Method for the electrolysis of water and electrolysis system

Abstract

The invention relates to a method for converting water into oxygen and hydrogen in a proton exchange membrane electrolysis process in one or more electrolysis cells (110.1, 110.2, 110.3, 110.4), wherein a fluid stream (b) containing water is supplied from an oxygen separator (120) to one or more electrolysis cells, wherein an oxygen-containing fluid stream (c) is supplied from an oxygen side (114) of one or more electrolysis cells to the oxygen separator (120), wherein a hydrogen-containing fluid stream (e) is supplied from a hydrogen side (116) of one or more electrolysis cells to a water separator (130), and wherein a pressure on the oxygen side (114) is set or adjusted to a value at least approximately the same or slightly higher than on the hydrogen side (116).

Description

[0001] The invention relates to a method for converting water into oxygen and hydrogen using proton exchange membrane electrolysis, and to an electrolysis plant which can be used, for example, to produce hydrogen. State of the art

[0002] Hydrogen can be produced using electrolysis, in which, for example, water is split or converted into oxygen and hydrogen using electrical energy. This is also referred to as water electrolysis. One example of a suitable method is proton exchange membrane electrolysis (PEM electrolysis). In this context, the terms PEM electrolysis plants or PEM electrolyzers are also used. The fundamentals of this technology are well-established, for example, in "Bessarabov et al: PEM electrolysis for Hydrogen production. CRC Press."

[0003] Water is supplied to the anode side of a PEM electrolyzer at a pressure only slightly above atmospheric pressure for hydrolysis. The H+ ions produced during this process diffuse through the electrolyzer membrane to the cathode side, from where, after combining to form hydrogen molecules, they are drawn off in a hydrogen-rich and water-containing gas mixture at a pressure of at least 20 bar. The oxygen produced on the anode side dissolves in the unreacted water, from which it is typically separated in a container called an oxygen separator. The oxygen-free water is recycled and reused in the PEM electrolysis process.

[0004] During PEM electrolysis, it is unavoidable that some hydrogen will diffuse back to the oxygen side, for example, due to defects or cracks in the membrane. Together with oxygen, this can create an explosive mixture in the oxygen separator, potentially leading to damage.

[0005] Against this background, the task arises to improve a PEM electrolysis system in such a way that safer operation is possible compared to the state of the art.

[0006] This problem is solved by a method for electrolysis and an electrolysis plant with the features of the independent claims. Embodiments are the subject of the dependent claims and the following description. Advantages of the invention

[0007] The present invention focuses on proton exchange membrane electrolysis, in which water, in particular demineralized water, is supplied as a feed medium to one or more electrolysis cells of a PEM electrolyzer in order to be split into hydrogen and oxygen.

[0008] As mentioned previously, a large portion of the water supplied during PEM electrolysis is not converted and is drawn off the anode side of the membrane along with oxygen, while the hydrogen is carried away from the cathode side in a hydrogen-rich gas stream. In a container called an oxygen separator, the oxygen is separated from the water to be released into the atmosphere or used for other purposes. The oxygen-free water is then fed back into the PEM electrolysis process, thus establishing a water cycle.

[0009] The hydrogen-rich fluid stream drawn from the cathode side of the electrolysis cell is fed to a water separator to remove water transported through the membrane and obtain a hydrogen stream, which is typically further purified and / or stored. To facilitate the use of the hydrogen stream obtained in the water separator, the electrolyzer is usually operated at a pressure above 20 bar on the cathode side. In contrast, the pressure on the anode side is only slightly above atmospheric pressure.

[0010] During PEM electrolysis, it is unavoidable that some hydrogen will diffuse back to the oxygen side or otherwise reach it, for example due to defects or tears in the membrane, as explained earlier. This is strongly promoted by the aforementioned high pressure difference between the anode and cathode sides.

[0011] An explosive mixture can therefore form on the oxygen side of the electrolysis unit, which can potentially ignite at numerous points in the electrolysis system downstream of the electrolysis cells, either on the way to the oxygen separator or downstream from it. An explosion or detonation can damage or even destroy the electrolysis system.

[0012] Ignition of the mixture can occur, in particular, if the proportion of hydrogen in the gas (the water content in the liquid phase is irrelevant here) exceeds a certain predetermined level, the so-called lower explosive limit (LEL); this is typically around 4% for hydrogen in oxygen. The (lower) explosive limit of a gas indicates the concentration in a gas mixture above which ignition or explosion is possible in the presence of an ignition source.

[0013] Since an ignition source in the electrolysis unit or downstream cannot generally be ruled out or avoided, the potential for ignition, explosion, or detonation must always be considered during the operation of an electrolysis plant. An explosion is an uncontrolled combustion of an ignitable gas mixture with a laminar flame front. An explosion can manifest as a deflagration or a detonation; these differ primarily in the speed of propagation of the reaction front.

[0014] The reaction between hydrogen and oxygen occurs very rapidly and generates very high flame velocities, meaning the flames propagate at high speeds, for example, in fluid compounds or fluid lines. Explosions of gases or gas mixtures result in a massive pressure increase. Typically, a deflagration of a hydrogen / oxygen mixture can lead to a pressure increase tenfold. The effects of a detonation are significantly more severe and less predictable.

[0015] Against this background, it is now proposed that during electrolysis, i.e., the conversion of water within the framework of a proton exchange membrane electrolysis, and thus in an electrolysis plant, the pressure on the oxygen side should be set or regulated to a value that is at least approximately the same as, or slightly higher than, that on the hydrogen side. In other words, the electrolysis plant should be operated in such a way that the pressure on the anode side is approximately the same as, or slightly higher than, the pressure on the cathode side.

[0016] Approximately equal pressure can be understood, for example, as a difference of less than 5%, preferably less than 2%, or less than 1% between the anode-side pressure and the cathode-side pressure. For example, at a pressure of 30 bar, a difference of preferably 50 mbar to a maximum of 1.5 bar is acceptable. Operating at nearly identical pressures on the anode and cathode sides of the PEM membrane reduces potential leakage currents from one electrode to the other in the event of membrane damage.

[0017] The pressure on the anode side is slightly higher if its value is between 50 mbar and 1.5 bar higher than the pressure on the cathode side. Such a pressure difference, in particular, reduces potential leakage currents and offers greater safety and longer response times for the control systems designed to prevent an explosive gas mixture or to initiate a shutdown. Therefore, it is preferable to set or regulate the pressure on the oxygen side to a slightly higher value than on the hydrogen side.

[0018] In this case, it is also relevant that the lower explosive limit for oxygen in hydrogen is approximately 6%, and therefore higher than the lower explosive limit for hydrogen in oxygen (which, as mentioned, is approximately 4%). This makes it particularly easy to prevent a potential explosion.

[0019] In one embodiment, the pressure on the anode and cathode sides is set by measuring and adjusting the differential pressure between an oxygen stream from the oxygen separator and a hydrogen stream from the water separator. This allows for the absolute pressure of either the oxygen stream from the oxygen separator or the hydrogen stream from the water separator to be set or regulated. The differential pressure can be adjusted or regulated, in particular, on the side where the absolute pressure is not being adjusted or regulated.

[0020] This allows for particularly precise monitoring and maintenance of the differential pressure. The differential pressure control can be set, for example, to maintain a small pressure difference in the range of 50 mbar to 1.5 bar (or possibly even up to 2 bar) to ensure a defined direction of the leakage flow. An advantage of this control concept is the separation of the two separators or containers, thus eliminating the risk of gas transfer from one container to the other via connecting pipes.

[0021] In one embodiment, the water separator and the oxygen separator are fluid-connected, particularly in their respective liquid-carrying areas (typically near the bottom), and the pressure on the oxygen side is set or regulated to the same or a slightly higher value than on the hydrogen side. Preferably, the difference between the liquid levels in the oxygen and water separators is monitored for this purpose. The absolute pressure in the system can be set or regulated on either the oxygen or the hydrogen side.

[0022] One advantage of pressure control via the difference in liquid levels is a more consistent control of the pressure differential. Fill levels are a very precise indicator of the pressure differential. Changes in liquid volume result in a greater inertia in the pressure change. Pressure control with less deviation from the setpoint is possible, even during transient operation of the electrolysis plant. This allows for higher ramp rates for certain pressure differential limits.

[0023] In one embodiment, water separated in the water separator is supplied directly to the cathode side of the electrolysis cell(s), i.e., without passing through the oxygen separator. This can be described as cathode water recirculation. This allows pressure profiles along the electrolysis cells to be controlled in order to reduce local pressure gradients within the electrolysis cells or a stack of electrolysis cells.

[0024] The invention is explained in more detail below with reference to the accompanying drawing, which shows a system according to a preferred embodiment of the present invention.

[0025] Brief description of the drawing Figure 1 , 2 and 3 schematically show electrolysis plants in various designs. Figure 4 , 5a and 5b show diagrams to illustrate the invention. Detailed description of the drawings

[0026] In Figure 1Figure 100 schematically depicts an electrolysis plant in an embodiment suitable for carrying out the process according to the invention. This is an electrolysis plant for water electrolysis using PEM. In particular, the electrolysis plant shown here, as well as the electrolysis plant generally described within the scope of the invention, is suitable for producing, for example, hydrogen on an industrial scale. The typical power output of such an electrolysis plant is, for example, more than 10 MW or even more than 20 MW.

[0027] The electrolysis system 100 features two electrolysis units or stacks 110a and 110b as examples, of which only stack 110a is shown in more detail, namely with four electrolysis cells 110.1, 110.2, 110.3, and 110.4, each containing a proton exchange membrane (PEM) 112. The PEM 112 separates each electrolysis cell into an oxygen side 114 and a hydrogen side 116. The oxygen sides 114 and the hydrogen sides 116 can be considered together as the oxygen side and hydrogen side, respectively, of a stack or of the entire electrolysis system.

[0028] Stack 110b can, for example, be constructed in the same way as Stack 110a, and further stacks of this type can also be provided. Generally, an electrolysis system can have one or more electrolysis cells, whereby in the case of multiple electrolysis cells, some of them, e.g., two, four, six, eight or more, are arranged as a stack and, for example, are supplied with electrical energy together.

[0029] The electrolysis system 100 also includes a tank 120, which serves as an oxygen separator or oxygen-water separator. The oxygen separator 120 is connected via a fluid connection to the stacks 110a and 110b, and within each stack, to the electrolysis cells 110.1, 110.2, 110.3, and 110.4. This allows a fluid flow b to be pumped from the oxygen separator 120 to the electrolysis cells via a suitable fluid connection 122. Depending on the type of electrolysis system and the number of electrolysis cells and / or stacks, multiple fluid connections (i.e., separate lines) may be provided. For example, two or four electrolysis cells can be supplied via a single fluid connection.

[0030] The stacks or electrolysis cells are also connected to the oxygen separator 120 via a fluid connection 126, e.g., pipes. A fluid flow c can be pumped from the oxygen sides 114 of the electrolysis cells to the oxygen separator 120 through the fluid connection 126; the pump 124 may also be sufficient for this purpose. Depending on the type of electrolysis system and the number of electrolysis cells, several fluid connections (i.e., separate lines) may be provided. For example, one fluid connection may be provided for two or four electrolysis cells.

[0031] Furthermore, the electrolysis system 100 includes an additional container 130, which serves as a water separator or hydrogen-water separator. The stacks or electrolysis cells are connected to the water separator 130 via a fluid connection 132. A fluid flow e can be conveyed from the hydrogen sides 116 of the electrolysis cells to the water separator 130 through the fluid connection 132. Depending on the type of electrolysis system and the number of electrolysis cells, several fluid connections may be provided. For example, one fluid connection may be provided for two or four electrolysis cells. Multiple water separators are also conceivable.

[0032] During operation of the electrolysis plant 100, the fluid stream b, which consists largely of water, is pumped from the oxygen separator 120 to the stacks or their electrolysis cells. There, the water is converted into oxygen and hydrogen by applying an electric current. The hydrogen is transported by the PEM 112 to the hydrogen side 116 and can then be fed, possibly mixed with water vapor and a liquid water phase, as stream e – a hydrogen-containing fluid stream – to the water separator 130. There, the hydrogen is separated and discharged as stream f, for example, for further use or storage. Water separated in the water separator 130 can, for example, be treated (not shown here) and then returned to the main water circuit as fluid stream g.

[0033] The oxygen remains on the oxygen side 114 along with most of the water. The resulting fluid flow c therefore contains water and oxygen – it is an oxygen-containing fluid flow. As mentioned, the fluid flow c is fed to the oxygen separator 120.

[0034] It should be mentioned here that the fluid stream c may also contain a certain small proportion of hydrogen, although this is to be avoided as far as possible within the scope of the present invention. Furthermore, the fluid stream e may also contain a certain small proportion of oxygen.

[0035] Since water is converted into oxygen and hydrogen in the electrolysis cells, water (so-called make-up water) must be supplied externally to maintain continuous operation.

[0036] Water separated in water separator 130, i.e., the fluid flow g already mentioned, can be fed to flow a and then back to oxygen separator 120, possibly also after prior treatment.

[0037] As mentioned, oxygen is separated from water in the oxygen separator 120. The separated or separated oxygen can be discharged as an oxygen stream, for example for further use and possibly stored.

[0038] Furthermore, the electrolysis system 100 includes a controllable or adjustable valve 142, by means of which the oxygen flow d from the oxygen separator can be controlled or regulated. The electrolysis system 100 also includes a pressure sensor 144, which can detect or measure a differential pressure between the oxygen flow d and the hydrogen flow f. Based on the measured value, the controllable or adjustable valve 142 can be set accordingly.

[0039] Furthermore, the electrolysis system 100 has a controllable or adjustable valve 152, by means of which the hydrogen flow f from the water separator can be controlled or regulated. In addition, the electrolysis system 100 has a pressure sensor 144, which can detect or measure the absolute pressure in the hydrogen flow f. Based on the measured value, the controllable or adjustable valve 152 can be set or regulated.

[0040] Furthermore, the electrolysis plant 100 has a control unit 160, which can be used to control and, if necessary, monitor, for example, the stacks 110a and 110b or their power supply. The control unit 160 can also be configured, for example, to control or regulate the controllable or adjustable valves 142 and 152 based on the respective measured values.

[0041] For the operation of the electrolysis plant 100, the pressure on the oxygen side 114 is set or regulated to a value that is equal to or slightly higher than the pressure on the hydrogen side 116. For this purpose, the pressure on the hydrogen side, i.e., in this case in the hydrogen flow f, is set or regulated to a specific absolute value, e.g., to a value of 30 bar, using the pressure sensor 154 and the valve 152.

[0042] Furthermore, the differential pressure between the oxygen flow d and the hydrogen flow f is measured by means of pressure sensor 144. This differential pressure is set or regulated to a value between 0 and 1 bar, so that the pressure on the oxygen side is equal to or slightly higher than on the hydrogen side. This is then achieved by actuating valve 142.

[0043] This prevents hydrogen from reaching the oxygen side and creating an explosive gas mixture on the oxygen side.

[0044] In Figure 2 A schematic representation of an electrolysis plant 200 in a further embodiment is shown, with which a method according to the invention can also be carried out. The electrolysis plant 200 corresponds in principle to the electrolysis plant 100 according to Figure 1 Therefore, only differences will be discussed below. Identical components are marked with the same reference symbol.

[0045] The water separator 130 is fluidly connected to the oxygen separator 120, so that the fluid flow g can be directed directly into the oxygen separator 120.

[0046] The electrolysis system 200 features the controllable or adjustable valve 142, by means of which the oxygen flow d from the oxygen separator 120 can be controlled or regulated. Instead of the pressure sensor 144, the electrolysis system 200 has a pressure sensor 244, which can detect or measure an absolute pressure in the oxygen flow d. Based on the measured value thus obtained, the controllable or adjustable valve 142 can be set. It should be noted that the pressure control or valve 142 can also be located on the side of the water separator 130, i.e., that the pressure control takes place there.

[0047] Furthermore, the electrolysis system 200 has a controllable or adjustable valve 246 by means of which the make-up water flow a into the oxygen separator 120 can be controlled or regulated. In addition, the electrolysis system 200 has a level sensor 248 that can detect or measure an absolute fill level in the oxygen separator 120. The electrolysis system 200 also has a level sensor 254 that can detect or measure an absolute fill level in the water separator 130.

[0048] The level sensor 254 is connected to the level sensor 248, allowing its values ​​to be transmitted. This enables the level sensor 248 to determine, for example, a differential level or differential fill level. Based on the fill level of both containers, the controllable valve 246 can be adjusted.

[0049] Furthermore, the electrolysis plant 200 features, for example, the controllable valve 152, by means of which the hydrogen flow f from the water separator 130 can be controlled or regulated. The valve 152 is regulated by the differential pressure between the two containers 120 and 130. That is, if the fill level in the water separator 130 rises relative to the fill level in the oxygen separator 120, the valve 152 closes to increase the hydrogen pressure. This forces liquid through line g into the oxygen separator 120, and the fill levels in the two containers equalize again. The aim here is to keep the pressure difference between the two containers small.

[0050] Furthermore, the electrolysis system 100 has the control unit 160, which can be installed here to control or regulate the controllable or adjustable valves 142, 246 and, if applicable, 152 based on the respective measured values.

[0051] For the operation of the electrolysis plant 200, the pressure on the oxygen side 114 is set or regulated to a value at least approximately the same as, or slightly higher than, that on the hydrogen side 116. For this purpose, the pressure on the oxygen side 114 is set or regulated to a specific absolute value, e.g., to a value of 30 bar or 32 bar, using the pressure sensor 244 and the valve 142.

[0052] Furthermore, the liquid level difference between the oxygen separator 120 and the water separator 130 is measured by means of the level sensors 248 and 254. This liquid level difference (relative to an absolute measurement outside the containers) is set or adjusted to zero, for example, but preferably to a slightly below zero value, i.e., to a slight gas overpressure in the oxygen system. This is then achieved by actuating the valve 152. A slightly below zero value for the liquid level difference means that the fill level in the oxygen separator is slightly lower than in the water separator. Since both separators are fluid-connected, this means that the gas pressure in the oxygen separator 120, and thus on the oxygen side, is slightly higher than in the water separator 130, or on the hydrogen side.

[0053] In this way, it is prevented, especially in the case of membrane defects, that hydrogen reaches the oxygen side and an explosive gas mixture is formed on the oxygen side.

[0054] In Figure 3 A schematic representation of an electrolysis plant 300 in a further embodiment is shown, with which a method according to the invention can also be carried out. The electrolysis plant 300 corresponds in principle to the electrolysis plant 100 according to Figure 1 Therefore, only differences will be discussed below. Identical components are marked with the same reference symbol.

[0055] The electrolysis system 300 comprises a fluid connection h through which water from the water separator 130 is supplied, for example, to the electrolysis cells 110.1 to 110.4, directly on the hydrogen side 116. The fluid flow h can also be supplied to only one or more of the electrolysis cells. A pump 334 is provided for this purpose.

[0056] In this way, a pressure profile within or along the electrolysis cell (or cells) can be controlled to reduce local pressure gradients within the electrolysis cells or a stack of electrolysis cells.

[0057] In Figure 4 A schematic diagram illustrating the invention is shown. Figure 400 represents an acceptable leakage size in mm², expressed as a differential pressure in bar between the hydrogen and oxygen sides of a PEM electrolysis cell, denoted by 402.

[0058] The points marked 411 represent a pressure gradient between the oxygen side and the hydrogen side at a proportion of 5 vol-% oxygen in the hydrogen separator.

[0059] The points marked 412 represent a pressure gradient between the oxygen side and the hydrogen side at a proportion of 3 vol-% oxygen in the hydrogen separator.

[0060] The points marked 413 represent a pressure gradient between the hydrogen side and the oxygen side at a hydrogen content of 2 vol-% in the oxygen separator.

[0061] The points marked 414 represent a pressure gradient between the hydrogen side and the oxygen side at a hydrogen content of 4 vol-% in the oxygen separator.

[0062] This diagram shows that the acceptable leakage size decreases with increasing differential pressure, as the leakage rate increases accordingly. However, it also shows that in cases where the pressure on the oxygen side is higher than on the hydrogen side, the acceptable leakage size is significantly larger than in cases where the pressure on the hydrogen side is higher than on the oxygen side. It should also be noted that this is due to the somewhat higher lower explosive limits of oxygen in hydrogen compared to hydrogen in oxygen.

[0063] Overall, this diagram shows that it is advantageous if the pressure on the oxygen side is slightly higher than on the hydrogen side, e.g. between 50 mbar or 100 mbar and 1 bar or 2 bar.

[0064] In the Figure 5a , 5bDiagrams illustrating the invention are shown. Figure 512 represents a PEM, with oxygen side 514 and hydrogen side 516. A small diagram is shown on both oxygen side 514 and hydrogen side 516, in which a pressure 502 is plotted over a position 500 along the PEM.

[0065] 541 indicates a regular diffusion of hydrogen from the hydrogen side 516 to the oxygen side 514, while 542 indicates a flow or leakage of oxygen from the oxygen side to the hydrogen side.

[0066] In Figure 5a Figure 531 shows a fluid flow that leads into the oxygen side 514 of the electrolysis cell; this fluid flow 531 can, for example, correspond to the fluid flow b according to Figures 1 to 3 or correspond to the relevant proportion for an electrolysis cell.

[0067] The pressure in the electrolysis cell is, for example, consistently around 30 bar on the hydrogen side, while on the oxygen side, the pressure is only around 30.1 bar at the higher position (in the figure above) (i.e., slightly higher than on the hydrogen side). Towards the bottom, i.e., the lower positions, the pressure increases to approximately 32 bar. This is due to the fluid flow 531 typically entering from below.

[0068] In this way, the overpressure at the lower positions is relatively high, e.g., approximately 2 bar. In the event of a rupture or other defect of the PEM, this results in a relatively high leakage rate, at least in the lower area, as also indicated by... Figure 4 as is evident.

[0069] In Figure 5b Figure 532 additionally shows a fluid flow that leads into the hydrogen side 516 of the electrolysis cell; this fluid flow 532 can, for example, correspond to the fluid flow h according to Figure 3or the corresponding proportion for an electrolysis cell. Due to this additional fluid flow, the pressure on the hydrogen side 516 increases downwards, e.g. also to approximately 32 bar, resulting in a comparable pressure profile across the position as on the oxygen side 514.

[0070] In this way, the pressure difference between the two sides remains consistently low, with a slightly higher pressure on the oxygen side in particular. Therefore, in the event of a rupture or other defect in the PEM, the leakage rate remains relatively low throughout, as also indicated by... Figure 4 This is evident. In this way, security can be increased.

Claims

1. A method for converting water, which is supplied to one or more electrolysis cells (110.1, 110.2, 110.3, 110.4) of a proton exchange membrane electrolyzer operated at a first pressure on the anode side and at a second pressure on the cathode side, in order to be decomposed into hydrogen and oxygen, wherein an oxygen-containing water stream (c) accumulating on the anode side (114) is supplied to an oxygen separator (120) for the separation of oxygen, while a hydrogen-rich and water-containing gas mixture obtained on the cathode side (116) is introduced into a water separator (130) for the separation of a hydrogen stream (f). characterized by the fact that The first pressure is the same as, or no more than, 2 bar higher than the second pressure.

2. Method according to claim 1, wherein the pressure difference between the anode and cathode sides is adjusted by measuring the differential pressure between the oxygen flow (d) from the oxygen separator (120) and the hydrogen flow (f) from the water separator (130).

3. Method according to claim 1, wherein the water separator (130) and the oxygen separator (120) are fluid-connected, particularly in a respective liquid-carrying area, and wherein the pressure on the oxygen side (314) is set or adjusted to at least approximately the same or a slightly higher value than on the hydrogen side (316) by setting or adjusting a liquid level difference in the oxygen separator (120) and the water separator (130).

4. Method according to claim 2 or 3, wherein an absolute pressure of the oxygen flow (d) from the oxygen separator (120) or of the hydrogen flow (f) from the water separator (130) is set or regulated.

5. Method according to one of the preceding claims, wherein a fluid stream (h) containing water is supplied directly to the hydrogen side (116) from the water separator (130) of one or at least one of the several electrolysis cells (110.4).

6. Method according to one of the preceding claims, wherein the pressure on the oxygen side (314) is set or adjusted to a value between 50 mbar and 1.5 bar higher than on the hydrogen side (316).

7. Method according to one of the preceding claims, wherein the pressure on the hydrogen side is set or regulated to a value between 5 barg and 50 barg, in particular between 10 barg and 32 barg.

8. Electrolysis system (100) comprising several electrolysis cells in which water can be converted into oxygen and hydrogen by means of proton exchange membrane electrolysis, comprising an oxygen separator (120) and a water separator (130), wherein the electrolysis system (100) is configured to supply a fluid stream (b) containing water from the oxygen separator to one or more electrolysis cells, to supply an oxygen-containing fluid stream (c) from an oxygen side of one or more electrolysis cells to the oxygen separator, and to supply a hydrogen-containing fluid stream (e) from a hydrogen side of one or more electrolysis cells to the water separator, wherein the electrolysis system (100) is further configured to set or regulate a pressure on the oxygen side to a value at least approximately equal to or slightly higher than on the hydrogen side.

9. Electrolysis system (100) according to claim 8, which is configured to adjust or regulate the pressure on the oxygen side to at least approximately the same or slightly higher value than on the hydrogen side by adjusting or regulating a differential pressure between an oxygen stream from the oxygen separator and a hydrogen stream from the water separator.

10. Electrolysis system (100) according to claim 8 or 9, wherein the water separator and the oxygen separator are fluid-connected, in particular in a respective liquid-carrying area, and wherein the electrolysis system (100) is configured to adjust or regulate the pressure on the oxygen side to at least approximately the same or slightly higher value than on the hydrogen side by adjusting or regulating a liquid level difference in the oxygen separator and the water separator.

11. Electrolysis system (100) according to one of claims 8 to 10, which is configured to adjust or regulate an absolute pressure of the oxygen flow from the oxygen separator or the hydrogen flow from the water separator.

12. Electrolysis system (100) according to one of claims 8 to 11, which is configured to supply a fluid stream containing water from the water separator of one or at least one of the several electrolysis cells directly to the hydrogen side (116).

13. Electrolysis system (100) according to one of claims 8 to 12, which is configured to carry out a method according to one of claims 1 to 7.

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