Water electrolysis system and method for operating water electrolysis system

Abstract

A water electrolysis system (100) comprises: a plurality of water electrolysis stacks (23) that generate hydrogen gas and oxygen gas from water as product gas; a water tank (11) that supplies water to each water electrolysis stack; a hydrogen gas tank (51) that stores hydrogen gas produced in each water electrolysis stack; an oxygen gas tank (52) that stores oxygen gas produced in each water electrolysis stack; and a system control unit (80) that controls operation of each unit of the system. During hydrogen production operation, the system control unit performs voltage monitoring for each water electrolysis stack, and performs processing for temporarily changing operation conditions and recovering the electrolysis voltage with respect to a water electrolysis stack in which an increase in the electrolysis voltage is detected by the voltage monitoring.

Description

Water electrolysis system and method for operating the water electrolysis system

[0001] The present invention relates to a water electrolysis system and a method for operating the water electrolysis system.

[0002] Unlike fossil fuels, hydrogen is a clean energy source that does not emit carbon dioxide when burned. For this reason, hydrogen has attracted attention as one of the clean energy sources for combating global warming, and technologies related to the production, transportation, and use of hydrogen are being developed. One such technology is the water electrolysis system described in Patent Document 1. The water electrolysis system described in Patent Document 1 electrolyzes water to generate hydrogen gas and oxygen gas from the water by connecting multiple water electrolysis stacks in series to achieve high voltage.

[0003] International Publication No. 2023 / 012945

[0004] However, in water electrolysis systems, factors such as deterioration of the water electrolysis stack can cause gases (hydrogen gas and oxygen gas generated from water) to accumulate in the electrode cells built into the water electrolysis stack, resulting in an increase in electrolysis voltage and a decrease in electrolysis efficiency (performance). In conventional water electrolysis systems, a decrease in the electrolysis efficiency (performance) of the water electrolysis stack is addressed by replacing the water electrolysis stack. In such conventional water electrolysis systems, frequent system shutdowns due to water electrolysis stack replacement can result in a decrease in hydrogen production efficiency. Furthermore, conventional water electrolysis systems increase the replacement costs for replacing a water electrolysis stack with a decreased electrolysis efficiency (performance) with another water electrolysis stack and the disposal costs for discarding the system.

[0005] The present invention has been made to solve the above-described problems, and a main object of the present invention is to provide a water electrolysis system that performs a process to restore the electrolysis efficiency (performance) of a water electrolysis stack when the electrolysis efficiency (performance) of the water electrolysis stack has decreased, and a method for operating the water electrolysis system.

[0006] To achieve the above object, the present invention provides a water electrolysis system comprising: a plurality of water electrolysis stacks that produce hydrogen gas and oxygen gas as product gases from water; a water tank that supplies water to each water electrolysis stack; a hydrogen gas tank that stores the hydrogen gas produced in each water electrolysis stack; an oxygen gas tank that stores the oxygen gas produced in each water electrolysis stack; and a system controller that controls operation of each part of the system, wherein the system controller monitors the voltage of each water electrolysis stack during hydrogen production operation, and temporarily changes the operating conditions of any water electrolysis stack that detects an increase in electrolysis voltage through the voltage monitoring to restore the electrolysis voltage. Other means will be described below.

[0007] According to the present invention, it is possible to reduce the frequency of replacing a water electrolysis stack whose electrolysis efficiency (performance) has decreased, thereby improving the efficiency of hydrogen production.

[0008] FIG. 1 is a configuration diagram of a water electrolysis system according to an embodiment; FIG. 2 is an internal block diagram of a system control unit of a water electrolysis system according to an embodiment; FIG. 3 is an explanatory diagram of electrolysis voltage in a water electrolysis stack before pressure adjustment; FIG. 4 is an explanatory diagram of electrolysis voltage in a water electrolysis stack after pressure adjustment; and FIG. 5 is a graph of stack current values ​​and cell voltages of each water electrolysis stack. FIG. 1 is a flowchart (1) illustrating the operation of a first embodiment when a process for restoring electrolysis voltage is performed. FIG. 2 is a flowchart (2) illustrating the operation of a second embodiment when a process for restoring electrolysis voltage is performed. FIG. 3 is a flowchart (1) illustrating the operation of a third embodiment when a process for restoring electrolysis voltage is performed. FIG. 4 is a flowchart (2) illustrating the operation of a third embodiment when a process for restoring electrolysis voltage is performed. FIG. 5 is a flowchart (1) illustrating the operation of a fourth embodiment when a process for restoring electrolysis voltage is performed. FIG. 6 is a flowchart (2) illustrating the operation of a fourth embodiment when a process for restoring electrolysis voltage is performed.

[0009] Hereinafter, an embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described in detail with reference to the drawings. Note that each drawing is merely a schematic illustration to allow a sufficient understanding of the present invention. Therefore, the present invention is not limited to the illustrated examples. Furthermore, in each drawing, common or similar components are designated by the same reference numerals, and redundant explanations thereof will be omitted.

[0010] <Configuration of Water Electrolysis System> The configuration of a water electrolysis system 100 according to this embodiment will be described below with reference to Fig. 1. Fig. 1 is a configuration diagram of the water electrolysis system 100 according to this embodiment.

[0011] As shown in Figure 1, the water electrolysis system 100 according to this embodiment includes a water tank 11, multiple water electrolysis units 20, a hydrogen gas tank 51, an oxygen gas tank 52, a system control unit 80, and a data communication unit 90. Note that Figure 1 shows water electrolysis units 20a to 20n as the multiple water electrolysis units 20.

[0012] The water electrolysis system 100 performs hydrogen production operation by supplying power from a power supply unit (not shown) to each water electrolysis unit 20. Although the power supply unit and power supply lines are not shown in the example shown in Fig. 1, the multiple water electrolysis units 20 are connected in series and parallel so that a general large-scale DC power supply can be used. There is no particular limit to the number of series-parallel connections.

[0013] The water tank 11 is a storage unit for water to be supplied to the water electrolysis unit 20. The water electrolysis unit 20 is a component that electrolyzes water to produce hydrogen gas and oxygen gas. The hydrogen gas tank 51 is a storage unit for hydrogen gas produced by the water electrolysis unit 20. The oxygen gas tank 52 is a storage unit for oxygen gas produced by the water electrolysis unit 20. The system control unit 80 is a component that controls the operation of each unit of the system. The data communication unit 90 is a component that transmits and receives data to and from the outside.

[0014] In this embodiment, the water electrolysis system 100 includes a low-pressure hydrogen gas tank 51a in addition to the hydrogen gas tank 51 as a storage unit for hydrogen gas generated in the water electrolysis unit 20. The low-pressure hydrogen gas tank 51a has a lower pressure than the hydrogen gas tank 51.

[0015] The water electrolysis system 100 also includes a low-pressure oxygen gas tank 52a, in addition to the oxygen gas tank 52, as a storage unit for oxygen gas generated in the water electrolysis unit 20. The low-pressure oxygen gas tank 52a has a lower pressure than the oxygen gas tank 52.

[0016] The water tank 11 is connected to each water electrolysis unit 20 via water piping 13. In the water electrolysis system 100, water in the water tank 11 is pressurized by a water pump 12 and sent to each water electrolysis unit 20. The water piping 13 branches along the way. Water sent out from the water tank 11 travels in the direction of arrow A10, then branches along the way and travels in the directions of arrows A10a and A10b.

[0017] Water flowing in the direction of arrow A10a is cooled by the large-sized cooler 14 and then taken into the hydrogen electrode side of the water electrolysis unit 20. Water flowing in the direction of arrow A10b is cooled by the large-sized cooler 14 and then taken into the oxygen electrode side of the water electrolysis unit 20. In the example shown in Fig. 1 , the water electrolysis system 100 is configured to supply water to both the hydrogen electrode side and the oxygen electrode side. However, the water electrolysis system 100 can be configured to supply water only to the oxygen electrode side by, for example, using a proton-permeable solid polymer water electrolysis stack 23.

[0018] 1 , the water electrolysis unit 20 includes two water flow rate control valves 21 on the hydrogen electrode side and the oxygen electrode side, and two small coolers 22. The water electrolysis unit 20 also includes a water electrolysis stack 23 that electrolyzes water to produce hydrogen gas and oxygen gas.

[0019] The water electrolysis unit 20 also includes a separation tank 31, pressure regulating valves 32 and 32a, and a water discharge valve 33 as hydrogen-related components on the hydrogen system 30 side. The separation tank 31 is a component that separates water and hydrogen gas. The pressure regulating valve 32 is a valve that adjusts the pressure of hydrogen gas sent from the water electrolysis unit 20 to the hydrogen gas tank 51 via the hydrogen gas pipe 61 by adjusting its aperture. The pressure regulating valve 32a is a valve that adjusts the pressure of low-pressure hydrogen gas sent from the water electrolysis unit 20 to the low-pressure hydrogen gas tank 51a via the low-pressure hydrogen gas pipe 61a by adjusting its aperture. The water discharge valve 33 is a valve that discharges water separated in the separation tank 31 to the outside of the water electrolysis stack 23. The water discharge valve 33 has a function of adjusting the internal pressure of the water electrolysis stack 23 to a predetermined pressure by adjusting its aperture.

[0020] The water electrolysis unit 20 also includes a separation tank 41, pressure adjustment valves 42 and 42a, and a water discharge valve 43 as oxygen-related components on the oxygen system 40 side. The separation tank 41 separates water and oxygen gas. The pressure adjustment valve 42 adjusts the opening degree of the pressure adjustment valve 42 to adjust the pressure of oxygen gas sent from the water electrolysis unit 20 to the oxygen gas tank 52 via the oxygen gas pipe 62. The pressure adjustment valve 42a adjusts the opening degree of the pressure adjustment valve 42a to adjust the pressure of low-pressure oxygen gas sent from the water electrolysis unit 20 to the low-pressure oxygen gas tank 52a via the low-pressure oxygen gas pipe 62a. The water discharge valve 43 discharges water separated in the separation tank 41 to the outside of the water electrolysis stack 23. The water discharge valve 43 adjusts the opening degree of the water discharge valve 43 to adjust the internal pressure of the water electrolysis stack 23 to a predetermined pressure.

[0021] In the water electrolysis system 100, water in a water tank 11 is pressurized by a water pump 12, and the water is cooled by a large cooler 14 while being sent to each water electrolysis unit 20. Each water electrolysis unit 20 takes in water sent from the water tank 11 by opening a water flow control valve 21, cools the water in a small cooler 22, and stores the water in a water electrolysis stack 23. The water electrolysis system 100 applies a predetermined voltage to the water electrolysis stack 23 of each water electrolysis unit 20 to electrolyze water, thereby producing hydrogen gas and oxygen gas.

[0022] Each water electrolysis unit 20 sends the hydrogen gas generated by the water electrolysis stack 23 together with unelectrolyzed water from the water electrolysis stack 23 to the separation tank 31. Each water electrolysis unit 20 then separates the medium sent from the water electrolysis stack 23 into water and hydrogen gas using the separation tank 31. Each water electrolysis unit 20 opens the water discharge valve 33 to a predetermined aperture to send water to the water tank 11 via the water piping 13. Each water electrolysis unit 20 also opens the pressure adjustment valve 32 to a predetermined aperture to send hydrogen to the hydrogen gas tank 51 via the hydrogen gas piping 61. Each water electrolysis unit 20 also opens the pressure adjustment valve 32a to a predetermined aperture to send low-pressure hydrogen to the low-pressure hydrogen gas tank 51a via the low-pressure hydrogen gas piping 61a.

[0023] Each water electrolysis unit 20 also sends the oxygen gas generated by the water electrolysis stack 23, together with water that has not been electrolyzed, from the water electrolysis stack 23 to the separation tank 41. Each water electrolysis unit 20 then separates the medium sent from the water electrolysis stack 23 into water and oxygen gas using the separation tank 41. Each water electrolysis unit 20 opens the water discharge valve 43 to a predetermined aperture to send water to the water tank 11 via the water pipe 13. Each water electrolysis unit 20 also opens the pressure adjustment valve 42 to a predetermined aperture to send oxygen to the oxygen gas tank 52 via the oxygen gas pipe 62. Each water electrolysis unit 20 also opens the pressure adjustment valve 42a to a predetermined aperture to send low-pressure oxygen to the low-pressure oxygen gas tank 52a via the low-pressure oxygen gas pipe 62a.

[0024] In the water electrolysis system 100, the water separated in the separation tank 41 is adjusted to a predetermined pressure by the water discharge valve 43, recovered in the water tank 11, and then supplied again to the water electrolysis stack 23 via the water pump 12.

[0025] In the water electrolysis system 100, the low-pressure hydrogen gas produced in each water electrolysis unit 20 is adjusted to a predetermined pressure (low pressure) using the pressure regulating valve 32a and recovered in the low-pressure hydrogen gas tank 51a via the low-pressure hydrogen gas pipe 61a. In the water electrolysis system 100, the high-pressure hydrogen gas produced in each water electrolysis unit 20 is adjusted to a predetermined pressure (high pressure) using the pressure regulating valve 32 and recovered in the hydrogen gas tank 51 via the hydrogen gas pipe 61.

[0026] The water electrolysis system 100 then uses a booster pump 70 to boost the hydrogen gas G1 stored in the low-pressure hydrogen gas tank 51a and sends the boosted hydrogen gas to the hydrogen gas tank 51. The water electrolysis system 100 supplies the hydrogen gas G1 stored in the hydrogen gas tank 51 to the outside by opening a pressure adjustment valve 71. At this time, the water electrolysis system 100 adjusts the pressure of the hydrogen gas G1 to a predetermined pressure by adjusting the aperture of the pressure adjustment valve 71 to a predetermined aperture.

[0027] In the water electrolysis system 100, the low-pressure oxygen gas produced in each water electrolysis unit 20 is adjusted to a predetermined pressure (low pressure) using the pressure regulating valve 32a and recovered in the low-pressure oxygen gas tank 52a via the low-pressure oxygen gas pipe 62a. In the water electrolysis system 100, the high-pressure oxygen gas produced in each water electrolysis unit 20 is adjusted to a predetermined pressure (high pressure) using the pressure regulating valve 32 and recovered in the oxygen gas tank 52 via the oxygen gas pipe 62.

[0028] The water electrolysis system 100 then uses the boost pump 70 to boost the oxygen gas G2 stored in the low-pressure oxygen gas tank 52a and sends the boosted oxygen gas to the oxygen gas tank 52. The water electrolysis system 100 supplies the oxygen gas G2 stored in the oxygen gas tank 52 to the outside by opening the pressure adjustment valve 72. At this time, the water electrolysis system 100 adjusts the pressure of the oxygen gas G2 to a predetermined pressure by adjusting the aperture of the pressure adjustment valve 72 to a predetermined aperture.

[0029] In particular, the low-pressure hydrogen gas tank 51a, the low-pressure oxygen gas tank 52a, the low-pressure hydrogen gas pipe 61a, the low-pressure oxygen gas pipe 62a, the pressure regulating valve 32a, and the pressure regulating valve 42a are equipment specific to the water electrolysis system 100 according to this embodiment. These components are provided for the following reasons.

[0030] For example, a typical water electrolysis stack 23 has variations in characteristics and variations in electrolysis voltage due to deterioration, which causes differences in electrolysis efficiency among the water electrolysis stacks 23 even when the same current is passed through them.

[0031] Therefore, the water electrolysis system 100 according to the present embodiment measures the electrolysis voltage of each water electrolysis stack 23, and adjusts the pressure regulating valve 32 a and the pressure regulating valve 42 a to reduce the pressure of the water electrolysis stack 23 with a high electrolysis voltage. In this way, the water electrolysis system 100 reduces the voltage of the water electrolysis stack 23 with a high electrolysis voltage, thereby reducing the voltage difference with the other water electrolysis stacks. The water electrolysis system 100 can operate with increased electrolysis efficiency of the water electrolysis stacks 23.

[0032] The pressures of the hydrogen gas and oxygen gas discharged from the water electrolysis stack 23 with a reduced pressure (the water electrolysis stack 23 with a high electrolysis voltage) are lower than the pressures of the hydrogen gas and oxygen gas discharged from the water electrolysis stack 23 with a low electrolysis voltage. Therefore, the hydrogen gas and oxygen gas discharged from both water electrolysis stacks 23 cannot be passed through the same gas piping system.

[0033] Therefore, the water electrolysis system 100 according to this embodiment is provided with a low-pressure hydrogen gas pipe 61a and a low-pressure oxygen gas pipe 62a as low-pressure gas lines. The water electrolysis system 100 then sends the hydrogen gas and oxygen gas discharged from the water electrolysis stack 23 (the water electrolysis stack 23 with a high electrolysis voltage) at a reduced pressure via these low-pressure gas lines to the low-pressure hydrogen gas tank 51a and the low-pressure oxygen gas tank 52a for storage. The water electrolysis system 100 then boosts the hydrogen gas and oxygen gas stored in the low-pressure hydrogen gas tank 51a and the low-pressure oxygen gas tank 52a using a booster pump 70, and sends the gas to the hydrogen gas tank 51 and the oxygen gas tank 52 for storage.

[0034] The water electrolysis system 100 also constantly monitors the opening and closing degrees of the pressure regulating valves 32, 32a, 42, 42a to prevent backflow of the produced gases (hydrogen gas and oxygen gas) due to a pressure difference between the reduced-pressure water electrolysis stack 23 (the water electrolysis stack 23 with a high electrolysis voltage) and the other water electrolysis stacks. The water electrolysis system 100 also controls the pressure regulating valves 71, 72 and the boost pump 70 according to the situation, thereby controlling the pressures in the hydrogen gas tank 51 and the oxygen gas tank 52.

[0035] <Configuration of System Control Unit> The configuration of the system control unit 80 will be described below with reference to Fig. 2. Fig. 2 is an internal block diagram of the system control unit 80.

[0036] 2, the system control unit 80 has a calculation unit 81 and a storage unit 82. The calculation unit 81 has an operation control unit 81a, a value setting unit 81b, and a replacement stack determination unit 81c.

[0037] The operation control unit 81a is a component that controls the hydrogen production operation of the water electrolysis system 100. The value setting unit 81b is a component that sets various parameters. The stack replacement determination unit 81c is a component that determines whether the water electrolysis stack 23 needs to be replaced.

[0038] The storage unit 82 also stores reference information 82a and a control program Pr. The reference information 82a is information that is referenced when performing hydrogen production operation or when setting various parameters. The control program Pr is a program that causes the calculation unit 81 to function as an operation control unit 81a, a value setting unit 81b, and an exchange stack determination unit 81c. The control program Pr is stored in a storage medium 99 and is installed directly or indirectly from the storage medium 99 into the system control unit 80.

[0039] Data measured in the water electrolysis system 100, such as the water electrolysis stack voltage, the water electrolysis cell voltage, and the temperatures, pressures, and flow rates of various parts of the apparatus, are managed by the system control unit 80. The system control unit 80 calculates operation variables required for stable operation of the water electrolysis stack 23 based on the data and reference information 82a ( FIG. 2 ) pre-registered in a storage unit 82 ( FIG. 2 ). The system control unit 80 then transmits control signals to each device in the water electrolysis system 100, such as valves, pumps, and coolers, to control the devices.

[0040] <Pressure adjustment system of water electrolysis system and control method thereof> The pressure adjustment system of the water electrolysis system 100 and its control method will be described below with reference to Fig. 3 and Fig. 4. Fig. 3A is an explanatory diagram of the electrolysis voltage in the water electrolysis stack 23 before pressure adjustment. Fig. 3B is an explanatory diagram of the electrolysis voltage in the water electrolysis stack 23 after pressure adjustment. Fig. 4 is a graph of the stack current value and cell voltage of each water electrolysis stack 23.

[0041] Here, a pressure adjustment system and a control method for the water electrolysis system 100 will be described based on the configuration of the water electrolysis system 100 shown in Fig. 1 . However, the pressure systems connected to the gas tank are not limited to the two systems of normal pressure and low pressure shown in Fig. 1 , and it is also possible to install a single or multiple pressure systems. The configuration of the water electrolysis system 100 is not limited to the configuration shown in Fig. 1 . Furthermore, the control point for the electrolysis voltage may be not only the electrolysis voltage of the water electrolysis stack 23 but also the cell voltage.

[0042] As described above, the water electrolysis system 100 measures the electrolysis voltage of each water electrolysis stack 23, and adjusts the pressure regulating valve 32a or 42a to reduce the pressure of the water electrolysis stack 23 with a high electrolysis voltage. In this way, the water electrolysis system 100 reduces the voltage of the water electrolysis stack 23 with a high electrolysis voltage, thereby reducing the voltage difference between the water electrolysis stack 23 with a high electrolysis voltage and the other water electrolysis stacks. Such a water electrolysis system 100 can operate with increased electrolysis efficiency of the water electrolysis stacks 23.

[0043] 3A and 3B show an example of pressure adjustment in the water electrolysis stack group. Note that although FIGS. 3A and 3B show an example configuration of four water electrolysis stacks 23a to 23d, they do not represent the actual number of water electrolysis stacks 23. In other words, the number of water electrolysis stacks 23 is not limited to four.

[0044] As shown in the upper diagrams of Figures 3A and 3B, when the water electrolysis system 100 is operated (performing hydrogen production operation), it is desirable to determine an operating voltage range and a warning voltage range for the water electrolysis stack 23. The operating voltage range is a voltage range in which hydrogen can be produced stably and efficiently over a long period of time. In contrast, the warning voltage range is a voltage range in which performance degradation or damage to the water electrolysis stack 23 is expected after a predetermined period of operation. Because these voltage ranges depend on the characteristics of the water electrolysis stack 23, it is desirable to measure the characteristics of the water electrolysis stack 23 to be used and set each voltage range before constructing an actual water electrolysis system 100.

[0045] The upper diagram of FIG. 3A shows the electrolysis voltages of the water electrolysis stacks 23a to 23d during operation. The lower diagram of FIG. 3A shows the pressures (internal pressures) of the water electrolysis stacks 23a to 23d during operation. In the water electrolysis system 100, as shown in the lower diagram of FIG. 3A , when the pressures of the water electrolysis stacks 23a to 23d during operation are equalized, the electrolysis voltages of the water electrolysis stacks 23a to 23d during operation fluctuate, as shown in the lower diagram of FIG. 3A . In this case, there is a possibility that a water electrolysis stack 23 will operate at an electrolysis voltage that exceeds the operational voltage range and falls within the warning voltage range, as in the case of the water electrolysis stack 23c shown in the upper diagram of FIG. 3A . In the example shown in the upper diagram of FIG. 3A , the water electrolysis stacks 23a, 23b, and 23d are operating at electrolysis voltages within the operational voltage range, but the water electrolysis stack 23c is operating at an electrolysis voltage within the warning voltage range.

[0046] Therefore, in the present embodiment, the pressure of the hydrogen gas or oxygen gas in the water electrolysis stack 23c is reduced by adjusting the pressure adjustment valves 32a, 42a connected to the water electrolysis stack 23c. The gas pressures of both gases may be adjusted to the same pressure, or may be changed within an allowable range for the pressure difference between the hydrogen generation electrode cell (not shown) and the oxygen generation electrode cell (not shown) built into the water electrolysis stack 23c. In this manner, the water electrolysis system 100 can adjust the pressure of the hydrogen gas or oxygen gas only in the water electrolysis stack 23c. As a result, as shown in the lower diagram of FIG. 3B , although the pressures of the water electrolysis stacks 23a to 23d during operation fluctuate slightly, as shown in the upper diagram of FIG. 3B , the electrolysis voltage of the water electrolysis stack 23c can be adjusted to drop from the warning voltage range to the operational voltage range while maintaining the voltages of the water electrolysis stacks 23a, 23b, and 23d.

[0047] FIG. 4 shows the characteristics of the stack current value (A) and the cell voltage value (V) before and after the pressure adjustment of the water electrolysis stacks 23a to 23d shown in FIGS. 3A and 3B.

[0048] 4, if the current value applied to the water electrolysis stacks 23a to 23d is A1 (amperes) and the voltage of the water electrolysis stacks 23a, 23b, and 23d at that time is V1 (volts), the water electrolysis stack 23c has a voltage of V2 (volts). It is predicted that the electrolysis voltage of the water electrolysis stack 23c is higher than that of the other water electrolysis stacks because the water electrolysis stack 23c has a high resistance.

[0049] Since the current flowing through the water electrolysis stacks 23a to 23d is constant at A1 (amperes), it is presumed that the water electrolysis stack 23c, which has a high resistance, has a higher temperature than the other water electrolysis stacks.

[0050] Although this embodiment shows an example of pressure adjustment by voltage measurement, it is preferable to also measure the temperature of each water electrolysis stack 23 in order to achieve more accurate pressure adjustment.

[0051] Furthermore, the voltage of the water electrolysis stack 23c after the pressure adjustment is predicted to drop to V2+ΔV (volts). Here, ΔV (volts) varies depending on the gas pressure difference before and after the pressure adjustment and is proportional to this pressure difference. The control program Pr stored in the storage unit 82 of the water electrolysis system 100 includes, as part of the control program Pr, a calculation algorithm for calculating the electrolysis voltage at the target pressure of each water electrolysis stack 23 when pressure adjustment of the individual water electrolysis stack 23 is required. The control program Pr stored in the storage unit 82 of the water electrolysis system 100 also includes, as part of the control program Pr, a control algorithm for controlling the pressure adjustment valves 32, 32a, 42, and 42a.

[0052] The water electrolysis system 100 controls the water flow control valve 21 and the small cooler 22 to cool the water sent from the water tank 11 to the water electrolysis stack 23 c and store the cooled water in the water electrolysis stack 23 c. In this way, the water electrolysis system 100 cools the water electrolysis stack 23 c. As a result, in the water electrolysis system 100, the amount of heat generated decreases due to a decrease in resistance caused by pressure adjustment, and the temperature of the water electrolysis stack 23 decreases. Therefore, it is desirable to adjust the amount of heat removed by these measures.

[0053] The amount of heat generated by the water electrolysis stack 23 is calculated by multiplying the current value by the electrolysis overvoltage, i.e., the value obtained by subtracting the theoretical electrolysis voltage from the electrolysis voltage. In the water electrolysis stack 23c, the amount of heat generated decreases by "A1 × ΔV" from "A1 × (V2 - theoretical electrolysis voltage)" to "A1 × (V2 + ΔV - theoretical electrolysis voltage)" by adjusting the pressure.

[0054] The water electrolysis system 100 adjusts the pressure of each water electrolysis stack 23. The pressure of each water electrolysis stack 23 may be adjusted multiple times. If the pressure of a water electrolysis stack 23 falls within the warning pressure range, the water electrolysis stack 23 can continue to operate without reducing the voltage efficiency or the amount of hydrogen produced by adjusting the pressure using a similar process.

[0055] While the setting of the operating temperature range and the warning temperature range when adjusting the pressure has been described above, it is desirable to set the operating temperature range and the warning temperature range when adjusting the temperature in the same way. For the water electrolysis stack 23 operating at an electrolysis voltage within the warning temperature range, the temperature of the water electrolysis stack 23 can be controlled to be within the operating temperature range by increasing the supply water flow rate.

[0056] Furthermore, during normal operation, the water electrolysis system 100 can control the electrolysis voltage of the water electrolysis stack 23 by adjusting the pressure regulating valves 32a, 42a. However, even when the pressure regulating valves 32a, 42a are adjusted, the electrolysis voltage of the water electrolysis stack 23 may not decrease from the caution voltage range to the operating voltage range. For example, when gas generated in many electrode cells of the water electrolysis stack 23 accumulates and inhibits electrode reactions, or when deterioration of the electrode cells progresses, causing a significant imbalance in the reaction distribution within the electrode surfaces of the electrode cells, adjustment of the pressure regulating valves 32a, 42a may not be enough to reduce the electrolysis voltage of the water electrolysis stack 23 from the caution voltage range to the operating voltage range.

[0057] Therefore, the water electrolysis system 100 performs an operation to change the pressure of the produced gas and the flow rate of water supplied to the water electrolysis stack 23 for the water electrolysis stack 23 in which the electrolysis voltage cannot be reduced from the warning voltage range to the operational voltage range. This change operation may be performed multiple times. Furthermore, there is no restriction on the order in which the change operations are performed.

[0058] By performing this changing operation, the water electrolysis system 100 can remove gas remaining in the electrode cells (not shown) of the water electrolysis stack 23 and change the reaction distribution within the electrode surfaces of the electrode cells (not shown). As a result, the water electrolysis system 100 can reduce the electrolysis voltage of the water electrolysis stack 23 from the warning voltage range to the operating voltage range. As a result, the water electrolysis system 100 can restore the electrolysis voltage of the water electrolysis stack 23. Then, the water electrolysis system 100 can operate the water electrolysis stack 23 by restoring the operating conditions of the water electrolysis stack 23 to the initial pressure, temperature, and flow rate.

[0059] Furthermore, the water electrolysis system 100 can change the parameters used as operating conditions to arbitrary operating values ​​for a water electrolysis stack 23 that is unable to restore its electrolysis voltage despite having performed a process to restore the electrolysis voltage of the water electrolysis stack 23. The parameters used as operating conditions may be, for example, any one of the pressure of the produced gas (hydrogen gas and oxygen gas), the flow rate of water supplied to the water electrolysis stack 23, and the temperature of the water electrolysis stack 23, or a combination of multiple parameters. The parameters used as operating conditions are not the initial operating conditions of the water electrolysis stack 23, such as the pressure, temperature, and flow rate. The water electrolysis system 100 includes, as part of the control program Pr stored in the storage unit 82, a control algorithm for changing the parameters used as operating conditions to arbitrary operating values. As a result, the water electrolysis system 100 changes the parameters to arbitrary operating values, resets the operating conditions, and reduces the electrolysis voltage of the water electrolysis stack 23 from the warning voltage range to the operational voltage range.

[0060] The water electrolysis system 100 also includes a replacement stack determination unit 81c that determines whether to replace the water electrolysis stack 23 when the electrolysis voltage of the water electrolysis stack 23 cannot be restored despite the process of restoring the electrolysis voltage of the water electrolysis stack 23 having been performed.

[0061] <Operation of Water Electrolysis System> During hydrogen production operation, the water electrolysis system 100 monitors the voltage of each water electrolysis stack 23, and for any water electrolysis stack 23 for which an increase in electrolysis voltage is detected by the voltage monitoring (hereinafter sometimes referred to as the "relevant stack"), temporarily changes the operating conditions to restore the electrolysis voltage. This operation is mainly performed by the system control unit 80 of the water electrolysis system 100.

[0062] Here, "temporarily" means that the hydrogen production operation ("operation in default state" in step S105 shown in FIG. 5A, etc.) is interrupted, a process for recovering the electrolysis voltage is performed, and then the hydrogen production operation (step S105 shown in FIG. 5A, etc.) is resumed. Also, "temporarily changing the operating conditions" means, for example, in the examples shown in FIGS. 5A and 5B, the changes made between steps S125 and S185 (steps S125, S135, S155).

[0063] The process of recovering the electrolysis voltage of the corresponding stack is performed to remove gas accumulated in the electrode cells (not shown) built into the corresponding stack. The removed gas is discharged from the corresponding stack to the outside (hydrogen gas tank 51, low-pressure hydrogen gas tank 51a, oxygen gas tank 52, low-pressure oxygen gas tank 52a) via separation tank 31 or separation tank 41.

[0064] Here, the process of restoring the electrolysis voltage of the corresponding stack will be described so as to be clear. In the process of restoring the electrolysis voltage of the corresponding stack, any one parameter or a combination of parameters selected from the pressure of the produced gas (hydrogen gas and oxygen gas), the flow rate of water supplied to the water electrolysis stack 23, and the temperature of the water electrolysis stack 23 is changed to an arbitrary manipulated value, thereby lowering the electrolysis voltage to the operating voltage range. Here, first to fourth examples will be described with reference to Figures 5A to 8B as examples of the process of restoring the electrolysis voltage of the corresponding stack.

[0065] First Example Hereinafter, with reference to Figures 5A and 5B , the operation of a first example of the water electrolysis system 100 when performing a process to restore the electrolysis voltage of a corresponding stack will be described. Figures 5A and 5B are flowcharts illustrating the operation of the first example when performing a process to restore the electrolysis voltage. In the first example, the pressure of the generated gas is adjusted to perform a process to restore the electrolysis voltage of a corresponding stack.

[0066] The flows shown in Figures 5A and 5B are created assuming that the water electrolysis stack 23c shown in Figures 3A to 4 is the water electrolysis stack 23 that performs the process of restoring the electrolysis voltage. In an actual flow, if there is a water electrolysis stack 23 that performs the process of restoring the electrolysis voltage, it is preferable to perform the process of restoring the electrolysis voltage on that water electrolysis stack 23.

[0067] 5A, first, the water electrolysis system 100 performs a hydrogen production operation in a default state under the control of the operation control unit 81a (FIG. 2) (step S105). The hydrogen production operation in the default state is performed based on initial parameters for the hydrogen production operation that are pre-stored in the storage unit 82 (FIG. 2).

[0068] After step S105, the water electrolysis system 100 detects, via the operation control unit 81a ( FIG. 2 ), the presence of a water electrolysis stack (relevant stack) whose electrolysis voltage has risen to the warning voltage range (step S110). In this case, the relevant stack is the water electrolysis stack 23c shown in the upper diagram of FIG. 3A .

[0069] After step S110, the water electrolysis system 100 sets, using the value setting unit 81b ( FIG. 2 ), a target voltage for lowering the electrolysis voltage of the corresponding stack (water electrolysis stack 23c) to within the operating voltage range (step S115). At this time, the water electrolysis system 100 calculates an electrolysis voltage (V2+ΔV) volts ( FIG. 4 ) as the target voltage for adjusting the internal pressure of the corresponding stack (water electrolysis stack 23c) based on the electrolysis characteristics (pressure dependency) of the corresponding stack (water electrolysis stack 23c).

[0070] After step S115, the water electrolysis system 100 causes the value setting unit 81b ( FIG. 2 ) to determine the target pressure of the produced gas for the corresponding stack (water electrolysis stack 23c) as a value calculated from the operating pressure characteristics of the corresponding stack (water electrolysis stack 23c) (step S120). The target pressure determined at this time is lower than the initial pressure (the pressure at step S105). In the first embodiment, the target voltage set in step S115 and the target pressure determined in step S120 are used as operating conditions for performing a process to restore the electrolysis voltage of the corresponding stack.

[0071] After step S120, the water electrolysis system 100 controls the value setting unit 81b ( FIG. 2 ) to reduce the pressure of the product gas for the corresponding stack (water electrolysis stack 23c) to the target pressure, and the operation control unit 81a ( FIG. 2 ) performs hydrogen production operation (step S125). At this time, the water electrolysis system 100 adjusts the apertures of the pressure adjustment valves 32a and 42a connected to the water electrolysis stack 23c to reduce the pressures of the hydrogen gas and oxygen gas product from the water electrolysis stack 23c to the target pressure. The pressures of the hydrogen gas and oxygen gas may be adjusted to have similar pressure values, or may be adjusted to have different pressure values ​​within an allowable range for the pressure difference between the hydrogen generating electrode and the oxygen generating electrode.

[0072] The process of step S125 is a process for recovering the electrolysis voltage of the corresponding stack for the first time. In this embodiment, the process for recovering the electrolysis voltage of the corresponding stack for the first time will be described as being performed regardless of the degree of deterioration of the electrode cells (not shown) built into the corresponding stack.

[0073] After step S125, the water electrolysis system 100 determines, via the operation control unit 81a ( FIG. 2 ), whether the corresponding stack (water electrolysis stack 23c) is operating within the operational voltage range (step S130). The determination in step S130 is equivalent to determining whether the electrolysis voltage of the corresponding stack (water electrolysis stack 23c) has dropped from the warning voltage range to the operational voltage range, as shown in the upper diagrams of FIGS. 3A and 3B .

[0074] If it is determined in step S130 that the corresponding stack (water electrolysis stack 23c) is operating within the operating voltage range (if "Yes"), the process proceeds to step S185. In this case, the water electrolysis system 100 causes the value setting unit 81b ( FIG. 2 ) to restore the generated gas pressure, which was changed (reduced) in step S125, to the original state (initial pressure state) for the corresponding stack (water electrolysis stack 23c) (step S185). Thereafter, the process returns to step S105. Then, in step S105, the water electrolysis system 100 performs the hydrogen production operation again under the default state. In other words, if it is determined in step S130 that the corresponding stack (water electrolysis stack 23c) is operating within the operating voltage range (if "Yes"), the water electrolysis system 100 determines that the process of restoring the electrolysis voltage of the corresponding stack has been completed. The water electrolysis system 100 then terminates the process of restoring the electrolysis voltage of the corresponding stack, performs a pressure change operation to return the pressure of the generated gas that was changed (reduced) in step S125 to its original state (initial pressure state), and resumes hydrogen production operation in the default state.

[0075] On the other hand, if it is determined in step S130 that the corresponding stack (water electrolysis stack 23c) is not operating within the operating voltage range ("No"), the process proceeds to step S135. In this case, the water electrolysis system 100 causes the value setting unit 81b (FIG. 2) to change the generated gas pressure for the corresponding stack (water electrolysis stack 23c) to a value higher than the initial pressure, and the operation control unit 81a (FIG. 2) performs hydrogen production operation (step S135). The change in the generated gas pressure (the change to increase the pressure) can be performed multiple times.

[0076] After step S135, the water electrolysis system 100 determines, by the operation control unit 81a ( FIG. 2 ), whether the corresponding stack (water electrolysis stack 23c) is operating within the operating voltage range (step S140), similarly to step S130.

[0077] If it is determined in step S140 that the corresponding stack (water electrolysis stack 23c) is operating within the operating voltage range (if "Yes"), the process proceeds to step S185. Thereafter, the process returns to step S105. That is, if it is determined in step S140 that the corresponding stack (water electrolysis stack 23c) is operating within the operating voltage range (if "Yes"), the water electrolysis system 100 determines that the process of restoring the electrolysis voltage of the corresponding stack has been completed. The water electrolysis system 100 then terminates the process of restoring the electrolysis voltage of the corresponding stack, performs a pressure change operation to return the generated gas pressure changed (reduced) in step S135 to the original state (initial pressure state), and resumes hydrogen production operation in the default state.

[0078] On the other hand, if it is determined in step S140 that the corresponding stack (water electrolysis stack 23c) is not operating within the operating voltage range ("No"), the process proceeds to step S145 shown in Fig. 5B via reference sign "A11." In this case, the operation control unit 81a (Fig. 2) of the water electrolysis system 100 determines that the electrode cells (not shown) built into the corresponding stack (water electrolysis stack 23c) may be deteriorated (step S145 in Fig. 5B).

[0079] After step S145, the water electrolysis system 100, as in step S120, determines the target pressure of the produced gas for the corresponding stack (water electrolysis stack 23c) using the value setting unit 81b ( FIG. 2 ) to be a value calculated from the operating pressure characteristics of the corresponding stack (water electrolysis stack 23c) (step S150). The determined target pressure is lower than the initial pressure (the pressure at step S105).

[0080] After step S150, similarly to step S125, the water electrolysis system 100 controls the value setting unit 81b ( FIG. 2 ) to reduce the pressure of the produced gas for the relevant stack (water electrolysis stack 23c) to the target pressure, and controls the operation control unit 81a ( FIG. 2 ) to perform hydrogen production operation (step S155).

[0081] The process of step S155 is a process for recovering the electrolysis voltage of the corresponding stack for the second time. In this embodiment, the process for recovering the electrolysis voltage of the corresponding stack for the second time will be described as being performed when there is a possibility that the electrode cell (not shown) built into the corresponding stack has deteriorated.

[0082] After step S155, the water electrolysis system 100 determines, by the operation control unit 81a ( FIG. 2 ), whether the corresponding stack (water electrolysis stack 23c) is operating within the operating voltage range (step S160), similarly to step S130.

[0083] If it is determined in step S160 that the corresponding stack (water electrolysis stack 23c) is operating within the operating voltage range (if "Yes"), the process proceeds to step S185 shown in FIG. 5A via reference numeral "B11." In this case, the water electrolysis system 100 causes the value setting unit 81b ( FIG. 2 ) to restore the generated gas pressure for the corresponding stack (water electrolysis stack 23c) that was changed (reduced) in step S155 to its original state (initial pressure state) (step S185 in FIG. 5A ). Then, the process returns to step S105.

[0084] On the other hand, if it is determined in step S160 that the corresponding stack (water electrolysis stack 23c) is not operating within the operating voltage range ("No"), the process proceeds to step S165. In this case, the water electrolysis system 100, using the operation control unit 81a ( FIG. 2 ), determines whether the electrolysis voltage of the corresponding stack (water electrolysis stack 23c) has reached a voltage at which degradation of electrode cells (not shown) built into the corresponding stack is expected (hereinafter may be referred to as "degradation voltage") (step S165).

[0085] If it is determined in step S165 that the electrolysis voltage of the corresponding stack (water electrolysis stack 23c) has not reached the degradation voltage ("No"), the process proceeds to step S185 shown in FIG. 5A via reference numeral "B11." In this case, the water electrolysis system 100 causes the value setting unit 81b (FIG. 2) to restore the generated gas pressure of the corresponding stack (water electrolysis stack 23c) that was changed (reduced) in step S155 to its original state (initial pressure state) (step S185 in FIG. 5A). Then, the process returns to step S105.

[0086] On the other hand, if it is determined in step S165 that the electrolysis voltage of the corresponding stack (water electrolysis stack 23c) has reached the degradation voltage ("Yes"), the process proceeds to step S170. That is, in this case, the water electrolysis system 100 determines that the electrode cells (not shown) of the corresponding stack have deteriorated. In this case, it is desirable to reduce the current value of each water electrolysis stack 23 including the corresponding stack. Therefore, in this case, the water electrolysis system 100 controls the value setting unit 81b (FIG. 2) to reduce the overall current flowing through the water electrolysis stacks 23 connected in series with the corresponding stack (water electrolysis stack 23c) (hereinafter sometimes referred to as "series stack"), and then controls the operation control unit 81a (FIG. 2) to perform the hydrogen production operation (step S170).

[0087] In step S170, for a stack whose electrolysis voltage has not recovered despite the process of recovering the electrolysis voltage, hydrogen production operation is performed while reducing the reaction overvoltage by lowering the pressure of the produced gas to a value lower than the initial pressure. Note that the "reaction overvoltage" here refers to a phenomenon in which the produced gas (hydrogen gas and oxygen gas) accumulates in the built-in electrode cells (not shown), causing the electrolysis voltage to rise, as in the case of water electrolysis stack 23c shown in the upper diagram of FIG. 3A. The process of step S170 is performed to reduce the frequency of replacement of the stack and improve hydrogen production efficiency.

[0088] After step S170, the water electrolysis system 100 causes the replacement stack determination unit 81c (FIG. 2) to determine whether or not the corresponding stack (water electrolysis stack 23c) needs to be replaced (step S175).

[0089] The determination in step S175 is made based on, for example, the following conditional expression (1): Continued production revenue of electrolytic hydrogen < (production revenue of electrolytic hydrogen after replacement - replacement cost of the stack) ... (1) If conditional expression (1) is satisfied, it is determined that the stack needs to be replaced, and on the other hand, if conditional expression (1) is not satisfied, it is determined that the stack does not need to be replaced.

[0090] The determination in step S175 involves calculating and comparing the revenue that can be obtained from the amount of hydrogen that can be produced in a predetermined operating period if the stack is not replaced with the revenue that can be obtained from the amount of hydrogen that can be produced in a predetermined operating period if the stack is replaced. If the revenue that can be obtained from replacing the stack is higher than the revenue that can be obtained from not replacing the stack, even after the replacement cost, the water electrolysis system 100 issues a warning to the user urging them to replace the stack. In response to this, the user stops the water electrolysis system 100, replaces the stack with another water electrolysis stack 23, and restarts the water electrolysis system 100.

[0091] If it is determined in step S175 that replacement of the corresponding stack (water electrolysis stack 23c) is not necessary ("No"), the process returns to step S170. In this case, the process may proceed to step S185 in Fig. 5A when, for example, a predetermined condition is met or a predetermined timing arrives.

[0092] On the other hand, if it is determined in step S175 that replacement of the corresponding stack (water electrolysis stack 23c) is necessary (if "Yes"), the water electrolysis system 100 causes the stack replacement determination unit 81c ( FIG. 2 ) to issue a warning to the user urging replacement of the corresponding stack (water electrolysis stack 23c), and instructs the user to replace the corresponding stack (water electrolysis stack 23c).

[0093] In the first embodiment, if the electrolysis voltage of the corresponding stack drops from the warning voltage range to the operational voltage range after changing the target voltage and target pressure as the operating conditions ("Yes" in step S130), the process of restoring the electrolysis voltage of the corresponding stack is terminated and hydrogen production operation is performed (steps S185 and S105). On the other hand, in the first embodiment, if the electrolysis voltage of the corresponding stack does not drop from the warning voltage range to the operational voltage range after changing the target voltage and target pressure as the operating conditions ("No" in step S130), the process of restoring the electrolysis voltage of the corresponding stack by changing the operating conditions is performed (step S135). If the electrolysis voltage of the corresponding stack still does not recover ("No" in step S140), the first embodiment finally reduces the current flowing through the entire series stack and performs hydrogen production operation (step S170).

[0094] Furthermore, in the first embodiment, if the profit from replacing the stack in question is higher than the profit from not replacing it, even if replacement costs are incurred, the user can be made to replace the stack in question with another water electrolysis stack 23.

[0095] Furthermore, in the first embodiment, the water electrolysis system 100 has a multi-series configuration that can utilize a general large-scale DC power supply, and can produce hydrogen stably and continuously with high electrolysis efficiency and at low cost.

[0096] Second Example Hereinafter, with reference to Figures 6A and 6B , the operation of a second example of the water electrolysis system 100 when performing a process to restore the electrolysis voltage of a corresponding stack will be described. Figures 6A and 6B are flowcharts illustrating the operation of the second example when performing a process to restore the electrolysis voltage. In the second example, the electrolysis voltage of a corresponding stack is restored by adjusting the flow rate of water supplied to the water electrolysis stack 23.

[0097] As shown in FIGS. 6A and 6B, the second embodiment differs from the first embodiment (FIGS. 5A and 5B) in that the second embodiment performs the processes of steps S120a, S125a, S135a, S150a, and S155a instead of the processes of steps S120, S125, S135, S150, and S155.

[0098] 6A , in step S120a, the water electrolysis system 100 determines, using the value setting unit 81b ( FIG. 2 ), a target flow rate of water to be supplied to the relevant stack (water electrolysis stack 23c) based on the reference information 82a stored in the storage unit 82. The determined target flow rate is higher than the initial flow rate (the flow rate at step S105). In the second example, the target voltage set in step S115 and the target flow rate determined in step S120a are used as operating conditions for performing a process to restore the electrolysis voltage of the relevant stack.

[0099] In step S125a, the water electrolysis system 100 controls the operation control unit 81a (FIG. 2) to increase the flow rate of water supplied to the corresponding stack (water electrolysis stack 23c) to the target flow rate, and performs hydrogen production operation using the operation control unit 81a (FIG. 2).

[0100] In step S135a, the water electrolysis system 100 changes the flow rate of water supplied to the corresponding stack (water electrolysis stack 23c) to a value lower than the initial flow rate using the value setting unit 81b (FIG. 2), and performs hydrogen production operation using the operation control unit 81a (FIG. 2). The change in the flow rate of water supplied (i.e., the change to lower the flow rate) can be performed multiple times. The water electrolysis system 100 resets the operating conditions using the control program Pr (FIG. 2).

[0101] 6B , in step S150a, similarly to step S120a, the water electrolysis system 100 determines, using the value setting unit 81b ( FIG. 2 ), a target flow rate of water to be supplied to the relevant stack (water electrolysis stack 23c) from the reference information 82a stored in the storage unit 82. The determined target flow rate is higher than the initial flow rate (the flow rate at step S105).

[0102] In step S155a, similarly to step S125a, the water electrolysis system 100 controls the value setting unit 81b (FIG. 2) to reduce the pressure of the produced gas for the relevant stack (water electrolysis stack 23c) to the target pressure, and controls the operation control unit 81a (FIG. 2) to perform hydrogen production operation.

[0103] The second embodiment (FIGS. 6A and 6B) differs from the first embodiment (FIGS. 5A and 5B) in that, instead of changing the target voltage and target pressure, the operating conditions are changed by changing the target voltage and target flow rate.

[0104] Third Example Hereinafter, with reference to Figures 7A and 7B , the operation of a third example of the water electrolysis system 100 when performing a process to restore the electrolysis voltage of a corresponding stack will be described. Figures 7A and 7B are flowcharts illustrating the operation of the third example when performing a process to restore the electrolysis voltage. In the third example, the electrolysis voltage of a corresponding stack is restored by adjusting the pressure of the produced gas and the flow rate of water supplied to the water electrolysis stack 23.

[0105] As shown in FIGS. 7A and 7B, the third embodiment differs from the first embodiment (FIGS. 5A and 5B) in that the processing of steps S120b, S125b, S135b, S150b, and S155b is performed instead of the processing of steps S120, S125, S135, S150, and S155.

[0106] As shown in FIG. 7A , in step S120b, the water electrolysis system 100 determines, for the corresponding stack (water electrolysis stack 23c), a target pressure of the produced gas and a target flow rate of water to be supplied to the corresponding stack using the value setting unit 81b ( FIG. 2 ). The determined target pressure is a value calculated from the operating pressure characteristics of the corresponding stack (water electrolysis stack 23c) and is lower than the initial pressure (pressure at step S105). The determined target flow rate is a value stored in the storage unit 82 as reference information 82a and is higher than the initial flow rate (flow rate at step S105). In the third example, the target voltage set in step S115 and the target pressure and target flow rate determined in step S120b are used as operating conditions for performing a process to restore the electrolysis voltage of the corresponding stack.

[0107] In step S125b, the water electrolysis system 100 controls the value setting unit 81b ( FIG. 2 ) to reduce the pressure of the produced gas for the corresponding stack (water electrolysis stack 23c) to the target pressure. The water electrolysis system 100 also controls the value setting unit 81b ( FIG. 2 ) to increase the flow rate of water supplied to the corresponding stack (water electrolysis stack 23c) to the target flow rate. The water electrolysis system 100 then performs the hydrogen production operation using the operation control unit 81a ( FIG. 2 ).

[0108] Furthermore, in step S135b, the water electrolysis system 100 uses the value setting unit 81b ( FIG. 2 ) to change the pressure of the produced gas for the corresponding stack (water electrolysis stack 23c) to a value higher than the initial pressure. The water electrolysis system 100 also uses the value setting unit 81b ( FIG. 2 ) to change the flow rate of water supplied to the corresponding stack (water electrolysis stack 23c) to a value lower than the initial flow rate. The water electrolysis system 100 then performs the hydrogen production operation using the operation control unit 81a ( FIG. 2 ). The changes in the produced gas pressure and the flow rate of the supplied water (increasing the produced gas pressure and decreasing the flow rate) can be performed multiple times. At this time, the water electrolysis system 100 may simultaneously change the produced gas pressure and the flow rate of the supplied water. Alternatively, the water electrolysis system 100 may alternately change the produced gas pressure and the flow rate of the supplied water. Depending on the internal state of the water electrolysis stack 23, performing the change operations simultaneously may result in better removal of gas from the electrode cells, while performing the change operations alternately may result in better removal of gas from the electrode cells. When the change operations are performed alternately, there is no restriction on the order of the change operations. However, since the change operation of the produced gas pressure directly acts on the gas adhering to the electrode cells, it may be more effective than the change operation of the flow rate of the supplied water.

[0109] If it is determined in step S140 that the corresponding stack (water electrolysis stack 23c) is operating within the operating voltage range ("Yes"), the water electrolysis system 100 determines that the process of restoring the electrolysis voltage of the corresponding stack has been completed. The water electrolysis system 100 then terminates the process of restoring the electrolysis voltage of the corresponding stack, performs a pressure change operation to return the generated gas pressure changed (reduced) in step S125 to its original state (initial pressure state), and resumes hydrogen production operation in the default state.

[0110] 7B , in step S150b, similarly to step S120b, the water electrolysis system 100 determines, using the value setting unit 81b ( FIG. 2 ), a target pressure of the produced gas and a target flow rate of water to be supplied to the corresponding stack (water electrolysis stack 23c). The determined target pressure is a value calculated from the operating pressure characteristics of the corresponding stack (water electrolysis stack 23c) and is lower than the initial pressure (pressure at step S105). The determined target flow rate is a value stored as reference information 82a in the storage unit 82 and is higher than the initial flow rate (flow rate at step S105).

[0111] In step S155b, similarly to step S125b, the water electrolysis system 100 controls the value setting unit 81b ( FIG. 2 ) to reduce the pressure of the produced gas for the corresponding stack (water electrolysis stack 23c) to the target pressure. The water electrolysis system 100 also controls the value setting unit 81b ( FIG. 2 ) to increase the flow rate of water supplied to the corresponding stack (water electrolysis stack 23c) to the target flow rate. The water electrolysis system 100 then performs the hydrogen production operation using the operation control unit 81a ( FIG. 2 ).

[0112] The third embodiment (FIGS. 7A and 7B) differs from the first embodiment (FIGS. 5A and 5B) in that, instead of changing the target voltage and target pressure as operating conditions, the third embodiment changes the target voltage, target pressure, and target flow rate.

[0113] Fourth Example Hereinafter, with reference to Figures 8A and 8B , the operation of a fourth example of the water electrolysis system 100 when performing a process to restore the electrolysis voltage of a corresponding stack will be described. Figures 8A and 8B are flowcharts illustrating the operation of the fourth example when performing a process to restore the electrolysis voltage. In the fourth example, the electrolysis voltage of a corresponding stack is restored by adjusting the pressure of the produced gas, the flow rate of water supplied to the water electrolysis stack 23, and the temperature of the water electrolysis stack 23.

[0114] As shown in FIGS. 8A and 8B, the fourth embodiment differs from the first embodiment (FIGS. 5A and 5B) in that the fourth embodiment performs the processes of steps S120c, S125c, S135c, S150c, and S155c instead of the processes of steps S120, S125, S135, S150, and S155.

[0115] As shown in FIG. 8A , in step S120c, the water electrolysis system 100 determines, using the value setting unit 81b ( FIG. 2 ), a target pressure of the produced gas, a target flow rate of water to be supplied to the stack, and a target temperature of the stack for the stack (water electrolysis stack 23c). The determined target pressure is a value calculated from the operating pressure characteristics of the stack (water electrolysis stack 23c) and is lower than the initial pressure (pressure at step S105). The determined target flow rate is a value stored in the storage unit 82 as reference information 82a and is higher than the initial flow rate (flow rate at step S105). The determined target temperature is a value calculated from the operating pressure characteristics of the stack (water electrolysis stack 23c) and is lower than the initial temperature (temperature at step S105). In the fourth embodiment, the target voltage set in step S115 and the target pressure, target flow rate, and target temperature determined in step S120c are used as operating conditions for performing processing to restore the electrolysis voltage of the corresponding stack.

[0116] In step S125c, the water electrolysis system 100 controls the value setting unit 81b ( FIG. 2 ) to reduce the pressure of the produced gas for the corresponding stack (water electrolysis stack 23c) to the target pressure. The water electrolysis system 100 also controls the value setting unit 81b ( FIG. 2 ) to increase the flow rate of water supplied to the corresponding stack (water electrolysis stack 23c) to the target flow rate. The water electrolysis system 100 also controls the value setting unit 81b ( FIG. 2 ) to reduce the temperature of the corresponding stack (water electrolysis stack 23c) to the target temperature. The water electrolysis system 100 then performs a hydrogen production operation using the operation control unit 81a ( FIG. 2 ).

[0117] In step S135c, the water electrolysis system 100 uses the value setting unit 81b ( FIG. 2 ) to change the pressure of the produced gas for the corresponding stack (water electrolysis stack 23c) to a value higher than the initial pressure. The water electrolysis system 100 also uses the value setting unit 81b ( FIG. 2 ) to change the flow rate of water supplied to the corresponding stack (water electrolysis stack 23c) to a value lower than the initial flow rate. The water electrolysis system 100 also uses the value setting unit 81b ( FIG. 2 ) to change the temperature of the corresponding stack (water electrolysis stack 23c) to a value higher than the initial temperature. The water electrolysis system 100 then performs a hydrogen production operation using the operation control unit 81a ( FIG. 2 ). The changes in the produced gas pressure, the flow rate of water supplied, and the temperature of the corresponding stack (increasing the produced gas pressure, decreasing the flow rate, and increasing the temperature of the corresponding stack) can be performed multiple times.

[0118] 8B , in step S150c, similarly to step S120c, the water electrolysis system 100 determines, using the value setting unit 81b ( FIG. 2 ), a target pressure of the produced gas, a target flow rate of water to be supplied to the stack, and a target temperature of the stack for the stack. The determined target pressure is a value calculated from the operating pressure characteristics of the stack (water electrolysis stack 23c) and is lower than the initial pressure (pressure at step S105). The determined target flow rate is a value stored in the storage unit 82 as reference information 82a and is higher than the initial flow rate (flow rate at step S105). The determined target temperature is a value calculated from the operating pressure characteristics of the stack (water electrolysis stack 23c) and is lower than the initial temperature (temperature at step S105).

[0119] In step S155c, similarly to step S125c, the water electrolysis system 100 controls the value setting unit 81b ( FIG. 2 ) to reduce the pressure of the produced gas for the corresponding stack (water electrolysis stack 23c) to the target pressure. The water electrolysis system 100 also controls the value setting unit 81b ( FIG. 2 ) to increase the flow rate of water supplied to the corresponding stack (water electrolysis stack 23c) to the target flow rate. The water electrolysis system 100 also controls the value setting unit 81b ( FIG. 2 ) to reduce the temperature of the corresponding stack (water electrolysis stack 23c) to the target temperature. The water electrolysis system 100 then performs a hydrogen production operation using the operation control unit 81a ( FIG. 2 ).

[0120] The fourth embodiment (FIGS. 8A and 8B) differs from the first embodiment (FIGS. 5A and 5B) in that, instead of changing the target voltage and target pressure as operating conditions, the fourth embodiment changes the target voltage, target pressure, target flow rate, and target temperature.

[0121] <Main Features of the Water Electrolysis System> (1) As shown in FIG. 1 , a water electrolysis system 100 according to this embodiment includes multiple water electrolysis stacks 23 that generate hydrogen gas and oxygen gas as product gases from water, a water tank 11 that supplies water to each water electrolysis stack 23, a hydrogen gas tank 51 that stores the hydrogen gas generated in each water electrolysis stack 23, an oxygen gas tank 52 that stores the oxygen gas generated in each water electrolysis stack 23, and a system controller 80 that controls the operation of each component of the system. During hydrogen production operation, the system controller 80 monitors the voltage of each water electrolysis stack 23, and temporarily changes the operating conditions of any water electrolysis stack 23 for which an increase in electrolysis voltage is detected by the voltage monitoring to restore the electrolysis voltage. Here, the "water electrolysis stack 23 for which an increase in electrolysis voltage is detected by the voltage monitoring" refers to a water electrolysis stack 23 in which gas has accumulated in a built-in electrode cell (not shown) and the electrolysis efficiency (performance) has decreased.

[0122] The water electrolysis system 100 according to the present embodiment temporarily changes the operating conditions of the water electrolysis stack 23 when an increase in electrolysis voltage is detected by voltage monitoring, thereby restoring the electrolysis efficiency (performance). This allows the water electrolysis system 100 according to the present embodiment to remove gas accumulated in the electrode cells (not shown) built into the water electrolysis stack 23 to the outside, thereby restoring the electrolysis efficiency (performance) of the water electrolysis stack 23. The water electrolysis system 100 according to the present embodiment minimizes a decrease in the electrolysis efficiency (performance) of each water electrolysis stack 23 provided in the system, thereby reducing the frequency of replacing the water electrolysis stack 23 and improving hydrogen production efficiency. Furthermore, the water electrolysis system 100 according to the present embodiment reduces the replacement cost of a water electrolysis stack 23 with a reduced electrolysis efficiency (performance) and the disposal cost of the water electrolysis stack 23.

[0123] (2) As shown in step S125 and step S185 in FIG. 5A , in the water electrolysis system 100 according to this embodiment, the system control unit 80 may be configured to perform a process of lowering the pressure of the produced gas and then restoring the pressure to the initial pressure, as a process of restoring the electrolysis voltage of the water electrolysis stack 23 in which an increase in the electrolysis voltage has been detected.

[0124] The water electrolysis system 100 according to the present embodiment changes the pressure of the water electrolysis stack 23, for example, from 0.5 MPa to 0.4 MPa to 0.5 MPa, when gas accumulates in the electrode cells (not shown) of the water electrolysis stack 23 and the electrolysis voltage increases. In this way, the water electrolysis system 100 according to the present embodiment can remove the gas accumulated in the electrode cells (not shown) of the water electrolysis stack 23 from the water electrolysis stack 23, thereby restoring the electrolysis efficiency (performance). The water electrolysis system 100 according to the present embodiment can minimize the decrease in the electrolysis efficiency (performance) of each water electrolysis stack 23 provided in the system, thereby reducing the frequency of replacing the water electrolysis stack 23 and improving the hydrogen production efficiency.

[0125] (3) As shown in step S125, step S135, and step S185 in FIG. 5A , in the water electrolysis system 100 according to the present embodiment, when an increase in the electrolysis voltage is detected in the water electrolysis stack 23, the system control unit 80 may be configured to, after reducing the pressure of the produced gas, increase the pressure to a value higher than the initial pressure and then return the pressure to the initial pressure, or to perform the pressure increase operation multiple times before returning the pressure to the initial pressure.

[0126] In the water electrolysis system 100 according to the present embodiment, when gas accumulates in the electrode cells (not shown) of the water electrolysis stack 23 and the electrolysis voltage increases, the pressure is changed, for example, from 0.5 MPa to 0.4 MPa to 0.6 MPa to 0.5 MPa. Alternatively, the water electrolysis system 100 according to the present embodiment performs the pressure change operation multiple times on the water electrolysis stack 23 with the increased electrolysis voltage. In this manner, the water electrolysis system 100 according to the present embodiment can efficiently remove gas accumulated in the electrode cells (not shown) of the water electrolysis stack 23 from the water electrolysis stack 23, thereby restoring the electrolysis efficiency (performance). The water electrolysis system 100 according to the present embodiment can minimize the decrease in the electrolysis efficiency (performance) of each water electrolysis stack 23 provided in the system. Therefore, the water electrolysis system 100 according to the present embodiment can reduce the frequency of replacing the water electrolysis stack 23 and improve the hydrogen production efficiency.

[0127] (4) As shown in step S155 to step S170 in FIG. 5B , in the water electrolysis system 100 according to this embodiment, the system control unit 80 may be configured to perform an operation to produce hydrogen gas and oxygen gas from water while reducing the reaction overpotential by reducing the pressure of the produced gas to a value lower than the initial pressure in the water electrolysis stack 23 in which the electrolysis voltage does not recover despite the execution of the process to recover the electrolysis voltage.

[0128] The water electrolysis system 100 according to the present embodiment reduces the reaction overvoltage by lowering the pressure of the produced gas to a value lower than the initial pressure for the water electrolysis stack 23 whose electrolysis voltage has not recovered despite treatment to restore the electrolysis voltage. The water electrolysis system 100 according to the present embodiment then operates to produce hydrogen gas and oxygen gas from water in a state in which the reaction overvoltage of the water electrolysis stack 23 has been reduced. The water electrolysis system 100 according to the present embodiment can improve the hydrogen production efficiency at the same current value (i.e., improve the electrolysis efficiency (performance)).

[0129] (5) As shown in step S125a and step S185 in FIG. 6A , in the water electrolysis system 100 according to this embodiment, the system control unit 80 may be configured to perform, for the water electrolysis stack 23 in which an increase in electrolysis voltage has been detected, a process of increasing the flow rate of water supplied to the water electrolysis stack 23 and then returning the flow rate to the initial value, as a process of restoring the electrolysis voltage.

[0130] The water electrolysis system 100 according to the present embodiment changes the flow rate, for example, from 5 ml / s to 4 ml / s to 5 ml / s, for a water electrolysis stack 23 in which gas has accumulated in the electrode cells (not shown) of the water electrolysis stack 23 and the electrolysis voltage has increased. In this way, the water electrolysis system 100 according to the present embodiment can recover the electrolysis efficiency (performance) by removing the gas accumulated in the electrode cells (not shown) of the water electrolysis stack 23 from the water electrolysis stack 23. The water electrolysis system 100 according to the present embodiment can minimize the decrease in the electrolysis efficiency (performance) of each water electrolysis stack 23 provided in the system, thereby reducing the frequency of replacing the water electrolysis stack 23 and improving the hydrogen production efficiency.

[0131] (6) As shown in step S125a, step S135a, and step S185 in FIG. 6A , in the water electrolysis system 100 according to this embodiment, when increasing the flow rate of water supplied to the water electrolysis stack 23 and then returning it to the initial flow rate, the system control unit 80 may reduce the flow rate to a value lower than the initial flow rate and then return it to the initial flow rate, or may perform the flow rate reduction operation multiple times before returning it to the initial flow rate.

[0132] The water electrolysis system 100 according to the present embodiment changes the flow rate, for example, from 5 ml / s to 4 ml / s, then 6 ml / s, and finally to 5 ml / s, for the water electrolysis stack 23 in which gas accumulates in the electrode cells (not shown) of the water electrolysis stack 23 and the electrolysis voltage increases. Alternatively, the water electrolysis system 100 according to the present embodiment performs a flow rate reduction operation multiple times for the water electrolysis stack 23 in which the electrolysis voltage increases. In this way, the water electrolysis system 100 according to the present embodiment can efficiently remove gas accumulated in the electrode cells (not shown) of the water electrolysis stack 23 from the water electrolysis stack 23, thereby restoring the electrolysis efficiency (performance). The water electrolysis system 100 according to the present embodiment can minimize the decrease in the electrolysis efficiency (performance) of each water electrolysis stack 23 provided in the system, thereby reducing the frequency of replacing the water electrolysis stack 23 and improving the hydrogen production efficiency.

[0133] (7) As shown in step S155a to step S170 in FIG. 6B , in the water electrolysis system 100 according to this embodiment, for the water electrolysis stack 23 in which the electrolysis voltage has not recovered despite the process of recovering the electrolysis voltage having been performed, the system control unit 80 may be configured to perform an operation to produce hydrogen gas and oxygen gas from water while reducing the reaction overpotential by increasing the flow rate of water supplied to the water electrolysis stack 23 to a value higher than the initial flow rate.

[0134] The water electrolysis system 100 according to the present embodiment reduces the reaction overvoltage of a water electrolysis stack 23 whose electrolysis voltage has not recovered despite treatment to restore the electrolysis voltage by increasing the flow rate of water supplied to the water electrolysis stack 23 to a value higher than the initial flow rate. The water electrolysis system 100 according to the present embodiment then operates to produce hydrogen gas and oxygen gas from water in a state in which the reaction overvoltage of the water electrolysis stack 23 has been reduced. The water electrolysis system 100 according to the present embodiment can improve the hydrogen production efficiency at the same current value (i.e., improve the electrolysis efficiency (performance)).

[0135] (8) As shown in step S125b and step S185 in FIG. 7A , in the water electrolysis system 100 according to this embodiment, the system control unit 80 may be configured to restore the electrolysis voltage of the water electrolysis stack 23 in which an increase in the electrolysis voltage has been detected by changing the pressure of the produced gas and the flow rate of water supplied to the water electrolysis stack 23, and then returning the pressure and flow rate to the initial values.

[0136] The water electrolysis system 100 according to the present embodiment simultaneously changes the pressure of the produced gas and the flow rate of water supplied to the water electrolysis stack 23 when gas accumulates in the electrode cells (not shown) built in the water electrolysis stack 23, causing an increase in electrolysis voltage. The water electrolysis system 100 then returns the pressure of the produced gas and the flow rate of water supplied to the water electrolysis stack 23 to their initial values. This allows the water electrolysis system 100 according to the present embodiment to efficiently remove gas accumulated in the electrode cells (not shown) of the water electrolysis stack 23 from the water electrolysis stack 23, thereby restoring the electrolysis efficiency (performance). The water electrolysis system 100 according to the present embodiment minimizes a decrease in the electrolysis efficiency (performance) of each water electrolysis stack 23 provided in the system, thereby reducing the frequency of replacing the water electrolysis stack 23 and improving hydrogen production efficiency. The pressure of the produced gas and the flow rate of water supplied to the water electrolysis stack 23 may be changed simultaneously. Alternatively, the pressure of the produced gas and the flow rate of water supplied to the water electrolysis stack 23 may be changed alternately. Depending on the internal state of the water electrolysis stack 23, making either of the changes may result in successful removal of gas from the electrode cells.

[0137] (9) As shown in step S125b and step S185 in FIG. 7A , in the water electrolysis system 100 according to the present embodiment, the system control unit 80 may be configured to restore the electrolysis voltage of the water electrolysis stack 23 in which an increase in the electrolysis voltage has been detected by changing the pressure of one or both of the produced gases of hydrogen gas and oxygen gas and the flow rate of water supplied to the water electrolysis stack 23, and then returning the pressure and flow rate to the initial values.

[0138] The water electrolysis system 100 according to the present embodiment changes the pressure of one or both of the produced gases, hydrogen gas and oxygen gas, and the flow rate of water supplied to the water electrolysis stack 23 when gas accumulates in the electrode cells (not shown) of the water electrolysis stack 23, causing an increase in electrolysis voltage. The water electrolysis system 100 then restores the pressure of the produced gas and the flow rate of water supplied to the water electrolysis stack 23 to the initial pressure and initial flow rate. This allows the water electrolysis system 100 according to the present embodiment to remove the gas accumulated in the electrode cells (not shown) of the water electrolysis stack 23 from the water electrolysis stack 23, thereby restoring the electrolysis efficiency (performance). The water electrolysis system 100 according to the present embodiment minimizes a decrease in the electrolysis efficiency (performance) of each water electrolysis stack 23 provided in the system, thereby reducing the frequency of replacing the water electrolysis stack 23 and improving hydrogen production efficiency.

[0139] (10) As shown in step S155b to step S170 in FIG. 7B , in the water electrolysis system 100 according to this embodiment, for the water electrolysis stack 23 in which the electrolysis voltage has not recovered despite the execution of the process to recover the electrolysis voltage, the system control unit 80 may be configured to perform an operation to produce hydrogen gas and oxygen gas from water while reducing the pressure of the produced gas to a value lower than the initial pressure and increasing the flow rate of water supplied to the water electrolysis stack 23 to a value higher than the initial flow rate, thereby reducing the reaction overpotential.

[0140] The water electrolysis system 100 according to the present embodiment reduces the reaction overvoltage of the water electrolysis stack 23, which has not recovered its electrolysis voltage despite the electrolysis voltage recovery process, by reducing the pressure of the produced gas to a value lower than the initial pressure and increasing the flow rate of water supplied to the water electrolysis stack 23 to a value higher than the initial flow rate. The water electrolysis system 100 according to the present embodiment then operates to produce hydrogen gas and oxygen gas from water in a state in which the reaction overvoltage of the water electrolysis stack 23 is reduced. The water electrolysis system 100 according to the present embodiment can improve the hydrogen production efficiency at the same current value (i.e., improve the electrolysis efficiency (performance)).

[0141] (11) As shown in step S125c and step S185 in FIG. 8A , in the water electrolysis system 100 according to this embodiment, the system control unit 80 may be configured to restore the electrolysis voltage of the water electrolysis stack 23 in which an increase in the electrolysis voltage has been detected by changing the pressure of the produced gas, the flow rate of water supplied to the water electrolysis stack 23, and the temperature of the water electrolysis stack 23, and then returning the pressure, flow rate, and temperature to the initial values.

[0142] The water electrolysis system 100 according to the present embodiment adjusts the pressure of the produced gas, the flow rate of water supplied to the water electrolysis stack 23, and the temperature of the water electrolysis stack 23 for the water electrolysis stack 23 in which gas has accumulated in the electrode cells (not shown) built in the water electrolysis stack 23, resulting in an increase in electrolysis voltage. In this way, the water electrolysis system 100 according to the present embodiment can remove the gas accumulated in the electrode cells (not shown) of the water electrolysis stack 23 from the water electrolysis stack 23, thereby restoring the electrolysis efficiency (performance). The water electrolysis system 100 according to the present embodiment can minimize a decrease in the electrolysis efficiency (performance) of each water electrolysis stack 23 provided in the system, thereby reducing the frequency of replacing the water electrolysis stack 23 and improving hydrogen production efficiency.

[0143] (12) As shown in step S155c to step S170 in FIG. 8B , in the water electrolysis system 100 according to this embodiment, for the water electrolysis stack 23 in which the electrolysis voltage has not recovered despite the process of recovering the electrolysis voltage having been performed, the system control unit 80 may be configured to perform an operation to produce hydrogen gas and oxygen gas from water while reducing the pressure of the produced gas to a value lower than the initial pressure, increasing the flow rate of water supplied to the water electrolysis stack 23 to a value higher than the initial flow rate, and further reducing the temperature of the water electrolysis stack 23 to a value lower than the initial temperature, thereby reducing the reaction overpotential.

[0144] In the water electrolysis system 100 according to the present embodiment, when the electrolysis voltage of the water electrolysis stack 23 does not recover despite the electrolysis voltage recovery process, the pressure of the produced gas is reduced to a value lower than the initial pressure, the flow rate of water supplied to the water electrolysis stack 23 is increased to a value higher than the initial flow rate, and the temperature of the water electrolysis stack 23 is reduced to a value lower than the initial temperature, thereby reducing the reaction overvoltage. The water electrolysis system 100 according to the present embodiment then operates to produce hydrogen gas and oxygen gas from water in a state in which the reaction overvoltage of the water electrolysis stack 23 is reduced. The water electrolysis system 100 according to the present embodiment can improve the hydrogen production efficiency at the same current value (i.e., improve the electrolysis efficiency (performance)).

[0145] (13) As shown in Fig. 2 , in the water electrolysis system 100 according to this embodiment, the system control unit 80 may include a value setting unit 81b that sets values ​​for the pressure of the produced gas, the flow rate of water supplied to the water electrolysis stack 23, and the temperature of the water electrolysis stack 23. As shown in Fig. 3A and 3B , when it is detected that the electrolysis voltage of the water electrolysis stack 23 has increased beyond an arbitrarily determined operating voltage range and into an arbitrarily determined warning voltage range, the value setting unit 81b may change any one parameter or a combination of multiple parameters selected from the pressure of the produced gas, the flow rate of water supplied to the water electrolysis stack 23, and the temperature of the water electrolysis stack 23 to an arbitrary operating value, thereby reducing the electrolysis voltage to the operating voltage range.

[0146] The water electrolysis system 100 according to the present embodiment can use the value setting unit 81b to reduce the electrolysis voltage to the operating voltage range for the water electrolysis stack 23 when the electrolysis voltage increases due to gas accumulation in the electrode cells (not shown) built into the water electrolysis stack 23. The water electrolysis system 100 according to the present embodiment can improve the hydrogen production efficiency at the same current value (i.e., improve the electrolysis efficiency (performance)).

[0147] (14) As shown in FIG. 2 , in the water electrolysis system 100 according to this embodiment, the system control unit 80 may include a stack replacement determination unit 81c that determines whether to replace the water electrolysis stack 23, in which the electrolysis voltage has not recovered despite the execution of a process to recover the electrolysis voltage, with another water electrolysis stack 23.

[0148] In the water electrolysis system 100 according to the present embodiment, the replacement stack determination unit 81c can determine whether to replace the water electrolysis stack 23, in which the electrolysis voltage has not recovered despite the execution of the process to restore the electrolysis voltage, with another water electrolysis stack 23. The water electrolysis system 100 according to the present embodiment can reduce the frequency of replacement of the water electrolysis stack 23, thereby reducing the replacement cost for replacing the water electrolysis stack 23 with a new water electrolysis stack 23 and the disposal cost for discarding the water electrolysis stack 23.

[0149] (15) As shown in step S180 of FIG. 5B , a method for operating a water electrolysis system 100 including a plurality of water electrolysis stacks 23 may include issuing a warning to a user urging replacement of a water electrolysis stack 23 when the electrolysis voltage of the water electrolysis stack 23 has not been restored despite a process for restoring the electrolysis voltage of the water electrolysis stack 23 in which an increase in the electrolysis voltage has been detected.

[0150] The water electrolysis system 100 according to this embodiment can timely issue a warning to the user to prompt replacement of the water electrolysis stack 23 .

[0151] As described above, the water electrolysis system 100 according to this embodiment can reduce the frequency of replacing the water electrolysis stack 23 whose electrolysis efficiency (performance) has decreased, thereby improving the hydrogen production efficiency.

[0152] The present invention is not limited to the above-described embodiment, and includes various modifications. For example, the above-described embodiment has been described in detail to clearly explain the present invention, and is not necessarily limited to a configuration including all of the described configurations. Furthermore, it is possible to replace part of the configuration of the embodiment with another configuration, and it is also possible to add another configuration to the configuration of the embodiment. Furthermore, it is possible to add, delete, or replace part of each configuration with another configuration.

[0153] REFERENCE SIGNS LIST 11 Water tank 12 Water pump 13 Water piping 14 Large cooler 20 Water electrolysis unit 21 Water flow rate adjustment valve 22 Small cooler 23, 23a, 23b, 23c, 23d Water electrolysis stack 30 Hydrogen system 31 Separation tank 32 Pressure adjustment valve 32a Pressure adjustment valve 33 Water discharge valve 40 Oxygen system 41 Separation tank 42 Pressure adjustment valve 42a Pressure adjustment valve 43 Water discharge valve 51 Hydrogen gas tank 51a Low-pressure hydrogen gas tank 52 Oxygen gas tank 52a Low-pressure oxygen gas tank 61 Hydrogen gas piping 61a Low-pressure hydrogen gas piping 62 Oxygen gas piping 62a Low-pressure oxygen gas piping 70 Booster pump 71 Pressure adjustment valve 72 Pressure adjustment valve 80 System control unit 81 Calculation unit 81a Operation control unit 81b Value setting unit 81c Replacement stack determination unit 82 Storage unit 82a Reference information 90 Data communication unit 99 Storage medium 100 Water electrolysis system G1 Hydrogen gas G2 Oxygen gas Pr Control program

Claims

1. A water electrolysis system comprising: a plurality of water electrolysis stacks which generate hydrogen gas and oxygen gas as product gases from water; a water tank which supplies water to each water electrolysis stack; a hydrogen gas tank which stores the hydrogen gas generated in each water electrolysis stack; an oxygen gas tank which stores the oxygen gas generated in each water electrolysis stack; and a system control unit which controls operation of each unit of the system, wherein the system control unit performs voltage monitoring for each water electrolysis stack during hydrogen production operation, and for any water electrolysis stack for which an increase in electrolysis voltage is detected by the voltage monitoring, temporarily changes operating conditions to restore the electrolysis voltage.

2. The water electrolysis system according to claim 1, wherein the system control unit performs a process of lowering the pressure of the generated gas and then restoring it to the initial pressure as a process of restoring the electrolysis voltage to the water electrolysis stack in which an increase in the electrolysis voltage has been detected.

3. The water electrolysis system of claim 2, wherein the system control unit, when returning the pressure of the generated gas to the initial pressure after reducing it, increases the pressure to a value higher than the initial pressure and then returns it to the initial pressure, or performs the operation of increasing the pressure multiple times before returning it to the initial pressure.

4. The water electrolysis system of claim 2, wherein the system control unit operates a water electrolysis stack in which the electrolysis voltage does not recover despite having performed a process to recover the electrolysis voltage, to produce the hydrogen gas and the oxygen gas from the water while lowering the reaction overvoltage by lowering the pressure of the generated gas to a value lower than the initial pressure.

5. The water electrolysis system of claim 1, wherein the system control unit performs a process of increasing the flow rate of water supplied to a water electrolysis stack in which an increase in the electrolysis voltage has been detected, and then returning the flow rate to an initial value, as a process of restoring the electrolysis voltage.

6. A water electrolysis system according to claim 5, wherein the system control unit, when increasing the flow rate of water supplied to the water electrolysis stack and then returning it to the initial flow rate, reduces the flow rate to a value lower than the initial flow rate before returning it to the initial flow rate, or performs the operation of reducing the flow rate multiple times before returning it to the initial flow rate.

7. The water electrolysis system of claim 5, wherein the system control unit operates a water electrolysis stack in which the electrolysis voltage does not recover despite the process of recovering the electrolysis voltage to generate the hydrogen gas and the oxygen gas from the water while lowering the reaction overvoltage by increasing the flow rate of water supplied to the water electrolysis stack to a value higher than the initial flow rate.

8. The water electrolysis system of claim 1, wherein the system control unit performs a process of restoring the electrolysis voltage of a water electrolysis stack in which an increase in the electrolysis voltage has been detected by changing the pressure of the generated gas and the flow rate of water supplied to the water electrolysis stack, and then restoring the pressure and flow rate to the initial values.

9. The water electrolysis system of claim 1, wherein the system control unit performs a process of restoring the electrolysis voltage of a water electrolysis stack in which an increase in the electrolysis voltage has been detected by changing the pressure of the generated gas of either or both of the hydrogen gas and the oxygen gas and the flow rate of water supplied to the water electrolysis stack, and then restoring the pressure and flow rate to the initial values.

10. The water electrolysis system of claim 9, wherein the system control unit, for a water electrolysis stack in which the electrolysis voltage does not recover despite having performed a process to recover the electrolysis voltage, operates to produce the hydrogen gas and the oxygen gas from the water while reducing the pressure of the generated gas to a value lower than the initial pressure and increasing the flow rate of water supplied to the water electrolysis stack to a value higher than the initial flow rate to reduce the reaction overvoltage.

11. The water electrolysis system of claim 1, wherein the system control unit performs a process of restoring the electrolysis voltage of a water electrolysis stack in which an increase in the electrolysis voltage has been detected by changing the pressure of the generated gas, the flow rate of water supplied to the water electrolysis stack, and the temperature of the water electrolysis stack, and then restoring the pressure, flow rate, and temperature to their initial values.

12. The water electrolysis system of claim 11, wherein the system control unit, for a water electrolysis stack in which the electrolysis voltage does not recover despite having performed a process to recover the electrolysis voltage, operates to produce the hydrogen gas and the oxygen gas from the water by reducing the pressure of the generated gas to a value lower than the initial pressure, increasing the flow rate of water supplied to the water electrolysis stack to a value higher than the initial flow rate, and further reducing the temperature of the water electrolysis stack to a value lower than the initial temperature to reduce the reaction overvoltage.

13. The water electrolysis system of claim 1, wherein the system control unit has a value setting unit which sets values ​​for the pressure of the generated gas, the flow rate of water supplied to the water electrolysis stack, and the temperature of the water electrolysis stack, and the value setting unit performs a process for a water electrolysis stack in which it has been detected that the electrolysis voltage has exceeded an arbitrarily determined operating voltage range and reached an arbitrarily determined warning voltage range by changing any one parameter or a combination of multiple parameters selected from the pressure of the generated gas, the flow rate of water supplied to the water electrolysis stack, and the temperature of the water electrolysis stack to an arbitrary operating value, thereby lowering the electrolysis voltage to the operating voltage range.

14. The water electrolysis system of claim 1, wherein the system control unit has a stack replacement determination unit that determines whether or not to replace a water electrolysis stack in which the electrolysis voltage does not recover despite processing to recover the electrolysis voltage with another water electrolysis stack.

15. A method for operating a water electrolysis system having a plurality of water electrolysis stacks, comprising the steps of: if there is a water electrolysis stack in which an increase in electrolysis voltage has been detected and a process for restoring the electrolysis voltage has been performed on the water electrolysis stack, but the electrolysis voltage does not recover, issuing a warning to a user to urge replacement.

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