Method of compressing gas and gas compressor system

Abstract

A method of compressing gas comprises delivering pump fluid 138 to a first chamber 104 of a plurality of chambers to pressurise gas stored in the chamber. In an operation, pumping fluid is withdrawn f

Description

The present disclosure is directed towards a method of compressing gas, and a gas compressor system. The gas is compressed to pressurise the gas to a higher pressure such as for delivery to a receiver vessel. BACKGROUND Various gas compressors are known to compress gas received from a relatively low pressure delivery storage tank for delivery to a receiver vessel at a higher pressure. One application of gas compressors is in the compression of hydrogen gas. Hydrogen gas can be produced in a variety of ways including steam reforming of natural gas, partial oxidation of methane, coal gasification, biomass gasification, methane pyrolysis with carbon capture, and electrolysis of water by way of example. The hydrogen gas is produced at a relatively low pressure which is typically in the range of 5 bar to 30 bar. The hydrogen gas is required to be compressed to a higher pressure prior to transport, storage, or delivery to an end hydrogen consumer. The compressed hydrogen gas may be deployed at a fuelling station for fuelling hydrogen vehicles. Some existing methods for compressing hydrogen gas typically employ gaseous compressors and intercoolers to deliver hydrogen gas. These methods are relatively inefficient, expensive to manufacture and display heating issues in operation. International Patent Application Publication No. WO2023094711 A1 discloses a hydrogen gas compressor system that comprises a hydrogen gas storage unit that defines an internal volume for storing hydrogen gas. An operating fluid delivery means such as a pump delivers an operating fluid to the hydrogen gas storage unit. This causes the pressure of the hydrogen gas contained within the hydrogen gas storage unit to increase. A coolant fluid delivery means such as a pump delivers a coolant fluid to the hydrogen gas storage unit to absorb heat from the hydrogen gas. There is a need to improve the energy efficiency of gas compression. SUMMARY There is provided a method of compressing gas and gas compressor system as set out in the accompanying claims. Other features of the invention will be apparent from the dependent claims, and the description which follows. According to a first aspect of the disclosure, there is provided a method of compressing gas. The method comprises delivering pumping fluid to a first chamber of a plurality of chambers to pressurise gas stored in the first chamber. The method comprises withdrawing pumping fluid from the first chamber; and using the pressure of the pumping fluid withdrawn from the first chamber to facilitate delivery of pumping fluid to a second chamber of the plurality of chambers to pressurise the gas stored in the second chamber. Advantageously, pumping fluid is delivered to the first chamber to pressurise gas stored in the first chamber for delivery to a receiver vessel such as a storage tank or another chamber of the plurality of chambers. An operation is then performed which comprises an expansion stage where fluid is withdrawn from the first chamber and a compression stage where fluid is delivered to a second chamber to pressurise the gas stored therein so that it can be delivered to a receiver vessel such as a storage tank or another chamber of the plurality of chambers. The pumping fluid, and in particular, the pressure of the pumping fluid, withdrawn from the first chamber is used to facilitate delivery of the pumping fluid to the second chamber. The pressure of the withdrawn pumping fluid is higher than atmospheric pressure, is typically higher than the pressure of the gas from the source of gas. Initially, the pressure of the pumping fluid within the chamber is at or close to pressure of a receiver vessel that receives gas from the chambers but will reduce as pumping fluid is withdrawn from the chamber. The pressure of the pumping fluid is used to reduce the external energy input required to deliver pumping fluid to the second chamber. This means that energy is recovered from the withdrawn pumping fluid thereby reducing the requirement of external energy input to deliver pumping fluid to the compression stage chamber. The second chamber may be a different chamber to the first chamber or the same chamber as the first chamber. The pumping fluid withdrawn from the first chamber may be used to facilitate delivery of pumping fluid to more than one chamber including the second chamber. The pumping fluid withdrawn from the first chamber may be used to facilitate delivery of pumping fluid to the plurality of chambers including the first chamber and the second chamber. The operation may be a first operation. After the first operation, the method may comprise performing a second operation comprising: withdrawing pumping fluid from the second chamber; and using the pressure of the pumping fluid withdrawn from the second chamber to facilitate delivery of pumping fluid to the first chamber to pressurise the gas in the first chamber such as for delivery to the receiver vessel. Advantageously, each operation comprises an expansion stage where fluid is withdrawn from one of the first and second chambers and a compression stage where fluid is delivered to the other of the first and second chambers to pressurise the gas stored therein so that it can be delivered to the receiver vessel. The pumping fluid withdrawn from one of the first and second chambers is used to facilitate delivery of the pumping fluid to the other of the first and second chambers. This means that energy is recovered from the withdrawn pumping fluid thereby reducing the requirement of external energy input to deliver pumping fluid to the compression stage chamber. The method may further comprise repeating, in sequence, the first operation and the second operation. Advantageously, a plurality of cycles of the first operation and second operation may be performed such as until a desired amount of gas is delivered to the receiver vessel. In the first operation, pumping fluid may not be withdrawn from the second chamber. Likewise, in the second operation, pumping fluid may not be withdrawn from the first chamber. In the first operation, pumping fluid may not be delivered to the first chamber. Likewise, in the second operation, pumping fluid may not be delivered to the second chamber. Using the pumping fluid withdrawn from the first chamber to facilitate delivery of pumping fluid to the second chamber may comprise delivering the withdrawn pumping fluid to second chamber. Advantageously, the withdrawn pumping fluid is delivered to the second chamber to pressurise gas in the second chamber. This directly recovers energy from the withdrawn pumping fluid to pressurise the gas in the second chamber. As such, less energy from an external source is required to pressurise the gas in the second chamber. Delivering the withdrawn pumping fluid to the second chamber may comprise delivering the withdrawn pumping fluid to the second chamber via a fluid pump. The method may further comprise operating the fluid pump to deliver pumping fluid to the second chamber. During the process of delivering pumping fluid to the second chamber, the fluid pump may be operated to continuously deliver pumping fluid to the second chamber until a maximum level of pumping fluid is delivered to the second chamber. Delivering the pumping fluid withdrawn from the first chamber may comprise delivering the withdrawn pumping fluid to the second chamber via a first fluid path that bypasses the fluid pump, and delivering the withdrawn pumping fluid, via a second fluid path, to the fluid pump. The fluid may be delivered via the second fluid path in response to the pressure of the pumping fluid withdrawn from the first chamber being insufficient for delivery to the second chamber. The fluid may be delivered by the first fluid path in response to the pressure of the pumping fluid withdrawn from the first chamber being too high for safe operation of the fluid pump. Advantageously, the fluid pump may be bypassed while the withdrawn pumping fluid has sufficient pressure for delivery to the second chamber. Delivering pumping fluid to the second chamber cause the pressure within the second chamber to increase. Once the pressure within the second chamber is equal to the pressure of the withdrawn pumping fluid, the fluid pump can be operated to continue the delivery of pumping fluid to the second chamber. This arrangement reduces the amount of work required to be performed by the fluid pump. The operation may further comprise fluidly connecting a fluid outlet of the first chamber to the fluid inlet of the fluid pump to allow the withdrawn pumping fluid to flow to the fluid inlet of the fluid pump. Fluidly connecting may comprise controlling a valve associated with the fluid outlet to allow fluid flow out of the first chamber via the fluid outlet. The operation may further comprise disconnecting a fluid inlet of the first chamber from fluid communication with the fluid inlet of the fluid pump such that pumping fluid is unable to flow to the first chamber. Disconnecting the fluid inlet of the first chamber from fluid communication with the fluid inlet of the fluid pump may comprise controlling a valve associated with the fluid inlet to prevent fluid flow into the first chamber via the fluid inlet. The withdrawn pumping fluid may flow to the inlet of the fluid pump via a fluid reservoir. The withdrawn pumping fluid may flow to the inlet of the fluid pump via an energy convertor that recovers energy from the withdrawn pumping fluid for driving a fluid pump. The method may comprise operating the energy convertor to recover energy from the withdrawn pumping fluid. The method may comprise operating the energy convertor to drive the fluid pump using the recovered energy. It will be appreciated that the energy convertor may be unable to recover sufficient energy to drive the fluid pump independently. Power from an external source may still be required. However, the demand on external power is reduced as a result of the aspects of the present disclosure as energy is recovered from the withdrawn pumping fluid. Using the pressure of the pumping fluid withdrawn from the first chamber to facilitate delivery of pumping fluid to the second chamber may comprise delivering withdrawn pumping fluid to an energy convertor to recover energy from the withdrawn pumping fluid for driving a fluid pump, and operating the fluid pump using the recovered energy to deliver pumping fluid to the second chamber. The energy convertor may comprise a fluid turbine operatively coupled to the fluid pump for driving the fluid pump. The energy convertor may be mechanically coupled to the fluid pump. The energy convertor may be electrically coupled to the fluid pump. Using the pressure of the pumping fluid withdrawn from the first chamber may comprise delivering the pumping fluid withdrawn from the first chamber to a first portion of a pressure convertor so as to drive a force transmitting means to pressurise pumping fluid in a second portion of the pressure convertor for delivery to the second chamber. The first portion may have a smaller volume than the second portion. The first portion may have a smaller effective surface area for pressure / force transmission. The pressure convertor may convert the pressure of the withdrawn pumping fluid in the first portion of the pressure convertor into a lower pressure in the second portion of the pressure convertor. Advantageously, the pressure convertor captures energy from the withdrawn pumping fluid in an effective way particularly when the pressure of the withdrawn pumping fluid is greater than a target compression pressure for the second chamber. The method may further comprise operating a fluid pump to deliver pumping fluid to the pressure convertor so as to further drive the force transmitting means to pressurise pumping fluid in the second portion of the pressure convertor for delivery to the second chamber. The fluid pump may be operated in response to the pressure of the withdrawn pumping fluid being insufficient to cause pumping fluid in the second portion of the pressure convertor to be delivered to the second chamber. Withdrawing pumping fluid from the first chamber may cause a valve associated with a gas inlet of the first chamber to allow the first chamber to receive gas. Withdrawing pumping fluid from the second chamber may cause a valve associated with a gas inlet of the second chamber to open to allow the second chamber to receive gas. Delivering pumping fluid to the second chamber may cause a valve associated with a gas outlet of the second chamber to open to allow gas to flow from the second chamber to a receiver vessel. Delivering pumping fluid to the first chamber may cause a valve associated with a gas outlet of the first chamber to open to allow gas to flow from the first chamber to a receiver vessel. The first chamber may be the same as the second chamber. The first chamber may be different to the second chamber. Delivering pumping fluid to the first chamber of the plurality of chambers may comprise delivering pumping fluid to the first chamber and one or more other chambers of the plurality of chambers. The pumping fluid may be delivered to the plurality of chambers. Withdrawing pumping fluid from the first chamber may comprise withdrawing pumping fluid from the first chamber and one or more other chambers of the plurality of chambers. The pumping fluid may be withdrawn from the plurality of chambers. The method may further comprise connecting the first chamber to a source of gas. Withdrawing pumping fluid from the first chamber may allow gas to flow to the first chamber from the source of gas. Delivering pumping fluid to the first chamber may comprise delivering pumping fluid to the first chamber and the other chambers of the plurality of chambers. Withdrawing pumping fluid from the first chamber may comprise withdrawing pumping fluid from the first chamber and the other chambers of the plurality of chambers. Using the pressure of the pumping fluid withdrawn from the first chamber to facilitate delivery of pumping fluid to the second chamber may comprise using the pressure of the pumping fluid withdrawn from the first chamber and the other chambers of the plurality of chambers to facilitate delivery of pumping fluid to the second chamber and the other chambers of the plurality of chambers. Advantageously, pumping fluid is delivered to all of the plurality of chambers to pressurise gas therein. The pumping fluid is then withdrawn. The pressure of the withdrawn pumping fluid is used to facilitate a subsequent delivery of pumping fluid to the plurality of chambers. This reduces the external energy input for the subsequent delivery of pumping fluid to the plurality of chambers. The plurality of chambers may comprise the first chamber, the second chamber, and a third chamber. The method may further comprise connecting a gas outlet of the first chamber to a gas inlet of the second chamber, and connecting a gas outlet of the second chamber to a gas inlet of the third chamber. Delivering pumping fluid to the first chamber may comprise delivering pumping fluid sequentially to the first chamber, second chamber, and third chamber. Advantageously, the first, second, and third chambers are sequentially connected via their gas inlets / outlets such that gas delivered to a gas inlet of the first chamber from a source of gas allows gas to be delivered to the first chamber, second chamber, and third chamber and for the pressure to equalize across the chambers. Pumping fluid may be delivered to the first chamber which increases the pressure of the gas in the first chamber and allows gas to be transferred from the first chamber to the second chamber. Once the first chamber has had a maximum level of pumping fluid delivered, pumping fluid is then delivered to the second chamber which increases the pressure of the gas in the second chamber and allows gas to be transferred from the second chamber to the third chamber. Once the second chamber has reached a maximum level of pumping fluid delivered, pumping fluid is then delivered to the third chamber which increases the pressure of the gas in the third chamber and allows gas to be transferred from the third chamber to a receiver vessel which may be another chamber of the plurality of vessels or another receiver vessel such as a storage tank associated with an end consumer. Once the third chamber has received a maximum level of pumping fluid, the pumping fluid may be withdrawn from the first, second, and third chambers, and the pressure from the withdrawn pumping fluid is used to reduce the energy input required to deliver pumping fluid to the first, second, and third chambers in a subsequent operation. In this example, the pressure rises across all three of the chambers, then across the second and third chambers, and finally the third chamber on its own. This arrangement allows work input to be reduced by removing the need to input work to transfer gas from one chamber to the next. This improves overall efficiency. The plurality of chambers may comprise the first chamber, the second chamber, and a third chamber. The method may further comprises connecting a gas outlet of the first chamber to a gas inlet of the second chamber, and connecting a gas outlet of the second chamber to a gas inlet of the third chamber. The method may further comprise withdrawing pumping fluid from the second chamber and delivering the pumping fluid withdrawn from the second chamber to the third chamber. Withdrawing pumping fluid from the second chamber may be performed in response to delivering pumping fluid to the first chamber. Pressuring the first chamber allows gas to flow from the first chamber to the second chamber. The gas displaces pumping fluid from the second chamber, and the displaced pumping fluid is allowed to be delivered to the third chamber so as to pressure gas in the third chamber. In this example, a fluid pump may only be used to deliver pumping fluid to the first chamber. This arrangement enables gas to be transferred from the first chamber to the third chamber. More generally, this enables gas to be transferred across a plurality of stages where each stage may comprise one or more chambers. The plurality of chambers may be a first plurality of chambers. The method may further comprise connecting a second plurality of chambers to the first plurality of chambers such that the second plurality of chambers are able to receive a flow of gas from the first plurality of chambers. The method may further comprise delivering pumping fluid to a first chamber of the second plurality of chambers to pressurise gas stored in the first chamber. The method may further comprise performing an operation comprising: withdrawing pumping fluid from the first chamber of the second plurality of chambers; and using the pumping fluid withdrawn from the first chamber to facilitate delivery of pumping fluid to the second chamber of the second plurality of chambers to pressurise the gas in the second chamber such as for delivery to a receiver vessel. The operation may be a first operation. The method may comprise, after the first operation, performing a second operation comprising: withdrawing pumping fluid from the second chamber of the second plurality of chambers; and using the pumping fluid withdrawn from the second chamber to facilitate delivery of pumping fluid to the first chamber of the second plurality of chambers to pressurise the gas in the first chamber such as for delivery to the receiver vessel. The method may further comprise delivering a coolant fluid to the chambers to absorb heat from the gas. The coolant fluid and pumping fluid may be delivered to the chambers simultaneously. According to a second aspect of the disclosure, there is provided a gas compressor system. The gas compressor system comprises a plurality of chambers. The gas compressor system comprises a pumping system comprising a fluid pump. The gas compressor system comprises a controller. The controller is configured to control the fluid pump to deliver pumping fluid to a first chamber of the plurality of chambers to pressurise gas stored in the first chamber. The controller is configured to control the system to perform an operation comprising: withdrawing pumping fluid from the first chamber; and using the pressure of the pumping fluid withdrawn from the first chamber to facilitate delivery of pumping fluid to a second chamber of the plurality of chambers to pressurise the gas in the second chamber such as for delivery to the receiver vessel. The operation may be a first operation. The controller may be further configured to, after the first operation, control the system to perform a second operation comprising: withdrawing pumping fluid from the second chamber; and using the pumping fluid withdrawn from the second chamber to facilitate delivery of pumping fluid to the first chamber to pressurise the gas in the first chamber such as for delivery to the receiver vessel. The controller being configured to control the system to perform the first operation may comprise the controller being configured to fluidly connect a fluid outlet of the first chamber to a fluid inlet of a fluid pump, and fluidly connect a fluid inlet of the second chamber to a fluid outlet of the fluid pump. The controller may be configured to operate the fluid pump to deliver pumping fluid to the second chamber. The controller being configured to control the system to perform the second operation may comprise the controller being configured to fluidly connect a fluid outlet of the second chamber to a fluid inlet of a fluid pump, and fluidly connect a fluid inlet of the first chamber to the fluid outlet of the fluid pump. The controller may be configured to operate the fluid pump to deliver pumping fluid to the first chamber. The pumping system may comprise a fluid reservoir fluidly connected to a fluid inlet of the fluid pump. The fluid reservoir may be arranged such that the withdrawn pumping fluid flows to the fluid reservoir prior to delivery to the fluid inlet of the fluid pump. The pumping system may comprise an energy convertor arranged to recover energy from the withdrawn pumping fluid to drive the fluid pump. The energy convertor may comprise a fluid turbine operatively connected to the fluid pump for driving the fluid pump. The energy convertor may be mechanically coupled to the fluid pump. The mechanical coupling may be a fluid coupling. The fluid coupling may be provided by a torque convertor. The energy convertor may comprise a rotatable mechanical linkage arranged to be driven by the fluid turbine and mechanically coupled to the fluid pump for driving the fluid pump. The energy convertor may comprise a generator arranged to be driven by the fluid turbine so as to generate electrical energy for driving the fluid pump. The plurality of chambers may be a first plurality of chambers. The gas compressor system may comprise a second plurality of chambers. The second plurality of chambers may be arranged to receive a flow of gas from the first plurality of chambers. The second plurality of chambers may be arranged to deliver pressurised gas to a receiver vessel. The source of gas may be another chamber / plurality of chambers. The receiver vessel may be another chamber / plurality of chambers. The gas compressor system may further comprise a coolant fluid delivery means arranged to deliver a coolant fluid to the gas storage unit to absorb heat from the gas. The gas storage unit may comprise a coolant fluid circuit via which the coolant fluid may flow through the hydrogen storage unit. The coolant fluid may traverse through the first chamber and / or second chamber. BRIEF DESCRIPTION OF DRAWINGS Figures 1 to 4 show simplified schematic diagrams for an example gas compressor system according to aspects of the present disclosure. Figures 5A and 5B shows simplified schematic diagrams of example pumping fluid delivery arrangements for example gas compressor systems according to aspects of the present disclosure. Figure 6 shows a simplified schematic diagram of another example gas compressor system according to aspects of the present disclosure. Figure 7 shows a simplified schematic diagram of another example gas compressor system according to aspects of the present disclosure. Figure 8 shows a schematic diagram for an example control system for controlling a gas compressor system according to aspects of the present disclosure. Figure 9A-9C show example pressure-volume curves for a chamber of a gas storage unit according to aspects of the present disclosure. Figure 10 shows a flow diagram of an example method of compressing gas according to aspects of the present disclosure. Figure 11 shows a simplified schematic diagram of an example pumping fluid delivery arrangement for example gas compressor systems according to aspects of the present disclosure. Figure 12 shows an example gas compressor system according to aspects of the present disclosure. DETAILED DESCRIPTION The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of various embodiments of the disclosure as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the various embodiments described herein can be made without departing from the scope and spirit of the disclosure. In addition, descriptions of well-known functions and constructions may be omitted for clarity and conciseness. The terms and words used in the following description and claims are not limited to the bibliographical meanings, but are merely used by the inventor to enable a clear and consistent understanding of the disclosure. Accordingly, it should be apparent to those skilled in the art that the following description of various embodiments of the disclosure is provided for illustration purpose only and not for the purpose of limiting the disclosure as defined by the appended claims and their equivalents. It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Referring to Figures 1 to 4, there is shown an example gas compressor system 100. Figure 1 shows the overall system 100. Figures 2 to 4 show aspects of the system 100 so as to describe the operations performed by the system. For clarity, in Figures 2 to 4, only the components relevant to understand the operation of the gas compressor system 100 are shown. The example gas compressor system 100 described below is particularly suitable for compression of hydrogen gas. However, the present disclosure is not limited to hydrogen gas and other gases, as may be desired by the skilled person, can be used with the gas compressor system 100. The gas compressor system 100 comprises a gas storage unit 102 that comprises a plurality of chambers including a first chamber 104 and a second chamber 106. The chambers 104, 106 may be in the form of cylinders but are not required to be cylinders. The chambers 104, 106 may be vertically aligned along their axis. The plurality of chambers 104, 106 can have additional chambers to the first and second chambers 104, 106. The first chamber 104 and second chamber 106 each comprise a gas inlet 108, 110 which is connected to a source of gas 112 such that the first chamber 104 and second chamber 106 can receive a flow of gas from the source of gas 112. In this example, the first chamber 104 and second chamber 106 are both directly connected to the source of gas 112, but this is not required in all examples. The source of gas 112 in this example is a storage tank (or a plurality of storage tanks) such as a fixed or mobile storage tank which stores gas at a relatively low pressure and, in particular, at a lower pressure than required by a receiver vessel 117. The storage tank(s) typically have a far larger volume than the receiver vessel 117, first chamber 104, or second chamber 106. The gas inlets 108, 110 are each associated with a valve 114, 116 to allow for the selective delivery of gas to the first chamber 104 and the second chamber 106 from the source of gas 112. In some examples, the valves 114, 116 comprise one-way valves 114, 116 that open to allow gas to flow into the first chamber 104 and second chamber 106 when the pressure within the chambers 104, 106 is less than the delivery pressure of the gas from the source of gas 112. The valves 114, 116 are not required to comprise one-way valves in all examples. The valves 114,116 may be control valves that are selectively opened / closed by operation of a controller 802 (Figure 8) of the gas compressor system 100. The first chamber 104 and second chamber 106 each comprise a gas outlet 118, 120 via which gas may be delivered from the gas storage unit 102. The gas outlets 118, 120 are each associated with a valve 122, 124 to allow for the selective delivery of gas from the first chamber 104, 106. The gas outlets 118, 120 in this example are connected to a receiver vessel 117 that receives the gas from the gas storage unit 102. In some examples, the valves 122, 124 comprise one-way valves that open to allow gas to flow out of the first chamber 104 and second chamber 106 when the pressure within the chambers 104, 106 is greater than the pressure of the receiver vessel 117 connected to the gas outlets 118,120. The valves 122, 124 are not required to comprise one-way valves in all examples. The valves 122, 124 may be control valves that are selectively opened / closed by operation of a controller 802 (Figure 8) of the gas compressor system 100. The receiver vessel 117 may be a storage tank (or tanks). The receiver vessel 117 may be a mobile storage tank. The receiver vessel 117 may be an end consumer (e.g., a vehicle tank) of the gas or may deliver the compressed gas to an end consumer. The first chamber 104 and second chamber 106 each comprise a fluid inlet 128, 130 via which a pumping fluid may be delivered to the first chamber 104 and second chamber 106. The pumping fluid is delivered to the first chamber 104 and second chamber 106 so as to decrease the available volume in the first chamber 104 and the second chamber 106 to thereby cause the gas in the first chamber 104 and second chamber 106 to compress and the pressure of the gas to increase. The fluid inlets 128, 130 are each associated with a valve 129, 131 to allow for the selective delivery of pumping fluid to the first chamber 104, 106 and the second chamber 104, 106 via the fluid inlets 128, 130. The valves 129, 131 are independently controllable (by the controller 802) such that pumping fluid can be delivered to both of the chambers 104, 106 at the same time or only to one of the chambers 104, 106 at a time. In some examples, one-way valves are also associated with the fluid inlets 128, 130. The present disclosure is not limited to any particular pumping fluid. Typically, the pumping fluid is selected based on the gas being compressed. In applications where the gas is hydrogen, the pumping fluid is typically water or an ionic fluid. The first chamber 104 and second chamber 106 each comprise a fluid outlet 132, 134 via which the pumping fluid may be withdrawn from the first chamber 104 and the second chamber 106. The fluid outlets 132, 134 are each associated with a valve 182, 184 to allow for the selective withdrawal of fluid from the first chamber 104 and the second chamber 106 via the fluid outlets 132, 134. The valves 182, 184 are independently controllable (by the controller 802) such that fluid can be withdrawn from one of the chambers 104, 106 at a time. The pumping fluid is delivered to the base of each of the first chamber 104 and 106. As pumping fluid is delivered to the chambers 104, 106, the level of the pumping fluid in each of the chamber 104, 106 rises to decrease the available volume for gas within the chamber 104, 106. The pumping fluid acts as a liquid piston. The first chamber 104 and second chamber 106 may have the same volumes and be arranged to receive the same volume of pumping fluid. This is not required in all examples and the chambers 104, 106 may have different volumes and / or receive different volumes of pumping fluid. The gas outlets 118, 120 are positioned towards the top of the chambers 104, 106. The fluid inlets 128, 130 are typically positioned towards the base of the chambers 104, 106. However, the present disclosure is not restricted to having the fluid inlets 128, 130 positioned at the base of the chambers 104, 106 and the fluid inlets 128, 130 may be positioned anywhere on the chambers 104, 106, such as the top of, or near the top of the chambers 104, 106 for example. The fluid outlets 132,134 are positioned towards the base of the chambers 104, 106. Pumping fluid is delivered via the fluid inlets 128, 130 by a pumping system 136. The pumping system 136 comprises a main fluid reservoir 138 that stores pumping fluid at a low pressure The main fluid reservoir 138 may store pumping fluid in an unpressurised state (i.e. at atmospheric pressure) or may pressurise the pumping fluid to be at the same pressure as the gas flowing from the source of gas 112. In some examples, the main fluid reservoir 138 is connected to the source of gas 112 such that gas can be delivered to the main fluid reservoir 138 to pressurise the pumping fluid stored therein. A control valve 150 is provided to selectively control whether gas is delivered to the main fluid reservoir 138 from the source of gas. The pumping system 136 further comprises a feed pump 140 that receives pumping fluid from the fluid reservoir 138 and delivers pumping fluid to a buffer fluid reservoir 144 of the pumping system 136. The buffer reservoir 144 stores pumping fluid at a higher pressure than the main fluid reservoir 138 such as a pressure at or just below the pressure of gas output from the gas storage unit 102 to the receiver vessel 117. The buffer fluid reservoir 144 temporarily stores pumping fluid prior to delivery to the first chamber 104 and second chamber 106. The pumping fluid may be pressurised by the feed pump 140 prior to delivery to the buffer fluid reservoir 144. The buffer fluid reservoir 144 is not required in all examples, but is beneficial to temporarily store pumping fluid withdrawn from one of the chambers 104, 106 prior to delivery to the other of the chambers 104, 106. The feed pump 140 is not required in all examples. The pumping system 138 further comprises a fluid pump 146 that receives fluid from the buffer reservoir 144 and delivers pressurised pumping fluid to the chambers 104, 106 via the fluid inlets 128, 130. The operation of the feed pump 140 and fluid pump 144 is controlled by the controller 802 (Figure 8) of the gas compressor system 100. The feed pump 140 and fluid pump 146 may be any suitable pump for delivering pumping fluid such as a centrifugal pump or a positive displacement pump. In an example implementation, a control valve 148 arranged along the gas flow path between the source of gas 112 and the chambers 104, 106 is opened, by the controller 802, to allow gas to flow to the first chamber 104 and the second chamber 106. The gas flows into the first chamber 104 and second chamber 106 until pressure equilibrium is reached with the source of gas 112. The control valve 149 arranged between the gas outlets 118, 120 and the receiver vessel 117 is also opened and, in this example, remain open throughout the operations described below so that gas may flow from the chambers 104, 106 to the receiver vessel 117 when the gas pressure in the chambers 104, 106 is greater than the pressure of the receiver vessel 117. The control valve 149 may comprise a plurality of valves connected to a plurality of tanks of a receiver vessel 117 and / or a plurality of receiver vessels 117. The tanks / receiver vessels are preferably at different pressures. As shown in Figure 2, the fluid pump 146 is controlled, by controller 802, to deliver pumping fluid to the first chamber 104 via the fluid inlet 128. The pumping fluid is fed to the fluid pump 146 from the fluid reservoir 138 via the feed pump 140 and buffer reservoir 144. The valve 129 between the fluid inlet 128 and the fluid pump 146 is opened, by the controller 802, so that fluid may flow from the fluid pump 146 into the first chamber 104 via the fluid inlet 128. The control valve 150 may also be opened, by the controller 802, to allow gas to flow to the fluid reservoir 138 to pressurise the pumping fluid in the fluid reservoir 138. This helps reduce the amount of energy required to pump the fluid. The fluid inlet 130 of the second chamber 106 is not fluidly connected to the fluid pump 146. This typically means that the valve 131 associated with the fluid inlet 130 of the second chamber 106 is closed by the controller 802. In this way, fluid is not delivered to the second chamber 106 at this time. The gas compressor system 100 then performs a first operation and second operation in sequence to deliver gas to the receiver vessel 117. The sequence of the first operation and second operation is repeated a number of times such as until a desired volume of gas is delivered to the receiver vessel 117. During the performance of the first operation and second operation, the control valve 149 arranged in the fluid path between the gas outlets 118, 120 and the receiver vessel 117 is open to allow gas to flow to the receiver vessel 117 In the first operation as shown in Figure 3, pumping fluid is withdrawn from the first chamber 104. This causes gas within the clearance volume of the first chamber 104 to expand and reduce in pressure. Once the pressure drops to below the delivery gas pressure, gas flows into the first chamber 104 from the source of gas 112. This fills the first chamber 104 with gas for subsequent delivery to the receiver vessel 117. The pressure of the pumping fluid withdrawn from the first chamber 104 is used to facilitate delivery of pumping fluid to the second chamber 106. This means that energy is recovered from the withdrawn pumping fluid so as to reduce the external energy input required to deliver pumping fluid to the second chamber 106. In this example, the withdrawn pumping fluid is delivered to the second chamber 106. The pumping fluid may be delivered to the second chamber 106 via the fluid pump 146. The withdrawn pumping fluid may flow to the fluid pump 146 bypassing the fluid reservoir 138. The pumping fluid may flow directly to the fluid pump 146 or may flow to the buffer reservoir 144 prior to delivery to the fluid pump 146. In some examples, any excess pumping fluid withdrawn from the first chamber 104 may also flow to the fluid reservoir 138. Valves may be provided to direct the flow of the withdrawn pumping fluid appropriately to either the buffer reservoir 144 I fluid pump 146 or fluid reservoir 138. It will be appreciated that gas flows into the first chamber 104 as the pumping fluid is withdrawn and the pressure in the first chamber 104 drops below the pressure of the gas in the source of gas 112. In the first operation, the withdrawn pumping fluid is delivered to the second chamber 106 either directly, via the fluid pump 146, or a combination of direct delivery and delivery via the fluid pump 146.The withdrawn pumping fluid is delivered to the second chamber 106 to compress the gas stored in the second chamber 106. Once the gas pressure within the second chamber 106 is greater than the pressure of the receiver vessel 117, gas flows from the second chamber 106 to the receiver vessel 117. In some examples, the withdrawn pumping fluid may initially flow to the second chamber 106 while the fluid pump 146 is not operated. Once the pressure of the withdrawn pumping fluid is insufficient to further compress the gas, then the fluid pump 146 is operated, by the controller 802, to deliver pumping fluid (withdrawn pumping fluid and, if needed, additional pumping fluid from the main fluid reservoir 136) to the second chamber 106. An example of this arrangement is shown in Figure 5B. In other examples, the fluid pump 146 is operated throughout the process of delivering fluid to the second chamber 106. An example of this arrangement is shown in Figure 5A. In both examples, pumping fluid forms a liquid piston that decreases the available volume for gas in the second chamber 106 and thus increasing the pressure of the gas for delivery to the receiver vessel 117. As the withdrawn pumping fluid from the first chamber 104, which is at a high pressure, is delivered to the second chamber 106, less work is required to be performed by the fluid pump 146 to pressurise the second chamber 106. In this way, energy is recovered from the withdrawn pumping fluid from the first chamber 104 and the energy required to pressurise the second chamber 106 is reduced. This delivery of pumping fluid to the second chamber 106 is performed until the pumping fluid level in the second chamber 106 reaches a maximum level. The maximum level may be detected by a level sensor that feeds data back to the controller 802 (Figure 8). The maximum level may be set by the system so as to ensure, for example, the pumping fluid does not reach the top of the second chamber 106 and overflow. Pressurised gas remains in the second chamber 106 in a clearance volume between the liquid piston formed by the pumping fluid and the gas outlet 120. Once the pumping fluid has reached its maximum level in the second chamber 106, gas is no longer able to flow to the receiver vessel 117 and the remaining gas is trapped in the clearance volume. In the first operation, the first chamber 104 acts as an expansion chamber and the second chamber 106 acts as a compression chamber. In the first operation, the fluid inlet 128 of the first chamber 104 is not fluidly connected to the fluid pump 146. This typically means that a valve 129 associated with the fluid inlet 128 of the first chamber 104 is closed by the controller 802. In this way, pumping fluid is not delivered to the first chamber 104 in the first operation. In the first operation, the fluid outlet 134 of the second chamber 106 is not fluidly connected to the fluid pump 146. This typically means that a valve 184 associated with the fluid outlet 134 is closed by the controller 802. In this way, pumping fluid is not withdrawn from the second chamber 106. In the second operation as shown in Figure 4, pumping fluid is withdrawn from the second chamber 106. This allows gas within the clearance volume of the second chamber 106 to expand and reduce in pressure. Once the pressure drops to below the delivery gas pressure, gas flows into the second chamber 106 from the source of gas 112. This fills the second chamber 106 with gas for delivery to the receiver vessel 117. The pumping fluid withdrawn from the second chamber 106 flows to the first chamber 104. The pumping fluid flows to the fluid pump 146 bypassing the main fluid reservoir 138. The pumping fluid may flow directly to the fluid pump 146 or may flow to the buffer fluid reservoir 144 prior to delivery to the fluid pump 146. Any excess pumping fluid withdrawn from the second chamber 106 may also flow to the main fluid reservoir 138. It will be appreciated that gas flows into the second chamber 106 as the pumping fluid is withdrawn and the pressure in the second chamber 106 drops below the pressure of the gas in the source of gas 112. In the second operation, the withdrawn pumping fluid is delivered to the first chamber 104 directly, via the fluid pump 146, or via a combination of direct delivery and via the fluid pump 146. The withdrawn pumping fluid is delivered to the first chamber 104 to compress the gas stored in the first chamber 104. Once the gas pressure within the first chamber 104 is greater than the pressure of the receiver vessel 117, gas flows from the first chamber 104 to the receiver vessel 117. In some examples, the withdrawn pumping fluid may initially flow to the first chamber 104 while the fluid pump 146 is not operated. Once the pressure of the withdrawn pumping fluid is insufficient to further compress the gas, then the fluid pump 146 is operated, by the controller 802, to deliver pumping fluid (withdrawn pumping fluid and, if needed, additional pumping fluid from the main fluid reservoir 136) to the first chamber 104. In other examples, the fluid pump 146 is operated throughout the process of delivering fluid to the first chamber 104. In both examples, pumping fluid forms a liquid piston that decreases the available volume for gas in the first chamber 104 and thus increases the pressure of the gas for delivery to the receiver vessel 117. As the withdrawn pumping fluid from the second chamber 106, which is at a high pressure, is delivered to the first chamber 104, less work is required to be performed by the fluid pump 146 to pressurise the first chamber 104. In this way, energy is recovered from the withdrawn pumping fluid from the second chamber 106 and the energy required to pressurise the first chamber 104 is reduced. This delivery of pumping fluid to the first chamber 104 is performed until the pumping fluid level in the first chamber 104 reaches a maximum level. The maximum level may be detected by a level sensor that feeds data back to the controller 802 (Figure 8). The maximum level may be set by the system so as to ensure, for example, the pumping fluid does not reach the top of first chamber 104 and overflow. Pressurised gas remains in the first chamber 104 in a clearance volume between the liquid piston formed by the pumping fluid and the gas outlet 118. Once the pumping fluid has reached its maximum level in the first chamber 104, gas is no longer able to flow to the receiver vessel 117 and the remaining gas is trapped in the clearance volume. In the second operation, the second chamber 106 acts as an expansion chamber and the first chamber 104 acts as a compression chamber. In the second operation, the fluid inlet 130 of the second chamber 106 is not fluidly connected to the fluid pump 146. This typically means that the valve 131 associated with the fluid inlet 130 of the second chamber 106 is closed by the controller 802. In this way, pumping fluid is not delivered to the second chamber 106 in the second operation. In the second operation, the fluid outlet 132 of the first chamber 104 is not fluidly connected to the fluid pump 146. This typically means that a valve (not shown) associated with the fluid outlet 132 is closed by the controller 802. In this way, pumping fluid is not withdrawn from the first chamber 104. After completion of the second operation, the first operation is then performed again followed by the second operation. A plurality of cycles can therefore be performed where each cycle comprises, in sequence, the first operation and the second operation. In each cycle, energy is recovered from one of the chambers which acts as an expanding chamber for use in pressurising gas in the other chamber which acts as a compression chamber. It will be appreciated that as the receiver vessel 117 fills with gas, the pressure of the receiver vessel 117 may increase. During the initial stages of filling the receiver vessel, pressure will be lower and flow to the receiver vessel 117 will start sooner with a period of constant pressure delivery from the chamber 104, 106. As the pressure rises in the receiver vessel, the compression stage will be longer as the gas needs to reach a higher pressure in the chamber 104, 106 before it can flow to the receiver vessel 117. Thus, over time, when filling the receiver vessel 117 the gas pressure required for delivery to the receiver vessel 117 will increase and the duration of delivery of gas during the first operation or second duration will decrease. As the gas pressure required for delivery to the receiver vessel 117 rises, the pressure of the withdrawn pumping fluid rises, and thus a greater amount of energy can be recovered from the withdrawn pumping fluid to compensate, at least partially, for the increase in gas compression required. It is not required that the gas pressure within the receiver vessel 117 increases. Gas may be withdrawn from the receiver vessel 117 during the performance of first / second operations by an end consumer. In addition, the receiver vessel 117 may comprise a plurality of vessels operating at one or more pressures. In some examples, the receiver vessel 117 may implement a liquid piston similar to the first chamber 104 I second chamber 106 to maintain the receiver vessel 117 at a substantially constant pressure by withdrawing pumping fluid to increase the available volume for gas storage within the receiver vessel 117. The controller 802 (Figure 8) controls the operation of the gas compressor system 100, and in particular controls components such as control valves 129, 131, 148, 149, 150 to selectively permit or prevent flow of fluid I gas therethrough, as well as the operation of the fluid pump 146, and the feed pump 140. To maximise the energy recovery, the controller 802 controls the timing and sequence of the control valves, and in particular the valves 129, 131 which control whether a chamber 104, 106 is fluidly connected to the fluid pump 146. The controller 802 receives input from sensors indicating the operating point within the compression process and within the current compression cycle. This can include input pressures from the source of gas 112 (e.g., pressure of the storage tank) and pipework via which the gas flows to / from the gas storage unit 102. The inputs can include inputs from the chambers 104, 106 indicating their current point in the pumping cycle. The inputs can include inputs from flow sensors and the control valves 129, 131, 148, 149, 150, 182, 184. The system determines the current compression status and schedules the sequencing of the pumps 140, 146 and valves 129, 131, 148, 149, 150, 182, 184 to complete the process. This can include determining the operating point of the gas transfer process, the operating point within the cycle for each chamber 104,106, controlling the pumping rate for the process, controlling the recovery rate to match the pumping rate for optimised operation, and adjusting the sequence as required to control the compression process and the switching between chambers 104, 106. The system 100 may optionally determine the best compression rate and gas flow rate for optimised rate and energy efficiency. In the above example, the withdrawn pumping fluid from one of the chambers 104, 106 is delivered to another of the chambers 104, 106. Figures 5A and 5B show two examples of how the withdrawn pumping fluid may be delivered in such a way that the pressure of the withdrawn pumping fluid facilitates delivery of the pumping fluid and reduces the external energy input. In the example of Figure 5A, the first operation is shown where pumping fluid is withdrawn from the first chamber 104 for delivery to the second chamber 106. The pumping fluid withdrawn from the first chamber 104 flows to valve 156. The valve 156 is controlled to fluidly connect the withdrawn pumping fluid from the first chamber 104 to the fluid pump 146. The pumping fluid flows to the fluid pump 146 via pressure reducing valve 158 which acts to provide the fluid pump 146 with a consistent supply pressure. The fluid pump 146 delivers pumping fluid to the second chamber 106 via the valve 160 which is controlled to fluidly connect the outlet of the fluid pump 146 to the second chamber 106. Pumping fluid with excess pressure bypasses the fluid pump 146 and flows to the valve 160 via a pump bypass line I first fluid path 161 which comprises a control valve 162 and one-way valve 164. In this example, the pump 146 is operating throughout the process of delivering pumping fluid to the second chamber 106 to supply pumping fluid to the second chamber 106. Initially, when the pressure in the second chamber 106 is less than the pressure of the pumping fluid withdrawn from the first chamber 104, pumping fluid is also delivered via the first fluid path 161. Once the pressure within the second chamber 106 equalises with the pressure of the withdrawn pumping fluid, the fluid pump 146 becomes the sole means of delivering pumping fluid to the second chamber 106 to increase the pressure therein. It will be appreciated that in the first operation, the fluid outlet of the second chamber 106 is fluidly disconnected from the fluid pump 146. This means that pumping fluid is not able to flow from the second chamber 106 to the inlet of the fluid pump via control valve 156. In the second operation, pumping fluid withdrawn from the second chamber 106 will flow to the fluid pump 146 via valve 156 and pressure-relief valve 158 for delivery to the first chamber 104. Pumping fluid will also flow via the bypass line I first fluid path 161 until pressure equalisation occurs. In the example of Figure 5B, the first operation is shown where pumping fluid is withdrawn from the first chamber 104 for delivery to the second chamber 106. The pumping fluid withdrawn from the first chamber 104 flows to valve 156. The valve 156 is controlled to fluidly connect the withdrawn pumping fluid from the first chamber 104 to the fluid pump 146. Pumping fluid 146 flows from the fluid pump 146 to valve 160 which is controlled to fluidly connect the outlet of the fluid pump 146 to the second chamber 106. In this example, a pump by-pass line I first fluid path 161 is also provided which provides a route for pumping fluid to flow from the first chamber 104 to the second chamber 106 without flowing through the fluid pump 146. The pump by-pass line 161 comprises a valve 166 which is controlled to fluidly connect the fluid outlet of the first chamber 104 to the fluid inlet of the second chamber 106. In this example, pumping fluid flows from the first chamber 104 to the second chamber 106 via the pump by-pass line I first fluid path 161 until the withdrawn pumping fluid pressure is equal to the pressure within the second chamber 106. The fluid pump 146 is not operated while fluid is able to flow to the second chamber 106 via the first fluid path 161. Once the pressure is equalised, the fluid pump 146 is operated to pressurise pumping fluid for delivery to the second chamber 106. In the second operation, pumping fluid withdrawn from the second chamber 106 will flow to the first chamber 104 initially via first fluid path 161. Once pressure is equalised, the fluid pump 146 is operated to pressurise pumping fluid for delivery to the first chamber 104. Referring to Figure 6, there is shown another example gas compressor system 100a and method performed by the gas compressor system 100a. The gas compressor system 100a in this example comprises all of the features of the gas compressor system 100 of Figures 1 to 4 and may use the pumping fluid delivery arrangements shown in Figures 5A and 5B For simplicity, shared features between the gas compressor system 100a and the gas compressor system 100 are given the same reference numerals. A detailed description of the shared features is omitted from the description of the gas compressor system 100a. The gas compressor system 100a performs similar operations to the gas compressor system 100. The chambers 104, 106 are brought into fluid communication with the source of gas 112 to allow gas to flow into the first chamber 104 and second chamber 105. The fluid pump 146 pumps pumping fluid to the first chamber 104 via the fluid inlet 128. This corresponds to the description provided for Figure 2. The gas compressor system 100a then performs repeated cycles of the first operation and second operation as described above in relation to Figures 3 and 4. In contrast to the gas compressor system 100 of Figures 3 and 4, the pumping system 136 further comprises an energy convertor 154 which is used to drive the fluid pump 146. The energy convertor 154 comprises a fluid turbine 154. Pumping fluid withdrawn from the first chamber 104 or second chamber 106 flows to the fluid pump 146 via the fluid turbine 154. The withdrawn pumping fluid drives the fluid turbine 154 which in turn drives the fluid pump 146. The fluid turbine 154 may be mechanically connected to the fluid pump 146 such as via a rotatable linkage (e.g., a connecting shaft) of the energy convertor 154 to mechanically drive the fluid pump 146. A fluid coupling such as a mechanical torque convertor may be used. The fluid turbine 154 may be electrically connected to the fluid pump 146. The energy convertor 154 in this example comprises a generator which is driven by the fluid turbine 154 to generate electricity for powering the fluid pump 146. In this way, energy is recovered from the withdrawn pumping fluid to reduce the amount of external power (e.g., mains power) required to power the fluid pump 146. In this example, the withdrawn pumping fluid still flows to the inlet of the fluid pump 146 after flowing through the fluid turbine 154. In some examples, the pressure of the pumping fluid after it has flown through the fluid turbine 154 is at or close to atmospheric pressure. In other examples, the pressure of the pumping fluid after it has flown through the fluid turbine 154 may be at or around a maximum inlet pressure for the fluid pump 146. It will be appreciated that fluid turbine 154 extracts work from the pumping fluid causing the pumping fluid to drop in pressure, and the amount of work extracted to be controlled such that the pumping fluid output from the fluid turbine 154 is at a desired pressure for delivery to the fluid pump 146. Figure 6 shows the first operation of the pumping cycle performed by the gas compressor 100a. The first chamber 104 is in the expansion stage such that pumping fluid is being withdrawn from the first chamber 104 and flows to the fluid turbine 154 to drive the fluid turbine 154. The fluid turbine 154 is connected, mechanically or electrically as described above, to the fluid pump 146 to drive the fluid pump. The fluid pump 146 delivers pumping fluid to the second chamber 130 to pressurise the second chamber 106. The withdrawn pumping fluid flows from the fluid turbine to the fluid pump 146. The withdrawn pumping fluid may flow via the buffer fluid reservoir 144. Excess withdrawn pumping fluid may also flow to the main fluid reservoir 138. It will be appreciated that in the second operation is similar to the first operation described in relation to Figure 6, but the first chamber 104 is in the compression stage and the second chamber 106 is in the expansion stage. Either of the gas compressor systems 100, 100a as described above may comprise multiple gas storage units 102 arranged to form several pumping stages. Each gas storage unit 102 operates as described above. A first gas storage unit 102 may form a first pumping stage of the gas compressor system 100, 100a. The first gas storage unit 102 receives gas from the source of gas 112 and compresses the gas for delivery to a second gas storage unit 102. The second gas storage unit 102 receives gas from the first gas storage unit 102 and compresses the gas either to delivery to the receiver vessel 117 or a third gas storage unit 102. Further gas storage units may also be provided to provide additional pumping stages. The source of gas 112 and receiver vessel 117 described above in relation to Figures 1 to 6 may be gas storage units 102. That is, the gas storage unit 102 described in Figures 1 to 6, may be provided between two other gas storage units 102 as part of a multi-pumping stage gas compressor system 100, 100a. In an example, the delivery gas pressure from the source of gas 112 may be in the region of 10 bar. The gas storage unit 102 may be arranged to pressurise the gas to increase the pressure to over 100 bar, optionally to over 350 bar, and optionally over 650 bar. The present disclosure is not limited to any particular pressure values. While not required in all examples, the gas compressor system 100, 100a may further comprise a heat exchanger (not shown). The heat exchanger may be integrated with the gas storage unit 102 and comprises a coolant fluid circuit via which coolant fluid may flow through the gas storage unit 102 and extract heat from the gas. The coolant fluid may be water or other form of liquid coolant. The gas compressor system 100, 100a may further comprise a coolant fluid delivery means (e.g., coolant pump) for delivering the coolant fluid to the gas storage unit 102 via the coolant fluid inlet. A coolant fluid reservoir may also be provided for storing coolant fluid. In operation, the fluid pump 146 is controlled to deliver pumping fluid into a chamber 104, 106 so as to decrease the available volume for gas 104, 106 within the chamber 104, 106. This causes the pressure of the gas stored within the chamber 104, 106 to increase. The compression of the gas causes the temperature of the gas to increase. To combat this, the coolant fluid delivery means is controlled to deliver coolant fluid to the coolant fluid inlet. The coolant fluid flows through the coolant fluid circuit to absorb heat from the gas. Referring to Figure 7, there is shown another example gas compressor system 100b according to aspects of the present disclosure. Like the examples of Figure 6, the gas compressor system 100b uses the pressure of withdrawn pumping fluid to facilitate delivery of pumping fluid. In this example, the gas storage unit 102 comprises the first chamber 104, the second chamber 106, and a third chamber 108. The present disclosure is not limited to gas storage units 102 with two or three chambers and any number, N, of chambers may be provided where N is two or more. The gas inlet 108 of the first chamber 104 is connected to the source of gas 112. The control valve 148 selectively controls whether the gas inlet 108 is fluidly connected to the source of gas 112. The gas outlet 118 of the first chamber 104 is connected to the gas inlet 110 of the second chamber 106. A control valve 170 is provided to selectively control whether the gas outlet 118 of the first chamber 104 is in fluid communication with the gas inlet 110 of the second chamber 106. The gas outlet 120 of the second chamber 106 is connected to the gas inlet 172 of the third chamber 168. A control valve 174 is provided to selectively control whether the gas outlet 120 of the second chamber 106 is in fluid communication with the gas inlet 172 of the second chamber 106. The gas outlet 178 of the third chamber 168 is connected to the receiver vessel 117. The control valve 149 selectively controls whether the gas outlet 178 is fluidly connected to the receiver vessel 117. One way valves 114, 122, 116, 124, 176, 180 are associated with the gas inlets 108, 110, 172 and gas outlets 118, 120, 178 of each of the chambers 104, 106, 168. Unlike in the gas compressor systems 100 and 100a as described above, the chambers 104, 106, 168 are not all directly connected to the source of gas 112. Instead, only the first chamber 104 is directly connected to the source of gas 112 in this example. The second chamber 106 is indirectly connected to the source of gas 112 via the first chamber 104 and the third chamber 168 is indirectly connected to the source of gas 112 via the second chamber 106. In this example, the outlet of the fluid pump 146 is connected to the fluid inlets 128, 130, 186 of each of the chambers 104, 106, 168. Control valves 129, 131, 118 are associated with the fluid inlets 128, 130, 186 and are provided to selectively control whether the fluid inlets 128, 130, 186 are in fluid communication with the outlet of the fluid pump 146. One-way valves are also provided and associated with each fluid inlet 128, 130, 186. The inlet of the fluid pump 146 is connected to the fluid outlets 132, 134, 190 of each of the chambers 104, 106, 168. Control valves 182, 184, 192 are provided to selectively control whether the fluid outlets 132, 134, 190 are in fluid communication with the fluid pump 146. One-way valves are also provided and associated with each fluid outlet 132, 134, 190. The fluid buffer reservoir 144 is provided and pumping fluid withdrawn from any of the chambers 106, 108, 168 flows to the buffer reservoir 144 prior to delivery to the fluid pump 146. Other components of the pumping system 136 such as the fluid reservoir 138 and the feed pump 140 may be provided but are not shown in this Figure. In an example implementation, the control valves 148, 170, 174 are opened to allow gas to flow from the source of gas 112 into each of the chambers 104, 106, 168 via their gas inlets 108, 110, 172. The valves 182, 184, 192 are also opened to allow residual pumping fluid to be drained from the chambers 104, 106, 168. The residual pumping fluid flows to fluid buffer 144 such that it can be subsequently delivered to the chambers 104, 106, 168 such as via the fluid pump 146. This allows for energy to be recovered from the withdrawn pumping fluid. The valves 148, 182, 184, 192 are then closed to prevent further gas delivery to the gas storage unit 102a, and prevent pumping fluid (if present) from being withdrawn from the chambers 104, 106, 108. The pressure is equalized across all of the chambers 104, 106, 168. Valve 129 is opened and the fluid pump 146 is operated to deliver pumping fluid to the first chamber 104 via the fluid inlet 128. This compresses the gas in the first chamber 104, second chamber 106 and third chambers 168 as valves 170 and 174 are open. The delivery of the pumping fluid may utilise the pressure of the pumping fluid previously withdrawn from the chambers 104, 106, 168. The delivery of pumping fluid to the first chamber 104 continues until the pumping fluid has reached a maximum level within the first chamber 104. This means that the majority of the gas originally present in the first chamber 104 has been transferred to the second chamber 106 and third chamber 168 and minimal gas remains in the first chamber 104. The valve 170 is then closed to fluidly disconnect the gas outlet 118 of the first chamber 104 from the gas inlet 110 of the second chamber 106. The valve 129 is closed to fluidly disconnect the fluid pump 146 from the fluid inlet 128 of the first chamber 104. The valve 131 is opened to fluidly connect the fluid pump 146 to the fluid inlet 130 of the second chamber 106. The fluid pump 146 is operated to deliver pumping fluid to the second chamber 106 via the fluid inlet 130. The delivery of pumping fluid to the second chamber 106 continues until the pumping fluid has reached a maximum level within the second chamber 106. This means that the majority of the gas originally present in the second chamber 106 has been transferred to the third chamber 168 and minimal gas remains in the second chamber 106. The valve 174 is then closed to fluidly disconnect the gas outlet 120 of the second chamber 106 from the gas inlet 172 of the third chamber 168. The valve 131 is closed to fluidly disconnect the fluid pump 146 from the fluid inlet 130 of the second chamber 106. The valve 188 is opened to fluidly connect the fluid pump 146 to the fluid inlet 186 of the third chamber 168. The fluid pump 146 is operated to deliver pumping fluid to the third chamber 168 via the fluid inlet 186. The gas in the third chamber 168 is therefore pressurised for delivery to the receiver vessel 117. After the pumping fluid has reached a maximum level within the third chamber 168, the valves 148, 170, 174 may be opened and the pumping fluid is withdrawn from the chambers 104, 106, 168 to allow the chambers 104, 106, 168 to fill with gas. The withdrawn pumping fluid is stored in fluid buffer 144 such that it can be subsequently delivered to the chambers 104, 106, 168 such as via the fluid pump 146. This allows for energy to be recovered from the withdrawn pumping fluid. This approach utilises a multi-stage compression process where the energy requirements to transfer gas between stages is reduced due to the recovery of energy from the withdrawn pumping fluid. The multi-stage construction facilitates intercooling, and increased flexibility on design of the chambers as compared to a single stage setup. In this example, rather than leveraging expanding gas to recover energy from a cylinder post compression, the liquid piston is utilised to minimize initial work input. In some examples, the gas compressor system 100b may comprise two (or more) sets of chambers 104, 106, 168. The two sets of chambers 104, 106, 108 may operate out of phase. This means that pumping fluid withdrawn from a first set of chambers 104, 106, 108 is delivered to a second set of chambers 104, 106, 168 to compress the gas stored in the second set of chambers 104, 106, 108. Figure 8 shows a control system 800 used to control and / or monitor the operation of the gas compressor system 100, 100a according to aspects of the present disclosure. In the Figure, solid lines indicate control signals and dashed lines indicate feedback and / or sensor signals. The control system typically comprises a controller 802 which is typically implemented by one or more suitable programmed or configured hardware, firmware, and / or software controllers, e.g. comprising one or more suitable programmed or configured microprocessor, microcontroller or other processor, for example an IC processor such as an ASIC, DSP, or FPGA (not illustrated). In preferred examples, the control system 800 communicates control information to other components of the system such as the source of gas 112, gas storage unit 102, fluid pump 146, energy converter 154 and feed pump 140. Process settings may be received via a process setting interface unit 804. The process settings may specify environmental conditions, for example in relation totemperature(s), flow rate(s), and / or pressure(s). In the example shown in Figure 8, a gas flow control module 806 generates control signals for controlling the gas flow rate, a temperate control module 808 generates control signals for controlling the temperature, and a pressure control module 810 generates control signals for controlling the pressure. The control signals are supplied to a control and actuation loom 812 which routes the control system to the desired components of the gas compressor system 100, 100a. The control system 800 may also receive feedback information from other components such as sensors (e.g. incorporated into the storage unit 102), measurement devices (e.g. incorporated into the hydrogen storage unit 102), valves, energy convertor 154, and / or fluid pumps 140, 146, in response to which the control system 800 may issue control information to one or more relevant components. The feedback information is received via a feedback and sensor loom 814 in this example. The control system 800 may perform analysis of the measurements or other information provided. This analysis may be carried out automatically in real time by the control system 800. Alternatively, or in addition, analysis of the system measurements and performance may be made by an operator in real time or offline. The operator may make adjustments to the operation of the gas compressor system 100, 100a by providing control instructions via the process settings interface 804. A safety control module 816 may be provided, which may receive alarm signals from one or more alarm sensors (not shown), e.g. gas sensors, temperature sensors, leak detectors or emergency stops that may be included in the gas compressor system 100, 100a. The safety control module 816 provides alarm information to the controller 802 based on the alarm signals received from the alarm sensors. The safety control module 816 may also control an alarm and shutdown module 818 to generate an alarm for the operator and / or shutdown the operation of the hydrogen gas compressor system. In preferred examples, the control system 800, and more particularly the controller 802 is configured to implement system modelling logic, e.g.., by supporting mathematical modelling software or firmware 820, for enabling the control system 800 to mathematically model the behaviour of the gas compressor system 100, 100a, depending on the process settings and / or on feedback signals received from one or more system components during operation of the gas compressor system 100, 100a. Optionally, the control system 800 is configured to implement Model Predictive Control (MPC). Using MPC, the control system 800 causes the control action of the control modules 806, 808, 810, 816 to be adjusted before a corresponding deviation from a relevant process set point actually occurs. This predictive ability, when combined with traditional feedback operation, enables the control system 800 to make adjustments that are smoother and closer to the optimal control action values that would otherwise be obtained. A control model can be written in Matlab, Simulink, or Labview by way of example and executed by the controller 802. Advantageously, MPC can handle MIMO (Multiple Inputs, Multiple Outputs) systems. Referring to Figure 9A-9C, show example pressure-volume diagrams to demonstrate how energy is recovered by the gas compressor systems 100, 100a, 100b of the present disclosure. Figure 9A shows an example pressure-volume diagram from a chamber 104, 106, 168 of the gas compressor system 100, 100a, 100b. At point A, the liquid piston of the chamber 104, 106, 168 is at the top of its stroke meaning that the pumping fluid in the chamber 104, 106, 168 is at its maximum level. High pressure gas is trapped in the clearance volume above the liquid piston as the pressure of gas in the receiver vessel 117 is approximately the same as the pressure of the gas in the clearance space. As the pumping fluid is withdrawn from the chamber 104, 106, 168 the liquid piston moves downwards allowing the gas trapped in the clearance space to expand. The expansion takes place along lines A-B so that the pressure in the chamber 104, 106, 168 decreases as the volume of the gas increases. When the liquid piston reaches point B, the pressure of the expanded gas in the chamber 104, 106, 168 is less than the pressure of the gas in the storage tank 112 allowing gas to flow into the chamber 104, 106, 168 via the gas inlet 108, 110. The flow of gas into the chamber 104, 106, 168 continues until point C when the liquid piston is at the lowest point of its stroke. That is, the minimum level of pumping fluid is in the chamber 104, 106, 168. Along the line B - C, the chamber 104, 106, 168 is filled with gas and the pressure in the chamber 104, 106, 168 is the same as the pressure of the gas in the storage tank. At point C, the expansion stage ends and the compression stage begins. Pumping fluid is delivered to the chamber 104, 106, 168 via the fluid inlet to cause the liquid piston to rise. As the pressure increases due to movement of the liquid piston, further gas is not able to flow into the chamber 104, 106, 168 as the pressure inside the chamber 104, 106, 168 is greater than the gas pressure in the storage tank 112. The pressure of the gas within the chamber 104, 106, 168 increases as the liquid piston moves up the chamber 104, 106, 168. At point D, the pressure of the gas in the chamber 104, 106, 168 has increased to the point that it is greater than the pressure of the receiver vessel 117, causing gas to flow out of the chamber 104, 106, 168 to the receiver vessel 117. The flow of gas continues as the liquid piston continues to move upwards along line D-A while the pressure in the chamber 104, 106, 168 remains constant at the discharge pressure. At point A, the compression cycle is completed and the liquid piston is at its maximum level. Figure 9B represents the energy input that would be required from the fluid pump 146 if energy were not recovered by utilising withdrawn pumping fluid to facilitate delivery of pumping fluid to the chamber 104, 106, 168. The shaded area 902 represents the total energy input required to pump fluid from atmospheric pressure into the chamber 104, 106, 168 to pressurise the chamber 104, 106, 168. Figure 9C is an overlay of Figures 9A and 9B and illustrates the energy savings of the aspects of the present disclosure. The area 904 bounded by the curve A-B-C-D represents the total energy input required by the fluid pump 146 when energy recovery is used as per the present disclosure. The area 906 represents the energy that is recoverable by utilising the pressure of the withdrawn pumping fluid to facilitate delivery of pumping fluid to the chamber 104, 106, 168. By way of example, recovering energy from the withdrawn pumping fluid as per the present disclosure can save between 40% and 70% of the external energy input that would be required if energy was not recovered. More or less energy can be saved depending on factors such as the compression ratio. Referring to Figure 10, there is shown an example method performed by the gas compressor system 100, 100a, 100b. Step S1001 comprises delivering pumping fluid to the first chamber 104 to pressurise gas stored in the first chamber 104 such as for delivery to the receiver vessel 117. Step S1002 comprises withdrawing pumping fluid from the first chamber 104. Step S1003 comprises using the pressure of the pumping fluid withdrawn from the first chamber 104 to facilitate delivery of pumping fluid to the second chamber 106 to pressurise the gas in the in the second chamber 106 such as for delivery to the receiver vessel. Figure 11 shows another example of how the pressure of the withdrawn pumping fluid may be used to facilitate delivery of pumping fluid to a chamber 104, 106, 168. This may be used in any of the gas compressor systems 100, 100a, 100b, 100c described herein. The withdrawn pumping fluid is delivered to a pressure convertor 1102 which acts to convert the pressure of the withdrawn pumping fluid. The pressure convertor 1102 is structurally similar to a hydraulic intensifier but operates to reduce rather than intensify the pressure. The pressure convertor 1102 may be referred to as a de-intensifier. The pressure convertor 1102 comprises a first portion 1104 and a second portion 1104 which are fluidly isolated from one another. A force transmitting arrangement comprising a first piston 1108 arranged in the first portion 1104 and a second piston 1110 arranged in the second portion 1106 is provided. The first piston 1108 is mechanically coupled to the second piston 1110. The first portion 1104 has a smaller volume than the second portion 1106 such that a higher pressure in the first portion 1104 may be converted to a lower pressure in the second portion 1106. The withdrawn pumping fluid is delivered to area P1 of the first portion 1104 and acts on the first piston 1108. This causes, via the second piston 1110, a compression or fluid transfer process in the area P2 in the second portion 1106. This can allow pumping fluid in the area P2 to flow out of the outlet to a chamber 104, 106, 168. If used in a compression process, the pressure in P2 will increase as the pressure in the chamber 104, 106, 168 receiving the pumping fluid increases. The pressure may exceed the pressure that can be converted from the withdrawn pumping fluid in area P1. This means that additional force is required to drive the force transmitting arrangement. The additional force can be delivered by fluid pump 146 delivering pumping fluid to area P3 of the second portion 1106. The delivered pumping fluid acts on the second piston 1110 to increase the pressure in area P2. This allows for the maximum energy to be recovered from the withdrawn pumping fluid. While not required, the fluid pump 146 is the same as fluid pump 146 described above in relation to Figures 1 to 10. The pressure convertor 1102 can be provided within the gas compressor system 100, 100a, 100b or gas compressor system 100c described below to more energy efficiently drive the liquid piston process. In this example, a valve 1112 is provided to between the pressure convertor 1102 and the source of withdrawn pumping fluid such as to control the flow of withdrawn pumping fluid to the pressure convertor 1102. The valve 1112 may be a pressure-reducing valve, a flow control valve, a back pressure regulator or other form of valve 1112. Referring to Figure 12 shows another example gas compressor system 100c. In this example, a first plurality of chambers 104a, 106a, 168a form a first stage of the system 100c, and a second plurality of chambers 104b, 106b, 168b form a second stage of the system 100c. The chambers are as described above, and include gas inlets / outlets and fluid inlets / outlets as described above. The gas compressor system 100c also comprises valves to control the flow of fluid / gas into I out of the chambers 104a,b, 106a,b, 168a,b similar to as described above. Figure 12 shows a particular operation of the gas compressor system 100c used to transfer gas from the first stage to the second stage using the delivery of pumping fluid to the first chamber 104a of the first stage via the fluid pump 146. In this configuration the fluid inlet of the first chamber 104a of the first stage is fluidly connected to the fluid pump 146. Pumping fluid is delivered to the first chamber 104a via the fluid pump 146. The gas outlet of the first chamber 104a of the first stage is fluidly connected to the gas inlet of the first chamber 104b of the second stage. Prior to the operation shown in Figure 12, chambers 104a, 106a, 168a may be connected to the fluid pump 146 so that pumping fluid is delivered to the chambers 104a, 106a, 168a to pressurise the gas stored therein. Once the pressure within the chambers 104a, 106a, 168a has reached target pressure, the valves of the gas compressor system 100c are controlled to configured the gas / fluid flow to have the arrangement as shown in Figure 12. Pumping fluid delivered to the first chamber 104a causes gas to be delivered from the first chamber 104a to the first chamber 104b of the second stage. The fluid outlet of the first chamber 104b is fluidly connected to the fluid inlet of the second chamber 106a of the first stage. Gas being delivered to the first chamber 104b allows for pumping fluid in the first chamber 104b to be displaced and delivered to the second chamber 106a of the first stage. The gas outlet of the second chamber 106a is fluidly connected to the gas inlet of the second chamber 106b of the second stage. The gas within the second chamber 106a is pressurised due to the delivery of pumping fluid to the second chamber 106a. This allows the gas in the second chamber 106a to be delivered to the second chamber 106b of the second stage. The fluid outlet of the second chamber 106b is fluidly connected to the fluid inlet of the third chamber 168a of the first stage. Gas being delivered to the second chamber 106b allows for pumping fluid in the second chamber 106b to be displaced and delivered to the third chamber 168a of the first stage. The gas outlet of the third chamber 168a is fluidly connected to the gas inlet of the third chamber 168b of the second stage. The gas within the third chamber 168a is pressurised due to delivery of pumping fluid to the third chamber 168a. This allows the gas in the third chamber 168a to be delivered to the third chamber 168b of the second stage. The fluid outlet of the third chamber 168b allows for pumping fluid in the third chamber 168b to be withdrawn from the chamber 168b. Energy can be recovered from the withdrawn pumping fluid as described previously to facilitate delivery of pumping fluid in a subsequent operation. While this operation has been described as discrete steps, it is typically a continuous process. At the end of this operation, the chambers 104a, 106a, 168a of the first stage are filled within pumping fluid, and the chambers 104b, 106b, 168b of the second stage have comparatively more gas and less water than prior to the operation. In some examples of the gas compressor system 100c, the number of valves can be reduced as compared to the arrangement shown in Figure 12. The valve associated with the gas outlet of the first chamber 104a of the first stage and the valve associated with the gas inlet of the first chamber 104b of the second stage can be replaced with a single valve to control whether gas can flow from the gas outlet of the first chamber 104a to the gas inlet of the first chamber 104b. In other words, each of the pairs of valves shown in Figure 12 associated with a pumping fluid pathway or gas pathway between the first stage and the second stage can be replaced with a single valve. Various combinations of optional features have been described herein, and it will be appreciated that described features may be combined in any suitable combination. In particular, the features of any one example embodiment may be combined with features of any other embodiment, as appropriate, except where such combinations are mutually exclusive. Throughout this specification, the term “comprising” or “comprises” means including the component(s) specified but not to the exclusion of the presence of others. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, 5 equivalent, or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed 10 in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. A method of compressing gas, the method comprising:delivering pumping fluid to a first chamber of a plurality of chambers to pressurise gas stored in the first chamber; andperforming an operation comprising:withdrawing pumping fluid from the first chamber; andusing the pressure of the pumping fluid withdrawn from the first chamber to facilitate delivery of pumping fluid to a second chamber of the plurality of chambers to pressurise gas stored in the second chamber.

2. A method as claimed in claim 1, wherein using the pressure of the pumping fluid withdrawn from the first chamber comprises delivering the withdrawn pumping fluid to the second chamber.

3. A method as claimed in claim 2, wherein delivering the withdrawn pumping fluid to the second chamber comprises delivering the withdrawn pumping fluid to the second chamber via a fluid pump.

4. A method as claimed in claim 3, further comprising operating the fluid pump to deliver pumping fluid to the second chamber.

5. A method as claimed in claim 4, wherein delivering the pumping fluid withdrawn from the first chamber comprises delivering the withdrawn pumping fluid to the second chamber via a first fluid path that bypasses the fluid pump, and, delivering the withdrawn pumping fluid, via a second fluid path, to the fluid pump, and optionally wherein the withdrawn pumping fluid is delivered via the second fluid path in response to the pressure of the withdrawn pumping fluid being insufficient for delivery to the second chamber.

6. A method as claimed in any preceding claim, wherein using the pressure of the pumping fluid withdrawn from the first chamber comprises: delivering the withdrawn pumping fluid to an energy convertor to recover energy from thewithdrawn pumping fluid for driving a fluid pump; and operating the fluid pump using the recovered energy to deliver pumping fluid to the second chamber.

7. A method as claimed in claim 6, wherein the energy converter comprises a fluid turbine operatively coupled to the fluid pump for driving the fluid pump.

8. A method as claimed in claim 7, wherein the fluid turbine is mechanically coupled to the fluid pump.

9. A method as claimed in claim 7, wherein the fluid turbine is electrically coupled to the fluid pump.

10. A method as claimed in any preceding claim, wherein using the pressure of the pumping fluid withdrawn from the first chamber comprises delivering the pumping fluid withdrawn from the first chamber to a first portion of a pressure convertor so as to drive a force transmitting means to pressurise pumping fluid in a second portion of the pressure convertor for delivery to the second chamber.

11. A method as claimed in claim 10, wherein the first portion has a smaller volume than the second portion.

12. A method as claimed in claim 10 or 11, further comprising operating a fluid pump to deliver pumping fluid to the pressure convertor so as to further drive the force transmitting means to pressurise pumping fluid in the second portion of the pressure convertor for delivery to the second chamber.13.A method as claimed in claim 12, wherein the fluid pump is operated in response to the pressure of the withdrawn pumping fluid being insufficient to cause pumping fluid in the second portion of the pressure convertor to be delivered to the second chamber.

14. A method as claimed in any preceding claim, wherein the first chamber is the same as the second chamber or is different to the second chamber.

15. A method as claimed in any preceding claim, wherein delivering pumping fluid to the first chamber of the plurality of chambers comprises delivering pumping fluid to the first chamber and one or more other chambers of the plurality of chambers.

16. A method as claimed in claim 15, wherein withdrawing pumping fluid from the first chamber comprises withdrawing pumping fluid from the first chamber and one or more other chambers of the plurality of chambers.

17. A method as claimed in any preceding claim, further comprising connecting the first chamber to a source of gas, and wherein withdrawing pumping fluid from the first chamber allows gas to flow to the first chamber from the source of gas.

18. A method as claimed in any preceding claim, wherein the operation is a first operation, and the method further comprises performing a second operation comprising withdrawing pumping fluid from the second chamber; and using the pressure of the pumping fluid withdrawn from the second chamber to facilitate delivery of pumping fluid to the first chamber to pressurise gas in the first chamber.

19. A method as claimed in claim 18, further comprising repeating, in sequence, the first operation and the second operation.

20. A method as claimed in any preceding claim, wherein delivering pumping fluid to the first chamber comprises delivering pumping fluid to the first chamber and the other chambers of the plurality of chambers, wherein withdrawing pumping fluid from the first chamber comprises withdrawing pumping fluid from the first chamber and the other chambers of the plurality of chambers, and wherein using the pressure of the pumping fluid withdrawn from the first chamber to facilitate delivery of pumping fluid to the second chamber comprises using the pressure of the pumping fluid withdrawn from the first chamber and the other chambers of the plurality of chambers to facilitate delivery of pumping fluid to the second chamber and the other chambers of the plurality of chambers.21 .A method as claimed in claim 20, wherein the plurality of chambers comprises the first chamber, the second chamber, and a third chamber, wherein the method further comprises connecting a gas outlet of the first chamber to a gas inlet of the second chamber, and connecting a gas outlet of the second chamber to a gas inlet of the third chamber, and wherein delivering pumping fluid to the first chamber comprises delivering pumping fluid sequentially to the first chamber, second chamber, and third chamber.

22. A method as claimed in any of claims 1 to 19, wherein the plurality of chambers comprises the first chamber, the second chamber, and a third chamber, wherein the method further comprises connecting a gas outlet of the first chamber to a gas inlet of the second chamber, and connecting a gas outlet of the second chamber to a gas inlet of the third chamber, and wherein the method further comprises withdrawing pumping fluid from the second chamber and delivering the pumping fluid withdrawn from the second chamber to the third chamber.

23. A gas compressor system comprising:a plurality of chambersa pumping system comprising a fluid pump;a controller, wherein the controller is configured to:control the fluid pump to deliver pumping fluid to a first chamber of the plurality of chambers to pressurise gas stored in the first chamber;control the system to perform an operation comprising: withdrawing pumping fluid from the first chamber; and using the pressure of the pumping fluid withdrawn from the first chamber to facilitate delivery of pumping fluid to a second chamber of the plurality of chambers to pressurise gas in the second chamber.

24. A gas compressor system as claimed in claim 23, wherein the pumping system comprises an energy convertor arranged to recover energy from the withdrawn pumping fluid to drive the fluid pump.

25. A gas compressor system as claimed in claim 24, wherein the energy convertor comprises a fluid turbine operatively connected to the fluid pump for driving the fluid pump.

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