Electrolytic system

Abstract

The simplified configuration allows for faster warm-up of the electrolytic cells, thereby shortening the system startup time. [Solution] The electrolytic system comprises a solid oxide type electrolytic cell, a combustion unit, a heat-insulating case housing the electrolytic cell and the combustion unit, a water vapor supply system, a fuel supply system, a power supply unit, and a warm-up control unit which controls the fuel supply system so that combustion fuel is supplied to the combustion unit when system startup is requested, and starts warming up the electrolytic cell by burning the combustion fuel in the combustion unit, controls the water vapor supply system and the power supply unit so that when the temperature of the electrolytic cell reaches a predetermined temperature before warm-up is complete, starts supplying water vapor and power to the electrolytic cell, and controls the power supply unit so that the voltage of the electrolytic cell becomes greater than the thermoneutral voltage.

Description

Technical Field

[0001] This specification discloses an electrolysis system.

Background Art

[0002] Conventionally, a reformer for reforming a hydrocarbon-based fuel, a solid oxide fuel cell that generates electricity by an electrochemical reaction between hydrogen and carbon monoxide contained in the fuel gas generated by the reformer and oxygen contained in air, a warm-up combustor that burns warm-up fuel and warm-up air to warm up the fuel cell, and an exhaust gas combustor that burns off-fuel and off-air discharged from the fuel cell to warm up the reformer have been proposed in a fuel cell system (see, for example, Patent Document 1). In this system, by providing a warm-up combustor separately from the exhaust gas combustor, the fuel cell is warmed up by the warm-up combustor when the system is started up.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In an electrolysis system including a solid oxide electrolysis cell, similar to a fuel cell system including a solid oxide fuel cell, since the operating temperature is high (for example, 650 - 800°C), it is necessary to warm up the electrolysis cell when starting up the system. And when starting up the system, it is desirable to quickly complete the warm-up of the electrolysis cell and shorten the start-up time as much as possible. However, if a warm-up combustor is provided in addition to the exhaust gas combustor as in the fuel cell system described in Patent Document 1 above, the modules for housing them become complicated and large-sized. In addition, the heat dissipation area increases due to the enlargement of the module, and there is a possibility that the thermal efficiency of the system deteriorates.

[0005] The primary purpose of this disclosure is to shorten the system startup time by completing the warm-up of the electrolytic cell early with a simple configuration. [Means for solving the problem]

[0006] This disclosure employs the following means to achieve the primary objectives described above.

[0007] The electrolytic system of this disclosure comprises: a solid oxide type electrolytic cell that generates hydrogen by electrolyzing water vapor; a combustion unit that burns a combustion fuel; a case that has thermal insulation properties and houses the electrolytic cell and the combustion unit; a water vapor supply system that supplies water vapor to the electrolytic cell; a fuel supply system that supplies combustion fuel to the combustion unit; a power supply unit that supplies power to the electrolytic cell; and a warm-up control unit that, when a system startup is requested, controls the fuel supply system so that combustion fuel is supplied to the combustion unit and burns the combustion fuel in the combustion unit to start warming up the electrolytic cell, controls the water vapor supply system and the power supply unit so that when the temperature of the electrolytic cell reaches a predetermined temperature before the warm-up is complete, starts supplying water vapor and power to the electrolytic cell, and controls the power supply unit so that the voltage of the electrolytic cell becomes greater than the thermoneutral voltage.

[0008] In the electrolytic system of this disclosure, when system startup is requested, combustion fuel is supplied to the combustion section, and the combustion fuel is burned in the combustion section to start warming up the electrolytic cell. When the temperature of the electrolytic cell reaches a predetermined temperature before warming is complete, the supply of water vapor and power to the electrolytic cell is started, and the power supply section is controlled so that the voltage of the electrolytic cell becomes greater than the thermoneutral voltage. In other words, the electrolytic cell is reacted during warming up, and the power supply section is controlled so that the heat generated by the internal resistance of the electrolytic cell exceeds the heat absorbed by the reaction of the electrolytic cell. This allows the electrolytic cell to act as a heat source and accelerate its warming up. As a result, with a simple configuration, the warming up of the electrolytic cell can be completed early, and the system startup time can be shortened. [Brief explanation of the drawing]

[0009] [Figure 1] This is a schematic diagram of the electrolytic system 10 of this embodiment. [Figure 2] This is a flowchart showing an example of the startup process. [Figure 3] This diagram illustrates the time-dependent changes in stack temperature, supply current, and cell voltage during system startup. [Modes for carrying out the invention]

[0010] Next, the forms for implementing this disclosure will be described with reference to the drawings.

[0011] Figure 1 is a schematic diagram of the electrolysis system 10 of this embodiment. The electrolysis system 10 of this embodiment includes an electrolysis module 20 including an electrolysis cell stack 21 that generates hydrogen by steam electrolysis, a steam supply system 30 that supplies steam to the electrolysis module 20, a hydrogen supply system 40 that supplies hydrogen to the electrolysis module 20, an air supply system 50 that supplies air to the electrolysis module 20, a hydrogen recovery system 60 that recovers the hydrogen generated in the electrolysis cell stack 21, a power supply device 70 that supplies the power necessary for steam electrolysis to the electrolysis cell stack 21, and a control device 80 that controls the entire system.

[0012] The electrolytic module 20 includes an electrolytic cell stack 21, a combustor 22, a fuel preheater 24, and heat exchangers 25, 26, and 27, all of which are housed in an insulated module case 28.

[0013] The electrolytic cell stack 21 comprises a plurality of solid oxide type single cells, each containing a solid electrolyte, a hydrogen electrode positioned on one side of the solid electrolyte, and an oxygen electrode positioned on the other side of the solid electrolyte. The electrolytic cell stack 21 is supplied with water vapor from the hydrogen electrode inlet and power from the power supply unit 70, which electrolyzes the water vapor to produce hydrogen at the hydrogen electrode and oxygen at the oxygen electrode. The hydrogen produced at the hydrogen electrode (generated hydrogen) is discharged from the hydrogen electrode outlet as a hydrogen electrode off-gas along with unreacted water vapor, and the oxygen produced at the oxygen electrode is discharged from the oxygen electrode outlet as an oxygen electrode off-gas along with air, which is a sweep gas supplied from the oxygen electrode inlet. A temperature sensor 71 is provided near the electrolytic cell stack 21 to detect a temperature (stack temperature Tst) correlated with the temperature of the electrolytic cell stack 21.

[0014] Since the electrolytic cell stack 21 operates in a high-temperature environment, for example, 650-800°C, the solid electrolyte, hydrogen electrode, and oxygen electrode are made of ceramic material. Furthermore, because the catalyst decomposes water vapor into oxygen ions and hydrogen, a cermet made of a catalytic metal such as nickel and ceramic is used for the hydrogen electrode. In order to maintain good catalytic activity of the hydrogen electrode, it is necessary to keep the hydrogen electrode in a reducing atmosphere and prevent oxidation of the metal. For this reason, in this embodiment, hydrogen is mixed into the water vapor supplied to the hydrogen electrode to prevent oxidation.

[0015] One end of the hydrogen electrode inlet pipe 21a is connected to the hydrogen electrode inlet of the electrolytic cell stack 21, and the other end of the hydrogen electrode inlet pipe 21a is connected to the steam supply system 30 and the hydrogen supply system 40. One end of the oxygen electrode inlet pipe 21b is connected to the oxygen electrode inlet of the electrolytic cell stack 21, and the other end of the oxygen electrode inlet pipe 21b is connected to the air supply system 50. One end of the hydrogen electrode outlet pipe 21c is connected to the hydrogen electrode outlet of the electrolytic cell stack 21, and the other end of the hydrogen electrode outlet pipe 21c is connected to the hydrogen recovery system 60. One end of the oxygen electrode outlet pipe 21d is connected to the oxygen electrode outlet of the electrolytic cell stack 21, and the other end of the oxygen electrode outlet pipe 21d is connected to the combustor 22.

[0016] The combustor 22 receives a supply of generated hydrogen (hydrogen for combustion) contained in the hydrogen electrode off-gas and the oxygen electrode off-gas as combustible gases, and burns these mixed gases. The combustor 22 is equipped with an ignition device for igniting the mixed gas and a temperature sensor for detecting the internal temperature, although these are not shown in the diagram. The electrolytic cell stack 21 is heated by the combustion heat and combustion exhaust gas generated in the combustor 22. The combustion exhaust gas discharged from the combustor 22 is heat-exchanged with water vapor flowing through the hydrogen electrode inlet pipe 21a and air flowing through the oxygen electrode inlet pipe 21b, etc., before being discharged outside the electrolytic module 20.

[0017] The steam supply system 30 generates steam from raw water (pure water) and supplies it to the electrolytic module 20. The steam supply system 30 comprises a steam generation unit 31 that evaporates raw water to generate steam, a water tank 32 that stores raw water, and a water pump 33 that supplies the raw water in the water tank 32 to the steam generation unit 31. The steam generated in the steam generation unit 31 is introduced into the hydrogen electrode inlet piping 21a, where it is heated by heat exchange with combustion heat from the combustor 22 and combustion exhaust gas in a fuel preheater 24 installed in the hydrogen electrode inlet piping 21a, and then supplied to the hydrogen electrode of the electrolytic cell stack 21.

[0018] The air supply system 50 includes an air supply pipe 51, one end of which is connected to a filter 52 and the other end of which is connected to an oxygen electrode inlet pipe 21b, and an air blower 53 installed on the air supply pipe 51. By driving the air blower 53, the air drawn into the air supply pipe 51 via the filter 52 is introduced into the oxygen electrode inlet pipe 21b, where it is heated by heat exchange with combustion exhaust gas in a heat exchanger 27 installed on the oxygen electrode inlet pipe 21b, and then heated by heat exchange with hydrogen electrode off-gas in a heat exchanger 25 before being supplied to the oxygen electrode of the electrolytic cell stack 21.

[0019] The hydrogen recovery system 60 recovers hydrogen from the hydrogen electrode off-gas containing the generated hydrogen and unreacted water vapor discharged from the hydrogen electrode outlet. It includes a hydrogen tank 61 for storing hydrogen, a hydrogen recovery pipe 63 connected to the electrolysis module 20 (hydrogen electrode outlet pipe 21c) and the hydrogen tank 61, and a condenser 62 installed in the hydrogen recovery pipe 63 for condensing the water vapor contained in the hydrogen electrode off-gas. A booster 64 is installed in the hydrogen recovery pipe 63. The hydrogen electrode off-gas containing the generated hydrogen and water vapor is heat-exchanged with cooling water in the condenser 62, causing the water vapor in the hydrogen electrode off-gas to be condensed and the generated hydrogen and condensed water to be separated into gas and liquid. Then, the generated hydrogen passes through the hydrogen recovery pipe 63, is pressurized by the booster 64, and is recovered into the hydrogen tank 61. Also, the condensed water is stored in the water tank 32. The water stored in the water tank 32 is used as raw water for generating electrolysis water vapor.

[0020] The hydrogen supply system 40 includes an antioxidant hydrogen supply pipe 41 that branches from the downstream side of the condenser 62 in the hydrogen recovery pipe 63 and is connected to the hydrogen electrode inlet pipe 21a of the electrolysis module 20, and a hydrogen blower 42 installed in the antioxidant hydrogen supply pipe 41. A part of the generated hydrogen flowing through the hydrogen recovery pipe 63 after passing through the condenser 62 is supplied as antioxidant hydrogen to the hydrogen electrode of the electrolysis cell stack 21 via the antioxidant hydrogen supply pipe 41 by driving the hydrogen blower 52. Also, the hydrogen tank 61 is connected to the antioxidant hydrogen supply pipe 41. By opening an on-off valve (not shown) installed at the outlet of the hydrogen tank 61 and driving the hydrogen blower 42, the hydrogen in the hydrogen tank 61 can be supplied to the hydrogen electrode of the electrolysis cell stack 21.

[0021] The hydrogen supply system 40 also includes a combustion hydrogen supply pipe 43 that branches from the downstream side of the condenser 62 in the hydrogen recovery pipe 63 and is connected to the combustor 22, and a hydrogen blower 44 installed in the combustion hydrogen supply pipe 43. Another part of the generated hydrogen flowing through the hydrogen recovery pipe 63 after passing through the condenser 62 is supplied as combustion hydrogen to the combustor 22 via the combustion hydrogen supply pipe 43 by driving the hydrogen blower 54.

[0022] The power supply device 70 has a power conversion unit (DC / DC converter) that converts the power input to the electrolysis system 10 from a grid power supply, a renewable energy device (e.g., a solar power generation device), a storage battery, etc. into DC power of a desired current or voltage and outputs it to the electrolytic cell stack 21. As control modes, the power supply device 70 sets a target value of the current (current target value) to be supplied to the electrolytic cell stack 21, and controls the power conversion unit so that the supply current (current detection value) of the electrolytic cell stack 21 detected by the current sensor 72 becomes the set current target value. It also has a voltage control mode in which a target value of the voltage (voltage target value) to be applied to the electrolytic cell stack 21 is set, and the power conversion unit is controlled so that the voltage (voltage detection value) of the electrolytic cell stack 21 detected by the voltage sensor 73 becomes the set voltage target value.

[0023] The control device 80 is configured as a microprocessor centered on the CPU 81. In addition to the CPU 81, it includes a ROM 82 that stores a processing program, a RAM 83 that temporarily stores data, and an input / output port (not shown). Detection signals from a flow rate sensor (not shown) that detects the flow rate of hydrogen flowing through the hydrogen supply pipe 41 for antioxidant, a flow rate sensor (not shown) that detects the flow rate of hydrogen flowing through the hydrogen supply pipe 43 for combustion, a flow rate sensor (not shown) that detects the flow rate of air flowing through the air supply pipe 51, a temperature sensor 71 installed near the electrolytic cell stack 21 that detects the temperature of the electrolytic cell stack 21 (stack temperature Tst), a current sensor 72 that detects the current (supply current I) supplied to the electrolytic cell stack 21, a voltage sensor 73 that detects the terminal voltage (stack voltage Vst) of the electrolytic cell stack 21, etc. are input through the input port. On the other hand, control signals from the control device 80 are output through the output port to the hydrogen blowers 42, 44, the air blower 53, the booster 64, the power supply device 70 (power conversion unit), etc.

[0024] Next, the operation of the electrolytic system 10 of this embodiment, as configured in this way, will be described in particular, the operation during system startup. Figure 2 is a flowchart of an example of a startup process executed by the CPU 81 of the control device 80. This process is executed when a request to start the electrolytic system 10 is received from a higher-level system.

[0025] When the startup process is executed, the CPU 81 of the control device 80 first controls the hydrogen blowers 42, 44 and the air blower 53 so that hydrogen is supplied from the hydrogen tank 61 to the combustor 22 via the hydrogen electrode and condenser 62 of the electrolytic cell stack 21, and air is supplied to the combustor 22 via the oxygen electrode of the electrolytic cell stack 21, thereby igniting the hydrogen-air mixture in the combustor 22 (S100). As a result, the electrolytic cell stack 21 is warmed up by the combustion heat and combustion exhaust gas generated by the combustion in the combustor 22. Next, the CPU 81 waits for the stack temperature Tst from the temperature sensor 71 to reach a predetermined temperature Tref or higher (S102). Here, the predetermined temperature Tref is lower than the warm-up completion temperature of the electrolytic cell stack 21 and is near the lower limit temperature required for the reaction of the electrolytic cell stack 21, for example, set to 500°C.

[0026] When the CPU 81 determines that the stack temperature Tst has risen above a predetermined temperature Tref, it controls the water vapor supply system 30 so that it starts supplying water vapor to the hydrogen electrode of the electrolytic cell stack 21 in addition to supplying hydrogen (S104), and controls the power supply unit 70 so that it starts supplying power to the electrolytic cell stack 21 (S106). The process in S106 is performed by controlling the power supply unit 70 in current control mode by gradually increasing the current target value over time so that the supply current I to the electrolytic cell stack 21 gradually increases over time. The reaction in the electrolytic cell stack 21 is started when water vapor commensurate with the power supplied to the electrolytic cell stack 21 is supplied. Then, the CPU 81 waits for the cell voltage to rise above a threshold that is greater than the thermoneutral voltage by a predetermined value α (S108). The thermoneutral voltage is the voltage at which the heat absorbed by the reaction of the electrolytic cell and the heat generated by the internal resistance of the electrolytic cell (Joule heating) are balanced, and it is known to be approximately 1.3V. The process in S108 is performed, for example, by determining whether the stack voltage Vst from the voltage sensor 73 has become greater than or equal to a threshold Vref calculated by (1.3 + α) × n, given that the number of single cells in the electrolytic cell stack 21 is n.

[0027] When the CPU 81 determines that the cell voltage has risen to a threshold value greater than the thermoneutral voltage by a predetermined value α, it performs constant voltage control in voltage control mode, setting a threshold Vref as the voltage target value to control the power supply unit 70 so that the voltage of the electrolytic cell stack 21 (stack voltage Vst) is maintained (S110). As a result, the heat generated by the internal resistance of the electrolytic cell (Joule heating) exceeds the heat absorbed by the reaction of the electrolytic cell, and the electrolytic cell stack 21 becomes a heat source, thus accelerating the warm-up of the electrolytic cell stack 21. As the warm-up of the electrolytic cell stack 21 progresses, the internal resistance of the electrolytic cell stack 21 decreases, and the supply current I to the electrolytic cell stack 21 increases due to the action of constant voltage control. The CPU 81 then waits for the supply current I detected by the current sensor 72 to become equal to or greater than the rated current Iref (S112). When the CPU 81 determines that the supplied current I is equal to or greater than the rated current Iref, it performs constant current control in current control mode to control the power supply unit 70 by setting the rated current Iref as the current target value so that the current supplied to the electrolytic cell stack 21 is maintained (S114). Then, the CPU 81 starts rated operation (S116) and finishes the startup process.

[0028] Figure 3 is an explanatory diagram showing the time changes of the stack temperature Tst, supply current I, and cell voltage during system startup. After the electrolytic cell stack 21 is warmed up by supplying hydrogen and air to the combustor 22, when the stack temperature Tst reaches or exceeds a predetermined temperature Tref before warming is complete at time t1, the supply of water vapor to the electrolytic cell stack 21 is started, and power is supplied from the power supply unit 70. This starts the reaction in the electrolytic cell stack 21. The supply current I to the electrolytic cell stack 21 is then gradually increased, and when the cell voltage reaches or exceeds a threshold value obtained by adding a predetermined value α to the thermoneutral voltage at time t2, the system switches to constant voltage control to maintain the cell voltage. Then, when the supply current I reaches or exceeds the rated current Iref at time t3, the system switches to constant current control to maintain the supply current I at the rated current Iref, and rated operation begins.

[0029] Thus, after starting the warm-up of the electrolytic cell stack 21, the electrolytic system 10 starts supplying water vapor and power to the electrolytic cell stack 21 when the stack temperature Tst exceeds a predetermined temperature Tref, thereby initiating the reaction in the electrolytic cell stack 21, and controls the supply current to the electrolytic cell stack 21 so that the cell voltage becomes greater than the thermoneutral voltage. This allows the electrolytic cell stack 21 to act as a heat source, accelerating its warm-up. As a result, with a simple configuration, the warm-up of the electrolytic cell stack 21 can be completed early, shortening the startup time of the electrolytic system 10.

[0030] Furthermore, the electrolytic system 10 gradually increases the supply current I to the electrolytic cell stack 21 until the cell voltage exceeds the thermoneutral voltage, and after the cell voltage exceeds the thermoneutral voltage, it switches to constant voltage control to maintain the cell voltage, thereby enabling the electrolytic cell stack 21 to function as a heating element more effectively.

[0031] Furthermore, after the electrolysis system 10 starts constant voltage control, it continues constant voltage control until the supply current I to the electrolytic cell stack 21 becomes equal to or greater than the rated current Iref. After the supply current I becomes equal to or greater than the rated current Iref, it switches to constant current control. This allows for a smooth transition to rated operation after the electrolytic cell stack 21 has finished warming up.

[0032] Furthermore, since the electrolysis system 10 performs both the warming up of the electrolytic cell stack 21 during system startup and temperature control of the electrolytic cell stack 21 during electrolysis operation using a single combustor 22, the electrolysis module 20 can be miniaturized. In addition, the electrolysis system 10 passes the hydrogen electrode off-gas discharged from the electrolytic cell stack 21 through the condenser 62 to condense the water vapor in the hydrogen electrode off-gas, and then supplies it to the combustor 22 as combustion hydrogen. This reduces the water vapor contained in the combustion hydrogen, thereby further improving the combustion performance in the combustor 22.

[0033] The above describes the forms for implementing this disclosure using embodiments, but this disclosure is not limited in any way to these embodiments, and can of course be implemented in various forms without departing from the gist of this disclosure. [Industrial applicability]

[0034] This disclosure can be used in industries such as the manufacturing of electrolytic systems. [Explanation of symbols]

[0035] 10 Electrolysis system, 21 Electrolytic cell stack (electrolytic cell), 22 Combustor (combustion section), 24 Fuel preheater (heat exchange section), 25, 26, 27 Heat exchanger (heat exchange section), 28 Module case (case), 30 Steam supply system, 40 Hydrogen supply system (fuel supply system), 43 Combustion hydrogen supply pipe (combustion fuel supply line), 50 Air supply system, 62 Condenser (condensing section), 70 Power supply unit (power supply section), 80 Control unit (warm-up control unit).

Claims

1. A solid oxide type electrolytic cell that generates hydrogen by electrolyzing water vapor, A combustion section for burning fuel, A case having thermal insulation properties, which houses the electrolytic cell and the combustion unit, A water vapor supply system that supplies water vapor to the electrolytic cell, A fuel supply system that supplies combustion fuel to the combustion section, A power supply unit that supplies power to the electrolytic cell, A warm-up control unit controls the fuel supply system so that combustion fuel is supplied to the combustion unit when a system startup is requested, and starts warming up the electrolytic cell by burning the combustion fuel in the combustion unit, and when the temperature of the electrolytic cell reaches a predetermined temperature before the warm-up is complete, controls the steam supply system and the power supply unit so that the supply of steam and power to the electrolytic cell is started, and controls the power supply unit so that the voltage of the electrolytic cell becomes greater than the thermoneutral voltage, An electrolytic system equipped with the following features.

2. The electrolytic system according to claim 1, The warm-up control unit, after starting to supply water vapor and power to the electrolytic cell, controls the power supply unit so that the current supplied to the electrolytic cell gradually increases until the voltage of the electrolytic cell becomes greater than the thermoneutral voltage, and after the voltage of the electrolytic cell becomes greater than the thermoneutral voltage, controls the power supply unit by constant voltage control so that the voltage of the electrolytic cell is maintained. Electrolytic system.

3. The electrolytic system according to claim 2, The warm-up control unit continues the constant voltage control after starting the constant voltage control until the current supplied to the electrolytic cell reaches the rated current, and after the current supplied to the electrolytic cell reaches the rated current, it controls the power supply unit by constant current control so that the current supplied to the electrolytic cell is maintained. Electrolytic system.

4. An electrolytic system according to any one of claims 1 to 3, A heat exchange unit is disposed within the case and performs heat exchange between the supply gas supplied to the electrolytic cell and the exhaust gas discharged from the electrolytic cell, A condensing unit is located outside the aforementioned case and condenses the water vapor in the exhaust gas that has passed through the heat exchange unit, A combustion fuel supply passage supplies at least a portion of the exhaust gas that has passed through the condensation section to the combustion section as combustion fuel, Equipped with, The fuel supply system supplies fuel to the electrolytic cell, thereby supplying the combustion to the combustion section via the heat exchange section, the condensing section, and the combustion fuel supply passage in that order. Electrolytic system.

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