Method for operating an electrolysis plant, and electrolysis plant

Abstract

The invention relates to a method for operating an electrolysis plant (1), comprising a stack (2) having an anode (3) and a cathode (4), wherein in normal operation of the electrolysis plant (1), water is supplied to the anode (3) via a water circuit (5) with an integrated pump (6), said water being split in the stack (2) by electrolysis into hydrogen and oxygen, and wherein the hydrogen produced by electrolysis is supplied to a gas-liquid separator (8) via a cathode outlet (10) of the stack (2) and a media line (7) connected thereto. According to the invention, when the electrolysis plant (1) is switched off, the current density is reduced to 0 A / cm² and the media line (7) is shut off with the aid of a valve (9), while the anode (3) continues to be supplied with water via the water circuit (5) with the aid of the pump (6). The invention further relates to an electrolysis plant (1) that is suitable for carrying out the method or can be operated according to the method.

Description

[0001] Description

[0002] Title:

[0003] Method for operating an electrolysis plant, electrolysis plant

[0004] The invention relates to a method for operating an electrolysis plant. Furthermore, the invention relates to an electrolysis plant suitable for carrying out the method or operable according to the method.

[0005] The preferred field of application of the invention is the production of hydrogen by electrolysis.

[0006] State of the art

[0007] Electrolysis is a chemical process in which electrical current is used to break down a chemical compound. This allows hydrogen and oxygen to be extracted from water.

[0008] To carry out the electrolysis, at least one electrolysis cell is required, for example a PEM (“Proton Exchange Membrane”) electrolysis cell, an AEL (“Liquid Alkaline Electrolysis”) electrolysis cell, a solid oxide electrolysis cell or SOEC (“Solid Oxide Electrolyzer Cell”) or an AEM C (Anion Exchange Membrane”) electrolysis cell, whereby the electrolysis cells mentioned differ in the type of ionic conductivity.

[0009] Typically, a large number of identical electrolysis cells are stacked and connected to form a cell stack, the so-called stack. This requires additional components, particularly those for power and water supply, as well as for gas processing. The power supply components typically include a rectifier for converting AC to DC and a DC / DC converter for adjusting the DC voltage. The water supply components include at least one water circuit and a pump. The gas processing components include gas-liquid separators to separate the product gases hydrogen and oxygen from water. Furthermore, means for controlling and / or regulating the system are required.

[0010] Against the backdrop of decarbonization, hydrogen is becoming increasingly important as an energy carrier. The current goal is to produce green hydrogen, i.e., hydrogen with a very low carbon dioxide (CO2) footprint, through electrolysis on a large industrial scale. Renewable energy sources such as wind and solar energy are required to produce green hydrogen. Since these energy sources fluctuate, electrical energy is not always available, and the electrolysis plant must be shut down temporarily. This negatively impacts the service life of the electrolysis plant for the reasons listed below.

[0011] When the system is shut down, the water supply to the anode side is interrupted. Due to the higher pressure on the cathode side, hydrogen diffuses from the cathode to the anode side through the not completely gas-tight membranes of the electrolysis cells. As a result, the cell voltage drops steadily from the usual 1.5 V to 2 V during operation to as low as 0 V. At the same time, the electrochemical potential on the anode-side catalyst drops from over 1.3 V to as low as 0 V. A potential drop to below 1 V is accompanied by a reduction in the anode-side catalyst, which is in an oxidized state during normal operation. The reduction and renewed oxidation when electrolysis operation is restarted leads to a significant acceleration of the aging of the anode-side catalyst, which reduces catalyst activity. As a result, for a given current density, the electrolysis cell voltage increases and the electrolysis efficiency decreases accordingly.Frequent shutdown and restart of an electrolysis plant can therefore reduce its service life to an uneconomical level.

[0012] The present invention is concerned with the task of minimizing or completely eliminating the disadvantages that occur when shutting down an electrolysis plant.

[0013] To achieve this objective, the method having the features of claim 1 and the electrochemical electrolysis system having the features of claim 6 are proposed. Advantageous developments of the invention can be found in the respective subclaims. Disclosure of the invention

[0014] A method for operating an electrolysis system comprising a stack with an anode and a cathode. During normal operation of the electrolysis system, water is supplied to the anode via a water circuit with an integrated pump, which is split into hydrogen and oxygen in the stack by electrolysis. The hydrogen produced by electrolysis is fed to a gas-liquid separator via a cathode outlet of the stack and a media line connected thereto. According to the invention, when the electrolysis system is shut down, the current density is reduced to 0 A / cm 2 reduced and the media line is blocked by means of a valve, while water continues to be supplied to the anode via the water circuit with the help of the pump.

[0015] The water that continues to be supplied to the anode during shutdown ensures that hydrogen that diffuses from the cathode side to the anode side is transported away from the anode along with the water. This prevents the anode potential from falling below 1 V. Consequently, the disadvantages associated with a potential drop below 1 V, as described above, are also avoided.

[0016] By simultaneously blocking the media line with the help of the valve, the pressure on the cathode side in the stack is separated from the pressure in the gas-liquid separator. During longer shutdown periods, this leads to the pressure on the cathode side in the stack decreasing over time, and less hydrogen diffusing from the cathode side to the anode side. This means that the hydrogen permeation rate decreases. The less hydrogen that diffuses from the cathode side to the anode side, the less water is required to transport the hydrogen away. Accordingly, the pumping capacity of the pump integrated into the water circuit can be reduced, thereby increasing system efficiency.

[0017] To further reduce pumping power or increase system efficiency, a further development of the invention proposes that the water flow toward the anode be reduced when the electrolysis system is shut down. This means that the water flow during shutdown is lower than during normal operation. The pumping power required to supply water to the anode is correspondingly low. Preferably, the water flow is reduced to a minimum water flow required to transport the hydrogen from the anode side. The water flow must therefore be just high enough to transport the hydrogen that has diffused from the cathode side to the anode side. Furthermore, it is preferably proposed that the minimum water flow be estimated. This can be done as follows:

[0018] Typical hydrogen permeation rates are approximately 0.85 mA cm' 2 p_H2_bara' 1 , where the cm 2refer to the active membrane area in the stack. Using these values ​​and a known solubility of hydrogen in water, for example, 0.0012 g_H2 / kg_H2O at 60 °C, the minimum water flow to the anode can be calculated. At a temperature of 60 °C, this is 87 mg_H2O cm' 2 s' 1 p_H2_bara' 1 (1 bara = 1 bar absolute = 0.1 MPa absolute).

[0019] According to a preferred embodiment of the invention, when the electrolysis system is shut down, the pressure on the cathode side in the stack is actively reduced by venting hydrogen from the cathode. Compared to passive pressure reduction, active pressure reduction leads to a significantly faster reduction in hydrogen permeation rate and thus to a reduction in the required pumping power. In this way, a reduction in the required pumping power can be achieved even with shorter shutdown times.

[0020] Advantageously, the pressure is reduced to ambient pressure so that the hydrogen permeation rate reaches a minimum.

[0021] To actively reduce the pressure on the cathode side, a gas line connected directly or indirectly to the stack's cathode outlet can be used. The gas line can, for example, be connected indirectly to the cathode outlet via the media line. In this case, the valve integrated into the media line can be designed as a directional control valve or be preceded by another valve, preferably a directional control valve.

[0022] Furthermore, it is proposed that when the electrolysis system is switched off, the pump integrated into the water circuit is operated intermittently. By operating the pump intermittently rather than continuously, the required pumping power can be further reduced and the system efficiency can be further increased. According to a further preferred embodiment of the invention, it is proposed that when the electrolysis system is switched off, the cathode is purged with an inert gas, preferably nitrogen. By purging the cathode with inert gas, hydrogen permeation can be completely stopped. The water flow towards the anode then only serves to provide a defined oxygen partial pressure on the anode. Accordingly, the water flow towards the anode can be minimized, for example by operating the pump integrated into the water circuit intermittently.

[0023] The inert gas can be stored in a separate container connected to the stack's cathode via an inert gas line with an integrated valve. Opening the valve allows inert gas to enter the cathode and displace the hydrogen present there. This hydrogen can be removed from the cathode via the gas line connected directly or indirectly to the cathode outlet.

[0024] During long shutdown periods, it is preferable to purge the cathode with inert gas at regular intervals. This ensures that the cathode is always filled with inert gas, even if there are gas leaks in the system. The intervals can range from one hour to 24 hours, for example.

[0025] To solve the aforementioned problem, an electrolysis system is also proposed. This system comprises: a stack with an anode and a cathode, a water circuit with an integrated pump to supply the anode with water, and a media line connecting a cathode outlet of the stack to a gas-liquid separator.

[0026] According to the invention, a valve for blocking the media line is integrated into the cathode outlet or into the media line. By blocking the media line, the cathode is separated from the gas-liquid separator and thus from the pressure in the gas-liquid separator. This leads to a drop in the cathode-side pressure in the stack due to leaks in the system, particularly during longer shutdown times. As the pressure drop, the hydrogen permeation rate also decreases, so that less hydrogen reaches the anode side by diffusion from the cathode side. If, in addition, water continues to be supplied to the anode via the water circuit when the electrolysis system is shut down, the hydrogen that has passed from the cathode side to the anode side can be transported away from the anode. This prevents the anode potential from falling below 1 V and the disadvantages described above from occurring.

[0027] The proposed electrolysis system is therefore particularly suitable for carrying out the method according to the invention described above, or can be operated according to this method. The electrolysis system can therefore also achieve the same advantages. In particular, the degradation of the electrolysis cells can be counteracted, thus increasing the service life of the stack.

[0028] According to a preferred embodiment of the invention, the valve integrated into the media line is designed as a directional valve, in particular as a 3-way valve, wherein a gas line is connected to the media line via the directional valve. With the help of the valve integrated into the media line, the gas line can be opened, while the media line in the direction of the gas-liquid separator is blocked. Hydrogen can then be vented from the cathode via the gas line, thus actively reducing the pressure in the cathode. With the help of the active pressure reduction, the desired low hydrogen permeation rate can be achieved much more quickly. In addition, the water flow in the direction of the anode can be minimized, since less hydrogen needs to be transported away from the anode. With the water flow, the required pump power also decreases, while system efficiency increases.

[0029] According to an alternative embodiment of the invention, the valve integrated into the media line is connected downstream of a directional control valve, in particular a 3-way valve. The gas line can then be connected to the media line via the directional control valve. The valve downstream of the directional control valve, however, can be designed as a simple shut-off valve.

[0030] Furthermore, it is proposed that a pressure sensor be integrated into the cathode outlet or the media line. The pressure sensor can be used to measure the pressure on the cathode side. The pressure sensor can be designed as an absolute pressure sensor or as a differential pressure sensor with respect to the ambient pressure. This is particularly advantageous if the pressure on the cathode side is to be reduced to ambient pressure. If the pressure sensor is an absolute pressure sensor, a further pressure sensor is preferably present to measure the ambient pressure.

[0031] In a further development of the invention, it is proposed that an inert gas container be connected to the cathode of the stack via an inert gas line with an integrated shut-off valve. This configuration of the electrolysis system enables the cathode to be purged with inert gas, for example, nitrogen, during shutdown. The volume of the inert gas container preferably corresponds to at least five to ten times the volume of the cathode, so that sufficient inert gas is available to purge the cathode, in particular for repeated purging of the cathode.

[0032] The proposed electrolysis system advantageously comprises a control unit that is configured to carry out steps of a method according to the invention. With the aid of the control unit, the valve integrated into the media line can then be closed when the electrolysis system is switched off and - if present - the gas line connected to the media line via the valve or another valve can be opened in order to reduce the pressure on the cathode side and thus the hydrogen permeation rate. At the same time, the control unit can be used to control the pump integrated into the water circuit so that it continues to supply water to the anode. Furthermore, the control unit can be used to estimate a minimum water flow that is necessary to remove the hydrogen that has passed from the cathode side to the anode side. The pump is then controlled accordingly. If the hydrogen permeation rate is very low, the pump can then be switched off if necessary.operated intermittently. If an inert gas container and an inert gas line with an integrated shut-off valve are available, the control unit can be used to control or open the shut-off valve to purge the cathode with the inert gas.

[0033] The invention and its advantages are described in more detail below with reference to the accompanying drawings. These show:

[0034] Fig. 1 is a schematic representation of a first electrolysis plant according to the invention,

[0035] Fig. 2 is a schematic representation of a second electrolysis plant according to the invention,

[0036] Fig. 3 shows a section of a schematic representation of a third electrolysis plant according to the invention and

[0037] Fig. 4 is a schematic representation of a fourth electrolysis plant according to the invention.

[0038] Detailed description of the drawings

[0039] Figure 1 shows a first electrolysis system 1 according to the invention. The electrolysis system 1 is used to produce hydrogen. For this purpose, the electrolysis system 1 comprises a stack 2 with an anode 3 and a cathode 4. During operation of the system, water is supplied to the anode 3 via a water circuit 5 with an integrated pump 6. In the stack 2 or in the electrolysis cells of the stack 2, the water is then split into hydrogen and oxygen. The hydrogen produced on the cathode side is fed to a gas-liquid separator 8 via a media line 7, which is connected to a cathode outlet 10 of the stack 2. The gas separated from the water is then passed through a droplet eliminator 17. The water is recirculated and fed back to the stack 2. The oxygen produced on the anode side is discharged from the anode 3 of the stack 2 together with water and likewise fed to a gas-liquid separator 18.In this case, a further droplet separator 19 is connected downstream of the gas-liquid separator 18. The water separated from the oxygen in the gas-liquid separator 18 is also recirculated and fed back to the stack 2. Furthermore, a fish water supply 21 is provided, through which fresh water is fed into the water circuit 5.

[0040] The electrical energy required for electrolysis is provided by power electronics 20 connected to the stack 2. The electrolysis process is controlled or regulated by a control unit 16 of the electrolysis system 1.

[0041] The electrolysis system 1 shown in Figure 1 has a valve 9, which is designed as a shut-off valve and integrated into the media line 7. With the aid of the valve 9, the connection between the cathode 4 and the gas-liquid separator 8 can be interrupted. The valve 9 thus enables the implementation of the method according to the invention when the electrolysis system 1 is shut down, the method comprising the following steps:

[0042] Reduction of current density to 0 A / cm 2 ,

[0043] Blocking the media line 7 using the valve 9 and maintaining the operation of the pump 6 so that water continues to be supplied to the anode 3 via the water circuit 5.

[0044] Due to the water flow on the anode 3, hydrogen permeated from the cathode 4 to the anode 3 is transported away from the anode 3 and fed to the gas-liquid separator 18. The majority of the permeated hydrogen reacts directly at the cathode catalyst with the dissolved oxygen present in the recirculated water (recombination), resulting in an extremely low hydrogen partial pressure in the recirculated water. This prevents the anode potential from falling below 1 V. By blocking the media line 7, the pressure on the cathode side drops over time due to the lack of gas tightness and hydrogen permeation to the anode side, which leads to a reduction in the hydrogen permeation rate. This means that less hydrogen diffuses and thus less hydrogen passes from the cathode side to the anode side.Accordingly, the water flow toward the anode 3 can be reduced, which in turn enables a reduction in pumping power and thus an increase in system efficiency. By blocking an anode-side gas outlet 22, it can also be ensured that the oxygen present in the gas-liquid separator 18 always ensures a sufficient amount of physically dissolved oxygen in the recirculated water.

[0045] Figure 2 shows a second electrolysis system 1 according to the invention. This differs from the system in Figure 1 by the valve 9 integrated into the media line 7, which in Figure 2 is not designed as a simple shut-off valve, but as a directional control valve. A gas line 11 is connected to the media line 7 via the directional control valve. Furthermore, a pressure sensor 12 is integrated into the media line 7 upstream of the valve 9. The pressure on the cathode side can be measured using the pressure sensor 12.

[0046] The electrolysis system 1 shown in Figure 2 enables an active pressure reduction in the cathode 4 when switched off. For this purpose, the media line 7 in the direction of the gas-liquid separator 8 is blocked using the valve 9, which is designed as a directional control valve. At the same time, the gas line 11 is opened so that hydrogen can be released from the cathode 4 via it. As hydrogen is released, the pressure in the cathode 4 drops. This also results in a decrease in the hydrogen permeation rate, so that less water needs to be supplied to the anode 3 and the pumping power can be reduced. Compared to the passive pressure drop in the cathode 4, the active pressure reduction has the advantage that the pressure is reduced much more quickly and thus the desired effect occurs much more quickly. Ideally, the pressure on the cathode side is reduced to ambient pressure in order to minimize the hydrogen permeation rate.When ambient pressure is reached, this can be monitored using pressure sensor 12. This is preferably designed as a differential pressure sensor relative to the ambient pressure.

[0047] A modification of the electrolysis system 1 in Figure 2 is shown in Figure 3. The modification, and therefore also the illustration, is limited to the area of ​​the media line 7. Valve 9 is designed here—analogous to Figure 1—as a simple shut-off valve. In addition, a further valve 9' is provided, which is connected upstream of valve 9 and is designed as a directional control valve. The media line 7 in the direction of the gas-liquid separator 8 can therefore be blocked independently of the release of the gas line 11.

[0048] A fourth electrolysis plant 1 according to the invention is shown in Figure 4. This has all the components of the electrolysis plant 1 of Figure 2 as well as further

[0049] Components. The other components include an inert gas container 13, which is connected to the cathode 4 of the stack 2 via an inert gas line 14 with an integrated shut-off valve 15. By opening the shut-off valve 15, inert gas flows from the inert gas container 13 via the inert gas line 14 into the cathode 4 and displaces the hydrogen present there. With the help of the

[0050] The cathode 4 can be purged with inert gas so that no hydrogen is present and hydrogen permeation is stopped. The hydrogen displaced from the cathode 4 can be discharged via the cathode outlet 10, the media line 7 and the gas line 11. For this purpose, the valve 9, which is designed as a directional control valve, must be set so that the

[0051] Media line 7 in the direction of the gas-liquid separator 8 is blocked and the gas line 11 is released.

Claims

Claims 1. A method for operating an electrolysis plant (1), comprising a stack (2) with an anode (3) and a cathode (4), wherein, during normal operation of the electrolysis plant (1), water is supplied to the anode (3) via a water circuit (5) with an integrated pump (6), which water is split into hydrogen and oxygen in the stack (2) by electrolysis, and wherein the hydrogen produced by electrolysis is supplied to a gas-liquid separator (8) via a cathode outlet (10) of the stack (2) and a media line (7) connected thereto, characterized in that when the electrolysis plant (1) is switched off, the current density is reduced to 0 A / cm 2 reduced and the media line (7) is blocked by means of a valve (9), while water continues to be supplied to the anode (3) via the water circuit (5) with the aid of the pump (6).

2. Method according to claim 1, characterized in that when the electrolysis plant (1) is switched off, the water flow in the direction of the anode (3) is reduced, preferably reduced to a minimum water flow required to transport the anode-side hydrogen away, which minimum water flow is further preferably estimated.

3. Method according to claim 1 or 2, characterized in that when the electrolysis plant (1) is switched off, the cathode-side pressure in the stack (2) is actively reduced by releasing hydrogen from the cathode (4), preferably reduced to ambient pressure.

4. Method according to one of the preceding claims, characterized in that when the electrolysis plant (1) is switched off, the pump (6) is operated intermittently.

5. Method according to one of the preceding claims, characterized in that when the electrolysis plant (1) is switched off, the cathode (4) is flushed with an inert gas, preferably with nitrogen.

6. Electrolysis plant (1), comprising a stack (2) with an anode (3) and a cathode (4), a water circuit (5) with an integrated pump (6) for supplying the anode (3) with water, a media line (7) via which a cathode outlet (10) of the stack (2) is connected to a gas-liquid separator (8), characterized in that a valve (9) for blocking the media line (7) is integrated into the cathode outlet (10) or into the media line (7).

7. Electrolysis plant (1) according to claim 6, characterized in that the valve (9) integrated into the media line (7) is designed as a directional valve, in particular as a 3-way valve, or is connected downstream of a directional valve (9'), in particular a 3-way valve, wherein a gas line (11) is connected to the media line (7) via the directional valve (9, 9').

8. Electrolysis plant (1) according to claim 6 or 7, characterized in that a pressure sensor (12) is integrated into the cathode outlet (10) or into the media line (7).

9. Electrolysis plant (1) according to one of claims 6 to 8, characterized in that an inert gas container (13) is connected to the cathode (4) of the stack (2) via an inert gas line (14) with an integrated shut-off valve (15), wherein the volume of the inert gas container (13) preferably corresponds to five to ten times the volume of the cathode (4). 10 Electrolysis plant (1) according to one of claims 6 to 9, characterized in that the electrolysis plant (1) comprises a control device (16) which is designed to carry out steps of a method according to one of claims 1 to 5.

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