Water electrolysis system and activation method for water electrolysis system

Abstract

A water electrolysis system (10) comprises a plurality of water electrolysis devices (12), a power supply device (14) for supplying current to the plurality of water electrolysis devices, a tank (16) that can accommodate oxygen gas generated at the plurality of water electrolysis devices, and a control unit (64) that controls supply of current from the power supply device to the plurality of water electrolysis devices. The control unit starts supply of current from the power supply device to a first water electrolysis device (12a) of the plurality of water electrolysis devices and then starts supply of current from the power supply device to a second water electrolysis device (12b) of the plurality of water electrolysis devices.

Description

Water electrolysis system and method for starting water electrolysis system

[0001] The present disclosure relates to a water electrolysis system and a method for starting a water electrolysis system.

[0002] In recent years, research and development have been conducted on water electrolysis systems that contribute to energy efficiency in order to enable more people to secure access to affordable, reliable, sustainable and advanced energy.

[0003] Japanese Unexamined Patent Application Publication No. 2023-140407 discloses a method for starting a water electrolysis device (water electrolyzer).

[0004] In a water electrolysis system provided with a plurality of water electrolyzers, there is a long-felt need for a favorable method for starting the water electrolysis system.

[0005] An object of the present disclosure is to solve the above-described problems.

[0006] A first aspect of the present disclosure provides a water electrolysis system comprising: a plurality of water electrolyzers, each water electrolyzer comprising a water electrolysis cell having a membrane electrode assembly formed by sandwiching an electrolyte membrane between a first electrode catalyst layer and a second electrode catalyst layer, the water electrolysis cell generating oxygen gas in the first electrode catalyst layer and hydrogen gas in the second electrode catalyst layer by electrolyzing water; a power supply device for supplying electric current to the plurality of water electrolyzers; a tank capable of containing oxygen gas generated by the plurality of water electrolyzers; and a control unit that controls supply of electric current from the power supply device to the plurality of water electrolyzers, wherein the control unit starts supply of electric current from the power supply device to a first water electrolyzer among the plurality of water electrolyzers, and then starts supply of electric current from the power supply device to a second water electrolyzer among the plurality of water electrolyzers.

[0007] A second aspect of the present disclosure is a method for starting a water electrolysis system, wherein the water electrolysis system comprises a plurality of water electrolysis devices, each having a membrane electrode assembly formed by sandwiching an electrolyte membrane between a first electrode catalyst layer and a second electrode catalyst layer, and a water electrolysis cell that generates oxygen gas in the first electrode catalyst layer and hydrogen gas in the second electrode catalyst layer by electrolyzing water; a power supply device for supplying current to the plurality of water electrolysis devices; a tank capable of containing the oxygen gas generated by the plurality of water electrolysis devices; and a control unit that controls the supply of current from the power supply device to the plurality of water electrolysis devices, wherein the control unit starts supplying current from the power supply device to the first water electrolysis device among the plurality of water electrolysis devices, and then the control unit starts supplying current from the power supply device to the second water electrolysis device among the plurality of water electrolysis devices.

[0008] According to the present invention, a better water electrolysis system and a method for starting a water electrolysis system can be provided.

[0009] Figure 1 is a schematic diagram of a water electrolysis system. Figure 2 is a schematic diagram of a water electrolysis cell. Figure 3 is a control block diagram of a water electrolysis system. Figure 4 is a flowchart showing an example of how to start up a water electrolysis system. Figure 5 is a timing chart showing an example of the operation described by the flowchart in Figure 4. Figure 6 is a flowchart showing how to start up a water electrolysis system according to the first modified example. Figure 7 is a timing chart showing an example of the operation described by the flowchart in Figure 6. Figure 8 is a flowchart showing how to start up a water electrolysis system according to the second modified example. Figure 9 is a timing chart showing an example of the operation described by the flowchart in Figure 8. Figure 10 is a timing chart showing an example of the operation described by the flowchart in Figure 8.

[0010] The water electrolysis apparatus comprises an electrolytic cell having a membrane electrode assembly formed by sandwiching an electrolyte membrane between a first electrode catalyst layer and a second electrode catalyst layer. In the electrolytic cell, oxygen gas is generated in the first electrode catalyst layer and hydrogen gas is generated in the second electrode catalyst layer by electrolysis of water. In such an electrolytic cell, the hydrogen gas generated in the second electrode catalyst layer may permeate through the electrolyte membrane (cross-leak) and be guided to the first electrode catalyst layer. When such cross-leak occurs, the hydrogen gas mixes with the oxygen gas generated in the first electrode catalyst layer.

[0011] Hydrogen gas permeation through the electrolyte membrane is more likely to occur when the current supplied to the water electrolysis device (current density) is relatively low. In other words, the hydrogen gas concentration in the oxygen gas discharged from the water electrolysis device tends to increase as the current supplied to the water electrolysis device rises to its rated current value.

[0012] Incidentally, in a water electrolysis system equipped with multiple water electrolysis devices, when the oxygen gas generated by the multiple water electrolysis devices is stored in a tank, it is necessary to manage the hydrogen gas concentration in the oxygen gas in the tank so that it does not exceed a predetermined threshold. According to the following disclosure, it is possible to suppress an excessive rise in the hydrogen gas concentration in the oxygen gas in the tank in such a water electrolysis system.

[0013] Figure 1 is a schematic diagram of a water electrolysis system 10. As shown in Figure 1, the water electrolysis system 10 comprises a plurality of water electrolyzers 12, a power supply unit 14 (see Figures 2 and 3), a tank 16, a supply line 18, a pump 20, an outlet line 22, and a purge device 24. In this embodiment, the water electrolysis system 10 has three water electrolyzers 12. Hereinafter, the three water electrolyzers 12 may be referred to as the first water electrolyzer 12a, the second water electrolyzer 12b, and the third water electrolyzer 12c. The water electrolysis system 10 may have two or more water electrolyzers 12.

[0014] Figure 2 is a schematic diagram of a water electrolysis cell 26. The water electrolysis apparatus 12 is, for example, a water electrolysis stack formed by stacking multiple water electrolysis cells 26 as shown in Figure 2. However, the water electrolysis apparatus 12 may also consist of only one water electrolysis cell 26.

[0015] As shown in Figure 2, the water electrolysis cell 26 comprises a membrane electrode assembly 28, a first power supply 30, a second power supply 32, a first support member 34, and a second support member 36. The membrane electrode assembly 28 is also called a catalyst-coated membrane (CCM).

[0016] The membrane electrode assembly 28 includes an electrolyte membrane 38, a first electrode catalyst layer 40, and a second electrode catalyst layer 42. The membrane electrode assembly 28 is formed by sandwiching the electrolyte membrane 38 between the first electrode catalyst layer 40 and the second electrode catalyst layer 42. The electrolyte membrane 38 is an ion exchange membrane capable of exchanging ions. The electrolyte membrane 38 is, for example, an anion exchange membrane (AEM). The electrolyte membrane 38 may also be a proton exchange membrane (PEM).

[0017] The first electrode catalyst layer 40 is bonded to one side of the electrolyte membrane 38. The first electrode catalyst layer 40 is, for example, an anode electrode catalyst layer. The second electrode catalyst layer 42 is bonded to the other side of the electrolyte membrane 38. The second electrode catalyst layer 42 is, for example, a cathode electrode catalyst layer. The first power supply unit 30 is adjacent to the first electrode catalyst layer 40. The first power supply unit 30 is porous so that liquid and gas can flow in the thickness direction of the first power supply unit 30. The second power supply unit 32 is adjacent to the second electrode catalyst layer 42. The second power supply unit 32 is porous so that gas can flow in the thickness direction of the second power supply unit 32.

[0018] A first channel 44 is formed in the first support member 34. The first channel 44 is adjacent to the first power supply unit 30. A second channel 46 is formed in the second support member 36. The second channel 46 is adjacent to the second power supply unit 32. The first channel 44 and the second channel 46 are provided on both sides of the electrolyte membrane 38.

[0019] An electrolytic liquid containing water is supplied to the first channel 44. The electrolytic liquid is, for example, an aqueous potassium hydroxide solution (KOH aqueous solution). If the electrolyte membrane 38 is a proton exchange membrane, the electrolytic liquid is, for example, pure water. The electrolytic liquid supplied to the first channel 44 passes through the first power supply 30 and is supplied to the membrane electrode assembly 28.

[0020] The water contained in the electrolytic liquid supplied to the membrane electrode assembly 28 is electrolyzed by the supply of current from the power supply unit 14 to the water electrolysis cell 26. In this embodiment, oxygen gas is generated in the first electrode catalyst layer 40 and hydrogen gas is generated in the second electrode catalyst layer 42. A portion of the hydrogen gas generated in the second electrode catalyst layer 42 can permeate (cross-leak) through the electrolyte membrane 38 from the second electrode catalyst layer 42 towards the first electrode catalyst layer 40. The oxygen gas generated in the first electrode catalyst layer 40 and the cross-leaked hydrogen gas pass through the first power supply unit 30 and are led to the first channel 44, and are discharged to the outside of the water electrolysis device 12 together with the electrolytic liquid. The hydrogen gas generated in the second electrode catalyst layer 42 passes through the second power supply unit 32 and is led to the second channel 46, and is discharged to the outside of the water electrolysis device 12.

[0021] The water electrolysis cell 26 may be equipped with components other than those described above. For example, the water electrolysis cell 26 may be equipped with a spring member that biases the second power supply 32 toward the electrolyte membrane 38. In this case, even if the thickness of the electrolyte membrane 38 decreases due to the aging of the water electrolysis device 12, the components of the water electrolysis cell 26 can be brought into close contact with each other. Note that the water electrolysis device 12 is subjected to a clamping load in the thickness direction of the electrolyte membrane 38.

[0022] The water electrolysis device 12 can increase the pressure of hydrogen gas in the second channel 46 by continuously generating hydrogen gas in the second channel 46. The water electrolysis device 12 is, for example, a differential pressure water electrolysis device (differential pressure water electrolysis stack). In a differential pressure water electrolysis device, when the pressure of hydrogen gas in the second channel 46 is increased, the pressure of the fluid in the first channel 44 is lower than the pressure of hydrogen gas. The water electrolysis device 12 may also be, for example, an isobaric water electrolysis device (isobaric water electrolysis stack). In an isobaric water electrolysis device, when the pressure of hydrogen gas in the second channel 46 is increased, the pressure of the fluid in the first channel 44 is equal to the pressure of hydrogen gas.

[0023] Figure 3 is a control block diagram of the water electrolysis system 10. As shown in Figure 3, the power supply unit 14 is electrically connected to each of the multiple water electrolysis devices 12. The power supply unit 14 is a DC power supply capable of supplying DC current to each water electrolysis device 12. The power supply unit 14 can supply current to the multiple water electrolysis devices 12 individually.

[0024] As shown in Figure 1, the tank 16 can store an electrolytic liquid. The tank 16 is configured to allow water to be supplied to its interior.

[0025] The supply line 18 connects each of the multiple water electrolyzers 12 to the tank 16. The supply line 18 guides the electrolytic liquid stored in the tank 16 to each of the multiple water electrolyzers 12. The supply line 18 is connected to the first flow path 44 (see Figure 2) of each water electrolyzer 12. The pump 20 is provided in the supply line 18. The pump 20 sends the electrolytic liquid stored in the tank 16 to each of the multiple water electrolyzers 12.

[0026] The outlet 22 connects each of the multiple water electrolyzers 12 to the tank 16. The outlet 22 leads the oxygen gas (oxygen gas containing cross-leaked hydrogen gas) and unreacted electrolytic liquid generated in each water electrolyzer 12 to the tank 16. The tank 16 also functions as a gas-liquid separator, separating the fluid containing the oxygen gas and electrolytic liquid led from the outlet 22 into gas-liquid. The tank 16 can contain oxygen gas. The tank 16 is connected to an oxygen gas outlet 54 for leading the gas-liquid separated oxygen gas to a gas tank 52.

[0027] The purge device 24 can supply purge gas to the tank 16 to exhaust the gas inside the tank 16 to the outside. The purge device 24 is, for example, a blower. For example, air can be used as the purge gas. The purge gas may also be an inert gas such as nitrogen gas.

[0028] Tank 16 is connected to an exhaust passage 48 for discharging the oxygen gas inside Tank 16, along with the purge gas, to the outside of the water electrolysis system 10. The exhaust passage 48 is provided with an on / off valve 50 for opening and closing the exhaust passage 48. Tank 16 is provided with an oxygen gas outlet passage 54 for guiding the oxygen gas to the gas tank 52. Tank 16 is provided with a hydrogen gas sensor 56 for measuring the hydrogen gas concentration in the oxygen gas inside Tank 16.

[0029] As shown in Figure 3, the water electrolysis system 10 further includes a control device 58. The control device 58 includes an arithmetic unit 60 and a storage unit 62. The arithmetic unit 60 is composed of a processor such as a CPU (Central Processing Unit) or a GPU (Graphics Processing Unit). In other words, the arithmetic unit 60 is composed of processing circuitry.

[0030] The arithmetic unit 60 includes a control unit 64, a purge control unit 66, and a determination unit 68. The control unit 64, the purge control unit 66, and the determination unit 68 can be realized by the arithmetic unit 60 executing a program stored in the storage unit 62.

[0031] Furthermore, at least a portion of the control unit 64, the purge control unit 66, and the determination unit 68 may be implemented using integrated circuits such as ASICs (Application Specific Integrated Circuits) and FPGAs (Field-Programmable Gate Arrays). Alternatively, at least a portion of the control unit 64, the purge control unit 66, and the determination unit 68 may be composed of electronic circuits including discrete devices.

[0032] The storage unit 62 is composed of a volatile memory (not shown) and a non-volatile memory (not shown). Examples of volatile memory include RAM (Random Access Memory). The volatile memory is used as the working memory of the processor and temporarily stores data necessary for processing or calculations. Examples of non-volatile memory include ROM (Read Only Memory) and flash memory. The non-volatile memory is used as storage memory and stores programs, tables, maps, etc. At least a part of the storage unit 62 may be provided in the processor, integrated circuit, etc. as described above.

[0033] The control unit 64 controls the power supply unit 14. Specifically, the control unit 64 controls the supply of current from the power supply unit 14 to the multiple water electrolysis devices 12. The control unit 64 can also control the pump 20. The purge control unit 66 controls the purge device 24 to supply purge gas from the purge device 24 to the tank 16.

[0034] The memory unit 62 stores, for example, the rated current values ​​of each water electrolyzer 12. The rated current values ​​of each water electrolyzer 12 may be the same or different. Hereinafter, the rated current value of the first water electrolyzer 12a may be referred to as the first rated current value Ir1, the rated current value of the second water electrolyzer 12b may be referred to as the second rated current value Ir2, and the rated current value of the third water electrolyzer 12c may be referred to as the third rated current value Ir3.

[0035] Next, the method for starting the water electrolysis system 10 will be described. Figure 4 is a flowchart showing an example of the method for starting the water electrolysis system 10. As shown in Figure 4, in step S1, the control unit 64 drives the pump 20. This supplies the electrolytic liquid stored in the tank 16 to the multiple water electrolysis devices 12. Specifically, the electrolytic liquid is supplied to the membrane electrode assembly 28 of each water electrolysis device 12. After this, the process proceeds to step S2.

[0036] In step S2, the control unit 64 controls the power supply unit 14 to start supplying current to the first water electrolyzer 12a. As a result, the water contained in the electrolytic liquid supplied to the membrane electrode assembly 28 is electrolyzed in the first water electrolyzer 12a. Consequently, oxygen gas is generated in the first electrode catalyst layer 40 and hydrogen gas is generated in the second electrode catalyst layer 42 in the first water electrolyzer 12a. In this case, some of the hydrogen gas generated in the second electrode catalyst layer 42 permeates (cross-leaks) through the electrolyte membrane 38 and is guided to the first electrode catalyst layer 40. Therefore, the oxygen gas generated in the first electrode catalyst layer 40 may contain hydrogen gas.

[0037] In the first water electrolysis apparatus 12a, the oxygen gas (oxygen gas containing hydrogen gas) generated in the first electrode catalyst layer 40 is guided to the tank 16 via the first flow path 44 and the outlet path 22, together with the electrolytic liquid. In other words, the oxygen gas (oxygen gas containing hydrogen gas) generated in the first water electrolysis apparatus 12a is contained in the tank 16.

[0038] In step S2, the control unit 64 gradually increases the current value supplied to the first water electrolyzer 12a (hereinafter referred to as the "first current value").

[0039] Furthermore, in step S3, the purge control unit 66 starts purge control. Specifically, the purge control unit 66 controls the purge device 24 to supply purge gas into the tank 16 and controls the on / off valve 50 to open the exhaust passage 48. As a result, the oxygen gas (oxygen gas containing hydrogen gas) in the tank 16 is supplied to downstream equipment or discharged to the outside via the exhaust passage 48.

[0040] The start of the purge control in step S3 may occur, for example, simultaneously with the start of the supply of current to the first water electrolyzer 12a. The timing of the start of the purge control may also be before or after the start of the supply of current to the first water electrolyzer 12a. After step S3, the process transitions to step S4.

[0041] In step S4, the determination unit 68 determines whether or not the first current value has reached the first rated current value Ir1. In step S4, for example, the determination unit 68 may determine that the first current value has reached the first rated current value Ir1 when an elapsed time from starting supply of current to the first water electrolysis device 12a reaches a first predetermined time. The first predetermined time is a time period from starting supply of current to the first water electrolysis device 12a until the first current value reaches the first rated current value Ir1, and is determined in advance and stored in the storage unit 62.

[0042] When the determination unit 68 determines that the first current value has not reached the first rated current value Ir1 (NO in step S4), the process of step S4 is repeated. When the determination unit 68 determines that the first current value has reached the first rated current value Ir1 (YES in step S4), the process proceeds to step S5. Note that when the first current value reaches the first rated current value Ir1, the control unit 64 maintains the first current value at the first rated current value Ir1.

[0043] In step S5, the purge control unit 66 stops purge control. That is, the purge control unit 66 controls the purge device 24 to stop supply of purge gas from the purge device 24 to the tank 16. After step S5, the process proceeds to step S6.

[0044] In step S6, the control unit 64 starts supply of current to the second water electrolysis device 12b. Accordingly, in the second water electrolysis device 12b, water contained in the electrolysis liquid supplied to the membrane electrode assembly 28 is electrolyzed. Therefore, in the second water electrolysis device 12b, oxygen gas is generated in the first electrode catalyst layer 40, and hydrogen gas is generated in the second electrode catalyst layer 42.

[0045] In the second water electrolysis device 12b, the oxygen gas (oxygen gas containing hydrogen gas) generated in the first electrode catalyst layer 40 is guided to the tank 16 via the first flow path 44 and the outlet path 22 together with the electrolysis liquid. In other words, the oxygen gas (oxygen gas containing hydrogen gas) generated in the second water electrolysis device 12b is stored in the tank 16.

[0046] In step S6, the control unit 64 gradually increases the current value supplied to the second water electrolysis device 12b (hereinafter referred to as "the second current value"). After that, the process proceeds to step S7.

[0047] In step S7, the determination unit 68 determines whether or not the second current value has reached the second rated current value Ir2. In step S7, for example, the determination unit 68 may determine that the second current value has reached the second rated current value Ir2 when the elapsed time from the start of current supply to the second water electrolysis device 12b reaches a second predetermined time. The second predetermined time is the time from the start of current supply to the second water electrolysis device 12b until the second current value reaches the second rated current value Ir2, and is predetermined and stored in the storage unit 62.

[0048] If the determination unit 68 determines that the second current value has not reached the second rated current value Ir2 (NO in step S7), the process of step S7 is repeated. If the determination unit 68 determines that the second current value has reached the second rated current value Ir2 (YES in step S7), the process proceeds to step S8. Note that when the second current value reaches the second rated current value Ir2, the control unit 64 maintains the second current value at the second rated current value Ir2.

[0049] In step S8, the control unit 64 supplies current to the third water electrolysis device 12c. Accordingly, in the third water electrolysis device 12c, water contained in the electrolysis liquid supplied to the membrane electrode assembly 28 is electrolyzed. Therefore, in the third water electrolysis device 12c, oxygen gas is generated in the first electrode catalyst layer 40 and hydrogen gas is generated in the second electrode catalyst layer 42.

[0050] In the third water electrolysis device 12c, the oxygen gas (oxygen gas containing hydrogen gas) generated in the first electrode catalyst layer 40 is guided to the tank 16 together with the electrolysis liquid via the first flow path 44 and the outlet path 22. In other words, the oxygen gas (oxygen gas containing hydrogen gas) generated in the third water electrolysis device 12c is stored in the tank 16.

[0051] In step S8, the control unit 64 gradually increases the current value supplied to the third water electrolyzer 12c (hereinafter referred to as the "third current value"). After this, the process proceeds to step S9.

[0052] In step S9, the determination unit 68 determines whether the third current value has reached the third rated current value Ir3. In step S9, the determination unit 68 may determine, for example, that the third current value has reached the third rated current value Ir3 when the elapsed time since the start of supplying current to the third water electrolyzer 12c has reached a third predetermined time. The third predetermined time is the time from the start of supplying current to the third water electrolyzer 12c until the third current value reaches the third rated current value Ir3, and is predetermined and stored in the storage unit 62.

[0053] If the determination unit 68 determines that the third current value has not reached the third rated current value Ir3 (NO in step S9), the process in step S9 is repeated. If the determination unit 68 determines that the third current value has reached the third rated current value Ir3 (YES in step S9), the process shown in Figure 4 (startup process of the water electrolysis system 10) is completed. When the third current value reaches the third rated current value Ir3, the control unit 64 controls the power supply unit 14 to maintain the third current value at the third rated current value Ir3. After this, the rated operation of the water electrolysis system 10 continues.

[0054] Figure 5 is a timing chart showing an example of the operation described by the flowchart in Figure 4. In Figure 5, the pump 20 is driven before time t0, supplying the electrolytic liquid in the tank 16 to the first water electrolyzer 12a, the second water electrolyzer 12b, and the third water electrolyzer 12c, respectively.

[0055] As shown in Figure 5, at time t0, the supply of current from the power supply unit 14 to the first water electrolysis unit 12a begins, and thereafter, the first current value increases at the first current increase rate. At time t0, no current is supplied to the second water electrolysis unit 12b and the third water electrolysis unit 12c. Also at time t0, purge gas is supplied from the purge device 24 to the tank 16. At this time, the supply of purge gas may be at a preset flow rate. At this time, the supply of purge gas may also be at a preset time interval.

[0056] Subsequently, at time t1, when the first current value reaches the first rated current value Ir1, the first current value is maintained at the first rated current value Ir1. Also at time t1, the supply of current from the power supply unit 14 to the second water electrolysis unit 12b begins, and thereafter, the second current value increases at the second current increase rate. Furthermore, at time t1, the supply of purge gas from the purge device 24 to the tank 16 is stopped. In other words, the operation of the purge device 24 is stopped almost simultaneously with the first current value reaching the first rated current value Ir1.

[0057] In other words, the control unit 64 starts supplying current to the second water electrolyzer 12b when the first current value reaches the first rated current value Ir1. In this case, the amount of oxygen gas generated in the first electrode catalyst layer 40 increases in conjunction with the rise in the first current value to the first rated current value Ir1. At this time, the amount of hydrogen gas permeating through the electrolyte membrane 38 (cross-leakage amount) does not change. Therefore, the supply of current to the second water electrolyzer 12b is started after the hydrogen gas concentration in the oxygen gas introduced from the first water electrolyzer 12a into the tank 16 has become lower than the hydrogen gas concentration in the oxygen gas introduced from the first water electrolyzer 12a into the tank 16 immediately after the start of water electrolysis at time t0. As a result, an excessive rise in the hydrogen gas concentration in the oxygen gas in the tank 16 can be suppressed. Therefore, the hydrogen gas concentration in the oxygen gas in the tank 16 will not exceed a predetermined threshold c0.

[0058] Furthermore, in this embodiment, the purge control unit 66 controls the purge device 24 to supply purge gas to the tank 16 from the time the power supply unit 14 starts supplying current to the first water electrolyzer 12a until the power supply unit 14 starts supplying current to the second water electrolyzer 12b. This makes it possible to remove oxygen gas (oxygen gas with a relatively high hydrogen gas concentration) generated in the initial stage when the current supply to the first water electrolyzer 12a is started from inside the tank 16 using the purge gas. Therefore, it is possible to further suppress an excessive rise in the hydrogen gas concentration in the oxygen gas inside the tank 16.

[0059] Furthermore, in this embodiment, at time t1, the supply of purge gas from the purge device 24 to the tank 16 is stopped. In this case, since power to drive the purge device 24 is not required, the power consumption required to start the water electrolysis system 10 can be reduced. Also, when supplying oxygen gas to equipment downstream of the tank 16, the amount of oxygen gas removed to the outside by supplying purge gas can be reduced.

[0060] At time t1, the first current value in the first water electrolyzer 12a has reached the first rated current value Ir1, so the hydrogen gas concentration in the oxygen gas introduced from the first water electrolyzer 12a into the tank 16 remains relatively low. On the other hand, at time t1, the amount of oxygen gas generated in the first electrode catalyst layer 40 of the second water electrolyzer 12b is low. Therefore, the hydrogen gas concentration in the oxygen gas introduced from the second water electrolyzer 12b into the tank 16 is relatively higher than the hydrogen gas concentration in the oxygen gas introduced from the first water electrolyzer 12a into the tank 16.

[0061] In this case, even if the hydrogen gas concentration in the oxygen gas introduced into the tank 16 from the second water electrolyzer 12b is high, it is diluted by the oxygen gas with a lower hydrogen gas concentration introduced into the tank 16 from the first water electrolyzer 12a. Therefore, even if the purge control is stopped at time t1, it is possible to suppress an excessive rise in the hydrogen gas concentration in the oxygen gas in the tank 16. In other words, the oxygen gas containing hydrogen gas generated in the second water electrolyzer 12b is diluted by the oxygen gas with a lower hydrogen gas concentration introduced into the tank 16 from the first water electrolyzer 12a. As a result, the hydrogen gas concentration in the oxygen gas in the tank 16 does not exceed the threshold c0, and therefore, oxygen gas with an excessively high hydrogen gas concentration is not removed to the outside.

[0062] At time t2, when the second current value reaches the second rated current value Ir2, the second current value is maintained at the second rated current value Ir2. Also at time t2, the supply of current from the power supply 14 to the third water electrolyzer 12c begins, and thereafter the third current value increases at the third current increase rate.

[0063] At time t2, the first current value of the first water electrolyzer 12a has reached the first rated current value Ir1, and the second current value of the second water electrolyzer 12b has reached the second rated current value Ir2. Therefore, the hydrogen gas concentration in the oxygen gas introduced from the second water electrolyzer 12b into the tank 16 is lower than the hydrogen gas concentration in the oxygen gas introduced from the second water electrolyzer 12b into the tank 16 immediately after the start of water electrolysis in the second water electrolyzer 12b at time t1. On the other hand, at time t2, it is immediately after the start of water electrolysis in the third water electrolyzer 12c, so the hydrogen gas concentration in the oxygen gas introduced from the third water electrolyzer 12c into the tank 16 is relatively higher than the hydrogen gas concentration in the oxygen gas introduced from the first water electrolyzer 12a and the second water electrolyzer 12b into the tank 16.

[0064] In this case, even if the hydrogen gas concentration in the oxygen gas introduced into the tank 16 from the third water electrolyzer 12c is high, it is diluted by the oxygen gas with a lower hydrogen gas concentration introduced into the tank 16 from the first water electrolyzer 12a and the second water electrolyzer 12b. Therefore, it is possible to suppress an excessive rise in the hydrogen gas concentration in the oxygen gas in the tank 16 at time t2. In other words, the oxygen gas containing hydrogen gas generated in the third water electrolyzer 12c is diluted by the oxygen gas with a lower hydrogen gas concentration introduced into the tank 16 from the first water electrolyzer 12a and the second water electrolyzer 12b. As a result, the hydrogen gas concentration in the oxygen gas in the tank 16 does not exceed the threshold c0, and therefore, oxygen gas with an excessively high hydrogen gas concentration is not removed to the outside.

[0065] At time t3, when the third current value reaches the third rated current value Ir3, the third current value is maintained at the third rated current value Ir3. This completes the startup of the water electrolysis system 10. In other words, the startup of the first water electrolysis device 12a, the second water electrolysis device 12b, and the third water electrolysis device 12c is completed.

[0066] In this embodiment, the control unit 64 starts supplying current from the power supply unit 14 to the first water electrolyzer 12a, and after the first current value reaches the first rated current value Ir1, it starts supplying current from the power supply unit 14 to the second water electrolyzer 12b, and after the second current value reaches the second rated current value Ir2, it starts supplying current from the power supply unit 14 to the third water electrolyzer 12c. In this case, the first water electrolyzer 12a, the second water electrolyzer 12b, and the third water electrolyzer 12c can be started while suppressing an excessive rise in the hydrogen gas concentration in the oxygen gas in the tank 16.

[0067] This makes it possible to lower the peak hydrogen gas concentration in the oxygen gas in the tank 16 compared to when the current is started simultaneously to the first water electrolyzer 12a, the second water electrolyzer 12b, and the third water electrolyzer 12c (multiple water electrolyzers 12). Therefore, it is possible to suppress an excessive rise in the hydrogen gas concentration in the oxygen gas in the tank 16. In addition, it is possible to reduce the amount of purge gas supplied to lower the peak hydrogen gas concentration and reduce the amount of oxygen gas removed to the outside.

[0068] (First Modification) Next, the method for starting the water electrolysis system 10 according to the first modification will be described. In this modification, explanations that overlap with the embodiments described above will be omitted, and the same reference numerals will be used for steps that are the same as those described in the embodiments described above. The same applies to the second modification described later.

[0069] Figure 6 is a flowchart showing the method for starting the water electrolysis system 10 according to the first modified example. As shown in Figure 6, in the first modified example, after step S5, the process transitions to step S20.

[0070] In step S20, the control unit 64 controls the power supply unit 14 to supply current to the second water electrolyzer 12b and the third water electrolyzer 12c. After this, the process proceeds to step S21.

[0071] In step S21, the determination unit 68 determines whether the second current value has reached the second rated current value Ir2 and the third current value has reached the third rated current value Ir3. In step S21, the determination unit 68 may, for example, determine that the second current value has reached the second rated current value Ir2 when the elapsed time since the start of supplying current to the second water electrolyzer 12b has reached the second predetermined time, and determine that the third current value has reached the third rated current value Ir3 when the elapsed time since the start of supplying current to the third water electrolyzer 12c has reached the third predetermined time.

[0072] If the determination unit 68 determines that the second current value has not reached the second rated current value Ir2 or the third current value has not reached the third rated current value Ir3 (NO in step S21), the process in step S21 is repeated. If the determination unit 68 determines that the second current value has reached the second rated current value Ir2 and the third current value has reached the third rated current value Ir3, the process shown in Figure 6 (startup process of the water electrolysis system 10) is completed. After this, the rated operation of the water electrolysis system 10 continues.

[0073] Figure 7 is a timing chart showing an example of the operation described by the flowchart in Figure 6. As shown in Figure 7, in the first modified example, at time t1, when the first current value reaches the first rated current value Ir1, the supply of current from the power supply 14 to the second water electrolyzer 12b starts, and the supply of current from the power supply 14 to the third water electrolyzer 12c also starts. Subsequently, the second current value increases to the second rated current value Ir2 at the second current increase rate, and the third current value increases to the third rated current value Ir3 at the third current increase rate.

[0074] In this modified example, the control unit 64 simultaneously starts supplying current from the power supply unit 14 to the second water electrolyzer 12b and to the third water electrolyzer 12c. With this configuration, compared to the case where the supply of current to the third water electrolyzer 12c is started after the supply of current to the second water electrolyzer 12b, the timing at which the third current value reaches the third rated current value Ir3 can be accelerated. This can shorten the time required to start up the water electrolysis system 10.

[0075] (Second Modification) Next, a method for starting the water electrolysis system 10 according to the second modification will be described. Figure 8 is a flowchart showing the method for starting the water electrolysis system 10 according to the second modification. As shown in Figure 8, in the second modification, after step S3, the process transitions to step S30.

[0076] In step S30, the determination unit 68 determines whether the first current value has reached the first current threshold Ia. The first current threshold Ia is predetermined and stored in the storage unit 62. The first current threshold Ia is smaller than the first rated current value Ir1. In step S30, similar to step S4 described above, the determination of whether the first current value has reached the first current threshold Ia may be made based on the elapsed time since the supply of current to the first water electrolyzer 12a was started.

[0077] If the determination unit 68 determines that the first current value has not reached the first current threshold Ia (NO in step S30), the process in step S30 is repeated. If the determination unit 68 determines that the first current value has reached the first current threshold Ia (YES in step S30), the processes in steps S5 and S6 are performed. That is, the control unit 64 starts supplying current from the power supply unit 14 to the second water electrolyzer 12b before the first current value rises to the first rated current value Ir1. After the process in step S6, the process proceeds to step S31.

[0078] In step S31, the determination unit 68 determines whether the concentration of hydrogen gas in the oxygen gas in the tank 16 is equal to or greater than a predetermined concentration c1. The concentration of hydrogen gas in the oxygen gas in the tank 16 can be obtained, for example, by a hydrogen gas sensor 56. The predetermined concentration c1 is lower than the threshold c0 for the hydrogen gas concentration in the oxygen gas. The predetermined concentration c1 is predetermined and stored in the storage unit 62.

[0079] If the determination unit 68 determines that the hydrogen gas concentration in the oxygen gas inside the tank 16 is lower than a predetermined concentration c1 (NO in step S31), the process proceeds to step S32.

[0080] In step S32, the determination unit 68 determines whether the second current value has reached the second current threshold Ib. The second current threshold Ib is predetermined and stored in the storage unit 62. The second current threshold Ib is smaller than the second rated current value Ir2. In step S32, similar to step S7 described above, the determination of whether the second current value has reached the second current threshold Ib may be made based on the elapsed time since the supply of current to the second water electrolyzer 12b was started.

[0081] If the determination unit 68 determines that the second current value has not reached the second current threshold Ib (NO in step S32), the process proceeds to step S31. If the determination unit 68 determines that the second current value has reached the second current threshold Ib (YES in step S32), the processes in steps S8 and S9 are performed, and then the process shown in Figure 8 is completed.

[0082] If the determination unit 68 determines that the hydrogen gas concentration in the oxygen gas in the tank 16 is equal to or greater than a predetermined concentration c1 (YES in step S31), the process proceeds to step S7.

[0083] In step S7, the determination unit 68 determines whether the second current value has reached the second rated current value Ir2. If the determination unit 68 determines that the second current value has not reached the second rated current value Ir2 (NO in step S7), the process in step S7 is repeated. If the determination unit 68 determines that the second current value has reached the second rated current value Ir2 (YES in step S7), the processes in steps S8 and S9 are performed, and then the process shown in Figure 8 is completed.

[0084] Figures 9 and 10 are timing charts showing an example of the operation described by the flowchart in Figure 8. As shown in Figure 9, in the second modified example, at time t1a, when the first current value reaches the first current threshold Ia, the supply of current from the power supply 14 to the second water electrolyzer 12b begins, and thereafter the second current value increases at the second current increase rate.

[0085] In other words, the control unit 64 starts supplying current from the power supply unit 14 to the second water electrolyzer 12b before the first current value rises to the first rated current value Ir1. This allows the timing at which the second current value reaches the second rated current value Ir2 to be accelerated compared to the case where the supply of current to the second water electrolyzer 12b is started after the first current value reaches the first rated current value Ir1. Therefore, the time required to start up the water electrolysis system 10 can be shortened.

[0086] In the example shown in Figure 9, after time t1a, the hydrogen gas concentration in the oxygen gas in tank 16 is lower than the predetermined concentration c1. In this case, at time t2a, when the second current value reaches the second current threshold Ib, the supply of current from the power supply unit 14 to the third water electrolyzer 12c is started, and thereafter the third current value increases at the third current increase rate.

[0087] In other words, the control unit 64 starts supplying current from the power supply unit 14 to the third water electrolyzer 12c before the second current value rises to the second rated current value Ir2. This allows the timing at which the third current value reaches the third rated current value Ir3 to be accelerated compared to the case where the supply of current to the third water electrolyzer 12c is started after the second current value reaches the second rated current value Ir2. Therefore, the time required to start up the water electrolysis system 10 can be shortened.

[0088] In the example shown in Figure 10, after time t1a, the hydrogen gas concentration in the oxygen gas in tank 16 is equal to or greater than a predetermined concentration c1. In this case, at time t2b, which is later than time t2a, when the second current value reaches the second rated current value Ir2, the supply of current from the power supply unit 14 to the third water electrolysis unit 12c is started, and thereafter the third current value increases at the third current increase rate.

[0089] In other words, in the example shown in Figure 10, if the hydrogen gas concentration in the oxygen gas in tank 16 reaches a predetermined concentration c1 or higher between the start of current supply from power supply 14 to the second water electrolyzer 12b and the start of current supply from power supply 14 to the third water electrolyzer 12c, the control unit 64 starts supplying current to the third water electrolyzer 12c when the second current value reaches the second rated current value Ir2. This allows the supply of current to the third water electrolyzer 12c to decrease the hydrogen gas concentration in the oxygen gas in tank 16 by delaying the supply of current to the third water electrolyzer 12c when the hydrogen gas concentration in the oxygen gas in tank 16 is likely to reach a predetermined concentration c1 or higher and reach a threshold c0.

[0090] The water electrolysis system 10 according to this embodiment can be started using the following methods as appropriate: the startup method shown in the flowchart in Figure 4 (referred to as the first startup method), the startup method shown in the flowchart in Figure 6 (referred to as the second startup method), and the startup method shown in the flowchart in Figure 8 (referred to as the third startup method).

[0091] Specifically, in the water electrolysis device 12, if the thickness of the electrolyte membrane 38 decreases due to aging or other factors, the amount of hydrogen gas permeating through the electrolyte membrane 38 (cross-leakage) increases. Also, if the thickness of the electrolyte membrane 38 decreases, the clamping load of the water electrolysis device 12 decreases. Therefore, if the clamping load of the water electrolysis device 12 is greater than a predetermined load, the second or third starting method may be selected, and the first starting method may be selected when the clamping load falls below the predetermined load. In this case, it is possible to shorten the time required to start up multiple water electrolysis devices 12 while preventing an excessive increase in the hydrogen gas concentration in the oxygen gas in the tank 16.

[0092] In addition to the disclosures mentioned above, the following further notes are made:

[0093] (Note 1) This disclosure relates to a water electrolysis system (10), comprising a plurality of water electrolysis devices (12) each having a membrane electrode assembly (28) formed by sandwiching an electrolyte membrane (38) between a first electrode catalyst layer (40) and a second electrode catalyst layer (42), and a water electrolysis cell (26) that generates oxygen gas in the first electrode catalyst layer and hydrogen gas in the second electrode catalyst layer by electrolyzing water; a power supply device (14) for supplying current to the plurality of water electrolysis devices; a tank (16) capable of containing oxygen gas generated in the plurality of water electrolysis devices; and a control unit (64) that controls the supply of current from the power supply device to the plurality of water electrolysis devices, wherein the control unit starts supplying current from the power supply device to the first water electrolysis device (12a) of the plurality of water electrolysis devices, and then starts supplying current from the power supply device to the second water electrolysis device (12b) of the plurality of water electrolysis devices.

[0094] With this configuration, the supply of current to the second water electrolyzer is initiated after the supply of current to the first water electrolyzer. This allows for a lower peak hydrogen gas concentration in the oxygen gas within the tank compared to the case where the supply of current to the second water electrolyzer is initiated simultaneously with the supply to the first water electrolyzer. Therefore, an excessive rise in hydrogen gas concentration in the oxygen gas within the tank can be suppressed. Thus, a better water electrolysis system can be provided.

[0095] (Note 2) The water electrolysis system described in Note 1 may also include a purge device (24) that supplies purge gas to the tank and discharges the gas in the tank to the outside, and a purge control unit (66) that controls the purge device and supplies purge gas to the tank from the time the power supply unit starts supplying current to the first water electrolysis device until the power supply unit starts supplying current to the second water electrolysis device.

[0096] With this configuration, oxygen gas (oxygen gas with a relatively high hydrogen gas concentration) generated in the initial stages when current is supplied to the first water electrolyzer can be removed from the tank by a purge gas. This further suppresses an excessive rise in the hydrogen gas concentration in the oxygen gas within the tank.

[0097] (Note 3) In the water electrolysis system described in Note 1 or 2, the control unit may start supplying current to the second water electrolysis device when the current value supplied from the power supply device to the first water electrolysis device reaches the rated current value (Ir1) of the first water electrolysis device.

[0098] With this configuration, the supply of current to the second water electrolyzer begins after the hydrogen gas concentration in the oxygen gas supplied from the first water electrolyzer has become relatively low, thus further suppressing an excessive rise in the hydrogen gas concentration in the oxygen gas in the tank.

[0099] (Note 4) In the water electrolysis system described in any one of Notes 1 to 3, the control unit may, after starting to supply current from the power supply unit to the first water electrolysis unit, start to supply current from the power supply unit to the third water electrolysis unit (12c) among the plurality of water electrolysis units.

[0100] With this configuration, the first, second, and third water electrolyzers can be started while suppressing an excessive rise in the hydrogen gas concentration in the oxygen gas inside the tank.

[0101] (Note 5) In the water electrolysis system described in Note 4, the control unit may start supplying current from the power supply unit to the third water electrolysis unit after starting to supply current from the power supply unit to the second water electrolysis unit.

[0102] With this configuration, the supply of current to the third water electrolyzer is started after the supply of current to the second water electrolyzer is started. This makes it possible to lower the peak value of the hydrogen gas concentration in the oxygen gas in the tank compared to when the supply of current to the third water electrolyzer is started simultaneously with the supply of current to the second water electrolyzer. Therefore, it is possible to suppress an excessive rise in the hydrogen gas concentration in the oxygen gas in the tank.

[0103] (Note 6) In the water electrolysis system described in Note 5, the control unit may start supplying current from the power supply to the third water electrolysis device when the current value supplied from the power supply to the second water electrolysis device reaches the rated current value (Ir2) of the second water electrolysis device.

[0104] With this configuration, the supply of current to the third water electrolyzer begins after the hydrogen gas concentration in the oxygen gas supplied from the second water electrolyzer has become relatively low, thus further suppressing an excessive rise in the hydrogen gas concentration in the oxygen gas in the tank.

[0105] (Note 7) In the water electrolysis system described in Note 4, the control unit may simultaneously start supplying current from the power supply to the second water electrolysis device and start supplying current from the power supply to the third water electrolysis device.

[0106] With this configuration, compared to the case where the supply of current to the third water electrolyzer is started after the supply of current to the second water electrolyzer is started, the timing at which the current supplied to the third water electrolyzer reaches its rated current value can be accelerated. This can shorten the time required to start up the water electrolysis system.

[0107] (Note 8) In the water electrolysis system described in Note 1, the control unit may start supplying current from the power supply to the second water electrolysis device before the current value supplied from the power supply to the first water electrolysis device rises to the rated current value of the first water electrolysis device.

[0108] With this configuration, compared to the case where the current supplied to the first water electrolyzer reaches the rated current value before the current supply to the second water electrolyzer begins, the timing at which the current supplied to the second water electrolyzer reaches the rated current value can be accelerated. This can shorten the time required to start up the water electrolysis system.

[0109] (Note 9) The water electrolysis system described in Note 8 may be a water electrolysis system in which the control unit can start supplying current from the power supply unit to the third water electrolysis unit among the plurality of water electrolysis units before the current value supplied from the power supply unit to the second water electrolysis unit rises to the rated current value of the second water electrolysis unit.

[0110] With this configuration, compared to the case where the current supplied to the second water electrolyzer reaches the rated current value before the current supply to the third water electrolyzer begins, the timing at which the current supplied to the third water electrolyzer reaches the rated current value can be accelerated. This can shorten the time required to start up the water electrolysis system.

[0111] (Note 10) In the water electrolysis system described in Note 9, if the hydrogen gas concentration in the oxygen gas in the tank reaches a predetermined concentration (c1) between the start of current supply from the power supply to the second water electrolysis device and the start of current supply from the power supply to the third water electrolysis device, the control unit may start supplying current to the third water electrolysis device when the current value supplied from the power supply to the second water electrolysis device reaches the rated current value of the second water electrolysis device.

[0112] With this configuration, if the hydrogen gas concentration in the oxygen gas in the tank exceeds a predetermined concentration and is likely to reach a hydrogen gas concentration threshold, the supply of current to the third water electrolysis device can be delayed to lower the hydrogen gas concentration in the oxygen gas in the tank.

[0113] (Note 11) This disclosure relates to a method for starting a water electrolysis system, wherein the water electrolysis system comprises a plurality of water electrolysis devices having a membrane electrode assembly formed by sandwiching an electrolyte membrane between a first electrode catalyst layer and a second electrode catalyst layer, and a water electrolysis cell that generates oxygen gas in the first electrode catalyst layer and hydrogen gas in the second electrode catalyst layer by electrolyzing water; a power supply device for supplying current to the plurality of water electrolysis devices; a tank capable of containing the oxygen gas generated by the plurality of water electrolysis devices; and a control unit that controls the supply of current from the power supply device to the plurality of water electrolysis devices, wherein the control unit starts supplying current from the power supply device to the first water electrolysis device among the plurality of water electrolysis devices, and then starts supplying current from the power supply device to the second water electrolysis device among the plurality of water electrolysis devices.

[0114] With this configuration, a method for starting a water electrolysis system that produces the same effects as described in Appendix 1 can be obtained. Therefore, a better method for starting a water electrolysis system can be provided.

[0115] While this disclosure has been described in detail, it is not limited to the individual embodiments described above. These embodiments can be added, replaced, modified, partially deleted, etc., in any way that does not depart from the gist of this disclosure or from the spirit of this disclosure derived from the claims and their equivalents. These embodiments can also be implemented in combination. For example, the order of operations and processes in the embodiments described above are given as examples only and are not limited thereto. The same applies when numerical values ​​or mathematical formulas are used in the description of the embodiments described above.

[0116] 10...Water electrolysis system 12...Water electrolysis device 12a...First water electrolysis device 12b...Second water electrolysis device 12c...Third water electrolysis device 14...Power supply unit 16...Tank 24...Purge device 26...Water electrolysis cell 28...Membrane electrode assembly 38...Electrolyte membrane 40...First electrode catalyst layer 42...Second electrode catalyst layer 64...Control unit 66...Purge control unit c1...Predetermined concentration Ir1...First rated current value Ir2...Second rated current value Ir3...Third rated current value

Claims

1. A water electrolysis system (10) comprising: a plurality of water electrolysis devices (12) each having a membrane electrode assembly (28) formed by sandwiching an electrolyte membrane (38) between a first electrode catalyst layer (40) and a second electrode catalyst layer (42), and a water electrolysis cell (26) that generates oxygen gas in the first electrode catalyst layer and hydrogen gas in the second electrode catalyst layer by electrolyzing water; a power supply device (14) for supplying current to the plurality of water electrolysis devices; a tank (16) capable of containing the oxygen gas generated by the plurality of water electrolysis devices; and a control unit (64) that controls the supply of current from the power supply device to the plurality of water electrolysis devices, wherein the control unit starts supplying current from the power supply device to the first water electrolysis device (12a) of the plurality of water electrolysis devices, and then starts supplying current from the power supply device to the second water electrolysis device (12b) of the plurality of water electrolysis devices.

2. A water electrolysis system according to claim 1, comprising: a purge device (24) that supplies purge gas to the tank and discharges the gas in the tank to the outside; and a purge control unit (66) that controls the purge device and supplies purge gas to the tank from the time the power supply unit starts supplying current to the first water electrolysis device until the power supply unit starts supplying current to the second water electrolysis device.

3. A water electrolysis system according to claim 1, wherein the control unit starts supplying current to the second water electrolysis device when the current value supplied from the power supply device to the first water electrolysis device reaches the rated current value (Ir1) of the first water electrolysis device.

4. A water electrolysis system according to claim 1, wherein the control unit, after starting to supply current from the power supply to the first water electrolysis device, starts to supply current from the power supply to the third water electrolysis device (12c) among the plurality of water electrolysis devices.

5. A water electrolysis system according to claim 4, wherein the control unit starts supplying current from the power supply to the second water electrolysis device, and then starts supplying current from the power supply to the third water electrolysis device.

6. A water electrolysis system according to claim 5, wherein the control unit starts supplying current from the power supply to the third water electrolysis device when the current value supplied from the power supply to the second water electrolysis device reaches the rated current value (Ir2) of the second water electrolysis device.

7. A water electrolysis system according to claim 4, wherein the control unit simultaneously initiates the supply of current from the power supply to the second water electrolysis device and the supply of current from the power supply to the third water electrolysis device.

8. A water electrolysis system according to claim 1, wherein the control unit starts supplying current from the power supply to the second water electrolysis device before the current value supplied from the power supply to the first water electrolysis device rises to the rated current value of the first water electrolysis device.

9. A water electrolysis system according to claim 8, wherein the control unit can initiate the supply of current from the power supply unit to a third water electrolysis unit among a plurality of water electrolysis units before the current value supplied from the power supply unit to the second water electrolysis unit rises to the rated current value of the second water electrolysis unit.

10. A water electrolysis system according to claim 9, wherein, if the hydrogen gas concentration in the oxygen gas in the tank becomes equal to or greater than a predetermined concentration (c1) between the start of supplying current from the power supply to the second water electrolysis device and the start of supplying current from the power supply to the third water electrolysis device, the control unit starts supplying current to the third water electrolysis device when the current value supplied from the power supply to the second water electrolysis device reaches the rated current value of the second water electrolysis device.

11. A method for starting a water electrolysis system, the water electrolysis system comprising: a plurality of water electrolysis devices having a membrane electrode assembly formed by sandwiching an electrolyte membrane between a first electrode catalyst layer and a second electrode catalyst layer, and a water electrolysis cell that generates oxygen gas in the first electrode catalyst layer and hydrogen gas in the second electrode catalyst layer by electrolyzing water; a power supply device for supplying current to the plurality of water electrolysis devices; a tank capable of containing the oxygen gas generated by the plurality of water electrolysis devices; and a control unit that controls the supply of current from the power supply device to the plurality of water electrolysis devices, wherein the control unit starts supplying current from the power supply device to the first water electrolysis device among the plurality of water electrolysis devices, and then the control unit starts supplying current from the power supply device to the second water electrolysis device among the plurality of water electrolysis devices.

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