Method and compressor arrangement for compressing hydrogen

Abstract

The invention relates to a compressor arrangement and method for compressing hydrogen produced by an electrolyser, wherein hydrogen is compressed to an intermediate pressure (20) by a first liquid ring machine (15) and the hydrogen is compressed from the intermediate pressure (20) to an outlet pressure by a second liquid ring machine (25, 35, 41, 42, 43, 44, 45). Pressure compensation between the outlet side (21) and the inlet side (22) of the first liquid ring machine (15) is permitted via a bypass line (16). The inlet pressure (18) on the inlet side (22) of the first liquid ring machine (15) is kept constant by changing the cross-section of the bypass line (16) with an adjustable valve (17). The hydrogen is compressed from the intermediate pressure (20) to the outlet pressure without the intermediate pressure (20) being kept constant.

Description

[0001] METHOD AND COMPRESSOR ARRANGEMENT FOR COMPRESSING HYDROGEN

[0002] The invention relates to a method and a compressor arrangement for compressing hydrogen produced by an electrolyzer.

[0003] Hydrogen produced by electrolysis is usually highly compressed for storage or transport, for example to a pressure between 600 bar and 700 bar above atmospheric pressure. The invention concerns the initial compression, starting from the pressure at which the hydrogen exits the electrolyzer, down to a higher outlet pressure, which can, for example, be in the order of magnitude between 5 bar and 10 bar above atmospheric pressure. To achieve the significantly higher final pressure, further compression stages can be added, which can be designed, for example, as piston compressors.

[0004] For the proper operation of an electrolyzer, it is advantageous if the inlet pressure with which the hydrogen is transferred from the electrolyzer to the compressor arrangement is kept constant.

[0005] It is advantageous to use a liquid ring machine arrangement for the initial compression of hydrogen. Liquid ring machines are well suited to conveying the water-containing mixture released by an electrolyzer and are cheaper to manufacture than, for example, piston compressors. However, liquid ring machines have so far had poor energy efficiency, especially during partial load operation, which is the norm for electrolyzers using solar or wind energy. There are regularly longer operating phases during which the electrolyzer's output is well below its maximum output.

[0006] The invention is based on the object of presenting a method and a compressor arrangement for compressing hydrogen produced by electrolysis, which avoids these disadvantages. This object is achieved by the features of the independent patent claims. Advantageous embodiments are specified in the subclaims.

[0007] In the method according to the invention for compressing hydrogen produced with an electrolyzer, the hydrogen is compressed to an intermediate pressure using a first liquid ring machine. The hydrogen is compressed from the intermediate pressure to an outlet pressure using a second liquid ring machine. A backflow from the outlet side to the inlet side of the first liquid ring machine is permitted via a bypass line. The inlet pressure on the inlet side of the first liquid ring machine is kept constant by changing the cross-section of the bypass line using an adjustable valve.

[0008] In one embodiment, the hydrogen is compressed from the intermediate pressure to the outlet pressure without the intermediate pressure being kept constant. The inlet pressure of the first liquid ring machine is regulated by adjusting the adjustable valve in the bypass line. Since the second liquid ring machine is not equipped with a bypass control and therefore the second liquid ring machine does not react quickly to changes in the intermediate pressure, the intermediate pressure drops when the bypass valve of the first liquid ring machine is opened. The fluctuation in the intermediate pressure during operation of the compressor arrangement can be greater than 0.1 bar, preferably greater than 0.2 bar, more preferably greater than 0.5 bar.The fluctuation of the intermediate pressure is the difference between the highest and lowest values ​​of the intermediate pressure during continuous operation of the compressor arrangement consisting of the first liquid ring machine and the second liquid ring machine. The fluctuations are primarily a result of the fluctuating volume flow supplied from the electrolyzer to the compressor arrangement.

[0009] The invention departs from the conventional approach in which pressure control for the inlet side of a liquid ring machine is carried out via a bypass line when the pressure on the outlet side of the liquid ring machine is also kept constant. With two liquid ring machines connected in series, this normally means that the second liquid ring machine is also provided with a bypass line, via which a backflow occurs from the outlet side to the inlet side of the second liquid ring machine.

[0010] The invention has recognized that a backflow via the bypass line of the second liquid ring machine is a significant reason for the unfavorable energy efficiency of a series connection of liquid ring machines and that in the first liquid ring machine the possibility of pressure control via a bypass line during the compression of hydrogen can be used even when the pressure on the output side of the first liquid ring machine is subject to fluctuations.

[0011] An adjustable valve is arranged in the bypass line, with which the backflow from the outlet side to the inlet side of the first liquid ring machine can be regulated. The adjustable valve can be continuously adjustable between an open state and a closed state. The adjustable valve can be integrated into a control loop in which the switching state of the valve is adjusted depending on the pressure on the inlet side of the first liquid ring machine. The control loop can be set up so that the adjustable valve closes further as the pressure on the inlet side of the first liquid ring machine increases, and vice versa.

[0012] A buffer tank can be arranged between the outlet of the first liquid ring machine and the inlet of the second liquid ring machine. The buffer tank can dampen short-term fluctuations in the intermediate pressure. The buffer tank can, for example, have a volume between 0.2 m 3 and 5 m 3 , preferably between 0.5 m 3 and 2 m 3 The buffer tank can simultaneously be used as a liquid separator to separate liquid quantities contained in the volume flow coming from the first liquid ring machine. Separated liquid quantities can be returned to the first liquid ring machine, in particular to the liquid ring of the first liquid ring machine.

[0013] A check valve can be arranged between the outlet of the first liquid ring machine and the inlet of the second liquid ring machine in order to prevent a pressure increase in the second liquid ring machine from having a feedback effect on the intermediate pressure. The check valve can be arranged between the branch to the bypass line and the inlet of the second liquid ring machine. The check valve can be arranged between the buffer tank and the inlet of the second liquid ring machine. In particular, however, when liquid ring machines are connected in parallel in the second stage, such a check valve enables individual machines to be switched off in order to adapt the output to demand.

[0014] In one embodiment, an upper limit is specified for the intermediate pressure. An intermediate pressure that is too high is disadvantageous because the first liquid ring machine may then no longer be able to keep the pressure on the inlet side constant. This in turn can impair the electrolysis, which depends on the hydrogen being able to be transferred to the first liquid ring machine at a certain pressure. On the other hand, a liquid ring machine can then be used for the first compression stage which is optimized for lower outlet pressures but has a limited maximum outlet pressure. The second liquid ring machine can be designed and operated in such a way that the upper limit for the intermediate pressure is not exceeded.In particular, the second liquid ring machine can be operated such that the speed of the second liquid ring machine is increased in order to counteract an increase in the intermediate pressure beyond the upper limit.

[0015] One discovery according to the invention is that it can be advantageous if the intermediate pressure is not kept constant. Reducing the intermediate pressure saves energy in the first stage; however, at the same time, care must be taken to ensure that the intermediate pressure does not become too low in order to maintain a control reserve for the bypass control and to avoid cavitation in the second compressor. If the bypass line of the first liquid ring machine is opened, this very quickly leads to a reduction in the intermediate pressure. This reduction can only be compensated for with a delay by reducing the speed of the second liquid ring machine.Since the intermediate pressure must never fall below a minimum value and the liquid ring machines always require a minimum speed in order to build up a stable liquid ring - these two values ​​are also related - a constant intermediate pressure has always been used when two liquid ring compressors are connected in series.

[0016] One possibility for keeping the intermediate pressure constant would be to also provide the second liquid ring machine with a bypass line which allows a backflow from the outlet side to the inlet side of the second liquid ring machine. By coordinated control of the two bypass lines, the intermediate pressure could be kept at a predetermined value. This is not provided for in the context of the invention because the energy efficiency of the compressor arrangement is impaired by a bypass line above the second liquid ring machine. The compression of the hydrogen from the intermediate pressure to the outlet pressure can take place without allowing a backflow via a return line arranged between the outlet side and the inlet side of the second liquid ring machine.

[0017] The second liquid ring machine can be controlled in such a way that energy consumption is kept low without exceeding the upper limit for the intermediate pressure. This procedure can be limited by a requirement that the permissible operating range of the second liquid ring machine must be observed. In particular, a lower limit for the speed of the second liquid ring machine can be provided. The lower speed limit can be based on the operating requirements of the second liquid ring machine. The lower speed limit can be dimensioned such that undesirable vibration conditions in the second liquid ring machine are avoided.The functionality of the compressor arrangement as a whole is not impaired by the lower speed limit because it can be compensated by opening the valve in the bypass line of the first liquid ring machine more when the second liquid ring machine is running at a higher speed than would be necessary to maintain the intermediate pressure.

[0018] The energy efficiency can be further improved if the hydrogen is compressed from the intermediate pressure to the outlet pressure using a plurality of second liquid ring machines connected in parallel. This opens up the additional possibility of varying the number of second liquid ring machines used to compress from the intermediate pressure to the outlet pressure. If the output of some of the second liquid ring machines is sufficient to maintain the intermediate pressure within the desired range, it is often more energy efficient to keep only this part of the second liquid ring machines in operation and to switch off the other second liquid ring machines.In particular, depending on the instantaneous power of the electrolyzer, a part of the second liquid ring machines can be switched off and the volume flow coming from the electrolyzer via the first liquid ring machine can be compressed with the other part of the second liquid ring machines.

[0019] Compression from the intermediate pressure to the initial pressure can be achieved with at least two, preferably at least three, more preferably at least five second liquid ring machines connected in parallel. The number of liquid ring machines that are subject to temporary shutdown can be variable. There can be operating states in which a single liquid ring machine is shut down, and operating states in which only one liquid ring machine is still operating. All operating states in between are also possible.

[0020] The control of the second compression stage, i.e. the control of the second liquid ring machine or the plurality of second liquid ring machines, can be carried out by means of a control system (open loop) or a closed control circuit (closed loop). The control can depend on the power at which the electrolyzer is operated, the position of the adjustable valve in the bypass line of the first liquid ring machine, the intermediate pressure, the speed of the first liquid ring machine and / or other parameters of the compressor arrangement. The control can be carried out in such a way that a table is stored in a control unit responsible for the second compression stage, from which table, based on measured values ​​relating to the current operating state of the compressor arrangement and in particular the power of the electrolyzer, default values ​​for the operation of the second compression stage can be read.The control unit can be configured to send control commands to the second liquid ring machine(s) to adjust operation according to the specified values ​​in the table. The specified values ​​can be based on a state estimate of the compressor arrangement or on a simulation of a digital twin. In the case of a closed control loop, the specified values ​​can be determined directly based on measured values ​​of the operating state of the compressor arrangement.

[0021] Additionally or alternatively, predictive control or regulation is also possible, in which predictions about the future operating state of the electrolyzer are incorporated. This can take into account predictions about the amount of renewable energy expected at a future point in time. An expectation for future amounts of wind energy or solar energy can, for example, be derived from a weather forecast.

[0022] The pressure at which the electrolyzer releases the hydrogen can be slightly above atmospheric pressure, for example between 0.1 bar and 0.3 bar higher than atmospheric pressure. The intermediate pressure between the first compression stage and the second compression stage can be, for example, between 0.5 bar and 2 bar higher than atmospheric pressure. The outlet pressure on the output side of the second compression stage can be, for example, between 6 bar and 12 bar higher than atmospheric pressure. The second compression stage can be followed by one or more further compression stages, with which the hydrogen is compressed to a pressure of at least 100 bar, preferably of at least 200 bar, more preferably of at least 500 bar above atmospheric pressure.A liquid separator can be arranged between the second compression stage and a third compression stage to separate liquid quantities contained in the volume flow coming from the second liquid ring machine. Separated liquid quantities can be returned to the second liquid ring machine, in particular to the liquid ring of the second liquid ring machine.

[0023] The compressor arrangement can be designed for high hydrogen volume flows, which are transferred from the electrolyzer to the first liquid ring machine. For example, the first liquid ring machine can be designed to deliver a volume flow of at least 2000 m 3 / h, preferably a volume flow of at least 5000 m 3 / h, more preferably a volume flow of at least 10,000 m 3 / h from the electrolyzer.

[0024] The volume flow of hydrogen delivered by an electrolyzer often depends on the extent to which renewable energy is available to operate the electrolyzer. Since the amount of available renewable energy is known to be highly variable, it is advantageous if the compressor arrangement is designed to deliver highly fluctuating volume flows. The compressor arrangement can be operated in such a way that the maximum volume flow is used for less than 20%, preferably less than 10%, more preferably less than 5% of the operating time. In other phases of the operating time, a smaller volume flow can be delivered which is less than 50%, preferably less than 20%, more preferably less than 10% of the maximum volume flow.The operating time in which a small volume flow is conveyed can amount to at least 10%, preferably at least 20%, more preferably at least 50% of the total operating time.

[0025] At the second compression stage, adaptation to the fluctuating volume flows can be achieved by varying the number of second liquid ring machines in operation.

[0026] At the first compression stage, the compressor arrangement can comprise exactly one liquid ring machine. A rapid response to a changed volume flow supplied from the electrolyzer to the inlet of the first liquid ring machine can be achieved by adjusting the cross-section of the bypass line. However, it is detrimental to energy efficiency if there is a strong backflow through the bypass line when the volume flow from the electrolyzer is low.

[0027] In addition to adjusting the cross-section of the bypass line, the pressure on the inlet side of the first liquid ring machine can be kept constant by operating the first liquid ring machine at a variable speed. During operating phases in which the volume flow coming from the electrolyzer is low, the speed of the first liquid ring machine can be reduced, thus reducing energy consumption. The operating limits of the first liquid ring machine should be observed, for example, by maintaining a lower speed limit below which stable operation of the first liquid ring machine can no longer be guaranteed.

[0028] If we assume a first liquid ring machine that can handle volume flows up to 10,000 m 3 / h, the compressor arrangement can operate electrolyzers with a capacity of up to approximately 50 MW. For electrolyzers with higher capacities, several compressor arrangements can be operated in parallel. This opens up the possibility of varying the number of compressor arrangements used to compress the hydrogen coming from the electrolyzer, which can further contribute to increasing energy efficiency.

[0029] The invention encompasses a compression system in which several compressor assemblies of the type described are connected to an electrolyzer. The compression system can be operated such that the number of compressor assemblies used to compress the hydrogen is varied depending on the volume flow supplied from the electrolyzer to the compression system.

[0030] The invention also relates to a compressor arrangement for compressing hydrogen produced by an electrolyzer, comprising a first liquid ring machine for compressing hydrogen supplied from the electrolyzer to an intermediate pressure, and a second liquid ring machine for compressing the hydrogen from the intermediate pressure to an outlet pressure. A bypass line is provided between the outlet side and the inlet side of the first liquid ring machine in order to keep the inlet pressure on the inlet side of the first liquid ring machine constant. The compressor arrangement is designed to compress the hydrogen from the intermediate pressure to the outlet pressure without the intermediate pressure being kept constant. The disclosure includes developments of the compressor arrangement with features that are described in connection with the method according to the invention.The disclosure includes further developments of the method which are described in connection with the compressor arrangement according to the invention.

[0031] The invention is described below by way of example with reference to the accompanying drawings using advantageous embodiments. They show:

[0032] Fig. 1: a compressor arrangement according to the invention designed to carry out the method according to the invention;

[0033] Fig. 2: the view according to Fig. 1 in an alternative

[0034] Embodiment of the invention,

[0035] Fig. 3: an alternative embodiment of a compressor arrangement according to the invention;

[0036] Fig. 4: a compressor system with a plurality of compressor arrangements according to the invention.

[0037] Fig. 1 shows a compressor arrangement connected to an electrolyzer 14. Hydrogen produced in the electrolyzer 14 is transferred to the compressor arrangement at an inlet pressure which may be, for example, 0.2 bar above atmospheric pressure. In the compressor arrangement, the hydrogen is compressed and discharged via an outlet line 32 at an outlet pressure of, for example, 10 bar above atmospheric pressure. The electrolyzer 14 has a maximum power of 25 MW, which corresponds to a volume flow of hydrogen in the order of 5000 m 3 / h, which is fed to the inlet of the compressor assembly. For the operation of the electrolyzer 14, it is important that a constant pressure is maintained at the outlet of the electrolyzer 14, with which the produced hydrogen can be transferred to the compressor assembly.

[0038] The compressor arrangement comprises a first compression stage with a first liquid ring machine 15 and a second compression stage with a second liquid ring machine 25. The first liquid ring machine 15 and the second liquid ring machine 25 are connected in series. With the first liquid ring machine 15, the hydrogen is compressed from the initial pressure to an intermediate pressure 20, which can be, for example, between 0.5 bar and 2 bar above atmospheric pressure. With the second liquid ring machine 25, the hydrogen is compressed from the intermediate pressure 20 to the final pressure.

[0039] The first liquid ring machine 15 is driven by a motor 23, which in this exemplary embodiment is operated at a constant speed. A bypass line 16 extends between the outlet side 21 and the inlet side 22 of the first liquid ring machine 15, which bypass line 16 allows a backflow from the outlet side 21 to the inlet side 22. The bypass line 16 is provided with an adjustable valve 17 that is continuously adjustable. The adjustable valve 17 changes the cross-section of the bypass line 16, which is available for a backflow from the outlet side 21 to the inlet side 22.

[0040] The adjustable valve 17 is controlled by a control unit (not shown). The control takes place as a function of the inlet pressure 18 present on the inlet side 22 of the first liquid ring machine 15. The control unit is designed to keep the inlet pressure 18 constant. The control unit therefore controls the adjustable valve 17 in such a way that an increase in the inlet pressure is counteracted by an increase in the cross-section of the bypass line 16, and a drop in the inlet pressure is counteracted by a reduction in the cross-section of the bypass line 16.

[0041] Between the first liquid ring machine 15 and the second liquid ring machine 25, a buffer tank 19 is arranged, which has a volume of 0.5 m 3The buffer tank 19 dampens the fluctuations in the intermediate pressure 20 that result from the actuation of the adjustable valve 17. The buffer tank 19 also serves as a liquid separator, with which liquid quantities are separated from the volume flow coming from the first liquid ring machine 15. The liquid quantities are returned to the liquid ring of the first liquid ring machine 15 via a heat exchanger 24.

[0042] The second liquid ring machine 25, at whose inlet 33 the intermediate pressure 20 is applied, is driven by a variable-speed motor 28, which is supplied with electrical energy via a converter 29 so that a desired speed is established. Between the outlet side 26 of the second liquid ring machine 25 and the outlet line 32 of the compressor arrangement, the volume flow is passed through a liquid separator 30. Quantities of liquid separated from the volume flow are returned to the liquid ring of the second liquid ring machine 25 via a heat exchanger 27.

[0043] The control unit controls the inverter 29 such that, on the one hand, the intermediate pressure 20 is kept within a range between 0.5 bar and 2 bar above atmospheric pressure and, on the other hand, energy consumption is kept as low as possible. In doing so, the control unit takes into account the operating limits of the second liquid ring machine 25 and, for example, does not reduce the speed so far that the second liquid ring machine 25 starts to oscillate. The control unit reads the relevant control commands for the inverter 29 from a table in which the control commands are stored as a function of parameters such as the output of the electrolyzer 14, the position of the adjustable valve 17 and the intermediate pressure 20.

[0044] For the operation of the compressor arrangement, this means that the adjustable valve 17 is only actuated as a function of the inlet pressure 18 and that fluctuations in the intermediate pressure 20 associated with actuation of the adjustable valve 17 are accepted. The speed control of the second liquid ring machine 25 counteracts fluctuations in the intermediate pressure 20, but without completely compensating them. The effect of actuation of the adjustable valve 17 on the intermediate pressure 20 is significantly faster than a subsequent change in the speed of the second liquid ring machine 25. However, the speed control of the second liquid ring machine 25 is sufficient to keep the intermediate pressure 20 within a predetermined range, which can be, for example, between 0.5 bar and 2 bar above atmospheric pressure.

[0045] If the second liquid ring machine 25 is operated at a higher speed than would be necessary to maintain the intermediate pressure 20, the reduced intermediate pressure 20 means that the first liquid ring machine 15 can demand a higher volume flow, which in turn leads to a falling inlet pressure 18 and the adjustable valve 17 being opened further. A higher speed of the second liquid ring machine 25 therefore only leads to higher energy consumption, without the electrolyzer 14 being affected by a change in the inlet pressure 18. Against this background, the converter 29 is controlled in such a way that the speed of the second liquid machine 25 is kept as low as the operating state of the compressor arrangement permits.

[0046] In Fig. 2 an alternative embodiment is shown in which the electrolyzer 14 has a maximum power of 50 MW, which corresponds to a volume flow of hydrogen of about 10,000 m 3 / h. A first liquid ring machine 15 is provided in the first compression stage, which can convey this volume flow. The second compression stage comprises two second liquid ring machines 25, 35, which are operated in parallel.

[0047] The outlet line 32 of the compressor arrangement is followed by further compression stages, which are indicated schematically in Fig. 2 as a block 36. From the pressure in the outlet line 32, which may be, for example, 10 bar above atmospheric pressure, the hydrogen is compressed by the further compression stages 36 to a significantly higher pressure of, for example, 700 bar. At this pressure, the hydrogen is stored in a tank 37.

[0048] The two second liquid ring machines 25, 38 of the second compression stage are driven by motors 28, 38, both of which are controlled at variable speeds by a converter 29, 39. If the power of the electrolyzer 14 is close to the maximum power, both second liquid ring machines 25, 38 operate in parallel and are controlled according to the same principles that were described in connection with the exemplary embodiment in Fig. 1.

[0049] In the electrolyzer 14 powered by renewable energy, there are longer operating phases in which the power is well below the maximum power, for example at 20% or 10% of the maximum power. The parallel connection of second liquid ring machines 25, 35 on the second compression stage opens up a further possibility for increasing energy efficiency by switching off one of the two second liquid ring machines 25, 35 and conveying the reduced volume flow only with the other of the two second liquid ring machines 25, 35.

[0050] The ability to operate only a portion of the second liquid ring machines 25, 35 provides the control unit with a further degree of freedom that can be used to increase energy efficiency. Instead of operating two second liquid ring machines 25, 35 at a very low speed, one of the two second liquid ring machines 25, 35 is operated at a higher speed. The control unit reads the relevant control commands from a table based on parameters relating to the operating state of the compressor arrangement, in particular based on the instantaneous output of the electrolyzer 14.

[0051] In addition, the liquid ring machine 15 of the first compression stage is also driven by a variable-speed drive 23, 34. Instead of accepting a very high backflow through the bypass line 16 when the volume flow coming from the electrolyzer 14 is low, the speed of the first liquid ring machine 15 can be reduced. This leads to greater energy efficiency because the backflow through the bypass line 16 decreases. The control unit controls the adjustable valve 17 and the inverter 34 accordingly, whereby the control can take place depending on various parameters of the operating state, in particular depending on the instantaneous power of the electrolyzer 14. In Fig.3 shows a compressor arrangement 40 which comprises a single liquid ring machine 15 on the first compression stage 46 and five second liquid ring machines 41, 42, 43, 44, 45 connected in parallel to one another on the second compression stage 47. The plurality of second liquid ring machines 41, 42, 43, 44, 45, which can be started up or switched off independently of one another, makes it possible to more finely adjust the compression power available on the second compression stage 47 to the instantaneous power of the electrolyzer.

[0052] In Fig. 4, the electrolyzer 14 has a higher maximum output of, for example, 250 MW. In order to be able to convey the volume flow of hydrogen generated thereby, a compressor system is connected to the electrolyzer 14, in which five of the compressor assemblies 40 shown in Fig. 3 are operated in parallel to one another. Because the compressor assemblies 40 can be started up or switched off individually from one another and because the liquid ring machines of the second compression stage 47 in each compressor assembly can also be started up or switched off individually from one another, the compressor system can be tuned even more finely to the current output of the electrolyzer 40, which enables a further increase in energy efficiency.

Claims

Patent claims 1. A method for compressing hydrogen produced by an electrolyzer, in which hydrogen is compressed to an intermediate pressure (20) using a first liquid ring machine (15) and in which the hydrogen is compressed from the intermediate pressure (20) to an outlet pressure using a second liquid ring machine (25, 35, 41, 42, 43, 44, 45), wherein a backflow between the outlet side (21) and the inlet side (22) of the first liquid ring machine (15) is permitted via a bypass line (16), wherein the inlet pressure (18) on the inlet side (22) of the first liquid ring machine (15) is kept constant by changing the cross-section of the bypass line (16) using an adjustable valve (17), and in which the hydrogen is compressed from the intermediate pressure (20) to the outlet pressure without the intermediate pressure (20) being kept constant.

2. The method according to claim 1, wherein an adjustable valve (17) is arranged in the bypass line (16) to regulate the backflow from the output side (21) to the input side (22) of the first liquid ring machine (15).

3. Method according to claim 1 or 2, wherein a buffer tank (19) is arranged between the outlet side (21) of the first liquid ring machine (15) and the inlet (33) of the second liquid ring machine (25), said buffer tank having a volume of between 0.2 m 3 and 5 m 3 , preferably between 0.5 m 3 and 2 m 3 has .

4. Method according to one of claims 1 to 3, wherein an upper limit is predetermined for the intermediate pressure (20) and wherein the second liquid ring machine (25) is operated in such a way that that the upper limit for the intermediate pressure (20) is not exceeded.

5. The method according to any one of claims 1 to 4, wherein the hydrogen is compressed from the intermediate pressure (20) to the outlet pressure by a plurality of second liquid ring machines (25, 35, 41, 42, 43, 44, 45) and wherein the second liquid ring machines (25, 35, 41, 42, 43, 44, 45) are connected in parallel to one another.

6. The method according to claim 5, wherein, depending on the instantaneous power of the electrolyzer (14), a portion of the second liquid ring machines (25, 35, 41, 42, 43, 44, 45) is switched off and the volume flow coming from the electrolyzer (14) via the first liquid ring machine (15) is compressed with the other portion of the second liquid ring machines (25, 35, 41, 42, 43, 44, 45).

7. The method according to any one of claims 1 to 6, wherein the hydrogen is compressed by the first liquid ring machine (15) from an inlet pressure (18) which is between 0.1 bar and 0.3 bar higher than atmospheric pressure to an intermediate pressure (20) which is between 0.5 bar and 2 bar higher than atmospheric pressure.

8. The method according to any one of claims 1 to 7, wherein the hydrogen is compressed by the second liquid ring machine (25) or the second liquid ring machines (25, 35, 41, 42, 43, 44, 45) to an outlet pressure which is between 6 bar and 12 bar higher than atmospheric pressure.

9. The method according to claim 8, wherein the hydrogen is compressed with one or more subsequent compression stages (36) to a pressure of at least 100 bar, preferably of at least 200 bar, more preferably at least 500 bar above atmospheric pressure.

10. The method according to any one of claims 1 to 9, wherein the first liquid ring machine (15) is designed to achieve a volume flow of at least 2000 m 3 / h, preferably a volume flow of at least 5000 m 3 / h, further preferably a volume flow of at least 10,000 m 3 / h from the electrolyzer (14).

11. Method according to one of claims 1 to 10, wherein the first liquid ring machine (15) is operated at a variable speed.

12. Compressor arrangement for compressing hydrogen produced by an electrolyzer, with a first liquid ring machine (15) for compressing hydrogen supplied from the electrolyzer (14) to an intermediate pressure (20), with a second liquid ring machine (25, 35, 41, 42, 43, 44, 45) for compressing the hydrogen from the intermediate pressure (20) to an outlet pressure, and with a bypass line (16) between the outlet side (21) and the inlet side (22) of the first liquid ring machine (15) for keeping the inlet pressure (18) on the inlet side (22) of the first liquid ring machine (15) constant, wherein the compressor arrangement is designed to compress the hydrogen from the intermediate pressure (20) to the outlet pressure without the intermediate pressure (20) being kept constant.

Images