Multi-stage compression system with compressions mounted in series

Abstract

Multi-stage compression system with compression stages connected in series. The invention relates to a gas compression system (2) comprising: - a gas inlet (10), - two compression stages (20) connected in series, each comprising: - a rotary electric motor (60) configured to compress the gas, - a speed control unit for the electric motor (27, 37), the control units each being configured to control the rotational speed of the electric motor (60) corresponding at least as a function of the gas pressure upstream of the corresponding compression stage, or the control units each being configured to control the rotational speed of the electric motor (60) corresponding at least as a function of the gas pressures downstream of the corresponding compression stage. Figure for the abstract: Fig. 1

Description

Title of the invention: Compression system with several compression stages connected in series

[0001] The present invention relates to gas compression systems, in particular hydrogen, and methods of compressing gas using such compression systems. Technical field

[0002] The emergence of hydrogen fuel cells has given rise to a need to compress a large quantity of gas into a small storage volume at low energy cost to power such systems.

[0003] Currently, different methods for compressing a gas exist. They consist of carrying out a volumetric compression of the gas in a compression chamber. The objective of this compression is to reduce the volume of the stored gas compared to the volume that it would occupy at atmospheric pressure. The volumetric ratio of the compression system (or compression ratio) is the parameter that makes it possible to modify the pressure between the inlet (or intake pressure) and the outlet (or discharge pressure) of the compression system.

[0004] Each compressor has a possible operation for a specific pressure range upstream of the compressor and a specific gas pressure range downstream of the compressor and is characterized by its compression ratio. In particular, it has an energy-optimized operation for a specific upstream pressure in the possible intake pressure range. Outside these ranges, the compressor may not compress the gas, may wear out prematurely, or even deteriorate, and has increased energy consumption.

[0005] In the case of using a single compressor, the compression is limited by the characteristics of the compressor itself and it is not possible to obtain significant pressure differentials between the upstream pressure and the downstream pressure.

[0006] It may be advantageous to have several compression stages connected in series. The operating characteristics of the compression stages, in particular their optimal intake pressures, are then different, in order to be adapted to the particular compression range to be achieved for each of them.

[0007] This multi-stage compression configuration improves efficiency and therefore reduces energy consumption compared to a single-compressor solution. It also makes it possible to achieve significant compressions that are not achievable or difficult to achieve at low energy cost with a single compression stage.

[0008] State-of-the-art compression stages operate at a constant rotational speed of their electric motor. The gas flow rate at the outlet of each compression stage is therefore almost constant over time, with variations related to cyclic irregularities, the closing and opening times of the valves and the compressibility factor of the gases which can vary depending on the pressure at the outlet of the compression system.

[0009] However, when variations in pressure at the inlet of the compression stage, for example linked to a variation in pressure generated by a system for producing said gas, or variations in the flow rate in the system, for example generated by a variation in consumption of the gas at the outlet, appear, they have repercussions on the different compression stages connected in series. The pressures at the inlet of the different compression stages may no longer be optimized. This can lead to increased energy consumption, faster wear, or even damage to the systems.

[0010] When flow rate variations occur, it is known from patent EP 1 291 525 B1 to apply a "Start / Stop" operating strategy of a compression stage. In this patent, the compressors are stopped or restarted according to predetermined pressure thresholds. Such repeated stops and restarts generate premature aging of the compressors, in particular of the parts deforming in the compressors, such as the membranes in membrane compressors.

[0011] In this case, the control of the compressors in series is generally carried out centrally by a connection of these to a central control unit remote from the compressors. This connection can be interrupted for hardware or software problems, leading to malfunctions of the compressors. This risk increases with the distances between the compressors and the central control unit, which complicates the regulation of a chain of compressors distant from each other, in particular in different locations and can generate a degradation of the system.

[0012] It is also known from patent EP 0 431 287 B1 to regulate the gas flow rate of compressors by varying the geometry of compression blades in centrifugal compressors.

[0013] There is a need to have a compression system having optimized efficiency and consequently reduced energy consumption. Statement of the invention

[0014] The invention meets this need using a gas compression system comprising: - a gas inlet - a first compression stage supplied with gas by the gas inlet and delivering compressed gas, - a second compression stage supplied with compressed gas by the first compression stage and delivering compressed gas, the first and second compression stages each comprising - a rotary electric motor configured to compress the gas, and - a unit for controlling the rotational speed of the electric motor, the control unit of the first compression stage being configured to control the rotational speed of the corresponding electric motor at least as a function of the gas pressure at the inlet of the first compression stage and the control unit of the second compression stage being configured to control the rotational speed of the corresponding electric motor at least as a function of the gas pressure at the inlet of the second compression stage Or the control unit of the first compression stage being configured to control the rotation speed of the corresponding electric motor at least as a function of the gas pressure at the outlet of the first compression stage and the control unit of the second compression stage being configured to control the rotation speed of the corresponding electric motor at least as a function of the gas pressure at the outlet of the second compression stage.

[0015] The fact that the inlet of the second compression stage is supplied by the gas at the outlet of the first compression stage is characteristic of having two compression stages in series forming a chain of compression stages.

[0016] With such a system of compression stages in series, any variation in the gas flow rate of the first compression stage causes a disagreement with the gas flow rate of the second compression stage and vice versa, which has an impact on the inlet pressure of the second compression stage. This disagreement can prevent the system from operating correctly, degrade its service life and its efficiency. The control units of the two compression stages make it possible to compensate for these variations and return the system to a predetermined optimal operating regime for each of the compression stages.

[0017] Such a device also makes it possible to optimize the system in the case where the flow rate varies during compression due to the variation in the compressibility factor of the gas and / or the temperature of the gas.

[0018] Finally, the fact that the regulation of the speeds of the motors is done locally at the level of each compression stage with local data (pressure at the inlet and / or outlet of the corresponding compression stage) makes it possible to have efficient regulation even when the compression stages of the chain are far from each other, in particular in the case of a network of compressors of a certain scale or an environment making control communications difficult. System

[0019] The system may comprise one or more additional compression stages connected in series downstream of the second compression stage, the additional compression stage(s) each comprising: - a rotary electric motor configured to compress the gas from the compression stage arranged upstream of said additional compression stage, - a unit for controlling the rotational speed of the electric motor as a function of the inlet gas pressure, when the control unit of the first and second compression stages controls the speed of the corresponding motor as a function at least of the inlet gas pressure of the corresponding first or second compression stages, or of the outlet gas pressure, when the control unit of the first and second compression stages controls the speed of the corresponding engine as a function of at least the outlet gas pressure of the corresponding first or second compression stages.

[0020] Thus, the system comprises a chain of compression stages connected in series with each other, the compression stages all being regulated at least by the pressure upstream of each compression stage or all regulated at least by the pressure downstream of each compression stage.

[0021] Preferably, the system comprises a gas reservoir at least between two consecutive compression stages, in particular between the first and second compression stages. Such a gas reservoir makes it possible to absorb a variation in flow rate of one of the two compression stages while preventing damage to the system.

[0022] Preferably, the system supplies gas to a closed gas storage tank or a gas consumption device which may be part of the system.

[0023] Preferably, the system comprises at the inlet a gas source, in particular an electrolyzer supplying gas to the gas inlet or a finite volume gas reservoir.

[0024] When the gas source is an electrolyzer, the second compression stage control unit may be configured to turn on the second compression stage when the inlet or outlet pressure of the second compression stage is greater than a predefined minimum set pressure of the second compression stage. The second compression stage may include a second compression stage gas bypass circuit between the gas outlet of the first compression stage and the gas outlet of the second stage when the second compression stage is turned off.

[0025] When the gas source is a compressed gas reservoir of finite volume, the first compression stage control unit may be configured to turn on the first compression stage when the inlet or outlet pressure of the first compression stage pressure is lower than a pressure respectively at the inlet or at the outlet of the maximum setpoint of the first compression stage. The first compression stage may include a gas bypass circuit of the first compression stage between the gas inlet and the inlet of the second compression stage open when the first compression stage is switched off.

[0026] The device may comprise a sensor of the pressure at the gas inlet and a central control unit, configured to stop the system when the pressure at the gas inlet reaches a minimum inlet pressure, in particular a pressure less than or equal to a minimum inlet pressure admissible for the first compression stage. The minimum inlet pressure may be less than or equal to 30 bars, preferably less than or equal to 10 bars, better still less than or equal to 5 bars. Such a shutdown of the system is particularly useful when the gas source is a gas reservoir with a finite volume.

[0027] The device may comprise a sensor for the pressure at the outlet of the last compression stage and a central control unit, configured to stop the system when the outlet pressure is greater than or equal to a predefined final outlet pressure. The predefined final outlet pressure may be between 400 and 1000 bars, better still between 600 and 850 bars, even better still between 700 and 800 bars. Such a system stoppage is particularly useful when the system is used to fill a closed gas tank. This is particularly linked to automotive tank applications, the typical hydrogen filling pressure of which is 350 bars or 700 bars according to current technologies. A hydrogen station tank with a pressure of around 750 bars is therefore suitable for this use.

[0028] The system may include a device for cooling the gases at the outlet of one or each compression stage. This makes it possible to increase the density of the gas and to limit the thermal stresses on the compression stages.

[0029] The system may include a measurement of the position of the flywheels and the control units may be high-frequency pilot units configured to vary the speed of the engine as a function of the position of the engine in real time. This makes it possible to reduce the rotational speed of the engines when passing the top and bottom dead centers of the piston corresponding to the opening and closing of the intake and discharge valves, which smooths out cyclic irregularities and increases the life of the compressors by reducing internal stresses. Compression stages

[0030] The first and / or the second compression stage and / or the additional compression stage(s) where applicable may be formed from a single compressor.

[0031] Alternatively, the first and / or the second compression stage and / or the additional compression stage(s) where applicable are formed from a plurality of com compressors connected in parallel. Each compressor may comprise a rotating electric motor. The control unit of the or each compression stage, having a plurality of compressors in parallel, may control the speed of all the electric motors of the compression stage. In this case, the compressors of the compression stage(s) having several compressors are preferably identical and the control unit generates the same speed setpoint for all the electric motors of the corresponding compression stage. Alternatively, a different rotational speed setpoint may be applied to each of the compressors of the same stage.

[0032] Preferably, the or each compressor each comprises at least one compression chamber receiving at its inlet a gas at an inlet pressure, compressing it by reducing its volume and delivering at its outlet a so-called compressed gas at a discharge pressure higher than the inlet pressure. The inlet and outlet of gas into the compression chamber is preferably done via inlet and discharge valves.

[0033] Preferably, the compressor(s) are diaphragm or slide compressors. For example, the electric motors of the compressor(s) compress the gas indirectly via a crankshaft driven in rotation by the electric motor and connected to a piston which it moves during its rotation in an alternating translational movement in a cylinder. The movement of the piston generates the compression of the gas, in particular by an alternating variation of the volume of a compression chamber. The piston can compress a liquid, in particular a hydraulic oil, separated from the compression chamber by at least one deformable membrane. When the piston moves in the cylinder, the liquid is compressed and deforms the deformable membrane, which reduces the volume of the compression chamber and compresses the gas.The electric motor of the or each compressor can generate the compression of the gas in a plurality of compression chambers in parallel, in particular by means of several pistons mounted on the same crankshaft. Preferably, all of the compression chambers have the same characteristics. They may or may not be out of phase with each other. For example, the electric motor can generate the compression of the gas in two compression chambers mounted in opposition, preferably having the same characteristics but being in compression phase inversion. Control unit

[0034] Preferably, the control units regulate the speed of the corresponding electric motors in real time and continuously over time.

[0035] The control unit of the first compression stage can be configured to control the rotational speed of the corresponding electric motor only as a function of the gas pressure at the inlet of the first compression stage and the unit of control of the second compression stage is configured to control the rotation speed of the corresponding electric motor only as a function of the gas pressure at the input of the second compression stage, where appropriate the control unit of the or each additional compression stage is configured to control the rotation speed of the corresponding electric motor only as a function of the gas pressure at the input of said additional compression stage.

[0036] As a variant, the control unit of the first compression stage is configured to control the rotation speed of the corresponding electric motor only as a function of the gas pressure at the outlet of the first compression stage and the control unit of the second compression stage is configured to control the rotation speed of the corresponding electric motor only as a function of the gas pressure at the outlet of the second compression stage, where appropriate the control unit of the or each additional compression stage is configured to control the rotation speed of the corresponding electric motor only as a function of the gas pressure at the outlet of said additional compression stage.

[0037] Preferably, the first and second compression stages each comprise one or more speed variators controlled by the corresponding control unit and each supplying an electric motor.

[0038] Thus, in the case where the first and / or the second compression stage comprise several compressors each comprising an electric motor as described previously, a speed variator can supply all the electric motors simultaneously or the electric motors can be supplied by different speed variators.

[0039] The control units may each comprise a supervision device, in particular a supervision automaton controlling the electric motor(s), in particular via the speed variator(s), as a function of the pressure data at the input of the compression stage or the pressure data at the output of the corresponding compression stage.

[0040] The control unit of at least one of the compression stages may comprise at least one temperature sensor at the gas outlet and / or inlet of said compression stage. Preferably, the control unit of the compression stage having a temperature sensor is configured to slow down the rotation speed of the electric motor(s) of the corresponding compression stage when the temperature measured by the corresponding temperature sensor is higher than a predetermined threshold temperature.

[0041] Such a speed reduction as a function of temperature makes it possible to protect the system from overheating and therefore from degradation linked to an excessively high system temperature.

[0042] Preferably, the control units are configured so that the rotation speed of the electric motor(s) can take at least one intermediate speed value between the maximum speed and stopping the electric motor. Preferably, the control units are configured to vary the rotation speed of the electric motor(s) continuously between at least two rotation speed values, in particular between a maximum value and a minimum rotation value of the electric motor, in addition to the possibility of stopping the electric motor.

[0043] Preferably, the control units of the compression stages are independent of each other. By "independent of each other", it is understood that the control units do not communicate directly with each other other than by means of the pressure variations detected at the inlet and outlet of the compression stage to which they belong. Thus, in operation, each control unit regulates the speed of the electric motor of the corresponding compression stage from local data measured at said compression stage to maintain the inlet or outlet pressure as the case may be at a predefined setpoint pressure. According to a variant, the control units of the compression stages are connected to a central control unit which provides each control unit with the pressure setpoint curve depending in particular on the nature of the gas source and / or the device at the outlet of the system.

[0044] According to another variant, the control units are completely independent and are not connected to any central control unit. They regulate the gas pressure downstream or upstream according to a predefined pressure curve. Regulation by inlet pressure

[0045] When the control units of the first and second compression stages, and where appropriate of the additional compression stages, are configured to control the rotation speed of the corresponding electric motor as a function of the gas pressure at the inlet of the corresponding compression stage, the system preferably comprises sensors of the gas pressure at the inlet of the corresponding compression stage.

[0046] In this case, the control units may be configured to maintain the inlet pressure of each compression stage at a predefined inlet setpoint pressure. Preferably, the inlet setpoint pressure is maintained substantially constant over time. Preferably, the inlet setpoint pressures of the compression stages are substantially constant over time at optimal pressure values ​​for the operation of the corresponding compression stage.

[0047] Thus, having a unit for controlling the first compression stage as a function of the pressure at the inlet of the first compression stage makes it possible to detect directly a variation in the inlet pressure and, by varying the flow rate of the gas at the outlet of the first compression stage, to keep the pressure at the inlet of the first compression stage according to a predetermined curve, in particular substantially constant at an optimal inlet pressure value for the first compression stage, or for the gas source, even in the event of a variation in the inlet pressure generated by the gas source. This optimization of the inlet pressure is reflected, via the variation in the flow rate at the outlet of the first compression stage, on the second compression stage by a variation in the pressure at the inlet of the latter.The control unit of the second compression stage as a function of the inlet pressure of the second compression stage makes it possible, by varying the flow rate of the second compression stage, to bring the inlet pressure of the second compression stage according to the predetermined curve, in particular to an optimal inlet pressure value in view of the characteristics specific to the second compression stage. Where appropriate, the control unit(s) of the additional compression stage(s) operate in the same way, so that the inlet pressure optimization is reflected in all the compression stages. Thus, whatever the flow rate of the gas source at the inlet of the compression stages, the operation of the system can be optimized.

[0048] It is then possible to finely and locally regulate the pressure at the inlet of the first compression stage and / or the gas flow rate at the inlet of the first compression stage according to predetermined setpoint values ​​and to pass this regulation on to all the downstream compression stages by regulations of the compression stages which are each carried out locally at the level of each compression stage without any communication between the different control units being necessary.

[0049] Preferably, the gas source is an electrolyzer.

[0050] In this case, the second compression stage may include a de-energizing circuit gas rivation between the outlet of the first compression stage and the gas outlet of the second compression stage as described previously.

[0051] The second compression stage control unit may be configured to start the second compression stage and close the bypass circuit when the inlet pressure of the second compression stage is greater than the inlet set pressure for starting the second compression stage. The same may be true for each of the additional compression stages, if applicable.

[0052] Alternatively, the gas source is a compressed gas reservoir of finite volume, the first compression stage may comprise a gas bypass circuit between its gas inlet and the inlet of the second compression stage as described above and the control unit of the first compression stage may be configured to start the first compression stage and close the bypass circuit when the inlet pressure of the first compression stage is lower than the maximum inlet pressure set for the first compression stage. This can be done for each of the additional compression stages, if applicable. Regulation by outlet pressure

[0053] Preferably, when the compression stage control units are configured to control the rotation speed of the corresponding electric motor as a function of the gas pressure at the output of the corresponding compression stage, each compression stage comprises a gas pressure sensor at the output of said compression stage.

[0054] The control unit of the second compression stage is preferably configured to maintain the outlet pressure of the second compression stage at a predefined outlet setpoint pressure, preferably substantially constant over time at an optimal pressure value for the operation of the second compression stage or the gas consuming systems. Alternatively, the outlet setpoint pressure of the second compression stage may be variable. The outlet setpoint pressure may follow a predefined pressure curve over time.

[0055] The system can supply gas to an external system having a predetermined gas flow curve, the set pressure at the outlet of the second compression stage can follow a curve adapted to the gas flow curve of the external system.

[0056] Thus, having a system for controlling the second compression stage as a function of the outlet pressure of the second compression stage makes it possible to detect in real time a variation in the outlet pressure and, by regulating the rotation speed of the electric motor of the second compression stage, to keep the outlet pressure of the second compression stage according to the set pressure, in particular at an optimal outlet pressure value in view of the characteristics specific to the second compression stage, even in the event of a variation in the outlet pressure. This optimization of the outlet pressure is reflected, via the variation in the flow rate at the inlet of the second compression stage, on the first compression stage by a variation in the outlet pressure of the latter.The system for controlling the first compression stage as a function of the outlet pressure of the first compression stage makes it possible, by varying the flow rate of the first compression stage, to bring the outlet pressure of the first compression stage back to an optimal outlet pressure value in view of the characteristics specific to the first compression stage. Where appropriate, the control unit(s) of the additional compression stage(s) operate in the same way, so that the optimization of the outlet pressure of the last compression stage in the chain is reflected in all of the . upstream compression stages. Thus, regardless of the variation in gas pressure at the outlet of the compression stages, the operation of the system can be optimized.

[0057] It is then possible to finely and locally regulate the pressure at the outlet of the last compression stage of the chain and / or the gas flow rate at the outlet of the last compression stage of the chain according to predetermined setpoint values ​​and to pass on this regulation to all the upstream compression stages by regulation of the compression stages which are each carried out locally at the level of each compression stage without any communication between the different control units being necessary.

[0058] Preferably the gas outlet is connected to a gas reserve of finite volume, which must be brought to a certain set pressure.

[0059] Preferably, the gas source is an electrolyzer.

[0060] In this case, the second compression stage may include a de-energizing circuit gas rivation between the outlet of the first compression stage and the gas outlet of the second compression stage as described previously.

[0061] The second compression stage control unit may be configured to start the second compression stage and close the bypass circuit when the inlet pressure of the second compression stage is greater than the inlet set pressure for starting the second compression stage.

[0062] Alternatively, the gas source is a compressed gas reservoir of finite volume, the first compression stage may comprise a gas bypass circuit between its gas inlet and the inlet of the second compression stage as described previously and the control unit of the first compression stage may be configured to start the first compression stage and close the bypass circuit when the pressure at the outlet of the first compression stage is lower than the maximum set pressure of the first compression stage. Gas

[0063] Preferably, the gas is hydrogen. Process

[0064] The invention also meets this need using a gas compression method, in particular using a compression system as described above, comprising: - the compression of a gas by a first compression stage comprising a rotary electric motor, - compression by a second compression stage of the compressed gas delivered by the first compression stage, the second compression stage comprising a rotary electric motor, - controlling the rotation speed of the electric motor of the first compression stage as a function of at least the gas pressure at the inlet of the first compression stage and controlling the rotation speed of the electric motor of the second compression stage as a function of at least the gas pressure at the inlet of the second compression stage, or controlling the rotation speed of the electric motor of the first compression stage as a function of at least the gas pressure at the outlet of the first compression stage and controlling the rotation speed of the electric motor of the second compression stage as a function of at least the gas pressure at the outlet of the second compression stage.

[0065] All of the characteristics described previously in the context of the compression system apply to the compression method independently of the device.

[0066] Preferably, the control is in real time and continuous over time.

[0067] The control of the rotation speed of the electric motor of the first compression stage can be carried out as a function of the gas pressure at the outlet of the first compression stage and the control of the rotation speed of the electric motor of the second compression stage can be carried out as a function of the gas pressure at the outlet of the second compression stage so that the pressure at the gas outlet of the last compression stage of the chain is regulated to a set pressure, preferably substantially constant.

[0068] The control of the rotation speed of the electric motor of the first compression stage can be carried out as a function of the gas pressure at the inlet of the first compression stage and the control of the rotation speed of the electric motor of the second compression stage can be carried out as a function of the gas pressure at the inlet of the second compression stage so that the pressure at the gas inlet is regulated to a set pressure, preferably substantially constant.

[0069] The method may comprise compression by one or more compression stages, downstream of the second compression stage, arranged in series, each compression stage comprising a rotary electric motor, and the control of the rotation speed of the electric motor of the or each additional compression stage as a function of the gas pressure at the inlet of said additional compression stage when the first and second compression stages are regulated by the corresponding upstream pressure, or as a function of the gas pressure at the outlet of said compression stage when the first and second compression stages are regulated by the corresponding downstream pressure.

[0070] Preferably, the rotational speed of the electric motor(s) takes at least one intermediate speed value between the maximum speed and the stopping of the electric motor. Preferably, the rotational speed of the electric motor(s) may vary continuously between at least two rotational speed values, in particular between a maximum value and a minimum value, in addition to stopping the electric motor.

[0071] Preferably, the method comprises storing the gas compressed by the first compression stage in an intermediate gas reservoir between the two compression stages, said intermediate reservoir supplying gas to the second compression stage.

[0072] The method may comprise steps of compressing the gas by additional compression stages mounted in series and each comprising a rotary electric motor and the control of each additional compression stage by a unit for controlling the rotation speed of the corresponding electric motor as a function of the inlet gas pressure, when the control unit of the first and second compression stages controls the speed of the corresponding motor as a function of the inlet gas pressure of the first or second corresponding compression stages, or of the outlet gas pressure, when the control unit of the first and second compression stages controls the speed of the corresponding motor as a function of the outlet gas pressure of the first or second corresponding compression stages.

[0073] The method may include storing the compressed gas at the outlet in a closed tank or supplying a gas consumption device.

[0074] Preferably, the compression of the gas by the two compression stages comprises an alternating variation of the volume of a compression chamber of each compression stage.

[0075] Preferably, the rotation of the electric motor of each compression stage directly or indirectly generates the alternating variation of the volume of the compression chamber of each compression stage. Preferably, the compression comprises the movement in a cylinder of a piston connected to a crankshaft driven in rotation by the electric motor and the compression of the gas in the compression chamber by said piston. Preferably, the compression of the gas by said piston is carried out by compression of a liquid, in particular a hydraulic oil, separated from a compression chamber containing the gas by a deformable membrane, the deformation of the membrane generating a variation of the volume of the compression chamber.

[0076] The method comprises reducing the rotational speed of the electric motors when the inlet pressure of the compression stages decreases below a set inlet pressure. This makes it possible to increase the inlet gas pressure to a set inlet pressure for each compression stage.

[0077] The method comprises increasing the rotation speed of the electric motors when the pressure at the inlet of the compression stages increases beyond a set inlet pressure. This allows the inlet gas pressure to be reduced to a set inlet pressure for each compression stage.

[0078] The method comprises increasing the rotational speed of the electric motors when the output pressure of the compression stages decreases below a set output pressure. This makes it possible to increase the output gas pressure to a set output pressure for each compression stage.

[0079] The method comprises reducing the rotational speed of the electric motors when the pressure at the outlet of the compression stages increases beyond a set outlet pressure. This makes it possible to reduce the outlet gas pressure to a set outlet pressure for each compression stage.

[0080] The setpoint inlet or outlet pressures are predefined over time. They can be constant or follow a predefined curve.

[0081] The method comprises measuring the temperature at the compression outlet and reducing the speed of the engines when this temperature exceeds a predetermined threshold temperature.

[0082] The method may comprise supplying the first compression stage with gas by an electrolyzer or a reservoir of finite size.

[0083] When the gas source is an electrolyser, the method may comprise starting the second compression stage and closing a bypass circuit of the second compression stage when the inlet pressure of the second compression stage is greater than a set inlet pressure for starting the second compression stage.

[0084] When the source is a reservoir of finite size, the method may comprise starting the first compression stage and closing a bypass circuit of the first compression stage when the inlet pressure of the first compression stage is lower than a set inlet pressure for the first compression stage. It will be assumed here that, by design of the system, the second compression stage has been designed to withstand the pressures P2 experienced until the start of the first compression stage. Brief description of the drawings

[0085] The invention may be better understood by reading the detailed description which follows, non-limiting examples of its implementation, and by examining the appended drawings, in which:

[0086] [fig. 1] [fig.l] schematically represents an example of a compression system,

[0087] [fig.2] [fig.2] is a variant of a compression system,

[0088] [fig.3] [fig.3] is a variant of the compression system,

[0089] [fig.4] [fig.4] schematically represents an example of a compression stage,

[0090] [fig.5] [fig.5] is an example of a compressor, and

[0091] [fig.6] [fig.6] schematically represents a variant of the com system pressure. Detailed description

[0092] An example of a compression system 2 is illustrated in [fig.l]. The compression system 2 comprises a gas inlet 10 supplying a first compression stage 20 as an input, the compressed gas outlet of which supplies a second compression stage 30 as an input. The second compression stage 30 delivers the compressed gas into a gas outlet 15.

[0093] The gas inlet is supplied with gas by a gas source 40. The gas source 40 is preferably a gas production unit such as an electrolyzer. Alternatively, it may be a gas reservoir.

[0094] The gas outlet 15 can be connected to a compressed gas storage or consumption device 42, preferably a compressed gas storage device.

[0095] An intermediate gas reservoir 45 between the compression stages 20 and 30 facilitates the regulation of the pressure P2. The pressure P2 of the gas in the reservoir 45 depends on several parameters, in particular - the gas pressure at the outlet of the first compression stage 20 which itself depends on the gas pressure PI at the inlet of the first compression stage 20, - the gas flow rate at the outlet of the first compression stage 20 and - the gas flow rate at the inlet of the second compression stage 30. In particular, if the gas flow rate of the first compression stage 20 is greater than the gas flow rate of the second compression stage 30, the pressure P2 will increase and vice versa if the gas flow rate of the first compression stage 20 is less than the gas flow rate of the second compression stage 30. If the two gas flow rates are identical, the pressure P2 will depend solely on the pressure of the gas at the outlet of the first compression stage 20 and therefore on the pressure of the gas PI and the compression ratio.

[0096] The first and second compression stages 20 and 30 each comprise a compressor denoted respectively 25 and 35 having a rotary electric motor and a unit for controlling the rotation speed of the electric motor denoted respectively 27 and 37 which controls the rotation speed of the electric motor corresponding to a predetermined set speed denoted respectively Vie and V2c as a function of the upstream pressures PI and P2 of the corresponding compression stage 20 or 30.

[0097] The speed of the electric motors can be regulated by means of supervisory automatons (not shown) controlling speed variators (not shown) which supply current to the electric motors.

[0098] The first and second compression stages 20 and 30 are configured to each operate with an upstream pressure between a minimum pressure Plmin and P2min respectively and a maximum pressure Plmax and P2max respectively. Their operation is optimized in terms of energy consumption and wear for an optimal upstream pressure Plopt and P2opt respectively.

[0099] Preferably, as illustrated in [fig. 1], the second compression stage 30 comprises a gas bypass circuit 48 making it possible to bypass the second compression stage 30 and the first compression stage 20 comprises a gas bypass circuit 50 making it possible to bypass the first compression stage 20. Such bypass circuits 48 and 50 make it possible to avoid passing through the compressor(s) of the compression stages 20 and 30 under certain conditions of use which we will detail below, in particular during system start-up. The gas bypass circuit 48 connects the inlet of the second compression stage 30, in particular the reservoir 45, directly to the gas outlet. The gas bypass circuit 50 connects the inlet of the first compression stage 20 directly to the gas outlet of the first compression stage 20.Such branch circuits 48 and 50 each comprise an inlet valve, not shown, enabling their opening and closing to be controlled.

[0100] In the example illustrated in [fig.l], the system comprises a gas reservoir 45 between the two compression stages 20 and 30 which acts as a buffer between them. Such a reservoir 45 allows the system in particular to absorb the pulsations linked to the cyclic irregularities between the two compression stages 20 and 30 without damaging the latter. The reservoir 45 can be formed by a connecting conduit between the two compression stages. Its volume is defined according to the parameters specific to the installation, in particular the volumes of the compression chambers and / or the rotation speed ranges over which the electric motors can rotate.

[0101] When using the illustrated system with a gas production unit, in particular an electrolyser, at the system inlet and a gas storage tank 42 at the outlet, the system will first operate at a start-up speed at a predefined value. The gas source 40 and the system 2 are switched on and the bypass circuits 48 and 50 are opened until the pressure PI is equal to the operating pressure Plmin of the first compression stage. The control unit 27 will then switch on the first compression stage 20 and the corresponding bypass circuit 50 is closed. The bypass circuit 48 of the second compression stage remains open and the second compression stage 30 is switched off. The control unit 27 then runs the electric motor at low speed so that the output flow rate of the first compression stage is lower than the supply flow rate by the gas source 40.The PI pressure will then increase until it reaches a constant set pressure Peak, in particular the optimized pressure. Plopt. Once this pressure Pic is reached upstream of the first compression stage 20, the control unit will adapt the rotation speed of the engine to maintain this pressure Pic, that is to say so that the gas flow rate in the first compression stage is substantially equal to the gas flow rate of the gas source. In parallel, as long as the pressure P2 is lower than a predefined ignition pressure P2start of the second compression stage, in particular the pressure P2min or a pressure between P2min and P2max, the second compression stage 30 remains off and the bypass circuit 48 remains open. As soon as this pressure P2start is reached, the bypass circuit 48 is closed and the control unit 37 starts the electric motor of the second compression stage 30 at a reduced setpoint speed V2c so that the pressure P2 increases to a constant setpoint pressure P2c, in particular the pressure P2opt.The reduced setpoint speed corresponds to a gas flow rate in the second compression stage 30 lower than the gas flow rate of the first compression stage 20. When the pressure P2c is reached, the control unit increases the speed of the electric motor of the second compression stage 30 so that the pressure P2 is substantially constant at the pressure P2c, i.e. the gas flow rate of the second compression stage 30 is substantially equal to the gas flow rate of the first compression stage 20. When both compression stages 20 and 30 are operating and the pressures Pic and P2c are reached, the system switches to a routine operating regime in which the compression stages 20 and 30 are controlled so that the pressures PI and P2 upstream of the compression stages 20 and 30 are maintained at their predefined setpoint value continuously.

[0102] As a variant, the first compression stage does not have the bypass circuit 50 and as soon as the pressure Plmin is reached, the control unit 27 starts the electric motor of the first compression stage 20 at a reduced setpoint speed Vie so that the pressure PI increases up to a constant setpoint pressure Pic, in particular the pressure Plopt.

[0103] As a further variant, the second compression stage does not have the bypass circuit 48 and as soon as the pressure P2start is reached, the control unit 37 starts the electric motor of the second compression stage 30 at a reduced setpoint speed V2c so that the pressure P2 increases up to a constant setpoint pressure P2c, in particular the pressure P2opt.

[0104] When using the compression system in routine operating mode, the control unit of the first compression stage 27 measures the input pressure PI, in particular via an input pressure sensor not shown, and compares it to the predefined setpoint pressure Pic. It deduces therefrom a setpoint speed Vie to be applied to the electric motor to have the setpoint pressure Pic as input. Thus when the input pressure PI is lower than the setpoint pressure Pic, the control unit reduces the rotation speed of the electric motor of the compressor 25 and conversely, when it is greater than Pic, the control unit increases the rotation speed of the electric motor. The control unit of the second compression stage 37 measures the pressure P2 in the tank 45, in particular via a pressure sensor not shown, and compares it to the predetermined setpoint pressure P2c. It deduces therefrom a setpoint speed V2c to be applied to the electric motor of the second compression stage 30 to have the setpoint pressure P2c upstream of the latter. Thus when the inlet pressure P2 is lower than the setpoint pressure P2c, the control unit 37 reduces the rotation speed of the electric motor of the compressor 35 and conversely, when it is greater than P2c, the control unit increases the rotation speed of the electric motor of the compressor 35.

[0105] The pressures PI and P2 are then continuously regulated to the set pressures Pic and P2c respectively so that the system operates in an optimized manner and with low energy consumption. For example, the pressure Pic can be maintained at a pressure of 10 bars.

[0106] The setpoint pressures Pic and P2c may not be constant but may vary according to a predefined curve, in particular over time, depending on the characteristics of the system, in particular the gas source 40 and / or the technical characteristics of the compression stages 20 and 30, corresponding in particular to the most optimized operation of the start-up and / or the stop-down of the system. This may, for example, correspond to a fixed rotation speed of the compression motor(s) at the start-up of the system, for a duration ensuring its temperature rise, for example a few minutes, before switching to operation in variable mode at constant setpoint pressures Pic and P2c. The characteristics of the system may be preconfigured in the various control units.Alternatively, when starting the system, the user can select the source type on a user interface and the system then controls the compression stages according to a pre-recorded curve corresponding to the chosen source type.

[0107] In the case where the gas source is a finite reservoir, the inlet pressure of the first compression stage will decrease over time. The inlet pressure curve of the first compression stage cannot therefore be maintained at the pressure Plopt. The set pressures of the compression stages are therefore each controlled over time by an optimized predefined pressure curve in which, in particular, energy consumption is reduced. The system can be configured to stop when the inlet pressure PI reaches a threshold pressure, for example a pressure of 1 bar.

[0108] As illustrated in [fig.2], the system may include a temperature sensor temperature T upstream or downstream of the second compression stage 30. The control unit 37 can reduce the rotation speed of the electric motor when the temperature upstream or downstream exceeds a predetermined threshold temperature, in particular a critical temperature beyond which the system risks being damaged.

[0109] The system may include a gas cooling system not shown downstream of one or each compression stage.

[0110] The system may also comprise a sensor for the pressure P3 downstream of the second compression stage 30 and the control unit 37 may be configured to stop the electric motor when the pressure P3 reaches a predetermined pressure P3, in particular, in the case of a finite volume reservoir at the outlet of the system, when the pressure in the reservoir 42 is at the maximum outlet pressure P3max that the second compression stage 30 or the outlet reservoir 42 can withstand without being damaged, or when the desired final pressure value has been reached.

[0111] As illustrated in [fig.2], the control unit 27 can also regulate the speed of the electric motor of the first compression stage 20 as a function of the pressure P2 downstream of the first compression stage 20. This can in particular make it possible to stop the first compression stage 20 when the pressure P2 exceeds a maximum pressure P2max that the first compression stage 20 can withstand without being damaged, in particular after the second compression stage 30 has been stopped or in the event of failure of the second compression stage. This also makes it possible to avoid continuous operation of the first compression stage without it being able to compress the gas and therefore to avoid unnecessary energy consumption.

[0112] Alternatively, the system comprises a central control unit which stops the entire system when the pressure in the reservoir 42 has reached the desired value or when the pressure in the reservoir is at the maximum outlet pressure P3max which the second compression stage 30 or the outlet reservoir 42 can withstand without being damaged.

[0113] The control units 27 and 37 can also each be configured to stop the corresponding electric motor when the corresponding inlet pressure P1 and P2 respectively is less than or equal to a predefined minimum pressure P1min and P2min.

[0114] In the variant illustrated in [fig.3], the control units 27 and 37 control the rotation speed of the respective electric motors as a function of the downstream gas pressure P2 and P3 respectively of the first and second compression stages 20 and 30.

[0115] Such control of the compression stages 20 and 30 from downstream makes it possible in particular to adapt the behavior of the compression stages as a function of a variation in flow rate at the system outlet, or a storage pressure target in a tank 42.

[0116] When using the illustrated system with a gas tank at the inlet and a gas consumption device at the outlet, the control unit 37 controls the speed of the motor of the compressor 35 to have a downstream pressure P3 at a set pressure P3c and the control unit 27 controls the speed of the motor of the compressor 25 to have a pressure P2 at a set pressure P2c. A variation in the outlet flow rate will cause a variation in the pressure P3. The control unit 37 will then adapt the speed of the electric motor of the compressor 35 to bring the downstream pressure back to P3c. Consequently, the pressure P2 will also vary and the control unit 27 will bring the intermediate pressure back to P2c by adapting the speed of the motor of the compressor 25. Preferably, the starting speed is the same as described above.

[0117] When using the illustrated system with a gas production unit, in particular an electrolyser, at the inlet of the system and a gas storage tank 42 at the outlet, the system then regulates the operating regime as a function of the outlet pressures P2 and P3, within the limits of the production capacities of the electrolyser, and the desired outlet pressure P3 or a maximum admissible pressure P3.

[0118] In addition, the system as a whole and / or each of the compression stages can also be controlled to stop when the pressure PI, or respectively the upstream pressure for a given compression stage, falls below a low limit pressure for the source 40 or the compression stage considered. This limit pressure can be linked to the technical characteristics of the source, or to the low limit of operating pressure at the inlet of a given compression stage.

[0119] For transient operating phases of the system, for example during an initial start-up, pressure rise slopes of P2 and P3 over time can be predefined. If P2 does not follow the expected pressure rise slope, the control unit 27 increases the rotation speed of the compressor motor 25 within the limit of the gas flow rate that the electrolyser can provide. If P3 does not follow the expected pressure rise slope, the control unit 37 increases the rotation speed of the compressor motor 35.

[0120] The system may comprise more than two compression stages, each comprising an electric motor whose speed is controlled by a set speed depending on the upstream pressure or the measured downstream pressure. All the compression stages are controlled at least by the upstream or all the compression stages are controlled at least by the downstream.

[0121] Alternatively, as illustrated in [fig.4], the first compression stage 20 comprises a plurality of compressors 21a to 21c mounted in parallel with each other, that is to say whose inputs and outputs are respectively connected to the same input 10 and the same output 45. The control unit 27 controls the rotation speed of each of the electric motors according to the characteristics of each compressor 21a to 21c. Preferably, the compressors 21a to 21c are identical and the control unit 27 controls the different motors at the same set rotation speed Vie. Having a plurality of compressors in parallel makes it possible to increase the gas flow rate in the compression stage. The same may be true for the other compression stage(s) 30.

[0122] The compressors 25 and 35 are for example as illustrated in Figures 5.

[0123] In [fig.5], the compressor shown is a membrane compressor comprising a rotary electric motor 60 driving in rotation a crankshaft 62 connected by a connecting rod / crank system 64 to a piston 66.

[0124] The piston 66 is driven by the crankshaft 62 in translation in a cylinder 68 forming the lower part of a reservoir 70. The reservoir 70 comprises two cavities 72 and 74 separated from each other in a sealed manner by a deformable membrane 76. The first cavity 72 contains a liquid, in particular a hydraulic oil and the second cavity 74 forms a compression chamber intended to receive the gas to be compressed by means of the deformable membrane 76.

[0125] An inlet valve 78 allows gas to enter the cavity 74 and the compressed gas is discharged via a discharge valve 80.

[0126] When the engine 60 rotates the crankshaft 62, the piston 66 is driven in translation in the cylinder between a delivery position and an intake position.

[0127] When moving the piston 66 from the top dead center position to the bottom dead center, the system is in the intake phase.

[0128] When moving the piston 66 from the bottom dead center position to the top dead center, the system is in the compression phase.

[0129] During the compression phase, the liquid in the first cavity 72 is compressed, which causes the membrane 76 to deform towards the second cavity 74 so as to reduce the volume of the latter, which compresses the gas contained in the compression chamber formed by the second cavity 74. When the system is in the compression phase, the pressure in the chamber is such that the discharge valve 80 opens and the compressed gas is expelled from the chamber by the latter. When the piston 66 descends from the top dead center to the bottom dead center, the membrane 76 deforms in the direction of the first cavity 72, which causes the volume of the second cavity 74 to increase. The gas is then sucked into the compression chamber by the intake valve 78. When the piston is at bottom dead center, the volume of the second cavity 74 is maximum.

[0130] As a variant, the compressor(s) 25 and / or 35 may comprise a plurality of compression chambers as described previously, the compression of which is generated by the movement of several pistons 66. The corresponding pistons 66 are all connected to the same crankshaft 62. The pistons 66 may have movements in the cylinders 66 out of phase with each other.

[0131] Other compressor technologies can be used within the scope of the invention, for example drawer compressors.

[0132] Notably, the invention also covers the operation of chains of compression stages comprising more than two compression stages and all sharing a principle of regulation at least either by the pressure upstream of the compression stages or alternatively at least by the pressure downstream of the compression stages. These compression stages are not necessarily close to each other, or even not necessarily co-located on the same geographical site. This has the advantage of limiting and / or avoiding the need for a control link between them or with a central control unit, which may be subject to communication problems, or even simple distance problems for its establishment.

[0133] For example, as illustrated in [fig.6], the system may comprise at least one additional compression stage 100 downstream of the second compression stage 30 having the same characteristics as the first and second compression stages 20 and 30, that is to say comprising a control unit 107 regulating the speed of the electric motor Vaddc of a compressor 105 by the upstream pressure Pamont to maintain the upstream pressure Pamont at a set value Pamontc when the first and second compression stages are also regulated by the upstream pressure, as illustrated, or regulated in rotation speed of the electric motor 105 by the downstream pressure Pavai to maintain the downstream pressure Pavai at a set value Pavalc when the first and second compression stages are also regulated by the downstream pressure (not shown).

[0134] Such an additional compression stage may also include a bypass circuit 110 operating like the bypass circuits 50 and 48.

[0135] The invention is not limited to the exemplary embodiments which have just been described. For example, the compressors may be different from those described. The compression stages may comprise a plurality of different compressors whose motors are controlled at different set speeds, for example in connection with their differences in technical characteristics.

Claims

Claims

1. Gas compression system (2) comprising: - a gas inlet (10), - at least one first compression stage (20) supplied by the gas inlet (10) with gas and delivering a compressed gas, - at least one second compression stage (30) supplied by the first compression stage (20) with compressed gas and delivering a compressed gas into the gas outlet (15), the first and second compression stages (20, 30) each comprising - a rotary electric motor (60) configured to compress the gas, - a unit for controlling the rotation speed of the electric motor (27, 37), the control unit of the first compression stage (27) being configured to control the rotation speed of the corresponding electric motor (60) at least as a function of the gas pressure PI at the inlet of the first compression stage (20) and the control unit of the second compression stage (37) being configured to control the rotation speed of the corresponding electric motor (60) at least as a function of the gas pressure P2 at the inlet of the second compression stage (30), or the control unit of the first compression stage (27) being configured to control the rotation speed of the corresponding electric motor (60) at least as a function of the gas pressure P2 at the outlet of the first compression stage (20) and the control unit of the second compression stage (37) being configured to control the rotation speed of the corresponding electric motor at least as a function of the gas pressure P3 at the outlet of the second compression stage (30).

2. System according to claim 1, comprising one or more additional compression stages connected in series downstream of the second compression stage, the additional compression stage(s) each comprising: - a rotary electric motor configured to compress the gas from the compression stage arranged upstream of said additional compression stage, - a unit for controlling the rotational speed of the electric motor as a function at least of the inlet gas pressure, when the control unit of the first and second compression stages controls the speed of the corresponding motor as a function at least of the inlet gas pressure of the corresponding first or second compression stages, or as a function at least of the outlet gas pressure, when the control unit of the first and second compression stages controls the speed of the corresponding motor as a function at least of the outlet gas pressure of the corresponding first or second compression stages.

3. System according to claim 1 or 2, wherein the first and / or second compression stages (20, 30) and / or the additional compression stage(s) if applicable comprise one or more parallel-connected compressors (25, 35).

4. System according to any one of the preceding claims, in which the control units (27, 37) regulate the speed of the corresponding electric motors (60) in real time and continuously over time.

5. System according to any one of the preceding claims, in which the control units (27, 37) are configured so that the rotation speed of the electric motor (60) can take at least one intermediate speed value between the maximum speed and the stopping of the electric motor, in particular to vary the rotation speed of the electric motor (60) continuously between at least two rotation speed values, in particular between a maximum value and a minimum value or the stopping of the electric motor.

6. System according to any one of the preceding claims, in which the control unit of the first compression stage (27) is configured to control the rotation speed of the corresponding electric motor (60) only as a function of the gas pressure PI at the input of the first compression stage (20), the control unit of the second compression stage (37) is configured to control the rotation speed of the corresponding electric motor (60) only as a function of the gas pressure P2 at the input of the second compression stage (30), where appropriate the control unit of the or each additional compression stage (100) is configured to control the rotation speed of the corresponding electric motor (60) only depending on the gas pressure Pamonten input of said additional compression stage, or the control unit of the first compression stage (27) is configured to control the rotation speed of the corresponding electric motor (60) only depending on the gas pressure P2 at the output of the first compression stage (20) and the control unit of the second compression stage (37) is configured to control the rotation speed of the corresponding electric motor only depending on the gas pressure P3 at the output of the second compression stage (30), where appropriate the control unit of the or each additional compression stage (100) is configured to control the rotation speed of the corresponding electric motor (60) only depending on the gas pressure Pavaien output of said additional compression stage.

7. System according to any one of the preceding claims, in which the control unit of the first and / or second compression stage comprises a sensor of the temperature at the outlet of said first and / or second compression stage, the control unit of said first and / or second compression stage being configured to slow down the rotation speed of the electric motor of said first and / or second compression stage when the temperature measured by the corresponding temperature sensor is higher than a predetermined threshold temperature.

8. System according to any one of the preceding claims, in which the control units of the first and second compression stages and, where appropriate, of the additional compression stage(s) are independent of each other.

9. System according to any one of the preceding claims, comprising a gas reservoir (45) at least between two consecutive compression stages, in particular between the first and second compression stages (20, 30).

10. System according to any one of the preceding claims, comprising at the outlet (42) a closed gas storage tank or a device for consuming the gas at the outlet.

11. System according to claim 10, comprising a sensor of the pressure at the gas outlet and a central control unit configured to stop the system when the outlet pressure is greater than or equal to a predefined final outlet pressure.

12. System according to any one of the preceding claims, comprising at the inlet a gas source (40), in particular an electrolyzer supplying gas to the gas inlet or a finite volume gas reservoir.

13. The system of claim 12, comprising a pressure sensor at the gas inlet and a central control unit configured to stop the system when the inlet pressure is less than or equal to a predefined minimum inlet pressure.

14. System according to any one of the preceding claims, in which the control unit of the first compression stage (27) is configured to control the rotation speed of the corresponding electric motor (60) at least as a function of the gas pressure PI at the inlet of the first compression stage (20) and the control unit of the second compression stage (37) is configured to control the rotation speed of the corresponding electric motor (60) at least as a function of the gas pressure P2 at the inlet of the second compression stage (30), the control units (27, 37) each being configured to maintain the pressure at the inlet of the first and second compression stages (20, 30) corresponding to a corresponding predefined setpoint pressure variable or constant over time, preferably constant over time.

15. System according to any one of the preceding claims, in which the control unit of the first compression stage (27) is configured to control the rotation speed of the corresponding electric motor (60) at least as a function of the gas pressure P2 at the outlet of the first compression stage (20) and the control unit of the second compression stage (37) is configured to control the rotation speed of the corresponding electric motor at least as a function of the gas pressure P3 at the outlet of the second compression stage (30), the control units of the first and second compression stages (20, 30) each being configured to maintain the pressure at the outlet of the corresponding compression stage at a corresponding predefined setpoint pressure variable or constant over time, preferably constant over time.

16. A method of compressing gas using a compression system according to any one of the preceding claims comprising: - compressing a gas by a first compression stage (20) comprising a rotary electric motor (60), - compression by a second compression stage (30) of the compressed gas delivered by the first compression stage (20), the second compression stage (30) comprising a rotary electric motor (60), - control of the rotation speed of the electric motor of the first compression stage as a function at least of the inlet gas pressure PI of the first compression stage (20) and control of the rotation speed of the electric motor of the second compression stage (30) as a function at least of the inlet gas pressure P2 of the second compression stage, or control of the rotation speed of the electric motor of the first compression stage (20) as a function at least of the outlet gas pressure P2 of the first compression stage (20) and control of the rotation speed of the electric motor of the second compression stage (30) as a function at least of the outlet gas pressure P3 of the second compression stage (30).

17. Method according to claim 16, in which the control of the rotation speed of the electric motor of the first compression stage is carried out as a function of the gas pressure at the outlet of the first compression stage and the control of the rotation speed of the electric motor of the second compression stage is carried out as a function of the gas pressure at the outlet of the second compression stage so that the pressure at the gas outlet is regulated to a set pressure, preferably substantially constant.

18. Method according to claim 16, in which the control of the rotation speed of the electric motor of the first compression stage is carried out as a function of the gas pressure at the inlet of the first compression stage and the control of the rotation speed of the electric motor of the second compression stage is carried out as a function of the gas pressure at the inlet of the second compression stage so that the pressure at the gas inlet is regulated to a set pressure, preferably substantially constant.

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