Water electrolysis system, and operation method of water electrolysis system

Abstract

To reduce an exchange frequency of a water electrolysis stack whose electrolytic efficiency (performance) is degraded.SOLUTION: A water electrolysis system 100 comprises: multiple water electrolysis stacks 23 for generating a hydrogen gas and an oxygen gas from water as generated gas; a water tank 11 for supplying water to each water electrolysis stack; a hydrogen gas tank 51 for storing the hydrogen gas generated in each water electrolysis stack; an oxygen gas tank 52 for storing the oxygen gas generated in each water electrolysis stack; and a system control unit 80 for controlling an operation of each part of the system. The system control unit includes: monitoring a voltage of each water electrolysis stack during hydrogen production operation; and performing processing to recover an electrolytic voltage by temporarily changing operation conditions with respect to the water electrolysis stack in which the electrolytic voltage is detected to have increased by monitoring the voltage.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] Unlike fossil fuels, hydrogen is a clean energy source that does not emit carbon dioxide when burned. For this reason, hydrogen has been attracting attention as one of the clean energies for combating global warming, and the development of technologies related to the production, transportation, and use of hydrogen is underway. As one such technology, for example, a water electrolysis system described in Patent Document 1 is provided. The water electrolysis system described in Patent Document 1 electrolyzes water and generates hydrogen gas and oxygen gas from the water by connecting multiple water electrolysis stacks in series to achieve high voltage. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2023 / 012945 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in water electrolysis systems, gas (hydrogen gas and oxygen gas generated from water) may accumulate in the electrode cells built into the water electrolysis stack due to factors such as deterioration of the water electrolysis stack, causing an increase in the electrolysis voltage and a decrease in the electrolysis efficiency (performance). In conventional water electrolysis systems, when the electrolysis efficiency (performance) of the water electrolysis stack decreases, the system is replaced. In such conventional water electrolysis systems, frequent system shutdowns due to replacement of the water electrolysis stack decrease the 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 disposal.

[0005] The present invention has been made to solve the above-mentioned 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 decreases, and a method for operating the water electrolysis system. [Means for solving the problem]

[0006] In order to achieve the above-mentioned object, the present invention provides a water electrolysis system comprising a plurality of water electrolysis stacks that generate 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 generated in each water electrolysis stack, an oxygen gas tank that stores the oxygen gas generated in each water electrolysis stack, and a system control unit that 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. Other means will be described later. Effect of the Invention

[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 hydrogen production efficiency. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a configuration diagram of a water electrolysis system according to an embodiment. [Diagram 2] FIG. 2 is an internal block diagram of a system control unit of the water electrolysis system according to the embodiment. [Figure 3A] FIG. 4 is an explanatory diagram of an electrolysis voltage in the water electrolysis stack before pressure adjustment. [Figure 3B] FIG. 4 is an explanatory diagram of an electrolysis voltage in the water electrolysis stack after pressure adjustment. [Figure 4]FIG. 4 is a graph showing stack current values ​​and cell voltages of each water electrolysis stack. [Figure 5A] 1 is a flowchart (1) showing the operation of the first embodiment when performing a process for recovering the electrolysis voltage. [Figure 5B] 13 is a flowchart (2) showing the operation of the first embodiment when performing a process for recovering the electrolysis voltage. [Figure 6A] 1 is a flowchart (1) showing the operation of the second embodiment when performing a process for recovering the electrolysis voltage. [Figure 6B] 13 is a flowchart (2) showing the operation of the second embodiment when performing a process for recovering the electrolysis voltage. [Figure 7A] 10 is a flowchart (1) showing the operation of the third embodiment when performing a process for recovering the electrolysis voltage. [Figure 7B] 13 is a flowchart (2) showing the operation of the third embodiment when performing a process for recovering the electrolysis voltage. [Figure 8A] 13 is a flowchart (1) showing the operation of the fourth embodiment when performing a process for recovering the electrolysis voltage. [Figure 8B] 13 is a flowchart (2) showing the operation of the fourth embodiment when performing a process for recovering the electrolysis voltage. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[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 only the illustrated examples. In addition, in each drawing, common or similar components are given the same reference numerals, and duplicated explanations thereof will be omitted.

[0010] <Water electrolysis system configuration> The configuration of a water electrolysis system 100 according to this embodiment will now be described 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 Fig. 1, a 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 in Fig. 1, water electrolysis units 20a to 20n are shown 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. Note that, 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 and 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 generate hydrogen gas and oxygen gas. The hydrogen gas tank 51 is a storage unit for the hydrogen gas generated in the water electrolysis unit 20. The oxygen gas tank 52 is a storage unit for the oxygen gas generated in the water electrolysis unit 20. The system control unit 80 is a component that controls the operation of each part 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] Furthermore, the water electrolysis system 100 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 midway. Water sent out from the water tank 11 travels in the direction of arrow A10, then branches midway and travels in the directions of arrow A10a and arrow 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, configuring the water electrolysis stack 23 to be a proton-permeable solid polymer structure.

[0018] 1, the water electrolysis unit 20 has therein two water flow rate regulating valves 21 on the hydrogen electrode side and the oxygen electrode side, and two small coolers 22. The water electrolysis unit 20 also has a water electrolysis stack 23 that electrolyzes water to generate 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 components of the hydrogen system 30 related to hydrogen. 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 the opening degree. 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 the opening degree. The water discharge valve 33 is a valve that drains the 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 the opening degree.

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

[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 the water sent from the water tank 11 by opening a water flow rate control valve 21, cools the water by 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 generating hydrogen gas and oxygen gas.

[0022] Each water electrolysis unit 20 sends the hydrogen gas generated by the water electrolysis stack 23 together with the water that was not electrolyzed from the water electrolysis stack 23 to the separation tank 31. Then, each water electrolysis unit 20 separates the medium sent from the water electrolysis stack 23 into water and hydrogen gas by the separation tank 31. Each water electrolysis unit 20 opens the water discharge valve 33 to a predetermined aperture and sends water to the water tank 11 via the water piping 13. Also, each water electrolysis unit 20 opens the pressure regulating valve 32 to a predetermined aperture and sends hydrogen to the hydrogen gas tank 51 via the hydrogen gas piping 61. Also, each water electrolysis unit 20 opens the pressure regulating valve 32a to a predetermined aperture and sends 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 the water that was not electrolyzed from the water electrolysis stack 23 to the separation tank 41. Then, each water electrolysis unit 20 separates the medium sent from the water electrolysis stack 23 into water and oxygen gas by the separation tank 41. Each water electrolysis unit 20 opens the water discharge valve 43 to a predetermined aperture and sends water to the water tank 11 via the water pipe 13. Each water electrolysis unit 20 also opens the pressure regulating valve 42 to a predetermined aperture and sends oxygen to the oxygen gas tank 52 via the oxygen gas pipe 62. Each water electrolysis unit 20 also opens the pressure regulating valve 42a to a predetermined aperture and sends 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] Moreover, in the water electrolysis system 100, the low-pressure hydrogen gas generated in each water electrolysis unit 20 is adjusted to a predetermined pressure (low pressure) by the pressure regulating valve 32a, and the hydrogen gas is recovered in the low-pressure hydrogen gas tank 51a via the low-pressure hydrogen gas piping 61a. Moreover, in the water electrolysis system 100, the high-pressure hydrogen gas generated in each water electrolysis unit 20 is adjusted to a predetermined pressure (high pressure) by the pressure regulating valve 32, and the hydrogen gas is recovered in the hydrogen gas tank 51 via the hydrogen gas piping 61.

[0026] The water electrolysis system 100 then boosts the hydrogen gas G1 stored in the low-pressure hydrogen gas tank 51a using the boost pump 70 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 the pressure regulating 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 regulating valve 71 to a predetermined aperture.

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

[0028] The water electrolysis system 100 then boosts the oxygen gas G2 stored in the low-pressure oxygen gas tank 52a using the boost pump 70 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 regulating 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 regulating 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 piping 61a, the low-pressure oxygen gas piping 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 from the following viewpoints.

[0030] For example, there is variation in characteristics and variation in electrolysis voltage due to deterioration in a typical water electrolysis stack 23. Therefore, even if the same current is passed through the water electrolysis stack 23, there is a difference in the electrolysis efficiency.

[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 32a and the pressure regulating valve 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 with the other water electrolysis stacks. Such a water electrolysis system 100 can operate the water electrolysis stacks 23 with increased electrolysis efficiency.

[0032] The pressure 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) is lower than the pressure 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 flowed 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 having a high electrolysis voltage) at a reduced pressure via these low-pressure gas lines to a low-pressure hydrogen gas tank 51a and a 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 boost 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 constantly monitors the opening and closing degrees of the pressure regulating valves 32, 32a, 42, 42a to prevent backflow of generated 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 to control the pressures in the hydrogen gas tank 51 and the oxygen gas tank 52.

[0035] <System Control Unit Configuration> 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 an exchange 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 replacement stack determination unit 81c is a component that determines whether or not the water electrolysis stack 23 needs to be replaced.

[0038] Further, the memory unit 82 stores reference information 82a and a control program Pr. The reference information 82a is information that is referenced when performing a hydrogen production operation, when setting various parameters, and the like. 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 in the system control unit 80 from the storage medium 99 directly or indirectly.

[0039] Data measured in the water electrolysis system 100, such as the water electrolysis stack voltage, the water electrolysis cell voltage, and the temperature, pressure, and flow rate of each part of the apparatus, are managed by a system control unit 80. The system control unit 80 calculates operation amounts required for stable operation of the water electrolysis stack 23 based on these 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 of the water electrolysis system 100, such as valves, pumps, and coolers, to control each device.

[0040] <Pressure adjustment system for water electrolysis system and its control method> The pressure adjustment system and its control method for the water electrolysis system 100 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 system connected to the gas tank is 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 pressure system or multiple pressure systems. The configuration of the water electrolysis system 100 is not limited to the configuration shown in Fig. 1. Furthermore, it is also possible to use the cell voltage as a control point for the electrolysis voltage, in addition to the electrolysis voltage of the water electrolysis stack 23.

[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 and the pressure regulating valve 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 the water electrolysis stack 23 with an increased electrolysis efficiency.

[0043] An example of pressure adjustment of the water electrolysis stack group is shown in Figures 3A and 3B. Note that, although Figures 3A and 3B show an example configuration of four water electrolysis stacks 23a to 23d, they do not show 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 diagram of Fig. 3A and the upper diagram of Fig. 3B, it is desirable to determine an operating voltage range and a warning voltage range for the water electrolysis stack 23 when the water electrolysis system 100 is operated (hydrogen production operation is performed). The operating voltage range is a voltage range in which hydrogen can be produced stably with high efficiency over a long period of time. In contrast, the warning voltage range is a voltage range in which a performance degradation or damage to the water electrolysis stack 23 is expected after a predetermined period of operation. Since 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-23d during operation. The lower diagram of FIG. 3A shows the pressures (internal pressures) of the water electrolysis stacks 23a-23d during operation. As shown in the lower diagram of FIG. 3A, when the pressures of the water electrolysis stacks 23a-23d during operation are equalized in the water electrolysis system 100, the electrolysis voltages of the water electrolysis stacks 23a-23d during operation fluctuate as shown in the lower diagram of FIG. 3A. In this case, there is a possibility that the water electrolysis stack 23 operates at an electrolysis voltage in the warning voltage range, exceeding the operating voltage range, as in 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 operate at an electrolysis voltage in the operating voltage range, but the water electrolysis stack 23c operates at an electrolysis voltage in the warning voltage range.

[0046] Therefore, in this 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 and 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 of the pressure difference between the electrode cell (not shown) on the hydrogen generation side and the electrode cell (not shown) on the oxygen generation side built in the water electrolysis stack 23c. In this way, the water electrolysis system 100 can adjust the pressure of the hydrogen gas or the oxygen gas only in the water electrolysis stack 23c. As a result, as shown in the lower diagram of FIG. 3B, although the pressure during operation of each of the water electrolysis stacks 23a to 23d fluctuates 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 operating voltage range while maintaining the voltage 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 pressure adjustment of the water electrolysis stacks 23a to 23d shown in FIGS. 3A and 3B.

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

[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 based on 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 water electrolysis system 100 includes, as part of the control program Pr stored in the storage unit 82, a calculation algorithm for calculating the electrolysis voltage at the target pressure of the water electrolysis stack 23 when pressure adjustment of each water electrolysis stack 23 is required. The water electrolysis system 100 also includes, as part of the control program Pr stored in the storage unit 82, a control algorithm for controlling the pressure adjustment valves 32, 32a, 42, 42a.

[0052] The water electrolysis system 100 controls the water flow rate control valve 21 and the small cooler 22 to cool the water sent from the water tank 11 to the water electrolysis stack 23c and store the water in the water electrolysis stack 23c. In this way, the water electrolysis system 100 cools the water electrolysis stack 23c. 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, so it is desirable to adjust the amount of heat removed by these factors.

[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 electrolysis voltage minus the theoretical electrolysis voltage. In the water electrolysis stack 23c, the amount of heat generated is reduced 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 adjustment may be performed multiple times for each water electrolysis stack 23. By adjusting the pressure by a similar process, the water electrolysis stack 23 that has entered the warning pressure range can continue to operate without reducing the voltage efficiency and the amount of hydrogen production.

[0055] Although the setting of the operating temperature range and the warning temperature range in the case of pressure adjustment has been described here, it is desirable to set the operating temperature range and the warning temperature range in the case of temperature adjustment in the same manner. 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 controlling to increase 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 drop 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 and a significant bias occurs in the reaction distribution within the electrode surfaces of the electrode cells, the electrolysis voltage of the water electrolysis stack 23 may not be able to drop from the caution voltage range to the operating voltage range by adjusting the pressure regulating valves 32a, 42a.

[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 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 can 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 operational voltage range. As a result, the water electrolysis system 100 can recover the electrolysis voltage of the water electrolysis stack 23. Then, the water electrolysis system 100 can operate the water electrolysis stack 23 by returning its operating conditions 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 the water electrolysis stack 23 in which the electrolysis voltage cannot be restored despite the process of restoring the electrolysis voltage of the water electrolysis stack 23. The parameters used as operating conditions can be, for example, any one of the pressure of the generated 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 has, as a 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 ​​and resets the operating conditions, thereby lowering the electrolysis voltage of the water electrolysis stack 23 from the warning voltage range to the operating voltage range.

[0060] The water electrolysis system 100 also includes a replacement stack determination unit 81c that determines whether or not to replace a water electrolysis stack 23 that is unable to restore the electrolysis voltage despite having undergone processing to restore the electrolysis voltage of the water electrolysis stack 23.

[0061] <Water electrolysis system operation> During hydrogen production operation, the water electrolysis system 100 performs voltage monitoring for each water electrolysis stack 23, and for any water electrolysis stack 23 for which an increase in electrolysis voltage has been detected by the voltage monitoring (hereinafter may be referred to as the "corresponding stack"), the water electrolysis system 100 performs a process to restore the electrolysis voltage by temporarily changing the operating conditions. Such an operation is mainly performed by the system control unit 80 of the water electrolysis system 100.

[0062] Note that "temporarily" here means that the hydrogen production operation ("operation in default state" in step S105 shown in FIG. 5A etc.) is interrupted, a process is performed to restore the electrolysis voltage, 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 example shown in FIG. 5A and FIG. 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 an electrode cell (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 recovering the electrolysis voltage of the stack is described so as to be clear. In the process of recovering the electrolysis voltage of the stack, any one parameter or a combination of multiple parameters among the pressure of the generated 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 operation value, and the electrolysis voltage is reduced to the operating voltage range. Here, with reference to Figs. 5A to 8B, first to fourth examples will be described as examples of the process of recovering the electrolysis voltage of the stack.

[0065] (First embodiment) Hereinafter, with reference to Figures 5A and 5B, the operation of a first embodiment of the water electrolysis system 100 when performing a process to restore the electrolysis voltage of the stack will be described. Figures 5A and 5B are flowcharts showing the operation of the first embodiment when performing a process to restore the electrolysis voltage. In the first embodiment, the pressure of the generated gas is adjusted to perform a process to restore the electrolysis voltage of the 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 a process to restore the electrolysis voltage. In an actual flow, if there is a water electrolysis stack 23 that performs a process to restore the electrolysis voltage, it is preferable to perform the process to restore the electrolysis voltage on that water electrolysis stack 23.

[0067] 5A, first, the water electrolysis system 100 performs hydrogen production operation in a default state by 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 the presence of a water electrolysis stack whose electrolysis voltage has increased to the warning voltage range (a relevant stack) via the operation control unit 81a (FIG. 2) (step S110). In this example, 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, by 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 operation voltage range (step S115). At this time, the water electrolysis system 100 calculates an electrolysis voltage (V2+ΔV) volts (FIG. 4) as a 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 determines, for the relevant stack (water electrolysis stack 23c), the target pressure of the generated gas to be a value calculated from the operating pressure characteristics of the relevant stack (water electrolysis stack 23c) by the value setting unit 81b (FIG. 2) (step S120). The target pressure determined at this time is a value 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 relevant stack.

[0071] After step S120, the water electrolysis system 100 reduces the pressure of the generated gas for the relevant stack (water electrolysis stack 23c) to the target pressure using the value setting unit 81b (FIG. 2), and performs hydrogen production operation using the operation control unit 81a (FIG. 2) (step S125). At this time, the water electrolysis system 100 adjusts the apertures of the pressure regulating valves 32a and 42a connected to the water electrolysis stack 23c to reduce the pressures of hydrogen gas and oxygen gas, which are generated gases in 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 is described as being performed regardless of the degree of deterioration of the electrode cell (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 or not the corresponding stack (water electrolysis stack 23c) is operating within the operation voltage range (step S130). The determination in step S130 is equivalent to determining whether or not the electrolysis voltage of the corresponding stack (water electrolysis stack 23c) has dropped from the warning voltage range to the operation voltage range, as shown in the upper diagrams of FIG. 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 return 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 in 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 is completed. The water electrolysis system 100 then terminates the process of recovering the electrolysis voltage of the 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 changes the generated gas pressure for the corresponding stack (water electrolysis stack 23c) to a value higher than the initial pressure using the value setting unit 81b (FIG. 2), and performs hydrogen production operation using the operation control unit 81a (FIG. 2) (step S135). The change in generated gas pressure (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 (the water electrolysis stack 23c) is operating within the operating voltage range (step S140), in the same manner as in 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 recovering the electrolysis voltage of the corresponding stack has been completed. Then, the water electrolysis system 100 ends the process of recovering 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 performs hydrogen production operation in the default state again.

[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 symbol "A11". In this case, the operation control unit 81a (Fig. 2) of the water electrolysis system 100 determines that the electrode cell (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 determines, as in step S120, a target pressure of the generated gas for the relevant stack (water electrolysis stack 23c) by the value setting unit 81b (FIG. 2) (step S150), which is calculated from the operating pressure characteristics of the relevant stack (water electrolysis stack 23c). The target pressure determined at this time is a value lower than the initial pressure (the pressure at step S105).

[0080] After step S150, the water electrolysis system 100 reduces the pressure of the generated gas for the relevant stack (water electrolysis stack 23c) to a target pressure using the value setting unit 81b (Figure 2), as in step S125, and performs hydrogen production operation using the operation control unit 81a (Figure 2) (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 is described as being performed when there is a possibility that the electrode cell (not shown) built into the corresponding stack is 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), in the same manner as in 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 symbol "B11". In this case, the water electrolysis system 100 causes the value setting unit 81b (Fig. 2) to return the generated gas pressure, which was changed (reduced) in step S155 for the corresponding stack (water electrolysis stack 23c), to the original state (initial pressure state) (step S185 in Fig. 5A). Thereafter, 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 determines, via the operation control unit 81a (FIG. 2), whether the electrolysis voltage of the corresponding stack (water electrolysis stack 23c) has reached a voltage at which degradation of the electrode cells (not shown) built in 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 (in the case of "No"), the process proceeds to step S185 shown in Fig. 5A via symbol "B11". In this case, the water electrolysis system 100 causes the value setting unit 81b (Fig. 2) to return the generated gas pressure, which was changed (reduced) in step S155, to the original state (initial pressure state) for the corresponding stack (water electrolysis stack 23c) (step S185 in Fig. 5A). Thereafter, 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 (if "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. In this case, the water electrolysis system 100 reduces the current flowing through the entire water electrolysis stacks 23 (hereinafter sometimes referred to as "series stacks") connected in series with the corresponding stack (water electrolysis stack 23c) by the value setting unit 81b (FIG. 2), and performs hydrogen production operation by the operation control unit 81a (FIG. 2) (step S170).

[0087] The process of step S170 is for performing hydrogen production operation while lowering the reaction overvoltage by lowering the pressure of the generated gas to a value lower than the initial pressure for the stack in question, in which the electrolysis voltage has not been restored despite the process of restoring the electrolysis voltage. Note that the "reaction overvoltage" here refers to a phenomenon in which the generated gas (hydrogen gas and oxygen gas) accumulates in the built-in electrode cells (not shown), causing the electrolysis voltage to rise, as in the 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 in question and improve the hydrogen production efficiency.

[0088] After step S170, the water electrolysis system 100 determines, via the stack replacement determination unit 81c (FIG. 2), whether or not the corresponding stack (the 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 electrolytic hydrogen production revenue < (electrolytic hydrogen production revenue after replacement - replacement cost of the relevant stack) …… (1) When the conditional expression (1) is satisfied, it is determined that the corresponding stack needs to be replaced, whereas when the conditional expression (1) is not satisfied, it is determined that the corresponding stack does not need to be replaced.

[0090] The determination in step S175 involves calculating and comparing the profit that can be obtained from the amount of hydrogen that can be produced in a predetermined operation period if the stack is not replaced with the profit that can be obtained from the amount of hydrogen that can be produced in a predetermined operation period if the stack is replaced. If the profit that can be obtained by replacing the stack is higher than the profit that can be obtained by not replacing the stack even after the replacement cost is incurred, the water electrolysis system 100 issues a warning to the user to urge the user 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 the stack (the water electrolysis stack 23c) does not need to be replaced (if "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 stack (water electrolysis stack 23c) is necessary (if "Yes"), the water electrolysis system 100 causes the replacement stack determination unit 81c (Figure 2) to issue a warning to the user to prompt replacement of the stack (water electrolysis stack 23c) and instructs the user to replace the stack (water electrolysis stack 23c).

[0093] In the first embodiment, if the electrolysis voltage of the stack falls from the warning voltage range to the operating voltage range after the target voltage and target pressure are changed as the operating conditions (if "Yes" in step S130), the process of recovering the electrolysis voltage of the stack is terminated and hydrogen production operation is performed (steps S185, S105). On the other hand, in the first embodiment, if the electrolysis voltage of the stack does not fall from the warning voltage range to the operating voltage range after the target voltage and target pressure are changed as the operating conditions (if "No" in step S130), the process of recovering the electrolysis voltage of the stack is performed by changing the operating conditions (step S135). Then, if the electrolysis voltage of the stack does not recover even after that (if "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 is higher than the profit from not replacing it, even incurring the replacement cost, the user can be made to replace the stack with another water electrolysis stack 23.

[0095] Moreover, in the first embodiment, the water electrolysis system 100 having a multiple series configuration that can utilize a general large DC power supply 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 the second embodiment of the water electrolysis system 100 when performing a process to restore the electrolysis voltage of the stack will be described. Figures 6A and 6B are flowcharts showing the operation of the second embodiment when performing a process to restore the electrolysis voltage. In the second embodiment, the flow rate of water supplied to the water electrolysis stack 23 is adjusted to perform a process to restore the electrolysis voltage of the stack.

[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] As shown in Fig. 6A, in step S120a, the water electrolysis system 100 determines, by 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 target flow rate determined at this time is higher than the initial flow rate (the flow rate at step S105). In the second embodiment, 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] Furthermore, in step S125a, the water electrolysis system 100 increases the flow rate of water supplied to the relevant stack (water electrolysis stack 23c) to a target flow rate using the operation control unit 81a (Figure 2), and performs hydrogen production operation using the operation control unit 81a (Figure 2).

[0100] Furthermore, in step S135a, the water electrolysis system 100 changes the flow rate of water supplied to the relevant 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 (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, by 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 memory unit 82. The target flow rate determined at this time is higher than the initial flow rate (the flow rate at step S105).

[0102] Furthermore, in step S155a, similar to step S125a, the water electrolysis system 100 reduces the pressure of the generated gas for the relevant stack (water electrolysis stack 23c) to a target pressure using the value setting unit 81b (Figure 2), and performs hydrogen production operation using the operation control unit 81a (Figure 2).

[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 as operating conditions, the second embodiment changes the target voltage and target flow rate.

[0104] (Third Example) Hereinafter, with reference to Figures 7A and 7B, the operation of the third embodiment of the water electrolysis system 100 when performing a process to restore the electrolysis voltage of the stack will be described. Figures 7A and 7B are flowcharts showing the operation of the third embodiment when performing a process to restore the electrolysis voltage. The third embodiment performs a process to restore the electrolysis voltage of the stack by adjusting the pressure of the generated 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 third embodiment performs the processes of steps S120b, S125b, S135b, S150b, and S155b instead of the processes 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 generated gas and a target flow rate of water to be supplied to the corresponding stack by the value setting unit 81b (FIG. 2). The target pressure determined at this time is a value calculated from the operating pressure characteristics of the corresponding stack (water electrolysis stack 23c) and is a value lower than the initial pressure (pressure at step S105). The target flow rate determined at this time is a value stored in the storage unit 82 as reference information 82a and is a value higher than the initial flow rate (flow rate at step S105). In the third embodiment, 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 reduces the pressure of the generated gas for the corresponding stack (water electrolysis stack 23c) to a target pressure using the value setting unit 81b (FIG. 2). The water electrolysis system 100 also increases the flow rate of water supplied to the corresponding stack (water electrolysis stack 23c) to a target flow rate using the value setting unit 81b (FIG. 2). The water electrolysis system 100 then performs hydrogen production operation using the operation control unit 81a (FIG. 2).

[0108] In step S135b, the water electrolysis system 100 changes the generated gas pressure for the corresponding stack (water electrolysis stack 23c) to a value higher than the initial pressure using the value setting unit 81b (FIG. 2). The water electrolysis system 100 also 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). The water electrolysis system 100 then performs a hydrogen production operation using the operation control unit 81a (FIG. 2). The generated gas pressure and the flow rate of water to be supplied (the generated gas pressure can be increased and the flow rate can be decreased) can be changed multiple times. At this time, the water electrolysis system 100 may simultaneously change the generated gas pressure and the flow rate of water to be supplied. Alternatively, the water electrolysis system 100 may alternately change the generated gas pressure and the flow rate of water to be supplied. Depending on the internal state of the water electrolysis stack 23, gas may be more effectively removed from the electrode cells by performing the change operations simultaneously, or may be more effectively removed from the electrode cells by performing the change operations alternately. When performing the change operations alternately, there is no restriction on the order of the change operations. However, since the change operation of the generated 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 (in the case of "Yes"), the water electrolysis system 100 determines that the process of recovering the electrolysis voltage of the corresponding stack has been completed. Then, the water electrolysis system 100 ends the process of recovering the electrolysis voltage of the corresponding stack, performs a pressure change operation to return the generated gas pressure changed (reduced) in step S125 to the 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 a target pressure of the generated gas and a target flow rate of water to be supplied to the stack by the value setting unit 81b (FIG. 2) for the stack (water electrolysis stack 23c). The target pressure determined at this time is a value calculated from the operating pressure characteristics of the stack (water electrolysis stack 23c) and is a value lower than the initial pressure (pressure at step S105). The target flow rate determined at this time is a value stored in the storage unit 82 as reference information 82a and is a value higher than the initial flow rate (flow rate at step S105).

[0111] In step S155b, similarly to step S125b, the water electrolysis system 100 reduces the pressure of the generated gas for the corresponding stack (water electrolysis stack 23c) to a target pressure using the value setting unit 81b (FIG. 2). In addition, the water electrolysis system 100 increases the flow rate of water supplied to the corresponding stack (water electrolysis stack 23c) to a target flow rate using the value setting unit 81b (FIG. 2). The water electrolysis system 100 then performs 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 (FIGS. 7A and 7B) changes the target voltage, target pressure, and target flow rate.

[0113] (Fourth Example) Hereinafter, with reference to Fig. 8A and Fig. 8B, the operation of the fourth embodiment of the water electrolysis system 100 when performing a process to restore the electrolysis voltage of the stack will be described. Fig. 8A and Fig. 8B are flowcharts showing the operation of the fourth embodiment when performing a process to restore the electrolysis voltage. The fourth embodiment performs a process to restore the electrolysis voltage of the stack by adjusting the pressure of the generated 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, for the stack (water electrolysis stack 23c), a target pressure of the generated gas, a target flow rate of water to be supplied to the stack, and a target temperature of the stack, using the value setting unit 81b (FIG. 2). The target pressure determined at this time is a value calculated from the operating pressure characteristics of the stack (water electrolysis stack 23c) and is a value lower than the initial pressure (pressure at step S105). The target flow rate determined at this time is a value stored in the storage unit 82 as reference information 82a and is a value higher than the initial flow rate (flow rate at step S105). The target temperature determined at this time is a value calculated from the operating pressure characteristics of the stack (water electrolysis stack 23c) and is a value 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 a process to recover the electrolysis voltage of the stack.

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

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

[0118] As shown in FIG. 8B, in step S150c, the water electrolysis system 100 determines, by the value setting unit 81b (FIG. 2), a target pressure of the generated 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), in the same manner as in step S120c. The target pressure determined at this time is a value calculated from the operating pressure characteristics of the stack (water electrolysis stack 23c), and is a value lower than the initial pressure (pressure at step S105). The target flow rate determined at this time is a value stored in the storage unit 82 as the reference information 82a, and is a value higher than the initial flow rate (flow rate at step S105). The target temperature determined at this time is a value calculated from the operating pressure characteristics of the stack (water electrolysis stack 23c), and is a value lower than the initial temperature (temperature at step S105).

[0119] In step S155c, similarly to step S125c, the water electrolysis system 100 reduces the pressure of the generated gas for the corresponding stack (water electrolysis stack 23c) to a target pressure using the value setting unit 81b (FIG. 2). The water electrolysis system 100 also increases the flow rate of water supplied to the corresponding stack (water electrolysis stack 23c) to a target flow rate using the value setting unit 81b (FIG. 2). The water electrolysis system 100 also reduces the temperature of the corresponding stack (water electrolysis stack 23c) to a target temperature using the value setting unit 81b (FIG. 2). The water electrolysis system 100 then performs 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 a plurality of water electrolysis stacks 23 that generate hydrogen gas and oxygen gas as generated 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 part of the system. During hydrogen production operation, the system controller 80 performs voltage monitoring on each water electrolysis stack 23, and performs a process of temporarily changing the operating conditions of a water electrolysis stack 23 in which an increase in electrolysis voltage is detected by the voltage monitoring to restore the electrolysis voltage. Here, the "water electrolysis stack 23 in 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 performs a process of recovering the electrolysis efficiency (performance) of the water electrolysis stack 23 for which an increase in the electrolysis voltage has been detected by voltage monitoring by temporarily changing the operating conditions. In this way, the water electrolysis system 100 according to the present embodiment can recover the electrolysis efficiency (performance) of the water electrolysis stack 23 by removing gas accumulated in the electrode cells (not shown) built in the water electrolysis stack 23 to the outside. The water electrolysis system 100 according to the present embodiment can prevent a decrease in the electrolysis efficiency (performance) of each water electrolysis stack 23 provided in the system as much as possible, thereby reducing the frequency of replacing the water electrolysis stack 23 and improving the hydrogen production efficiency. In addition, the water electrolysis system 100 according to the present embodiment can reduce the replacement cost of a water electrolysis stack 23 with a reduced electrolysis efficiency (performance) for another water electrolysis stack 23 and the disposal cost for disposal.

[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 reducing the pressure of the produced gas and then restoring the pressure to the initial pressure, as a process of restoring the electrolysis voltage to the water electrolysis stack 23 in which an increase in the electrolysis voltage has been detected.

[0124] In the water electrolysis system 100 according to the present embodiment, when gas accumulates in the electrode cells (not shown) built in 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 0.5 MPa. 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 to recover 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 steps S125, S135, and S185 in FIG. 5A , in the water electrolysis system 100 according to this embodiment, when the pressure of the generated gas in the water electrolysis stack 23 in which an increase in the electrolysis voltage has been detected is reduced, the system control unit 80 may be configured to increase the pressure to a value higher than the initial pressure and then return the pressure to the initial pressure, or to perform the operation of increasing the pressure multiple times and then return 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) built in 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 operation of changing the pressure multiple times for the water electrolysis stack 23 with the increased electrolysis voltage. In this way, the water electrolysis system 100 according to the present embodiment can efficiently remove the gas accumulated in the electrode cells (not shown) of the water electrolysis stack 23 from the water electrolysis stack 23 to recover 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 steps S155 to S170 in FIG. 5B , in the water electrolysis system 100 of this embodiment, for the water electrolysis stack 23 in which the electrolysis voltage does not recover despite the execution of a 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 lowering the reaction overpotential by reducing the pressure of the produced gas to a value lower than the initial pressure.

[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 in which the electrolysis voltage does not recover despite the treatment to recover 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 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)).

[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 controller 80 may be configured to perform a process of increasing the flow rate of water supplied to the water electrolysis stack 23 in which an increase in electrolysis voltage has been detected, and then returning the flow rate to the initial flow rate, 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 the water electrolysis stack 23 in which gas has accumulated in the electrode cells (not shown) built in 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 remove the gas accumulated in the electrode cells (not shown) of the water electrolysis stack 23 from the water electrolysis stack 23 to recover 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.

[0131] (6) As shown in steps S125a, S135a, and S185 in FIG. 6A , in the water electrolysis system 100 of 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 be configured to reduce the flow rate to a value lower than the initial flow rate and then return it to the initial flow rate, or to perform an operation of reducing the flow rate multiple times before returning it to the initial flow rate.

[0132] In the water electrolysis system 100 according to the present embodiment, when gas accumulates in the electrode cells (not shown) built in the water electrolysis stack 23 and the electrolysis voltage increases, the flow rate is changed, for example, from 5 ml / s to 4 ml / s to 6 ml / s to 5 ml / s. Alternatively, the water electrolysis system 100 according to the present embodiment performs an operation of reducing the flow rate multiple times for the water electrolysis stack 23 with the increased electrolysis voltage. In this way, the water electrolysis system 100 according to the present embodiment can efficiently remove the gas accumulated in the electrode cells (not shown) of the water electrolysis stack 23 from the water electrolysis stack 23 to recover 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 of this embodiment, for the water electrolysis stack 23 in which the electrolysis voltage does not recover despite the process of recovering the electrolysis voltage, the system control unit 80 may be configured to perform 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 in which the electrolysis voltage does not recover despite treatment to recover 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 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)).

[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 perform a process of restoring the electrolysis voltage to 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 restoring 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 generated 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 and the electrolysis voltage increases. The water electrolysis system 100 according to the present embodiment then returns the pressure of the generated gas and the flow rate of water supplied to the water electrolysis stack 23 to the initial pressure and initial flow rate. In this way, the water electrolysis system 100 according to the present embodiment can efficiently remove the gas accumulated in the electrode cells (not shown) of the water electrolysis stack 23 from the water electrolysis stack 23 to recover 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. The pressure of the generated gas and the flow rate of water supplied to the water electrolysis stack 23 may be changed simultaneously. Alternatively, the pressure of the generated gas and the flow rate of the 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 this embodiment, the system control unit 80 may be configured to perform a process of restoring the electrolysis voltage to 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 generated gases of hydrogen gas and oxygen gas and the flow rate of water supplied to the water electrolysis stack 23, and then restoring the pressure and flow rate to the initial values.

[0138] In the water electrolysis system 100 according to the present embodiment, the pressure of one or both of the generated gases of hydrogen gas and oxygen gas and the flow rate of water supplied to the water electrolysis stack 23 are changed for the water electrolysis stack 23 in which gas has accumulated in the electrode cells (not shown) built in the water electrolysis stack 23 and the electrolysis voltage has increased. Thereafter, the water electrolysis system 100 according to the present embodiment returns the generated gas pressure and the flow rate of water supplied to the water electrolysis stack 23 to the initial pressure and initial flow rate. 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 to recover 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.

[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 a 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 overvoltage.

[0140] The water electrolysis system 100 according to the present embodiment reduces the pressure of the produced gas to a value lower than the initial pressure and increases 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 overvoltage of the water electrolysis stack 23 in which the electrolysis voltage does not recover despite the treatment to recover 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 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 perform a process of restoring 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 restoring the pressure, flow rate, and temperature to the initial values.

[0142] The water electrolysis system 100 according to the present embodiment changes the pressure of the generated 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 and the electrolysis voltage has increased. 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 to recover 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.

[0143] (12) As shown in step S155c to step S170 in FIG. 8B , in the water electrolysis system 100 of 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 to reduce the reaction overvoltage.

[0144] In the water electrolysis system 100 according to the present embodiment, for a water electrolysis stack 23 in which the electrolysis voltage does not recover despite the treatment for recovering the electrolysis voltage, the pressure of the generated 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 ​​of the generated gas pressure, 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 Fig. 3B, for the water electrolysis stack 23 in which it has been detected that the electrolysis voltage has increased beyond an arbitrarily determined operating voltage range to an arbitrarily determined warning voltage range, the value setting unit 81b may change any one parameter or a combination of multiple parameters among the generated gas pressure, 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 to reduce the electrolysis voltage to the operating voltage range.

[0146] The water electrolysis system 100 according to the present embodiment can, by using the value setting unit 81b, reduce the electrolysis voltage to the operating voltage range for the water electrolysis stack 23 in which the electrolysis voltage has increased 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 controller 80 may be configured to include a stack replacement determination unit 81c that determines whether to replace the water electrolysis stack 23, in which the electrolysis voltage has not been restored despite the performance of a process to restore 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 been restored despite the execution of a 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, and therefore can reduce the replacement cost for replacing the 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, when an increase in electrolysis voltage has been detected in a water electrolysis stack 23 and a process for restoring the electrolysis voltage has been performed on the water electrolysis stack 23, a warning is issued to a user to prompt replacement of the water electrolysis stack 23.

[0150] The water electrolysis system 100 according to this embodiment can timely notify the user of a warning urging 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 modified examples. For example, the above-described embodiment has been described in detail to easily explain the present invention, and is not necessarily limited to having all of the configurations described. In addition, it is possible to replace a 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. In addition, it is possible to add, delete, or replace a part of each configuration with another configuration. [Explanation of symbols]

[0153] 11 Water tank 12 Water Pump 13 Water piping 14 Large cooler 20 Water electrolysis section 21 Water flow control valve 22 Small cooler 23, 23a, 23b, 23c, 23d Water electrolysis stack 30 Hydrogen system 31 Separation Tank 32 Pressure Regulating Valve 32a Pressure Regulating Valve 33 Water discharge valve 40 Oxygen System 41 Separation Tank 42 Pressure Regulating Valve 42a Pressure Regulating Valve 43 Water drain 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 Regulating Valve 72 Pressure Regulating Valve 80 System control section 81 Arithmetic section 81a Operation control unit 81b Value setting section 81c Exchange stack judgement unit 82 Memory section 82a References 90 Data Communications Department 99 Storage medium 100 Water electrolysis system G1 Hydrogen gas G2 Oxygen gas Pr control program

Claims

1. a plurality of water electrolysis stacks for generating hydrogen gas and oxygen gas as product gases from water; a water tank for supplying water to each of the water electrolysis stacks; a hydrogen gas tank for storing the hydrogen gas generated in each water electrolysis stack; an oxygen gas tank for storing the oxygen gas generated in each water electrolysis stack; A system control unit that controls the operation of each unit of the system, 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, performs a process of temporarily changing operating conditions to restore the electrolysis voltage. A water electrolysis system comprising:

2. The water electrolysis system according to claim 1, the system control unit performs a process of lowering the pressure of the generated gas and then restoring the pressure to an 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. A water electrolysis system comprising:

3. The water electrolysis system according to claim 2, When returning the pressure of the generated gas to the initial pressure after reducing the pressure of the generated gas, the system control unit increases the pressure to a value higher than the initial pressure and then returns the pressure to the initial pressure, or performs an operation of increasing the pressure multiple times and then returns the pressure to the initial pressure. A water electrolysis system comprising:

4. The water electrolysis system according to claim 2, the system control unit performs an operation to generate the hydrogen gas and the oxygen gas from the water while reducing a reaction overvoltage by reducing a pressure of the generated gas to a value lower than an initial pressure for a water electrolysis stack in which the electrolysis voltage does not recover despite the process of recovering the electrolysis voltage. A water electrolysis system comprising:

5. 2. The water electrolysis system of claim 1, the system control unit performs a process of increasing a flow rate of water supplied to the 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 to the water electrolysis stack. A water electrolysis system comprising:

6. The water electrolysis system according to claim 5, the system control unit, when increasing the flow rate of the 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 and then returns it to the initial flow rate, or performs an operation of reducing the flow rate multiple times and then returns it to the initial flow rate; A water electrolysis system comprising:

7. The water electrolysis system according to claim 5, the system control unit performs an operation to generate the hydrogen gas and the oxygen gas from the water while reducing a reaction overvoltage by increasing a flow rate of water supplied to the water electrolysis stack in which the electrolysis voltage does not recover despite the process of recovering the electrolysis voltage being performed, to the water electrolysis stack. A water electrolysis system comprising:

8. 2. The water electrolysis system of claim 1, the system control unit performs a process of restoring the electrolysis voltage of the 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 returning the pressure and the flow rate to the initial values. A water electrolysis system comprising:

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

10. 10. The water electrolysis system of claim 9, the system control unit, for a water electrolysis stack in which the electrolysis voltage does not recover despite the process of recovering the electrolysis voltage, performs an operation to produce the hydrogen gas and the oxygen gas from the water while reducing a reaction overvoltage by reducing the pressure of the generated gas to a value lower than an initial pressure and increasing a flow rate of water supplied to the water electrolysis stack to a value higher than an initial flow rate. A water electrolysis system comprising:

11. 2. The water electrolysis system of claim 1, the system control unit performs a process of restoring the electrolysis voltage of the 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 returning the pressure, flow rate, and temperature to their initial values. A water electrolysis system comprising:

12. The water electrolysis system according to claim 11, the system control unit, for a water electrolysis stack in which the electrolysis voltage does not recover despite having performed the process of recovering the electrolysis voltage, performs an operation 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, 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 a reaction overvoltage; A water electrolysis system comprising:

13. 2. The water electrolysis system of claim 1, the system control unit includes a value setting unit that sets values ​​of a pressure of the generated gas, a flow rate of water supplied to the water electrolysis stack, and a temperature of the water electrolysis stack; the value setting unit performs a process of changing any one or a combination of 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 operation value for the water electrolysis stack in which it has been detected that the electrolysis voltage has exceeded an arbitrary operation voltage range and reached an arbitrary warning voltage range, thereby lowering the electrolysis voltage to the operation voltage range. A water electrolysis system comprising:

14. 2. The water electrolysis system of claim 1, the system control unit includes a stack replacement determination unit configured to determine whether to replace a water electrolysis stack in which the electrolysis voltage is not restored despite the process for restoring the electrolysis voltage with another water electrolysis stack. A water electrolysis system comprising:

15. A method for operating a water electrolysis system including a plurality of water electrolysis stacks, comprising: When 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 but the electrolysis voltage has not yet been restored, a warning is issued to a user to urge replacement. A method for operating a water electrolysis system comprising the steps of:

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