Electrolysis system, in particular for atmospheric water electrolysis

Abstract

The invention relates to an electrolysis system (1) with a plurality of electrolysis installations (3, 3A, 3B, 3C), which each have an electrolyser with a respective product gas line (5, 5A, 5B, 5C) by means of which a product gas can be conveyed out of the electrolysis installation (3, 3A, 3B, 3C), and with a compressor installation (7) comprising a pre-compressor device (9) with a plurality of pre-compressors (11, 11A, 11B), a post-compressor station (13) and a central collection line (15), wherein the product gas lines (5, 5A, 5B, 5C) are connected at the input side to a pre-compressor (11, 11A, 11B) and the pre-compressors (11, 11A, 11B) are connected at the output side to the central collection line (15), and wherein the post-compressor station (13) is connected to the central collection line (15).

Description

[0001] Description

[0002] Electrolysis system, especially for atmospheric water electrolysis

[0003] The invention relates to an electrolysis system, in particular for atmospheric water electrolysis, with a plurality of electrolysis plants, each having an electrolyzer.

[0004] In electrolysis, a chemical reaction, in particular a redox reaction, is initiated by means of an electric current, whereby electrical energy is converted into chemical energy. The electrical energy required for this is supplied by a direct voltage source which has two electrical poles, an anode and a cathode. A direct electrical current is passed through two electrodes, the anode and the cathode, into a conductive electrolyte, for example a conductive liquid. In proton exchange electrolysis (PEM electrolysis), the conduction takes place at least partially through a proton exchange membrane which is permeable to positively charged protons.At the electrodes, reaction products of the electrochemical reaction are formed through electrolysis—that is, by conducting the direct current from the direct voltage source, for example, using protons, from one electrode to the other—from the substances or elements contained in the electrolyte or liquid. In addition to PEM electrolysis systems, which operate in acidic environments, alkaline electrolyzers are also known and frequently used.

[0005] The DC voltage source causes a deficiency of electrons in the electrode connected to the positive pole, the anode, and an excess of electrons in the electrode connected to the negative pole, the cathode. At the cathode, electroneutral or positively charged substances absorb electrons and are thereby reduced. At the anode, electrons are released into the electrode or anode, where the substance or substances are oxidized.

[0006] A common form of electrolysis is water electrolysis, by means of which water is split into hydrogen and oxygen, whereby a simple reaction equation can be 2H2O -> 2H2+O2. This can be achieved using proton exchange electrolysis (PEM electrolysis), in which water is electrochemically split in an electrolysis cell with a solid polymer electrolyte. The polymer electrolyte, or more precisely a polymer electrolyte membrane or proton exchange membrane, is responsible for the conduction of protons and hydrogen ions, the separation of product gases, and the electrical insulation of the electrodes. The product gases produced in low-pressure PEM electrolysis are molecular oxygen O2 and molecular hydrogen H2 from water H2O.Due to its solid structure, the polymer electrolyte membrane exhibits a low gas transfer rate, resulting in very high product gas purity. Maintaining high gas purity is important for storage safety and the direct use of the product gas, for example, in a fuel cell.

[0007] Water electrolysis using PEM electrolysis, for example, is a promising technology for energy storage in conjunction with renewable energy sources due to its dynamic response times and high efficiencies. For example, in so-called power-to-gas concepts, it can store excess electricity in the form of energy when there is a temporary electricity surplus that exceeds demand, such as in the production of renewable energy.

[0008] In addition to pressureless operation (atmospheric operation), the pressure operation of commercial PEM electrolyzers can be divided into equal pressure and differential pressure. Atmospheric water electrolysis produces hydrogen and oxygen at a slight overpressure and, from today's perspective, represents a preferred solution for achieving high performance flexibility while simultaneously minimizing costs. Therefore, for the use of these product gases generated by electrolysis, pressure increase by means of a compressor is required in many downstream processes or for hydrogen storage.

[0009] Therefore, a key process step is the mechanical compression of the hydrogen to a required storage pressure. The maximum storage pressure depends on the specific application of the electrolysis plant. For example, the use of hydrogen in an industrial plant in conjunction with a nearby electrolysis plant may require a pressure of between 5 and 15 bar, such as in a steelworks. Supplying a pipeline requires pressures of between 50 and 100 bar. Filling station applications require pressure levels of up to 850 bar. For the large-scale storage of hydrogen, pressure accumulators are usually used which can be charged up to a pressure of 200 bar, which can sometimes require very complex and cost-intensive investment in the construction and operation of compression plants in an electrolysis system. The further use of the hydrogen in chemical processes, such as e.g.Ammonia synthesis requires pressures between about 150 and 350 bar.

[0010] An isothermal compression process requires less compressor work than the isentropic process. The specific compressor work from atmospheric pressure to 50 bar is calculated to be around 20 kJ / mol, which is almost 50 percent higher than the work required for the isothermal process. This amount of energy corresponds to around 8 percent of the lower calorific value of hydrogen. Due to the heat transfer in real compressors, in practice an atmospheric electrolysis plant for technical applications results in compressor work that lies between the two borderline cases of isentropic and isothermal. In view of the expected and required increase in plant sizes of atmospheric electrolysis systems on an industrial scale with an output of more than 100 MW, there is therefore a growing need to design an adapted compressor plant within large-scale electrolysis systems.This is intended to be improved from a cost perspective compared to previously known solutions, while at the same time ensuring the necessary operational flexibility as required for operation on an industrial scale.

[0011] The object of the present invention is therefore to provide an electrolysis system which combines the requirements of efficient and cost-effective compression with high operational flexibility at high electrolysis capacities.

[0012] This object is achieved according to the invention by an electrolysis system with a plurality of electrolysis plants, each of which has an electrolyzer with a respective product gas line, by means of which a product gas can be discharged from the electrolysis plant, and with a compressor plant comprising a pre-compression device with a plurality of pre-compressors, a post-compressor station and a central collecting line, wherein the product gas line is connected on the inlet side to a pre-compressor and the pre-compressors are connected on the outlet side to the central collecting line, and wherein the post-compressor station is connected to the central collecting line.

[0013] The invention is based on the knowledge that, for larger plant capacities on an industrial scale, the known electrolysis systems cannot be easily scaled up further without incurring significant disadvantages due to the high compression capacities required, for cost reasons and due to systemic operating requirements.

[0014] From today's perspective, atmospheric water electrolysis represents a particularly preferred solution for achieving high performance flexibility while simultaneously minimizing costs, both from an economic and technological perspective. However, the use of these gases produced by electrolysis requires a pressure boost using a compressor in many downstream processes or for hydrogen storage. Due to the low density of hydrogen, piston or diaphragm compressors are predominantly used for this purpose. The conventional turbo compressors commonly used for compression processes would be uneconomical to manufacture and operate due to the high number of stages required, which is caused by the chemical and molecular properties of hydrogen.However, the design principle of piston compressors causes adverse pressure fluctuations on both the suction and discharge sides, which can reduce the service life of the electrolysis or lead to operational failures. The compression downstream of the electrolysis, as well as other subsequent process steps, also have dynamic behavior during transient processes that differs from that of the electrolysis and must be decoupled for stable operation. An intermediate storage facility with a fixed volume, whose operating principle for storage and discharge processes consists of pressure variation, is not available for this purpose, as this pressure variation would be too small for atmospheric applications and would result in enormous storage volumes and thus in considerable cost and space requirements.The intermediate storage usually takes place in a constant pressure piston gas storage device, whose piston weight keeps the pressure constant and the filling level describes the volume.

[0015] The invention proposes a two-stage compression concept in an atmospheric electrolysis system with multiple electrolysis plants, which comprises pre-compression in the pre-compression device and downstream central post-compression in the post-compressor station. In the compressor plant, the two subsystems are coordinated with one another and interact to achieve the most efficient and cost-effective compression possible with high operating flexibility and high availability at high electrolysis outputs of several 100 MW. The technical and functional separation of the pre-compression device and the post-compressor station enables decoupling, which promotes high flexibility in the selection and design of the respective compressors, especially from a cost perspective.Pre-compressed hydrogen from the pre-compression device can be fed into the central collecting line as product gas from the product gas line, at a specified operating pressure of 2 to 20 bar, preferably 8 bar as the outlet pressure from the pre-compression. The central collecting line is therefore designed for the supply and transport, and if necessary, for the temporary storage of large gas volumes from the majority of electrolysis plants connected via a respective product gas line, taking into account the operating pressure of the pre-compressed hydrogen from this first pressure stage of the pre-compression.

[0016] To reduce these costs of electrolysis plants, the electrolysis system proposed here, with its adapted, staged compression in the compressor system, achieves cost-effective pre-compression. Previously proposed approaches involve implementing a pressurized electrolysis process, which can provide a pre-pressure of 8 bar to 30 bar. However, since pressurized electrolysis systems are less flexible in terms of operation and have not yet been designed for systems with an electrolysis capacity exceeding 100 MW, the invention seeks to further develop proven atmospheric electrolysis for large-scale electrolysis systems and to demonstrate a technically feasible alternative to pressure electrolysis.

[0017] What is proposed in an atmospheric electrolysis system is a very advantageous combination of a pre-compression device and a preferably central post-compression station. The compressors of the pre-compression device serve as low-pressure compressors and are used to pre-compress the hydrogen from the electrolyzers to a predetermined and, if possible, constant output pressure, for example 8 bar. In large electrolysis systems, one or more electrolysis plants are each assigned a pre-compressor, which feeds into a common central collecting line. It is also possible for one electrolysis plant to be connected to two different pre-compressors, with two product gas lines then connecting from this electrolysis plant to one of the two pre-compressors.Several electrolysis plants, for example two electrolysis plants, can also be connected to one and the same pre-compressor via a respective product gas line, so that hydrogen product gas from two or more electrolyzers can flow into the pre-compressor on its suction side. This means that very simple redundancy or multiple use of the pre-compressors is achieved in the pre-compression plant, which enables continuous and at the same time flexible operation. The central collecting line can also be supplied in parallel by other electrolysis plants via the pre-compressors, so that the electrolysis system can be expanded and scalable for large electrolysis capacities. Due to the large number of pre-compressors in the pre-compression plant, the simplest and most cost-effective compressor solutions are selected for the first compression stage.

[0018] For the next process step, the central collection line is fluidically connected to the booster compressor station via a connecting line, where the pre-compressed hydrogen is recompressed to a required final pressure, which depends on the specific hydrogen utilization. In most cases, pressures of 30 bar or more are required.

[0019] Preferably, the pre-compressors are designed for approximately atmospheric inlet pressure and the compressor output of the pre-compressors is limited to an outlet pressure of maximum 10 bar.

[0020] Outlet pressures between 2 bar and 9 bar are advantageous; 8 bar in particular has proven particularly useful in the design of large electrolysis systems, for example for pre-compression and as the feed pressure into the central collecting line. This is also the inlet pressure on the suction side of the booster compressor station, so that a more favorable pressure ratio is set for the central booster compression of the large hydrogen volume flows from the collecting line, thus enabling further efficiency gains and an associated cost reduction. This operating window enables particularly efficient and low-loss operation on the hydrogen side for atmospheric electrolysis, and a precise outlet pressure can be set and maintained.

[0021] Further preferably, the pre-compressors are located in close proximity to the electrolysis plant, so that the route of the respective product gas line is minimized. In contrast, the post-compressor station can be located centrally in close proximity to the pre-compressors or, optionally, at a more distant location that makes sense for the respective application.

[0022] This can reduce the pressure loss. Since compressors operate on the basis of pressure ratios, the pressure loss in the hydrogen line to the compressor is crucial in electrolysis plants with low operating pressure in order to prevent the energy required for compression from increasing disproportionately. The pre-compressors are therefore preferably located in the immediate vicinity of the electrolysis plants, in particular the electrolyzers. A further advantage of the pre-compression concept is the high availability of the compressors selected for pre-compression and the associated option of being able to do without a complex redundant design - for example two pre-compressors per electrolysis plant - in the lower pressure range. A multiple redundant design would therefore only be necessary for the downstream booster compressor station at most.

[0023] In a particularly preferred embodiment of the electrolysis system, the compressors in the pre-compression device are designed as screw compressors.

[0024] Compared to piston compressors, screw compressors are significantly simpler in design, significantly more cost-effective, and can compress large quantities of hydrogen. A certain disadvantage is the occurrence of design-related internal leaks, which can lead to a significant drop in efficiency and additional energy consumption as pressure increases. This is particularly significant for hydrogen compression. Therefore, limiting the compression capacity of pre-compressors designed as screw compressors to an outlet pressure of less than 10 bar is preferable and should be adjusted accordingly. This can be achieved by appropriately designing the screw compressor.

[0025] Screw compressors are characterized by their high level of resistance and thus operational reliability to fluctuating operating conditions. Their design-related internal compression without free inertia forces makes them flexible long-distance runners with minimal maintenance requirements and comparatively low operating costs in full and part load operation. The operating principle is based on a main and a secondary rotor which, through continuous rotation, enclose gas portions from the so-called suction side and expel them against a smaller outlet geometry on the so-called pressure side. While the main rotor generally takes on the drive function, the task of the secondary rotor is to form chambers and seal them. Basically, a distinction can be made between oil-free and oil-flooded screw compressor types.Both are used in single- or multi-stage concepts in hydrogen applications; however, in different, design-dependent operating ranges, concerning circumferential speed of the rotors, pressure difference and delivery volumes).

[0026] In a further preferred embodiment of the electrolysis system, the booster compressor station is designed as a central booster compressor station and is designed for a final pressure of at least 30 bar. Preferably, the booster compressor station is designed for a final pressure of at least 20 bar, in particular at least 30 bar.

[0027] A central booster station has the advantage that a single supply line connects to the booster station from the central manifold at a central connection point, and at the same time, a large mass flow of hydrogen gas pre-compressed to the initial pressure can be transferred through this supply line. From a plant engineering perspective, this has the advantage that, due to the pressure in the supply line, the hydrogen density increases compared to the product gas line for pre-compression, allowing a smaller pipeline diameter to be selected, which consequently leads to lower investment and production costs.In addition to reducing investment costs, the combination of pre- and post-compression allows for more robust operation of the electrolysis system, as a higher pressure loss is permissible between pre- and post-compression, which in a particularly advantageous design also allows for a central post-compression station at a greater distance from the electrolysis system.

[0028] In a particularly preferred embodiment of the electrolysis system, the secondary compressor station is designed as a piston compressor station with a number of piston compressors.

[0029] The combination of a screw compressor in the pre-compression device and a downstream central piston compressor station has proven particularly advantageous here. The screw compressors serve as low-pressure compressors and are used for pre-compression of the hydrogen. In large electrolysis systems, one or more electrolysis plants are each assigned a screw compressor which feeds into the common central collecting line, which can also be supplied in parallel by other electrolysis plants with screw compressors. In the subsequent process step, the post-compression in the second compression stage, the central collecting line is connected to a central piston compressor station where the post-compression takes place to a predetermined final pressure that depends on the specific hydrogen usage.In most applications, transfer pressures of 30 bar are required for the hydrogen in electrolysis systems and are therefore particularly advantageous for further transport and industrial or energetic use.

[0030] Compared to piston compressors, screw compressors are simpler in design, significantly more cost-effective, and can compress large quantities of hydrogen. A certain disadvantage of screw compressors is the occurrence of structurally inherent internal leaks, which, as pressure increases, can lead to a significant drop in efficiency and additional energy consumption. Therefore, limiting the compression power in the pre-compressor to less than 10 bar, as proposed here, is advantageous in order to avoid leaks or keep them as low as possible.

[0031] Since compressors operate based on pressure ratios, the pressure loss in the hydrogen line to the compressor is a crucial factor in electrolysis systems with low operating pressure, preventing a disproportionate increase in the energy required for compression. The screw compressor is therefore preferably located in close proximity to the electrolysis units.

[0032] A further advantage of the screw compressor is its high availability and the associated option of being able to do without a redundant design in the lower pressure range. A redundant design would therefore preferably only be provided for the downstream piston compressor station, by keeping an appropriate number of piston compressors on hand as redundancy. By pre-compressing the hydrogen product gas from the pre-compression device to, for example, 6 bar outlet pressure, the performance-limiting pistons in the lower pressure range on the piston compressor can even be omitted. This not only allows for a considerable reduction in size but also enables an increase in the performance of the individual compressors. This means that the number of centrally arranged piston compressors can be reduced and greater flexibility achieved.

[0033] The invention advantageously utilizes the knowledge that, due to the material properties of hydrogen, piston or diaphragm compressors are predominantly used for this purpose, since the conventional turbo compressors usually used for compression processes would be uneconomical to manufacture due to the high number of stages required. In the case of piston compressors, on the other hand, the first compression stages, which compress the hydrogen from an atmospheric pressure level of approximately 1 bar to, for example, 7-8 bar, are particularly influential in terms of size and cost. The higher the inlet pressure selected for the compressor, the lower the energy consumption of the compressor and the lower the investment costs of the piston compressor.

[0034] Thus, the number of piston compressors is essentially defined only by the redundancy requirements described above. This can now be implemented centrally as an N+1 unit rather than as 2 N units thanks to the central piston compressor station.

[0035] In a particularly preferred embodiment of the electrolysis system, a compressed gas reservoir is connected to the central collecting line. This means that the compressed gas reservoir can be loaded with pre-compressed hydrogen as product gas at the working pressure and at the same time acts as a buffer storage. This pressurised gas reservoir advantageously serves to better compensate for fluctuations in performance, so that hydrogen can be fed from the compressed gas reservoir into the central collecting line as required, or can also be fed from the compressed gas reservoir into the central collecting line in a well-dosed manner. This promotes particularly stable operation of the two-stage compression concept and precise pressure maintenance at the specified output pressure, for example a target pressure of around 8 bar, which is also the specified inlet pressure on the suction side of the booster compressor station. The compressed gas reservoir can be used to maintain the pressure orPressure reduction can be equipped with a throttle, i.e. a pressure reducer, or if necessary with a bidirectional control valve, so that either storage or discharge can be achieved.

[0036] Preferably, the electrolysis system has a gas return line which branches off from the booster compressor station on the pressure side and is connected to the compressed gas storage unit.

[0037] This makes it possible to charge the compressed gas storage tank with additional highly compressed hydrogen from the second compression stage by extracting compressed hydrogen at approximately 30 bar as needed and feeding it to the compressed gas storage tank via the gas return line. This also allows charging of the gas pressure storage tank at a pressure greater than the pressure in the central manifold, advantageously between approximately 8 bar and 30 bar.

[0038] To set a desired loading pressure, a pressure-reducing device is installed in the return line, which can preferably be designed as a gas expansion turbine. This provides a particularly advantageous option for adjusting the pressure in the compressed gas storage tank to the specified loading pressure. The final pressure of the two-stage compressed hydrogen, for example, 30 bar after the booster compressor station, can thus be reduced to the desired storage pressure during recirculation.

[0039] The design of the pressure reduction device as a gas expansion turbine is particularly advantageous in the present case, since it still allows energetic use of the hydrogen gas expanding in the expander, for example by driving an electric generator coupled to the turbine shaft, or by exploiting the temperature reduction of the hydrogen gas, for example by thermal coupling to a heat exchanger or a cooling device in the electrolysis system.

[0040] An expander, also called a turbo expander, gas expansion turbine or expansion turbine, belongs to the family of turbines in which a pressurised gas expands and does work in the process. In contrast to a gas turbine in the broader sense, an expander only consists of the actual turbine and does not have a compressor or combustion chamber as an integral part of the machine. The gas to be expanded is therefore not generated by the machine itself; in this case, compression is carried out by the post-compressor station, i.e. it arises from the upstream compression process. Unlike pressure reduction by a simple throttle, the expander also makes advantageous use of the energy in the gas. The expansion turbine can be single-stage or multi-stage, axial or radial. The dissipated mechanical work can be used to drive a generator, a pump or a compressor, as described above.Alternatively, the expansion can also be implemented in a gas piston engine. In a further preferred embodiment, the pre-compression device in the electrolysis system has a bypass line into which a gas buffer is connected, wherein the bypass line is connected to the product gas line.

[0041] The installation of an intermediate gas storage unit as a low-pressure gas storage unit in a bypass line to the product gas line of an electrolysis plant is particularly advantageous due to the possibility of loading and operating the intermediate gas storage unit at a constant pressure. To maintain the pressure, operation at constant pressure should be provided. In this case, the intermediate gas storage unit can advantageously effectively regulate even the smallest and short-term pressure changes in the range of seconds to the setpoint pressure. Any pressure changes, in particular short-term pressure fluctuations, therefore no longer have an impact on the downstream compression. The intermediate gas storage unit can be designed as a central intermediate gas storage unit, i.e. lines branch off from the respective product gas lines of several electrolysis plants, so that a common bypass line is formed in which a central intermediate gas storage unit is arranged.

[0042] The preferred method proposed here is the use of a gas buffer under constant pressure operation for low-pressure gas storage of the produced hydrogen, the operating principle of which is preferably the change in volume rather than the change in pressure. For this purpose, the storage volume to be stored or buffered is changed at constant pressure and can thus decouple the dynamic behavior of the processes and in particular the interactions between electrolysis and compression and avoid pressure fluctuations particularly efficiently. Buffering is advantageously achieved by a geometric change in the volume of the storage space of the gas buffer, whereby the operating pressure in the system remains almost constant.The design principle of the gas buffer allows the geometric volume of the tank to be changed at a nearly constant pressure using a membrane and an actuator, making it particularly advantageous as a buffer between the electrolysis unit and the downstream compressor. Furthermore, the respective control mechanisms of both units enable the mass flows of the electrolysis unit and the compressor to be adjusted, thus achieving stable operation.

[0043] Depending on the requirements, a particularly simpler implementation could also be a low-pressure gas buffer, a diaphragm accumulator that does not require constant pressure regulation. The storage container of the gas buffer is lined with a diaphragm or bladder that can expand toward the container wall when pressure slowly increases. The diaphragm acts merely as a passive damping element, dampening any pressure fluctuations that occur. However, it does not keep the pressure constant at a setpoint or return it to a setpoint after the damping or pressure change has occurred.

[0044] In a particularly preferred embodiment, the gas intermediate storage device is designed as a constant-pressure piston storage device, so that the pressure after the electrolysis and before the compression can be set to a constant inlet pressure and can be maintained at the inlet pressure.

[0045] Preferably, a cooling device for cooling the product gas is provided, which is connected downstream of the pre-compressors.

[0046] This cooling device is used for the intermediate cooling of the product gas compressed in the pre-compressors, in particular the hydrogen produced in the electrolyzer and subsequently compressed. The pre-compression leads to a rise in the temperature of the hydrogen, so that intermediate cooling is very advantageous. An appropriately designed cooling device can be arranged in each of the lines leading off the pressure side of a pre-compressor. However, it is also possible and advantageous to install a central cooling device designed for larger volume flows immediately upstream of the post-compressor station, in the line that branches off from the central collecting line to the post-compressor station.

[0047] Cooling can preferably be achieved by adding water and / or hydrogen as a coolant to initiate and maintain heat exchange in the cooling device, i.e., the coolant absorbs heat from the compressed and correspondingly heated hydrogen via a heat exchanger surface. This allows for particularly cost-effective and technically simple cooling, since both water and hydrogen are available as coolants in the electrolysis system.

[0048] The water storage capacity of hydrogen H2 decreases with increasing pressure, so that for direct cooling, i.e., injection cooling, liquid water would have to be injected into the compressed product gas, which, however, would have to be removed again, which is a complex process. After compression, the dryest possible hydrogen H2 is desired, which is why cooling by means of a heat exchanger is preferred.

[0049] According to an advantageous embodiment, during operation, during cooling of the product gas in the cooling device, at least a portion of the water vapor contained in the hydrogen condenses, and the condensate can be fed to the electrolyzer via a line and used after cleaning. Utilization of the released condensation heat is also possible.

[0050] The downstream cooling device therefore not only serves to condition the gas temperature of the hydrogen for the subsequent second compression stage in the booster compressor station or the following process steps, but is also designed to condense a portion of the water vapor contained in the gas. This condensate can be reused by feeding it to the electrolysis plant, e.g., to reduce the electrolysis water requirement. A return line with purification can be provided for this purpose. Preferably, a cooling device for cooling the product gas is connected downstream of the booster compressor station on the pressure side of the electrolysis system.

[0051] Thus, two-stage cooling is implemented in the electrolysis system, with intermediate cooling after the pre-compressors for gas conditioning and further cooling after the post-compression stage, and if required, further intermediate cooling in intermediate stages of the pre- and post-compression stages. For this purpose, a cooling device is installed in the pressure-side line of the post-compressor station, which cools the hydrogen gas compressed to the final pressure, for example, 30 bar. During the cooling of the hydrogen, at least a portion of the water vapor in the gas condenses, and the condensate is preferably fed to the electrolyzer after cleaning via a return line.

[0052] In a preferred embodiment of the electrolysis system, the cooling device, in particular the cooling device designed as a heat exchanger, is therefore designed for the addition of water and / or hydrogen as a coolant via a cooling line.

[0053] This design is applicable to a cooling device both for intermediate cooling downstream of the pre-compressors and, particularly preferably, for use after the post-compression in the cooling device downstream of the post-compressor station. Thus, the design with two-stage compression is also advantageously accompanied by two- or multi-stage cooling.

[0054] Preferably, a control device can be provided to which the pre-compressors are connected, so that the pre-compressors can be regulated to an inlet-side electrolysis pressure. The pressure setpoint at the inlet of the pre-compressors is set in the atmospheric pressure range, i.e. at a setpoint of 1 bar, or in particular between 0.9 bar and 1.5 bar. Preferably, as an alternative or in addition to the control device, an intermediate gas storage device designed as a constant-pressure reservoir is provided, which is connected in the product gas line and arranged upstream of the pre-compression device, i.e. on its suction side. This is a particularly simple and robust way of keeping the inlet pressure for the pre-compression device constant and of avoiding adverse reactions between the electrolysis and the pre-compressor. The pre-compressors can thus be subjected to a constant pressure on the inlet side.

[0055] In the case of a control device, the quantity control device is preferably designed in such a way that speed and bypass control can be carried out.

[0056] In a further preferred embodiment, the control device is designed as a two-stage control device, which has a leading power control as the first control stage and a stabilizing pressure control as the second control stage, thus achieving a controlled interaction between electrolysis and compression. A two-stage constant pressure control enables particularly stable operation of the electrolysis system in conjunction with the two-stage compression.

[0057] Further advantages, features and details of the invention will become apparent from the following description of the preferred embodiments and from the drawings. The features and combinations of features mentioned above in the description as well as the features and combinations of features mentioned below in the description of the figures and / or shown alone in the single figures can be used not only in the respective combination specified, but also in other combinations or on their own, without departing from the scope of the invention. Examples of embodiments of the invention are explained in more detail with reference to a drawing. These show schematically and in a highly simplified manner:

[0058] FIG 1 shows a schematic representation of an electrolysis system, comprising an electrolysis plant, a gas buffer and a compressor plant;

[0059] FIG 2 shows an electrolysis system with a compressor system, comprising a pre-compression device and a post-compression station;

[0060] FIG 3 an electrolysis system according to FIG 2 with further technical features.

[0061] The same reference symbols have the same meaning in the figures.

[0062] 1 shows a schematic representation of an electrolysis system 1 according to the invention. The electrolysis system 1 has at least one electrolysis plant 3 with at least one electrolyzer (not shown in detail in FIG. 1), a gas buffer 27 and a compressor plant 7, which are fluidically connected via a product gas line 5. A channel 39 and an outlet channel 41 are formed by the product gas line 5. The gas buffer 27 is fluidically connected between the electrolysis plant 3 and the compressor plant 7 via the channel 39 and the outlet channel 41, so that the product gas hydrogen H2 from the electrolysis can be fed to the compressor plant 7 via the gas buffer 27. The compressor plant 7 is connected downstream in the flow direction of the hydrogen H2 produced to the gas buffer 27 via the outlet channel 41.The compressor system 7 is designed in two stages and has a pre-compression device 9 and a post-compression station 13 which are coordinated with one another and configured accordingly. This is described in detail below with reference to FIG. 2. For pressure monitoring, a pressure monitoring device 43 is connected in FIG. 1 in the gas-carrying product gas line 5 between the electrolysis system 3 and the gas intermediate storage device 27. Furthermore, a pressure monitoring device 43 is connected in the gas-carrying product gas line 5 between the gas intermediate storage device 27 and the compressor system 7. The electrolysis system 3 can be designed with an electrolyzer for PEM electrolysis, for low-pressure proton exchange electrolysis, or be based on alkaline electrolysis. Atmospheric electrolysis produces hydrogen and oxygen at low excess pressures.

[0063] The electrolysis system 1 is characterized by a two-stage control device 37, which has a first control stage 37A and a second control stage 37B. The first control stage 37A forms the leading power control with a power setpoint L. The power setpoint L characterizes the electrolysis power in the electrolysis system 1 as the leading variable for the operation of the electrolysis system 3 or the electrolysis power of the electrolyzer. The electrolysis current intensity I is a suitable physical operating measurement variable and parameter for the first control stage 37A. Alternative setpoints are given, for example, by the current density in the electrolyzer or the hydrogen product mass flow of the electrolysis.

[0064] A pressure control which stabilizes the electrolysis operation is provided as the second control stage 37B, so that a controlled interaction of the electrolysis process and the downstream compression is brought about in the two-stage compressor system 7. In the second control stage 37B, the working pressure is predetermined as the pressure setpoint P, as it is to be set and maintained in the gas intermediate storage device 27 or in the channel 39 and the outlet channel 41. Typically, for low-pressure proton exchange electrolysis, pressure setpoints P of significantly less than 10 bar are desired, in particular between 1.0 and 1.5 bar are preferred, i.e. a slight overpressure compared to atmospheric pressure. For low-pressure proton exchange electrolysis, a pressure setpoint of approximately 1.1 bar is particularly advantageous. With the pressure control of the second control stage 37B, a predetermined inlet pressure which is kept as constant as possible can be set at the compressor system 7.The pre-compression device 9 is connected to the control device 37 and is designed accordingly for the pressure setpoint P as the inlet pressure and can be controlled accordingly. The plurality of pre-compressors 11, 11A, 11B incorporated in the pre-compression device 9 - see FIG. 2 and corresponding embodiments - are therefore connected to the control device 37. These can each be controlled to the inlet-side electrolysis pressure with the predetermined pressure setpoint P, whereby a speed and bypass control is implemented and used for the pre-compressors 11, 11A, 11B designed as screw compressors.During operation of the electrolysis system 1, a reliable constant pressure control in the control concept is additionally supported within certain limits by the gas intermediate storage 27, since the large storage volume and its change enable the compensation of small and smallest pressure fluctuations in the range of seconds in a fluctuation range of 10 mbar up to several 100 mbar around the pressure setpoint of, for example, P = 1.1 bar.

[0065] During operation of the electrolysis system 1 during electrolysis in the electrolysis plant 3, hydrogen generated as product gas by means of a gas extraction unit 45 is introduced into the storage container of the gas intermediate storage 27 via the channel 39 of the product gas line 5, and the hydrogen introduced into the storage container is brought to a predetermined setpoint pressure P by means of a pressure control device and maintained at the setpoint pressure P. The hydrogen with the setpoint pressure P is then fed to the compressor plant 7, specifically to the pre-compression device 9. In the pre-compression device 9, the hydrogen is pre-compressed from the inlet pressure at the setpoint pressure P to a predetermined and constant outlet pressure between 2 bar and 9 bar, for example 8 bar.This output pressure is also kept as constant as possible, which is achieved by precise inlet pressure control in the control device 37 and a constant compression ratio of the pre-compression device 9. The hydrogen is finally compressed in the compressor system 7 in a second compression stage of the compression process to the desired final pressure, for example, 30 bar or even higher if necessary, and is then available for various applications or for further transport.

[0066] In terms of control technology, the electrolysis process and the subsequent compression of hydrogen produced in the electrolysis are carried out in a combined two-stage operation. A first control stage 37A for the electrolysis forms the leading power control with a power setpoint L for the electrolysis power, which is controlled accordingly. A pressure control is implemented as the second control stage 37B with a constant pressure setpoint P as an optionally predeterminable inlet pressure for the subsequent compression of the hydrogen in the two-stage compressor system 7 with the pre-compression device 9 and the post-compression station 13. During pressure control, when control intervention is required, the pressure is regulated to the pressure setpoint P by the pressure control device by bringing about an isobaric change in the volume of the hydrogen in the gas intermediate storage 27.A target / actual comparison 47 with respect to the specified pressure setpoint P can additionally be carried out using a control state Z or the proportional storage volume of the gas buffer 27 as a measured variable, and the values ​​can be compared for any necessary control intervention on the control state Z to maintain the pressure setpoint P.

[0067] If necessary, or for safety-relevant shutdown operations, it is possible to safely blow off the hydrogen from the system via a blow-off valve 49A, for example when the electrolysis system 3 is shut down for servicing purposes. It is particularly advantageous here that the gas buffer storage unit 27 is designed and operated in such a way that the storage space is closed and brought into a rest position when the electrolysis system 3 is shut down. This closes the storage space of the gas buffer storage unit 27 for hydrogen and reduces the volume in the storage space - the hydrogen is forced out. This means that there is no longer any hydrogen product gas in the storage space and the amount of nitrogen required as purge gas for inerting is correspondingly reduced for shutdown management, for example when servicing is necessary.If necessary, additional hydrogen can be blown off from the electrolysis system 1 via the blow-off devices 49A, 49B.

[0068] FIG 2 shows an electrolysis system 1 with a compressor system 7, on the basis of which the basic concept of the invention for staged compression with the pre-compression device 9 and the post-compressor station 13 is explained in more detail below and how it can be used particularly advantageously for large electrolysis outputs of several 100 MW. FIG 2 shows an electrolysis system 1 with a plurality of electrolysis systems 3, 3A, 3B, 3C - four systems for example - which each have at least one electrolyzer, but generally several electrolyzers. The electrolysis systems 3, 3A, 3B, 3C have a nominal output of e.g. 100 MW each. The electrolysis system 1 also has a compressor system 7 which comprises a pre-compression device 9 and a central post-compressor station 13.The pre-compression device 9 comprises a plurality of pre-compressors 11, 11A, 11B - for example three pre-compressors - which are assigned to the electrolysis systems 3, 3A, 3B, 3C in a corresponding manner and are connected to these via a respective product gas line 5. The system is interconnected in such a way that each of the pre-compressors 11, 11A, 11B is connected to two of the electrolysis systems 3, 3A, 3B, 3C via a respective product gas line 5. Thus, each of the pre-compressors 11, 11A, 11B is supplied with hydrogen from the

[0069] Electrolysis can be fed via a product gas line 5 for pre-compression. The pre-compressors 11, 11A, 11B are each designed as screw compressors and are located in close proximity to the electrolysis systems 3, 3A, 3B, 3C. The screw compressors are designed for an almost atmospheric inlet pressure, and the compressor output is limited to an outlet pressure of less than 10 bar. Screw compressors as pre-compressors 11, 11A, 11B are characterized by particularly high availability and the associated option of being able to dispense with a redundant design in the pre-compression device 9. The pre-compressors 11, 11A, 11B are each connected on the output side, i.e. on their pressure side, to a central collecting line 15, so that during operation pre-compressed hydrogen from the pre-compression device 9 can be fed into the central collecting line 15 at a predetermined output pressure of, for example, 8 bar.The booster compressor station 13 is also connected to the central collecting line via a connecting line 59. Thus, pre-compressed hydrogen at the specified outlet pressure can be supplied from the central collecting line 15 to the connecting line 59 of the booster compressor station 13. The booster compressor station 13 is designed as a central booster compressor station 13 on the connecting line 59 and is designed for a final pressure of at least 30 bar. Piston compressors are used here.

[0070] By pre-compressing in the pre-compression device 9 by means of screw compressors to typically greater than 6 bar, for example 8 bar outlet pressure, the power-limiting pistons of the lower pressure range on the piston compressor of the secondary compressor station 13 can be omitted because a pre-pressure is already provided. In addition to a considerable structural downsizing, this also enables an increase in the flow rate of the individual piston compressors. This means that the number of centrally arranged piston compressors in the piston compressor station 13 can be reduced and greater flexibility achieved. The number of piston compressors is therefore essentially only defined by redundancy requirements, which is now designed as a central piston compressor station 13 and can be implemented as a unit with N+1 piston compressors and not as a unit with 2N piston compressors.For buffering and as a pressurized gas reservoir, a compressed gas reservoir 17 is connected to the central collecting line 15, which can be charged with hydrogen as required both via the collecting line 15 and on the pressure side via the booster compressor station 13. For this purpose, a gas return line 19 branches off from a pressure-side outlet line at the output of the booster compressor station 13 and connects to the compressed gas reservoir 17. A valve 51 is connected to the return line 19, and a pressure reducing device 21 is installed, so that an adapted charging pressure for the compressed gas reservoir 17 can be set, which is reduced compared to the final pressure of 30 bar of the booster compressor station 13. The pressure reducing device 21 is designed as an expansion turbine 23, which enables a precise charging pressure of the compressed gas reservoir 17.Furthermore, it is possible to use the mechanical work performed by the expansion turbine 23 during expansion for other purposes in the electrolysis system 1, for example to drive a generator. Furthermore, the cooling of the hydrogen expanding in the expansion turbine 23 can be used for energy purposes, for example as cooling power in a refrigeration unit or in a heat exchanger. If required, hydrogen can be fed to the central collecting line 15 at the predetermined outlet pressure via the compressed gas reservoir 17. Stable and uninterrupted continued operation is thus possible in the event that the hydrogen supply from the pre-compression device 9 is temporarily reduced or even interrupted by a malfunction. This maintains the pressure in the central collecting line 15 and ensures stable operating behavior of the piston compressor in the post-compression station 13.This also makes stand-by operation for the booster compressor station 13 possible if required.

[0071] The start-up behavior during commissioning of the compressor system 7 or one or more of the electrolysis systems 3, 3A, 3B, 3C in the electrolysis system 1 is also improved by the hydrogen reserve in the compressed gas storage 17. This can be particularly advantageous for the piston compressors of the booster compressor station 13, since a sufficiently large quantity of pressurized hydrogen is already stored in the compressed gas storage 17, which can be fed into the collecting line 15 under a pre-pressure and made available there as needed.

[0072] An electrolysis system 1 with additional technical features compared to FIG 2 is shown in FIG 3 . The basic system concept corresponds to the electrolysis system 1 shown in FIG 2 and FIG 3: The system concept is characterized by two-stage compression, with a plurality of pre-compressors 11, 11A, 11B designed as screw compressors, downstream of which a central post-compressor station 13 designed as a piston compressor station is fluidically connected. The two compression stages are coupled via the central collecting line 15 and the connecting line 59, which is centrally connected to the post-compressor station 13.

[0073] On the side of the electrolysis plants 3, 3A, 3B, 3C, a bypass line 25 is provided in FIG. 3, into which a gas intermediate storage device 27 is connected. The gas intermediate storage device 27 is designed as a low-pressure gas storage device for hydrogen. The bypass line 25 has a number of branch lines, one branch line being connected to a respective product gas line 5 of an electrolysis plant 3, 3A, 3B, 3C. The connection to the product gas line 5 is made in a line section before entering the corresponding assigned pre-compressor 11, 11A, 11B. A changeover valve 61 is installed in each branch line, so that bidirectional operation and use of the bypass line 25 is achieved. It is therefore possible to use the branch line to load or unload the central gas storage facility 27 with hydrogen.The changeover valve 61 can, if required, be completely closed in both flow directions, so that a branch line and the corresponding line path can then be optionally blocked. The gas intermediate storage device 27 fulfills an important buffer function with regard to pressure fluctuations in the electrolysis pressure. In order to utilize the buffer function of the gas intermediate storage device 27, the changeover valve 61 is operated in the open state in the direction of the product gas line. As already shown in FIG. 1, during operation of the electrolysis system 1, reliable constant pressure control is additionally supported in the control concept within certain limits by the gas intermediate storage device 27, since the large storage volume and its variation enable compensation of small and extremely small pressure fluctuations in the range of seconds in a fluctuation range of 10 mbar up to several 100 mbar around the pressure setpoint of, for example, P = 1.1 bar.Thus, the inlet pressure of the pre-compressors 11, 11A, 11B, which are designed as screw compressors, can be kept constant at a predetermined target pressure value.

[0074] In addition, a gas return line 57, which leads from the compressed gas storage 17, is connected to the bypass line 25. A control valve 63 is installed in the gas return line 57 so that a loading pressure can be set that is lower than the pressure in the compressed gas storage 17. This allows compressed hydrogen from the compressed gas storage 17 to be fed to the intermediate gas storage 27. In particular, when starting up the electrolysis systems 3, 3A, 3B, 3C in the electrolysis system 1 - i.e. with even reduced hydrogen production - hydrogen can thus be selectively decoupled from the intermediate gas storage 27 or from the compressed gas storage 17 in order to run the compressor system 7 at a favorable operating point and to supply it with hydrogen at a sufficient volume flow and pressure.This also makes it possible to operate an electrolysis plant 3, 3A, 3B, 3C at partial load without having to significantly reduce the compressor plant 3.

[0075] Furthermore, a cooling device 29 is connected to the connecting line 59, which connects to the central collecting line 15, and is designed as a heat exchanger with a primary side and a secondary side. This cools the hydrogen from the pre-compressors 11, 11A, 11B, which has been pre-compressed and thus already heated to a higher temperature. Cooling is preferably carried out by adding water and / or hydrogen as a cooling medium, which can be supplied via a cooling line 33. This enables cost-effective and technically simple cooling to be carried out, since both water and hydrogen are available in the electrolysis plant. A second cooling device 31 is installed in the outlet line 55 on the pressure side of the post-compressor station 13 and is connected downstream of the post-compressor station 13. Here the hydrogen can finally be pressurized to a final pressure of e.g.Hydrogen, highly compressed at 30 bar and heated to over 80 °C, is cooled again as product gas. Cooling in the cooling device 31 is achieved by adding water and / or hydrogen through a cooling line 33. The cooling medium then leaves the cooling device 31 through line 53. During the cooling of the hydrogen product stream, at least a portion of the water vapor in the gas condenses, and the condensate can be returned to the electrolyzer 3, 3A, 3B, 3C via a return line 35.

[0076] The present invention is distinguished in particular by a particularly advantageous implementation of a two-stage compression concept in an electrolysis system 1 for high outputs above 100 MW. The concept advantageously comprises a pre-compression device 9, with pre-compressors 11, 11A, 11B preferably designed as screw compressors, and with a central post-compressor station 13, which is preferably designed as a piston compressor station. The electrolysis system 1, in its particularly designed form, is thus adapted and can be used for large-scale electrolysis plants. This advantageous plant concept makes it possible to achieve cost savings both through the elimination of the low-pressure stages on the preferred piston compressor of the post-compressor station 13 and through a possible reduced redundancy of the preferred screw compressors.A further advantage is the possible increase in performance of the piston compressor due to the higher inlet pressure caused by the hydrogen already pre-compressed in the pre-compression device 9. This also means that the number of compressors can be reduced in high-performance electrolysis plants, with corresponding cost advantages for construction and operation. In addition, a large, conventional piston compressor is assembled on site on a complex foundation - both geometrically and in terms of vibration control - with considerable assembly work. Screw compressors can be brought to the construction site as a so-called skid-based solution and only require a simple foundation. If re-compression in a piston compressor begins at higher pressure, a skid-based setup can also be used here, or a few large central compressor systems can be used for very large electrolysis mass flows.The expected cost savings are considerable.

Claims

Patent claims 1. Electrolysis system (1) with a plurality of electrolysis plants (3, 3A, 3B, 3C), each having an electrolyzer with a respective product gas line (5), by means of which a product gas can be discharged from the electrolysis plant (3, 3A, 3B, 3C), and with a compressor plant (7) comprising a pre-compression device (9) with a plurality of pre-compressors (11, 11A, 11B), a post-compressor station (13) and a central collecting line (15), wherein the product gas line (5, 5A, 5B, 5C) is connected on the inlet side to a pre-compressor (11, 11A, 11B) and the pre-compressors (11, 11A, 11B) are connected on the outlet side to the central collecting line (15), and wherein the post-compressor station (13) is connected to the central collecting line (15).

2. Electrolysis system (1) according to claim 1, wherein the pre-compressors (11, 11A, 11B) are designed for approximately atmospheric inlet pressure and the compressor output of the pre-compressors (11, 11A, 11B) is limited to an outlet pressure of maximum 10 bar.

3. Electrolysis system (1) according to claim 1 or 2, wherein the pre-compressors (11, 11A, 11B) are arranged in spatial proximity to the electrolysis plant (3, 3A, 3B, 3C), wherein the line path of the respective product gas line (5) is minimized.

4. Electrolysis system (1) according to one of the preceding claims, in which the compressors (11, 11A, 11B) in the pre-compression device (9) are designed as screw compressors.

5. Electrolysis system (1) according to one of the preceding claims, in which the recompressor station (13) is designed as a central recompressor station (13) and is designed for a final pressure of at least 20 bar, in particular at least 30 bar.

6. Electrolysis system (1) according to one of the preceding claims, wherein the secondary compressor station (13) is designed as a piston compressor station with a number of piston compressors.

7. Electrolysis system (1) according to one of the preceding claims, with a compressed gas storage device (17) connected to the central collecting line (15).

8. Electrolysis system (1) according to claim 7, with a gas return line (19) branching off from the pressure side of the secondary compressor station (13) and connected to the compressed gas storage (17).

9. Electrolysis system (1) according to claim 8, with a pressure reducing device (21) built into the return line (19), which is designed in particular as a gas expansion turbine (23).

10. Electrolysis system (1) according to one of the preceding claims, in which the pre-compression device (9) has a bypass line (25) into which a gas intermediate storage (27), wherein the bypass line (25) is connected to the product gas line (5).

11. Electrolysis system (1) according to claim 10, wherein the intermediate gas storage device (27) is designed as a constant-pressure piston storage device, so that the pressure after the electrolysis and before the compression can be set to a constant inlet pressure.

12. Electrolysis system (1) according to one of the preceding claims, in which a cooling device (29) for cooling the product gas is provided, which is connected downstream of the pre-compressors (11, 11A, 11B).

13. Electrolysis system (1) according to one of the preceding claims, in which a cooling device (31) for cooling the Product gas of the booster compressor station (13) is connected downstream of the pressure side.

14. Electrolysis system (1) according to claim 12 or 13, wherein the cooling device (29, 31), in particular a heat exchanger, is designed for the addition of water and / or hydrogen as a coolant via a cooling line (33).

15. Electrolysis system (1) according to claim 12, 13 or 14, wherein the cooling device (29, 31) is designed for at least partial condensation of the water vapor present in the product gas and has a condensate return line (35) for the condensate, which opens into an electrolysis plant (3, 3A, 3B, 3C).

16. Electrolysis system (1) according to one of the preceding claims, in which a control device (37) is provided to which the pre-compressors (11, 11A, 11B) are connected, so that the pre-compressors (11, 11A, 11B) can be controlled to an inlet-side electrolysis pressure.

17. Electrolysis system (1) according to claim 16, wherein the control device (37) is designed such that a speed and bypass control can be carried out.

18. Electrolysis system (1) according to claim 16 or 17, characterized by a two-stage control device (37) which has a leading power control as the first control stage (37A) and a stabilizing pressure control as the second control stage (37B), so that a controlled interaction of electrolysis and compression is effected.

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