Water electrolysis system and water electrolysis method

Abstract

This water electrolysis system has a plurality of DC power supplies and a plurality of water electrolysis stacks connected to the plurality of DC power supplies. The water electrolysis system comprises an operation control unit that individually controls the currents of the plurality of DC power supplies. The operation control unit is configured to individually control the currents of the plurality of DC power supplies so that either the average value of the stack voltages or the average value of the cell voltages of the plurality of water electrolysis stacks decreases according to a designated hydrogen production amount. Accordingly, hydrogen can be produced with high efficiency in consideration of variations in the initial performance and performance deterioration of the water electrolysis stacks.

Description

Water electrolysis system and water electrolysis method

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

[0002] Hydrogen is a clean energy that does not emit carbon dioxide during combustion. Therefore, hydrogen is attracting attention as one of the clean energies for countermeasures against global warming. As an apparatus for producing hydrogen, the development of an apparatus for producing hydrogen on a large scale by electrolyzing water using renewable energy or the like has been promoted. Hydrogen produced using renewable energy is called CO2-free hydrogen.

[0003] CO2-free hydrogen is produced based on energy sources such as renewable energy and nuclear power. Renewable energy such as wind power and solar power has a large time variation in output, and when its power generation capacity increases, an adjustment power for adjusting the power supply-demand balance is required. On the other hand, a water electrolysis system for producing hydrogen has a large power consumption. From this, a water electrolysis system that can adjust the power consumption and serve as a power adjustment power has been proposed.

[0004] Patent Document 1 describes a technique for setting the number of water electrolysis stacks to be operated according to the input power so that the overall power efficiency is maximized within a current range capable of reducing the amount of cross leakage, improving the durability of the water electrolysis stacks, and making the power efficiency of the water electrolysis stacks appropriate.

[0005] Japanese Patent Application Laid-Open No. 2021-181605

[0006] As described in Patent Document 1, the power efficiency of the water electrolysis stack can be made appropriate by changing the number of water electrolysis stacks to be operated according to the input power. By the way, due to variations in the initial performance and performance degradation of the water electrolysis stacks, the hydrogen production efficiency of each water electrolysis stack is different. However, conventionally, hydrogen has not been efficiently produced in consideration of such variations in the initial performance and performance degradation of the water electrolysis stacks.

[0007] The object of the present invention is to provide a water electrolysis system and a water electrolysis method that can produce hydrogen with high efficiency, while taking into account variations in the initial performance and performance degradation of the water electrolysis stack.

[0008] To solve the above problems, for example, the configuration described in the claims is adopted. The present invention includes multiple means for solving the above problems, but one example is a water electrolysis system having multiple DC power supplies and multiple water electrolysis stacks connected to the multiple DC power supplies, and comprising an operation control unit that individually controls the current of the multiple DC power supplies. The operation control unit individually controls the current of the multiple DC power supplies so that the average value of the cell voltages of the multiple water electrolysis stacks or the average value of the stack voltages of the multiple water electrolysis stacks becomes lower, according to a specified hydrogen production amount.

[0009] According to the present invention, in a large-scale water electrolysis system that operates multiple water electrolysis stacks simultaneously, hydrogen can be produced with high efficiency even when using water electrolysis stacks with variations in initial performance and performance degradation. Other problems, configurations, and effects will be clarified by the following description of embodiments.

[0010] This is a configuration diagram showing an example of a water electrolysis system according to the first embodiment of the present invention. This is a block diagram showing an example of the configuration of the system control unit of the water electrolysis system according to the first embodiment of the present invention. This is a flowchart showing an example of current setting by the water electrolysis system according to the first embodiment of the present invention. This is a diagram showing an example of the relationship between the current of the water electrolysis stack and the amount of hydrogen produced. This is a configuration diagram showing an example of a water electrolysis system according to the second embodiment of the present invention.

[0011] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings illustrating each embodiment, the same components and processes are denoted by the same reference numerals, and the description of redundant parts is omitted. Furthermore, the components and processes shown in the drawings are only schematic in order to allow for a sufficient understanding of the present invention. Therefore, the present invention is not limited to the illustrated examples.

[0012] <First Embodiment Example> A water electrolysis system and water electrolysis method according to the first embodiment example of the present invention will be described with reference to Figures 1 to 4.

[0013] [About the Water Electrolysis Stack] Figure 1 shows the configuration of the water electrolysis system 100 in this embodiment. As shown in Figure 1, the water electrolysis system 100 is equipped with multiple (many) water electrolysis stacks 10. The water electrolysis stacks 10 generate hydrogen from water when power is supplied to them. The water electrolysis stacks 10 in this embodiment include alkaline type, solid polymer type, anion exchange membrane type, etc. The water electrolysis stacks 10 in this embodiment are applicable to any type of electrolytic cell.

[0014] In typical water electrolysis stacks, the energy conversion efficiency from electricity to hydrogen is around 60-80%, with the remainder being converted into heat, which heats the components of the water electrolysis stack and the water flowing inside. Due to variations in initial performance and performance degradation, even with the same configuration, the hydrogen production efficiency of each water electrolysis stack differs. Therefore, in large-scale water electrolysis systems that operate multiple water electrolysis stacks simultaneously, it is necessary to operate water electrolysis stacks in a way that produces hydrogen with high efficiency.

[0015] The hydrogen production efficiency of a water electrolysis stack is proportional to the average cell voltage during hydrogen production. In water electrolysis systems with multiple orthogonal transducers, variations occur in the total hydrogen production amount of the water electrolysis system, the initial performance of the water electrolysis stack, and performance degradation. Therefore, it is important to improve hydrogen production efficiency by supplying a current amount controlled according to the variations in the total hydrogen production amount of the water electrolysis system, the initial performance of the water electrolysis stack, and performance degradation. In this embodiment, the hydrogen production efficiency is improved by appropriately controlling the current amount through the process described below.

[0016] [Configuration of the Water Electrolysis System] The configuration of the water electrolysis system 100 shown in Figure 1 will be described below. As shown in Figure 1, the water electrolysis system 100 according to this embodiment includes a transformer 21, a quadrature converter 22, a harmonic filter 23, a switch 24, a water electrolysis stack 10, a system control unit 80, and a data communication unit 90. The water electrolysis system 100 in this embodiment is a large-scale system in which multiple (many) water electrolysis stacks 10 are installed, and multiple transformers 21 and quadrature converters 22 are installed. In particular, in this embodiment, a quadrature converter 22 is installed in pairs with each water electrolysis stack 10.

[0017] In other words, the grid power 30, which is high-voltage AC power, is transformed to a predetermined voltage by a multi-stage transformer 21 via a switch 24, and then branched into multiple systems. Then, the AC power is stepped down to the required voltage by a transformer 21 connected to each system via a switch 24, and supplied to the orthogonal converter 22, which is installed in pairs with each water electrolysis stack 10.

[0018] Each orthogonal converter 22 converts the supplied AC power into DC power and supplies the converted DC power to the water electrolysis stack 10. Therefore, the orthogonal converters 22 function as DC power sources. When the orthogonal converters 22 convert to DC power, the operation control unit 81a (see Figure 2) of the system control unit 80, which will be described later, individually controls the amount of current of the converted DC power in each orthogonal converter 22. In the example shown in Figure 1, the orthogonal converters 22 and the water electrolysis stack 10 are connected as a pair. There are no particular restrictions on the number of series-parallel connections of the transformer 21 and the number of parallel connections of the orthogonal converters 22. The harmonic filter 23 is connected to the grid to suppress the high frequencies generated by the orthogonal converters 22.

[0019] The transformer 21, orthogonal converter 22, and switch 24 shown in Figure 1 are controlled by the system control unit 80. The system control unit 80 controls the transformer 21, orthogonal converter 22, and switch 24 based on hydrogen production commands obtained via the data communication unit 90. These hydrogen production commands include, for example, commands to produce hydrogen at 100% production capacity, 90% production capacity, and 80% production capacity. These commands allow for multi-stage or stepless adjustment of the production capacity. These hydrogen production commands are transmitted from a terminal (not shown) operated by an operator giving instructions for hydrogen production. Alternatively, instructions for hydrogen production capacity may be automatically given from a terminal that monitors the status of electricity generation as renewable energy.

[0020] [Configuration of the System Control Unit] Figure 2 is a block diagram showing the configuration of the system control unit of the water electrolysis system. As shown in Figure 2, the system control unit 80 is composed of a computer having 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 orthogonal converter current calculation unit 81c. The operation control unit 81a performs operation control processing to control the hydrogen production operation of the water electrolysis system 100.

[0021] The value setting unit 81b performs value setting processing to set various parameters during operation control. The orthogonal converter current calculation unit 81c calculates the DC current amount of the orthogonal converter 22. The DC current amount of the orthogonal converter 22 here is the DC current amount of each of the multiple orthogonal converters 22 provided, as shown in Figure 1.

[0022] The memory unit 82 stores reference information 82a and a control program Pr. The reference information 82a is referenced when running hydrogen production operations and setting various parameters. This reference information 82a includes the initial hydrogen production efficiency of each water electrolysis stack 10 installed in the water electrolysis system 100, as well as data on hydrogen production efficiency measured at any given time.

[0023] The control program Pr is a program that makes the calculation unit 81 function as the operation control unit 81a, the value setting unit 81b, and the orthogonal converter current calculation unit 81c. The control program Pr is stored in the storage medium 85 and installed directly or indirectly from the storage medium 85 to the storage unit 82 of the system control unit 80. Reference information 82a may also be transferred from the storage medium 85 to the storage unit 82 of the system control unit 80.

[0024] Data such as the water electrolysis stack voltage, water electrolysis cell voltage, temperature, pressure, flow rate, and current of each part of the water electrolysis system 100 are managed by the system control unit 80. The system control unit 80 controls the DC power using the orthogonal converter 22 based on the operation command from the operation control unit 81a. The system control unit 80 then calculates the operation quantities necessary for the stable operation of the water electrolysis stack 10 based on this data and the reference information 82a that has been pre-registered in the storage unit 82.

[0025] Furthermore, the system control unit 80 transmits control signals to each component of the water electrolysis system 100, such as valves, pumps, and coolers, to control each component. Note that in the system configuration of the water electrolysis system 100 shown in Figure 1, only the configuration for supplying DC power to the water electrolysis stack 10 is shown, and other components of the water electrolysis system 100, such as valves, pumps, and coolers, are not shown.

[0026] [Operation Control Process] Figure 3 is a flowchart showing the process flow of the system control unit 80 in this embodiment controlling hydrogen production in the water electrolysis stack 10. First, the operation control unit 81a of the system control unit 80 determines whether the instruction received via the data communication unit 90 is an instruction to operate at 100% hydrogen production capacity (step S11).

[0027] When the instruction for 100% hydrogen production capacity is given in step S11 (YES in step S11), the operation control unit 81a sets the DC power converted by each orthogonal converter 22 to the current amount for 100% hydrogen production capacity (step S12). With this setting of the current amount for 100% hydrogen production capacity, the current amount for 100% hydrogen production capacity is supplied to each water electrolysis stack 10, and each water electrolysis stack 10 performs the maximum hydrogen production process.

[0028] Furthermore, if a hydrogen production capacity other than 100% is instructed in step S11 (NO in step S11), the operation control unit 81a acquires reference information 82a, such as the cell voltage of each water electrolysis stack 10, stored in the memory unit 82 (step S13). Then, the orthogonal converter current calculation unit 81c calculates the current amount of each orthogonal converter 22 such that, when the hydrogen production capacity is instructed to be other than 100%, the average value of the cell voltages of all water electrolysis stacks or the average value of the stack voltages is the lowest (step S14).

[0029] Here, the current amount at which the average value of the cell voltage of the entire water electrolysis stack, or the average value of the stack voltage, is lowest is determined, for example, by setting the current of each water electrolysis stack 10 based on the difference in the rise of the cell voltage from the lowest value of the average cell voltage at the rated current value of the water electrolysis stack 10, according to the specified hydrogen generation amount. In other words, water electrolysis stacks with a high average cell voltage are set to a low current amount, and water electrolysis stacks with a low average cell voltage are set to a high current amount. This lowers the average value of the cell voltage for the entire system.

[0030] Once the current amount is calculated in step S14, the operation control unit 81a sets the calculated current amount to the individual orthogonal converters 22 and supplies DC power to each water electrolysis stack 10 (step S15). Based on this setting of the current amount for hydrogen production capacity other than 100%, each water electrolysis stack 10 is supplied with the corresponding current amount for its production capacity, and each water electrolysis stack 10 performs the hydrogen production process corresponding to the current amount.

[0031] After hydrogen production begins due to the setting of the current amount in step S12 or step S15, the operation control unit 81a determines whether or not it has received an instruction to change the hydrogen production capacity according to the instruction received via the data communication unit 90 (step S16). If an instruction to change the hydrogen production capacity is not received in step S16 (NO in step S16), the operation control unit 81a controls the current amount converted by each orthogonal converter 22 to maintain the current amount set in step S12 or step S15. On the other hand, if an instruction to change the hydrogen production capacity is received in step S16 (YES in step S16), the operation control unit 81a returns to the determination in step S11.

[0032] The above control processes enable hydrogen production by each water electrolysis stack 10. Next, we will explain how the current is adjusted according to the hydrogen production capacity to lower the average current value. Generally, the water electrolysis stacks 10 have variations in initial performance and performance degradation, resulting in different hydrogen production efficiencies for each water electrolysis stack.

[0033] For example, Figure 4 shows the characteristics of a certain water electrolysis stack. In Figure 4, the vertical axis represents the voltage [V] of the cells constituting the water electrolysis stack, and the horizontal axis represents the current density [A / cm²] of the cells. Characteristic a shown in Figure 4 is the initial characteristics of the cells constituting the water electrolysis stack, called BOL (Beginning of Life). Characteristic c is the characteristics of the cells constituting the water electrolysis stack when they are at their most degraded, called EOL (End of Life), and characteristic b is the characteristics of a certain water electrolysis stack 10 that were actually measured.

[0034] As shown in Figure 4, in characteristics a, b, and c, the current density increases as the cell voltage increases, and the amount of hydrogen produced also increases proportionally to the current. In a single water electrolysis system, the hydrogen production efficiency during hydrogen production in the water electrolysis stack is uniquely determined by the cell voltage of the water electrolysis stack, and it can be seen that the lower the cell voltage, the higher the hydrogen production efficiency. Note that the characteristics of BOL (characteristic a) and EOL (characteristic c) shown in Figure 4 also differ due to variations in the water electrolysis stack. Note that the characteristics shown in Figure 4 are merely examples.

[0035] By the way, as can be seen by comparing the characteristics a of BOL and characteristics c of EOL shown in Figure 4, even at the same cell voltage, the current density differs depending on whether or not the water electrolysis stack has deteriorated. This current density differs even further when considering the variability of the cells themselves. For example, in the cells with the characteristics shown in Figure 4, the BOL with characteristic a, which has no deterioration, has a current density of approximately 1.3 [A / cm2] when the cell voltage is 1.9V, and it can be seen that hydrogen is produced with high efficiency at a higher current density compared to characteristics b and c. On the other hand, the deteriorated EOL with characteristic c has a current density of approximately 0.8 [A / cm2] when the cell voltage is 1.9V, and it can be seen that the hydrogen production efficiency is lower compared to the BOL with characteristic a.

[0036] In this embodiment, the system control unit 80 increases the current to the water electrolysis stack 10 with high conversion efficiency and decreases the current to the water electrolysis stack 10 with low conversion efficiency, according to the instructed hydrogen production amount (production capacity), so that the overall average cell voltage or stack voltage is as low as possible. In other words, the water electrolysis system 100 according to this embodiment does not supply the same amount of current to the water electrolysis stack 10 from each orthogonal converter 22, but rather supplies a controlled amount of current according to the hydrogen production amount of the entire water electrolysis system 100, according to the initial performance and the variation in performance degradation. Specifically, the operation control unit 81a, for example, controls the current of the water electrolysis stack 10 by changing it according to the difference in the rise of the cell voltage from the lowest value of the average cell voltage at the rated current value of the water electrolysis stack 10, according to the specified hydrogen production amount. As a result, water electrolysis stacks 10 with high average cell voltages are controlled with a low current, and water electrolysis stacks with low average cell voltages are controlled with a high current, so that the average value of the cell voltage of the entire system is low. As a result, according to the water electrolysis system 100 of this embodiment, it is possible to generate the desired hydrogen with minimal current consumption.

[0037] Furthermore, it is preferable that the characteristics of the water electrolysis stack 10 be based on the latest data. For this reason, the accuracy of the control can be improved by updating the current amount calculation program of the orthogonal converter current calculation unit 81c based on the latest operating data stored in the memory unit 82. Also, when the operation control unit 81a sets the current amount of the water electrolysis stack 10 variably based on the calculation by the orthogonal converter current calculation unit 81c, it is preferable to operate in a way that the overall voltage is as low as possible from the standpoint of reducing power consumption, but operating in a way that is as low as possible is just one example. In this embodiment, if the average cell voltage or stack voltage of the entire system can be reduced by changing the current amount of some or all of the orthogonal converters 22 to be lower than when all orthogonal converters 22 are set to the same current amount, the effect of reducing power consumption can be obtained.

[0038] <Second Embodiment Example> Next, with reference to Figure 5, a water electrolysis system and water electrolysis method according to a second embodiment example of the present invention will be described. In Figure 5 illustrating this embodiment example, the same reference numerals are used for parts corresponding to Figure 1 described in the first embodiment example, and redundant explanations are omitted. In this embodiment example, the connection configuration of the water electrolysis stack 10 to the power supply differs from that of the water electrolysis system 100 shown in Figure 1.

[0039] [Configuration of the Water Electrolysis System] The configuration of the water electrolysis system 100 shown in Figure 5 is as follows: The water electrolysis system 100 consists of multiple water electrolysis stacks 10 connected in series and parallel to a single DC power supply, the orthogonal converter 22. Multiple units connected in series and parallel are connected together. Specifically, as shown in Figure 5, two water electrolysis stacks 10 connected in series and three sets of water electrolysis stacks 10-1a, ..., 10-1n (where n is an arbitrary integer) connected in parallel are connected in series and connected to one orthogonal converter 22. In addition, another group of water electrolysis stacks 10-2a, ..., 10-2n (where n is an arbitrary integer) with the same connection configuration are connected to one orthogonal converter 22.

[0040] Each orthogonal converter 22 is supplied with power from the grid power 30 via a switch 24 and a transformer 21. Note that the two orthogonal converters 22 shown in Figure 5 are just one example; a configuration with more water electrolysis stacks and orthogonal converters 22 is also possible. Furthermore, the number of series-parallel connections of the water electrolysis stack 10 is also just one example and is not limited to the example shown in Figure 5.

[0041] In the water electrolysis system 100 shown in Figure 5, the amount of DC power supplied from each orthogonal converter 22 to each water electrolysis stack group is individually controlled by the system control unit 80. The process by which the system control unit 80 individually controls the amount of current is the same as the process described in the flowchart of Figure 3 in the first embodiment. That is, the system control unit 80 supplies a controlled amount of current to each water electrolysis stack group according to the hydrogen production amount of the entire water electrolysis system 100, and according to the variation in initial performance and performance degradation.

[0042] In other words, the system control unit 80 controls the current of the water electrolysis stack by changing it according to the specified hydrogen generation amount, based on the difference in the rise of the cell voltage from the lowest value of the average cell voltage at the rated current value of the water electrolysis stack. As a result, water electrolysis stacks with high average cell voltages are controlled with a low current, and water electrolysis stacks with low average cell voltages are controlled with a high current, thereby operating the system in a way that lowers the average value of the cell voltage for the entire system.

[0043] Thus, also in the case of the present embodiment example, similar to the first embodiment example, desired hydrogen generation can be performed with minimal power consumption. However, in the case of the present embodiment example, since the orthogonal converter 22 is installed for each of the water electrolysis stacks 10 connected in series-parallel, the amount of current of the power source converted by the orthogonal converter 22 becomes the optimal amount of current when the series-parallel connected water electrolysis stack group is handled collectively. Therefore, compared with the example of FIG. 1, although there is a possibility that the amount of suppression of the overall current consumption may slightly decrease, the number of orthogonal converters 22 can be reduced, so that the system configuration can be simplified accordingly. Also, when the system control unit 80 calculates the amount of current or measures each water electrolysis stack, it may be performed in units of the series-parallel connected water electrolysis stack group, and thus the processing and configuration for control can also be simplified.

[0044] <Modification Example> The present invention is not limited to the embodiment examples described so far, and includes various modification examples. For example, the above-described embodiment examples have been described in detail for easy understanding of the present invention, and are not necessarily limited to those having all the configurations described. Also, a part of the configuration of one embodiment example can be replaced with the configuration of another embodiment example, and the configuration of another embodiment example can also be added to the configuration of one embodiment example. Also, for a part of the configuration of each embodiment example, addition, deletion, or replacement with other configurations is possible.

[0045] Furthermore, when calculating the current amount for individual orthogonal converters 22 in step S14 of the flowchart in Figure 3 so that the average value of the cell voltage or stack voltage of all orthogonal converters 22 is lowest, some inefficient water electrolysis stacks 10 may be shut down. Alternatively, if it is not operationally desirable to completely shut down only some of the water electrolysis stacks 10, the inefficient water electrolysis stacks 10 may be allowed to generate hydrogen with the minimum amount of current, while still achieving the lowest average value of the current amounts for all orthogonal converters 22. Another example of step S14 in the flowchart in Figure 3 is calculating and controlling the current amount for individual orthogonal converters 22 so that the average value of the cell voltage or stack voltage of all orthogonal converters 22 is lowest. In practice, it is preferable to appropriately set the current amount for each orthogonal converter 22 by comprehensively considering the desired effect of current reduction and various requirements such as the lifespan of the water electrolysis stacks 10.

[0046] Furthermore, in the configuration diagrams shown in Figures 1, 2, and 5, only control lines and information lines deemed necessary for explanation are shown, and not all control lines and information lines are necessarily shown in the actual product. In reality, it can be assumed that almost all components are interconnected. Also, the flowchart shown in Figure 3 is just one example, and if the processing result is the same, some processing orders may be changed or multiple processes may be executed simultaneously. In addition, the system control unit 80 shown in each embodiment example may be configured by implementing a program that executes the processing method described in the flowchart of Figure 3 on a general-purpose computer device, for example. In this case, the program may be stored on an external recording medium such as memory, IC card, SD card, or optical disc, and transferred to a computer that functions as the system control unit 80.

[0047] 10...Water electrolysis stack, 10-1a to 10-1n, 10-2a to 10-12...Water electrolysis stack group, 21...Transformer, 22...Orthogonal converter, 23...Harmonic filter, 24...Switch, 30...System power, 80...System control unit, 81...Calculation unit, 81a...Operation control unit, 81b...Value setting unit, 81c...Orthogonal converter current calculation unit, 82...Storage unit, 82a...Reference information, 85...Storage medium, 90...Data communication unit, 100...Water electrolysis system

Claims

1. A water electrolysis system having a plurality of DC power supplies and a plurality of water electrolysis stacks connected to the plurality of DC power supplies, comprising an operation control unit that individually controls the current of the plurality of DC power supplies, wherein the operation control unit individually controls the current of the plurality of DC power supplies such that the average value of the cell voltages of the plurality of water electrolysis stacks or the average value of the stack voltages of the plurality of water electrolysis stacks becomes lower, according to a specified hydrogen production amount.

2. The water electrolysis system according to claim 1, wherein each water electrolysis stack is connected to a pair of DC power supplies.

3. The water electrolysis system according to claim 1, wherein one DC power supply is connected to each unit in which multiple water electrolysis stacks are connected in series and parallel.

4. The water electrolysis system according to claim 1, wherein the operation control unit controls the water electrolysis stack by changing the current of the water electrolysis stack according to a specified hydrogen generation amount, based on the difference in the rise of the cell voltage from the lowest value of the average cell voltage at the rated current value of the water electrolysis stack, thereby controlling water electrolysis stacks with high average cell voltages with low currents and controlling water electrolysis stacks with low average cell voltages with high currents, so as to lower the average value of the cell voltage of the entire system.

5. The water electrolysis system according to claim 1, wherein the operation control unit individually controls the current of the plurality of DC power supplies so that the average value of the overall cell voltage of the plurality of water electrolysis stacks becomes low when the hydrogen production amount is less than 100% of the specified amount.

6. A water electrolysis method for generating hydrogen using a plurality of DC power supplies and a plurality of water electrolysis stacks connected to the plurality of DC power supplies, wherein the operation control process is performed to individually control the current of the plurality of DC power supplies, and as the operation control process, the current of the plurality of DC power supplies is individually controlled so that the average value of the cell voltages of the plurality of water electrolysis stacks or the average value of the stack voltages of the plurality of water electrolysis stacks becomes lower, according to a specified amount of hydrogen generated.

Images