Water electrolysis system

Abstract

To solve a problem that, when a water electrolysis system is shut down, pressure on a hydrogen electrode side becomes lower than pressure on an oxygen electrode side, which may deteriorate sealing integrity of a water electrolysis cell.SOLUTION: A water electrolysis system that performs water electrolysis by applying a voltage between a hydrogen electrode and an oxygen electrode of a water electrolysis cell in which an electrolyte membrane is sandwiched between the hydrogen electrode and the oxygen electrode is provided with a back pressure valve and a control part to control the back pressure valve. The back pressure valve is connected to the hydrogen electrode and controls communication between the hydrogen electrode and the atmosphere. The control part performs back pressure valve control during stoppage of water electrolysis by the water electrolysis cell, by controlling an opening degree of the back pressure valve such that pressure on the hydrogen electrode side exceeds pressure on the oxygen electrode side.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a water electrolysis system. [Background technology]

[0002] Water electrolysis systems that electrolyze water to produce hydrogen and oxygen are known (see, for example, Patent Document 1 below). In such water electrolysis systems, while the system is stopped, the exhaust valve on the hydrogen gas side is opened to exhaust the hydrogen gas from the hydrogen electrode to the outside of the system during the shutdown period to prevent hydrogen from migrating from the hydrogen electrode (cathode) to the oxygen electrode (anode) and causing undesired reactions. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-111981 Summary of the Invention [Problem to be solved by the invention]

[0004] However, repeated opening of the exhaust valve connected to the hydrogen electrode to discharge hydrogen gas from the hydrogen electrode outside the system could reduce the durability of the cell's seal structure, which sandwiches the electrolyte membrane between the hydrogen electrode and the oxygen electrode. This is because the cell's seal structure is typically designed on the assumption that the pressure at the hydrogen electrode is higher than the pressure at the oxygen electrode. When the hydrogen gas is discharged by opening the exhaust valve on the hydrogen side, the pressure at the hydrogen electrode drops, and the pressure at the hydrogen electrode becomes lower than the pressure at the oxygen electrode. Repeated occurrence of this condition could reduce the durability of the seal structure within the cell. [Means for solving the problem]

[0005] The present disclosure can be realized in the following forms or application examples.

[0006] (1) A water electrolysis system disclosed herein performs water electrolysis by applying a voltage between a hydrogen electrode and an oxygen electrode of a water electrolysis cell having an electrolyte membrane sandwiched between the hydrogen electrode and the oxygen electrode, and includes: a backpressure valve connected to the hydrogen electrode and controlling communication between the hydrogen electrode and the atmosphere; and a control unit that controls the aperture of the backpressure valve while water electrolysis by the water electrolysis cell is stopped, so that the pressure on the hydrogen electrode side exceeds the pressure on the oxygen electrode side. This controls the aperture of the backpressure valve when water electrolysis is stopped, thereby suppressing or avoiding deterioration of the sealing performance of the water electrolysis cell. (2) The above configuration may further include a pressure detection unit that detects the pressure on the hydrogen electrode side and the pressure on the oxygen electrode side. This allows the pressures on the hydrogen electrode side and the oxygen electrode side to be directly detected, thereby reliably achieving the above-described back pressure valve control. Because the amounts of hydrogen gas and oxygen gas produced can be estimated from the amount of electricity consumed by the water electrolysis cell, instead of providing a pressure detection unit, it is also possible to employ a configuration in which the pressure on the hydrogen electrode side is estimated from the amount of electricity consumed by the water electrolysis cell and the opening degree of the back pressure valve, or a configuration in which the pressure on the oxygen electrode side is estimated from the amount of electricity consumed by the water electrolysis cell and the amount of pure water supplied to the water electrolysis cell. (3) In the configuration described in (1) or (2), after the water electrolysis by the water electrolysis cell is stopped, the controller may control the back pressure valve to purge pure water present on the oxygen electrode side, and shut down the water electrolysis system when the pressure difference between the hydrogen electrode side and the oxygen electrode side reaches a predetermined shut-down threshold. This ensures that the hydrogen electrode side and the oxygen electrode side are purged when the water electrolysis system is shut down. (4) In the configurations (1) to (3) above, the control unit may be configured to start the water electrolysis cell and perform protective operation to increase the pressure on the hydrogen electrode side when the pressure difference between the hydrogen electrode side and the oxygen electrode side falls below a predetermined start-up determination threshold while the water electrolysis system is stopped. This can suppress or avoid a situation in which the pressure on the hydrogen electrode side decreases and the sealing performance of the water electrolysis cell deteriorates while the water electrolysis system is stopped. (5) In the configurations (1) to (4) above, the control unit may measure the elapsed time from the stop of water electrolysis to the execution of the protection process, and when water electrolysis is next stopped, set the magnitude of the stop judgment threshold in accordance with the elapsed time. In this way, the stop judgment threshold is set according to the elapsed time until protective operation was performed when the water electrolysis system was last stopped, and therefore, when the time until protective operation is performed is short, learning can be made to increase the stop judgment threshold to make protective operation less likely to occur. [Brief explanation of the drawings]

[0007] [Figure 1] 1 is a schematic configuration diagram of a water electrolysis system according to an embodiment. [Figure 2] FIG. 2 is an external view showing the appearance of a water electrolysis stack. [Figure 3] 1 is a schematic diagram illustrating the configuration of a fuel cell according to an embodiment. [Figure 4] 4 is a flowchart showing an example of water electrolysis control including back pressure valve control in the first embodiment. [Figure 5] 4 is a timing chart showing an example of control performed by a control unit. [Figure 6] 10 is a flowchart showing an example of control in the second embodiment. [Figure 7] 10 is a timing chart showing an example of control according to the second embodiment. [Figure 8] 10 is a flowchart showing an example of water electrolysis control including back pressure valve control in a third embodiment. [Figure 9] 10 is a timing chart showing an example of control according to the fourth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] A. First embodiment: (A1) System configuration of water splitting system: FIG. 1 is a schematic diagram of a water electrolysis system 100. As shown in the figure, the water electrolysis system 100 is mainly composed of a water electrolysis stack 30 and includes a power supply 81 that supplies the power required for water electrolysis, and a controller 82 that monitors the status of the water electrolysis cells that make up the water electrolysis stack 30 and controls the operation of the water electrolysis system 100. The hydrogen electrode side of the water electrolysis stack 30 is equipped with a hydrogen gas-liquid separator 83 connected by a pipe, a backpressure valve 86 provided in a pipe 87 that extracts hydrogen gas obtained by separating water in the hydrogen gas-liquid separator 83, and a hydrogen gas exhaust port 89 at the end of the pipe 87 through which hydrogen gas is extracted. The hydrogen electrode side of the water electrolysis stack 30 is equipped with a hydrogen outlet pressure gauge 84 that measures the hydrogen outlet pressure Pho of the water electrolysis stack 30, and a hydrogen exhaust pressure gauge 85 that measures the pressure of hydrogen gas in the hydrogen gas-liquid separator 83. The hydrogen exhaust pressure gauge 85 is not necessarily provided for the control of this embodiment.

[0009] The oxygen electrode side of the water electrolysis stack 30 is provided with a pure water supply port 98 for taking in pure water resulting from electrolysis, an oxygen gas-liquid separator 91 connected to the oxygen electrode side of the water electrolysis stack 30 via a supply path 93 and a circulation path 96, a water circulation pump 92 and an ion exchanger 94 provided on the supply path 93 for pure water from the oxygen gas-liquid separator 91 to the water electrolysis stack 30, and an oxygen gas exhaust port 99 connected to the oxygen gas-liquid separator 91 for extracting oxygen gas. The oxygen electrode side of the water electrolysis stack 30 is also provided with a water inlet pressure meter 95 for measuring the water inlet pressure Pwi of the water electrolysis stack 30 and an oxygen exhaust pressure meter 97 for measuring the pressure of oxygen gas in the oxygen gas-liquid separator 91. Of these, the hydrogen outlet pressure meter 84 for detecting the hydrogen outlet pressure Pho and the water inlet pressure meter 95 for detecting the water inlet pressure Pwi correspond to pressure detection units. The oxygen exhaust pressure meter 97 is not necessarily provided for the control of this embodiment.

[0010] As described below, the water electrolysis stack 30 has a stack structure in which multiple water electrolysis cells (hereinafter simply referred to as unit cells) 60 are stacked. Pure water, in which ions have been reduced by an ion exchanger 94, is supplied to the oxygen electrode side. When a predetermined amount of power is supplied from a power source 81, the power is used to decompose water, producing a hydrogen-containing gas containing hydrogen and moisture at the hydrogen electrode side, and an oxygen-containing gas containing oxygen and moisture at the oxygen electrode side. Here, moisture includes water vapor and water mist. The hydrogen-containing gas at the hydrogen electrode side is separated from moisture by a hydrogen-gas-liquid separator 83, and the gaseous hydrogen gas (H) is discharged from a hydrogen-gas outlet 89 via a back-pressure valve 86. The oxygen-containing gas at the oxygen electrode side is sent to an oxygen-gas-liquid separator 91 via a circulation path 96. The oxygen-gas-liquid separator 91 separates the gaseous oxygen gas (O) from moisture and discharges it from a gas outlet 99.

[0011] The control unit 82 is connected to a monitoring terminal (not shown) provided on the water electrolysis stack 30, and is also connected to a hydrogen electrode-side hydrogen outlet pressure gauge 84, a hydrogen exhaust pressure gauge 85, and a pipe 87, as well as a water electrode-side water inlet pressure gauge 95, an oxygen exhaust pressure gauge 97, and a water circulation pump 92. The control unit 82 can monitor the power output from the power supply 81 connected to the water electrolysis stack 30, such as the voltage of the power supply 81, via the monitoring terminal. The water electrolysis system 100 of this embodiment uses DC power generated by renewable energy, such as solar power or wind power, as the power supply 81. Therefore, if the amount of power generation decreases, the control unit 82 detects this and stops water electrolysis. When the amount of power generation recovers, the control unit 82 resumes water electrolysis. The control unit 82 determines whether to resume water electrolysis. However, as will be described later, even if there is no power supply from the power supply 81, protective operation may be performed to protect the water electrolysis stack 30. For this reason, the power supply 81 is equipped with a battery 81a. The battery 81a is charged when there is surplus power, and supplies power during protective operation, which will be described later. The operation of the water electrolysis system 100 performed by the control unit 82 will be described in detail later using flowcharts and timing charts.

[0012] The water electrolysis system 100 is compactly assembled around the water electrolysis stack 30. An external view of the water electrolysis system 100 is shown in Fig. 2. As shown in the figure, the water electrolysis system 100 is housed in a case 100a, and a power line from an external power supply 81 is connected to a power supply connection terminal 100b, and a battery 81a is connected to a standby power supply terminal 100c. In addition, the hydrogen gas exhaust port 89, gas exhaust port 99, and pure water supply port 98 described above are provided on a side surface 100d of the case 100a.

[0013] (A2) Structure of water electrolysis stack: The configuration of the water electrolysis stack 30, particularly the structure of the unit cell 60, will be described with reference to Figure 3. The water electrolysis stack 30 is a solid polymer water electrolysis stack that receives a supply of pure water to the oxygen electrode side and electrolyzes water to produce oxygen gas and hydrogen gas by applying a DC voltage from a power source 81 between the oxygen electrode and the hydrogen electrode. The water electrolysis stack 30 has a stack structure in which a plurality of unit cells 60, which are water electrolysis cells, are stacked one on top of the other.

[0014] To roughly explain the structure of the unit cell 60, the unit cell 60 includes a membrane electrode assembly (MEA) in which electrodes (catalytic electrodes) carrying a catalyst for promoting the water electrolysis reaction are arranged on both sides of an electrolyte membrane. Gas recovery layers for recovering the gas produced by water electrolysis are arranged on both sides of the MEA. A seal portion is integrally formed on the outer periphery of the MEGA on which the gas recovery layer is arranged to prevent leakage of the produced gas (so-called cross leak) and electrical short circuits between the catalytic electrodes. The MEGA and the seal portion are sandwiched between separators.

[0015] The single cell 60 comprises an MEGA 20 in which an anode-side gas recovery layer 22a is arranged on one side of an MEA 10 and a cathode-side gas recovery layer 22c is arranged on the other side, an anode-side separator 33a and a cathode-side separator 33c that sandwich the MEGA 20, a sealing member 40, a bonding layer 50, and a gasket 70.

[0016] The MEA 10 includes an electrolyte membrane 11, a solid polymer thin film that exhibits good proton conductivity in a wet state, and an anode catalyst layer 12a and a cathode catalyst layer 12c, each supporting a catalyst for promoting water electrolysis. As shown in FIG. 1 , the anode catalyst layer 12a is provided on one side of the electrolyte membrane 11 so that the edges of the anode catalyst layer 12a and the electrolyte membrane 11 are substantially aligned. The cathode catalyst layer 12c is provided on the other side of the electrolyte membrane 11 so that the edge of the cathode catalyst layer 12c is located inward relative to the edge of the electrolyte membrane 11. Therefore, the peripheral edge of the electrolyte membrane 11 is exposed further outward than the peripheral edge of the cathode catalyst layer 12c on the side where the cathode catalyst layer 12c is provided. The electrolyte membrane 11 can be formed, for example, of a fluorine-based ion-exchange membrane such as Nafion (registered trademark). The anode catalyst layer 12a and the cathode catalyst layer 12c can be formed by applying, to the electrolyte membrane 11, a carbon support carrying platinum (Pt) or the like and an electrolyte resin having proton conductivity.

[0017] The MEGA 20 includes an anode-side gas collection layer 22a on the side of the MEA 10 where the anode-side catalyst layer 12a is provided, and a cathode-side gas collection layer 22c on the side where the cathode-side catalyst layer 12c is provided. The anode-side gas collection layer 22a and the cathode-side gas collection layer 22c (hereinafter collectively referred to as "gas collection layers 22") are formed of a material that is gas permeable and electrically conductive. As shown in FIG. 1, the end of the anode-side gas collection layer 22a is approximately aligned with the end of the anode-side catalyst layer 12a. The end of the cathode-side gas collection layer 22c is also approximately aligned with the end of the cathode-side catalyst layer 12c. That is, MEGA 20 has a stepped structure in which the peripheral edge of the electrolyte membrane 11 is exposed outward from the cathode catalyst layer 12c and the cathode-side gas collection layer 22c on the side where the cathode catalyst layer 12c is provided, and the peripheral edges of the cathode catalyst layer 12c and the cathode-side gas collection layer 22c are located inward from the peripheral edges of the anode catalyst layer 12a and the anode-side gas collection layer 22a. The gas collection layer 22 can be made of a porous fibrous base material that is electrically conductive and gas permeable / gas diffusible, such as carbon fiber or graphite fiber.

[0018] The sealing member 40 is a member for preventing cross-leakage and electrical short circuits between catalytic electrodes. The sealing member 40 is impregnated into a bonding layer 50 (described below) formed on the peripheral edge of the electrolyte membrane 11, and is bonded to the electrolyte membrane 11 via the bonding layer 50. The sealing member 40 has a three-layer structure in which an acid-modified thermoplastic resin 42 is provided on both sides of a thermoplastic resin 41. The acid-modified thermoplastic resin 42 generally has a lower viscosity and melting point than thermoplastic resins, and has the property of high adhesiveness with other substances. In this embodiment, polypropylene is used as the thermoplastic resin 41. Furthermore, Admer (registered trademark), a modified polyolefin, is used as the acid-modified thermoplastic resin 42.

[0019] The bonding layer 50 is a member for bonding the sealing member 40 and the electrolyte membrane 11. The bonding layer 50 includes an electrolyte resin and carbon particles. The bonding layer 50 has an uneven surface (not shown) formed by the electrolyte resin and carbon particles, and the uneven surface is impregnated with the acid-modified thermoplastic resin 42 included in the sealing member 40. The bonding layer 50 includes an electrolyte resin similar to the catalyst layer 12, and is bonded to the electrolyte membrane 11 in the same way that the catalyst layer 12 is bonded to the electrolyte membrane 11. In other words, the bonding layer 50 functions to bond (adhere) the sealing member 40 and the electrolyte membrane 11. In this embodiment, DE2020 (manufactured by Dupont) is used as the electrolyte resin included in the bonding layer 50.

[0020] The anode-side separator 33a and the cathode-side separator 33c that make up the water electrolysis stack 30 are made of gas-blocking and electron-conductive materials. The anode-side separator 33a and the cathode-side separator 33c (hereinafter collectively referred to as "separators 33") sandwich the MEGA 20 and the seal member 40. The separator 33 can be made of a thin plate-like member made of press-formed stainless steel, or a metal such as titanium or a titanium alloy, or a carbon member such as dense carbon.

[0021] The oxygen gas flow channel 34a is formed between the anode-side gas recovery layer 22a and the anode-side separator 33a, and the hydrogen gas flow channel 34c is formed between the cathode-side gas recovery layer 22c and the cathode-side separator 33c. The oxygen gas flow channel 34a is a flow channel through which pure water and oxygen gas generated by electrolysis flow, and the hydrogen gas flow channel 34c is a flow channel through which hydrogen gas generated by water electrolysis and water produced on the anode-side catalyst layer 12a side flow.

[0022] In manufacturing the water electrolysis stack 30, a bonding layer 50 is formed on the periphery of the electrolyte membrane 11, and the bonding layer 50 is impregnated with the sealing member 40. The sealing member 40 is formed from a thermoplastic resin 41 and an acid-modified thermoplastic resin 42, and is impregnated into the irregularities formed by the carbon carrier of the bonding layer 50 and the general-type electrolyte resin by heat treatment. This allows the sealing member 40 to be firmly bonded to the bonding layer 50 due to an anchor effect. The sealing member 40 is bonded (adhered) to the electrolyte membrane 11 via the bonding layer 50.

[0023] The MEGA 20 thus obtained, in which the peripheral edge of the electrolyte membrane 11 is joined to the sealing member 40, is sandwiched between an anode-side separator 33a and a cathode-side separator 33c to form a unit cell 60. In the unit cell 60, the outer periphery of the MEGA 20 is sealed by the sealing member 40 that is integrally joined to the bonding layer 50 on the exposed portion of the electrolyte membrane 11. The water electrolysis stack 30 is formed by stacking these unit cells 60.

[0024] The unit cells 60 used in this embodiment, and in turn the water electrolysis stack 30 formed by stacking these cells, employ the above-described configuration, so that the sealing member 40 is preferably configured such that the pressure Pho of hydrogen gas in the hydrogen gas flow path 34c is higher than the water inlet pressure Pwi of oxygen gas in the oxygen gas flow path 34a, more precisely, the water inlet pressure Pwi of pure water containing oxygen. Pho <Pwi 3, the force applied to the sealing member 40 from the hydrogen gas flow path 34a side becomes greater than the force applied from the oxygen gas flow path 34c side, which may weaken the bond at the bonding layer 50. Therefore, it is desirable that the pressure Pho of the hydrogen gas in the hydrogen gas flow path 34c is higher than the pressure Pwi of the oxygen gas in the oxygen gas flow path 34a, or more precisely, the pressure Pwi of the pure water containing oxygen.

[0025] (A3) Control of water electrolysis: Next, the control of water electrolysis in the water electrolysis system 100 configured as described above, particularly the process for stopping water electrolysis, will be described. As described above, the water electrolysis system 100 uses renewable energy such as solar power generation, and therefore stops water electrolysis when it determines that the power supply from the power source 81 is insufficient. Such stopping of water electrolysis can be easily determined by, for example, monitoring the output voltage and outputtable current of the power source 81. When it determines that a condition for stopping water electrolysis is met, the control unit 82 activates the water electrolysis stop control process routine shown in FIG. 4. The illustrated water electrolysis stop control process routine shows control for stopping water electrolysis in the water electrolysis system 100, and this control includes backpressure valve control. Changes in various pressures and other parameters due to the water electrolysis stop control will be described below with reference to the timing chart in FIG. 5.

[0026] 5 shows the hydrogen outlet pressure Pho and the water inlet pressure Pwi in the top row. The middle row shows changes in the opening degree Vha of the back pressure valve 86, and the bottom row shows changes in the amount of pure water (w) supplied to the oxygen electrode side. As will be described in detail later, tee indicates the timing at which water electrolysis ends, tps indicates the timing at which pure water purging starts, tpe indicates the timing at which pure water purging ends, tsp indicates the timing at which the water electrolysis system 100 is stopped, tw indicates the timing at which the pure water flow rate Qw is increased toward the start of water electrolysis, and tse indicates the timing at which water electrolysis starts.

[0027] 4 is started, i.e., during water electrolysis, the water electrolysis system 100 operates in a state in which the pressure at the hydrogen electrode of the water electrolysis stack 30, i.e., the hydrogen outlet pressure Pho, is higher than the pressure at the inlet side of the oxygen electrode of the stack 30, i.e., the water inlet pressure Pwi. Therefore, pressure is applied to the sealing member 40 in a direction that maintains the seal. Furthermore, the opening Vha of the back pressure valve 86 at the hydrogen electrode is small, and a sufficient amount of pure water flow Qw required for water electrolysis is ensured at the oxygen electrode.

[0028] If the water electrolysis process cannot be continued due to some factor, such as a voltage drop in the power supply 81, the control unit 82 stops the water electrolysis process and starts the process required to shut down the water electrolysis system 100 ( FIG. 4 ). When the illustrated process routine is started, the control unit 82 switches the rotation speed of the water circulation pump 92 to a low speed required for purging the pure water, thereby reducing the pure water flow rate Qw, and increases the opening degree θ of the back-pressure valve 86 ( FIG. 5 , timing tee) (step S101). As a result, as shown in FIG. 5 , the pure water flow rate Qw becomes low, and the hydrogen outlet pressure Pho gradually decreases. Therefore, the control unit 82 monitors the differential pressure of the water electrolysis system (step S111). The differential pressure monitoring is a process of obtaining the differential pressure ΔP between the hydrogen outlet pressure Pho detected by the hydrogen outlet pressure gauge 84 and the water inlet pressure Pwi detected by the water inlet pressure gauge 95. Next, a determination is made as to whether the monitored differential pressure ΔP is greater than a predetermined first threshold Tu1 (step S121). If the determination result is ΔP>Tu1, that is, if the hydrogen outlet pressure Pho exceeds the water inlet pressure Pwi by more than the first threshold Tu1, no special action is taken, and the process returns to step S111 to repeat the process from differential pressure monitoring. The first threshold Tu1 is a value corresponding to the pressure loss caused by the pure water flow rate Qw during the pure water purge.

[0029] On the other hand, if ΔP>Tu1 is not satisfied, that is, if the pressure difference ΔP between the hydrogen outlet pressure Pho and the water inlet pressure Pwi becomes equal to or less than the first threshold value Tu1, the opening θ of the back pressure valve 86 is reduced (step S131). This timing corresponds to the timing tps of the start of the pure water purge shown in FIG. 5. As a result of reducing the opening θ of the back pressure valve 86, the decrease in the hydrogen outlet pressure Pho stops, and the hydrogen outlet pressure Pho is thereafter maintained substantially constant. In this way, the pure water purge of the oxygen electrode is started, and the process of purging pure water from the oxygen electrode continues from timing tps to timing tpe, with the hydrogen outlet pressure Pho maintained higher than the water inlet pressure Pwi. The above control is the back pressure valve control.

[0030] Next, it is determined whether the pure water purge has finished (step S141), and the process waits until the pure water purge has finished. The finish of the pure water purge may be determined by time, or may be triggered by the integrated value of the water flow rate Qw. If it is determined that the pure water purge has finished (step S141: "YES"), the water circulation pump 92 is stopped to stop the flow of pure water, and the opening θ of the back pressure valve 86 is reduced (step S151). Because the water circulation pump 92 is stopped, the water inlet pressure Pwi drops from the pressure during the purge and is maintained at atmospheric pressure. In response to this, the opening θ of the back pressure valve 86 is increased, and the hydrogen outlet pressure Pho gradually drops while being maintained higher than Pw (timings tpe to tsp).

[0031] During this time, the control unit 82 monitors the differential pressure (step S161) and determines whether the differential pressure ΔP is greater than a predetermined shutdown determination threshold Tu2 (step S171). If the determination result is ΔP>Tu2, that is, if the hydrogen outlet pressure Pho is greater than the water inlet pressure Pwi by more than the shutdown determination threshold Tu2, the control unit 82 does not take any particular action and returns to step S161 to repeat the process from the differential pressure monitoring. The shutdown determination threshold Tu2 is a differential pressure required when the water electrolysis system 100 is shut down in order to maintain the sealing performance of the stack 30 during system shutdown.

[0032] If the determination of ΔP>Tu2 is "NO," that is, if it is determined that the differential pressure has reached the value required for shutting down the water electrolysis system 100, the backpressure valve 86 is fully closed, the water electrolysis system 100 is shut down (step S181), and this processing routine is terminated. Thereafter, if water electrolysis is resumed, for example, due to recovery of power supply from the power source 81, the water circulation pump 92 is started (timing tw) with the backpressure valve 86 kept closed, pure water is supplied to the stack 30, and water electrolysis is initiated using power from the power source 81 (timing tse). Thereafter, hydrogen is generated on the hydrogen electrode side, causing an increase in pressure. Therefore, the backpressure valve 86 is appropriately opened, and hydrogen gas is exhausted to the outside through the hydrogen gas exhaust port 89. The exhausted hydrogen gas is stored in a gas cylinder or the like and used for applications that use hydrogen as an energy source, such as supply to fuel cell vehicles. The shutdown processing (step S200), connected by a dashed line from step S181 in the figure, will be described in detail later as a second embodiment.

[0033] According to the first embodiment described above, the water electrolysis system 100 maintains the pressure on the hydrogen electrode side (i.e., the hydrogen outlet pressure Pho) greater than the pressure on the oxygen electrode side (i.e., the clogged water inlet pressure Pwi) by at least the differential pressure ΔP greater than the stop-determination threshold Tu2 not only during water electrolysis but also throughout the period from when the decision to stop water electrolysis is made to shut down the water electrolysis system 100. This prevents a load from being placed on the sealing members 40 of the stack 30, thereby suppressing deterioration of the sealing performance.

[0034] B. Second embodiment: Next, a second embodiment of the water electrolysis system 100 will be described. The water electrolysis system 100 of the second embodiment has the same hardware configuration as the first embodiment, and the control performed when the water electrolysis system 100 is shut down is also the same. The second embodiment differs from the first embodiment in that a shutdown process (step S200) indicated by a dashed line in FIG. 5 is performed after step S181. An example of this shutdown process is shown in FIG. 6. This process is performed while the water electrolysis system 100 is shut down. Since it is expected that the power supply from the normal power source 81 will be insufficient while the water electrolysis system 100 is shut down, the shutdown process is performed using power from the battery 81a. To reduce power consumption from the battery 81a, the shutdown process, which will be described later, is activated and executed intermittently at predetermined time intervals, for example, every few minutes.

[0035] When the shutdown process is intermittently initiated, first, a differential pressure monitoring process is performed (step S201). As in the example described above, the differential pressure ΔP is detected, and it is determined whether this differential pressure ΔP is equal to or less than a predetermined startup determination threshold Tu3 (step S202). The startup determination threshold Tu3 is set as a value that determines whether the differential pressure ΔP between the pressure on the hydrogen electrode side in the unit cell 60, i.e., the hydrogen outlet pressure Pho, and the pressure on the oxygen electrode side, i.e., the water inlet pressure Pwi, has become small enough to affect the sealing performance within the unit cell 60. In this embodiment, the startup determination threshold Tu3 is set to approximately 0.

[0036] If the differential pressure ΔP is greater than the start-up determination threshold Tu3, nothing is done, the process exits to "NEXT", the process routine is terminated, and preparations for the next intermittent operation are made. On the other hand, if it is determined that the differential pressure ΔP is equal to or less than the start-up determination threshold Tu3 (step S202: "YES"), the process from step S211 onward is performed to protect the stack 30 while the water electrolysis system 100 is stopped. This protection process prevents the pressure on the hydrogen electrode side of each unit cell 60 constituting the stack 30 from falling below the pressure on the oxygen electrode side. Pressure changes resulting from this process are illustrated in FIG. 7, which will be referred to as appropriate for the following description.

[0037] The stack 30 of the water electrolysis system 100 may not be used for an extended period of time due to a power shortage in the power supply 81 or other reasons. During such an extended system shutdown, the pressure difference ΔP between the hydrogen electrode side pressure and the oxygen electrode side pressure may decrease and become equal to or less than the start-up determination threshold Tu3 set for detecting the lower limit of the pressure difference (timing thw in FIG. 7 ). When this is detected in step S202, the control unit 82 starts up the water electrolysis system 100 and first starts up the water circulation pump 92 to start the flow of pure water (timing tw in FIG. 7 ). As a result, as shown in FIG. 7 , the water inlet pressure Pwi increases, and water electrolysis becomes possible (timing tse). Therefore, water electrolysis is started using power from the battery 81a (step S221). Even when the power supply 81 cannot supply sufficient power, the power from the battery 81a can be used to perform water electrolysis to a degree sufficient for protective operation.

[0038] If the hydrogen outlet pressure Pho increases upon the start of water electrolysis, the opening degree θ of the backpressure valve 86 is appropriately controlled (step S231) to prevent excessive pressure on the hydrogen electrode side. The differential pressure monitoring continues (step S241) and waits until the differential pressure ΔP between the hydrogen outlet pressure Pho and the water inlet pressure Pwi becomes higher than the stop determination threshold Tu2 (step S251). If the differential pressure ΔP becomes higher than the stop determination threshold Tu2 (step S251: "YES"), water electrolysis is terminated (timing tsp), the opening degree θ of the backpressure valve 86 is reduced, and the rotation speed of the water circulation pump 92 is reduced to reduce the pure water flow rate (step S261), and pure water purging is performed (timing tps). The pure water purging continues for a predetermined time (step S271), after which the water circulation pump 92 is stopped to terminate the pure water purging (timing tpe). As a result, the water inlet pressure Pwi decreases, and the pressure difference between the water inlet pressure Pwi and the hydrogen outlet pressure Pho returns to a level favorable for the sealing of the stack 30. This means that the water electrolysis system 100 has returned to the initial state at the time when operation was normally stopped.

[0039] According to the water electrolysis system 100 of the second embodiment described above, when the operation is stopped as in the first embodiment and the operation is stopped for a long period of time, causing a decrease in pressure on the hydrogen electrode side due to gas permeation through the unit cell 60 or other reasons, the water electrolysis system 100 is temporarily started up to perform water electrolysis in the stack 30 for a short time, thereby restoring the pressure difference ΔP between the hydrogen electrode side pressure and the oxygen electrode side pressure. This temporary operation of the water electrolysis system 100 is referred to as protective operation. In the second embodiment, the protective operation can be performed to avoid or suppress a situation in which a decrease in the pressure difference ΔP adversely affects the sealing performance within the stack 30.

[0040] C. Third embodiment: Next, a water electrolysis system 100 according to a third embodiment will be described. The water electrolysis system 100 according to the third embodiment has the same hardware configuration as those according to the first and second embodiments. The water electrolysis system 100 according to the third embodiment executes a water electrolysis stop control routine shown in FIG. 8. This routine has been rewritten to execute both the same routine according to the first embodiment ( FIG. 4 ) and the stop processing routine according to the second embodiment ( FIG. 6 ). The same processing from steps S101 to S151 is abbreviated as "partial system shutdown processing." Therefore, as in the previous examples, backpressure valve control is also executed. In the third embodiment, in addition to the processing according to the first and second embodiments, step S100 is executed at the start of the water electrolysis stop processing, step S155 is executed immediately after the partial system shutdown processing shown in the figure, and step S185 is executed after step S200 ( FIG. 6 ).

[0041] First, when the water electrolysis stop control processing routine is started, the control unit 82 starts measuring time (step S100). The time is measured using a clock built into the control unit 82. The time is measured from the start of stop control of the water electrolysis system 100 to the completion of the processing in step S200. That is, in the processing in step S185 performed after the stop processing (step S200), the elapsed time Tm from the start of measurement is measured. As described in the second embodiment, the stop processing (step S200) temporarily starts the water electrolysis system 100 to protect the sealing performance of the seal member 40 of the unit cell 60 of the stack 30 when the pressure difference ΔP between the hydrogen electrode side pressure and the oxygen electrode side pressure becomes substantially zero due to gas permeation or the like during system shutdown. In the third embodiment, the elapsed time Tm from the timing of stopping the water electrolysis system 100 to the protective operation of the water electrolysis system 100 is measured.

[0042] The measured elapsed time Tm is stored in a memory (not shown) of the controller 82 and is used to set the stop determination threshold Tu2 based on the elapsed time Tm measured in the previous process in the water electrolysis stop control process routine after the water circulation pump is stopped and the aperture of the back-pressure valve 86 is increased (step S151) to stop operation of the water electrolysis system 100. That is, the water electrolysis system 100 of the third embodiment records the elapsed time Tm from the shutdown of the water electrolysis system (timing tee in FIG. 5 ) to the end of the protective operation (step S200) (timing tpe in FIG. 7 ). When water electrolysis in the water electrolysis system 100 is stopped, the stop determination threshold Tu2 used to determine the pressure difference ΔP at which to start the protective operation is set using this elapsed time Tm or the elapsed time Tm(n-1) measured previously (here, the previous time). That is, the system learns situations in which protective operation is required. Here, n indicates the order in which protective operation is performed, and "n" indicates that the order in which it is performed is the most recent.

[0043] In the third embodiment described above, the time from when the water electrolysis system 100 is stopped until the protective operation is completed is reflected in the stop determination threshold Tu2 used in the process to next stop the water electrolysis system 100. Therefore, if the differential pressure ΔP is likely to decrease while water electrolysis is stopped, the hydrogen electrode side pressure at the time of stopping in the process to stop water electrolysis, i.e., the hydrogen outlet pressure Pho, is set to a higher value. This makes it possible to learn and lengthen the time required for the pressure on the hydrogen electrode side to become equal to the pressure on the oxygen electrode side while water electrolysis is stopped. This prevents a situation in which the differential pressure ΔP is unnecessarily decreased while water electrolysis is stopped, thereby reducing the sealing performance of the stack 30. Furthermore, this makes it less likely that water electrolysis will be temporarily started while water electrolysis is stopped, thereby reducing unnecessary power consumption.

[0044] D. Fourth embodiment: Next, a fourth embodiment will be described. The water electrolysis system 100 of the fourth embodiment has the same hardware configuration as the first to third embodiments, and the control during operation shutdown is the same as that of the third embodiment. The water electrolysis system 100 of the fourth embodiment maintains sufficient sealing by maintaining the pressure on the hydrogen electrode side higher than the pressure on the oxygen electrode side, and performs similar control in an emergency. An example of such emergency control will be described using the timing chart of FIG. 9. In the fourth embodiment, while water electrolysis is being performed, the pressure on the hydrogen electrode side, i.e., the hydrogen outlet pressure Pho, is maintained higher than the water inlet pressure Pwi, as in the other embodiments. In this state, as shown in the figure, the opening θ of the backpressure valve 86 is small, and the pure water flow rate Qw required for water electrolysis flows through the oxygen electrode.

[0045] If any abnormality occurs in this state, it is determined that an emergency has occurred and the water electrolysis system 100 is shut down. In this shutdown process, the pure water flow rate Qw is throttled and the opening degree θ of the back pressure valve 86 is simultaneously increased (timing tee). As a result, both the hydrogen outlet pressure Pho and the water inlet pressure Pwi decrease. The control unit 82 cuts off the current from the power supply 81 to the stack 30. When both the hydrogen outlet pressure Pho and the water inlet pressure Pwi decrease (timing tns), purging is continued for a predetermined time (timings tns to tne). The purging performed here is pure water purging for the oxygen electrode and N2 purging for the hydrogen electrode. Nitrogen gas for N2 purging is supplied from a nitrogen gas tank (not shown) for emergency use. The purging time is set to 10 minutes in this embodiment.

[0046] After purging is complete, the water circulation pump 92 is stopped and the pure water flow rate Qw is set to 0, thereby shutting down the water electrolysis system 100. Repairs to the abnormality that caused the emergency event are then carried out. Once the repairs are complete, the water circulation pump 92 is started to circulate the pure water at the flow rate Qw, preparing for startup of the water electrolysis system 100.

[0047] In the fourth embodiment described above, when an emergency event occurs, water electrolysis is immediately stopped and the hydrogen electrode side and oxygen electrode side of the stack 30 are purged. In particular, in this example, considering that hydrogen is a flammable gas, the hydrogen electrode side is purged with N2. Therefore, although control is not performed to maintain the pressure difference between the hydrogen electrode side pressure and the oxygen electrode side pressure at a predetermined value or more, control may be performed to increase the pressure on the N2-purged hydrogen electrode side by a predetermined value higher than the pressure on the oxygen electrode side.

[0048] According to the fourth embodiment described above, if it is determined that an emergency event has occurred due to an abnormality such as a breakdown during water electrolysis, the water electrolysis system 100 can be stopped without delay.

[0049] In each of the above embodiments, some of the configurations realized by hardware may be replaced with software. At least a portion of the configurations realized by software may also be realized by a discrete circuit configuration. Furthermore, when some or all of the functions of the present disclosure are realized by software, the software (computer program) may be provided in a form stored on a computer-readable recording medium. The term "computer-readable recording medium" is not limited to portable recording media such as floppy disks and CD-ROMs, but also includes internal storage devices within a computer, such as various RAMs and ROMs, and external storage devices fixed to a computer, such as a hard disk. In other words, the term "computer-readable recording medium" has a broad meaning, including any recording medium that can fix data packets, not just temporarily.

[0050] The present disclosure is not limited to the above-described embodiments and can be realized in various configurations without departing from the spirit thereof. For example, the technical features in the embodiments corresponding to the technical features in each aspect described in the Summary of the Invention section can be appropriately replaced or combined to solve some or all of the above-described problems or achieve some or all of the above-described effects. Furthermore, if a technical feature is not described as essential in this specification, it can be appropriately deleted. [Explanation of symbols]

[0051] 11...electrolyte membrane, 12...catalyst layer, 12a...anode side catalyst layer, 12c...cathode side catalyst layer, 22...gas recovery layer, 22a...anode side gas recovery layer, 22c...cathode side gas recovery layer, 30...water electrolysis stack, 33...separator, 33a...anode side separator, 33c...cathode side separator, 34a...oxygen gas flow path, 34c...hydrogen gas flow path, 40...sealing member, 41...thermoplastic resin, 42...acid-modified thermoplastic resin, 50...bonding layer, 60...single cell, 70...gasket, 81...power source, 81a...battery, 82...control part, 83...hydrogen gas-liquid separator, 84...hydrogen outlet pressure gauge, 85...hydrogen exhaust pressure gauge, 86...back pressure valve, 87...piping, 89...hydrogen gas exhaust port, 91...oxygen gas-liquid separator, 92...water circulation pump, 93...supply line, 94...ion exchanger, 95...water inlet pressure gauge, 96...circulation line, 97...oxygen exhaust pressure gauge, 98...pure water supply port, 99...gas exhaust port, 100...water electrolysis system, 100a...case, 100b...power supply connection terminal, 100c...backup power supply terminal, 100d...side, ΔP...differential pressure Δ, Pho...hydrogen outlet pressure, Pwi...water inlet pressure, Qw...pure water flow rate

Claims

1. A water electrolysis system that performs water electrolysis by applying a voltage between a hydrogen electrode and an oxygen electrode of a water electrolysis cell having an electrolyte membrane sandwiched between the hydrogen electrode and the oxygen electrode, a back pressure valve connected to the hydrogen electrode and controlling communication between the hydrogen electrode and the atmosphere; a control unit that performs back pressure valve control by controlling an aperture of the back pressure valve while the water electrolysis by the water electrolysis cell is stopped, so that the pressure on the hydrogen electrode side exceeds the pressure on the oxygen electrode side; and A water electrolysis system equipped with

2. The water electrolysis system according to claim 1 , further comprising a pressure detector that detects the pressure on the hydrogen electrode side and the pressure on the oxygen electrode side.

3. 3. The water electrolysis system according to claim 1 or 2, wherein after the water electrolysis by the water electrolysis cell is stopped, the controller controls the back pressure valve to purge pure water present on the oxygen electrode side, and stops the water electrolysis system in a state in which a differential pressure between the hydrogen electrode side and the oxygen electrode side is maintained at a predetermined stop determination threshold.

4. 4. The water electrolysis system according to claim 3, wherein, when a pressure difference between the hydrogen electrode side pressure and the oxygen electrode side pressure falls below a predetermined start-up determination threshold while the water electrolysis by the water electrolysis cell is stopped, the controller performs a protective operation by starting the water electrolysis cell to increase the pressure on the hydrogen electrode side.

5. 5. The water electrolysis system according to claim 4, wherein the controller measures an elapsed time from the stop of the water electrolysis to the execution of the protective operation, and sets the magnitude of the stop determination threshold in accordance with the elapsed time when water electrolysis is next stopped.

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