JP7360352B2 - SOFC/SOEC system - Google Patents

Description

The present invention relates to SOFC/SOEC systems.

Conventionally, SOFC/SOEC systems equipped with SOFC/SOEC cells have been known that have two operating modes: an SOEC mode in which hydrogen is generated by electrolyzing water, and an SOFC mode in which electricity is generated by an oxidation reaction of hydrogen. . For example, Patent Document 1 discloses a technology that uses two types of heat storage devices to recover the heat generated in the SOFC/SOEC cell during the SOFC mode, and supplies the recovered heat to the SOFC/SOEC cell during the SOEC mode. has been done.

Japanese Patent Application Publication No. 2018-116834

However, even with the above-mentioned prior art, there is still room for improvement in terms of technology for improving power generation efficiency. For example, in the technology described in Patent Document 1, in the air supplied to the SOFC/SOEC cell during the SOFC mode, as it flows along the flow path of the air electrode, oxygen is consumed and the oxygen concentration decreases, while the nitrogen concentration increases. Therefore, the concentration resistance determined by the gas concentration in the flow path of the air electrode increases, and the output voltage of the SOFC/SOEC cell decreases. Therefore, the power generation efficiency of the entire SOFC/SOEC system may decrease.

In addition, in the technology described in Patent Document 1, heat is exchanged between two types of heat storage devices and the SOFC/SOEC cell by either convection heat transfer or radiation heat transfer, so that the heat storage in the heat storage device is Density is relatively small. Therefore, in order to continuously perform electrolysis and power generation of water, it is necessary to separately input energy for heating in SOEC mode and cooling in SOFC mode. For this reason, there is a possibility that the power generation efficiency of the entire SOFC/SOEC system will decrease.

The present invention was made to solve the above-mentioned problems, and an object of the present invention is to provide a technology for improving power generation efficiency in a SOFC/SOEC system.

The present invention has been made to solve at least part of the above-mentioned problems, and can be realized as the following forms.

(1) According to one embodiment of the present invention, a SOFC/SOEC system is provided. This SOFC/SOEC system includes a cell that has two operating modes: an SOEC mode in which hydrogen and oxygen are generated by electrolysis of water, and an SOFC mode in which hydrogen is oxidized to generate electricity, and a voltage applied to the cell. an oxygen storage section in which oxygen generated in the cell in SOEC mode is stored; and a control section for switching an operation mode of the cell; The voltage regulator is controlled to apply a voltage higher than a preset voltage value as a value for thermoneutral state to the cell, and the hydrogen supplied to the cell is controlled to be applied to the cell in the SOFC mode. The oxygen storage section is controlled so as to supply a larger amount of oxygen than a preset amount to completely oxidize the oxygen.

According to this configuration, when the cell is operated in the SOFC mode, oxygen stored in the oxygen storage section is supplied as an oxidizing agent for hydrogen. In SOFC mode, the cell's output is the voltage that can be obtained as electrical energy converted from the chemical energy of the raw materials supplied to the cell, minus the voltage loss obtained by multiplying the cell's internal resistance by the current. voltage. The internal resistance of a cell includes a concentration resistance determined by the gas concentration supplied to the cell, and when air is used as the hydrogen oxidizer, the concentration resistance increases as the nitrogen concentration increases due to oxygen consumption. Become. On the other hand, when oxygen is used as an oxidizing agent for hydrogen, the oxygen concentration does not easily change even if oxygen is consumed for oxidizing hydrogen, and therefore it is possible to suppress an increase in concentration resistance. Thereby, the output voltage of the cell can be increased, so that a relatively high power generation voltage can be obtained. Therefore, power generation efficiency can be improved.

The control unit also controls the voltage adjustment unit to apply a voltage higher than a preset voltage value to the cell in the thermoneutral state to the cell in the SOEC mode. As a result, heat due to the internal resistance of the cell is generated in the cell, which in the thermoneutral state would lower its temperature due to the progression of the endothermic steam electrolysis reaction, making it impossible to stabilize the cell temperature. can. Further, the control unit controls the oxygen storage unit to supply oxygen in an amount larger than a preset amount to oxidize all of the hydrogen supplied to the cell in the SOFC mode. As a result, the heat of the cell is released from the cell, where the temperature rises due to the oxidation reaction of hydrogen, which is an exothermic reaction, to the outside as sensible heat of the oxygen discharged from the cell without reaction. Cell temperature can be stabilized. In this way, in the SOEC mode, heat is generated by the internal resistance of the cell, and in the SOFC mode, the heat of the cell is released to the outside of the cell by supplying a large amount of oxygen. Thereby, the heat balance of the cell can be established in each of the SOEC mode and the SOFC mode, so that temperature changes in the cell can be suppressed.

(2) The SOFC/SOEC system of the above embodiment further includes a storage amount detection section that detects the amount of oxygen stored in the oxygen storage section, and an air supply section that supplies air to the cell, The control unit uses the storage amount of oxygen detected by the storage amount detection unit to control the supply of oxygen stored in the oxygen storage unit to the cell and the air supply unit to supply air to the cell. You may switch between supply and . According to this configuration, the control unit uses the storage amount of oxygen detected by the storage amount detection unit to supply oxygen stored in the oxygen storage unit to the cells, and to supply air to the cells by the air supply unit. supply. Thereby, when the amount of stored oxygen is small, power generation in the cell can be continued by supplying air to the cell instead of oxygen.

(3) The SOFC/SOEC system of the above embodiment further includes a heat storage unit connected to the cell, storing heat generated in the cell in SOFC mode, and supplying heat to the cell in SOEC mode. , the control unit may switch between storage of heat generated in the cell and supply of heat to the cell by the heat storage unit. According to this configuration, the heat storage unit connected to the cell stores heat generated in the SOFC mode cell and supplies heat to the SOEC mode cell. As a result, in SOFC mode, the cell heat, whose temperature rises due to the exothermic hydrogen oxidation reaction, is transferred from the cell to the heat storage section and stored in the heat storage section, thereby increasing the cell temperature. can be stabilized. Furthermore, in the SOEC mode, the cell temperature can be stabilized by supplying heat to the cell whose temperature is lowered by the progression of the steam electrolysis reaction, which is an endothermic reaction. Thereby, the heat balance of the cell can be established in each of the SOEC mode and the SOFC mode, so that temperature changes in the cell can be suppressed.

(4) In the SOFC/SOEC system of the above form, the heat storage section stores heat generated in the cell or stores heat to the cell by being supplied with hydrogen or water supplied to the cell. supply, may be switched. According to this configuration, the heat storage section stores heat generated in the cell by being supplied with hydrogen used for power generation in the SOFC mode cell, and is supplied with water used for electrolysis by the SOEC mode cell. heat is supplied to the cell. In this way, the heat storage can switch between storing heat and supplying heat using the fluids handled by the SOFC/SOEC system. Thereby, the heat storage section can be driven without significantly changing the system, so that the heat balance of the cell can be established relatively easily.

(5) In the SOFC/SOEC system of the above embodiment, the heat storage section includes a heat flux measurement section that measures the heat flux between the heat storage section and the cell, and the control section controls the heat flux measurement section that measures the heat flux between the heat storage section and the cell. The rate of change in temperature of the cell is calculated using the heat flux calculated, and the amount of oxygen supplied to the cell by the oxygen storage unit and the amount of oxygen supplied to the cell by the voltage adjustment unit are calculated using the calculated rate of change in cell temperature. At least one of the voltage values applied to the cell may be adjusted. According to this configuration, the control unit calculates the rate of change in cell temperature using the heat flux between the heat storage unit and the cell measured by the heat flux measurement unit. As a result, the cell temperature can be predicted using the calculated rate of change in cell temperature, so the control unit uses the calculated rate of change in cell temperature to control the supply of oxygen from the oxygen storage unit to the cell. By adjusting at least one of the amount and the voltage value applied to the cell by the voltage adjustment section, the heat balance of the cell can be established. Therefore, temperature changes in the cell can be suppressed.

(6) In the SOFC/SOEC system of the above embodiment, the control unit controls the flow rate of oxygen supplied to the cell when the absolute value of the rate of change of the cell temperature calculated in the SOFC mode is equal to or higher than a predetermined value. May be increased. According to this configuration, in the SOFC mode, when the absolute value of the calculated cell temperature change rate is equal to or higher than a predetermined value, the control unit increases the flow rate of oxygen supplied to the cell to increase the sensible heat amount of oxygen. Make it bigger. In SOFC mode, if the absolute value of the rate of change in cell temperature is greater than or equal to a predetermined value, this indicates that the temperature rise is relatively fast due to the relatively large amount of heat generated in the cell. The sensible heat of the oxygen exhausted from the cell is used to cool the cell. Thereby, rapid temperature changes in the cell can be suppressed.

(7) In the SOFC/SOEC system of the above embodiment, the control unit increases the voltage applied to the cell when the absolute value of the rate of change of the cell temperature calculated in the SOEC mode is equal to or higher than a predetermined value. It's okay. According to this configuration, in the SOEC mode, when the absolute value of the calculated rate of change in cell temperature is equal to or higher than a predetermined value, the control unit increases the voltage applied to the cell to generate heat due to internal resistance of the cell. Increase the amount of In SOEC mode, if the absolute value of the rate of change in cell temperature is greater than or equal to a predetermined value, this indicates that the temperature of the cell is decreasing relatively quickly, so the voltage applied to the cell is increased to The cell is heated using heat generated by internal resistance. Thereby, rapid temperature changes in the cell can be suppressed.

Note that the present invention can be realized in various ways, such as a method for controlling an SOFC/SOEC system, a computer program for executing the method for controlling an SOFC/SOEC system, and a method for distributing this computer program. It can be realized in the form of a server device, a non-temporary storage medium storing a computer program, etc.

FIG. 1 is a block diagram of a SOFC/SOEC system according to a first embodiment. FIG. 2 is a diagram illustrating the relationship between current density and heat flow rate in a SOFC/SOEC cell. It is a figure explaining heat balance in a SOFC/SOEC stack. It is a figure explaining the effect of a 1st embodiment. FIG. 2 is a block diagram of a SOFC/SOEC system according to a second embodiment. FIG. 3 is an explanatory diagram of heat exchange between the SOFC/SOEC stack and the heat storage section. It is a figure explaining heat balance in a SOFC/SOEC stack. FIG. 2 is a block diagram of a SOFC/SOEC system according to a third embodiment. It is a flowchart explaining the operation of the SOFC/SOEC system. It is a figure explaining heat balance in a SOFC/SOEC stack. It is a schematic diagram of a SOFC/SOEC stack and a heat storage part of a 4th embodiment. It is an explanatory diagram of a stack heat balance model. It is a block diagram of the SOFC/SOEC system of a 5th embodiment. It is a flowchart explaining the operation of the SOFC/SOEC system. It is a block diagram of a SOFC/SOEC system of a 6th embodiment. It is a flowchart explaining the operation of the SOFC/SOEC system.

<First embodiment>
FIG. 1 is a block diagram of the SOFC/SOEC system 1 of the first embodiment. The SOFC/SOEC system 1 is a system that generates hydrogen by electrolyzing water vapor (hereinafter simply referred to as "steam electrolysis") and generates electricity from a chemical reaction between the generated hydrogen and oxygen. As a result, the SOFC/SOEC system 1 can, for example, store hydrogen generated by steam electrolysis using surplus electricity, and generate electricity by oxidizing the hydrogen when electricity is needed. You can get electricity. The SOFC/SOEC system 1 of this embodiment includes a SOFC/SOEC stack 10, an oxygen side heat exchanger 20, an oxygen storage section 30, a fuel side heat exchanger 40, a water vapor generation section 50, and a hydrogen storage section 60. , a plurality of pipes connecting these, and a control section 100.

The SOFC/SOEC stack 10 includes multiple SOFC/SOEC cells. Each of the plurality of SOFC/SOEC cells functions as a solid oxide electrolytic cell (SOEC) that performs water vapor electrolysis by applying a voltage. Each of the plurality of SOFC/SOEC cells also functions as a solid oxide fuel cell (SOFC) that generates electricity by being supplied with hydrogen and oxygen. Here, in the SOFC/SOEC system 1, the state in which the SOFC/SOEC stack 10 is performing steam electrolysis is assumed to be operating in "SOEC mode", and the state in which the SOFC/SOEC stack 10 is generating electricity is "SOFC mode". Suppose you are driving at In this embodiment, Ni-YSZ is used as the electrode on the fuel side 11 (fuel side electrode) of the SOFC/SOEC stack 10, and a perovskite oxide (for example, ( (La,Sr) _CoO3 , (La,Sr)(Co,Fe) _O3 , _LaSrMnO3, etc.) are used. As the electrolyte 13 separating the fuel side 11 and the oxygen side 12, YSZ (yttria stabilized zirconia) is used. Note that in other embodiments, known materials may be used for the fuel side electrode, the oxygen side electrode, and the electrolyte 13. The SOFC/SOEC stack 10 of this embodiment performs steam electrolysis in an atmosphere of approximately 650° C. in SOEC mode, and generates power in an atmosphere of approximately 700° C. in SOFC mode.

The SOFC/SOEC stack 10 is electrically connected to an external power supply source (not shown) via a transformer 14. The transformer 14 is electrically connected to the control unit 100 and changes the voltage of the power supplied to the SOFC/SOEC stack 10 by an external power supply source according to a command from the control unit 100. The transformer 14 corresponds to a "voltage adjustment section" in the claims.

The oxygen side heat exchanger 20 includes an exhaust flow path 21 through which oxygen side off-gas flows, a fuel gas flow path 22 through which a mixed gas of water vapor and hydrogen (hereinafter referred to as "fuel gas") flows, and an oxygen flow path through which oxygen flows. It has 23. The oxygen side heat exchanger 20 exchanges heat between the oxygen side off-gas discharged from the oxygen side 12 of the SOFC/SOEC stack 10 and the fuel gas supplied to the fuel side 11 or the oxygen supplied to the oxygen side 12. I do.

The oxygen storage unit 30 includes an oxygen booster 31 and an oxygen tank 32. The oxygen pressure booster 31 boosts the pressure of the oxygen side off-gas that has passed through the exhaust flow path 21 of the oxygen side heat exchanger 20 to, for example, 20 MPa. The oxygen tank 32 stores the oxygen side off-gas whose pressure has been increased by the oxygen booster 31. The oxygen off-gas stored in the oxygen tank 32 is mainly composed of oxygen, and in the SOFC mode in the SOFC/SOEC stack 10, oxygen in the oxygen side heat exchanger 20 is used as an oxidizing agent to oxidize hydrogen. It is supplied to the SOFC/SOEC stack 10 via the flow path 23. An on-off valve 33 that turns on and off the supply of oxygen off-gas to the oxygen booster 31 is arranged between the exhaust flow path 21 of the oxygen-side heat exchanger 20 and the oxygen booster 31. A flow control valve 34 that controls the flow rate of oxygen supplied to the oxygen side 12 of the SOFC/SOEC stack 10 is arranged between the oxygen tank 32 and the oxygen flow path 23 of the oxygen side heat exchanger 20. The on-off valve 33 and the flow control valve 34 operate according to commands from the control section 100 that is electrically connected to them.

The fuel-side heat exchanger 40 has an exhaust flow path 41 through which fuel-side off-gas flows, and a fuel flow path 42 through which fuel gas flows. The fuel side heat exchanger 40 performs heat exchange between the fuel side off-gas discharged from the fuel side 11 of the SOFC/SOEC stack 10 and the fuel gas supplied to the fuel side 11.

The steam generation unit 50 includes a water tank 51 that stores water, an external heat source 52 that supplies heat for heating water, a heat medium flow path 53 through which a heat medium supplied by the external heat source 52 flows, and an evaporation flow path. It has 54. In the water vapor generation unit 50, water in the water tank 51 flowing through the evaporation channel 54 is heated by the heat medium in the heat medium channel 53, thereby generating water vapor. The generated water vapor is supplied to the fuel side 11 of the SOFC/SOEC stack 10 in SOEC mode. A flow control valve 55 that adjusts the amount of water sent from the water tank 51 to the evaporation flow path 54 is arranged between the water tank 51 and the evaporation flow path 54 . The flow rate control valve 55 operates in response to a command from the control unit 100 that is electrically connected thereto.

Hydrogen storage unit 60 includes a hydrogen booster 61 and a hydrogen tank 62. The hydrogen booster 61 pressurizes hydrogen to, for example, 20 MPa. The hydrogen tank 62 stores hydrogen pressurized by the hydrogen booster 61. The amount of hydrogen stored in the hydrogen tank 62 is supplied to the fuel side 11 of the SOFC/SOEC stack 10 by a flow rate control valve 63 disposed between the hydrogen tank 62 and a flow rate distributor 73, which will be described later. adjusted. The flow rate control valve 63 operates according to a command from the control unit 100 that is electrically connected thereto.

The radiator 71 is arranged downstream of the exhaust flow path 41 of the fuel-side heat exchanger 40. The radiator 71 radiates heat from the fuel-side off-gas that has passed through the exhaust flow path 41.

The gas-liquid separator 72 is disposed downstream of the radiator 71 and separates water and hydrogen contained in the fuel-side off-gas heat radiated by the radiator 71. The water separated by the gas-liquid separator 72 is sent to the water tank 51 of the steam generator 50. Hydrogen separated by the gas-liquid separator 72 is sent to the hydrogen tank 62 via the hydrogen booster 61.

The flow rate distributor 73 is a mass flow controller arranged between the steam generation section 50 and the hydrogen storage section 60, and the oxygen side heat exchanger 20 and the fuel side heat exchanger 40. The flow rate distributor 73 distributes fuel gas, which is a mixture of steam sent from the steam generation section 50 and hydrogen sent from the hydrogen storage section 60, to the oxygen side heat exchanger 20 and the fuel side heat exchanger 40, respectively. Distribute and supply. A flow rate control valve 74 is arranged between the flow rate distributor 73 and the oxygen side heat exchanger 20 to adjust the flow rate of the fuel gas sent to the oxygen side heat exchanger 20. The flow rate control valve 74 operates in response to a command from the control unit 100 that is electrically connected thereto.

The control unit 100 is a computer including a ROM, a RAM, and a CPU, and controls each unit of the SOFC/SOEC system 1. In the present embodiment, the control unit 100 controls voltage at the transformer 14, flow rate control at the flow rate control valves 34, 55, 63, and 74, opening/closing control at the on/off valve 33, and flow rate control at the flow rate distributor 73. Performs distribution control, etc.

Although not shown in FIG. 1, each part of the SOFC/SOEC system 1 is provided with a mass flow sensor that detects the flow rate in the piping and a temperature sensor that detects the temperature of the gas. The control section 100 acquires the detection values of these sensors and controls each section of the SOFC/SOEC system 1.

Next, the operation of the SOFC/SOEC system 1 of this embodiment will be explained. In the SOFC/SOEC system 1, the control unit 100 causes the SOFC/SOEC stack 10 to operate in either the SOEC mode or the SOFC mode, depending on the occurrence of surplus power externally or a request for power, for example. The SOFC/SOEC system 1 is controlled so that it is operated in the operation mode.

First, the operation when the SOFC/SOEC stack 10 operates in SOEC mode will be described. Under the control of the control unit 100, in the steam generation unit 50, water is supplied from the water tank 51 to the evaporation channel 54. The water supplied to the evaporation channel 54 is heated by the heat medium supplied by the external heat source 52 and becomes water vapor. The generated water vapor is mixed with hydrogen supplied from the hydrogen storage section 60 and sent to the flow rate distributor 73 in order to avoid oxidation of the fuel-side electrode during electrolysis (deterioration of electrode performance due to oxidation of the Ni electrode). The fuel gas sent to the flow rate distributor 73 is divided between, for example, the oxygen side heat exchanger 20 and the fuel side heat exchanger 40 in consideration of the amount of heat exchanged in the oxygen side heat exchanger 20 and the fuel side heat exchanger 40. The flow rate is distributed to the oxygen side heat exchanger 20 and the fuel side heat exchanger 40 such that the flow rate ratio with respect to the oxygen side heat exchanger 20 is 0.82/0.18. The fuel gases heat exchanged in each of the oxygen side heat exchanger 20 and the fuel side heat exchanger 40 merge at the inlet of the fuel side 11 of the SOFC/SOEC stack 10 and flow into the flow path of the fuel side 11.

In the SOFC/SOEC stack 10, steam electrolysis is performed using power supplied from an external power source via a transformer 14. In this embodiment, the control unit 100 applies the voltage (1.37V), which is the thermoneutral voltage (1.29V) plus the additional voltage (0.08V), to the SOFC/SOEC stack 10. The transformer 14 is controlled. Here, the thermoneutral voltage is a state in which hydrogen can be generated on the fuel side 11 by only the heat generated in the SOFC/SOEC stack 10 during steam electrolysis in the SOFC/SOEC stack 10 (hereinafter referred to as "thermo neutral voltage"). This refers to the voltage required to maintain the "neutral state". Further, the additional voltage refers to a voltage that is added to the thermoneutral voltage in order to generate heat due to the internal resistance of the SOFC/SOEC stack 10, in addition to heat generation in the thermoneutral state. In this embodiment, the control unit 100 sets the voltage for generating heat by the internal resistance of the SOFC/SOEC stack 10 from a map input in advance. Specifically, the control unit 100 controls, for example, the sensible heat of the fuel-side offgas and oxygen-side offgas discharged from the SOFC/SOEC stack 10 and the sensible heat of the fuel gas and oxygen flowing into the SOFC/SOEC stack 10. From the difference, the amount of heat released from the SOFC/SOEC stack 10 by the gas is calculated. The control unit 100 determines the additional voltage from the above-described map using the calculated amount of heat radiation.

In the SOFC/SOEC stack 10, hydrogen produced by steam electrolysis and unelectrolyzed water vapor are discharged from the outlet of the fuel side 11, and oxygen produced by steam electrolysis is discharged from the outlet of the oxygen side 12. be discharged. The fuel side off-gas (mixed gas of water vapor and hydrogen) discharged from the fuel side 11 recovers exhaust heat in the fuel side heat exchanger 40, is cooled by a radiator 71, and is then separated into water and water by a gas-liquid separator 72. It is separated into hydrogen gas. The separated water is stored in a water tank 51, and the separated hydrogen gas is pressurized by a hydrogen booster 61 and then stored in a hydrogen tank 62.

Exhaust heat of the oxygen side off-gas (oxygen) discharged from the oxygen side 12 is recovered in the oxygen side heat exchanger 20 and sent to the oxygen storage section 30. In the oxygen storage section 30 , the oxygen pressure is increased to 20 MPa by the oxygen booster 31 and then stored in the oxygen tank 32 .

Next, the operation when the SOFC/SOEC stack 10 operates in the SOFC mode will be explained. Under the control of the control unit 100, the oxygen stored in the oxygen tank 32 is supplied to the oxygen side 12 of the SOFC/SOEC stack 10 after flowing through the oxygen side heat exchanger 20. At this time, the control unit 100 controls the flow rate control valve 34 so that a larger amount of oxygen is supplied to the oxygen side 12 than a preset amount to oxidize all the hydrogen supplied to the SOFC/SOEC stack 10. control. In this embodiment, the control unit 100 supplies hydrogen to the oxygen side 12 from a map input in advance using the flow rate of hydrogen supplied to the fuel side 11 detected using a mass flow controller (not shown). Determine the amount of oxygen. Under the control of the control unit 100, the hydrogen stored in the hydrogen tank 62 flows through the flow rate distributor 73, and then through the oxygen side heat exchanger 20 or the fuel side heat exchanger 40, and then flows into the fuel of the SOFC/SOEC stack 10. It is supplied to side 11. As a result, in the SOFC/SOEC stack 10, water is generated by the oxidation reaction of hydrogen, and electricity is also generated. The generated electricity is sent to an external power receiving device (not shown).

From the fuel side 11, unreacted hydrogen gas is discharged together with the generated water. The exhaust heat of the gas exhausted from the fuel side 11 is recovered in the fuel side heat exchanger 40, cooled by the radiator 71, and then separated into water and hydrogen gas by the gas-liquid separator 72. The separated water is stored in a water tank 51, and the separated hydrogen gas is pressurized by a hydrogen booster 61 and then stored in a hydrogen tank 62.

Exhaust heat of the oxygen side off-gas discharged from the oxygen side 12 is recovered in the oxygen side heat exchanger 20 and sent to the oxygen storage section 30. In the oxygen storage section 30 , the oxygen pressure is increased to 20 MPa by the oxygen booster 31 and then stored in the oxygen tank 32 .

FIG. 2 is a diagram illustrating the relationship between current density and heat flux in a SOFC/SOEC cell. Here, in the operation of the SOFC/SOEC system 1 described above, the reason why a voltage higher than the thermoneutral voltage is applied to the SOFC/SOEC stack 10 when operating the SOFC/SOEC stack 10 in the SOEC mode will be explained. In FIG. 2, the horizontal axis shows the current density in the electrolyte 13 of the SOFC/SOEC stack 10, and the vertical axis shows the heat flux corresponding to this current density. Furthermore, on the horizontal axis indicating current density, the plus side indicates the current density in SOFC mode, and the minus side indicates current density in SOEC mode. The heat flux in the SOFC/SOEC stack 10 is the sum of the amount of heat generated by the internal resistance of the SOFC/SOEC stack 10 and the amount of heat generated by the steam electrolysis reaction or the oxidation reaction of hydrogen.

In FIG. 2, the relationship between current density and heat flux in the SOFC/SOEC stack 10 is shown by two thick lines (signs Lf0 and Lf1). The symbol Lf0 indicated by a dotted line indicates the relationship between current density and heat flux in the thermoneutral state described above, and the symbol Lf1 indicated by a solid line indicates the relationship between current density and heat flux in this embodiment. It shows a relationship. Furthermore, the amount of heat generated by the internal resistance of the SOFC/SOEC stack 10 is shown by medium-thick lines (signs Lm0, Lm1), and the amount of heat generated by the steam electrolysis reaction or the oxidation reaction of hydrogen is shown by the thin line (signs Lm1). Lt0).

In the thermoneutral state, a thermoneutral voltage is applied to the SOFC/SOEC stack 10 in the SOEC mode (region on the negative side of current density) shown in FIG. For example, when the current density at the thermoneutral voltage is -1.65 A/cm ² (dotted line SOEC1 shown in FIG. 2), the amount of heat generated due to the internal resistance of the SOFC/SOEC stack 10 is approximately 0.5 W/cm ² ( This is a point Pm0) where the dotted line labeled Lm0 and the dotted line SOEC1 shown in FIG. 2 intersect. In addition, the reaction heat (the amount of change in free energy subtracted from the amount of enthalpy change) due to the steam electrolysis reaction is about -0.5 W/cm ² (point Pm0 where the dotted line with symbol Lt0 and the dotted line intersect in Fig. 2). Therefore, the total heat flux in the SOFC/SOEC stack 10 is 0 W/cm ² (point Pf0 where the dotted line labeled Lf0 and the dotted line SOEC1 shown in FIG. 2 intersect).

On the other hand, in the present embodiment, in the SOEC mode, the SOFC/SOEC stack 10 is supplied with a voltage higher than the thermoneutral state as described above, so the resistance heat generation amount increases compared to the thermoneutral voltage. Specifically, the amount of heat generated by the internal resistance of the SOFC/SOEC stack 10 is about 0.6 W/cm ² (point Pm1 where the solid line labeled Lm1 and the dotted line SOEC1 in FIG. 2 intersect). Since the reaction heat (code Lt0) due to the steam electrolysis reaction at this time does not change, the heat flux in the SOFC/SOEC stack 10 in this embodiment becomes code Lf1 and increases compared to the thermoneutral state (Fig. A point Pf1) where the dotted line with the symbol Lf1 shown in FIG. 2 intersects with the dotted line SOEC1. That is, in the present embodiment, the amount of resistance heat generated in the SOFC/SOEC stack 10 is increased by increasing the voltage applied to the SOFC/SOEC stack 10 in the SOEC mode.

FIG. 3 is a diagram illustrating the heat balance in the SOFC/SOEC stack 10. FIG. 3(a) shows the heat balance of the SOFC/SOEC stack 10 in SOEC mode with an output of 50 kWe, and FIG. 3(b) shows the heat balance of the SOFC/SOEC stack 10 in SOFC mode with an output of 3.8 kWe. shows the heat balance.

In the SOEC mode (see FIG. 3(a)), as explained in FIG. 2, by making the voltage applied to the SOFC/SOEC stack 10 higher than the thermoneutral voltage, the amount of resistance heat generated in the SOFC/SOEC stack 10 is reduced. It is larger than the neutral state. As a result, the value of "A-1: Resistance heat generation" shown in FIG. 3(a) becomes a value (3.13 kW) higher than that in the thermoneutral state. As a result, the amount of heat generated in the SOFC/SOEC stack 10 "A: stack heat generation" is the sum of "A-1: resistance heat generation" and "A-2: reaction heat" (0.84 kW). On the other hand, the amount of heat dissipated from the SOFC/SOEC stack 10 is "B: Fuel side ΔH" (0.36kW) which is the sensible heat amount of the off gas on the fuel side, and "C: Oxygen side ΔH" which is the sensible heat amount of the oxygen side off gas. (0.43kW) and "D: heat radiation loss" (0.05kW), which is 0.84kW. That is, A-(B+C+D)=0, and by applying a voltage higher than the thermoneutral voltage to the SOFC/SOEC stack 10, heat balance in the SOEC mode is established.

In the SOFC mode (see FIG. 3(b)), as described above, the control unit 100 controls to supply oxygen to the oxygen side 12 in an amount larger than that required for reaction with hydrogen. As a result, a relatively large amount of oxygen is discharged from the oxygen side 12, so the amount of heat dissipated by the sensible heat of oxygen increases compared to the case where only the amount of oxygen required for reaction with hydrogen is supplied. . In this embodiment, the control unit 100 controls the amount of oxygen supplied so that the sum of the amount of heat generated and the amount of heat dissipated in the SOFC/SOEC stack 10 in the SOFC mode becomes zero. Specifically, as shown in FIG. 3(b), since the amount of heat generated in the SOFC/SOEC stack 10 in the SOFC mode is 2.27 kW, the control unit 100 sets "B: fuel side ΔH". The amount of oxygen supplied is controlled so that the sum of "C: oxygen side ΔH" and "D: heat radiation loss" is 2.27 kW. As a result, as shown in FIG. 3(b), A-(B+C+D)=0, and the heat balance in SOFC mode is also established. In this embodiment, the sensible heat of oxygen is recovered in the oxygen side heat exchanger 20 and used to heat the fuel gas supplied to the fuel side 11 and the oxygen supplied to the oxygen side 12.

FIG. 4 is a diagram illustrating the effects of this embodiment. FIG. 4(a) shows a comparison of Nernst potentials, and FIG. 4(b) shows a comparison of power generation efficiency. Both FIGS. 4(a) and 4(b) show a SOFC/SOEC system that generates power using air as a comparative example with respect to this embodiment. Note that the data shown in FIG. 4 shows simulation results under constant current control at 700°C.

A comparison of the Nernst potential shown in FIG. 4(a) reveals that the Nernst potential of this embodiment is 1.15 times higher than that of the comparative example. Looking in detail, in this embodiment, since the partial pressure of oxygen supplied to the oxygen side is high, the voltage is 0.74V in the comparative example, whereas in this embodiment, the voltage is 0.85V. It is increasing. Further, in the comparative example, a gas containing nitrogen in addition to oxygen is supplied to the oxygen side, whereas in the present embodiment, only oxygen is supplied, so that the overvoltage on the oxygen side is 0. While it is 14V, in this embodiment, it is reduced to 0.08V.

Moreover, as shown in FIG. 4(b), when the power generation efficiency ratio of the present embodiment is 100%, it has become clear that the power generation efficiency ratio of the comparative example is 95%. From this, it has become clear that in SOFC mode, the power generation efficiency ratio is improved by using oxygen as an oxidizing agent for hydrogen.

In a SOFC/SOEC system, in a SOFC/SOEC cell in SOFC mode, (a) ohmic resistance of constituent members such as the electrolyte and electrode material of the SOFC/SOEC cell, and (b) H ₂ /H ₂ O/O in the flow field. ₍₂₎ Concentration resistance determined by concentration, and (c) reaction resistance determined by catalyst activity on the fuel side/oxygen side. Therefore, the output voltage of the SOFC/SOEC cell is calculated by subtracting the product of the sum of the resistances and current in (a), (b), and (c) above from the voltage that can be extracted as electrical energy from chemical energy. will be the value. Here, when air is used as the hydrogen oxidizing agent, oxygen is consumed by power generation, so the oxygen concentration decreases from upstream to downstream of the oxidizing agent. Therefore, among the three types of resistance described above, the concentration resistance (b) increases. In particular, when the air utilization rate in the SOFC/SOEC stack 10 is set high, the N ₂ concentration increases due to oxygen consumption, and the concentration resistance component in (b) increases.

As described above, in the SOFC/SOEC system 1 of the present embodiment, when the SOFC/SOEC stack 10 is operated in the SOFC mode, the oxygen stored in the oxygen storage section 30 is supplied as an oxidizing agent for hydrogen. As a result, as described above, compared to the case where air is used as the hydrogen oxidizing agent, increase in concentration resistance can be suppressed, and the output voltage of the cell can be increased. A relatively high power generation voltage can be obtained. Therefore, power generation efficiency can be improved.

Further, in the SOFC/SOEC system 1 of the present embodiment, the control unit 100 applies a voltage higher than a value preset as a value for thermoneutral state to the SOFC/SOEC stack 10 in the SOEC mode. Then, the transformer 14 is controlled. As a result, heat due to the internal resistance of the SOFC/SOEC stack 10 is generated in the SOFC/SOEC stack 10, where the temperature decreases due to the progress of the steam electrolysis reaction, which is an endothermic reaction, at the thermoneutral voltage. The temperature of the SOFC/SOEC stack 10 can be stabilized. Further, the control unit 100 controls the oxygen storage unit 30 to supply oxygen in an amount larger than a preset amount to oxidize all the hydrogen supplied to the SOFC/SOEC stack 10 in the SOFC mode. do. As a result, from the SOFC/SOEC stack 10, where the temperature rises due to the oxidation reaction of hydrogen, which is an exothermic reaction, the sensible heat of the oxygen discharged from the SOFC/SOEC stack 10 without reacting is transferred to the SOFC/SOEC. Since the heat of the stack 10 is released to the outside, the temperature of the SOFC/SOEC stack 10 can be stabilized. In this way, in the SOEC mode, heat is generated by the internal resistance of the SOFC/SOEC stack 10 by applying a voltage higher than the thermoneutral voltage, and in the SOFC mode, the amount of hydrogen supplied to the SOFC/SOEC stack 10 is greater than the amount of oxidation. By supplying a large amount of oxygen, the heat of the SOFC/SOEC stack 10 is released to the outside of the SOFC/SOEC stack 10. Thereby, the heat balance of the SOFC/SOEC stack 10 can be established in each of the SOEC mode and the SOFC mode, so that temperature changes in the SOFC/SOEC stack 10 can be suppressed.

<Second embodiment>
FIG. 5 is a block diagram of the SOFC/SOEC system 2 according to the second embodiment. FIG. 6 is an explanatory diagram of heat exchange between the SOFC/SOEC stack 10 and the heat storage section 80. The SOFC/SOEC system 2 of the second embodiment differs from the SOFC/SOEC system 1 of the first embodiment in that it includes a heat storage section 80.

The SOFC/SOEC system 2 of this embodiment includes a SOFC/SOEC stack 10, an oxygen side heat exchanger 20, an oxygen storage section 30, a fuel side heat exchanger 40, a water vapor generation section 50, and a hydrogen storage section 60. , a heat storage section 80 , a plurality of pipes connecting these, and a control section 100 .

The heat storage section 80 includes a heat storage section 81 , a heat transport section 82 , a flow control valve 83 , and an on/off valve 84 . The heat storage unit 80 stores the heat of the SOFC/SOEC stack 10 in the SOFC mode, and supplies the stored heat to the SOFC/SOEC stack 10 in the SOEC mode.

The heat storage section 81 includes a pipe 75 from the fuel side heat exchanger 40 to the inlet side of the fuel side 11 of the SOFC/SOEC stack 10, and a pipe 75 between the fuel side heat exchanger 40 and the fuel side heat exchanger from the outlet side of the fuel side 11 of the SOFC/SOEC stack 10. The connecting pipe 85 connects the pipe 76 up to the container and the connecting pipe 85. The heat storage section 81 includes a heat storage material 81a and a plurality of channels 81b installed in a container electrically insulated from the outside (see FIG. 6). The heat storage material 81a can repeatedly store and release heat using the redox reaction (MxOy+yH ₂ ⇔xM+yH ₂ O) of the metal oxide MxOy. As the metal M, Fe, Mn, Ni, Cu, etc. are generally used. In this embodiment, the metal M is Fe.

The heat transport section 82 is a plate-shaped material having heat resistance, high thermal conductivity, and good electrical conductivity, and is made of SiC, AlN, stainless steel, or the like. The heat transport section 82 is formed from a material with high thermal conductivity, so that the temperature difference required for heat transport between the heat storage section 81 and the SOFC/SOEC stack 10 is relatively small while reducing thermal resistance. Even if it is small, it can transport heat.

As shown in FIG. 6, the heat transport section 82 has one end connected to the fuel side interconnect flow path 11a/fuel side electrode 11b/electrolyte 13/oxygen side electrode 12b/oxygen side interconnect flow path of the SOFC/SOEC stack 10. It is sandwiched between the laminates formed by 12a. The other end of the heat transport section 82 is sandwiched between heat storage materials 81a (see FIG. 6).

The flow rate control valve 83 is arranged in the connecting pipe 85 between the pipe 75 from the fuel side heat exchanger to the inlet side of the fuel side 11 of the SOFC/SOEC stack 10 and the heat storage section 81 . The flow rate control valve 83 controls the flow rate of water or hydrogen gas supplied to the heat storage section 81 . The on-off valve 84 is arranged in the connecting pipe 85 between the heat storage section 81 and the pipe 76 from the outlet side of the fuel side 11 of the SOFC/SOEC stack 10 to the fuel side heat exchanger. The on-off valve 84 turns on and off the supply of water or hydrogen gas discharged from the heat storage section 81 .

In the SOEC mode, fuel gas (H ₂ O + H ₂ ) is supplied to the fuel-side interconnect flow path 11a in the SOFC/SOEC stack 10 under the control of the control unit 100 (as indicated by the white arrow F11 in FIG. 6(a)). ), oxygen (O ₂ ) is supplied to the oxygen side interconnect flow path 12a (white arrow F12 in FIG. 6(a)). Thereby, a steam electrolysis reaction is performed in the SOFC/SOEC stack 10. On the other hand, under the control of the control unit 100, in the heat storage unit 80, the fuel gas flowing through the pipe 75 flows into the flow path 81b of the heat storage unit 80 via the connection pipe 85 (white arrow F13 in FIG. 6(a)). . At this time, an oxidation reaction (formula (1)) progresses between the heat storage material 81a, which is Fe, and the water flowing into the flow path 81b, and 284 kJ/mol of heat is generated. Note that the generated hydrogen (white arrow F14 in FIG. 6(a)) is mixed with the fuel side off-gas flowing through the pipe 76 via the connection pipe 85.
3Fe+4H ₂ O⇒Fe ₃ O ₄ +4H ₂ ...(1)

The heat generated in the heat storage material 81a is transmitted to the heat transport section 82 mainly by conductive heat transfer (outline arrow F15 in FIG. 6(a)). The heat transmitted to the heat transport section 82 is transmitted to the SOFC/SOEC stack 10 via the heat transport section 82 (white arrow F16 in FIG. 6(a)). The heat transferred to the SOFC/SOEC stack 10 is used to maintain the temperature of the SOFC/SOEC stack 10, whose temperature tends to decrease due to the endothermic steam electrolysis reaction (white arrow F17 in FIG. 6(a)). As a result, in the SOEC mode, the temperature of the heat storage section 81 that generates heat according to equation (1) is maintained at about 667°C, and the temperature of the SOFC/SOEC stack 10 where the steam electrolysis reaction proceeds is maintained at about 650°C.

In the SOFC mode, under the control of the control unit 100, in the SOFC/SOEC stack 10, hydrogen is supplied to the fuel-side interconnect flow path 11a (white arrow F21 in FIG. 6(b)), and the oxygen-side interconnect flow path Oxygen is supplied to 12a (white arrow F22 in FIG. 6(b)). Thereby, in the SOFC/SOEC stack 10, power generation is performed by the oxidation reaction of hydrogen. At this time, heat is generated in the SOFC/SOEC stack 10 due to the oxidation reaction of hydrogen. The heat generated in the SOFC/SOEC stack 10 is transmitted to the heat transport section 82 mainly by conduction heat transfer (white arrow F23 in FIG. 6(b)). The heat transmitted to the heat transport section 82 is transmitted to the heat storage section 81 via the heat transport section 82 (outline arrow F24 in FIG. 6(b)).

On the other hand, under the control of the control unit 100, in the heat storage unit 80, hydrogen gas flowing through the pipe 75 flows into the flow path 81b of the heat storage unit 80 via the connection pipe 85 (white arrow F23 in FIG. 6(b)). . At this time, the heat storage material 81a, which is Fe ₃ O ₄ , absorbs 284 kJ/mol of heat because a reduction reaction (formula (2)) progresses with the hydrogen gas flowing into the flow path 81b. I can do it.
Fe ₃ O ₄ +4H ₂ ⇒3Fe+4H ₂ O...(2)
The heat transmitted from the SOFC/SOEC stack 10 to the heat storage section 81 via the heat transport section 82 spreads throughout the heat storage material 81a (white arrow F26 in FIG. 6(b)), and the reaction of equation (2) progresses. Heat is stored by the heat storage material 81a. As a result, in the SOFC mode, the temperature of the SOFC/SOEC stack 10, which generates heat due to the oxidation reaction of hydrogen, is maintained at approximately 700°C, and the temperature of the heat storage section 81, which absorbs heat according to equation (2), is maintained at approximately 665°C. . In this way, in the SOFC/SOEC system 2 of the present embodiment, the heat storage unit 80 uses the fuel gas (H ₂ O+H ₂ or H ₂ ) handled by the system to store hydrogen in the SOFC/SOEC stack 10. Heat storage and heat supply are switched to complement the heat generated by the oxidation reaction and the heat absorbed by the steam electrolysis reaction. In addition, the off-gas from the heat storage section 80 (white arrow F27 in FIG. 6(b)) is mixed with the fuel-side off-gas and collected together with the fuel-side off-gas, so it is not discharged outside the system. .

Next, the operation of the SOFC/SOEC system 2 of this embodiment will be explained. Here, points that are particularly different from the operation of the SOFC/SOEC system 1 of the first embodiment will be explained.

In the SOEC mode, a part of the fuel gas sent to the flow rate distributor 73 is sent to the inlet of the fuel side 11 of the SOFC/SOEC stack 10 via the fuel side heat exchanger 40. It flows through the connecting pipe 85 under the control of the flow rate control valve 83 . Water in the fuel gas flowing through the connection pipe 85 undergoes an oxidation reaction with the heat storage material 81a, which is Fe, to generate heat in the heat storage section 81. On the other hand, in the SOFC/SOEC stack 10 to which a thermoneutral voltage is applied, the temperature tends to decrease due to heat absorption in the steam electrolysis reaction. Therefore, even if the temperature difference between the heat storage section 81 and the SOFC/SOEC stack 10 is relatively small (for example, about 17 degrees Celsius), the heat transport section 82 having high thermal conductivity can transfer the SOFC/SOEC stack from the heat storage section 81 to the SOFC/SOEC stack 10. The heat of the heat storage material 81a is transported to the SOEC stack 10. At this time, the control unit 100 determines that the heat transport rate from the heat storage unit 81 to the SOFC/SOEC stack 10 is based on the sensible heat amount of the off-gas on the fuel side, the sensible heat amount of the off-gas on the oxygen side, and the heat radiation loss of the SOFC/SOEC stack 10. The flow rate of water in the fuel gas is controlled by the flow rate control valve 83 so that it becomes equal to the time change of the sum of . At this time, the control of the flow rate of water in the fuel gas by the flow rate control valve 83 is determined, for example, in the same manner as in the first embodiment using a map input to the control unit 100 in advance. As a result, in the SOFC/SOEC stack 10 in SOEC mode, the thermoneutral voltage (Vth = 1.29V, 650°C) is reached by the amount of heat from the heat storage section 81 without increasing resistance heat generation as in the first embodiment. This makes it possible to establish a stack heat balance.

In the SOFC mode, the SOFC/SOEC stack 10 generates electricity and heat due to the oxidation reaction of hydrogen. In the SOFC/SOEC stack 10, when a high oxygen utilization rate (for example, 0.5) is set, the amount of heat generated within the SOFC/SOEC stack 10 is determined by the amount of heat released (sensible heat amount of the off-gas on the fuel side and (sensible heat amount of off-gas and heat radiation loss of SOFC/SOEC stack 10). On the other hand, out of the hydrogen gas sent to the flow rate distributor 73, a portion of the hydrogen gas sent to the inlet of the fuel side 11 of the SOFC/SOEC stack 10 via the fuel side heat exchanger 40 is controlled by the control unit 100. It flows through the connecting pipe 85 under the control of the control valve 83 . The hydrogen flowing through the connecting pipe 85 is used for the reduction reaction of the heat storage material 81a, which is an endothermic reaction, and the temperature of the heat storage section 81 tends to decrease. Therefore, even if the temperature difference between the heat storage section 81 and the SOFC/SOEC stack 10 is relatively small (for example, about 35° C.), the heat transport section 82 transfers the heat from the SOFC/SOEC stack 10 to the heat storage section 81. Transports heat in the SOFC/SOEC stack 10. At this time, the control unit 100 determines that the transport rate of heat from the SOFC/SOEC stack 10 to the heat storage unit 81, that is, the heat storage rate of the heat storage unit 81, is the amount of heat released from the amount of heat generated within the SOFC/SOEC stack 10. The flow rate of hydrogen by the flow rate control valve 83 is controlled so as to be equal to the time change of the subtracted value. At this time, the control of the hydrogen flow rate by the flow rate control valve 83 is determined using a map that has been input to the control unit 100 in advance. As a result, in the SOFC/SOEC stack 10 in SOFC mode, the heat of the SOFC/SOEC stack 10 is transported to the heat storage section 80 without increasing the oxygen flow rate as in the first embodiment, thereby improving the stack heat balance. It becomes possible to establish this.

FIG. 7 is a diagram illustrating the heat balance in the SOFC/SOEC stack 10. FIG. 7(a) shows the heat balance of the SOFC/SOEC stack 10 in SOEC mode with an output of 50 kWe, and FIG. 7(b) shows the heat balance of the SOFC/SOEC stack 10 in SOFC mode with an output of 3.8 kWe. shows the heat balance. In the heat balance in the SOFC/SOEC stack 10 in the second embodiment, in addition to the amount of heat generated in the SOFC/SOEC stack 10, the amount of heat radiated by sensible heat of the off-gas discharged from the SOFC/SOEC stack 10, and the heat radiation loss, The amount of heat transferred to the heat storage section 80 via the transport section 82 is also added.

In SOEC mode (see Figure 7(a)), power is supplied at the thermoneutral voltage, so "A: stack heat generation" is different from "A-1: resistance heat generation" and "A-2 : heat of reaction" and the deduction is 0. On the other hand, regarding the amount of heat dissipation, as in the first embodiment, "B: fuel side ΔH" (0.34 kW) is the sensible heat of the off-gas on the fuel side, and "C: sensible heat of the off-gas on the oxygen side" is the sensible heat of the off-gas on the oxygen side. ΔH" (0.43 kW) and "D: heat radiation loss" (0.05 kW), which is 0.82 kW. In this embodiment, the heat storage unit 80 is further controlled by the control unit 100 so that the amount of heat is 0.82 kW, so the heat balance of the entire SOFC/SOEC stack 10 is A−(B+C+D)− E=0, and this holds true.

In the SOFC mode (see Figure 7(b)), the amount of heat generated in the SOFC/SOEC stack 10 is 2.27 kW, which is the amount of heat dissipated by the sensible heat of the off-gas discharged from the SOFC/SOEC stack 10 and the amount of heat dissipated. The total power including the loss is 0.52kW. This is different from the first embodiment in which the amount of heat dissipated is increased by the sensible heat of oxygen, and the flow rate of gas flowing through the oxygen side 12 is smaller than that in the first embodiment. As a result, "C: oxygen side ΔH" This is because it is smaller than in the first embodiment. In this embodiment, the heat storage unit 80 is controlled by the control unit 100 so that in addition to the sensible heat and heat radiation loss of the off-gas, the amount of heat transferred to the heat storage unit 80 is 1.75 kW. The heat balance for the entire stack 10 is A-(B+C+D)-E=0. Therefore, a heat balance is established for the entire SOFC/SOEC stack 10 in the SOFC mode.

As explained above, in the SOFC/SOEC system 2 of the second embodiment, the heat storage section 81 connected to the SOFC/SOEC stack 10 stores the heat generated in the SOFC/SOEC stack 10 in SOFC mode, Heat is supplied to the SOFC/SOEC stack 10 in SOEC mode. As a result, in the SOFC mode, the heat of the SOFC/SOEC stack 10, whose temperature increases due to the oxidation reaction of hydrogen, which is an exothermic reaction, is transferred from the SOFC/SOEC stack 10 to the heat storage section 81. By storing heat in the storage section 81, the temperature of the SOFC/SOEC stack 10 can be stabilized. Furthermore, in the SOEC mode, the temperature of the SOFC/SOEC stack 10 can be stabilized by supplying heat to the SOFC/SOEC stack 10, whose temperature decreases as the steam electrolysis reaction, which is an endothermic reaction, progresses. can. Thereby, the heat balance of the SOFC/SOEC stack 10 can be established in each of the SOEC mode and the SOFC mode, so that temperature changes in the SOFC/SOEC stack 10 can be suppressed.

Furthermore, in the SOFC/SOEC system 2 of the present embodiment, the heat storage unit 81 stores the heat generated in the SOFC/SOEC stack 10 by being supplied with hydrogen used for power generation in the SOFC/SOEC stack 10 in the SOFC mode. Store heat. Furthermore, the heat storage unit 81 supplies heat to the SOFC/SOEC stack 10 by being supplied with water used for electrolysis in the SOFC/SOEC stack 10 in SOEC mode. In this way, the heat storage section 81 can switch between heat storage and heat supply using the fluid handled by the SOFC/SOEC system 2. Thereby, the heat storage section 81 can be driven without significantly changing the system, so that the heat balance of the SOFC/SOEC stack 10 can be established relatively easily.

<Third embodiment>
FIG. 8 is a block diagram of the SOFC/SOEC system 3 according to the third embodiment. The SOFC/SOEC system 3 of the third embodiment is different from the SOFC/SOEC system 2 of the second embodiment in that it includes an air intake section 90 and that the oxygen storage section 30 has a pressure gauge 35 and an exhaust valve 36. They differ in that they have

The SOFC/SOEC system 3 of this embodiment includes a SOFC/SOEC stack 10, an oxygen side heat exchanger 20, an oxygen storage section 30, a fuel side heat exchanger 40, a water vapor generation section 50, and a hydrogen storage section 60. , a heat storage section 80 , an air intake section 90 , a plurality of pipes connecting these, and a control section 100 .

The air intake section 90 is connected to the downstream side of the flow rate control valve 34 in a pipe 77 that connects the oxygen tank 32 and the oxygen-side heat exchanger 20. The air intake section 90 has a pipe 91 and a flow rate control valve 92. The pipe 91 has an air intake port at one end and is connected to the pipe 77 at the other end. The flow control valve 92 is disposed on the pipe 91 and adjusts the flow rate of air flowing through the pipe 91 in accordance with a command from the control unit 100. The air intake section 90 corresponds to an "air supply section" in the claims.

A pressure gauge 35 included in the oxygen storage section 30 detects the pressure of oxygen within the oxygen tank 32 . The detected oxygen pressure is output to the control section 100. The exhaust valve 36 is arranged in an exhaust pipe 37 that is connected to a pipe 78 that connects the oxygen side heat exchanger 20 and the oxygen booster 31. The exhaust valve 36 is an on-off valve that turns on and off exhaust of oxygen off gas using the exhaust pipe 37 in accordance with commands from the control unit 100. The pressure gauge 35 corresponds to a "storage amount detection section" in the claims.

FIG. 9 is a flowchart illustrating the operation of the SOFC/SOEC system 3. Next, the operation of the SOFC/SOEC system 3 of this embodiment will be explained. Here, particularly different points from the operation of the SOFC/SOEC system 2 of the second embodiment will be explained.

When the operation is performed according to the flowchart shown in FIG. 9, the SOFC/SOEC system 3 is generating power in the same SOFC mode as in the second embodiment (step S10). At this time, the control unit 100 determines whether the pressure P of oxygen in the oxygen tank 32 output by the pressure gauge 35 is equal to or higher than the oxygen lower limit pressure P _limit (step S11). Here, the oxygen lower limit pressure P _limit is, for example, 0.01 times the maximum pressure Pmax of oxygen that can be stored in the oxygen tank 32 . Note that the oxygen lower limit pressure P _limit is not limited to this. When the control unit 100 determines that the oxygen pressure P is equal to or higher than the oxygen lower limit pressure P _limit , the control unit 100 outputs a command to close the flow rate control valve 92 and the exhaust valve 36 and open the on-off valve 33 (step S12). On the other hand, if the control unit 100 determines that the oxygen pressure P is lower than the oxygen lower limit pressure P _limit , the process proceeds to step S15, which will be described later.

After step S12, the control unit 100 sets the flow rate of oxygen flowing through the pipe 77 to the flow rate control valve 34 (step S13). As a result, the oxygen side 12 of the SOFC/SOEC stack 10 is supplied with oxygen at a set flow rate from the oxygen tank 32.

Next, the control unit 100 determines whether the pressure P of oxygen in the oxygen tank 32 is lower than the oxygen lower limit pressure P _limit (step S14). When the control unit 100 determines that the oxygen pressure P is lower than the oxygen lower limit pressure P _limit , it outputs a command to close the flow rate control valve 34 and the on-off valve 33 and open the exhaust valve 36 (step S15). This stops the supply of oxygen from the oxygen tank 32 to the oxygen side 12 of the SOFC/SOEC stack 10. If it is determined that the oxygen pressure P is equal to or higher than the oxygen lower limit pressure P _limit , the process returns to step S12, and a command to close the flow rate control valve 92 and the exhaust valve 36 and open the on-off valve 33 is output.

After step S15, the control unit 100 sets the flow rate of air flowing through the pipe 91 to the flow rate control valve 92 (step S16). Thereby, oxygen contained in atmospheric air is supplied to the oxygen side 12 of the SOFC/SOEC stack 10 for power generation. At this time, the control unit 100 sets the air flow rate to the flow rate control valve 92 so that the same amount of oxygen as the oxygen flow rate set in step S13 is supplied to the oxygen side 12.

Next to step S16, the control unit 100 determines whether the SOFC/SOEC system 3 is operating in the SOEC mode (step S17). When the control unit 100 determines that the SOFC/SOEC system 3 is operating in the SOEC mode, it outputs a command to continue operating in the SOEC mode (step S18). When the control unit 100 determines that the SOFC/SOEC system 3 is not operating in the SOEC mode, the control unit 100 returns to step S10 and executes steps S10 to S16 again.

FIG. 10 is a diagram illustrating the heat balance in the SOFC/SOEC stack of this embodiment. FIG. 10(a) shows the heat balance of the SOFC/SOEC stack 10 in SOFC mode using oxygen at an output of 3.8 kWe, and FIG. 10(b) shows the heat balance of the SOFC/SOEC stack 10 using oxygen at the same output of 3.8 kWe. The heat balance of the SOFC/SOEC stack 10 in the SOFC mode used is shown.

In the SOFC mode using oxygen (see FIG. 10(a)), the amount of heat generated in the SOFC/SOEC stack 10 is 2.27 kW, as in the case of the SOFC mode in the second embodiment. The amount of heat dissipated from No. 10 is 0.52 kW. Therefore, by setting the amount of heat transferred to the heat storage section 80 (the amount of heat stored in the heat storage material 81a) to 1.75 kW, the heat balance of the entire SOFC/SOEC stack 10 is established.

In the SOFC mode using air (see FIG. 10(b)), the amount of heat generated in the SOFC/SOEC stack 10 is 2.25 kW, which is almost the same as in the SOFC mode using oxygen. However, the amount of heat dissipated from the SOFC/SOEC stack 10 increases compared to the case where only oxygen flows (the case of FIG. 10(a)), so the flow rate of the gas flowing through the oxygen side 12, that is, air, increases compared to the case where only oxygen flows (the case of FIG. 10(a)). Oxygen side ΔH" increases, and the total becomes 1.55 kW. Therefore, by setting the amount of heat transferred to the heat storage section 80 (the amount of heat stored in the heat storage material 81a) to 0.70 kW, the heat balance of the entire SOFC/SOEC stack 10 is established.

In this way, in SOFC mode, when the gas supplied to the oxygen side 12 of the SOFC/SOEC stack 10 is switched from oxygen to air, the flow rate of the gas flowing through the oxygen side 12 increases, so the amount of heat dissipated by sensible heat changes. do. Therefore, in the present embodiment, the control unit 100 controls the flow rate control valve 83 of the heat storage unit 80 to adjust the heat storage rate of the heat storage material 81a, and controls the SOFC/SOEC stack 10 in both oxygen and air. Establish an overall heat balance.

As described above, in the SOFC/SOEC system 3 of the third embodiment, the control unit 100 uses the amount of stored oxygen detected by the pressure gauge 35 to supply the oxygen storage unit 30 to the SOFC/SOEC stack 10. The oxygen storage section 30 and the air intake section 90 are controlled to switch between the supply of oxygen stored in the oxygen storage section 30 and the supply of air by the air intake section 90. Thereby, when the amount of stored oxygen is small, power generation in the cell can be continued by supplying air to the SOFC/SOEC stack 10 instead of oxygen.

<Fourth embodiment>
FIG. 11 is a schematic diagram of the SOFC/SOEC stack 10 and the heat storage section 81 of the fourth embodiment. FIG. 12 is an explanatory diagram of a stack heat balance model in the SOFC/SOEC system of this embodiment. The SOFC/SOEC system of the fourth embodiment differs from the SOFC/SOEC system 2 of the second embodiment in that the temperature of the SOFC/SOEC stack 10 is predicted.

The SOFC/SOEC system of this embodiment includes a SOFC/SOEC stack 10, an oxygen side heat exchanger 20, an oxygen storage section 30, a fuel side heat exchanger 40, a water vapor generation section 50, and a hydrogen storage section 60. , a heat storage section 80, a plurality of pipes connecting these sections, and a control section 200.

Two thermocouples TC1 and Tc2 are arranged in the heat transport section 82 of the heat storage section 80. Thermocouple TC1 and thermocouple TC2 are arranged with an interval of δ [m] as shown in FIG. 11(a). Temperatures T1 and T2 detected by thermocouples TC1 and Tc2, respectively, are output to control section 200. The two thermocouples TC1 and TC2 correspond to a "heat flux measuring section" in the claims.

In the SOFC/SOEC stack 10, as shown in FIG. 11(b), which is a cross-sectional view of the fuel-side interconnect flow path 11a and the oxygen-side interconnect flow path 12a, there are gases at the inlet and outlet of each flow path. Thermocouples Tf1, Tf2, To1, and Tf2 are arranged to measure the temperature of. The temperatures Tf_in, Tf2_out, To_in, and To_out detected by the thermocouples Tf1, Tf2, To1, and Tf2, respectively, are output to the control unit 200.

The control unit 200 of the SOFC/SOEC system of the fourth embodiment implements a stack heat balance model that calculates the heat balance between the SOFC/SOEC stack 10 and the heat storage unit 81. In the SOFC/SOEC system of the fourth embodiment, the control unit 200 uses this stack heat balance model to control the SOFC/SOEC based on the prior prediction of the heat balance between the SOFC/SOEC stack 10 and the heat storage unit 81. The temperature of the stack 10 is predicted, and the heat balance of the entire SOFC/SOEC stack 10 is established using the temperature prediction. The method will be explained below.

The following parameters are input to the control unit 200 as detected values.
・Temperatures Tf_in[K], Tf2_out[K], To_in[K], To_out[K] detected by thermocouples Tf1, Tf2, To1, and Tf2, respectively
・Temperature difference ΔT (=T1-T2) [K] between temperatures T1 and T2 detected by thermocouples TC1 and Tc2, respectively
Note that in the SOFC mode, a heat flux from the SOFC/SOEC stack 10 toward the heat storage section 81 is generated, so the temperature difference ΔT is negative; in the SOEC mode, the heat flux from the heat storage section 81 toward the SOFC/SOEC stack 10 occurs, so the temperature difference ΔT becomes positive.

Further, the following parameters are input to the control unit 200 as set values.
・Single cell voltage V [V] applied to one SOFC/SOEC cell configuring the SOFC/SOEC stack 10
・Temperature T [K] of SOFC/SOEC cell
・Inlet side gas amount mi _H2 [mol/s], mi _H2O [mol/s], mi _O2 [mol/s] flowing into the fuel side interconnect flow path 11a and the oxygen side interconnect flow path 12a
・Heat conduction section cross-sectional area Ac [m ² ] in the direction perpendicular to the heat conduction direction of the heat transport section 82
The heat conduction section cross-sectional area Ac is expressed by the following equation (3).
Ac=Wc×tc...(3)
Here, Wc [m] is the width of the heat transport section 82, and tc [m] is the thickness of the heat transport section 82 (see FIG. 11(a)).

Furthermore, the following constants are input to the control unit 200.
Power generation/electrolysis effective area of SOFC/SOEC cell Ae [cm ² ]
Faraday constant F [C/mol]
Standard temperature Tc [K]
Stack model heat capacity Ks [J/K]

The control unit 200 uses these detected values, set values, and constants to calculate (I) heat flux qc [W/m ₂ ], (II) heat amount residual Q _res [W], and (III ) The stack temperature change rate dT_dt [K/s] is calculated. The details of these calculation methods will be explained below.

(I) Heat flux qc [W/m ₂ ]
Regarding heat transfer from the SOFC/SOEC stack 10 to the heat storage section 81, as described above, the temperature difference ΔT is negative in the SOFC mode and positive in the SOEC mode, so that the thermal conductivity λc of the heat transport section 82 It is calculated by equation (4) using [W/(m·K)].
qc=λc×ΔT/δ...(4)

(II) Calorific value residual Q _res [W]
The calorific value residual Q _res is calculated by the following equation (5).
Q _res =Q _ASR +Q _IRV +ΔHf+ΔHo+Ac×qc+Q _loss (T)...(5)
Q _ASR , which is a component on the right side of equation (5), is the internal heat generation amount and is calculated by the following equation (6).
Q _ASR [W] = i ² × ASR (T) ... (6)
i in equation (6) is current density [A/cm ² ], and is expressed by equation (7) below.
i=(V-Voc(T))/ASR(T)...(7)
Here, Voc(T) is a function representing the closed circuit voltage [V] at temperature T, and ASR(T) is a function representing the stack overall resistance [Ω·cm ² ] at temperature T.

Q _IRV , which is a component on the right side of equation (5), is calculated by equation (8) below.
Q _IRV = T x ΔS (T) ... (8)
ΔS(T) in equation (8) is the amount of change in entropy at temperature T.

ΔHf and ΔHo, which are the components on the right side of equation (5), are determined using the amount of outlet side gas flowing out from the fuel side interconnect flow path 11a and the oxygen side interconnect flow path 12a. Specifically, the hydrogen outlet side gas amount mo _H2 [mol/s], the water outlet side gas amount mo _H2O [mol/s], and the oxygen outlet side gas amount mo _O2 [mol/s] are as follows. Inlet side gas amounts of hydrogen, water, and oxygen flowing into the fuel side interconnect flow path 11a and the oxygen side interconnect flow path 12a mi _H2 [mol/s], mi _H2O [mol/s], mi _O2 [mol/s] ] and the flow rate change. At this time, the flow rate change is calculated from the reaction amount of hydrogen (i x Ae/F/2 [mol/s]) and the reaction amount of oxygen (i x Ae/F/4 [mol/s]). . In each of the following equations (9), (10), and (11), the outlet side gas amount mo _H2 [mol/s] of hydrogen, the outlet side gas amount mo _H2O [mol/s] of water, and the The outlet side gas amount mo _O2 [mol/s] is shown.
mo _H2 =mi _H2 ±i×Ae/F/2 (9)
mo _H2O =mi _H2O +i×Ae/F/2...(10)
mo _O2 =mi _O2 ±i×Ae/F/4 (11)
Note that "±" shown in equations (9) and (11) becomes "+" because in SOEC mode, the amount of gas on the outlet side increases from the amount of gas on the inlet side, and in SOFC mode, the amount of gas on the outlet side increases. The gas amount is "-" because it is smaller than the gas amount on the inlet side.

Therefore, ΔHf and ΔHo can be calculated using the difference between the outlet enthalpy amount and the inlet enthalpy amount (standard temperature T ₀ [K], low pressure specific heat Cpi (T) of gas type i), and the following formula (12) is used. , (13).
ΔHf=(Tf_out−T ₀ )
× {(mi _H2 ±i×Ae/F/2)×Cp _H2 (Tf_out)
+(mi _H2O +i×Ae/F/2)×Cp _H2O (Tf_out)}
-(Tf_in-T ₀ )
×{mi _H2 ×Cp _H2 (Tf_in)+mi _H2O ×Cp _H2O (Tf_in)}
...(12)
ΔHo=(To_out−T ₀ )
×(mi _O2 ±i×Ae/F/4)×Cp _O2 (To_out)
-(To_in-T ₀ )×mi _O2 ×Cp _O2 (To_in)...(13)
Here, Cp is the specific heat [J/mol/K] for each target molecule and temperature. Furthermore, Q _loss , which is a component on the right side of equation (5), is a function representing outer heat radiation [W].

(III) Stack temperature change rate dT_dt [K/s]
The stack temperature change rate dT_dt is expressed by the following equation (14) using the heat flux qc and the heat amount residual Q _res calculated by the method described above.
dT_dt=Q _res /Ks...(14)

In the SOFC/SOEC system of this embodiment, the control unit 200 calculates the stack temperature change rate dT_dt using the method described above. Using the calculated stack temperature change rate dT_dt, the control unit 200 controls the amount of oxygen supplied by the oxygen storage unit 30 to the SOFC/SOEC stack 10 and the amount of oxygen supplied to the transformer so that the heat balance of the SOFC/SOEC stack 10 is established. 14 to adjust the value of the voltage applied to the SOFC/SOEC stack 10.

The heat balance in the SOFC/SOEC stack is determined by the internal resistance of the SOFC/SOEC stack, irreversible heat, the amount of sensible heat change on the fuel side/air side, the heat balance of heat radiation from the stack surface, the heat flow from the heat storage part, etc. In particular, when the temperature of the SOFC/SOEC stack changes, such as a change in the amount of heat supplied from the heat storage section or a change in the inflow gas temperature on the fuel side/air side, the heat balance in the SOFC/SOEC stack changes. changes from a steady state to either positive (increase in temperature) or negative (increase in temperature). Therefore, it is important to predict the rate of temperature change in the SOFC/SOEC stack when operating in SOFC mode or SOEC mode.

As explained above, in the SOFC/SOEC system 4 of the fourth embodiment, the control unit 200 calculates the temperature from the two locations measured by the two thermocouples TC1 and TC2 arranged in the heat transport unit 82. The temperature change rate dT_dt of the SOFC/SOEC stack 10 is calculated using the heat flux between the heat storage section 81 and the SOFC/SOEC stack 10. As a result, the temperature of the SOFC/SOEC stack 10 can be predicted using the calculated temperature change rate dT_dt of the SOFC/SOEC stack 10. Based on this prediction result, the heat balance of the SOFC/SOEC stack 10 is can be established. Therefore, temperature changes in the SOFC/SOEC stack 10 can be suppressed.

Further, when the heat capacity of the SOFC/SOEC stack is large and heat transfer occurs through convective heat transfer, a deviation from the zero heat balance cannot be immediately detected using a value detected by a temperature detection means such as a thermocouple. This is due to a delay in temperature response due to the heat capacity of the SOFC/SOEC stack, and there is a possibility that it will take time for the temperature change to appear in the SOFC/SOEC stack. In the SOFC/SOEC system of the fourth embodiment, the temperature change rate dT_dt of the SOFC/SOEC stack 10 is calculated using the temperature detected by thermocouples TC1, TC2, etc., so the temperature change of the SOFC/SOEC stack 10 is calculated. Can be predicted. As a result, even if the internal temperature of the SOFC/SOEC stack 10 changes or a disturbance occurs, the internal heat balance of the SOFC/SOEC stack 10 can be quickly predicted, and the internal temperature of the SOFC/SOEC stack 10 can be predicted. It can be detected faster than it can be measured.

<Fifth embodiment>
FIG. 13 is a block diagram of the SOFC/SOEC system 5 of the fifth embodiment. The SOFC/SOEC system 5 of the fifth embodiment differs from the SOFC/SOEC system 3 of the third embodiment in that it includes a bypass pipe 93 and that it includes a control unit 200 instead of the control unit 100. .

The SOFC/SOEC system 5 of this embodiment includes a SOFC/SOEC stack 10, an oxygen side heat exchanger 20, an oxygen storage section 30, a fuel side heat exchanger 40, a water vapor generation section 50, and a hydrogen storage section 60. , a heat storage section 80 , an air intake section 90 , a plurality of pipes including a bypass pipe 93 and connecting these, and a control section 200 .

One end of the bypass pipe 93 is connected to a pipe 79 that connects the outlet of the oxygen side 12 of the SOFC/SOEC stack 10 and the exhaust flow path 21 of the oxygen side heat exchanger 20. The other end of the bypass pipe 93 is connected to a pipe 78 to which the exhaust pipe 37 is connected. A flow rate control valve 94 is arranged in the bypass pipe 93 to adjust the flow rate of gas flowing through the bypass pipe 93 in accordance with a command from the control unit 200.

FIG. 14 is a flowchart illustrating the operation of the SOFC/SOEC system 5. Next, the operation of the SOFC/SOEC system 5 of this embodiment will be explained. Here, particularly different points from the operation of the SOFC/SOEC system 3 of the third embodiment will be described. The flowchart shown in FIG. 14 is always performed in the SOFC mode.

First, the stack temperature change rate dT_dt in SOFC mode is calculated (step S20). The control unit 200 calculates the stack temperature change rate dT_dt in the SOFC mode using the installed stack heat balance model (see the fourth embodiment).

Next, the control unit 200 determines whether the absolute value of the calculated stack temperature change rate dT_dt is smaller than the upper limit value dT_dt _limit of the stack temperature change rate (step S21). Here, the upper limit value dT_dt _limit of the stack temperature change rate is a value set from the ratio of the heat radiation amount to the heat capacity of the SOFC/SOEC stack 10. The upper limit value dT_dt _limit of the stack temperature change rate is, for example, when the heat radiation amount of the SOFC/SOEC stack 10 is 50 kW, and when the heat capacity of the SOFC/SOEC stack 10 is 1×10 ⁴ [J/K], dT_dt limit is dT_dt _limit . can be set as a ratio of the heat radiation amount to the heat capacity of the SOFC/SOEC stack 10, such as 5×10 ⁻³ . If the control unit 200 determines that the absolute value of the calculated stack temperature change rate dT_dt is smaller than the upper limit value dT_dt _limit , it outputs a command to close the flow rate control valve 94 and open the on-off valves 33 and 84 (step S22). On the other hand, if the control unit 200 determines that the absolute value of the calculated stack temperature change rate dT_dt is greater than or equal to the upper limit value dT_dt _limit , the process proceeds to step S25, which will be described later.

Next to step S22, the control unit 200 sets the flow rate of oxygen flowing through the pipe 77 to the flow rate control valve 34, and sets the flow rate of hydrogen gas flowing through the connecting pipe 85 to the flow rate control valve 83 (step S23). As a result, the oxygen side 12 of the SOFC/SOEC stack 10 is supplied with the amount of oxygen necessary for power generation from the oxygen tank 32, and the heat generated in the SOFC/SOEC stack 10 is transferred to the oxygen side 12 of the heat storage section 80 by the heat storage material 81a. Hydrogen gas is supplied to the heat storage section 81 so that heat is stored therein.

Next, the control unit 200 determines whether the absolute value of the calculated stack temperature change rate dT_dt is greater than or equal to the upper limit value dT_dt _limit of the stack temperature change rate (step S24). When the control unit 200 determines that the absolute value of the calculated stack temperature change rate dT_dt is greater than or equal to the upper limit value dT_dt _limit , the control unit 200 closes the exhaust valve 36, the flow rate control valve 83, and the on-off valve 84, and outputs a command to open the on-off valve 33. (Step S25). On the other hand, if the control unit 200 determines that the absolute value of the calculated stack temperature change rate dT_dt is smaller than the upper limit value dT_dt _limit , the control unit 200 returns to step S22 and outputs a command to close the flow rate control valve 94 and open the on-off valves 33 and 84. do.

Next to step S25, the control unit 200 outputs a command to the flow rate control valve 34 to change the flow rate of oxygen flowing through the pipe 77, and also outputs a command to the flow rate control valve 94 to change the flow rate of the gas flowing through the bypass pipe 93. settings (step S26). Here, changing the oxygen flow rate using the flow rate control valve 34 will be explained. In step S24, if it is determined that the absolute value of the calculated stack temperature change rate dT_dt is greater than or equal to the upper limit value dT_dt _limit , the heat of the SOFC/SOEC stack 10 cannot be stored in the heat storage section 81; temperature increases. Therefore, in step S26, by increasing the flow rate of oxygen supplied to the oxygen side 12 of the SOFC/SOEC stack 10, the heat of the SOFC/SOEC stack 10 is released by the sensible heat of oxygen. suppress the temperature rise. At this time, the control unit 200 establishes heat balance in the entire SOFC/SOEC stack 10 by setting the oxygen flow rate to the original flow rate plus the corrected oxygen flow rate Δm _O2 . The corrected oxygen flow rate Δm _O2 [mol/s] can be calculated using the following equation (15) using the stack heat balance model described in the fourth embodiment.
Δm _O2
= {Q _res +i×Ae/F/4×Cp(To_out)×(To_out−T ₀ )}
/{(To_out-T ₀ )-(To_in-T ₀ )×Cp(To_in)}
...(15)
Here, Cp(To_in) is the low-pressure specific heat of the gas at the inlet of the oxygen-side interconnect flow path 12a, and Cp(To_out) is the low-pressure specific heat of the gas at the outlet of the oxygen-side interconnect flow path 12a. The upper limit value dT_dt _limit corresponds to a "predetermined value" in the claims.

Next to step S26, the control unit 200 determines whether the SOFC/SOEC system 5 is operating in the SOEC mode (step S27). When the control unit 200 determines that the SOFC/SOEC system 5 is operating in the SOEC mode, it outputs a command to continue operating in the SOEC mode (step S28). When the control unit 200 determines that the SOFC/SOEC system 5 is not operating in the SOEC mode, the control unit 200 returns to step S20 and executes steps S20 to S26 again.

When the SOFC/SOEC system 5 is operated in the SOFC mode, the heat generated in the SOFC/SOEC stack 10 is radiated to the heat storage section 81, so that the heat balance for the entire SOFC/SOEC stack 10 is established. . However, since there is a limit to the amount of heat that the heat storage section 81 can store, if the heat storage section 81 is no longer able to store heat, such as when operating in SOFC mode for a long time, the SOFC/SOEC stack 10 cannot radiate heat. Otherwise, the temperature of the SOFC/SOEC stack 10 may rise.

As described above, in the SOFC/SOEC system 5 of the fifth embodiment, when the absolute value of the calculated temperature change rate of the SOFC/SOEC stack 10 is equal to or higher than the upper limit value dT_dt _limit in the SOFC mode, the control unit 200 controls the SOFC/SOEC system 5 to The amount of sensible heat of oxygen is increased by increasing the flow rate of oxygen supplied to the SOEC stack 10. In the SOFC mode, the fact that the absolute value of the temperature change rate of the SOFC/SOEC stack 10 is greater than or equal to the upper limit value dT_dt _limit means that the temperature rise is relatively fast because the amount of heat generated in the SOFC/SOEC stack 10 is relatively large. As shown, the amount of oxygen supplied is increased to cool the SOFC/SOEC stack 10 using the sensible heat of the oxygen discharged from the SOFC/SOEC stack 10. Thereby, rapid temperature changes in the SOFC/SOEC stack 10 can be suppressed. Therefore, even if heat storage in the heat storage section 81 is completed, temperature changes can be suppressed by predicting the heat dissipation state using the stack heat balance model and establishing the heat balance of the SOFC/SOEC stack 10 using the corrected oxygen flow rate _ΔmO2 . I can do it.

<Sixth embodiment>
FIG. 15 is a block diagram of the SOFC/SOEC system 6 of the sixth embodiment. The SOFC/SOEC system 6 of the sixth embodiment differs from the SOFC/SOEC system 1 of the first embodiment in that it includes a control section 200 instead of the control section 100.

The SOFC/SOEC system 6 of this embodiment includes a SOFC/SOEC stack 10, an oxygen side heat exchanger 20, an oxygen storage section 30, a fuel side heat exchanger 40, a water vapor generation section 50, and a hydrogen storage section 60. , a heat storage section 80 , a plurality of pipes connecting these, and a control section 200 . That is, the SOFC/SOEC system 6 of the sixth embodiment includes a control unit 200 that performs calculations using a stack heat balance model instead of the control unit 100 in the SOFC/SOEC system 2 of the second embodiment. .

FIG. 16 is a flowchart illustrating the operation of the SOFC/SOEC system 6. Next, the operation of the SOFC/SOEC system 6 of this embodiment will be explained. Here, points that are particularly different from the operation of the SOFC/SOEC system 1 of the first embodiment will be explained. The flowchart shown in FIG. 16 is always performed in SOEC mode.

First, the control unit 200 calculates the stack temperature change rate dT_dt in the SOEC mode using the installed stack heat balance model (see the fourth embodiment) (step S30). calculate.

Next, the control unit 200 determines whether the absolute value of the calculated stack temperature change rate dT_dt is smaller than the upper limit value dT_dt _limit of the stack temperature change rate (step S31). Here, the upper limit value dT_dt _limit of the stack temperature change rate is a value set from the ratio of the heat radiation amount to the heat capacity of the SOFC/SOEC stack 10, as in the fifth embodiment. If the control unit 200 determines that the absolute value of the calculated stack temperature change rate dT_dt is smaller than the upper limit value dT_dt _limit , it outputs a command to close the flow control valve 34 and open the on-off valves 33 and 84 (step S32). On the other hand, if the control unit 200 determines that the absolute value of the calculated stack temperature change rate dT_dt is greater than or equal to the upper limit value dT_dt _limit , the process proceeds to step S35, which will be described later. The upper limit value dT_dt _limit corresponds to a "predetermined value" in the claims.

Next to step S32, the control unit 200 sets the flow rate of the fuel gas sent from the flow rate distributor 73 to the oxygen side heat exchanger 20 in the flow rate control valve 74, and sets the flow rate of the fuel gas flowing through the connection pipe 85 in the flow rate control valve 83. The flow rate is set (step S33). As a result, the fuel side 11 of the SOFC/SOEC stack 10 is supplied with steam subjected to steam electrolysis, and the heat that supplements the heat absorption by steam electrolysis in the SOFC/SOEC stack 10 is transferred to the heat storage material 81a of the heat storage section 80. Water is supplied to the heat storage 81 as occurs at .

Next, the control unit 200 determines whether the absolute value of the calculated stack temperature change rate dT_dt is greater than or equal to the upper limit value dT_dt _limit of the stack temperature change rate (step S34). When the control unit 200 determines that the absolute value of the calculated stack temperature change rate dT_dt is greater than or equal to the upper limit value dT_dt _limit , it closes the flow rate control valves 34 and 83 and the on-off valve 84, and outputs a command to open the on-off valve 33 ( Step S35). On the other hand, if the control unit 200 determines that the absolute value of the calculated stack temperature change rate dT_dt is smaller than the upper limit value dT_dt _limit , the control unit 200 returns to step S32 and outputs a command to close the flow control valve 34 and open the on-off valves 33 and 84. do.

Next to step S35, the control unit 200 outputs a setting for increasing the voltage applied to the SOFC/SOEC stack 10 to the transformer 14 (step S36). Here, a change in the supply voltage to the SOFC/SOEC stack 10 by the transformer 14 will be explained. In step S34, if it is determined that the absolute value of the calculated stack temperature change rate dT_dt is equal to or greater than the upper limit value dT_dt _limit , the heat that can be supplied to the SOFC/SOEC stack 10 cannot be generated in the heat storage section 81. The temperature of stack 10 decreases. Therefore, in step S36, the supply voltage to the SOFC/SOEC stack 10 is increased to increase the resistance heat generation amount in the SOFC/SOEC stack 10, thereby suppressing the temperature rise in the SOFC/SOEC stack 10. At this time, the control unit 200 establishes heat balance in the entire SOFC/SOEC stack 10 by setting the voltage supplied to the SOFC/SOEC stack 10 to a voltage obtained by adding the correction voltage ΔV to the thermoneutral voltage. The correction voltage ΔV [V] can be calculated using the following equation (16) using the stack heat balance model described in the fourth embodiment.
ΔV=-(ASR/Qres) ^0.5 ...(16)

Next to step S36, the control unit 200 determines whether the SOFC/SOEC system 5 is operating in the SOFC mode (step S37). When the control unit 200 determines that the SOFC/SOEC system 5 is operating in the SOFC mode, it outputs a command to continue operating in the SOFC mode (step S38). When the control unit 200 determines that the SOFC/SOEC system 5 is not operating in the SOFC mode, the process returns to step S30 and calculates the stack temperature change rate.

When the SOFC/SOEC system 6 is operated in the SOEC mode, the heat generated in the heat storage section 81 is transported to the SOFC/SOEC stack 10, so that the heat balance in the entire SOFC/SOEC stack 10 is To establish. However, since there is a limit to the amount of heat that the heat storage section 81 can generate, if the heat storage section 81 is unable to supply heat, for example when the operating time in SOEC mode becomes long, the SOFC/SOEC stack 10 Temperature may drop.

As described above, in the SOFC/SOEC system 6 of the sixth embodiment, when the absolute value of the calculated temperature change rate of the SOFC/SOEC stack 10 is equal to or greater than the upper limit value dT_dt _limit in the SOEC mode, the control unit 200 controls the SOFC /By increasing the voltage applied to the SOEC stack 10, the amount of heat generated by the internal resistance of the SOFC/SOEC stack 10 is increased. In the SOEC mode, if the absolute value of the temperature change rate of the SOFC/SOEC stack 10 is greater than or equal to the upper limit value dT_dt _limit , this indicates that the temperature decrease of the SOFC/SOEC stack 10 is relatively fast. The voltage applied to the SOEC stack 10 is increased to heat the SOFC/SOEC stack 10 using heat generated by the internal resistance of the SOFC/SOEC stack 10. Thereby, rapid temperature changes in the SOFC/SOEC stack 10 can be suppressed. Therefore, even when the heat dissipation from the heat storage section 81 is completed, the heat dissipation state can be predicted using the stack heat balance model and the heat generation component can be compensated for by correcting the single cell voltage. Therefore, temperature changes can be suppressed by establishing a stack heat balance.

<Modification of this embodiment>
The present invention is not limited to the above-described embodiments, and can be implemented in various forms without departing from the spirit thereof. For example, the following modifications are also possible.

[Modification 1]
In the embodiment described above, when operating in the SOFC mode, the SOFC/SOEC stack 10 is supplied with oxygen stored in the oxygen tank 32, i.e., pure oxygen, to the oxygen side 12 of the SOFC/SOEC stack 10. did. However, in the SOFC mode of operation, pure oxygen may not be supplied to the oxygen side 12 of the SOFC/SOEC stack 10. It is sufficient that the oxygen concentration is higher than that when air is supplied. This reduces the influence of concentration resistance on the SOFC/SOEC stack 10, so power generation efficiency can be improved.

[Modification 2]
In the first embodiment, the additional voltage in the SOEC mode and the oxygen flow rate in the SOFC mode are determined from a map input to the control unit 100 in advance. However, the method of determining the additional voltage and the flow rate of oxygen is not limited to this. For example, in the case of additional voltage in SOEC mode, it may be determined using a map from the amount of water supplied to the fuel side 11. Further, the flow rate of oxygen in the SOFC mode may be determined using a map based on the magnitude of the voltage output by the SOFC/SOEC stack 10. The same applies to the control of the amount of water or hydrogen supplied to the heat storage section 81 using the flow rate control valve 83 by the control section 100 in the second embodiment.

[Modification 3]
In the second embodiment, the heat storage section 81 includes a heat storage material 81a that generates heat when Fe is oxidized by water contained in the fuel gas and absorbs heat when Fe ₃ O ₄ is reduced by hydrogen. . However, the combination of materials that perform an exothermic reaction and an endothermic reaction in the heat storage material 81a is not limited to this.

[Modification 4]
In the third embodiment, the pressure of oxygen in the oxygen tank 32 is detected by the pressure gauge 35, and the amount of oxygen stored in the oxygen tank 32 is detected. However, the "storage amount detection unit" is not limited to this. Any means that can detect the amount of oxygen stored in the oxygen tank 32 may be used.

[Modification 5]
In the fourth embodiment, an example of the stack heat balance model that calculates the stack temperature change rate dT_dt using the heat flux in the heat transport section 82 was shown, but the stack heat balance model is not limited to this.

[Modification 6]
In the fifth embodiment and the sixth embodiment, the upper limit value dT_dt _limit of the stack temperature change rate is a value set from the ratio of the heat radiation amount to the heat capacity of the SOFC/SOEC stack 10. However, the "predetermined value" is not limited to this.

[Modification 7]
In the fifth embodiment, control using the stack temperature change rate dT_dt _limit calculated by the stack heat balance model is applied to the SOFC/SOEC system 3 including the air intake section 90. It may be applied to the SOFC/SOEC system 1 that does not include the heat storage section 80 or the SOFC/SOEC system 2 that does not include the air intake section 90.

[Modification 8]
In the sixth embodiment, control using the stack temperature change rate dT_dt _limit calculated by the stack heat balance model is applied to the SOFC/SOEC system 1. It may be applied to the SOFC/SOEC system 2 including the heat storage section 80 and the SOFC/SOEC system 3 including the air intake section 90.

[Modification 9]
In the embodiments described above, the first embodiment establishes the heat balance of the SOFC/SOEC stack 10 by heat generation due to the internal resistance of the SOFC/SOEC stack 10 and an increase in the flow rate of oxygen, and the heat storage section 80 is used to improve the SOFC. /SOFC/SOEC stack by combining the second to sixth embodiments in which the heat balance of the SOFC/SOEC stack 10 is established by supplying and storing heat in response to heat absorption and heat generation in the SOEC stack 10. A heat balance of 10 may be established.

Although the present aspect has been described above based on the embodiments and modified examples, the embodiments of the above-described aspect are for facilitating understanding of the present aspect, and do not limit the present aspect. This aspect may be modified and improved without departing from the spirit and scope of the claims, and this aspect includes equivalents thereof. Furthermore, if the technical feature is not described as essential in this specification, it can be deleted as appropriate.

1, 2, 3, 5, 6...SOFC/SOEC system 10...SOFC/SOEC stack 14...Transformer 20...Oxygen side heat exchanger 30...Oxygen storage section 35...Pressure gauge 40...Fuel side heat exchanger 50...Steam Generation section 60... Hydrogen storage section 71... Heat radiator 72... Gas-liquid separator 73... Flow rate distributor 80... Heat storage section 82... Heat transport section 90... Air intake section 100, 200... Control section dT_dt... Stack temperature change rate

Claims (7)

SOFC/SOEC system,
A cell having two operating modes: a SOEC mode in which hydrogen and oxygen are generated by electrolysis of water, and an SOFC mode in which hydrogen is oxidized to generate electricity;
a voltage adjustment unit that adjusts the voltage applied to the cell;
an oxygen storage unit in which oxygen generated in the cell in SOEC mode is stored;
A control unit that switches the operation mode of the cell,
The control unit includes:
Controlling the voltage regulator so as to apply a voltage higher than a voltage value preset as a value for thermoneutral state to the cell in SOEC mode;
controlling the oxygen storage unit to supply the cell in SOFC mode with an amount of oxygen greater than a preset amount to oxidize all the hydrogen supplied to the cell;
SOFC/SOEC system. The SOFC/SOEC system according to claim 1 further comprises:
a storage amount detection unit that detects the storage amount of oxygen stored in the oxygen storage unit;
an air supply unit that supplies air to the cell,
The control unit controls the supply of oxygen stored in the oxygen storage unit to the cell and the air supply unit to supply air to the cell using the storage amount of oxygen detected by the storage amount detection unit. supply and switch between,
SOFC/SOEC system. The SOFC/SOEC system according to claim 1 or 2 further comprises:
A heat storage unit connected to the cell, storing heat generated in the cell in SOFC mode, and supplying heat to the cell in SOEC mode,
The control unit switches between storage of heat generated in the cell and supply of heat to the cell by the heat storage unit.
SOFC/SOEC system. The SOFC/SOEC system according to claim 3,
The heat storage unit switches storage of heat generated in the cell or supply of heat to the cell when hydrogen or water is supplied to the cell.
SOFC/SOEC system. The SOFC/SOEC system according to claim 3 or 4,
The heat storage unit includes a heat flux measurement unit that measures heat flux with the cell,
The control unit includes:
Calculating the rate of change in temperature of the cell using the heat flux measured by the heat flux measurement unit,
adjusting at least one of the amount of oxygen supplied to the cell by the oxygen storage unit and the voltage value applied to the cell by a voltage adjustment unit using the calculated rate of change in the cell temperature;
SOFC/SOEC system. The SOFC/SOEC system according to claim 5,
The control unit increases the flow rate of oxygen supplied to the cell when the absolute value of the rate of change of the cell temperature calculated in the SOFC mode is equal to or higher than a predetermined value.
SOFC/SOEC system. The SOFC/SOEC system according to claim 5 or 6,
The control unit increases the voltage applied to the cell when the absolute value of the rate of change of the cell temperature calculated in the SOEC mode is equal to or higher than a predetermined value.
SOFC/SOEC system.

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