WO2024194699A1

WO2024194699A1 - Hydraulic gas compressor

Info

Publication number: WO2024194699A1
Application number: PCT/IB2024/051347
Authority: WO
Inventors: Mohammed BOUHERAOUA; Kais Atallah; Panagiotis LAZARI
Original assignee: Bouheraoua Mohammed
Priority date: 2023-03-21
Filing date: 2024-02-13
Publication date: 2024-09-26

Abstract

A hydraulic compressor comprising a liquid pump to move incompressible liquid in and out of compression chambers, and in doing so displace gas which is then moved into a gas reservoir tank through transfer of mass. The gas is compressed in the gas reservoir to the desired pressure. The system comprises a plurality of compression tanks, each of which comprises interdependent volumes of liquid and gas. One or more pumps move liquid through the first compression tank, forcing gas to be expelled from the first compression tank and directed to gas reservoir where it is compressed. The liquid in each of said compression tanks is contained within an expandable bladder.

Description

HYDRAULIC GAS COMPRESSOR

TECHNICAL FIELD

The present invention relates to the field of gas compression using liquid pumps and a plurality of compression tanks.

BACKGROUND

Gas compressors are being used in a wide range of applications including industrial, health, and residential sectors. The majority of gas compressors are air compressors, but there are many other applications where gas compression is needed, including refrigerant compressors, natural gas compressors, oxygen compressors and compressors for highly reactive gases. Several studies have identified that the energy consumption from the use of air compressors amounts to -10% of the total energy consumption worldwide. Thus, energy savings in compressing air or gases in general could deliver a very significant economic benefit. Most common types of positive displacement compressors include reciprocating (piston, or diaphragm) or rotary (screw, vane) compressors and the use of each type of compressor depends on the fluid being compressed and the pressure and volume requirements. All positive displacement compressors use solid elements to compress the fluid and most energy is converted to heat. The inherently low efficiency of conventional air compressor systems, which lies in the region of 5-15% represents a huge waste. Certainly, any improvement in the energy efficiency of air compressor systems would contribute considerably to sustainable growth worldwide by reducing industrial energy consumption for air compression. Furthermore, conventional positive displacement compressors often use oil lubrication and expensive filtration is needed in order to prevent oil mist and gaseous hydrocarbons from ending up in the compressed gas reservoir.

Hydro-pneumatic gas compression, where the gas is compressed using a liquid piston and a pump, is a novel concept the inventors have developed and has many advantages over conventional compressors that use solid elements to compress the fluid. Hydro-pneumatic gas compression enables isothermal cycles during compression, resulting in higher energy efficiencies when compared to adiabatic cycles. The concept of hydro-pneumatic gas compression (i.e. using liquids to compress air) is not new. Indeed, it was first described in 1879 (Mekarski, Louis, ‘Improvement in devices for using compressed air for motive power.’, 8683, Apr. 22, 1879). Although it has received relatively little attention to date, the quest for energy efficiency is driving a renewed interest in hydro-pneumatic systems for energy storage (EP3789609A1 , US20210075296A1 , WO2021250666A1 , CN1 10985356B) and gas compression in general (US5073090A, DE4430716A1 , CN102840183B). However, known prior art does not take full advantage of the potential of the principle of hydro-pneumatic gas compression for increased energy efficiency and do not possess the necessary robustness that would allow efficient and reliable commercial operation.

For example, US5073090A teaches a system wherein the compressor has two hollow chambers which are interconnected by a conduit system having a pump located in it. The compressor contains a sufficient volume of non-compressible transfer fluid to completely fill one of the cylinders and the conduit system. A switching system causes the pump to pump the transfer fluid into a first chamber and then pump the transfer fluid from the first chamber and into the second chamber. In such system the liquid is in open contact with the gas and potentially there is the risk of liquid inadvertently entering the storage tank as a cylinder fills with liquid. Hence, in practice, such a system is forced to operate well-below the maximum compression ratio.

Similarly, CN102840183B teaches a system where the liquid is also in open contact with gas, and liquid level sensors are used to determine the maximum fill level of the compression tank. Therefore, there is the need for an energy-efficient system that allows the system to operate near maximum theoretical efficiency, and which is robust enough to avoid risk of liquid migration into the storage tank and which is simple and reliable in its operation. The present invention aims to address many of these problems known in the art.

ADVANTAGES OF THE INVENTION

Some of the advantages of the present disclosure, which at least one embodiment herein satisfies, are as follows.

A first advantage of the invention is the provision of a method and system for compressing gases using liquid movement in a way that provides improved efficiency and reliability.

Another advantage of the present invention is the provision of a method and system for compressing gases using liquid movement in a way that enables maintenance without loss of productive time.

SUMMARY

The present invention utilizes a liquid pump to move incompressible liquid in and out of compression chambers, and in doing so displace gas which is then moved into a gas reservoir tank through transfer of mass. The gas is compressed in the gas reservoir to the desired pressure. The system comprises a plurality of compression tanks, each of which comprises interdependent volumes of liquid and gas. One or more pumps move liquid through the first compression tank, forcing gas to be expelled from the first compression tank and directed to gas reservoir where it is compressed. The liquid in each of said compression tanks is contained within an expandable bladder. Dedicated passages through enable incoming liquid to be fed to the expandable bladder in the compression tank and cause it to volumetrically expand, thus forcing outflow of gas from each compression tank towards the gas reservoir communicatively connected to the compression tanks; The liquid is circulating in a closed system wherein during the first part of the cycle liquid moved out of the expandable bladder of the one compression tank and into the expandable bladder of the second compression tank, and the operation is reversed when the expandable bladder in first compression tank reaches the end of its forward stroke. Gas is supplied to the compression tanks through a low-pressure gas source. If the gas is atmospheric air, the low-pressure gas source is typically the atmosphere.

In a first aspect of the invention, a hydraulic compressor system for compressing gas, comprises at least one pump; a gas reservoir; a first compression tank, and a second compression tank. Each of the compression tanks comprises: an expandable bladder defining a bladder cavity, a tank cavity outside of the expandable bladder, wherein the cavity comprises a compressible gas; a liquid line configured to fluidly couple the expandable bladder cavity with at least one of the one or more pumps such that the liquid is pumped into or out of the expandable bladder via the liquid line; and a gas line configured to fluidly couple the gas reservoir with the tank cavity such that the gas flows into or out of the tank cavity via the gas line. The expandable bladder contains a liquid in the expandable bladder cavity and the expandable bladder is configured to expand when liquid is input into the expandable bladder cavity. The expansion of the expandable bladder causes gas to be pressurised and achieves mass transfer of the gas from the compression tank into the gas reservoir. Mass transfer (m) of gas into the gas reservoirs results in an increase of pressure in the gas reservoir according to the gas law PV=mRT where P is the pressure, V is the volume of the gas reservoir, m is the mass of gas in the reservoir, T is the absolute temperature, and R is a constant. During operation of the system, liquid is pumped by the one or more pumps to the expandable bladder in the first compression tank and gas is expelled from the tank cavity of the first compression tank and directed to gas reservoir where it is compressed.

Because liquid flows into the inner cavity of the expandable bladder, gas and liquid are not in direct contact. At the end of the compression stroke, there is no risk of liquid getting transferred into the transfer lines that connect the compression tank and the gas reservoir. The expandable bladder expands to the maximum extent and allows greater efficiency because nearly all quantity (mass) of gas in the compression tank can be transferred to the gas reservoir.

The first compression tank is configured to operate in a compression stroke and in a return stroke. During the compression stroke, liquid is configured to be pumped into the expandable bladder, via the liquid line, such that the expandable bladder expands into the tank cavity and causes compression of the gas; and wherein during return stroke, liquid is configured to flow out from the expandable bladder via the liquid line such that the expandable bladder contracts and the tank cavity expands.

The liquid line of the first compression tank is fluidly coupled to the liquid line of the second compression tank such that when the first compression tank is configured to operate in the return stroke and the second compression tank is configured to operate in the compression stroke, liquid is configured to flow out from the expandable bladder of the first compression tank and to be fed, by the pump, into the expandable bladder of the second tank; and when the second compression tank is configured to operate in the return stroke and the first compression tank is configured to operate in the compression stroke, liquid is configured to flow out from the expandable bladder of the second compression tank and fed, by the pump, into the expandable bladder of the first tank.

In some embodiments the system further comprises: an inlet line extending between a low- pressure gas source and the tank cavity; a check valve that controls the flow of gas between the tank cavity and the low pressure gas source; and a check valve that controls the flow of gas between the tank cavity and the gas reservoir in the gas line.

In preferred embodiments, during operation of the system, the volumetric ratio of the volume of the tank cavity at the end of the compression stroke to the volume of the tank cavity at the beginning of the compression stroke (Vc/Vo) is <=0.1

Each compression tank comprises a pressure sensor, wherein said pressure sensor is arranged to measure the pressure of the liquid within the expandable bladder.

The pump is characterised by a pump stall pressure and the maximum pressure of the liquid in the expandable bladder is configured to be less than 10% of the pump stall pressure, and the minimum pressure of the liquid in the expandable bladder is less than 5% of the pump stall pressure.

In one embodiment, the pump comprises a variable speed pump wherein the speed of said pump is adjusted so that towards the end of the compression stroke the pump speed and hence the liquid flowrate is reduced to allow a smooth change-over from one compression tank to another compression tank as the automatically controlled isolation valves are opened and closed.

In another embodiment, the system comprises a pair of two pumps, said two pumps connected in parallel and operating as a single unit, wherein of said pair of pumps, the first pump of said pair of pumps has a higher flow capacity and can operate to a lower pressure and the second pump of said pair of pumps has a lower flow capacity and can operate at a higher pressure.

The system comprises a plurality of liquid lines, wherein each liquid line is configured to couple a hydraulic compression tank with one or more of the one or more pumps; wherein each liquid line comprises at least two isolation valves; wherein each liquid line is arranged to connect to each compression tank to one or more of the one or more pumps in a closed circuit; wherein each liquid line comprises a forward isolation valve and a return isolation valve; wherein when the hydraulic compression tank is configured to operate in the compression stroke, the forward isolation valve is open and the return isolation valve is closed; and wherein when the hydraulic compression tank is configured to operate in the return stroke the forward isolation valve is closed and the return isolation valve is open.

The isolation valves are controlled by a control system; wherein the control system is configured to open or close the isolation valves based on data received from a sensor system. In a preferred embodiment the sensor system comprises of a pressure sensor arranged to measure the pressure of the liquid within the expandable bladder of each hydraulic compression chamber and a pressure sensor arranged to measure the pressure within the gas reservoir.

In another embodiment, the sensor system comprises a flowmeter in fluid communication with the expandable bladder of each hydraulic compression chamber, and a pressure sensor arranged to measure the pressure within the gas reservoir.

Yet in another embodiment, the sensor system CONSISTS of one flowmeter fluidically connected with pump output, and one pressure sensor fluidically connected with the reservoir no other sensors are necessary when the gas intake pressure is constant.

In one embodiment, the control system is configured to: receive data corresponding to the pressure of the liquid inside the expandable bladder before the compression stroke is started; receive date corresponding to the pressure of the liquid inside the expandable during the compression stroke is started; calculate the compression ratio (Vc/Vo); determine whether the compression ratio has reached the maximum value; and switch the isolation valves from being open to being closed, or being closed to being open, when the maximum value of the compression ratio has been reached.

In another embodiment, the control system is configured to switch the isolation valves from being open to being closed, or being closed to being open, when the pump reaches a stalling condition defined by the pump current.

Yet in another embodiment, automatic switchover of system from a forward compression cycle of the first compression tank to a forward compression cycle of the second tank and reverse stroke of the first compression tank is happening with a time difference between the opening of the operation of the 2nd forward valve and the 1 st reverse valve and the operation of closing the 1 st forward valve and the closing of the 2nd reverse valve wherein this time difference is between 1/1 Oth and 1/2 of the time needed for full opening of a said isolation valve.

Another aspect of the system relates to the number of compression tanks. In one embodiment there are two compression tanks. In another embodiment, there are three or more tanks wherein each tank is matched with dedicated passages through which incoming liquid may be fed forcing outflow of gas, with pressure sensors to measure liquid pressure or flowmeters to measure liquid flow and a pair of isolation valves to control the flow of liquid from the pump into to the compression tank during the compression stroke from the compression tank to the pump during the return stroke and wherein the timing of the operation of said isolation valves corresponding to each of said plurality of tanks is arranged so that the pump or pumps do not reach stalling conditions.

The expandable bladder within a compression tank is elastic and is characterized by a spring constant that defines a resistance to expansion.

The expandable bladder may be of any size depending on the overall capacity of the system. The volume of the compression tanks and the expandable bladder in said expandable tanks is a function of the volume of the gas reservoir tank and the target pressure in the gas reservoir tank, divided by the number of switch-over cycles. The number of switch-over cycles is the number of cycles that liquid flows from one bladder to the other.

The system is operated in a way as to achieve maximum energy efficiency for compressing the gas. According to a preferred embodiment, the method for compressing gas according to the invention comprises: a. providing a first compressor tank comprising : i. an expandable bladder defining a bladder cavity, wherein the expandable bladder contains a liquid in the expandable bladder cavity; ii. a tank cavity outside of the expandable bladder, wherein the cavity comprises a gas; iii. a liquid input; and iv. a gas port; v. connecting the gas port of the first compressor tank to a gas reservoir via a gas line; vi. connecting the liquid input of the first compressor tank to a first liquid line; b. operating the first compression tank in either a compression stroke or a return stroke, wherein operating the hydraulic compression tank in the compression stroke comprises: i. compressing the gas in the tank cavity of the first compressor tank by pumping a liquid into the expandable bladder cavity of the first compressor tank via the first liquid line such that the expandable bladder of the first compressor tank expands into the tank cavity; ii. displacing the gas out of the tank cavity of the first compressor tank, via the gas port, and collecting said gas in the gas reservoir; iii. further compressing the gas in the gas reservoir; and c. wherein operating the hydraulic compression tank in the return stroke comprises: i. pumping a liquid out of the expandable bladder cavity of the first compressor tank via the first liquid line such that the expandable bladder of the first compressor tank contracts, which in turn increases the volume of the tank cavity of the first compressor tank; ii. filling the tank cavity of the first compressor tank with gas from a low- pressure gas source via the gas port.

According to one embodiment, the method further comprises: a. providing a second compressor tank comprising: i. an expandable bladder defining a bladder cavity, wherein the expandable bladder contains a liquid in the expandable bladder cavity; ii. a tank cavity outside of the expandable bladder, wherein the cavity comprises a gas; iii. a liquid input; and iv. a gas port; b. connecting the gas port of the second compressor tank to a gas reservoir via a gas line, wherein the first and second compressor tanks may be connected to the same gas reservoir or different gas reservoirs; c. connecting the liquid input of the second compressor tank to a second liquid line; wherein the first liquid line and the second liquid line are in fluid communication; d. operating the first compressor tank in a compression stroke and the second compressor tank in a return stroke comprising: e. compressing the gas in the tank cavity of the first compressor tank by pumping a liquid from the expandable bladder cavity of the second compressor tank and into the expandable bladder cavity of the first compressor tank such that the expandable bladder of the second compressor tank contracts and the expandable bladder of the first compressor tank expands; f. displacing the gas out of the tank cavity of the first compressor tank, via the gas port of the first hydraulic chamber, and collecting said gas in the gas reservoir; g. filling the tank cavity of the second compressor tank with gas from a low- pressure gas source via the gas port of the second hydraulic chamber; and h. further compressing the gas in the gas reservoir.

In another embodiment, the method further comprises: a) determining that the compression stroke of the first compressor tank has been completed and/or the return stroke of the second compressor tank has been completed; and b) switching the operation of the first and second compressor tanks such that the second compressor tank is operated in a compression stroke and the first compressor tank in a return stroke comprising: compressing the gas in the tank cavity of the second compressor tank by pumping a liquid from the expandable bladder cavity of the first compressor tank and into the expandable bladder cavity of the second compressor tank such that the expandable bladder of the first compressor tank contracts and the expandable bladder of the second compressor tank expands; c) displacing the gas out of the tank cavity of the second compressor tank, via the gas port of the second hydraulic chamber, and collecting said gas in the gas reservoir; d) filling the tank cavity of the first compressor tank with gas from a low pressure gas source via the gas port of the second hydraulic chamber; and e) further compressing the gas in the gas reservoir.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of preferred embodiments of the invention will now be described by referring to the accompanying drawings:

Figure 1. is a schematic representation of the basic arrangement of a hydraulic gas compression system according to one embodiment of the present with two compression tanks.

Figure 2. is a schematic illustration of the gas volumes in a typical compression tank at start and end state of the compression stroke.

Figure 3 is a schematic representation of the system showing operation of compression stroke in the first tank and corresponding flow of liquid.

Figure 4 is a schematic representation of the system showing utilisation of three-way valves.

Figures 5a-5c is a schematic representation of the system showing alternative means of taking pressure and flow measurements.

Figure 6 is a schematic representation of the system showing aspects of the control system.

Figure 7 is an example of a flowchart showing the control operation.

Figure 8 is an example of typical curves of pressure over time.

Figure 9 is a schematic representation of the system showing an embodiment with multiple pumps used in parallel.

Figure 10 is a schematic representation showing utilisation of more than two compression tanks.

Figure 11 is a schematic representation showing utilisation of more than two pumps and more than two compression tanks.

Figure 12 is a schematic representation showing utilisation of valves at the intake and exit of the pump. REFERENCES IN THE DRAWINGS

The following is a list of references made in the figures.

1a: First gas line from a first low-pressure gas source to the first compression tank

1b: First gas line from a second low-pressure gas source to the second compression tank

10: Liquid pump

10a: Additional pump

10n: Nth additional pump

11 : Inlet low-pressure side of pump

12: Outlet high-pressure side of pump

2: Hydraulic compression tank

21 : First hydraulic compression tank

210: Tank cavity of the first hydraulic compression tank

22: Second hydraulic compression tank

22n: Nth additional hydraulic compression tank

220: Tank cavity in the second hydraulic compression tank

3: Expandable bladder inside hydraulic compression tank

30: Bladder cavity

31 : Expandable bladder in the first hydraulic compression tank

310: Liquid inside first bladder

32: Expandable bladder in the second hydraulic compression tank

320: Liquid inside second bladder

4: Gas reservoir

51 : First isolation valve of first hydraulic compression tank

52: First isolation valve of second hydraulic compression tank

53: Second isolation valve of first hydraulic compression tank

54: Second isolation valve of second hydraulic compression tank

55: Three-way valve of first hydraulic compression tank

56: Three-way valve of the second hydraulic compression tank

57: Isolation valve at the pump intake

61 : First check valve of first compression tank

62: Second check valve of first compression tank

63: First check valve of second compression tank

63n: First check valve of Nth additional compression tank

64: Second check valve of second compression tank

64n: Second check valve of Nth compression tank 65: Check valves at pump exit used when multiple pumps are utilised

66: Check valve at the exit of the pump.

70: Pressure sensor for gas reservoir tank

700: Measured of gas reservoir tank

71 : Pressure sensor for bladder in first compression tank

710: Measured pressure corresponding to bladder in first compression tank, during compression stroke of first compression tank

72: Pressure sensor for bladder in second compression tank

72n: Pressure sensor for bladder in Nth additional compression tank

720: Measured pressure corresponding to bladder in second compression tank, during compression stroke of first compression tank and return stroke of second compression tank.

73: Pressure sensor at pump exit

73a: Pressure sensor at pump exit of additional pump

73n: Pressure sensor at pump exit of additional Nth pump

730: Pump pressure measured at pump exit

731 : Peak (maximum) liquid pressure at pump exit

732: The slope of the curve of tank pressure

733: Pressure of near zero value of second compression tank.

740: Pressure of the liquid at the exit of the pump

74: Flowmeter associated with first compression tank

75: Flowmeter associated with second compression tank

76: Flowmeter associated with exit of liquid pump

80: Controller

81 : Wireless communication

82: Computing device

91 : Liquid pipeline having low pressure liquid

92: Liquid pipeline having high pressure liquid

93a: Second gas line from the first hydraulic compression chamber to the gas reservoir

93b: Second gas line from the second hydraulic compression chamber to the gas reservoir.

DETAILED DESCRIPTION

The present invention refers to a hydraulic gas compression system.

The system comprises of the following main elements: one or more liquid pumps, two or more compression tanks each having an internal expandable bladder, one or more gas reservoir tanks that can be pressurised, sets of isolation valves each set corresponding to a compression tank, sets of check valves each set corresponding to a compression tank, dedicated flow channels, measuring instruments, and a control system.

Details of the principles and the structure of the invention are hereinafter explained with reference to examples of alternative embodiments. These embodiments serve to highlight the principles of the invention and are not limiting in any way by means of structural arrangements.

Figure 1. is a schematic representation of the basic arrangement of a hydraulic gas compression system according to one embodiment of the present with two hydraulic compression tanks. The system comprises a liquid pump 10, a first compression tank 21 and a second compression tank 22, and a gas reservoir 4. Inside each of the compression tanks, there is an expandable bladder 31 , 32. As the pump 10 moves liquid into the first hydraulic compression tank 31 , this liquid is fed into the expandable bladder in the first hydraulic compression tank 31 and the liquid in the first bladder 310 causes the expandable bladder to expand thus displacing the gas in the gas space of the first compression tank 210. Gas flows through dedicated gas lines 93a, 93b into the gas reservoir 4 where it is compressed. Each hydraulic compression tank 21 , 22 is associated with a set of isolation valves 51 , 52, 53, 54 and a set of check valves 61 , 62, 63, 64. Various instruments are used to measure pressure or flow. Figure 1 shows a sensing arrangement for a preferred embodiment wherein a pressure sensor 70 for the gas reservoir tank 4, and pressure sensors 71 , 72 to measure the pressure inside each of the expandable bladders of the first and second compression tanks 21 , 22. The gas reservoir 4 may comprise a bank of several individual tanks. Hydraulic compression tanks 21 , 22 comprise interdependent volumes wherein the total tank volume is equal to the gas volume 210 plus the volume of the expandable bladder 310 filled with liquid. The use of expandable bladder helps to enable utilisation of full stroke compression without risk of liquid carry-over into said gas reservoir. In the opposite case, when no bladder is used, the liquid and the gas in the hydraulic compression tank are in contact. As such, there is a high risk of liquid carry-over into the gas reservoir tank 4 when the remaining gas volume 210 in the compression tank becomes small and tends to zero.

The gas can be any gas including but not limited to air, nitrogen, hydrogen, oxygen or even refrigeration gases. The use of a bladder also enables the efficient compression of gases that are soluble in liquids without worrying about microbubbles of gas in the liquid. The use of a bladder is also important for the compression of gases that are highly reactive or corrosive such as oxygen for example. Figure 2. is a schematic illustration of the gas volumes in a typical compression tank at start and end state of the compression stroke. The total volume of a compression tank 2 is denoted by V and comprises (e.g. consists of) the gas volume and the volume taken by the expandable bladder 3 which is filled with liquid 30. The gas volume at the starting condition when the expandable bladder is effectively empty of liquid is denoted by Vo. The gas volume at the end condition when the expandable bladder is fully expanded to its maximum volume (e.g. is filled to maximum capacity of liquid) is denoted by Vc. For maximum system efficiency, Vc isl as small as possible. According to this invention the ratio Vc/Vo is less than or equal to 0.1 , i.e. the expandable bladder can expand to accommodate approximately 9 times its size compared to when it is empty.

Figure 3 is a schematic representation of the system showing operation of compression stroke in the first tank and corresponding flow of liquid. The thickened line shows the flow of liquid during the compression stroke of the first compression tank and consequently the return stroke of the second compression tank. Liquid flows in a closed circuit. During the part of the cycle when the first compression tank 21 is in a stage of a compression stroke, the pump 10 moves the liquid from the pump exit port 12 through the high-pressure liquid pipeline 92 into the expandable bladder 31 of the first compression tank to add to the liquid inside first bladder 310 and expand the expandable bladder. The gas in the first compression tank 210 is displaced and forced to move into the gas reservoir tank 4 via the dedicated gas passage 93. In this case the first check valve 61 is open to allow gas passage into the gas reservoir tank, but the second check valve 62 is held closed so no gas can escape. At the liquid side, the first isolation valve of first compression tank 51 that is connected to the low-pressure side 91 is closed and the second isolation valve 52 that is connected to the high-pressure side 92 is open so that liquid is directed into the expandable bladder 31 to cause its expansion. Liquid flows in a closed liquid and the liquid that is supplied to the intake of the pump 1 1 flows from the expandable bladder of the second compression tank 32 whereby liquid inside the expandable bladder of the second tank 320 flows through the second isolation valve of the second compression tank 54, said isolation vale is connected to the low-pressure side 91 . As liquid moves out of the expandable bladder of the second compression tank 32, the volume of the liquid of the expandable bladder 320 reduces, the expandable bladder contracts and the gas pressure inside the second compression tank reduces, thus allowing gas from the low- pressure supply 1 b to flow into the second compression chamber 22 and the volume of the tank cavity 220 increases. The second check valve of second compression tank 64 opens to let gas in, while the first check valve of second compression tank 63 remains closed. The second compression tank 22 is in a return stroke operation while the first compression tank 21 is in compression stroke operation.

The flow of gas to the gas reservoir tank 4 via the hydraulic compression tanks 21 , 22 is achieved via gas lines, wherein each gas line comprises a first gas line 93a, 93b and a second gas line 1 a, 1 b. The first gas line 93a, 93b extends between the tank cavities 210, 220 of the hydraulic compression tanks 21 , 22 and the gas reservoir. The second gas line 1 a, 1 b, extends between the low-pressure gas sources and the tank cavities 210, 220 of the hydraulic compression tanks 21 , 22. The gas lines comprise all pairs of check valves 61 , 62, 63, 64 as well as the ports for pressure sensors 70.

Figure 4 is a schematic representation of the system showing utilisation of three-way valves. According to this embodiment, the pairs of isolation valves associated with the first compression tank 51 , 52 and the pair of isolation valves associated with the second compression tank 53, 54 are being replaced by a first three-way valve 55 and a second three- way valve 56.

Figures 5a-5c is a schematic representation of the system showing alternative means of taking pressure and flow measurements. Figure 5a shows an embodiment where pressure sensors are used to measure pressure and control the operation of the system. In this embodiment four pressure sensors are utilised. A pressure sensor for measuring pressure in the gas reservoir tank 70, a pressure sensor for measuring liquid pressure in the expandable bladder of the first compression tank 71 , a pressure sensor for measuring liquid pressure in the expandable bladder of the second compression tank 72, and a pressure sensor for measuring liquid pressure at the exit of the liquid pump 73. Pressure sensors for measuring liquid pressure in the expandable bladder 71 , 72 are placed in fluidic communication with said expandable bladder to measure the liquid pressure. The fluidic connection may be at a port of a fluid line feeding to said bladder. The pressure in the expandable bladder is equal to the liquid pressure measured. This pressure is equal to the gas pressure of the gas inside the tank cavity 210, 220 plus the spring constant of the expandable bladder 31 , 32 itself as it expands. The expandable bladder is typically made of flexible material which behaves like a spring when it expands. Thus, the total liquid pressure is equal to gas pressure plus equivalent bladder resistance and can be expressed as P_L= PG + Ps wherein P is the liquid pressure, PG is the gas pressure in the tank cavity 210, 220 and Ps is the pressure due to the spring-like behaviour of the expandable bladder 31 , 32. The maximum pressure generated in the expandable bladder 31 , 32 as a result of the spring effect at full stroke is less than 10% (as a fraction of pump stall pressure) and the minimum pressure generated in the expandable bladder as a result of the spring effect at the end of the return stroke when the expandable bladder is at its minimum volume is less than 5%.

In an alternative embodiment, instead of pressure sensors that measure the pressure of the liquid inside each of the expandable bladders 31 , 32, flow meters are used to measure liquid flow in and out of each of the expandable bladders associated with the first and second compression tanks. The flowmeter associated with first compression tank 74 is installed just before liquid pipe connection to the first compression tank, and the flowmeter associated with second compression tank 75 is installed just before liquid pipe connection to the second compression tank.

It is possible that the operation of the system may be controlled only measurement of gas pressure in the storage tank and measurement of flow from the liquid pump. In this embodiment, one pressure sensor 70 is used to measure gas pressure in the storage tank, and one flowmeter 76 is used to measure from at the exit of the pump. Since liquid is in a closed system, one flowmeter is sufficient to indicate quantity of liquid moved into either the first or the second compression tank.

The system can be controlled by using values from the sensors in any of the embodiments described in figures 5a to 5c. In addition, the electric current to the liquid pump can also be used to indicate when the pump is approaching maximum pressure and reaching stalling conditions.

Figure 6 is a schematic representation of the system showing the microcontroller and system controls. The isolation valves 51 , 52, 53, 54 and sensors 70, 71 , 72, 73, 74, 75, 76 are all connected to the input/output of a control system 80. The control system comprises processor, memory, and an electronic controller. The electronic controller is connected via control lines to the pump, and the isolation valves to control their operation. The control system also comprises wireless communication to enable it to communicate with a remote computer. A remote computer may be a handheld device, a smart phone, or a remote server.

The logic for the management of the operation of the system is illustrated in Figure 7 which is an example of a flowchart showing the control operation. According to this example, the operation for the compression of each of the compression tanks goes through a loop wherein the pressure of the liquid in the expandable bladder 31 of the first compression tank 21 is checked and if it is less that a threshold of maximum pressure, the pump operates to continue feeding liquid into the expandable bladder 31 of the first compression tank 21 until the maximum pressure is reached. When the maximum pressure is reached, the operation “Compress Tankl” terminates and the operation “Compress Tank2” starts. In operation “Compress Tank2”, the control system utilises pressure sensors 71 , 72 as the main control inputs. Based on the decision to run the operations “Compress Tankl” or “Compress Tank2” the corresponding isolation valves are opened and closed to achieve the correct flow of liquid to the expandable bladder of first compression tank 31 with liquid coming from the expandable bladder of the second compression tank 32 and vice-versa.

In one embodiment, the opening and closing of isolation valves, once a decision is made to run the operation “Compress Tankl” or “Compress Tank2”, and switch from the compression stroke of the first tank to a compression stroke of the second tank, may be done with a time difference between the opening of the operation of the 2nd forward valve and the 1 st reverse valve and the operation of closing the 1 st forward valve and the closing of the 2nd reverse valve wherein this time difference is between 1 /1 Oth and 1 /2 of the time needed for full opening of a said isolation valve.

Figure 8 is an example of typical curves of pressure over time. Four curves are shown on the same timeline. These are the pressure of gas reservoir tank 700, the measured pressure corresponding to bladder in first compression tank 710, during compression stroke of first compression tank, the measured pressure corresponding to bladder in second compression tank 720, during compression stroke of first compression tank and return stroke of second compression tank, and the measured pressure of the liquid at the exit of the pump 740. The pump pressure reaches a peak 701 at the end of the compression stroke. This peak denotes the point when the system will switch over from compressing the first tank to compressing the second tank. The controller, upon detecting the rapid pressure increase and the achievement of a threshold reference maximum pressure, will give instructions to the electronically controlled valves to switch operation from “Compress Tankl” to “Compress Tank2”.

It is possible that the system may determine that maximum compression ratio Vc/Vo is reached irrespective of the gas pressure in the gas reservoir 70 by a computation of rate of increase of pressure inside the expandable bladder together with a time delay. This rate of increase of pressure corresponds to the slope of the curve of tank pressure 732 as illustrated by means of a specific example in figure 8. At the end of the compression stroke the rate of increase of pressure of the liquid inside the expandable bladder increases rapidly to reach a maximum before falling to a near zero value 733.

The controller may utilise pressure measurements to switch from operation “Compress TankT’ terminates and the operation “Compress Tank2” as explained above. It may also utilise measurement of pump current to determine that the pump has reached stalling conditions as indicated by a rapid increase in current. Stalling is achieved because the expandable bladder can no longer expand, and the pressure reaches its maximum or because the pump itself has reached it maximum hydraulic head and is unable to provide further flow to expand the expandable bladder further.

Figure 9 is a schematic representation of the system showing an embodiment with multiple pumps used in parallel. Additional pumps 10a to 10n may be utilised with corresponding pressure sensors 73a to 73n. In this configuration set of check valves 65 are used at the exit of each pump. Multiple pumps can improve efficiency and compression speed. For example, at lower pressure a one pump may operate that can deliver high liquid flow, albeit up to a lower pressure. Above a threshold pressure, a second pump may operate that can deliver liquid flow at a lower flowrate but achieve higher compression pressure. In this manner, speed is improved, energy efficiency is improved, and pump durability may also be improved. In one embodiment, the multiple pumps are two pumps operating as a pair. Said pair of two pumps is connected in parallel and they are operating as a single unit, wherein of said pair of pumps, the first pump of said pair of pumps has a higher flow capacity and can operate to a lower pressure and the second pump of said pair of pumps has a lower flow capacity and can operate at a higher pressure.

Efficiency improvements may also be achieved by utilising more than two compression tanks. Figure 10 is a schematic representation showing utilisation of more than two compression tanks. Figure 10 shows an additional compression tank 22n and the corresponding isolation valves 53n, 54n and check valves 63n, 64n which are in fluid communication via fluid lines (as indicated by dotted lines). The system configuration of a compression tank with the corresponding isolation valves, check valves and pressure sensor is such that the system is modular and utilisation of more than two compression tanks is easy and can even be done after the initial installation.

In some embodiments, efficiency improvements may be gained by utilising a variable speed pump 10 wherein the speed of said pump is adjusted so that towards the end of the compression stroke the pump speed and hence the liquid flowrate is reduced to allow a smooth change-over from one compression tank to another compression tank as the automatically controlled isolation valves are opened and closed. The timing of the operation of the isolation valves corresponding to each of the plurality of tanks is arranged so that the pump 10 or pumps do not reach stalling conditions.

Figure 1 1 is a schematic representation showing utilisation of more than two pumps 10, 10a, 10n and more than two compression tanks 21 , 22, 22n. Depending on system demands for pressure and quantity of compressed gas, system configuration can be determined to achieve optimum performance regarding speed and energy efficiency.

To make the system more resilient to time delays between the opening and closing of isolation valves, valves at the fluid entry and fluid exit of the pump may be utilised.

Figure 12 is a schematic representation showing utilisation of valves at the intake and exit of the pump. A check valve 66 is placed at the high-pressure side, i.e. the exit side of the pump. An isolation valve 57 is optionally placed at the low-pressure side, i.e. the intake side of the pump.

Claims

1 . A hydraulic compressor system for compressing gas, comprising: at least one pump for pumping liquid; a. a gas reservoir for storing compressed gas; b. at least two compression tanks, each of said compression tanks comprises:

• an expandable bladder defining a bladder cavity, wherein the expandable bladder contains a liquid in the expandable bladder cavity and the expandable bladder is configured to expand when liquid is input into the expandable bladder cavity;

• and a tank cavity outside of the expandable bladder, wherein the cavity comprises a compressible gas; wherein the expandable bladder and the tank cavity outside the expandable bladder form interdependent volumes of liquid and gas; c. a liquid line configured to fluidly couple the expandable bladder cavity with at least one of the one or more pumps such that the liquid is pumped into or out of the expandable bladder via the liquid line ; d. a gas line configured to fluidly couple the gas reservoir with the tank cavity such that the gas flows into or out of the tank cavity via the gas line; wherein, during operation of the system, liquid is pumped by the one or more pumps to the expandable bladder in the first compression tank and gas is expelled from the tank cavity of the first compression tank and directed to gas reservoir where it is compressed.

2. The hydraulic compressor system according to claim 1 , wherein the first compression tank is configured to operate in a compression stroke and in a return stroke; wherein, during the compression stroke, liquid is configured to be pumped into the expandable bladder, via the liquid line, such that the expandable bladder expands into the tank cavity and causes compression of the gas; and wherein during return stroke, liquid is configured to flow out from the expandable bladder via the liquid line such that the expandable bladder contracts and the tank cavity expands.

3. The hydraulic compressor system according to any one of claims 1 or 2, wherein the liquid line of the first compression tank is fluidly coupled to the liquid line of the second compression tank such that: when the first compression tank is configured to operate in the return stroke and the second compression tank is configured to operate in the compression stroke, liquid is configured to flow out from the expandable bladder of the first compression tank and to be fed, by the pump, into the expandable bladder of the second tank; and when the second compression tank is configured to operate in the return stroke and the first compression tank is configured to operate in the compression stroke, liquid is configured to flow out from the expandable bladder of the second compression tank and fed, by the pump, into the expandable bladder of the first tank.

4. The hydraulic compressor system according to any preceding claim, wherein the system further comprises: o an inlet line extending between a low pressure gas source and the tank cavity; o a check valve that controls the flow of gas between the tank cavity and the low pressure gas source; and o a check valve that controls the flow of gas between the tank cavity and the gas reservoir in the gas line.

5. The hydraulic compressor system according to any one of claims 1 -4, wherein the volumetric ratio of the volume of the tank cavity at the end of the compression stroke to the volume of the tank cavity at the beginning of the compression stroke (Vc/Vo) is <=0.1

6. The hydraulic compressor system of any one of the preceding claims, wherein each compression tank comprises a pressure sensor, wherein said pressure sensor is arranged to measure the pressure of the liquid within the expandable bladder.

7. The hydraulic compressor system of any one of the preceding claims, wherein the expandable bladder is characterized by a spring constant that defines a resistance to expansion.

8. The hydraulic compressor system according to any one of the preceding claims, wherein the pump is characterised by a pump stall pressure and the maximum pressure of the liquid in the expandable bladder is configured to be less than 10% of the pump stall pressure, and the minimum pressure of the liquid in the expandable bladder is less than 5% of the pump stall pressure.

9. A hydraulic compressor system according to any one of the preceding claims, wherein the system comprises a plurality of liquid lines, wherein each liquid line is configured to couple a hydraulic compression tank with one or more of the one or more pumps; wherein each liquid line comprises at least two isolation valves; wherein each liquid line is arranged to connect to each compression tank to one or more of the one or more pumps in a closed circuit; wherein each liquid line comprises a forward isolation valve and a return isolation valve; wherein when the hydraulic compression tank is configured to operate in the compression stroke, the forward isolation valve is open and the return isolation valve is closed; and wherein when the hydraulic compression tank is configured to operate in the return stroke the forward isolation valve is closed and the return isolation valve is open.

10. A hydraulic compressor system according to claim 9, wherein the system further comprises a sensor system and wherein the isolation valves are controlled by a control system; wherein the control system is configured to open or close the isolation valves based on data received from the sensors.

11. A hydraulic compressor system according to claim 10, wherein the sensor system comprises of a pressure sensor arranged to measure the pressure of the liquid within the expandable bladder of each hydraulic compression chamber and a pressure sensor arranged to measure the pressure within the gas reservoir.

12. A hydraulic compressor system according to claim 10 or claim 1 1 , wherein said sensor system comprises a flowmeter in fluid communication with the expandable bladder of each hydraulic compression chamber, and a pressure sensor arranged to measure the pressure within the gas reservoir.

13. A hydraulic compressor system according to any one of claims 10-12, wherein said sensors CONSIST of one flowmeter fluidically connected with pump output, and one pressure sensor fluidically connected with the reservoir no other sensors are necessary when the gas intake pressure is constant

14. A hydraulic compressor system according to any one of claims 10-13, wherein the control system is configured to: a. receive data corresponding to the pressure of the liquid inside the expandable bladder before the compression stroke is started; b. receive date corresponding to the pressure of the liquid inside the expandable during the compression stroke is started c. calculate the compression ratio (Vc/Vo) d. determine whether the compression ratio has reached the maximum value; e. and switch the isolation valves from being open to being closed, or being closed to being open, when the maximum value of the compression ratio has been reached.

15. A hydraulic compressor system according to any one of claims claim 1 1 -15, wherein the control system is configured to switch the isolation valves from being open to being closed, or being closed to being open, when the pump reaches a stalling condition defined by the pump current.

16. A hydraulic compressor system according to any one of claims 1 1 -16, wherein automatic switchover of system from a forward compression cycle of the first compression tank to a forward compression cycle of the second tank and reverse stroke of the first compression tank is happening with a time difference between the opening of the operation of the 2nd forward valve and the 1 st reverse valve and the operation of closing the 1 st forward valve and the closing of the 2nd reverse valve wherein this time difference is between 1/1 Oth and 1/2 of the time needed for full opening of a said isolation valve.

17. A hydraulic compressor system according to claim 1 , wherein said system comprises a pair of two pumps, said two pumps connected in parallel and operating as a single unit, wherein of said pair of pumps, the first pump of said pair of pumps has a higher flow capacity and can operate to a lower pressure and the second pump of said pair of pumps has a lower flow capacity and can operate at a higher pressure.

18. A hydraulic compressor system according to claim 1 , wherein said pump comprises a variable speed pump wherein the speed of said pump is adjusted so that towards the end of the compression stroke the pump speed is reduced and hence the liquid flowrate is reduced to allow a smooth change-over from one compression tank to another compression tank as the automatically controlled isolation valves are opened and closed.

19. A hydraulic compressor system according to claim 1 , wherein said plurality of compression tanks comprises three or more tanks wherein each tank is matched with dedicated passages through which incoming liquid may be fed forcing outflow of gas, with pressure sensors to measure liquid pressure or flowmeters to measure liquid flow and a pair of isolation valves to control the flow of liquid from the pump into to the compression tank during the compression stroke from the compression tank to the pump during the return stroke and wherein the timing of the operation of said isolation valves corresponding to each of said plurality of tanks is arranged so that the pump or pumps do not reach stalling conditions.

20. A method for compressing gas, the method comprising: a. providing a first compressor tank comprising: i. an expandable bladder defining a bladder cavity, wherein the expandable bladder contains a liquid in the expandable bladder cavity; ii. a tank cavity outside of the expandable bladder, wherein the cavity comprises a gas; iii. a liquid input; and iv. a gas port; v. connecting the gas port of the first compressor tank to a gas reservoir via a gas line; vi. connecting the liquid input of the first compressor tank to a first liquid line; b. operating the first compression tank in either a compression stroke or a return stroke, wherein operating the hydraulic compression tank in the compression stroke comprises: i. compressing the gas in the tank cavity of the first compressor tank by pumping a liquid into the expandable bladder cavity of the first compressor tank via the first liquid line such that the expandable bladder of the first compressor tank expands into the tank cavity; ii. displacing the gas out of the tank cavity of the first compressor tank, via the gas port, and collecting said gas in the gas reservoir; iii. further compressing the gas in the gas reservoir; and c. wherein operating the hydraulic compression tank in the return stroke comprises: i. pumping a liquid out of the expandable bladder cavity of the first compressor tank via the first liquid line such that the expandable bladder of the first compressor tank contracts, which in turn increases the volume of the tank cavity of the first compressor tank; ii. filling the tank cavity of the first compressor tank with gas from a low pressure gas source via the gas port.

21 . The method of claim 20, wherein the method further comprises: a. providing a second compressor tank comprising: i. an expandable bladder defining a bladder cavity, wherein the expandable bladder contains a liquid in the expandable bladder cavity; ii. a tank cavity outside of the expandable bladder, wherein the cavity comprises a gas; iii. a liquid input; and iv. a gas port; b. connecting the gas port of the second compressor tank to a gas reservoir via a gas line, wherein the first and second compressor tanks may be connected to the same gas reservoir or different gas reservoirs. c. connecting the liquid input of the second compressor tank to a second liquid line; wherein the first liquid line and the second liquid line are in fluid communication. d. operating the first compressor tank in a compression stroke and the second compressor tank in a return stroke comprising: e. compressing the gas in the tank cavity of the first compressor tank by pumping a liquid from the expandable bladder cavity of the second compressor tank and into the expandable bladder cavity of the first compressor tank such that the expandable bladder of the second compressor tank contracts and the expandable bladder of the first compressor tank expands; f. displacing the gas out of the tank cavity of the first compressor tank, via the gas port of the first hydraulic chamber, and collecting said gas in the gas reservoir; g. filling the tank cavity of the second compressor tank with gas from a low pressure gas source via the gas port of the second hydraulic chamber; and h. further compressing the gas in the gas reservoir.

22. The method of claim 21 , wherein the method further comprises: a. determining that the compression stroke of the first compressor tank has been completed and/or the return stroke of the second compressor tank has been completed; and b. switching the operation of the first and second compressor tanks such that the second compressor tank is operated in a compression stroke and the first compressor tank in a return stroke comprising: compressing the gas in the tank cavity of the second compressor tank by pumping a liquid from the expandable bladder cavity of the first compressor tank and into the expandable bladder cavity of the second compressor tank such that the expandable bladder of the first compressor tank contracts and the expandable bladder of the second compressor tank expands; c. displacing the gas out of the tank cavity of the second compressor tank, via the gas port of the second hydraulic chamber, and collecting said gas in the gas reservoir; d. filling the tank cavity of the first compressor tank with gas from a low-pressure gas source via the gas port of the second hydraulic chamber; and e. further compressing the gas in the gas reservoir.