Water electrolysis system

Abstract

This invention provides a water electrolysis system that can suppress cross-leakage and minimize the decrease in electrolysis efficiency. [Solution] A water electrolysis system that performs water electrolysis using a water electrolysis cell, comprising a controller that controls the depressurization of the hydrogen electrode of the water electrolysis cell by applying a stop current lower than the current applied during water electrolysis when water electrolysis is stopped, the controller determines a target stop current value based on a cross-leak hydrogen concentration map determined by the pressure of the hydrogen electrode and the applied current, and applies a current of the target stop current value to the water electrolysis cell.

Description

Technical Field

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

Background Art

[0002] Patent Document 1 discloses that when the water electrolysis system is stopped, in the pressure reduction process on the cathode side, the supply current on the high-pressure side is made larger than the supply current on the low-pressure side (current is applied in two stages as the pressure is reduced).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, depending on the value of the low current density, cross leakage (mixing of hydrogen into the oxygen electrode side of the water electrolysis cell) may increase, and the hydrogen concentration on the oxygen electrode side may rise. Also, when the cross leakage is large, the electrolysis efficiency in the water electrolysis cell decreases.

[0005] In view of the prior art, an object of the present disclosure is to provide a water electrolysis system capable of suppressing cross leakage and suppressing a decrease in electrolysis efficiency.

Means for Solving the Problems

[0006] The present application discloses a water electrolysis system that performs water electrolysis in a water electrolysis cell, and includes a controller that applies a stop current lower than the applied current during water electrolysis and controls the pressure reduction of the hydrogen electrode of the water electrolysis cell when the water electrolysis is stopped. The controller determines a target stop current value based on a concentration map of cross-leakage hydrogen determined by the pressure of the hydrogen electrode and the applied current, and applies a current of the target stop current value to the water electrolysis cell.

[0007] The concentration map has multiple maps corresponding to the shape and / or degradation status of the electrolyte membrane of the water electrolysis cell. [Effects of the Invention]

[0008] According to this disclosure, it is possible to apply current to stop hydrogen cross-leakage within an acceptable range, thereby suppressing a decrease in electrolysis efficiency. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is a conceptual diagram illustrating the configuration of the water electrolysis system 10. [Figure 2] Figure 2 is a cross-sectional view illustrating the layer structure of the water electrolysis cell 11. [Figure 3] Figure 3 is a conceptual diagram illustrating the configuration of the controller 30. [Figure 4] Figure 4 illustrates the cross-leak hydrogen concentration map. [Figure 5] Figure 5 is a diagram illustrating the control process for Embodiment 1. [Figure 6] Figure 6 illustrates the control process for Embodiment 2. [Figure 7] Figure 7 illustrates a conventional control system. [Modes for carrying out the invention]

[0010] 1. Configuration of the water electrolysis system Figure 1 conceptually represents a water electrolysis system 10 in one configuration. The basic principles and concepts regarding the production of hydrogen and oxygen by water electrolysis in the water electrolysis system 10 can be based on known principles. In this embodiment, the water electrolysis system 10 has a water electrolysis stack 20 in which a plurality of water electrolysis cells 11 are stacked and sandwiched at both ends by end plates, and a water supply side path (oxygen electrode side path) on one side of the water electrolysis stack 20 and a hydrogen electrode side path on the other side. In the water electrolysis system 10, pure water is supplied to the water electrolysis cell 11 in the water electrolysis stack 20 from the water supply side path and energized by the power supply 29 to decompose the water into hydrogen and oxygen. The obtained hydrogen is discharged to the hydrogen electrode side path, recovered, and stored.

[0011] In the water supply path (oxygen electrode path), tap water is purified by passing it through an ion exchanger or the like and stored in a gas-liquid separator 21. This water is then supplied to the water electrolysis stack 20 via a water pump 22 through a cooler 23 and an ion exchanger 24. The oxygen and water that exit the water electrolysis stack 20 are returned to the gas-liquid separator 21, where the gas and liquid are separated. The gas (oxygen) is discharged, and the liquid (water) is reused for water electrolysis by the water pump 22. These components are connected by piping, and configured to allow water and oxygen to flow through the necessary paths.

[0012] In the hydrogen electrode side path, hydrogen and associated water from the water electrolysis stack 20 are collected in the gas-liquid separator 25, where the gas and liquid are separated. The gas (hydrogen) is sent to the hydrogen tank 26 via a dehumidifier for storage. Meanwhile, the water (associated water) separated in the gas-liquid separator 25 is returned to the gas-liquid separator 21 in the water supply side path via the ion exchanger 27. These components are also connected by piping, allowing water and hydrogen to flow through the necessary paths. Furthermore, in this embodiment, a pressure regulating valve 28 is connected to the path through which hydrogen discharged from the gas-liquid separator 25 flows, and the pressure in the hydrogen electrode side path can be adjusted by adjusting the opening degree of the pressure regulating valve 28. In this embodiment, this pressure can be measured by a pressure gauge 40 that measures the pressure inside the piping from the water electrolysis stack 20 to the gas-liquid separator 25.

[0013] A power supply 29 is connected to the two electrodes of the water electrolysis stack 20 via a power supply line. A voltage is applied from this power supply 29 to the water electrolysis stack 20, causing water electrolysis to occur in the water electrolysis cell 11. In this embodiment, the power supply 29 and the controller 30 are electrically connected, allowing the controller 30 to control the voltage applied by the power supply 29. The power supply 29 is a well-known, standard power supply used for water electrolysis.

[0014] 1.1. Water electrolysis stack As described above, the water electrolysis stack 20 is configured such that a plurality of water electrolysis cells 11 are stacked and sandwiched between end plates disposed at both ends thereof.

[0015] FIG. 2 shows a cross-section of a site where water electrolysis is performed in one water electrolysis cell 11. As can be seen from FIG. 2, the water electrolysis cell 11 has a stacked structure composed of a plurality of layers. The layer configuration is as known and is not particularly limited. However, as shown in FIG. 2, in the water electrolysis cell 11, a hydrogen electrode catalyst layer 13, a hydrogen electrode diffusion layer 15, and a hydrogen electrode separator 17 are stacked on one side of the electrolyte membrane 12, and an oxygen electrode catalyst layer 14, an oxygen electrode diffusion layer 16, and an oxygen electrode separator 18 are stacked on the other side of the electrolyte membrane 12. The hydrogen electrode separator 17 is wave-shaped in the cross-section, forms a groove-shaped hydrogen electrode flow path 17a between the hydrogen electrode diffusion layer 15, and hydrogen and accompanying water flow through this hydrogen electrode flow path 17a and are discharged to the hydrogen electrode side path. On the other hand, the oxygen electrode separator 18 is also wave-shaped in the cross-section and forms a groove-shaped oxygen electrode flow path 18a between the oxygen electrode diffusion layer 16. Water is supplied from the water supply side path to the oxygen electrode flow path 18a, and oxygen and the remaining water are discharged from the oxygen electrode flow path 18a to the water supply side path.

[0016] 1.2. Controller The controller 30 is a controller that controls the water electrolysis system 10 of the present embodiment. More specifically, in the present embodiment, it is a controller that at least acquires the pressure value from the pressure gauge 40 and controls the applied voltage (applied current) by the power supply 29 and the opening degree of the pressure regulating valve 28. However, it does not need to be a controller only for that purpose and can be provided with other functions for controlling the water electrolysis system 10. The mode of the controller 30 is not particularly limited, but typically it can be configured by a computer. FIG. 3 conceptually shows a configuration example of the computer 30 as the controller 30.

[0017] The computer 30 includes a CPU (Central Processing Unit) 31 as a processor, a RAM (Random Access Memory) 32 that functions as a working area, a ROM (Read-Only Memory) 33 as a storage medium, a receiving unit 34 that is an interface for receiving information into the computer 30 regardless of whether it is wired or wireless, and an output unit 35 that is an interface for sending information from the computer 30 to the outside regardless of whether it is wired or wireless. A power supply 29 is electrically connected to the receiving unit 34 and the output unit 35, and is configured to be able to control the applied voltage by the power supply 29 through the transmission and reception of information by signals. On the other hand, a pressure gauge is electrically connected to the receiving unit 34 so that the pressure value can be acquired as an electrical signal, and a pressure regulating valve 28 is electrically connected to the output unit 35 so that the opening degree of the pressure regulating valve 28 can be controlled.

[0018] In the computer 30, each process for the control performed in the electrolytic water system 10 of the present embodiment is stored as a specific command, and a computer program for executing this is stored. In the computer 30, the CPU 31, the RAM 32, and the ROM 33 as hardware resources cooperate with the computer program. Specifically, the CPU 31 realizes functions by executing a computer program recorded in the ROM 33 in the RAM 32 that functions as a working area based on the applied current information of the power supply 29 acquired through the receiving unit 34. Information acquired or generated by the CPU 31 is stored in the RAM 32. Then, based on the obtained result, commands are sent to the power supply 29 and the pressure regulating valve 28 through the output unit 35 as necessary. The content of the control by the specific electrolytic water system 10 will be described next.

[0019] 2. Control by the controller (applied voltage control) In a water electrolysis device, hydrogen is discharged through the hydrogen electrode side path and oxygen is discharged through the oxygen electrode side path with the water electrolysis cell interposed therebetween. However, a leak (cross leak) may occur such that a part of the hydrogen enters the oxygen electrode side path, increasing the hydrogen concentration (hydrogen concentration in oxygen) on the oxygen electrode side. It is preferable that such leaked hydrogen be minimized, and it is desirable to keep the hydrogen concentration on the oxygen electrode side as low as possible. In particular, when operating at a low current density, this tendency is prominent, so it is necessary to take measures when the water electrolysis is stopped. In this embodiment, control is performed by the controller 30.

[0020] 2.1. Concentration map of cross-leaked hydrogen Before explaining the specific control, the concentration map of cross-leaked hydrogen used for the control will be explained. FIG. 4 shows an example of the concentration map of cross-leaked hydrogen. The concentration map of cross-leaked hydrogen is a map representing three relationships: current density, hydrogen electrode side pressure, and hydrogen concentration in oxygen (concentration of hydrogen with respect to oxygen on the oxygen electrode side). The specific expression method is not particularly limited as long as it can represent these three relationships. Here, as shown in FIG. 4, it will be explained by a graph with current density on the horizontal axis and hydrogen electrode side pressure on the vertical axis. In this case, the hydrogen concentration in oxygen can be represented by concentration lines as shown by the dotted lines in FIG. 4. That is, in FIG. 4, each of the lines N1, N2, N3, N4, N5, N6,... is a line connecting equal concentrations. And the hydrogen concentration in oxygen increases as the current density decreases (the left side in FIG. 4). That is, N1 < N2 < N3 < N4 < N5 < N6 <.... In addition to this, the hydrogen concentration in oxygen has a steeper change gradient as the current density decreases (the interval between the concentration lines becomes narrower).

[0021] Since the concentration map of cross-leaked hydrogen varies depending on the thickness of the electrolyte membrane 12 of the water electrolysis cell 11, in this embodiment, it is preferable to have a map for each predetermined thickness. This can not only cope with differences in the water electrolysis cells 11 to be used, but also enable more accurate control by changing the concentration map to be applied according to the passage of use in response to the thinning of the electrolyte membrane 12 due to use.

[0022] The cross-leak hydrogen concentration map is obtained in advance through testing, and this data is stored as a database in the ROM 33 of the controller 30. The current density can be controlled by the controller 30 by controlling the power supply 29, and the hydrogen electrode side pressure can be controlled by the controller 30 by controlling the pressure regulating valve 28. The hydrogen electrode side pressure is also acquired by the controller 30 from the pressure gauge 40.

[0023] 2.2. Control Example 1 Next, we will explain control example 1. Figure 5 shows a diagram for explanation. The upper part of Figure 5 shows the concentration map of cross-leak hydrogen, and the lower part of Figure 5 shows the progression of each value. Here, we will explain the control of stopping water electrolysis, so it is necessary to ultimately set the current density to 0 (zero) and control the process to keep the hydrogen concentration in oxygen below the critical value C (not exceeding the allowable hydrogen concentration). This critical value C is at position C in the lower part of Figure 5. Note that all of the following controls are performed by controller 30, and therefore, no further description is provided.

[0024] When the control to stop water electrolysis is initiated, the current density is reduced to I1. This current density I1 is the target stop current value, and it is the applied current at which the hydrogen concentration in oxygen does not reach the critical value C, based on the cross-leak hydrogen concentration map. Therefore, as shown in the lower part of Figure 5, the hydrogen concentration in oxygen increases but does not reach the critical value C. Furthermore, in this configuration, the hydrogen electrode pressure is maintained constant when the current density decreases. In both figures of Figure 5, the corresponding position is indicated by T1.

[0025] Next, the hydrogen electrode pressure is reduced to complete the depressurization. In this configuration, the current density I1 is kept constant at this time. In both figures of Figure 5, the corresponding positions are indicated by T2. According to this, the hydrogen concentration in oxygen decreases.

[0026] Then, after the depressurization is complete, the current density is reduced to 0 to stop the water electrolysis. In both figures of Figure 5, the corresponding position is indicated by T3. According to this, although the hydrogen concentration in oxygen increases, by shortening the time until the current is stopped, the operation can be stopped without the hydrogen concentration in oxygen exceeding the critical value C.

[0027] 2.3. Control Example 2 Next, we will explain control example 2. Figure 6 shows a diagram for explanation. The upper part of Figure 6 shows the cross-leak hydrogen concentration map, and the lower part of Figure 6 shows the progression of each value.

[0028] When the control to stop water electrolysis is initiated, the current density is reduced to I1. This current density I1 is the applied current at which the hydrogen concentration in oxygen does not reach the critical value C, based on the cross-leak hydrogen concentration map. Therefore, as shown in the lower part of Figure 6, the hydrogen concentration in oxygen increases but does not reach the critical value C. Furthermore, in this configuration, the hydrogen electrode pressure is maintained constant when the current density decreases. In both figures of Figure 6, the corresponding position is indicated by T1.

[0029] Next, the pressure and current density on the hydrogen electrode side are reduced along the isoconcentration line to complete the decompression. In both figures of Figure 6, the corresponding positions are indicated by T2. As shown, the pressure and current density are controlled along the isoconcentration line, so the hydrogen concentration in oxygen remains constant.

[0030] Then, after the depressurization is complete, the current density is reduced to 0 to stop the water electrolysis. In both figures of Figure 6, the corresponding position is indicated by T3. According to this, although the hydrogen concentration in oxygen increases, by shortening the time until the current is stopped, the operation can be stopped without the hydrogen concentration in oxygen exceeding the critical value C.

[0031] 3. Effects, etc. According to this disclosure, it is possible to apply current to stop cross-leakage within the permissible range of hydrogen cross-leakage, thereby suppressing a decrease in electrolysis efficiency. Conventionally, as shown in Figure 7 for example, the hydrogen concentration in oxygen sometimes exceeded the critical value C as the current density and hydrogen electrode pressure decreased.

[0032] Furthermore, according to this disclosure, since the hydrogen concentration in oxygen remains at a low level after the water electrolysis shutdown is complete, the problem of hydrogen concentration in oxygen is less likely to occur during the next operation, which is advantageous from the perspective of system operation.

[0033] Furthermore, when the structure of the water electrolysis cell requires the hydrogen electrode pressure to be higher than the oxygen electrode pressure, control can be performed while lowering the hydrogen electrode pressure during control, while maintaining that it is higher than the oxygen electrode pressure. [Explanation of Symbols]

[0034] 10...Water electrolysis system, 11...Water electrolysis cell, 20...Water electrolysis stack, 21...Gas-liquid separator, 22...Pump, 23...Cooler, 24...Ion exchanger, 25...Gas-liquid separator, 26...Hydrogen tank, 27...Ion exchanger, 28...Pressure regulating valve, 29...Power supply, 30...Controller

Claims

1. A water electrolysis system that performs water electrolysis using a water electrolysis cell, The system includes a controller that, when stopping water electrolysis, applies a stopping current lower than the current applied during water electrolysis to control the depressurization of the hydrogen electrode of the water electrolysis cell. The controller is, A target stop current value is determined based on the cross-leak hydrogen concentration map determined by the pressure of the hydrogen electrode and the applied current, and the current of the target stop current value is applied to the water electrolysis cell. Water electrolysis system.

2. The water electrolysis system according to claim 1, wherein the concentration map has a plurality of maps corresponding to the shape and / or degradation status of the electrolyte membrane of the water electrolysis cell.

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