Bidirectional single-screw system, and method for compressing and expanding a gas

Abstract

A bidirectional single-screw system for compressing and expanding a gas, with at least one single-screw air end (10) which has a low-pressure line (15) on one side and a high-pressure line (16) on the other side. A motor / generator unit (30) which is coupled to the single-screw air end (10) conveys and compresses a gas in the compression mode from the low-pressure line (15) into the high-pressure line (16), by means of drive of the single-screw air end (10), and generates electrical energy in the expansion mode when the gas is conveyed from the high-pressure line (16) back into the low-pressure line (15). A liquid separator (21) which is connected to the low-pressure line (15) or the high-pressure line (16) separates the gas and a liquid downstream of the air end (10) in the flow direction, wherein the liquid is conducted via a liquid line (70) back to the single-screw air end (10) again. A second connection to the low-pressure line (15) or to the high-pressure line (16) is established by way of an injection valve (76) which is arranged in the liquid line (70) and has two outlets.

Description

Bidirectional single-screw system and method for Compression and expansion of a gas

[0001] The present invention relates to a bidirectional single-screw system and a method for compression and expansion of a gas according to the preamble of claims 1 and 7.

[0002] By reversing the direction of rotation of a single-screw compressor machine, the same machine can be used to compress and expand a gas. To accommodate a wider range of pressure differences, multiple single-screw machines can be series-connected in multiple stages. The machine is often lubricated with oil or other environmentally friendly fluids. Ideally, a fluid is used that eliminates the need for post-processing of the gas as it exits the machine, thus increasing the system's sustainability. The fluid is added during the process to provide lubrication and, at the same time, to seal the machine.

[0003] The single-screw compressor is a proven technology for gas compression. Unlike other rotary compressor technologies such as turbo or twin-screw compressors, the single-screw compressor can, if required, operate at a higher pressure level of approximately 10 bar. Furthermore, the nominal speed is low at approximately 3000 rpm, which, despite the high pressure ratio, keeps the temperature rise during compression low. and thus increases efficiency. Further advantages of single-screw technology include reduced noise and vibration, as well as low maintenance requirements. [4] A single-screw machine has been known since 1974 (US 3945778 A) which can be used both as a compressor and as an expander. [5] WO 2013078132 A1 also shows a bidirectional single-screw machine in which oil is injected directly into the compression or expansion head, the so-called air end, for sealing purposes. [6] CN 112253257 A shows a single-screw expander system that features water injection at the air end for lubrication and sealing. This is a single-stage expander system in which water is injected directly into the air end for lubrication and sealing. [7] Multi-stage single-screw machines are mainly used as compressors. The system according to CN 209510633 U shows a two-stage single-screw compressor, which is operated with water injection for lubrication and sealing. [8] CN 102352777 B shows a bidirectional, multi-stage single-screw system, which can also be used in an energy storage system. The multiple stages are mounted on the same shaft.

[0009] Efforts to realize bidirectional single-screw machines with water injection and lubrication are disclosed, for example, in CN 111271283 A and CN 102042225 A. Although these are bidirectional, they are primarily designed for compressor operation and are only single-stage.

[0010] State-of-the-art oil-free single-screw machines are usually single-stage and optimized for compression operation. If the system is designed bidirectionally, the physical layout and dimensions in the air end often result in the problem of over- / under-compression or over- / under-expansion, which can lead to energy losses. Over-compression occurs when the internal volume ratio in the air end is greater than the required pressure ratio, which means that the outlet pressure is greater than the target pressure. Conversely, under-compression occurs when the internal volume ratio in the air end is less than the required pressure ratio, which means that the outlet pressure is less than the target pressure.Similarly, during expansion, overexpansion occurs when the internal volume ratio at the air end is greater than the required pressure ratio, resulting in the outlet pressure being lower than the target pressure. Conversely, underexpansion occurs when the internal volume ratio at the air end is smaller than the required pressure ratio, resulting in the outlet pressure being higher than the target pressure.

[0011] The temperature differences associated with compression and expansion are also often problematic in a bidirectional system and can lead to overheating or overcooling of the machine. This can cause the machine to malfunction or, if water is used for lubrication and sealing, ice to form during overcooling. To avoid these problems, external temperature control measures must be taken or additives, such as antifreeze, must be added to the gas or liquid.

[0012] The present invention now has the object of improving a bidirectional single-screw system and a method for compressing and expanding a gas of the type mentioned at the outset in such a way that the advantages of the known devices and methods are retained, and with a control of the liquid supply and discharge, the temperature, volume and pressure conditions are optimized in such a way that both the compression and the expansion mode can be carried out efficiently and sustainably, and no external measures for temperature regulation and / or the supply / processing of the gas are required.

[0013] Accordingly, the object of the present invention is to create a bidirectional and oil-free single-screw machine for the compression and expansion of a gas, in which an intelligent fluid management concept ensures the lubrication and sealing of the single-screw machine on the one hand but on the other hand also enables the control of the internal volume ratio in the air end and the heat transfer between the gas and the liquid.

[0014] Another task is to optimize the single-screw system in order to increase its efficiency.

[0015] Thanks to bidirectionality, a gas can be compressed and expanded with the same machine. This significantly expands the application area while also roughly halving the investment costs and space requirements.

[0016] The multi-stage design allows application not only at low pressure levels, but also for medium and high pressure levels. The following description and drawings specifically show a system with two stages, but it can also be implemented with 1-n stages if required. A typical pressure level for a two-stage system is, for example, 40 bar, with a pressure ratio of slightly over 6:1 per stage.

[0017] The lubrication and sealing of the single-screw machine are realized by controlling the fluid supply and discharge to implement an intelligent fluid management concept. In addition, the crucial control options are based on the heat transfer between the gas and the fluid, as well as the internal volume ratio in the air end and the associated sealing or expansion ratio of the single-screw machine.

[0018] By using a liquid, specifically water, for lubrication and sealing, the single-screw machine is completely oil-free. This means the gas is not contaminated as it passes through the single-screw machine, eliminating the need for complex cleaning and post-processing of the gas.

[0019] The use of a fluid also eliminates the disadvantages of dry-running machines, which are typically used for oil-free compressed gas supply. Such machines are inefficient due to the lack of lubrication and sealing, require high speeds, and generate significant noise and vibration.

[0020] The fluid is mixed with the gas upstream of the single-screw machine as needed and / or injected directly into the pressure chamber within the single-screw machine itself at various times during compression or expansion. This also allows the sealing effect of the fluid to be adapted and optimized for compression and expansion operation.

[0021] A further advantage of adding the liquid is the ability to influence the heat transfer from the gas to the liquid. This allows the heating of the gas during compression or the cooling of the gas during expansion to be altered. A quasi-isothermal compression or ex- In this case, a system in which the gas heats up or cools down only minimally is preferred. This increases efficiency and enables high pressure ratios on a single stage without significant temperature differences.

[0022] Without the addition of a liquid, the compression or expansion is almost adiabatical with a polytropic exponent of k=1.4. This results in a compression with a pressure ratio of 6:1 and an initial temperature of 20°C, resulting in an outlet temperature of 216°C:

[0023] The addition of a liquid, especially water, enables heat transfer between gas and liquid, allowing the process to proceed quasi-isothermally with a polytropic exponent of, for example, k=1.04. This results in a significantly lower outlet temperature of 41°C for compression under the same conditions:

[0024] The heat generated during compression or the cold generated during expansion are thus at a non-critical, controllable and usable temperature level.

[0025] The small temperature difference during expansion allows the process to be operated above the freezing point of the liquid, so that neither preheating of the compressed gas nor the addition of Antifreeze is necessary. This is especially important when water is used as the liquid.

[0026] The efficiency is also increased with the addition of liquid, since, for example, more work is required for compression to a certain pressure level after cooling in the adiabatic process than in the quasi-isothermal process.

[0027] By adding liquid upstream of the single-screw machine, the internal volume ratio of the single-screw machine can also be controlled and optimized with a given geometry. Due to the incompressibility of the liquid, less volume remains for the compressible gas during the compression or expansion process when the liquid content is increased, thus increasing the internal volume ratio in the air end. This means that for a given geometry of the single-screw machine, the volume ratio can be adapted to the respective application and the desired pressure ratio. This prevents over- and under-compression or over- and under-expansion, and further increases efficiency. During expansion in particular, the volume ratio can be precisely matched to the pressure ratio, thus avoiding highly inefficient over- and under-expansion.

[0028] The bidirectionality of the system also enables its application in an energy storage system, in which a gas is compressed to charge the energy storage device, which is then can be expanded again at a later point in time to release the energy again. During compression, work is performed to drive the compressor, compressing the gas and at the same time heat can be generated due to the cooling requirements of the compressed gas. Conversely, during expansion, the compressed gas drives the expander, which performs work, and at the same time cold can be generated due to the heat requirements of the expanded gas. The heat during compression or cold during expansion can be efficiently dissipated through an internal fluid circuit in the single-screw machines via a heat exchanger.

[0029] At an ambient temperature of 20°C, the temperature levels of heating and cooling with single-screw technology with liquid injection are at a non-critical and directly usable level of approximately 30-80°C for heating (compression) and 0-15°C for cooling (expansion) due to the quasi-isothermal compression and expansion. The use of heating and cooling is therefore particularly efficient, as the temperature levels correspond directly to the room cooling or heating requirements. The efficiency of the overall system is further increased by the direct use of heating and cooling.

[0030] Further applications arise in other areas requiring oil-free or pure compressed gas. Because the present invention is oil-free, no complex post-processing of the compressed gas is necessary.

[0031] This object is achieved by a bidirectional single-screw system and a method for compressing and expanding a gas having the features of patent claims 1 and 7. Further features and embodiments emerge from the dependent claims, and their advantages are explained in the following description. Drawings Fig. 1 Schematic drawing of a two-stage single-screw System for the compression and expansion of a gas. Fig. 2 Schematic of the compression operating mode. Fig. 3 Schematic of the expansion operating mode. Fig. 4 Scheme of the compression operating mode with heat exchanger. Fig. 5 Scheme of the expansion operating mode with heat exchanger. Fig. 6 Scheme with two heat exchangers per stage. Fig. 7a Scheme with only one liquid separator per stage in Compression operating mode Fig. 7b Scheme with only one liquid separator per stage in Expansion operating mode The figures represent possible embodiments, which are explained in the following description. Description

[0032] Fig. 1 shows a two-stage preferred embodiment of the bidirectional single-screw system for the compression and expansion of a gas. The present invention can be implemented with at least one stage, preferably two or more stages. Two single-screw air ends 10, 10' are each driven or braked by a motor / generator unit 30, 30'. This allows a gas to be compressed from a low-pressure reservoir 50 with a lower pressure and transported to a high-pressure reservoir 60 with a higher pressure, or expanded in the opposite direction from the high-pressure reservoir 60 with a higher pressure and transported to a low-pressure reservoir 50 with a lower pressure.The pressure reservoirs are connected to the single-screw air ends 10, 10' via connecting lines, wherein the connecting lines of each stage on the side facing the low-pressure reservoir 50 are referred to as low-pressure lines 15, 15' and the connecting lines on the side facing the high-pressure reservoir 60 are referred to as high-pressure lines 16, 16'. The connecting line between two single-screw air ends 10, 10' is simultaneously the high-pressure line 16 to the single-screw air end 10 and the low-pressure line 15' to the single-screw air end 10'. The connecting lines 15, 16 between the components are suitable for either a gas, a liquid or a gas-liquid mixture and are generally bidirectional, i.e. the direction of flow can be... both sides are realized.

[0033] Furthermore, liquid separators 21, 22 are provided for separating gas and liquid; these are arranged on the connecting lines 15, 16. The liquid separators 21, 22', which are located closest to the reservoirs 50, 60, are not required depending on the operating direction. Thus, during compression, only the liquid separators 22, 22' are required, as shown in Fig. 2, and conversely, during expansion, only the liquid separators 21, 21' (Fig. 3) are required. The liquid separator 22 / 21', which is arranged between two single-screw air ends 10, 10', can be used in both operating directions. The liquid separators 21, 22 have an inlet for the gas-liquid mixture, an outlet for the gas, and an additional outlet for the separated liquid.A liquid line 70 connects the outlet of the liquid separator 21 or 22 for the liquid with the single-screw air end 10, so during compression the liquid separator 22 is connected to the single-screw air end 10 via the liquid line 70 and vice versa during expansion the liquid separator 21 is connected to the single-screw air end 10 via the liquid line 70. In the flow direction from the liquid separator 21 or 22 to the single-screw air end 10, a controllable injection valve 76 is arranged in front of the single-screw air end 10, which has a second outlet to the connecting line 15 or 16, wherein this outlet is in each case on the inlet side of the single-. Screw Air-Ends 10 opens into the connecting line 15 or 16, so that this second outlet of the injection valve 76 is located opposite the Air-End 10 on the opposite side of the liquid separator 21 or 22.

[0034] For the expansion working direction (Fig. 3), an additional pump 73 is arranged after the liquid separator 21 so that the liquid can be brought to the necessary pressure in order to be introduced into the single-screw air end 10 or the high-pressure line 16 at the higher pressure.

[0035] In a preferred embodiment, the liquid line 70 passes through a heat exchanger 75 so that the liquid can be heated or cooled as needed. The heat exchanger 75 can be used for both compression (Fig. 4) and expansion (Fig. 5). The heat exchanger 75 can be arranged in the liquid line 70 either upstream or downstream of the pump.

[0036] In a further preferred embodiment, the injection valve 76 is a valve arrangement with one inlet and three outlets, whereby only two outlets are available depending on the operating mode. To enable the liquid separator 22 / 21' to be used in a two- or multi-stage system for both compression and expansion, a directional valve 80 is arranged after the outlet of the liquid separator 22 / 21' for the separated liquid. This directional valve 80 directs the liquid, depending on the operating mode. capacity either to the heat exchanger 75 (compression) or to the pump 73 (expansion). In this configuration, an inlet valve 74 is arranged upstream of the heat exchanger 75, which has two inlets and only one outlet. During compression, the liquid comes from the liquid separator 22, which is arranged in the high-pressure line 16, and during expansion, from the liquid separator 21, which is arranged in the low-pressure line 15.

[0037] Each individual single-screw stage has its own liquid circuit, which is connected to at least one liquid separator 21, 21' and has a liquid line 70. Upon exiting the single-screw air end 10, the gas-liquid mixture is separated before the liquid can thermally interact with the environment via the heat exchanger 75. Heat exchangers 75, 75' of multiple stages can also be designed to be coupled or, for the expansion and compression operating modes, designed separately for each flow direction 751, 752 per stage (Fig. 6). After the heat exchanger 75, the liquid from the single-screw stage is again mixed with the gas in the direction of flow before entering the single-screw air end 10 and / or injected directly into the single-screw air end 10 in the compression or expansion chamber. The liquid for lubrication and sealing is then fed to the single-screw air end 10.Depending on the configuration, the injection valve 76 can inject fluid into the single-cylinder injection system alternately or simultaneously. Screw Air-End 10 , and during compression into the low pressure line - device 15, or during expansion into the high-pressure line 16, in order to change the volume ratio before entering the air end 10. In the preferred embodiment, the injection valve 76 is controllable and continuously adjustable so that the fluid quantity of each of the two active outlets can be adjusted as desired and independently of one another.

[0038] Fig. 2 and Fig. 4 show the compression operating mode, with the components not used not being shown. In this operating mode, a gas is compressed from a low-pressure reservoir 50 and conveyed into a high-pressure reservoir 60. In a special embodiment, the low-pressure reservoir 50 is the ambient air and / or a compressed air line is used instead of the high-pressure reservoir 60. The single-screw air ends 10, 10' rotate in the direction in which the volume of the closed pressure chamber is reduced, which enables compression. The motor / generator units 30, 30' are used as motors to drive the single-screw air ends 10, 10' using electrical energy. Due to the injection of liquid in the direction of flow in front of or directly in the Single-Screw Air-End 10, a gas-liquid mixture is created.

[0039] After exiting the single-screw air end 10, the gas-liquid mixture enters the liquid separator 22 where the liquid is separated from the gas. The gas is then fed to the next stage, or after the last stage, into the high-pressure reservoir 60. The separated liquid is first passed through the heat exchanger 75 in order to thermally interact with the environment or an external circuit, before it is added back to the gas through the injection valve 76. In compression mode, the gas and the liquid in the single-screw air end 10 are heated. The absorbed heat is released via the heat exchanger 75 and can be used for heating purposes, for example. The cooled liquid is then mixed back into the gas in the low-pressure line 15 before it enters the single-screw air end 10, and secondly used directly at the single-screw air end 10 for lubrication and sealing or injected directly into the pressure chamber.

[0040] The addition of the incompressible liquid in the low-pressure line 15 before entering the single-screw air-end 10 enables the increase of the internal volume ratio in the air-end 10. However, this also reduces the volume flow of the gas through the single-screw air-end 10. In the case of under-compression, i.e. if the volume ratio is smaller than the required pressure ratio, the pressure is brought to the required pressure level when expelled from the pressure chamber, which requires additional energy. On the other hand, in the case of over-compression, an unnecessarily high pressure level is reached, which leads to efficiency losses. Therefore, it is important to avoid over-compression and to operate the single-screw air-ends 10 ideally in a slight under-compression. This means that the addition of the Introducing liquid into the low-pressure line 15 before entering the single-screw air end 10 to increase the internal volume ratio is only useful in special configurations, for example, to reduce the gas mass flow and prevent excessive undercompression. However, in addition to lubrication and sealing, it is advisable to add the liquid directly to the single-screw air end 10 or to inject it into the pressure chamber to control the heat transfer between gas and liquid, thus achieving quasi-isothermal compression.

[0041] Fig. 3 and Fig. 5 show the expansion mode of operation without those components which are not required for this operation. In this mode, a gas is expanded from a high-pressure reservoir 60 and conveyed into a low-pressure reservoir 50. In a special embodiment, the low-pressure reservoir 50 is the ambient air. The single-screw air ends 10, 10' rotate in the opposite direction to that in the compression mode of operation. The volume of the closed pressure chamber is increased, which enables expansion. The motor / generator units 30, 30' are used as a generator to brake the single-screw air ends 10, 10' by releasing electrical energy.

[0042] After exiting the single-screw air end 10, the gas-liquid mixture is separated in the liquid separator 21. After the first expansion stage, the gas flows into the second expansion expansion stage or after the second expansion stage into the low-pressure reservoir 50. In contrast to the compression operating mode, the separated liquid must now first be brought to the higher pressure level that prevails before the respective stage using a pump 73. The liquid is then conveyed through the heat exchanger 75 in order to thermally interact with the environment or with an external circuit. In the expansion operating mode, the liquid is cooled during expansion because heat is transferred to the cooling gas. The liquid can be reheated via the heat exchanger 75 and thus used for cooling.The liquid is then mixed with the gas in the high-pressure line 16 before entering the single-screw air end 10 through the injection valve 76, and is used directly at the single-screw air end 10 for lubrication and sealing and is injected directly into the pressure chamber so that a quasi-isothermal expansion can be achieved.

[0043] The addition of the incompressible fluid in the high-pressure line 16 before entering the single-screw air end 10 allows the volume ratio in the high-pressure line 16 to be increased. This is important in the expansion mode because it prevents inefficient under-expansion. Under-expansion occurs if the volume ratio is smaller than the required pressure ratio. On the other hand, it is also important to prevent over-expansion, i.e. the case where the internal volume Ratio is greater than the required pressure ratio. Thus, by controlling the fluid admixture, the volume ratio and expansion can be optimized. Furthermore, fluid for lubrication and sealing is added directly to the Single-Screw Air-End 10 and injected directly into the pressure chamber to control the heat transfer between gas and fluid.

[0044] 1, the liquid separator 21 is not required during compression and the liquid separator 22' is not required during expansion. In order to avoid the costs and space requirements of this unnecessary component, a further embodiment variant provides that only one liquid separator 21 is required for each stage. Fig. 7a shows the corresponding arrangement for a single stage with only one liquid separator 21 for compression. The lines marked with an X in the figure from the low-pressure line 15 to the liquid separator 21 are not required. Likewise not required are the pump and the connection for injecting the liquid into the high-pressure line 16. Conversely, during expansion the lines from the high-pressure line 16 to the liquid separator 21 and the connection for injecting the liquid into the low-pressure line 15 are not required, as shown in Fig. 7b shown .

[0045] In a special embodiment, two separate heat exchangers 751, 752 can be used for compression and expansion, as shown in Fig. 6. For compression, for example, a heat exchanger 751 is used, which extracts the heat from the liquid, thus cooling it, and releases heat to the environment. Conversely, a heat exchanger 752 can be used during expansion, which heats the liquid again, thus cooling the environment. This is particularly advantageous when different requirements and / or external circuits are used due to the use of heat or cold. The heat released during compression can, for example, be used for space heating, hot water, as process heat (low temperature) or for preheating in an industrial process. During expansion, for example, a data center, food cold storage facility or another room can be cooled, cooling water can be provided for a machine, or the cold can be used directly for server or machine cooling.

[0046] In a particularly advantageous embodiment of the invention, it is provided that the injection valves 76, 76', the inlet valves 74, 74' and the directional valves 80, as well as the motor / generator units 30, 30', are controlled by a central controller and can be adjusted according to predefined criteria or automatically, depending on the operating mode. To optimize compression and expansion, it is also provided that various sensors and measuring devices are arranged in the individual components. These can be, for example, pressure sensors in the low-pressure lines 15, 15', the high-pressure lines 16, 16' and / or in the liquid separators 21, 22 and single-screw air- Ends 10, 10' to check the pressure ratio. Temperature sensors or flow sensors, located before and after the air ends 10, 10' and / or the heat exchangers 75, 75', can also be beneficial for monitoring temperature changes. By intelligently controlling the speeds of the motor / generator units 30, 30', the pumps 73, 73', and the injection valves 76, 76' depending on the pressure, temperature, and flow measurements, optimization can be achieved in the respective operating mode.

Claims

Patent claims 1. Bidirectional single-screw system for compression and Expansion of a gas, comprising at least one single-screw air end (10) which has a low-pressure line (15) on one side and a high-pressure line (16) on the other side, and a motor / generator unit (30) which is coupled to the single-screw air end (10), wherein by driving the single-screw air end (10) by the motor / generator unit (30), a gas can be conveyed and compressed from the low-pressure line (15) into the high-pressure line (16), and electrical energy can be generated in the reverse direction by expanding the gas by the motor / generator unit (30), characterized in that at least one liquid separator (21) is connected to the low-pressure line (15) and / or the high-pressure line (16), and a liquid line (70) for transporting a liquid exists between the liquid separator (21) and the single-screw air end (10),and an injection valve (76) is arranged between the liquid separator (21) and the single-screw air end (10), which has at least one second connection to the low-pressure line (15), or to the high pressure line (16).

2. Single-screw system according to claim 1, characterized in that at least one heat exchanger (75) is arranged between the liquid separator (21) and the injection valve (76). 3 Single-screw system according to claim 1 or 2, characterized in that at least one pump (73) is arranged between the liquid separator (21) and the injection valve (76). 4 Single-screw system according to claim 2 or 3, characterized in that a liquid separator (21, 22) is arranged on both sides of the single-screw air end (10), and the pump (73) is arranged between the liquid separator (21) on the low-pressure line (15) and the injection valve (76) or the heat exchanger (75). Single-screw system according to claim 2 or 3, characterized in that at least two air ends (10, 10') form a multi-stage system, wherein the liquid separator (22), which is used for separating the liquid in a first stage during compression, is used as a liquid separator (21') in a second stage during expansion, and a directional valve (80) is arranged downstream of the outlet of the liquid separator (22 / 21'), which guides the liquid into the liquid line (70) of the first stage during compression and into the liquid line (70') of the second stage during expansion, and an inlet valve (74) is arranged between the pump (73), the directional valve (80) and the injection valve (76) or the heat exchanger (75).Single-screw system according to claim 1 or 5, characterized in that the injection valve (76) has a connection to both the low-pressure line (15) and the high-pressure line (16). Method for the compression and expansion of a gas, wherein the gas is conveyed by means of a bidirectional single-screw machine from a low-pressure reservoir (50) into a high-pressure reservoir (60) or vice versa, the single-screw machine comprising at least one single-screw air end (10) which is connected on one side via a low-pressure line (15) to the low-pressure reservoir (50) and on the other side via a high-pressure line (16) to the high-pressure reservoir (60), as well as a motor / generator unit (30) which is coupled to the single-screw air end (10), wherein by driving the single-screw air end (10) by the motor / generator unit (30), the gas is conveyed and compressed from the side of the low-pressure reservoir (50) to the side of the high-pressure reservoir (60), and in the reverse direction, electrical energy is generated by expanding the gas by the motor / generator unit (30). is generated,wherein the single-screw air end (10) is lubricated and sealed by means of a liquid, and a liquid is added to the gas in the flow direction upstream of the single-screw air end (10) and directly in the single-screw air end (10). Method according to claim 7, characterized in that water is used as the liquid. Method according to claim 7, characterized in that air is used as the gas. Method according to claim 7, characterized in that the admixture of the liquid is regulated by means of a control system such that the compression or expansion process is quasi-isothermal. Method according to claim 7, characterized in that the admixture of the liquid is regulated by means of a control system such that the internal volume ratio in the air end (10) is controlled in order to prevent over- / undercompression or over- / underexpansion.

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