generation, and a gas transfer controller that performs gas transfer control of at least one of the following: fuel cell mode gas transfer control, which transfers a gas containing hydrogen and a gas containing oxygen from the high-temperature side to the low-temperature side of the stack in fuel cell mode, and electrolysis mode gas transfer control, which transfers water vapor and oxygen electrode gas from the low-temperature side to the high-temperature side of the stack, or from the high-temperature side to the low-temperature side, depending on the operation of the stack in electrolysis mode.

[0007] Furthermore, the method for operating a fuel cell system according to this disclosure is a method for operating a fuel cell system that performs hydrogen production and power generation by switching between an electrolysis mode, in which hydrogen and oxygen are produced by the electrolysis of water, and a fuel cell mode, in which hydrogen is produced by oxidizing hydrogen, in a stack in which a plurality of cells having fuel electrodes and oxygen electrodes are stacked, the method comprising: determining whether to set the operating mode to fuel cell mode or electrolysis mode; switching to either fuel cell mode or electrolysis mode based on the determined operating mode; if the operating mode is fuel cell mode, transferring hydrogen-containing gas and oxygen-containing gas from the high-temperature side to the low-temperature side of the stack; and if the operating mode is electrolysis mode, transferring water vapor and oxygen electrode gas from the low-temperature side to the high-temperature side, or from the high-temperature side to the low-temperature side of the stack, according to the operation of the stack.

[0008] According to this disclosure, a uniform temperature can be achieved within the stack with a simple device configuration.

[0009] This is a schematic block diagram showing a fuel cell system according to Embodiment 1. This is a schematic configuration diagram showing a stack according to Embodiment 1. This is an explanatory diagram showing an example of the direction of gas transfer within the stack in fuel cell mode according to Embodiment 1. This is an explanatory diagram showing an example of the direction of gas transfer within the stack in fuel cell mode according to Embodiment 1. This is an explanatory diagram showing an example of the direction of gas transfer within the stack in fuel cell mode according to Embodiment 1. This is an explanatory diagram showing an example of the direction of gas transfer within the stack in electrolysis switching operation from fuel cell mode to electrolysis mode according to Embodiment 2. This is an explanatory diagram showing an example of the direction of gas transfer within the stack in switching operation from electrolysis mode to fuel cell mode according to Embodiment 2. This is a schematic configuration diagram showing a stack according to Embodiment 3. This is a schematic block diagram showing a fuel cell system according to Embodiment 3. This is a schematic configuration diagram showing a stack according to Embodiment 3. This is a schematic configuration diagram showing a stack according to Embodiment 3. This is a flowchart showing the operation of a fuel cell system according to Embodiment 4. This is a flowchart showing an example of the operation of the fuel cell system according to Embodiment 4. This is a flowchart showing an example of the operation of the fuel cell system according to Embodiment 4. This is a flowchart showing an example of the operation of the fuel cell system according to Embodiment 4. This is a schematic diagram showing an example of a processing circuit that realizes each control function of the fuel cell system according to Embodiment 4.

[0010] Embodiments will be described with reference to the drawings. Here, the same parts and corresponding components are denoted by the same reference numerals, and their detailed descriptions are omitted.

[0011] Embodiment 1. A fuel cell system 1000 according to Embodiment 1 will be described with reference to the drawings. Figure 1 is a schematic block diagram showing a fuel cell system 1000 according to Embodiment 1. In Figure 1, the fuel cell system 1000 includes a stack 1 in which a plurality of cells 11 having fuel electrodes and oxygen electrodes are stacked and switch between an electrolysis mode in which hydrogen and oxygen are produced by the electrolysis of water and a fuel cell mode in which hydrogen is oxidized to generate electricity, and a gas transfer controller 7 that performs gas transfer control of at least one of the following: fuel cell mode gas transfer control in which hydrogen-containing gas 22 and oxygen-containing gas 32 are transferred from the high-temperature side to the low-temperature side of the stack 1 in fuel cell mode, and electrolysis mode gas transfer control in which water vapor 52 and oxygen electrode gas 62 are transferred from the low-temperature side to the high-temperature side or from the high-temperature side to the low-temperature side of the stack 1 in accordance with the operation of the stack.

[0012] Stack 1 constitutes a so-called reversible fuel cell, which operates a highly efficient solid oxide fuel cell (SOFC) that operates at temperatures of approximately 600 to 750°C, with the fuel cell mode set to SOFC mode and the electrolysis mode set to SOEC mode.

[0013] As shown in Figure 2, the stack 1 is composed of multiple cells 11 sandwiched between interconnectors 12, and each cell 11 has a structure in which an electrolyte is sandwiched between a fuel electrode and an air electrode. In fuel cell mode, hydrogen is supplied to the fuel electrode (anode) and oxygen to the oxygen electrode (cathode), causing the hydrogen to dissociate into hydrogen ions and electrons at the fuel electrode, and generating electricity as the electrons move to the oxygen electrode. The oxygen ions generated at the oxygen electrode pass through the electrolyte and combine with the hydrogen ions that have lost electrons on the anode side, that is, oxidizing the hydrogen to form water. The oxygen electrode may also be an air electrode to which oxygen-containing air is supplied. Since this electrochemical reaction is an exothermic reaction, the stack 1 is heated to the operating temperature and absorbs the reaction heat, resulting in the upper part of the stack 1 being hot and the lower part being cold. Therefore, as shown in Figure 1, in fuel cell mode, the gas transfer controller 7 introduces gas 22 containing relatively low-temperature hydrogen from the hydrogen supply unit 2 into the stack 1 and transfers it from the high-temperature side to the low-temperature side of the stack 1 using the control valve 71. Similarly, in fuel cell mode, the gas transfer controller 7 introduces gas 32 containing relatively low-temperature oxygen from the oxygen supply unit 3 into the stack 1 and transfers it from the high-temperature side to the low-temperature side of the stack 1 using the control valve 71. Here, the operation controller 10 of the control device 100 is responsible for determining whether to use fuel cell mode or electrolysis mode, switching operating modes, issuing gas supply commands, and control. The gas transfer controller 7 and temperature controller 8 receive commands from the operation controller 10 or perform control based on measured values ​​acquired by themselves. Here, the temperature of the hydrogen-containing gas 22 is, for example, about 500°C, and the temperature of the oxygen-containing gas 32 is, for example, about 500°C.

[0014] The hydrogen-containing gas 22 is obtained by separating hydrogen produced by the electrolysis of water in the electrolysis mode from unreacted water vapor by the gas separation unit 4. For example, the hydrogen gas separated by the gas separation unit 4 is introduced into the hydrogen supply unit 2, where its temperature, quantity, etc., are controlled. The hydrogen-containing gas 22 only needs to contain hydrogen that will cause the fuel cell reaction, and may be high-purity hydrogen gas introduced into the hydrogen supply unit 2 from a hydrogen cylinder, or hydrogen gas extracted from a stable gas such as methane using a reformer, etc., and introduced into the hydrogen supply unit 2. These may be mixed, or trace amounts of other gases may be present. Similarly, the oxygen-containing gas 32 is obtained by introducing air containing oxygen, nitrogen, carbon dioxide, etc., into the oxygen supply unit 3. For example, air containing oxygen gas taken in from the outside is introduced into the oxygen supply unit 3, where its temperature, quantity, etc., are controlled. The oxygen-containing gas 32 only needs to contain oxygen that will cause the fuel cell reaction, and may be high-purity oxygen gas introduced into the oxygen supply unit 3 from an oxygen cylinder, or oxygen produced by the electrolysis of water in the electrolysis mode. These may also be mixed together, and may contain trace amounts of other gases.

[0015] The gas transfer controller 7 transfers the hydrogen-containing gas 22 and the oxygen-containing gas 32 from the high-temperature side to the low-temperature side of the stack 1. As a result, the relatively colder hydrogen-containing gas 22 and oxygen-containing gas 32 are transferred while absorbing heat within the stack 1, thereby reducing the temperature distribution within the stack 1.

[0016] In the electrolysis mode, water heated to approximately 700°C to 900°C is supplied as steam 52 to the stack 1, which is composed of multiple cells 11 sandwiched between interconnectors 12. Under the influence of the current applied to the cells 11, the steam 52 generates hydrogen gas and oxygen ions at the fuel electrode (cathode). The oxygen ions pass through the electrolyte and become oxygen gas at the oxygen electrode (anode), and hydrogen gas is recovered at the fuel electrode and oxygen gas at the oxygen electrode. This reaction depends on the operation of the current, voltage, etc., and can be either an exothermic or endothermic reaction. Since water electrolysis is an endothermic reaction, the cell temperature decreases as the current density increases, but if the current density is further increased, the heat generated by overvoltage such as ohmic loss exceeds the heat absorbed. Therefore, the gas transfer controller 7, according to the operation of the stack 1, subtracts the heat absorbed by water electrolysis from the heat generated by overvoltage and transfers the steam 52 from the low-temperature side to the high-temperature side of the stack 1 if it is endothermic, and transfers it from the high-temperature side to the low-temperature side if it is exothermic. For example, the gas transfer controller 7 grasps the operating state of the stack 1 using a temperature measuring unit, a voltage measuring unit, etc. (not shown), and transfers steam 52 from the low-temperature side to the high-temperature side of the stack 1, or from the high-temperature side to the low-temperature side, using control valves 71, 72, etc. For example, when supplying steam 52 to the lower part of the stack 1, the gas transfer controller 7 opens control valve 73 and closes control valve 74, and when supplying steam 52 to the upper part of the stack 1, it closes control valve 73 and opens control valve 74.

[0017] Furthermore, the temperature of the steam 52 being transferred may be changed by the operation of the stack 1. For example, if the reference temperature of the steam 52 is 800°C, the temperature of the steam 52 supplied from the steam supply unit 5 is set by the temperature setter 9 to 820°C, which is, for example, 20°C higher than the reference temperature, and then heated by the temperature controller 8 to produce a high-temperature gas at the set temperature. For example, if the lower part of the stack 1 is at a low temperature and the upper part is at a high temperature, the gas transfer controller 7 opens the control valve 73 of the piping supplying to the lower part and closes the control valve 74 of the piping supplying to the upper part, thereby transferring the high-temperature steam 52 from the lower part to the upper part of the stack 1. If the water electrolysis reaction is an exothermic reaction, for example, if the reference temperature is 800°C, the temperature of the steam 52 supplied from the steam supply unit 5 is set by the temperature setter 9 to 780°C, which is, 20°C lower than the reference temperature, and then heated or cooled by the temperature controller 8 to produce a low-temperature gas at the set temperature. Then, the gas transfer controller 7 closes the control valve 73 of the piping supplying to the lower part and opens the control valve 74 of the piping supplying to the upper part, thereby transferring steam 52 at a temperature lower than the reference temperature from the upper part to the lower part of the stack 1.

[0018] The reaction condition in which endothermic and exothermic reactions are balanced is called the thermal neutral point, and theoretically, this is said to be true when the single-cell voltage is 1.3V. Therefore, the operating voltage of cell 11 is obtained, and if the operating voltage is below the thermal neutral voltage, control is performed to transfer steam 52 at a temperature above the reference temperature (high temperature) from the low temperature side to the high temperature side of stack 1, and if the operating voltage exceeds the thermal neutral voltage, control is performed to transfer steam 52 at a temperature below the reference temperature (low temperature) from the high temperature side to the low temperature side. Alternatively, if the operating voltage is below the thermal neutral voltage, the set temperature can be set to a predetermined temperature higher than the reference temperature, and the high-temperature steam 52 heated to the set temperature by the temperature controller 8 can be transferred from the low temperature side, for example, the bottom, to the high temperature side, for example, the top, of stack 1. Alternatively, if the operating voltage exceeds the thermal neutral voltage, the set temperature can be set to a predetermined temperature lower than the reference temperature, and the steam 52 heated or cooled to the set temperature by the temperature controller 8 can be transferred from the high temperature side, for example, the top, to the low temperature side, for example, the bottom, of stack 1. In this way, by increasing the temperature inside stack 1 in the case of an endothermic reaction and decreasing the temperature inside stack 1 in the case of an exothermic reaction, the temperature distribution inside stack 1 can be reduced.

[0019] It is preferable to set the temperature according to the exothermic and endothermic reactions, but it is also possible to reduce the temperature distribution within stack 1 by performing only one of these reactions. Although a reference temperature of 800°C, a predetermined temperature of 20°C, and a thermal neutral voltage of 1.3V are given as examples, the values ​​used in controlling the cell operation are not limited to these, and the reference temperature can be set to around 750 to 850°C, and the predetermined temperature to around 10 to 50°C. Furthermore, the predetermined temperatures for setting temperatures higher and lower than the reference temperature may differ between endothermic and exothermic reactions. The operating voltage may also differ depending on whether it is below the thermal neutral voltage or above the thermal neutral voltage.

[0020] Furthermore, while an example of using water vapor 52 to equalize the temperature distribution within the stack 1 has been described, the water electrolysis reaction can be stabilized not only by using water vapor 52, but also by introducing oxygen electrode gas 62 such as oxygen or air from the oxygen electrode gas supply unit 6 and transferring it to the oxygen electrode of the stack 1. The oxygen electrode gas 62 may be introduced from an oxygen cylinder, or air may be introduced.

[0021] An example of the gas transfer direction within the stack 1 by the gas transfer controller 7 will be described. In the schematic configuration diagram of the stack 1 shown in Figure 2, multiple manifolds 121, 122, 123, and 124 for passing gas are provided at the ends of the interconnector 12. For example, a manifold 121 provided at the end of one side of the interconnector 12 transfers gas to the upper or lower interconnector 12. A groove, for example, is provided on the surface of the interconnector 12 that is in contact with the cell 11, forming a gas transfer path. In fuel cell mode, hydrogen-containing gas 22 is transferred to the fuel electrode side of the cell 11, and oxygen-containing gas 32 is transferred to the oxygen electrode side of the cell 11. In electrolysis mode, water vapor 52, which is a gas containing water, is transferred to the fuel electrode side of the cell 11, and oxygen electrode gas 62, which contains oxygen, is transferred to the oxygen electrode side of the cell 11.

[0022] Figure 3 is an explanatory diagram showing an example of the direction of gas transfer within the stack 1 in fuel cell mode. In fuel cell mode, the gas transfer controller 7 controls the transfer of hydrogen-containing gas 22 from the top to the bottom of the stack 1 through a manifold 121 (not shown in Figure 3) provided at the end of the interconnector 12, and also transfers it between the fuel electrode of each stacked cell 11 and the interconnector 12. It also controls the transfer of oxygen-containing gas 32 from the top to the bottom of the stack through a manifold 121 provided at the end of the interconnector 12, and transfers it between the oxygen electrode of each stacked cell 11 and the interconnector 12 in the same direction as the hydrogen-containing gas 22. Furthermore, the hydrogen-containing gas 22 transferred between the fuel electrode and the interconnector 12, and the oxygen-containing gas 32 transferred between the oxygen electrode and the interconnector 12, are transferred to the bottom through a manifold 122 (not shown in Figure 3) provided at the opposite end of the interconnector 12, which has a manifold 121 at its end. In this way, by transferring hydrogen-containing gas 22 and oxygen-containing gas 32 from the high-temperature upper part to the low-temperature lower part, the low-temperature gas gradually becomes hotter, and the temperature within the stack 1 can be made uniform. By using the reaction gas used for power generation, the fuel cell system 1000 can have a simple device configuration.

[0023] Figure 4 is an explanatory diagram showing another example of the direction of gas transfer within the stack 1 in fuel cell mode. In fuel cell mode, the gas transfer controller 7 controls the transfer of hydrogen-containing gas 22 from the top to the bottom of the stack 1 through a manifold 121 (not shown in Figure 4) provided at the end of the interconnector 12, and also transfers it between the fuel electrodes of each stacked cell 11 and the interconnector, and transfers oxygen-containing gas 32 from the top to the bottom of the stack 1 through a manifold 123 (not shown in Figure 4) provided at the end of the interconnector 12, and also transfers it between the oxygen electrodes of each stacked cell 11 and the interconnector 12, crossing with the hydrogen-containing gas 22. The oxygen-containing gas 32 is transferred from the top to the bottom of the stack 1 through a manifold 123 provided on the side of the interconnector 12 adjacent to the side of the interconnector 12 through which the hydrogen-containing gas 22 passes. Furthermore, the hydrogen-containing gas 22 transported between the fuel electrode and the interconnector 12 is transported to the lower part through a manifold 122 (not shown in Figure 4) located at the opposite end of the interconnector 12, which has a manifold 121 at one end. The oxygen-containing gas 32 transported between the oxygen electrode and the interconnector 12 is transported to the lower part through a manifold 124 (not shown in Figure 4) located at the opposite end of the interconnector 12, which has a manifold 123 at one end. In this way, the hydrogen-containing gas 22 and the oxygen-containing gas 32 are transported from the high-temperature upper part to the low-temperature lower part, causing the low-temperature gas to gradually become high-temperature, thus equalizing the temperature within the stack 1. By using reaction gases used for power generation, the fuel cell system 1000 can have a simple device configuration.

[0024] Figure 5 is an explanatory diagram showing another example of the direction of gas transfer within the stack 1 in fuel cell mode. In fuel cell mode, the gas transfer controller 7 controls the transfer of hydrogen-containing gas 22 from the top to the bottom of the stack 1 through a manifold 121 (not shown in Figure 5) provided at the end of the interconnector 12, and also transfers it between the fuel electrodes of each stacked cell 11 and the interconnector 12. It also controls the transfer of oxygen-containing gas 32 from the top to the bottom of the stack 1 through a manifold 122 provided at the end of the interconnector 12, and transfers it between the oxygen electrodes of each stacked cell 11 and the interconnector 12 in the opposite direction to that of the hydrogen-containing gas 22. Furthermore, the hydrogen-containing gas 22 transported between the fuel electrode and the interconnector 12 is transported to the lower part through a manifold 122 (not shown in Figure 5) located at the opposite end of the interconnector 12, which has a manifold 121 at one end. The oxygen-containing gas 32 transported between the oxygen electrode and the interconnector 12 is transported to the lower part through a manifold 121 located at the opposite end of the interconnector 12, which has a manifold 122 at one end. In this way, the hydrogen-containing gas 22 and the oxygen-containing gas 32 are transported from the high-temperature upper part to the low-temperature lower part, causing the low-temperature gas to gradually become high-temperature, thus equalizing the temperature within the stack 1. By using reaction gases used for power generation, the device configuration of the fuel cell system 1000 can be simplified. The transport of the hydrogen-containing gas 22 and the oxygen-containing gas 32 in opposite directions allows for a uniform temperature within the same cell.

[0025] Figure 6 is an explanatory diagram showing an example of the direction of gas transfer within the stack 1 in electrolysis mode. In electrolysis mode, when the operating voltage is below the thermal neutral voltage, it shows an example of transferring water vapor 52 and oxygen electrode gas 62 at temperatures above the reference temperature from the low-temperature side to the high-temperature side of the stack. In electrolysis mode, when the operating voltage is below the thermal neutral voltage, the gas transfer controller 7 transfers water vapor 52 at temperatures below the reference temperature from the bottom to the top of the stack 1 through a manifold 122 (not shown in Figure 6) provided at the end of the interconnector 12, and also transfers it between the fuel electrode of each stacked cell 11 and the interconnector 12. It also transfers oxygen electrode gas 62 at temperatures above the reference temperature from the bottom to the top of the stack 1 through a manifold 122 provided at the end of the interconnector 12, and transfers it between the oxygen electrode of each stacked cell 11 and the interconnector 12 in the same direction as the water vapor 52. Furthermore, the steam 52 transported between the fuel electrode and the interconnector 12, and the oxygen electrode gas 62 transported between the oxygen electrode and the interconnector 12, are transported to the upper part through a manifold 121 (not shown in Figure 6) located at the opposite end of the interconnector 12, which has a manifold 122 at one end. In this way, by transporting the steam 52 and oxygen electrode gas 62 from the lower part (lower temperature side) to the upper part (higher temperature side), the high-temperature gas gradually cools down, making it possible to equalize the temperature within the stack 1. By using a water electrolysis reaction gas, the fuel cell system 1000 can have a simple device configuration.

[0026] Figure 7 is an explanatory diagram showing another example of the direction of gas transfer within the stack 1 in electrolysis mode. It shows an example in which, when the operating voltage exceeds the thermal neutral voltage in electrolysis mode, water vapor 52 and oxygen electrode gas 62 at temperatures below the reference temperature are transferred from the high-temperature side to the low-temperature side of the stack 1. In electrolysis mode, the gas transfer controller 7 controls the transfer of water vapor 52 at temperatures below the reference temperature from the top to the bottom of the stack 1 through a manifold 121 (not shown in Figure 7) provided at the end of the interconnector 12, and also transfers it between the fuel electrode of each stacked cell 11 and the interconnector 12. It also controls the transfer of oxygen electrode gas 62 at a temperature lower than the reference temperature from the top to the bottom of the stack 1 through a manifold 121 provided at the end of the interconnector 12, and transfers it between the oxygen electrode of each stacked cell 11 and the interconnector 12 in the same direction as the water vapor 52. Furthermore, the steam 52 transported between the fuel electrode and the interconnector 12, and the oxygen electrode gas 62 transported between the oxygen electrode and the interconnector 12, are transported to the lower part through a manifold 122 (not shown in Figure 7) located at the opposite end of the interconnector 12, which has a manifold 121 at one end. In this way, by transporting the steam 52 and oxygen electrode gas 62 at temperatures below the reference temperature from the high-temperature upper part to the low-temperature lower part, the low-temperature gas gradually becomes high-temperature, and the temperature within the stack 1 can be made uniform. By using water electrolysis reaction gas, the fuel cell system 1000 can have a simple device configuration.

[0027] Figure 8 is an explanatory diagram showing another example of the gas transfer direction within the stack 1 in electrolysis mode. It shows an example in which, in electrolysis mode, when the operating voltage is below the thermal neutral voltage, water vapor 52 and oxygen electrode gas 62 at temperatures above the reference temperature are transferred from the low-temperature side to the high-temperature side of the stack 1. In electrolysis mode, the gas transfer controller 7 controls the transfer of water vapor 52 at temperatures above the reference temperature from the bottom to the top of the stack 1 through a manifold 122 (not shown in Figure 8) provided at the end of the interconnector 12, and also transfers it between the fuel electrodes of each stacked cell 11 and the interconnector 12. It also controls the transfer of oxygen electrode gas 62 at temperatures above the reference temperature from the bottom to the top of the stack 1 through a manifold 124 (not shown in Figure 8) provided at the end of the interconnector 12, and transfers it between the oxygen electrodes of each stacked cell 11 and the interconnector 12, crossing with the water vapor 52. The oxygen electrode gas 62 is transferred from the bottom to the top of the stack 1 through a manifold 124 located on the side of the interconnector 12 adjacent to the side of the interconnector 12 through which the steam 52 passes. Furthermore, the steam 52 transferred between the fuel electrode and the interconnector 12 is transferred to the top through a manifold 121 (not shown in Figure 8) located on the opposite end of the interconnector 12, which has a manifold 122 at one end. The oxygen electrode gas 62 transferred between the oxygen electrode and the interconnector 12 is transferred to the top through a manifold 123 (not shown in Figure 8) located on the opposite end of the interconnector 12, which has a manifold 124 at one end. In this way, by transferring steam 52 and oxygen electrode gas 62 at temperatures above the reference temperature from the lower side (lower temperature) to the upper side (higher temperature), the high-temperature gas gradually cools down, making it possible to equalize the temperature within the stack 1. By using a water electrolysis reaction gas, the device configuration of the fuel cell system 1000 can be made simple.

[0028] Figure 9 is an explanatory diagram showing another example of the direction of gas transfer within the stack 1 in electrolysis mode. It shows an example in which, in electrolysis mode, when the operating voltage is below the thermal neutral voltage, water vapor 52 and oxygen electrode gas 62 at temperatures above the reference temperature are transferred from the low-temperature side to the high-temperature side of the stack 1. In electrolysis mode, the gas transfer controller 7 controls the transfer of water vapor 52 at temperatures above the reference temperature from the bottom to the top of the stack 1 through a manifold 122 (not shown in Figure 9) provided at the end of the interconnector 12, and also transfers it between the fuel electrode of each stacked cell 11 and the interconnector 12. It also controls the transfer of oxygen electrode gas 62 at temperatures above the reference temperature from the bottom to the top of the stack 1 through a manifold 121 provided at the end of the interconnector 12, and transfers it between the oxygen electrode of each stacked cell 11 and the interconnector 12 in the opposite direction to that of the water vapor 52. Furthermore, the steam 52 transported between the fuel electrode and the interconnector 12 is transported to the upper part through a manifold 121 (not shown in Figure 9) located at the opposite end of the interconnector 12, which has a manifold 122 at one end. The oxygen electrode gas 62 transported between the oxygen electrode and the interconnector 12 is transported to the upper part through a manifold 122 located at the opposite end of the interconnector 12, which has a manifold 121 at one end. In this way, by transporting steam 52 and oxygen electrode gas 62 at temperatures above the reference temperature from the lower part (lower temperature side) to the upper part (higher temperature side), the high-temperature gases gradually cool down, making it possible to equalize the temperature within the stack 1. By using water electrolysis reaction gas, the fuel cell system 1000 can have a simple device configuration. By transporting the steam 52 and oxygen electrode gas 62 in opposite directions, it is possible to equalize the temperature within the same cell.

[0029] In addition, in the electrolysis mode, when the operating voltage exceeds the thermal neutral voltage, the water vapor 52 and oxygen electrode gas 62 at a temperature lower than the reference temperature may be transferred in a cross direction, or the water vapor 52 and oxygen electrode gas 62 may be transferred in opposite directions. The gas transfer method by the gas transfer controller 7 is not limited to these. In the electrolysis mode, when the operating voltage is below the thermal neutral voltage, the gas transfer controller 7 transfers water vapor and oxygen electrode gas at a temperature above the reference temperature from the low temperature side to the high temperature side of the stack 1, and when the operating voltage exceeds the thermal neutral voltage, it transfers water vapor and oxygen electrode gas at a temperature below the reference temperature from the high temperature side to the low temperature side. The gas transfer controller 7 controls the gas temperature and switches the direction of gas transfer to the opposite direction depending on whether the operating voltage is below the thermal neutral voltage or above the thermal neutral voltage.

[0030] Here, an example is shown in which multiple manifolds 121 to 124 are provided at the ends of the interconnector 12, but some of these may be located in the center. The manifolds 121 to 124 may each or some of them have different shapes. Different numbers and shapes of manifolds 121 to 124 may be provided for each interconnector 12.

[0031] Thus, by configuring the fuel cell system 1000 to include a stack 1 in which a plurality of cells 11 having fuel electrodes and oxygen electrodes that generate hydrogen and electricity, and which switch between an electrolysis mode that generates hydrogen and oxygen by electrolysis of water and a fuel cell mode that generates electricity by oxidizing hydrogen, and a gas transfer controller 7 that performs at least one of the following gas transfer controls: fuel cell mode gas transfer control that transfers hydrogen-containing gas 22 and oxygen-containing gas 32 from the high-temperature side to the low-temperature side of the stack 1 in fuel cell mode, and electrolysis mode gas transfer control that transfers water vapor 52 and oxygen electrode gas 62 from the low-temperature side to the high-temperature side of the stack 1, or from the high-temperature side to the low-temperature side, according to the operation of the stack in electrolysis mode, the temperature inside the stack 1 can be made uniform with a simple device configuration.The low-temperature hydrogen-containing gas 22 and oxygen-containing gas 32 can be transferred from the high-temperature side to the low-temperature side of the stack 1, which becomes hot due to the exothermic reaction in fuel cell mode, and the temperature inside the stack 1 can be made uniform.By using reaction gases used for power generation or water electrolysis, the fuel cell system 1000 can have a simple device configuration.

[0032] Furthermore, in electrolysis mode, when the operating voltage is below the thermal neutral voltage, the control is performed to transfer water vapor 52 and oxygen electrode gas 62 at a temperature above the reference temperature from the low-temperature side to the high-temperature side of the stack 1, and when the operating voltage exceeds the thermal neutral voltage, the control is performed to transfer water vapor 52 and oxygen electrode gas 62 at a temperature below the reference temperature from the high-temperature side to the low-temperature side, thereby enabling heat exchange in accordance with the operation of the stack 1, which involves endothermic and exothermic reactions.

[0033] Embodiment 2. Embodiment 2 describes the control of gas transfer when switching from fuel cell mode to electrolysis mode, or from electrolysis mode to fuel cell mode, using the gas transfer controller 7. Figure 10 is an explanatory diagram showing an example of the direction of gas transfer within the stack 1 during the switching operation from fuel cell mode to electrolysis mode. In electrolysis mode, the entire stack 1 needs to be heated to a higher temperature than in fuel cell mode due to the endothermic reaction. Even after switching modes and heating, the temperature within the stack 1 does not immediately become constant, and a temperature distribution occurs, so the gas transfer controller 7 is used to control the direction of gas transfer. That is, when switching from fuel cell mode to electrolysis mode, the gas transfer controller 7 transfers steam 52, which has been adjusted to a set temperature predetermined to be higher than the electrolysis mode operating temperature, from the low-temperature side to the high-temperature side of the stack 1 until the temperature of the stack 1 reaches the preset electrolysis mode operating temperature. For example, if the temperature required to operate in electrolysis mode is 800°C, the electrolysis mode operating temperature is set to 800°C. Then, the temperature of the steam supplied from the steam supply unit 5 is set by the temperature setter 9 to a predetermined temperature, for example, 820°C, which is 20°C higher than the electrolysis mode operating temperature, and is heated by the temperature controller 8 to become a high-temperature gas adjusted to the set temperature of 820°C. Then, the control valve 73 is opened and the control valve 74 is closed, and the high-temperature steam 52 is transferred to the stack 1 from the bottom to the top via the control valve 72. For example, as shown in Figure 10, the high-temperature steam 52 is transferred from the bottom to the top of the stack 1 through a manifold 122 (not shown in Figure 10) provided at the end of the interconnector 12, and is also transferred between the fuel electrodes of each stacked cell 11 and the interconnector 12. Furthermore, the steam 52 that has been transferred between the fuel electrodes and the interconnector 12 is transferred to the top through a manifold 121 (not shown in Figure 10) provided at the opposite end of the interconnector 12, which has a manifold 122 at its end. In this way, when switching from fuel cell mode to electrolysis mode, steam 52 adjusted to a predetermined temperature higher than the electrolysis mode operating temperature is transferred from the low-temperature side to the high-temperature side of the stack 1 until the temperature of the stack 1 reaches the preset electrolysis mode operating temperature. This allows the temperature inside the stack 1 to be raised to the electrolysis mode operating temperature while maintaining a uniform temperature.

[0034] Figure 11 is an explanatory diagram showing an example of the direction of gas transfer within stack 1 during the switching operation from electrolysis mode to fuel cell mode. Conversely to the above, the operating temperature in fuel cell mode is lower than that in electrolysis mode, so it is necessary to return the entire stack 1 to a lower temperature. Even after switching modes, the temperature within stack 1 does not immediately become constant and a temperature distribution occurs, so the gas transfer controller 7 is used to control the direction of gas transfer in the same way. That is, when switching from electrolysis mode to fuel cell mode, the gas transfer controller 7 transfers at least one of the water vapor 52, oxygen electrode gas 62, hydrogen-containing gas 22, and oxygen-containing gas 32 at a temperature lower than the fuel cell mode operating temperature from the high-temperature side to the low-temperature side of stack 1 until the temperature of stack 1 reaches a preset fuel cell mode operating temperature. For example, as shown in Figure 11, hydrogen-containing gas 22 is transported from the top to the bottom of the stack 1 through a manifold 122 (not shown in Figure 11) provided at the end of the interconnector 12, and also transported between the fuel electrodes of each stacked cell 11 and the interconnector 12. Oxygen-containing gas 32 is transported from the top to the bottom of the stack 1 through a manifold 122 provided at the end of the interconnector 12, and also transported between the oxygen electrodes of each stacked cell 11 and the interconnector 12 in the same direction as the hydrogen-containing gas 22. Furthermore, the hydrogen-containing gas 22 transported between the fuel electrodes and the interconnector 12, and the oxygen-containing gas 32 transported between the oxygen electrodes and the interconnector 12, are transported to the bottom through a manifold 121 (not shown in Figure 11) provided at the opposite end of the interconnector 12, which has a manifold 122 at its end. Figure 11 illustrates an example using hydrogen-containing gas 22 and oxygen-containing gas 32 as reaction gases used in fuel cell mode. However, steam 52 and oxygen electrode gas 62 adjusted to a temperature lower than the fuel cell mode operating temperature may also be used.In this way, when switching from electrolysis mode to fuel cell mode, the temperature inside stack 1 can be uniformly reduced to the fuel cell mode operating temperature by transferring at least one of the following from the high-temperature side to the low-temperature side of stack 1: water vapor 52, oxygen electrode gas 62, hydrogen-containing gas 22, and oxygen-containing gas 32, which are lower than the fuel cell mode operating temperature, until the temperature inside stack 1 reaches a preset fuel cell mode operating temperature.

[0035] Embodiment 3. Embodiment 3 describes a modified version of the fuel cell system 1000 shown in Embodiments 1 and 2. The fuel cell system 1000 according to Embodiment 3 includes a plurality of temperature detection units 13 distributed from the top to the bottom in the stack 1, as shown in Figure 12. The gas transfer controller 7 performs at least one of the following gas transfer controls: in fuel cell mode, it transfers hydrogen-containing gas 22 and oxygen-containing gas 32 from the high-temperature side to the low-temperature side detected by the temperature detection units 13; and in electrolysis mode, it transfers water vapor 52 from the low-temperature side to the high-temperature side, or from the high-temperature side to the low-temperature side, according to the operation of the stack 1. In this way, by transferring gas and performing heat exchange based on the measured temperature, the temperature inside the stack 1 can be further made uniform.

[0036] As shown in Figure 13, flow regulators 21 and 31 are provided to adjust the flow rates of hydrogen-containing gas 22 and oxygen-containing gas 32, respectively, and the flow rates of hydrogen-containing gas 22 and oxygen-containing gas 32 supplied to the stack 1 can be adjusted according to the detected temperature. If the flow rates of these gases are increased and transported in the high-temperature section, the temperature in the high-temperature section can be lowered and the temperature distribution within the stack 1 can be reduced. Similarly, a flow regulator 51 is provided to adjust the flow rate of water vapor 52, and the flow rate of water vapor 52 supplied to the stack 1 can be adjusted according to the detected temperature. The flow rate of oxygen electrode gas 62 can also be adjusted. For example, when supplying these high-temperature gases, if the flow rate of the gas supplied to the low-temperature section is increased and transported, the temperature in the low-temperature section can be raised and the temperature distribution within the stack 1 can be reduced. When supplying these low-temperature gases, if the flow rate of the gas supplied to the high-temperature section is increased and transported, the temperature in the high-temperature section can be lowered and the temperature distribution within the stack 1 can be reduced. Thus, the stack 1 has a plurality of temperature detection units 13 distributed from top to bottom, and the gas transfer controller 7 can further equalize the temperature inside the stack 1 by adjusting the flow rate of at least one of the hydrogen-containing gas 22, oxygen-containing gas 32, water vapor 52, and oxygen electrode gas 62 supplied to the stack 1 according to the temperature detected by the temperature detection units 13.

[0037] As shown in Figure 14, for example, a stack heater 14 may be provided at the bottom of the stack 1 where the temperature is low. The stack heater 14 may be controlled in conjunction with the gas transfer control of Embodiments 1 and 2 by the gas transfer controller 7 or the gas transfer control described above, or the stack heater 14 may be controlled according to the temperature detected by the temperature detection unit 13 provided in the stack 1. By providing a stack heater 14 on the low-temperature side of the stack 1 and heating the low-temperature side of the stack 1 with the stack heater 14, the temperature inside the stack 1 can be made even more uniform.

[0038] As shown in Figure 15, a heat storage unit 15 may be provided to store at least one of the heat from the exhaust gas in the electrolysis mode and the excess heat in either the fuel cell mode or the electrolysis mode. In the electrolysis mode, the heat stored in the heat storage unit 15 may be used to heat at least one of the steam 52 and the oxygen electrode gas 62. By utilizing the heat stored in the heat storage unit 15, energy-saving operation becomes possible. In particular, it is highly effective to store heat in the heat storage unit 15 in the fuel cell mode and then use the stored heat to heat the steam 52 in the electrolysis mode. Materials such as NaCl (sodium chloride), KCl (potassium chloride), and CaCl2 (calcium chloride) can be used to constitute the heat storage unit 15.

[0039] Furthermore, if at least one of the oxygen-containing gas 32 and oxygen electrode gas 62 used in embodiments 1 to 4 is replaced with air, the gas can be used flexibly and inexpensively.

[0040] Embodiment 4. Embodiment 4 describes an operating method for running the fuel cell system 1000 using the fuel cell system 1000 shown in Embodiments 1 to 3. Figure 16 is a flowchart showing the operation of the fuel cell system 1000 according to Embodiment 4. In a stack 1 in which a plurality of cells 11 having fuel electrodes and oxygen electrodes are stacked, the fuel cell system 1000 performs hydrogen generation and power generation by switching between an electrolysis mode, which generates hydrogen and oxygen by electrolysis of water, and a fuel cell mode, which generates power by oxidizing hydrogen. First, it is determined whether to set the operating mode to fuel cell mode or electrolysis mode (step S101). Then, it is switched to the determined fuel cell mode or electrolysis mode (steps S102, S103). When the fuel cell mode is selected (YES in step S101, step S102), the hydrogen-containing gas 22 and the oxygen-containing gas 32 are transferred from the high-temperature side to the low-temperature side of the stack 1 (step S104). When electrolysis mode is activated (step S101 NO, step S103), water vapor 52 and oxygen electrode gas 62 are transferred from the low-temperature side to the high-temperature side of the stack 1, or from the high-temperature side to the low-temperature side, according to the operation of the stack 1 (step S105).

[0041] Further details of the operation of the fuel cell system 1000 will be explained using Figures 17 to 19. For example, as shown in Figure 17, if there is a shortage of grid power (YES in step S201), the system switches to fuel cell mode and generates electricity using the fuel cell system 1000 (step S202). If there is no shortage of grid power (NO in step S201), the system switches to electrolysis mode and generates hydrogen and oxygen using the fuel cell system 1000 (step S203). Here, a shortage of grid power is defined from the perspective of balancing power supply and demand, for example, if there is a shortage of grid power during the daytime when demand is high, and there is no shortage of grid power during the nighttime when demand is low. Whether or not there is a shortage of grid power may also be defined based on the shutdown of power plants, weather conditions, etc. Furthermore, if solar energy, stored energy, etc. are used in addition to grid power, the determination may also be made based on the surplus or shortage of these.

[0042] An example of operation when switching to fuel cell mode in step S202 will be explained using Figure 18. First, it is checked whether or not the system is currently in fuel cell mode (step S301). If it is in fuel cell mode (YES in step S301), operation continues as is (step S309). If it is not in fuel cell mode (NO in step S301), power supply is temporarily stopped (step S302). Then, it is checked whether the gas transfer direction in the gas transfer controller 7 is from the low temperature side to the high temperature side (step S303). If the gas transfer direction is not from the high temperature side to the low temperature side (NO in step S303), the gas transfer direction is switched from the high temperature side to the low temperature side (step S304). The supplied gas is then switched to the fuel cell mode gas, i.e., gas 22 containing hydrogen and gas 32 containing oxygen (step S305). The temperature of stack 1 is then measured (step S306). If the temperature of stack 1 is not the operating temperature for fuel cell mode (NO in step S307), it is cooled or heated until it reaches the operating temperature. As a result of this process, the fuel cell system 1000 becomes capable of generating power and supplies power to the power converter and the like (step S308).

[0043] An example of operation when switching to electrolysis mode in step S203 will be explained using Figure 19. First, it is checked whether or not the system is currently in electrolysis mode (step S401). If it is in electrolysis mode (YES in step S401), operation continues as is (step S411). If it is not in electrolysis mode (NO in step S401), it stops if power is being supplied to the power converter, etc. (step S402). Then, the gas transfer direction in the gas transfer controller 7 is switched from the low temperature side to the high temperature side (step S403). The supplied gas is then switched to the gas for electrolysis mode, i.e., water vapor 52 and oxygen electrode gas 62 (step S404). The temperature of stack 1 is then measured (step S405), and if the temperature of stack 1 is not the operating temperature for electrolysis mode (NO in step S406), it waits until the temperature drops or rises. If the temperature of stack 1 is the operating temperature for electrolysis mode (YES in step S406), it is checked whether or not the operating voltage is below the thermal neutral voltage (step S407). If the operating voltage is not below the thermal neutral voltage, i.e., exceeds the thermal neutral voltage (NO in step S407), the gas transfer direction is switched from the high-temperature side to the low-temperature side (step S408), and low-temperature gas, i.e., water vapor 52 and oxygen electrode gas 62, is supplied to stack 1 and a voltage is applied (step S409). If the operating voltage is below the thermal neutral voltage (YES in step S407), with the gas transfer direction from the low-temperature side to the high-temperature side, high-temperature gas, i.e., water vapor 52 and oxygen electrode gas 62, is supplied to stack 1 and a voltage is applied (step S410).

[0044] Thus, it is determined whether to set the operation mode to the fuel cell mode or the electrolysis mode, and the operation mode is switched to the fuel cell mode or the electrolysis mode based on the determined operation mode. When the operation mode is the fuel cell mode, the gas 22 containing hydrogen and the gas 32 containing oxygen are transferred from the high-temperature side to the low-temperature side of the stack 1. When the operation mode is the electrolysis mode, according to the operation of the stack 1, the water vapor 52 and the oxygen electrode gas 62 are transferred from the low-temperature side to the high-temperature side or from the high-temperature side to the low-temperature side of the stack 1, so that the temperature inside the stack can be made uniform. By transferring the low-temperature gas 22 containing hydrogen and the gas 32 containing oxygen from the high-temperature side to the low-temperature side of the stack 1, which becomes high temperature due to the heat generation reaction in the fuel cell mode, the temperature of the stack 1 can be made uniform. By using the reaction gas used for power generation or water electrolysis, the fuel cell system 1000 can have a simple device configuration.

[0045] Each control function of the fuel cell system 1000, such as the gas transfer controller 7, the temperature adjuster 8, and the operation controller 10, is realized by the processing circuit of the control device 100. FIG. 20 is a schematic configuration diagram showing an example of the processing circuit that realizes each control function of the fuel cell system 1000. The control device 100 includes a processor 90, a storage device 91, a communication I / F (interface) 92, and the like. For example, a CPU (Central Processing Unit) is used for the processor 90. The storage device 9 is configured to transmit and receive data to and from the processor 90 and store the data. Here, communication of commands, measurement data, etc. by the control device 100 is performed via a communication path. The communication path may be wireless or wired. The control device 100 may be inside the fuel cell system 1000 or may perform remote operation from a remote location. These communications are performed via the communication I / F 92. The calculation and determination in the control device 100 are executed by the processor 90. The acquired measurement data, arithmetic expressions, etc. are stored in the storage device 91.

[0046] The processor 90 and the storage device 91 may be shared and used singly or there may be a plurality of them. Also, the processor 90 may be provided with, for example, a logic circuit using an ASIC (Application Specific Integrated Circuit), IC (Integrated Circuit), DSP (Digital Signal Processor), FPGA (Field Programmable Gate Array), etc., and various signal processing circuits. As the processor 90, a plurality of the same type or different types may be provided so that each process may be executed in parallel by a plurality of arithmetic processing units.

[0047] As the plurality of storage devices 91, for example, a RAM (Random Access Memory) configured to be able to read and write data from the processor 90, a ROM (Read Only Memory) configured to be able to read data from the processor 90, a hard disk, etc. are provided.

[0048] Each control function of the fuel cell system 1000 is realized by the processor 90 executing software or a program stored in the storage device 91 and cooperating with hardware. The setting data set in the fuel cell system 1000 may be stored in the storage device 91 as part of software or a program, or may be input by the user. A non-temporary recording medium 911 in which the fuel cell system operation program 912 is recorded may be distributed and installed in the storage device 91.

[0049] Although various exemplary embodiments and examples are described in the present disclosure, the various features, aspects, and functions described in one or more of the embodiments are not limited to the application of a specific embodiment, but are applicable to the embodiments singly or in various combinations. Therefore, countless variations not illustrated are assumed to be within the scope of the technology disclosed in the present specification. For example, it includes the case where at least one component is modified, added, or omitted, and further, the case where at least one component is extracted and combined with the components of other embodiments.

[0050] 1. Stack, 2. Hydrogen supply unit, 3. Oxygen supply unit, 4. Gas separation unit, 5. Water vapor supply unit, 6. Oxygen electrode gas supply unit, 7. Gas transfer controller, 8. Temperature controller, 9. Temperature setter, 10. Operation controller, 11. Cell, 12. Interconnector, 13. Temperature detection unit, 14. Stack heater, 15. Heat storage unit, 21, 31, 51. Flow regulator, 22. Gas containing hydrogen, 32. Gas containing oxygen, 52. Water vapor, 62. Oxygen electrode gas, 71, 72, 73, 74. Control valve, 100. Control device, 121, 122, 123, 124. Manifold, 1000. Fuel cell system

Claims

1. A fuel cell system comprising: a stack of multiple cells having fuel electrodes and oxygen electrodes that switch between an electrolysis mode, which generates hydrogen and oxygen by electrolysis of water, and a fuel cell mode, which generates electricity by oxidizing hydrogen, to perform hydrogen generation and electricity generation; and a gas transfer controller that performs at least one of the following gas transfer controls: fuel cell mode gas transfer control, which transfers hydrogen-containing gas and oxygen-containing gas from the high-temperature side to the low-temperature side of the stack in the fuel cell mode, and electrolysis mode gas transfer control, which transfers water vapor and the gas of the oxygen electrode from the low-temperature side to the high-temperature side of the stack, or from the high-temperature side to the low-temperature side, in accordance with the operation of the stack in the electrolysis mode.

2. The fuel cell system according to claim 1, wherein the gas transfer controller controls the transfer of hydrogen-containing gas from the top to the bottom of the stack through a manifold provided in the interconnector, and transfers it between the fuel electrode of each stacked cell and the interconnector, and transfers oxygen-containing gas from the top to the bottom of the stack through a manifold provided in the interconnector, and transfers it between the oxygen electrode of each stacked cell and the interconnector in the same direction as the hydrogen-containing gas.

3. The fuel cell system according to claim 1, wherein the gas transfer controller controls the transfer of a hydrogen-containing gas from the top to the bottom of the stack through a manifold provided in the interconnector, and transfers it between the fuel electrode of each stacked cell and the interconnector, and transfers an oxygen-containing gas from the top to the bottom of the stack through a manifold provided in the interconnector, and transfers it between the oxygen electrode of each stacked cell and the interconnector, crossing it with the hydrogen-containing gas.

4. The fuel cell system according to claim 1, wherein the gas transfer controller controls the transfer of hydrogen-containing gas from the top to the bottom of the stack through a manifold provided in the interconnector, and between the fuel electrode of each stacked cell and the interconnector, and the transfer of oxygen-containing gas from the top to the bottom of the stack through a manifold provided in the interconnector, and between the oxygen electrode of each stacked cell and the interconnector in the opposite direction to the hydrogen-containing gas.

5. The fuel cell system according to any one of claims 1 to 4, wherein the gas transfer controller controls the transfer of water vapor at a temperature of reference temperature or higher from the low-temperature side to the high-temperature side of the stack when the operating voltage is less than or equal to the thermal neutral voltage, and when the operating voltage exceeds the thermal neutral voltage, the transfer of water vapor at a temperature of reference temperature or lower from the high-temperature side to the low-temperature side.

6. The fuel cell system according to any one of claims 1 to 5, wherein the gas transfer controller, in the electrolysis mode, when the operating voltage is below the thermal neutral voltage, transfers the water vapor at a temperature above the reference temperature from the bottom to the top of the stack through a manifold provided in the interconnector, and transfers it between the fuel electrode of each stacked cell and the interconnector, transfers the gas of the oxygen electrode at a temperature above the reference temperature from the bottom to the top of the stack through a manifold provided in the interconnector, and transfers it between the oxygen electrode of each stacked cell and the interconnector in the same direction as the water vapor, and when the operating voltage exceeds the thermal neutral voltage, controls the direction of the water vapor and the gas of the oxygen electrode at a temperature below the reference temperature to transfer them from top to bottom.

7. The fuel cell system according to any one of claims 1 to 5, wherein the gas transfer controller, in the electrolysis mode, when the operating voltage is below the thermal neutral voltage, transfers the water vapor at a temperature above the reference temperature from the bottom to the top of the stack through a manifold provided in the interconnector, and transfers it between the fuel electrode of each stacked cell and the interconnector; transfers the gas of the oxygen electrode at a temperature above the reference temperature from the bottom to the top of the stack through a manifold provided in the interconnector, and transfers it between the oxygen electrode of each stacked cell and the interconnector, crossing it with the water vapor; and when the operating voltage exceeds the thermal neutral voltage, controls the direction of the water vapor and the gas of the oxygen electrode at a temperature below the reference temperature to transfer them from top to bottom.

8. The fuel cell system according to any one of claims 1 to 5, wherein the gas transfer controller, in the electrolysis mode, when the operating voltage is below the thermal neutral voltage, transfers the water vapor at a temperature above the reference temperature from the bottom to the top of the stack through a manifold provided in the interconnector, and transfers it between the fuel electrode of each stacked cell and the interconnector, transfers the gas of the oxygen electrode at a temperature above the reference temperature from the bottom to the top of the stack through a manifold provided in the interconnector, and transfers it between the oxygen electrode of each stacked cell and the interconnector in the opposite direction to the water vapor, and when the operating voltage exceeds the thermal neutral voltage, controls the direction of the water vapor and the gas of the oxygen electrode at a temperature below the reference temperature to transfer from top to bottom.

9. The fuel cell system according to any one of claims 1 to 8, wherein when switching from the fuel cell mode to the electrolysis mode, the gas transfer controller transfers the steam, which has been adjusted to a set temperature higher than the electrolysis mode operating temperature, from the low-temperature side to the high-temperature side of the stack until the temperature of the stack reaches a preset electrolysis mode operating temperature.

10. The fuel cell system according to any one of claims 1 to 8, wherein when switching from the electrolysis mode to the fuel cell mode, the gas transfer controller transfers at least one of the following at a temperature lower than the fuel cell mode operating temperature: water vapor, the oxygen electrode gas, a gas containing hydrogen, and a gas containing oxygen, from the high-temperature side to the low-temperature side of the stack until the temperature of the stack reaches a preset fuel cell mode operating temperature.

11. The fuel cell system according to any one of claims 1 to 10, wherein the stack has a plurality of temperature detection units distributed from top to bottom, and the gas transfer controller performs at least one of the following gas transfer controls: fuel cell mode gas transfer control in the fuel cell mode, which transfers the hydrogen-containing gas and the oxygen-containing gas from the high-temperature side to the low-temperature side detected by the temperature detection units; and electrolysis mode gas transfer control in the electrolysis mode, which, in accordance with the operation of the stack, transfers water vapor from the low-temperature side to the high-temperature side, or from the high-temperature side to the low-temperature side detected by the temperature detection units.

12. The fuel cell system according to any one of claims 1 to 11, wherein the stack has a plurality of temperature detection units distributed from top to bottom, and the gas transfer controller adjusts the flow rate of at least one of the hydrogen-containing gas, oxygen-containing gas, water vapor, and oxygen electrode gas supplied to the stack according to the temperature detected by the temperature detection units.

13. A fuel cell system according to any one of claims 1 to 12, wherein a stack heating unit is provided on the low-temperature side of the stack to heat the stack, and the low-temperature side of the stack is heated by the stack heating unit.

14. A fuel cell system according to any one of claims 1 to 13, comprising a heat storage unit for storing at least the heat of the exhaust gas in the electrolysis mode and at least the excess heat in the fuel cell mode or the electrolysis mode, wherein in the electrolysis mode, the heat stored in the heat storage unit is used to heat at least one of the water vapor and the gas of the oxygen electrode.

15. The fuel cell system according to any one of claims 1 to 14, wherein at least one of the oxygen-containing gas and the gas of the oxygen electrode is air.

16. A method for operating a fuel cell system that generates hydrogen and generates electricity by switching between an electrolysis mode, in which hydrogen and oxygen are generated by electrolysis of water, and a fuel cell mode, in which hydrogen is generated by oxidizing hydrogen, in a stack in which a plurality of cells having fuel electrodes and oxygen electrodes are stacked, the method comprising: determining whether the operating mode should be the fuel cell mode or the electrolysis mode; switching to the fuel cell mode or the electrolysis mode based on the determined operating mode; if the operating mode is the fuel cell mode, transferring a gas containing hydrogen and a gas containing oxygen from the high-temperature side to the low-temperature side of the stack; and if the operating mode is the electrolysis mode, transferring water vapor and the gas of the oxygen electrode from the low-temperature side to the high-temperature side, or from the high-temperature side to the low-temperature side of the stack, according to the operation of the stack.

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