Electrolysis plant with a pressure electrolyzer and method for operating such an electrolysis plant

Description

[0001] The invention relates to an electrolysis plant comprising a pressure electrolyzer for producing hydrogen and oxygen as product gases, with a plurality of electrolysis cells, each having two half-cells separated by an ion-permeable membrane, thus forming an anode compartment and a cathode compartment. The invention further relates to a method for operating an electrolysis plant.

[0002] Hydrogen is currently produced, for example, using proton exchange membrane (PEM) electrolysis or alkaline electrolysis. The electrolyzers use electrical energy to produce hydrogen and oxygen from the supplied water.

[0003] An electrolyzer typically comprises a large number of electrolysis cells arranged adjacent to one another. Water is split into hydrogen and oxygen in these cells via electrolysis. In a PEM electrolyzer, distilled water is typically supplied as a reactant at the anode and split into hydrogen and oxygen across a proton-exchange membrane (PEM). The water is oxidized to oxygen at the anode. The protons pass through the proton-exchange membrane. Hydrogen is produced at the cathode. The water is usually pumped from one side into the anode and / or cathode compartment. Alkaline electrolysis also utilizes a membrane, designed as a semipermeable membrane or diaphragm, which selectively allows the passage of specific ions.The electrolyte is potassium hydroxide solution (KOH) with a typical concentration of 20-40%. The gas-tight membrane, the so-called diaphragm, allows the transport of OH⁻ ions but simultaneously prevents the mixing of the resulting product gases.

[0004] In terms of plant technology, the electrolysis process takes place in the so-called electrolysis stack, which is composed of several electrolysis cells. Water is introduced as the reactant into the electrolysis stack, which is under DC voltage. After passing through the electrolysis cells, two fluid streams emerge, consisting of water and gas bubbles (oxygen O₂ and hydrogen H₂, respectively). Subsequently, gas separation is necessary, i.e., a phase separation of water and the respective gaseous product gas in the phase mixture. It is common practice to connect several electrolysis cells and, furthermore, several electrolysis units via piping, and to feed the exiting gas-water mixture to a central gas separator.

[0005] Despite the comparatively high gas tightness of the ion-permeable membrane, for example, the ionomer of the proton-conducting membrane in PEM electrolysis, permeation of oxygen from the anode to the cathode and of hydrogen from the cathode to the anode occurs during operation. This is due, firstly, to the fact that complete gas impermeability of the ionomer cannot be achieved. Secondly, the membrane itself absorbs water through direct contact with it. The foreign gases produced by permeation cause undesirable side reactions that reduce the efficiency of the water electrolysis and can potentially damage the membrane.

[0006] In practice, therefore, small amounts of hydrogen are present in the oxygen gas stream and small amounts of oxygen in the hydrogen gas stream. The quantity of each foreign gas depends on the electrolysis cell design and also varies under the influence of current density, catalyst composition, aging, and the membrane material of the electrolysis cell. It is inherent to the system that the gas stream containing one product gas always contains very small amounts of the other. In subsequent process steps, even trace amounts of oxygen are typically removed from the hydrogen in gas purification steps, sometimes involving very complex and costly processes, especially when a particularly high product gas quality is required, as is the case, for example, when using the hydrogen in fuel cells.In some cases, it may be necessary to reduce the concentration of foreign gases directly at or after the electrolysis cell or electrolysis stack, e.g. in gas separators or gas separators downstream of the electrolyzer.

[0007] This problem is particularly relevant at high system pressures in electrolysis plants with pressurized electrolyzers, and is exacerbated during their transient operating phases, such as when starting up or ramping up the unpressurized system of a pressurized electrolyzer, during partial load operation, or generally during load changes, and is further amplified by aged membranes. This leads to operational limitations or even prevents continued operation and results in premature safety shutdowns. Consequently, especially with positive load gradients, such as during the electrolysis plant's startup or when switching from partial to full load operation, critical conditions arise for the membrane due to differential pressures across the cell separation between the anode and cathode compartments and the associated pressure gradients.

[0008] On the other hand, pressure electrolysis is of particular interest for large-scale industrial applications, and a development trend toward higher operating pressures is therefore clearly evident. For example, the water electrolyzer is the main component in so-called power-to-gas plants. In this context, one of the most important operating parameters is the electrolyzer's operating pressure. Pressure operation is justified, firstly, by the requirements of the application being served. Secondly, an increase in pressure is essential for efficient hydrogen storage due to its small specific volume. This becomes clear when considering available PEM or alkaline electrolyzers, as well as the demonstration projects carried out in recent years in the context of power-to-gas.A further advantage is the reduced water absorption capacity of the gas with increasing pressure, which leads to less effort required for gas drying. Therefore, extensive development activities are underway regarding electrolysis plants with pressure electrolyzers, and there is a need for operating concepts for pressure electrolyzers on an industrial scale.

[0009] In publication JP 2010121146 A (disclosing an electrolysis plant according to the preamble of claim 1), an electrolysis plant and a method for operating an electrolysis plant are described. An operational safety concept is proposed, which includes the installation of shut-off valves in the water circulation, hydrogen extraction, and oxygen extraction lines, as well as real-time monitoring and control mechanisms. This allows anomalies such as pressure and gas concentration deviations to be detected, and measures to be taken to prevent the mixing and backflow of hydrogen and oxygen.

[0010] The invention is therefore based on the objective of providing an electrolysis system with a pressure electrolyzer that enables improved operation with regard to safety and system efficiency.

[0011] The object of the invention is achieved by an electrolysis plant according to claim 1, comprising inter alia a pressure electrolyzer for producing hydrogen and oxygen as product gases at a nominal pressure, with a plurality of electrolysis cells, each having two half-cells separated by an ion-permeable membrane, so that an anode compartment and a cathode compartment are formed, wherein an oxygen product line is connected to the anode compartment on the anode side and a hydrogen product line is connected to the cathode compartment on the cathode side, wherein the hydrogen product line opens into a first gas separator and the oxygen product line into a second gas separator, wherein a first pressurized gas storage tank for hydrogen and a second pressurized gas storage tank for oxygen are connected to the pressure electrolyzer on the cathode side.so that pressurized gas can be drawn from each of the compressed gas storage tanks and supplied to the pressure electrolyzer on both the cathode and anode sides, wherein the first compressed gas storage tank is connected to the pressure electrolyzer via a first extraction line and the second compressed gas storage tank via a second extraction line, wherein a controllable regulating valve is connected in one extraction line so that a respective pre-charge pressure can be set on both the cathode and anode sides, and wherein a differential pressure regulating device with a differential pressure sensor is provided, which is connected at a respective receiving point of the first extraction line and the second extraction line, so that the differential pressure of the cathode-side pre-charge pressure and the anode-side pre-charge pressure can be determined and regulated, so that a predetermined respective pre-charge pressure can be set on both the cathode and anode sides.

[0012] The problem is further solved according to the invention by a method for operating an electrolysis plant according to claim 8, in which, inter alia, in a start-up phase, the pressure electrolyzer is pre-charged with pressurized gas to a pre-pressure, wherein hydrogen is drawn from the first pressurized gas storage and supplied to the pressure electrolyzer on the cathode side, and wherein oxygen is drawn from the second pressurized gas storage and supplied to the pressure electrolyzer on the anode side, wherein a predetermined respective pre-pressure is set on the cathode side and on the anode side.

[0013] The advantages and preferred designs listed below with regard to the electrolysis plant can be applied analogously to the method for operating an electrolysis plant.

[0014] The invention is based on the observation that in electrolysis plants with pressurized electrolyzers for water electrolysis, the pressurized electrolyzers are periodically depressurized after a certain period or before extended periods of inactivity, such as for service or repair work. When the depressurized electrolysis plant is restarted, the production of hydrogen and oxygen gases is used to build up pressure to the system pressure or nominal pressure. It has been found that this start-up phase can be very dynamic in pressurized electrolyzers. Therefore, the partial load range is traversed as quickly as possible during the start-up phase, as this is characterized by transient operating conditions with high concentrations of extraneous gases due to gas permeation across the membrane. These extraneous gas concentrations can reach safety-critical levels during the commissioning and start-up of the pressurized electrolyzer.The concentration of hydrogen as an extraneous gas in the oxygen-laden anode compartment is particularly critical and must be monitored due to hydrogen permeation from the cathode compartment across the ion-conducting membrane into the anode compartment. The extraneous gas concentration decreases with increasing load. Therefore, the pressurized electrolyzer is started up with a very steep positive load gradient, with a pressure increase of up to 10% of the nominal pressure per second. This brings the pressurized electrolyzer in the electrolysis plant up to operating capacity and nominal output. Maximum gas production is thus achieved in the electrolysis plant within approximately 10 seconds.

[0015] Furthermore, in a pressure electrolyzer, the pressure build-up lags significantly behind the increase in gas flow rate during the start-up phase. This results in very strong unsteady flow conditions within the electrolysis cell as well as in the connected peripherals, such as the gas separators and product lines. These unsteady flow conditions impose strong differential pressures and pressure gradients across the electrolysis cell, the corresponding half-cells, and thus the membrane. This is particularly noticeable during the first few seconds, when the rapidly increasing gas volume must accelerate an upstream column of process water. During this process, the gas is temporarily compressed locally until it finally breaks through the water column. This process repeats periodically and is the cause of severe alternating pressure loads across the electrolysis cell and the ion-selective membrane.This situation is further exacerbated by the fact that the gas flow rate on the cathode side (i.e., on the hydrogen side) is twice as high as on the anode side (i.e., on the oxygen side). The ion-selective membrane of the electrolysis cell, and especially one that has already aged due to operational wear, can suffer irreversible damage, leading to electrolysis failure.

[0016] With progressive aging, and also due to defects in the ion-conducting membrane in individual cells, this membrane can lose its essential property of differential pressure resistance. If this occurs, the electrolysis system must be overhauled, which is costly and time-consuming, even though it could still be used for steady-state operation. Current safety concepts with simple differential pressure limiting using individual pressure sensors are unable to resolve the highly dynamic processes during start-up. The system is hydraulically decoupled by the water columns between the electrolysis cells and the gas volumes in the gas separators. This is particularly important for future electrolysis plants with pressure electrolyzers requiring nominal pressures of at least 30 bar and well above.

[0017] The invention proposes integrating separate pressurized gas storage tanks for hydrogen and oxygen into an electrolysis system with a pressure electrolyzer. The oxygen pressurized gas storage tank is connected to the anode side, and the hydrogen pressurized gas storage tank to the cathode side. These pressurized gas storage tanks can be used to pressurize the pressure electrolyzer, particularly during the start-up phase, by filling the cathode side with hydrogen and the anode side with oxygen. The respective pre-charge pressures on the anode and cathode sides can be flexibly adjusted as needed. This design provides a particularly advantageous configuration of the electrolysis system, as the pressure electrolyzer is already pre-pressurized to a working pressure close to its nominal pressure before the actual electrolysis begins, i.e., before the electrolysis cells are energized with electrolytic current.This pre-pressurization procedure using gas from the compressed gas storage tanks is gentle on the membrane and avoids the highly dynamic effects of steep load gradients and differential pressures across the membrane. Pre-pressurization can be achieved with a defined pressure ramp over time to a predetermined pre-charge pressure for hydrogen and oxygen. The compressed gas storage tanks are accordingly filled with hydrogen and oxygen and kept under pressure. Storage pressures of up to 200 bar are possible, provided sufficient storage volumes are available for commissioning. Advantageously, the compressed gas storage tanks can be controlled with regard to the storage and withdrawal of compressed gas of the desired quantity and pressure level.

[0018] This novel plant concept of the invention makes it possible to separate the operation of the electrolysis plant into a start-up phase with a pre-pressurization procedure and a subsequent load phase with the actual electrolysis operation and the energizing of the pressure electrolyzer. In the start-up phase, the anode and cathode compartments are, for example, brought to a pre-pressurization pressure close to the nominal operating pressure and prepared for the actual electrolysis operation at a nominal pressure. This establishes operational readiness for energizing and electrolysis operation.

[0019] Thus, the electrolysis system of the invention makes it possible and intended for the first time to pre-pressurize the electrolysis system with a defined pressure on the hydrogen side and / or the oxygen side. This means that the corresponding pressurized gas – hydrogen or oxygen – is introduced into the pressure electrolyzer from a pressurized gas storage tank. Only then does the electrical load take effect. Due to the pre-pressurization, the resulting gas flow rate is only a fraction of that of previous system concepts, in which the product gases were generated at low pressure during the load phase. Furthermore, the dynamic stresses on the membrane are significantly reduced or minimized. The system concept and process control of a pressure electrolysis system are largely technology-independent and adaptable, meaning they can be advantageously used for various types of electrolysis, such as alkaline water electrolysis or PEM water electrolysis.

[0020] In the electrolysis system according to the invention, the first pressurized gas storage tank is connected to the pressure electrolyzer via a first extraction line, and the second pressurized gas storage tank is connected via a second extraction line. This achieves a separation of the gas spaces and piping systems for the hydrogen and oxygen supply at the desired pre-pressure. Furthermore, the local determination and optimization of the respective connection points for the first and second extraction lines on the pressure electrolyzer is possible and can be selected based on technical considerations in order to appropriately supply the anode and cathode spaces with the pressurized gas and pre-pressurize them to the required pressure.

[0021] In addition, in the inventive embodiment of the electrolysis system, a controllable regulating valve is connected in a withdrawal line, so that a respective pre-charge pressure can be set on the cathode and anode sides. During the start-up phase, this enables precise metering and monitoring of the external gas supply of the respective pressurized gas – hydrogen or oxygen – from the corresponding external pressurized gas storage into the pressure electrolyzer. Differential pressures across the ion-permeable membrane can thus be reduced or avoided, or limited to a permissible value.

[0022] Furthermore, in the electrolysis system according to the invention, the first extraction line is connected to the first gas separator and the second extraction line is connected to the second gas separator. Connection to the respective gas separator on both the anode and cathode sides is advantageous, with the extraction lines then connecting to and opening into the gas space of the respective gas separator. The gas space is understood to be the space or volume above the liquid phase in the gas separators resulting from the phase separation during operation of the pressure electrolyzer.

[0023] However, it is also possible, and preferred, for the first extraction line in the electrolysis system to be connected to the cathode compartment and the second extraction line to the anode compartment. Connecting these lines to the electrolysis stack, which consists of stacks of electrolysis cells, allows for direct and local pressurization of the anodic and cathodic half-cells, either within the cell or in the vicinity. This enables even more precise adjustment and monitoring of the local pressure values ​​across the ion-conducting membrane, particularly during the start-up phase when pre-pressurizing with the respective H₂ or O₂ gas to prepare the electrolysis process. Combinations of extraction line connections to the gas separators and / or to the anode or cathode compartment are also possible.

[0024] Furthermore, in the embodiment of the electrolysis system according to the invention, a differential pressure regulating device with a differential pressure sensor is provided, wherein the differential pressure sensor is connected to a respective receiving point of the first extraction line and the second extraction line, so that the differential pressure of the cathode-side pre-pressure and the anode-side pre-pressure can be determined and controlled.

[0025] This type of arrangement and coupling of the differential pressure sensor allows – depending on the chosen connection topology – the differential pressure across the connected gas chambers or volumes to be monitored directly or indirectly, be it the differential pressure between the anode and cathode chambers or the differential pressure between the first and second gas separators. The differential pressure control device is advantageously usable not only during the start-up phase when pressurizing with compressed gas from the gas storage tanks, but also during the subsequent normal electrolysis operation when the electrolysis cells are energized and operating at a high nominal pressure.

[0026] This design enables highly accurate in-situ differential pressure determination, even at high system pressures, directly within the pressure electrolyzer. As an in-situ status indicator, the electrolysis system incorporates direct differential pressure measurement via selected and representative measuring points. For this purpose, a differential pressure control unit is provided, which includes a differential pressure sensor configured to measure a differential pressure, for example, between the anode and cathode compartments. This value can then be processed by the differential pressure control unit. Thus, the differential pressure sensor is connected via two selected measuring points—one representative of the anode compartment and the other of the cathode compartment—and its differential pressure measurement signal is processed by the differential pressure control unit.Advantageously, the differential pressure sensor is connected to the first and second sampling lines, which simplifies implementation. Direct differential pressure measurement enables highly accurate and virtually instantaneous condition monitoring, thus ensuring reliable operation of the electrolysis plant. To determine and evaluate the differential pressure, the differential pressure control unit can also include a differential pressure sensor that measures the differential pressure directly between the half-cells. This measurement can be performed at different points, depending on the requirements.

[0027] Preferably, the differential pressure control device in the electrolysis plant is designed for differential pressure limitation, with a maximum value for the differential pressure being set.

[0028] This achieves a particularly simple yet effective differential pressure limitation between the anode and cathode compartments, thus limiting the differential pressure load across the diaphragm according to a set maximum differential pressure value, while simultaneously monitoring gas flow. Generally, the lowest possible differential pressure is preferable during operation.

[0029] A particularly advantageous feature of the differential pressure limiter is the safe operation of the electrolysis system, as the current differential pressure can be reliably monitored and controlled relative to the permissible maximum differential pressure. The maximum permissible differential pressure can also be preset and adjusted within the differential pressure control device.

[0030] Preferably, a pressure sensor is arranged at the first gas separator and at the second gas separator in the electrolysis plant.

[0031] Additionally, each gas separator is connected to a pressure measuring device with a pressure sensor to determine an absolute pressure value. This allows for the determination of not only the differential pressure but also the absolute pressure in the gas phase of the first gas separator and the absolute pressure in the gas phase of the second gas separator. Furthermore, the differential pressure between the gas separators can be determined directly and accurately via the differential pressure sensor. The measurement signals for the differential pressure and the determined absolute pressure values ​​in the gas separators can be processed in the differential pressure control unit. With absolute pressure measurement, the inlet pressure during the start-up phase and the system pressure during steady-state operation, i.e., the nominal pressure, can be determined by measuring the pressure in the first and second gas separators.The values ​​can also be advantageously read into and processed by the differential pressure control device, or this functionality can be integrated into a higher-level control unit of the electrolysis plant, which then includes the differential pressure control device.

[0032] In a preferred embodiment of the electrolysis plant, a first reactant line is connected to the cathode compartment on the cathode side and a second reactant line is connected to the anode compartment on the anode side.

[0033] This allows, for example, the implementation of two reactant circuits in an electrolysis system using a PEM pressure electrolyzer. Thus, water circulates not only through the anode compartment but also through the cathode compartment. However, it is also possible for the PEM pressure electrolyzer to be implemented with only one anode-side circuit. In this case, water, particularly demineralized water, can be used as the reactant for PEM electrolysis, and it can be supplied via the two reactant lines on the anode and cathode sides.

[0034] Preferably, the ion-permeable membrane in the electrolysis system is designed as a proton-permeable membrane, so that PEM electrolysis can be carried out.

[0035] In an alternative preferred embodiment of the electrolysis system, the ion-permeable membrane of the pressure electrolyzer is designed as a diaphragm, which selectively allows the passage of hydroxide ions, thus enabling alkaline electrolysis. In this alkaline electrolysis configuration, an electrolyte, for example potassium hydroxide with a concentration of 20%–40%, can be supplied to the electrolysis cell via the reactant lines. In principle, the electrolysis system can also incorporate combinations of pressure electrolyzers of different technologies, based on either alkaline or PEM electrolysis.

[0036] The invention is therefore advantageously applicable to various types of pressure electrolyzers in electrolysis plants, such as alkaline water electrolysis as well as PEM electrolysis, whereby the pressurized gas storage and the differential pressure control device with differential pressure sensors ensure safe commissioning and adjustment of a pre-charge pressure during the start-up phase. This minimizes and monitors, in particular, the risk of unwanted gas transfer from one side of the electrolysis half-cell to the other due to hydrodynamic effects and excessively high differential pressures.

[0037] Preferably, the ion-permeable membrane in the electrolysis system is designed as a proton-permeable membrane, enabling PEM electrolysis. In the case of PEM electrolysis, the proton-permeable membrane is preferably based on a gas- and liquid-tight fluoropolymer.

[0038] In an acidic or proton exchange membrane (PEM) electrolyzer, distilled or demineralized water is split into hydrogen and oxygen by an electric current. It consists of a proton-permeable polymer membrane (PEM). This membrane is coated on the cathode side with a porous electrode made of platinum supported on carbon and on the anode side with metallic or oxide noble metals (usually iridium and ruthenium). An external voltage is applied to these electrodes. Typically, water is supplied as the reactant on the anode side of the electrolyzer. However, both half-cells can be flooded with water, or only the cathode side, depending on the application. The catalytic effect of the noble metal electrode leads to the decomposition of the water on the anode side: oxygen, free electrons, and positively charged H+ ions are produced.The hydrogen ions diffuse through the proton-conducting membrane to the cathode side, where they combine with the electrons to form hydrogen as the product gas.

[0039] In a further preferred embodiment of the electrolysis plant, the ion-permeable membrane is designed as a diaphragm, which selectively allows the passage of hydroxide ions, so that alkaline electrolysis can be carried out.

[0040] In an alkaline electrolyzer, hydrogen is produced at the cathode and oxygen at the anode when a direct current voltage of at least 1.5 volts is applied. The electrolyte is potassium hydroxide solution (KOH) with a typical concentration of 20%–40%. A gas-tight membrane, called a diaphragm, is used as the ion-permeable membrane. This allows the transport of OH⁻ ions but prevents the mixing of the resulting product gases. Dimensionally stable anodes (DSAs), usually titanium electrodes with a ruthenium oxide coating, are used as electrodes. These are expanded metals coated with a noble metal catalyst oxide—for example, ruthenium or iridium oxide. However, systems with Raney nickel catalysts in a gas diffusion electrode also exist. Alkaline electrolyzers are used on a large scale worldwide.

[0041] Thus, in a method for operating such an electrolysis plant with a pressurized electrolyzer, it is provided that in a start-up phase the pressurized electrolyzer is pre-pressurized with pressurized gas to a predetermined pressure, whereby hydrogen is drawn from the first pressurized gas storage tank and fed to the pressurized electrolyzer on the cathode side, thereby establishing a pre-pressurized pressure on the cathode side. Simultaneously, oxygen is drawn from the second pressurized gas storage tank and fed to the pressurized electrolyzer on the anode side, thereby establishing a predetermined pre-pressurized pressure on the anode side. In this way, the pressurized electrolyzer in the electrolysis plant is initially pre-pressurized to the predetermined pressure for pressurized operation during the start-up phase. In this start-up phase, electrolysis is preferably not yet initiated, i.e., no electrolysis current flows during the start-up phase.

[0042] Preferably, in the method a differential pressure Δp is measured between the anode-side pre-pressure p 2 and the cathode-side pre-pressure p 1, wherein the measurement signal is read into the differential pressure control device and compared with a reference value.

[0043] The reference value can be a safety-relevant maximum permissible differential pressure or a setpoint for a predefined differential pressure. It is also possible to store multiple reference values ​​in the differential pressure control unit, adapted to different operating modes, particularly during commissioning. The differential pressure measurement signal is processed in the differential pressure control unit, and, depending on the stored control algorithm, a control intervention is performed as needed. This might be the case, for example, to ensure the intended pressure increase during the start-up phase with a time-based pressure ramp of the pre-charge pressures on both the anode and cathode sides.

[0044] In this process, hydrogen from the first compressed gas storage tank is preferentially supplied to the first gas separator, and oxygen from the second compressed gas storage tank is supplied to the second gas separator. Pressurizing the gas chambers of the gas separators is particularly easy in order to increase the system pressure during the start-up phase.

[0045] In principle, it is also possible to supply the electrolysis stack to the half-cells of the anode compartment and the cathode compartment as an alternative or additional measure to the gas supply to the gas separators.

[0046] In a particularly preferred embodiment of the process, the cathode-side pre-pressure of the hydrogen is set equal to the anode-side pre-pressure of the oxygen.

[0047] This operating mode during pressurization in the start-up phase and, if applicable, in normal operation would correspond to a differential pressure of zero or near zero across the ion-selective membrane. This results in a particularly material-friendly operating mode.

[0048] It is also preferably possible that the cathode-side pressure of hydrogen is set higher than the anode-side pressure of oxygen.

[0049] For example, pressure differences of the inlet pressures of a few tens of mbar up to about 500 mbar can be advantageous, depending on the specific plant design and the hydrostatic conditions in the tank structure and the location of the corresponding water-carrying systems and components.

[0050] In this process, the electrolysis current is preferably switched on only after the respective pre-pressure has been reached in the pressure electrolyzer, whereby hydrogen and oxygen are produced as product gases, and pressure electrolysis is carried out at a nominal pressure.

[0051] The electrolysis current is thus advantageously switched on only after the start-up phase and the pressurization to the predetermined anode-side and cathode-side pre-pressure. The nominal pressure during the subsequent pressure electrolysis is typically greater than 30 bar and can reach 100 bar or more. Nominal pressures of 35 bar to 80 bar are typically preferred.

[0052] Preferably, the pressure electrolysis is carried out at a nominal pressure of at least 30 bar.

[0053] The pre-charge pressure, which is set on both the anode and cathode sides, can range from 1% to 99% of the nominal pressure. For energy efficiency reasons and to ensure a stable transition to electrolysis after the start-up phase, the pre-charge pressure can be almost equal to the nominal pressure or only slightly lower; for example, the pressure can be set between 90% and 99% of the nominal pressure.

[0054] Exemplary embodiments of the invention are explained in more detail with reference to a drawing. This drawing schematically and in a highly simplified manner shows the FIG an electrolysis system with a pressure electrolyzer according to the invention.

[0055] The single figure shows a highly simplified section of an electrolysis plant 1, including its parts and components. The electrolysis plant 1 includes a pressure electrolyzer 3, which can be configured as either a PEM electrolyzer or an alkaline electrolyzer and is designed for a high nominal pressure pN of at least 25 bar as the operating pressure.

[0056] The electrolyzer 3 comprises a cathode compartment 9 and an anode compartment 7, separated by an ion-permeable membrane 5. The anode compartment 9 and the cathode compartment 7 are each composed of a plurality of anodic and cathodic half-cells, respectively, stacked axially (not shown in detail in the figure). The cathodic and anodic half-cells are combined to form electrolysis cells and are separated by an ion-conducting membrane 5. The figure thus shows a vertically oriented pressure electrolyzer 3, designed for the electrochemical splitting of water (H₂O) or an electrolyte as a reactant into hydrogen (H₂) and oxygen (O₂) as product gases by means of an electric current. In the case of acid electrolysis, demineralized water (H₂O) is used as the reactant.In alkaline electrolysis, an alkali is used, for example potassium hydroxide (KOH) in an aqueous solution with a concentration typically of 20% to 40%. Several such electrolysis cells can be connected in series in horizontally stacked electrolysis stacks.

[0057] In the embodiment shown in the FIG, the pressure electrolyzer 3 in the electrolysis system 1 is designed as a PEM electrolyzer for high nominal pressures pN. Each electrolysis cell has a proton-permeable membrane 5 based on a fluoropolymer, to which an electrode – an anode and a cathode – is connected on both sides. During operation, an external DC voltage is applied to these electrodes. On the cathode side, a first reactant line 21A is provided for supplying water (H2O) to the cathode chamber 9. On the anode side, a second reactant line 21B is connected for supplying water (H2O) to the anode chamber 7. Thus, two water circuits are realized in the pressure electrolyzer 3, which is designed as a PEM electrolyzer. Thus, water H₂O circulates not only through the anode compartment 7, but also through the cathode compartment 5. However, it is also possible that the pressure electrolyzer 3 is implemented with only one anode-side circuit.

[0058] During operation of electrolysis plant 1, the oxygen O₂ produced in the anode compartment 7 of the electrolysis cell is discharged via an oxygen product line 11B. On the cathode side, a hydrogen product line 11A is provided for the discharge of the hydrogen H₂ produced from a cathode compartment 5.

[0059] For the phase separation of the respective product gas and water mixture, a first gas separator 13A is connected downstream of the hydrogen product line 11A. Similarly, a second gas separator 13B is connected downstream of the oxygen product line 11B. Thus, during operation, phase separation is achieved such that the gaseous phase is present in the upper section of the gas separators 11A and 11B, while the liquid phase is present at the bottom of the gas separators 11A and 11B, i.e., at a certain water level. The product gases hydrogen (H₂) and oxygen (O₂), separated from the water in this way, are discharged via a hydrogen product line 11A and an oxygen product line 11B, respectively, leading from the corresponding gas separator 11A and 11B, and further processed in subsequent processes. For example, the hydrogen (H₂) is purified in a gas cleaning system and compressed for further applications.To regulate the withdrawal of the product gases hydrogen H 2 and oxygen O 2, a valve 23 is installed in each of the outgoing product lines 11A, 11B.

[0060] A first separate compressed gas storage tank 25A, filled with hydrogen (H₂) under high pressure during operation, is connected on the cathode side to the first gas separator 13A via a first withdrawal line 27A. Similarly, a second separate compressed gas storage tank 25B, filled with oxygen under high pressure during operation, is provided and is connected on the cathode side to the second gas separator 13B via a second withdrawal line 27B. Each of the withdrawal lines 27A and 27B is equipped with a controllable regulating valve 29, allowing the flow rate (volume or mass flow) and pressure level to be adjusted when gas is withdrawn from the compressed gas storage tanks 25A and 25B.

[0061] The electrolysis plant 1 has a differential pressure control device 15, which includes a differential pressure sensor 17. A differential pressure sensor 17 is thus connected across the gas chambers, taking measurements at a respective sampling point 19 on the first sampling line 27A and on the second sampling line 27B. Additionally, a pressure measuring device 31 is connected to the gas separators 13A and 13B for determining an absolute pressure value, so that, in addition to the differential pressure Δp with respect to the gases in the sampling lines 27A and 27B, an absolute pressure pA in the gas phase of the first gas separator 13A and an absolute pressure pB in the gas phase of the second gas separator 13B can also be determined. Indirectly, the differential pressure between the anode chamber 7 and the cathode chamber 5 can also be determined.It may also be provided that a further differential pressure sensor 17 - not shown in the FIG - is installed, which taps off via the anode chamber 7 and the cathode chamber 5, so that a value for the differential pressure Δp between the cathode chamber 9 and the anode chamber 7 can also be determined directly.

[0062] The measurement signals for the differential pressure and the pressure values ​​pA and pB in the gas separators 13A and 13B are processed in the differential pressure control unit 15. A maximum differential pressure Δpmax is stored as an input value for the differential pressure control unit, which can be read in or set. This value for the maximum differential pressure Δpmax can be adjusted as needed to the respective selected or typical operating conditions of the pressure electrolyzer 3 and the aging state of the ion-permeable membrane 5. In addition to the nominal pressure pN of the pressure electrolyzer 3, other adjustable setpoints include the preset pre-pressure p1 for the hydrogen H2 from the pressurized gas storage tank 25A and the pre-pressure p2 for the oxygen O2 from the pressurized gas storage tank 25B. The differential pressure control unit 15 is configured to output control signals S1 and S2, which can be transmitted to a higher-level control system of the electrolysis plant 1 (not shown in detail).This allows the physical operating parameters of the electrolysis system 1, such as the electrolysis current, the electrolysis current density, the reactant volume flows, the system pressure or the differential pressure Δp < Δp max, as well as the respective volume flows via the extraction lines 27A and 27B, to be set and adjusted. The latter is done via a control intervention at the control valve 29.

[0063] This results in a system concept that, during the start-up phase of electrolysis plant 1, initially supplies hydrogen (H₂) to the cathode side of pressure electrolyzer 3 at a pre-charge pressure p₁ and oxygen (O₂) to the cathode side at a pre-charge pressure p₂. Thus, during the start-up phase, pressure electrolyzer 3 is initially pressurized to a pre-charge pressure, with a predetermined pre-charge pressure p₁ and p₂ being set on the cathode and anode sides, respectively. No electrolysis takes place during this start-up phase; that is, the electrolysis cells are not energized with electrolysis current, and therefore no product gases are yet formed. Depending on the design of the electrolysis cells and the materials used, an operating mode in which the cathode-side pre-charge pressure p₁ is selected and set higher than the anode-side pre-charge pressure p₂ may be preferred.The pre-charge pressures p1 and p2 are set lower than the nominal operating pressure pN of the pressure electrolyzer 3 during electrolysis. The values ​​for the pre-charge pressures p1 and p2 are flexibly adjustable to the specific requirements and can be set between 1% and 99%. For energy efficiency and hydrodynamic reasons, setting high pre-charge pressures p1 and p2 close to the nominal pressure pN of the pressure electrolyzer 3 is preferable, i.e., a pre-charge pressure greater than 90% of the nominal pressure pN. Then, when the electrolysis cells are energized after the start-up phase and pressure build-up, the gas flow rate of the generated hydrogen (H2) and oxygen (O2) is significantly lower due to the already high pressure level than it would be without the pre-charge pressure from the pressurized gases introduced into the pressure electrolyzer 3.In normal operating mode after the start-up phase, nominal pressures pN of greater than 30 bar up to 200 bar are possible in the pressure electrolyzer 3, typically between 35 bar and 80 bar. The pressure sensors 31 allow the gas phase pressure in the gas separators 13A, 13B to be measured and processed, particularly during the start-up phase. A target / actual comparison of the measured value is performed with the specified cathode-side pre-pressure p1 and the anode-side pre-pressure p2. In this way, the set pressure ramp during the pressurization of the pressure electrolyzer 3 with hydrogen H2 and oxygen O2 from the pressurized gas storage tanks 25A, 25B can be controlled, especially during the start-up phase.Furthermore, during the start-up phase, the absolute pressure values ​​pA and pB in the gas phases of the first gas separator 13A and the second gas separator 13B can be determined via a pressure measuring device 31 installed in each separator and compared to the pre-pressures p1 and p2 according to the selected pressure ramp. In the pressure electrolysis operation following the start-up phase, this configuration also serves for safety-related monitoring and control, and, if necessary, correction and adjustment of the pressure states across the ion-selective membrane 5.

[0064] The differential pressure control device 15 can also be designed and integrated as part of the higher-level control or monitoring system of the electrolysis plant 1 (not shown in detail). The control system is thus designed to control the operation of the electrolysis stack in the pressure electrolyzer 3. With the help of the control system, for example, a predefinable absolute pressure pa can be set as a setpoint in the anode chamber 7 and a predefinable absolute pressure pk as a setpoint in the cathode chamber 9. It is also possible, for example, to operate the system in which the anode-side pressure pa is set higher than the pressure pk in the cathode chamber 9, if required. Corresponding pressure sensors are then connected to or installed in the anode chamber 7 and the cathode chamber 9 to determine the absolute pressure pk in the cathode chamber 9 and the pressure pa in the anode chamber 7.In this way, the potential negative consequences of membrane damage during the operation of a pressure electrolyzer 3 are minimized, since in the event of a breakthrough of the ion-permeable membrane 5, less hydrogen migrates through the membrane from the cathode compartment 9 to the anode compartment 7. This achieves an emergency running capability for the pressure electrolysis process until a scheduled overhaul and maintenance can be carried out.

[0065] After pre-pressurizing the pressure electrolyzer 3 with the desired pre-pressure p1, p2 during the start-up phase, the electrolysis process is initiated. During operation of the electrolysis plant 1, reactant water H2O is supplied to the pressure electrolyzer 3 via reactant lines 21A, 21B, and hydrogen H2 and oxygen O2 are produced as product gases. Using the differential pressure sensor 17, a differential pressure Δp between the first gas separator 13A and the second gas separator 13B can now be measured via the sampling points 19 on the sampling lines 27A, 27B, even during electrolysis. The measurement signal is read into the differential pressure control unit 15 and compared with the reference value Δpmax. If the differential pressure Δp is less than the maximum permissible reference value Δpmax, pressure electrolysis continues.

[0066] If the differential pressure Δp exceeds the reference value Δp max, the differential pressure control device 15 initiates a shutdown operating mode. This can also be done in the higher-level control and monitoring unit by transmitting corresponding control signals S1 and S2 from the differential pressure control device 15 to a higher-level control unit.

[0067] Particularly due to the more precise differential pressure measurement, pressure electrolyzers 3 with a high system pressure of at least 30 bar can be reliably and precisely started up and brought into electrolysis operation at nominal pressure pN. When restarting or starting up the depressurized system, the electrolysis is not activated and used for gas production and pressure build-up to the nominal pressure pN. Instead, the pressure electrolyzer 3 is kept de-energized with respect to electrolysis and initially pre-pressurized to a pre-pressure p1, p2.

Claims

1. An electrolysis plant (1) comprising a pressure electrolyser (3) for generating hydrogen (H2) and oxygen (O2) as product gases at a nominal pressure (PN) with a plurality of electrolysis cells, each of which has two half-cells which are separated by an ion-permeable membrane (5), such that an anode chamber (7) and a cathode chamber (9) are formed, wherein an oxygen product line (11B) is connected to the anode chamber (7) on the anode side, and a hydrogen product line (11A) is connected to the cathode chamber (9) on the cathode side, wherein the hydrogen product line (11A) opens into a first gas separator (13A) and the oxygen product line (11B) opens into a second gas separator (13B), characterised in that: a first compressed-gas reservoir (25A) for hydrogen (H2) is connected on the cathode side and a second compressed-gas reservoir (25B) for oxygen (O2) is connected on the anode side to the pressure electrolyser (3), such that pressure-loaded gas is able to be discharged from each of the compressed-gas reservoirs (25A, 25B) and is able to be supplied to the pressure electrolyser (3) on the cathode side and on the anode side, wherein the first discharge line (27A) is connected to the first gas separator (13A), and the second discharge line (27B) is connected to the second gas separator (13B), and wherein the first compressed-gas reservoir (25A) is connected via a first discharge line (27A) and the second compressed-gas reservoir (25B) is connected via a second discharge line (27B) to the pressure electrolyser (3), wherein a controllable regulating valve (29) is connected in one discharge line (27A, 27B), such that a respective inlet pressure (p1, p2) is adjustable on the cathode side and the anode side, and wherein a differential pressure regulator (15) comprising a differential pressure sensor (17) is provided, which is connected to the first discharge line (27A) and the second discharge line (27B) at a respective sensor location (19), such that the differential pressure (Δp) of the inlet pressure (p1) on the cathode side and the inlet pressure (p2) on the anode side is determinable and regulatable such that a respective specified inlet pressure (p1, p2) is adjustable on the cathode and anode sides.

2. The electrolysis plant (1) according to claim 1, wherein the first discharge line (27A) is connected to the cathode chamber (9), and the second discharge line (27B) is connected to the anode chamber (7).

3. The electrolysis plant (1) according to claim 1 or 2, wherein the differential pressure regulator (15) is configured to limit the differential pressure, wherein a maximum value for the differential pressure (Δpmax) is set.

4. The electrolysis plant (1) according to any one of the preceding claims, wherein a pressure sensor (31) is arranged on the first gas separator (13A) and the second gas separator (13B).

5. The electrolysis plant (1) according to any one of the preceding claims, wherein a first reactant line (21A) is connected to the cathode chamber (9) on the cathode side, and a second reactant line (21B) is connected to the anode chamber (7) on the anode side.

6. The electrolysis plant (1) according to any one of the preceding claims, wherein the ion-permeable membrane (5) is designed as a proton-permeable membrane such that PEM electrolysis is feasible.

7. The electrolysis plant (1) according to any one of the preceding claims, wherein the ion-permeable membrane (5) is designed as a diaphragm selectively allowing hydroxide ions to pass, such that alkaline electrolysis is feasible.

8. A method for operating an electrolysis plant (1) according to any one of the preceding claims, wherein in a starting phase, the pressure electrolyser (3) is preloaded with pressure-loaded gas to an inlet pressure, wherein hydrogen (H2) is discharged from the first compressed-gas reservoir (25A) and supplied to the pressure electrolyser (3) on the cathode side, and wherein oxygen (O2) is discharged from the second compressed-gas reservoir (25B) and supplied to the pressure electrolyser (3) on the anode side, wherein a respective specified inlet pressure (p1, p2) is set on the cathode side and on the anode side.

9. The method according to claim 8, wherein a differential pressure (Δp) between the inlet pressure (p2) on the anode side and the inlet pressure (p1) on the cathode side is measured, wherein the measurement signal is read into the differential pressure regulator (15) and compared to a reference value (Δpmax).

10. The method according to claim 8 or 9, wherein hydrogen (H2) from the first compressed-gas reservoir (25A) is supplied to the first gas separator (13A), and oxygen (O2) from the second compressed-gas reservoir (25B) is supplied to the second gas separator (13B).

11. The method according to claim 8, 9, or 10, wherein the inlet pressure (p1) of the hydrogen (H2) on the cathode side is set to be equal to the inlet pressure (p2) of the oxygen (O2) on the anode side.

12. The method according to claim 8, 9, or 10, wherein the inlet pressure (p1) of the hydrogen (H2) on the cathode side is set to be greater than the inlet pressure (p2) of the oxygen (O2) on the anode side.

13. The method according to any one of claims 8 to 12, wherein the electrolysis current is switched on in the pressure electrolyser (3) after the respective inlet pressure (p1, p2) has been reached, wherein hydrogen (H2) and oxygen (O2) are generated as product gases, wherein a pressure electrolysis is performed at a nominal pressure (pN).

14. The method according to claim 13, wherein a pressure electrolysis is performed at a nominal pressure (pN) of at least 30 bar.