GB2639264A

GB2639264A - Energy recovery system for fluid network

Info

Publication number: GB2639264A
Application number: GB2403730.1A
Authority: GB
Inventors: James Bennie Christopher
Original assignee: Energy Tech Ltd
Current assignee: Energy Tech Ltd
Priority date: 2024-03-14
Filing date: 2024-03-14
Publication date: 2025-09-17
Also published as: GB202403730D0; WO2025191250A1

Abstract

An energy recovery system 10 for a fluid (for example hydrogen, natural gas, nitrogen, air) network 12 comprises a multi-stage expander 16 having a plurality of expander stages 26a-c. The multi-stage expander has a controllable valve 60 between each adjacent expander stage which includes a valve member 72, a valve seat 70, and a valve actuator for producing a relative movement between the valve member and valve seat to open and close the valve. Expansion cylinders of each expander stage have a larger swept volume in sequence. A power output means 18 for extracting a power output from the multi-stage expander 20 is also provided. There may be an interstage reheater 32 provided between each adjacent expander stage. There may be an interstage scrubber 30 provided between each adjacent expander stage. The energy recovery system may generate electrical power at 50Hz or 60Hz.

Description

Energy Recovery System For Fluid Network The present invention relates to an energy recovery system for a fluid network, which is preferably but not necessarily suited for use on the domestic natural gas supply network. The invention further relates to a method for recovering energy from a fluid network.

Natural gas is transported over large distances at high pressure, typically 70 to 85 barg, as this results in significant reductions in frictional energy losses when compared with lower pressure transportation through a pipe of equivalent diameter. Fewer gas boosting stations are thus required. This is particularly important for transport of supply over long distance, particularly cross-continental and subsea flow, where boosting supply is more difficult. However, at the point of consumption, the supply is reduced in pressure to between 0.1 and 4 barg for domestic, commercial, and industrial use, not only because high-pressure end use is unnecessary, but also due to the increased danger associated with high-pressure supply. The drop in pressure at the point of supply results in energy waste.

Generally, compressors are used to compress the gas to a sufficiently high pressure for export in a high-pressure pipeline, though some natural reservoirs do have sufficiently high pressure to permit direct export without compression. To regulate from high pressure to low pressure, the natural gas is expanded via a regulator valve in an isenthalpic expansion process. This often leads to localised cooling; local regulation stations often can be seen icing over during expansion. This necessitates heating of the local regulator to prevent cold damage.

It is an object of the present invention to reduce or substantially obviate the aforementioned problems and to utilise the energy waste during the expansion process.

According to a first aspect of the invention, there is provided an energy recovery system for a fluid network, the energy recovery system comprising: a high-pressure fluid inlet associated with a high-pressure part of a fluid network; a low-pressure fluid outlet associated with a low-pressure part of the fluid network; a multi-stage expander having a plurality of expander stages between the high-pressure fluid inlet and the low-pressure fluid outlet, the multi-stage expander having a controllable valve between each adjacent expander stage of the plurality of expander stages, the or each controllable valve including a valve member, a valve seat, and a valve actuator for producing a relative movement between the valve member and valve seat to open and close the controllable valve, wherein each expander stage includes an expansion cylinder, and wherein each successive expander stage has an expansion cylinder of a larger swept volume to an expansion cylinder of a preceding expander stage; and a power output means for extracting a power output from the multi-stage expander.

The use of the specific valve arrangements of the present invention allows for much greater control over the fluid inlets and outlets of the expander stages. As such, flow control is much improved, which enables the present system to have much smoother electricity generation capabilities regardless of the incoming fluid flow rate. By increasing the size of the cylinders, specifically by increasing the swept diameter, sequentially through the multi-stage expander, a consistent amount of energy can be extracted via expansion through the various stages. Force is equal to the pressure multiplied by the area, and therefore a larger area is required to compensate for the reduced pressure at each stage to maintain the force output. If the force output is equalised across the various stages, the power output from the system is consistent for each stage, and the power output from the multi-stage expander can be extracted via a single crankshaft.

Preferably, each controllable valve may be a controllable plate valve in which the valve member is a valve plate.

Plate valves have the advantage of being compact, ensuring that the apparatus implementing the system can be contained in a relatively small volume, whilst also being controllable with high precision.

Optionally, the power output means may comprise a continuously variable 25 transmission.

A continuously variable transmission ensures that a correct frequency output from the power output means is provided, so that, regardless of flow rate of the fluid network, the electricity generated has a constant frequency. If the expander is operating at a non-synchronous speed for power generation, the continuously variable transmission ensures that the generator is spun at a synchronous speed regardless.

Preferably, each expander stage may comprise a double-acting reciprocating expander which expands on an inbound stroke and an outbound stroke.

A double-acting reciprocating expander can have a short-stroke design, typically being between 8cm and 15cm in length, which allows the expander to operate at high revolutions of 1500 RPM up to 1800 RPM. This makes the expander ideally suited to producing power for an electrical generator at 50 Hz or 60 Hz, which are the commonly used frequencies within national power grids. A short stroke design also makes for a compact unit, which can be easily transported by road or rail, as well as enabling the desirable high speed operation.

Optionally, the valve member may comprise an offset aperture array with respect to a valve opening of the valve seat.

A valve will have the best sealing properties where such an offset aperture array which seals against the valve seat.

The valve member may be spring-biased to inhibit recoil damage thereof.

Under the pressure of the upstream gas moving through the valve, there is a realistic risk of damage when the valve member collides with the valve seat. As such, the presence of a spring biasing means limits this risk by damping the motion of the valve member.

Preferably, the valve actuator may comprise an external piston configured to actuate the valve member relative to the valve seat.

An external piston is the simplest means of introducing a controllable valve into the system, as a moving piston is able to be controlled and timed with high precision.

The valve actuator may comprise an electromagnet co-located with the valve seat to actuate the valve member.

Electromagnetic control eliminates the need for a moving member penetrating the expansion cylinder, reducing the risk of leaks. Magnetic actuation is also capable of being controlled very precisely.

Optionally, each expansion cylinder may be devoid of a gland seal associated with the controllable valve, and specifically with the piston actuator of the controllable valve.

The absence of a gland seal or similar removes a potential leak path from the apparatus.

The energy recovery system may further comprise a programmable logic controller for controlling a timing of the or each valve actuator of the or each controllable plate valve.

A programmable logic controller is a microchip-based control device which based on a preset program can electronically control the rapid sequential movements of the cylinder intake and outlet valves.

A programmable logic controller allows for precision control of valve timing, which can change the speed of the expander in use to ensure that the operation is synchronized for constant speed of operation. This means that a consistent electricity generation can be achieved. The use of programmable logic controllers allows for highly accurate control of the valves without needing to use any type of cam or mechanical means to synchronize the opening and closing across different expander stages. This allows for precise control of the timings of the valves, as well as reducing the size of the apparatus.

Each expander stage may comprise two inlet said controllable plate valves, and two outlet said controllable plate valves, wherein the two inlet controllable plate valves are spaced either side of a central region of the expansion cylinder, so as to be either side of the piston head, and the two outlet controllable plate valves are spaced either side of the central region.

The multi-valve arrangement considered here has the advantage of enabling double-action reciprocating expansion, so that expansion occurs on both strokes of the piston in a usable manner. An increased energy generation efficiency is achieved.

Preferably, the multi-stage expander may comprise at least three expander stages.

The more stages of expansion that can be provided, particularly if interstage heating can be achieved merely with ambient heating, the greater the potential energy extraction from the energy recovery system.

Optionally, the multi-stage expander comprises an interstage reheater assembly between each adjacent expander stage.

Interstage reheating avoids the temperature becoming too low between expansion stages that might necessitate cryogenic-compatible construction materials. Additionally, the use of ambient or additional heaters increases the temperature of the gas to increase the available energy that can be extracted via downstream expansion.

This improves the efficiency of the process for extracting work from the working fluid during the expansion process, particularly if heating is performed using ambient air or water.

The interstage reheater assembly may comprise a higher-temperature medium stream, preferably comprising a heated waste fluid from the system.

The most efficient means of improving heat gain through the system for greater energy return is by using waste heat from, e.g., steam or hot water generated as a waste product from a nearby power station.

The multi-stage expander may comprise an inlet and interstage scrubber between each adjacent expander stage.

Interstage scrubbers allow for the removal of liquid phase fluid which is formed through the expansion process, so as to ensure efficient downstream expansion.

The energy recovery system may further comprise an electrical generator associated with the power output means.

The system is preferably supplied with an electrical generator, so that it can be provided as a full electricity generating apparatus when installed into a gas network, for instance. It will of course be appreciated that the output from the power output means could be used to drive a mechanical work apparatus, for instance, driving a hydraulic apparatus.

Optionally, a piston of each of the expansion stages of the multi-stage expander may be connected to a common crankshaft.

The output of power via a common crankshaft ensures that the power from the system is output in a manner which can directly drive a generator.

In one embodiment, the multi-stage expander may operate at either 1500 RPM or 1800 RPM for generating electrical power at either 50 Hz or 60 Hz respectively.

If the expander can be run at these frequencies, electricity which is directly usable in a standard power network can be generated without any need for transformation.

The fluid of the fluid network may be any of: hydrogen; natural gas; nitrogen; air; or a combination thereof.

Preferably, the expander stages of the multi-stage expander may be provided in a horizontal plane.

Multi-stage compressors are often formed such that the compressors are in a horizontal plane. In the present invention, it is possible to retrofit existing multi-stage compressors into a form which permits the present expansion process to occur, without necessarily needing to design extensive additional machines to compensate.

Furthermore, horizontal opposition of the expander stages serves to balance the crankshaft, reducing vibrational effects which might otherwise damage or deform the crankshaft.

The energy recovery system may further comprise at least one pulsation vessel associated with the multi-stage expander.

Pulsation vessels provide room for the fluid in the system to move, dampening pulsation and therefore the risk of vibrational damage to the system.

According to a second aspect of the invention, there is provided a method for recovering energy from a fluid network, the method comprising the steps of: a] connecting a multi-stage expander between a high-pressure part of a fluid network and a low-pressure part of the fluid network, the multi-stage expander having a plurality of expander stages, the multi-stage expander having a controllable valve between each adjacent expander stage of the plurality of expander stages, the or each controllable valve including a valve member, a valve seat, and a valve actuator for producing a relative movement between the valve member and valve seat to open and close the controllable valve, wherein each expander stage includes an expansion cylinder, and wherein each successive expander stage has an expansion cylinder of a larger swept volume to an expansion cylinder of a preceding expander stage; b] extracting a power output from the multi-stage expander; and c] generating electricity using the power output of the multi-stage expander.

The present system provides a unique option for energy generation using processes that are already necessary as part of gas transport systems for local distribution, and can therefore provide an environmentally friendly means of electricity generation.

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made by way of example only to the accompanying drawings, in which: Figure 1 shows a pictorial representation of an energy recovery system in accordance with a first aspect of the invention; Figure 2A shows a representation from the side of a representative embodiment of the energy recovery system of Figure 1; Figure 2B shows the energy recovery system of Figure 2A from above; Figure 3 shows a diagrammatic representation of the energy recovery system of Figure 2A; Figure 4 shows a detailed representation of an expander stage of the energy recovery system of Figure 3; Figure 5A shows a cross-sectional representation of a first embodiment of a controllable plate valve for an inlet of the energy recovery system of Figure 4; Figure 5B shows a cross-sectional representation of a first embodiment of a controllable plate valve for an outlet of the energy recovery system of Figure 4 Figure 6 shows a plan view of the valve plate of Figure 5; and Figure 7 shows a cross-sectional representation of a second embodiment of a controllable plate valve of the energy recovery system of Figure 4.

Referring firstly to Figure 1, there is shown a representative energy recovery system, referenced globally at 10, which is coupled to a fluid network 12, typically a gas supply network. The fluid network 12 has a high-pressure part 14a, most likely associated with a transport-side of the fluid network 12, and a low-pressure part 14b, most likely associated with a local distribution side of the fluid network 12.

The energy recovery system 10 has a multi-stage expander 16 connected between a high-pressure inlet 18a and a low-pressure outlet 18b of the fluid network 12, from which, there is a power output means 20 for extracting power from the expansion process during the transition of the fluid from high pressure to low pressure.

An indicative arrangement of an apparatus 22, including the multi-stage expander 16 forming the operating part of the energy recovery system 10 is shown in Figures 2A and 2B. Components are mounted onto a support structure 24, which may provide a convenient unit for transport. The apparatus 22 comprises the multi-stage expander 16 having a plurality of expander stages 26a, 26b, 26c, which is coupled to an electricity generator 28 via the power output means 20. Additional voluminous components may include suction scrubbers 30 associated with each of the expander stages 26a, 26b, 26c, and a heater assembly 32 for mitigating the effects of expansion-based cooling of the apparatus 22.

The apparatus 22 has a compact configuration which allows it to be readily transported to site either by road or rail as a single unit.

Figure 3 shows a schematic diagram of the energy recovery system 10 between the high-pressure inlet 18a and the low-pressure outlet 18b.

Fluid enters the multi-stage expander 16 via an inlet valve 34, and may pass through a preliminary scrubber 30 and heater 36, which may be part of the heater assembly 32, and may include ambient or elevated heating. This removes liquid from the incoming fluid and increases the possible energy extraction respectively. The fluid enters into a first expander stage 26a, preferably via a pulsation vessel 38.

The heater 36, and indeed any of the heaters described herein, may be provided having a higher-temperature medium stream such as steam or hot water, and if this is generated as part of the system, this can be an efficient means of extracting further energy or power from the working fluid being expanded. Alternatively, it may be possible to extract a slipstream of fluid from the fluid network which can power a fired heater, to further improve energy gains, if this proves thermodynamically efficient.

Upon expansion from the first expander stage 26a, the fluid preferably passes through a further pulsation vessel 38, a further scrubber 30 and heater 36. This again removes liquid, and increases the available energy that can be retrieved via expansion, before passing into a further pulsation vessel 38. This precedes the second expander stage 26b.

In a preferred embodiment, a dimension, here bore diameter BDa, of the expansion cylinder of the first expander stage 26a is less than a dimension BDb of the expansion cylinder of the second expander stage 26b. This is carefully determined to enable a consistent power output from each expander stage 26a, 26b despite the decreasing potential expansion energy at each expander stage 26a, 26b.

The fluid exiting the second expander stage 26b passes through a further pulsation vessel 38, a further scrubber 30 and heater 36. This again removes liquid, and increases the available energy that can be retrieved via expansion.

In the depicted embodiment, there are two third expander stages 26c, and the fluid flow is split prior to entry into the entry pulsation vessels 38 of said third expander stages 26c. A dimension BDb of the expansion cylinder of each of the third expander stages 26c is equal to one another, and greater than the dimension BDa, BDb of each of the first and second expander stages 26a, 26b.

The fluid exiting the third expander stages 26c passes through a further pulsation vessel 38, before the flow lines are recombined. The pressure of the fluid should therefore be sufficient to enter the low-pressure part 14b of the fluid network 12. A further let-down valve 43 may be provided to ensure matching of the pressure exiting the multi-stage expander 16 into the low-pressure outlet 18b. A heater 42 is also indicated, heated by ambient air, to ensure the low pressure gas is warmed to a sufficient extend so as not to case any damage associated with low temperature to the low pressure part 14b of the network.

Valves, as described in detail in Figures 4 to 7, associated with each of the expander stages 26a, 26b, 26c, are controlled by a centralised electronic control unit 40, which is here provided as a programmable logic controller. In particular, the electronic control unit 40 controls valve opening timing, but could also be used to adjust valve timing to prevent interstage overcooling. Specifically, the electronic control unit 40 will alter the timing of the valve opening is there is excessive interstage cooling, to prevent the system from reaching cryogenic temperatures, which might otherwise necessitate more expensive construction materials. If there was too much expansion or cooling happening on a single stage, the electronic control unit 40 will reduce the opening time of the upstream cylinder exhaust valve. This means that on the next stroke there is more unexpanded, or unswept, gas from the previous stroke, which means the incoming gas is not able to expand to the same extent and therefore this mechanism will keep the interstage temperature higher. However, reducing the valve opening time, will slow the expander speed, as it will act as a braking mechanism. Changing expander speed will result in a non-synchronous speed for power generation. To compensate for this, the continuously variable transmission 50 will automatically adjust to keep the generator at asynchronous speed aspect for power generation.

A bypass line may be provided which allows for ejection of fluid within the multi-stage expander 16 via a bypass valve 44. The bypass may be used for start-up or shut-down purposes of the apparatus, to ensure continuous flow of gas to the low pressure part of the network.

The power output means of the present embodiment comprises a crankshaft 48 which is connected to a piston 46a, 46b, 46c of each of the expander stages 26a, 26b, 26c simultaneously. The variation in the dimensions of the expansion cylinders of the expander stages 26a, 26b, 26c ensures that the simultaneous drive of the crankshaft by all pistons 46a, 46b, 46c is achieved.

A continuously variable transmission 50 is also provided, which ensures that the output frequency of the crankshaft 48 is produced so as to be directly exploitable. Preferably, the output frequency is either 50 Hz or 60 Hz, and the associated generator 28 is provided to generate electricity based on this frequency so as to be directly usable without further transformation. A continuously variable transmission 50 is an automated transmission capable of changing through a continuous range of gear ratios. This allows the continuously variable transmission 50 to enable operation of the electricity generator 28 at a constant angular velocity.

An indicative expander stage 26 is illustrated and described in Figure 4.

The expander stage 26 has an expansion cylinder 52 having a core chamber 54 within which a reciprocating piston 46 is operable. Fluid flow passes through the expansion cylinder 52 in a direction perpendicular to the direction of operation of the piston 46.

A flow inlet 58 of the expansion cylinder 52 is connected to an inlet region 56 having two spaced apart valves 60a which control flow into the core chamber 54. The valves 60a allow fluid introduction either side of the head 62 of the piston 46 so that the expander stage 26 can achieve expansion on both forward and backward strokes.

The piston head 62 is connected to the push rod 65, which connects to the crosshead 67. The crosshead 67 is connected by connecting rods to the crankshaft 48. The crosshead 67 is a well-known mechanism for converting the oscillating movements of the connecting rod and crankshaft 48 into a backwards and forwards motion in a single horizontal plane, to move the piston 46 backwards and forwards. The expansion cylinders 26 are connected to the crankshaft casing by a cylinder distance piece 69. The cylinder distance piece 69 houses the crosshead 67 and portions of the connecting rod and push rod 65. The push rod 65 passes through a gland seal packing 71 into the expansion cylinder 26, whereby the gland seal packing 71 isolates the high pressure of the expansion cylinder 26 from the cylinder distance piece 69 and crankshaft 48, which normally operate at just above atmospheric pressure.

The valves 60a, 60b are respectively positioned either side of a central region 63 of the core chamber 54 so that separate chamber regions are formed. This allows for the double-acting reciprocation to be achieved, allowing for short-stroke pistons 46 to be used. It will be apparent however that a single-acting piston could be used within the scope of the present invention.

A corresponding pair of spaced apart valves 60b is provided on an opposite side of the core chamber 54, to connect the core chamber 54 with an outlet region 64 in communication with a flow outlet 66 of the expansion cylinder 52.

Valve control is achieved by the electronic control unit 40, timing so as to ensure pressure reduction across the expander stage 26. The expansion process drives the piston 46, which in turns drives the crankshaft 48 to produce power from which electricity can be generated.

An exemplary valve arrangement is shown in Figures 5A and 5B, respectively showing inlet and outlet valves. The valve 60, 60' is a magnetically operated plate valve, having a valve member, shown in the form of a valve plate 68 and a valve seat 70. Other valve arrangements which are not controllable plate valves and which have different valve members may also be considered. Figure 6 shows an indicative configuration for the valve plate 68, having a plurality of concentric apertures 72.

The valve seat 70 has a plurality of inlets 72, which may also have a concentric arrangement, offset to those of the valve plate 68 so that closure of the valve 60, 60' is achievable when the valve plate 68 is directly in contact with a portion 76 of the valve seat 70 adjacent to the inlets 72. Closure of the valve 60, 60' is achieved in this arrangement by the use of a magnetic closure means, here in the form of a circular electromagnet 78 located in the portion 76 of the valve seat 70 adjacent to the inlets 72. Control of the electromagnet 78 is achieved via electrical connection to the electronic control unit 40 through a closed member 80 which extends through the expansion cylinder 52. Since there is no external moving part, no additional sealing is required, avoiding potential leak paths.

In an open condition of the valve 60, 60', the valve plate 68 is spaced from the portion 76 of the valve seat 70 adjacent to the inlets 72, and fluid can flow around the valve plate 68 and through one or more outlets 82 of the valve seat 70. The valve 60, 60' type can be used on the inlet or outlet of the core chamber 54, but the valve 60, 60' must be slightly reconfigured due to the unidirectionality of the valve 60, 60' so that the correct pressure flow through the core chamber 54 is maintained.

When the electromagnet 78 is deactivated, one or more springs 84 or similar biasing means will provide a dampening effect as the magnetic force is released so that the high-pressure fluid flow through the valve 60, 60' does not cause the valve plate 68 to become damaged by colliding with the valve seat 70 with great force.

An alternative valve embodiment is shown in Figure 7. Identical or similar components will be referenced using identical or similar reference numerals, and further detailed description is omitted for brevity.

The valve 160 comprises a valve seat 170 of the same form as described for the magnetic valve 60. The valve plate 168 also has a similar apertured structure to that shown in Figure 6. However, the mechanism of actuation of the valve plate 168 is different. An actuating member 186 is provided which is connected to the valve plate 168 and is moveable towards and away from the portion 176 of the valve seat 170 adjacent to the inlets 174.

Since there is an actuating member 188, there does need to be a connection to the exterior of the expansion cylinder 152, and therefore additional sealing is required. A gland seal 186 is indicated which seals to a shaft 190 of the actuating member 186; however, there is a greater risk of leak from the expansion cylinder 152.

Whilst two forms of plate valve are disclosed herein which can be accurately controlled for precise timing, it will be appreciated that other forms of valve, plate valve or otherwise, may be incorporated into the present invention without deviating from the overall inventive concept.

It is therefore possible to provide an energy recovery system for a fluid network which utilises the necessary expansion process at a local distribution level to generate electricity. The present invention overcomes the difficulty of synchronisation of the energy output by the various expansion stages by the use of very precise timing control over the valves used.

The words 'comprises/comprising' and the words 'having/including' when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps, or components, but do not preclude the presence or addition of one or more other features, integers, steps, components, or groups thereof.

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

The embodiments described above are provided by way of example only, and various changes and modifications will be apparent to persons skilled in the art without departing from the scope of the present invention as defined by the appended claims.

Claims

CLAIMS1. An energy recovery system for a fluid network, the energy recovery system comprising: a high-pressure fluid inlet associated with a high-pressure part of a fluid network; a low-pressure fluid outlet associated with a low-pressure part of the fluid network; a multi-stage expander having a plurality of expander stages between the high-pressure fluid inlet and the low-pressure fluid outlet, the multi-stage expander having a controllable valve between each adjacent expander stage of the plurality of expander stages, the or each controllable valve including a valve member, a valve seat, and a valve actuator for producing a relative movement between the valve member and valve seat to open and close the controllable valve, wherein each expander stage includes an expansion cylinder, and wherein each successive expander stage has an expansion cylinder of a larger swept volume to an expansion cylinder of a preceding expander stage; and a power output means for extracting a power output from the multi-stage expander.
2. An energy recovery system as claimed in claim 1, wherein the or each controllable valve is a controllable plate valve in which the valve member is a valve plate.
3. An energy recovery system as claimed in claim 1 or claim 2, wherein the power output means comprises a continuously variable transmission.
4. An energy recovery system as claimed in any one of the preceding claims, wherein each expander stage comprises a double-acting reciprocating expander which expands on an inbound stroke and an outbound stroke.
5. An energy recovery system as claimed in any one of the preceding claims, wherein the valve plate comprises an offset aperture array with respect to a valve opening of the valve seat.
6. An energy recovery system as claimed in any one of the preceding claims, wherein the valve member is spring-biased to inhibit recoil damage thereof.
7. An energy recovery system as claimed in any one of the preceding claims, wherein the valve actuator comprises an external piston configured to actuate the valve member relative to the valve seat.
8. An energy recovery system as claimed in any one of the preceding claims, wherein the valve actuator comprises an electromagnet co-located with the valve seat to actuate the valve member.
9. An energy recovery system as claimed in claim 8, wherein each expansion cylinder is devoid of a gland seal associated with the controllable valve.
10. An energy recovery system as claimed in any one of the preceding claims, further comprising a programmable logic controller for controlling a timing of the or each valve actuator of the or each controllable valve.
11. An energy recovery system as claimed in any one of the preceding claims, wherein each expander stage comprises two inlet said controllable valves, and two outlet said controllable valves, wherein the two inlet controllable valves are spaced either side of a central region of the expansion cylinder and either side of a piston head and the two outlet controllable valves are spaced either side of the central region and either side of the piston head.
12. An energy recovery system as claimed in any one of the preceding claims, wherein the multi-stage expander comprises at least three expander stages.
13. An energy recovery system as claimed in any one of the preceding claims, wherein the multi-stage expander comprises an interstage reheater assembly between each adjacent expander stage.
14. An energy recovery system as claimed in claim 13, wherein the interstage reheater assembly comprises a higher-temperature medium stream.
15. An energy recovery system as claimed in any one of the preceding claims, wherein the multi-stage expander comprises an interstage scrubber between each adjacent expander stage.
16. An energy recovery system as claimed in any one of the preceding claims, further comprising an electrical generator associated with the power output means.
17. An energy recovery system as claimed in any one of the preceding claims, wherein a piston of each of the expansion stages of the multi-stage expander is connected to a common crankshaft.
18. An energy recovery system as claimed in any one of the preceding claims, wherein the multi-stage expander operates at either 1500 RPM or 1800 RPM for generating electrical power at either 50 Hz or 60 Hz respectively.
19. An energy recovery system as claimed in any one of the preceding claims, wherein the fluid of the fluid network is any of: hydrogen; natural gas; nitrogen; air; or a combination thereof.
20. An energy recovery system as claimed in any one of the preceding claims, wherein the expander stages of the multi-stage expander are provided in a horizontal plane.
21. An energy recovery system as claimed in any one of the preceding claims, further comprising at least one pulsation vessel associated with the multi-stage expander.
22. A method for recovering energy from a fluid network, the method comprising the steps of: a] connecting a multi-stage expander between a high-pressure part of a fluid network and a low-pressure part of the fluid network, the multi-stage expander having a plurality of expander stages, the multistage expander having a controllable valve between each adjacent expander stage of the plurality of expander stages, the or each controllable valve including a valve member, a valve seat, and a valve actuator for producing a relative movement between the valve member and valve seat to open and close the controllable valve, wherein each expander stage includes an expansion cylinder, and wherein each successive expander stage has an expansion cylinder of a larger swept volume to an expansion cylinder of a preceding expander stage; b] extracting a power output from the multi-stage expander; and c] generating electricity using the power output of the multi-stage expander.