GB2407921A

GB2407921A - Electricity generating unit

Info

Publication number: GB2407921A
Application number: GB0325631A
Authority: GB
Inventors: Richard Martin
Original assignee: TOTAL RECLAIM SYSTEMS Ltd
Current assignee: TOTAL RECLAIM SYSTEMS Ltd
Priority date: 2003-11-04
Filing date: 2003-11-04
Publication date: 2005-05-11
Also published as: GB0325631D0

Abstract

There is disclosed an electricity generating unit and an electricity generating assembly comprising a plurality of such electricity generating units. In one embodiment, an electricity generating unit (10) is disclosed which is adapted to operate on a hazardous drive fluid, in particular natural gas (16). The electricity generating unit (10) comprises a fluid actuated driver (12), and a generator (14) coupled to the driver (12) and adapted to be driven by the driver (12) to generate electricity wherein, in use, at least part of the generator (14) is adapted to be exposed to the drive fluid (16) used to actuate the driver (12).

Description

1 2407921

ELECTRICITY GENERATING UNIT

The present invention relates to an electricity generating unit and to an electricity generating assembly comprising a plurality of electricity generating units.

In particular, but not exclusively, the present invention relates to an electricity generating unit having a fluid actuated driver and a generator coupled to and driven by the driver, and to an electricity generating assembly comprising a plurality of such electricity generating units.

Conventional fluid actuated electricity generating units comprise a fluid actuated driver, such as a turbine, and a generator coupled to and driven by the turbine. The generator is mechanically coupled to the turbine, but is isolated from drive fluid used to actuate the turbine. This is because the generator would be susceptible to damage if exposed to the drive fluid, which is often at a high pressure/temperature, or which may be corrosive to the internal elements of the generator.

Some electricity generating units employ potentially hazardous fluids as the drive fluid for actuating the turbine, which fluids may be flammable or explosive. In such situations, it is particularly important to ensure that the generator is isolated from the drive fluid, so that there is no accidental ignition of the drive fluid due to, for example, electrical discharges (sparks) emitted by the generator.

Isolation of the generator from the turbine in conventional units is achieved in one of two different ways. In one type of unit, a rotor of the generator is coupled to a drive shaft of the turbine, and is rotated to generate electricity. The turbine drive shaft passes through a seal which isolates the generator from the turbine drive fluid. However, in use, the seal tends to experience wear due to frictional contact with the shaft, which can result in leakage of drive fluid past the seal.

This is undesirable, particularly when the drive fluid is a hazardous fluid.

In alternative types of trait, a magnetic coupling is provided between the turbine drive shaft and the rotor, avoiding the requirement to seal the turbine drive shaft.

However, in use, magnetic couplings generate considerable heat, which is generally undesired, and which would be of particular concern if operating in a hazardous atmosphere.

One example of a potential use for electricity generating units operated using a hazardous drive fluid is in the oil and gas exploration and production industry. Completed gas wells are provided with control installations for controlling production of gas from the well. In the offshore environment (or in remote locations), these installations are typically unmanned, controlled from an onshore control Gentle, and may not be visited for long periods. The installations require a power supply for operation of electrical control systems and, especially in the offshore environment, it is not convenient to make a cost-cifective connection to an available power source. Accordingly, alternative power sources are required.

Petrol or diesel generators are avoided for safety reasons and due to the requirement to ensure a continued supply of fuel, therefore solar panels or wind generators tend to be used. However, these do not provide a sufficiently reliable source of power and, in particular in the case of installations located offshore or in other remote installations, are susceptible to theft.

It has therefore been proposed to use the recovered natural gas as a drive fluid for an electricity generating unit of the type described above, as the installation power requirements are relatively low and are unlikely to cause a significant reduction in the pressure of the recovered gas.

However, the problems of using hazardous fluids discussed above have lead to difficulties in production of a viable unit.

Another example of a potential use for electricity generating units operated using a hazardous drive fluid is in gas supply networks. Currently, it is common to supply natural gas for domestic (or low level industrial) use from a main storage/supply facility through high pressure transportation and distribution systems. This is particularly true of the USA where the distribution systems may include supply pipelines hundreds of miles in length.

Before the gas can be supplied to the end user, the gas pressure must be reduced to a safe operating level.

This is achieved by supplying the gas to an intermediate pressure let down station, which allows the gas to expand and reduce in pressure.

It has been proposed to generate power in these pressure let down stations using the natural gas as a drive fluid. However, the difficulties of operating using natural gas have led to problems in development of a viable unit.

It is amongst the objects of embodiments of the present invention to obviate or mitigate at least one of the foregoing disadvantages.

According to a first aspect of the present invention, there is provided an electricity generating unit comprising: a fluid actuated driver; and a generator coupled to the driver and adapted to be driven by the driver to generate electricity; wherein, in use, at least part of the generator is adapted to be exposed to a drive fluid used to actuate the driver.

By providing an electricity generating unit having a generator which is exposed to a drive fluid used to actuate the driver, problems associated with sealing shafts associated with the generator, and with magnetic couplings of known electricity generating units, are avoided. It will be understood that, to achieve this, the generator is designed to be capable of operating safely in the drive fluid, and may be designed such that there is little or no deterioration in performance or operation of the generator through its exposure to the drive fluid. Indeed, as will be discussed below, the drive fluid may be utilised to dissipate heat produced by the generator in operation, enhancing generator performance.

Preferably, the unit comprises a housing in which the driver and the generator are mounted.

Preferably also, the generator is directly coupled to the driver, and may be coupled to a drive shaft of the driver. The generator may comprise a rotor and a stator and the rotor may be directly coupled to the driver.

The unit may be adapted to operate at a pressure above ambient atmospheric pressure. Thus the driver may be adapted to be driven/actuated by drive fluid at a pressure above ambient pressure. The unit may be operated using a hazardous fluid, for example, a potentially flammable and/or explosive fluid. Thus the driver may be adapted to be driven/actuated by a hazardous fluid. In preferred embodiments of the invention, the unit may be operated using a natural gas (or equivalent), thus the driver may be adapted to be actuated by natural gas. This facilitates power generation utilizing the inherent pressure of recovered natural gas. The unit may be adapted to be coupled directly to a source of natural gas, and may be adapted to be provided as part of a production facility/installation for recovering the gas, for example, an offshore gas installation. It will be understood that the pressure of such gas recovered from depth in a gas well is considerably higher than ambient, surface atmospheric pressure.

Alternatively, the unit may be adapted to be provided in a gas supply network, between a supply of natural gas (or equivalent) and an end user. For example, the unit may be provided in a pressure let down facility adapted to reduce the pressure of the gas. The unit may be coupled to a gas storage facility by a relatively high pressure supply conduit, and may include a plurality of relatively low pressure outlet conduits, for supply to end users. This allows electricity generation by making use of the required pressure let down in gas supply networks.

In alternative embodiments, any other hazardous fluid may be used as the drive fluid, for example, alternative hydrocarbon containing fluids.

Preferably, the generator is in fluid communication with the driver, and the driver and the generator may be mounted in a fluid chamber of the unit. The fluid chamber may be defined by the housing, and may be sub divided into a driver chamber portion and a generator chamber portion, the driver and generator chamber portions in fluid communication.

The unit may comprise an inlet for flow of drive fluid into the unit, and an outlet for flow of drive fluid out of the unit. In embodiments of the invention, the unit may comprise a plurality of outlets. This may facilitate flow of drive fluid from the unit via a selected at least one of a plurality of outlets. The unit may define at least one drive fluid flow path for flow of drive fluid therethrough. In embodiments of the invention, the unit may comprise a plurality of drive fluid flow paths and may comprise a first drive fluid flow path for flow of fluid between an inlet and a first outlet, a second drive fluid flow path for flow of fluid between an inlet and a second outlet, and a third drive fluid flow path for flow of fluid between an inlet and a third outlet. This may facilitate flow of fluid along a desired path through the unit. The unit may comprise a single inlet common to each outlet/flow path.

Alternatively, the unit may comprise an inlet corresponding to each outlet, thus may include a dedicated inlet for each outlet. The unit may comprise at least one outlet arranged to define a flow path through at least part of the generator. Flow of drive fluid through the generator in this fashion may provided a cooling effect tending to cool the generator, in use.

At least one fluid flow path may be adapted to be closed by closing the outlet. Thus where there are a plurality of fluid flow paths, a selected one or more of the fluid flow paths may be adapted to be closed by closing the respective outlet(s). The unit may comprise a control assembly associated with the outlet(s) for controlling flow of fluid through the unit. The control assembly may comprise a valve and where there are a plurality of outlets, may comprise a manifold or the like connected between the outlets. This may facilitate flow of fluid from a selected one or more of the outlets to a desired, common location.

Preferably, the driver comprises a turbine and may comprise a radial flow turbine, that is, a turbine of a type where drive fluid is directed generally tangentially onto the turbine blades. Where natural gas or the like is used as the drive fluid, it will be understood that the turbine is an impulse turbine. The driver may comprise a turbine having a plurality of turbine stages, for example, a plurality of adjacent rotor/stator stages having respective rotor/stator blades. In embodiments of the invention, the driver may comprise a pelton wheel type turbine. A pelton wheel type turbine offers advantages in terms of simplicity of construction and operation, with associated reductions in maintenance and operating costs. Furthermore, where the unit is operated using a hazardous fluid such as a natural gas, the relatively simple construction of a pelton wheel type turbine may offer advantages over alternative types of driver. This is because, as is known in the oil and gas exploration and production industry, condensate (liquid that condenses out of natural gas) can form in recovered gas, particularly where the gas experiences a drop in pressure. Formation of such condensates will not affect operation of a pelton wheel type turbine, but may cause operational difficulties will alternative types of driver. However, in alternative embodiments, the driver may comprise an alternative radial flow turbine such as a Rushton or Smith type turbine, an axial flow turbine, a motor such as an air motor or any other suitable alternative driver.

In preferred embodiments of the invention, at least part of the generator may be shielded or isolated from the drive fluid. The generator may comprise a stator having stator windings and the stator windings may be shielded or isolated from the driver fluid. Where the unit is operated using a hazardous fluid such as natural gas, this may prevent generation of any electrical discharges (sparks) within the drive fluid during operation of the generator, thereby avoiding the potential for ignition of the drive fluid. The windings may be shielded or isolated from the drive fluid, and may be coated with a coating material such as a resin, adhesive, sealant or any other suitable material. The windings may be coated subsequent to assembly of the stator, or the material used to form the windings may be coated prior to assembly of the stator.

The unit may be adapted to be evacuated prior to use and in particular prior to flow of drive fluid into the unit. Alternatively, the unit may be adapted to be filled or at least partly filled with an inert fluid, such as Nitrogen, prior to flow of drive fluid into the unit. Where the drive fluid is a hazardous fluid such as natural gas, evacuation of the unit, or filling with an inert fluid, may ensure that there is no accidental ignition of the gas. This is because, as is well known, in order for a fuel such as natural gas to ignite, there must be a sufficient supply of air to enable a combustible mixture.

The unit may be adapted to be coupled to a producing gas well by suitable production tubing, and may be provided at surface in a supply line for the gas. The unit may be designed such that the entire production of gas from the well flows through the unit, or only part of the total production from the well may flow through the unit.

According to a second aspect of the present invention, there is provided an electricity generating assembly comprising a plurality of electricity generating units, each electricity generating unit having: a fluid actuated driver; and a generator coupled to the driver and adapted to be driven by the driver to generate electricity; wherein, in use, at least part of the generator is adapted to be exposed to drive fluid used to actuate the driver.

Further features of the electricity generating units are defined above.

Preferably, the electricity generating units are connected in series. Accordingly, an outlet of a first unit may be connected to an inlet of a further unit.

this fashion, drive fluid may be supplied from the outlet of a first unit to the inlet of a further unit. It will be understood that there is a drop in the pressure of the drive fluid across the driver of each unit, thus the pressure of the drive fluid supplied to the inlet of the further unit will be lower than the pressure of the drive fluid supplied to the previous unit.

In a further alternative, the unit may comprise a plurality of drivers, typically connected in series, the drivers each connected to the generator. The drivers may be coupled to a common drive shaft for driving the generator. There may be a progressive pressure drop across the drivers.

In a further alternative, the electricity generating units may be coupled in parallel such that each unit experiences a common pressure of the drive fluid.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a longitudinal cross-sectional view of an electricity generating unit in accordance with a preferred embodiment of the present invention; Fig. 2 is a schematic view of the electricity generating unit of Fig. 1; Fig. 3 is a schematic view of an electricity generating assembly comprising a plurality of electricity generating units) and Fig. 4 is a longitudinal cross-sectional view of an electricity generating unit in accordance with an alternative embodiment of the present invention.

Turning firstly to Fig. 1, there is shown a longitudinal cross-sectional view of an electricity generating unit in accordance with a preferred embodiment of the present invention, the unit indicated generally by reference numeral 10.

The electricity generating unit 10 is of a type adapted to operate on a hazardous drive fluid, in particular natural gas, and generally comprises a fluid actuated driver 12, and a generator 14 coupled to the driver 12 and adapted to be driven by the driver 12 to generate electricity. In use, the driver 12 is actuated by a drive fluid 16, and the generator 14 is exposed to the drive fluid.

By exposing the generator 14 to the drive fluid 16, it is not necessary to seal a drive shaft 18 of the driver 12, nor to provide a magnetic coupling for coupling the driver 12 to the generator 14, such that disadvantages of prior proposals are avoided.

The unit 10 has a particular utility in the oil and gas exploration and production industry, for example, on an unmanned offshore gas production installation or on an installation located in a remote environment.

Installations of this type may be used to control the flow of gas from a well, and to control supply of produced gas to an onshore storage/production facility, through a connected pipeline or the like. Using natural gas to actuate the driver 12 makes use of the inherent pressure of the gas recovered from depth in a gas well, which is considerably higher than ambient atmospheric pressure at surface outside the unit.

However, it will be understood that the unit 10 has alternative uses, for example, on a manned installation, or indeed in alternative environments where it is desired to generate electricity using a hazardous fluid as drive fluid for the driver 12.

The unit 10 has a particular alternative utility in a gas supply network, where the unit 10 is suitable for extracting power from the pressure let down required between high pressure transportation and distribution systems and low pressure local supply systems in common use for distributing gas to an end user. The unit 10 is then provided between a supply of natural gas (or equivalent) and the end user. The unit 10 is coupled to a gas storage facility by a relatively high pressure supply conduit, and may include a plurality of relatively low pressure outlet conduits, for supply to end users.

This allows electricity generation by making use of the required pressure let down in gas supply networks. The pressure let down in such systems is from a supply pressure of many thousands of psi to atmospheric or just above (typically 15-16psi). As will be described below, the unit is particularly suited to this application as several units may be linked in series, each operating at progressively lower pressures.

The electricity generating unit 10 and its method of operation will now be described in more detail with reference also to Fig. 2, which is a schematic view of the unit 10.

The driver 12 takes the form of a turbine which, in the illustrated embodiment, is a pelton wheel type turbine. As will be understood by persons skilled in the art, a pelton wheel turbine is a radial flow turbine having a number of curved turbine blades spaced around a circumference of the wheel, the blades typically oriented on respective radii of the wheel. The generator 14 includes a rotor 20 which carries a number of permanent magnets (not shown), and a stator 22 which has a number of stator windings 24, in a conventional fashion. The turbine 12 is activated to drive the generator thereby rotating the rotor 20 of the generator 14 within the stator 22 to generate an Alternating Current (AC) output. The stator windings 24 are isolated from drive fluid 16 to reduce the likelihood of ignition.

This is achieved by coating the stator windings 24 with an insulating coating material, such as a resin, adhesive or sealant, after the windings have been formed on a core of the stator, or by providing a preformed insulating sleeve or the like which is inserted into the generator around the stator windings 24 after forming the windings.

The pelton wheel turbine 12 and the generator 14 are both located in a chamber 26 defined by a housing 28 of the unit 10. The housing 28 comprises a number of separate housing portions coupled together, and includes a turbine housing portion 30, a coupling housing portion 32 and a generator casing 34, the turbine housing portion connected to the generator casing 34 via the coupling housing portion 32. This enables the turbine 12 and generator 14 to be entirely contained within a pressurised enclosure, with no seals on the turbine shaft 18 or rotor 20 to wear or leak. The generator 14 is thus integral with the turbine 12 and both are within the same pressure envelope. Indeed, the unit 10 is built to contain the design (operating) pressure of the drive fluid 16 plus a factor of safety as required by the applicable regulations governing its use.

The generator rotor 20 includes a rotor shaft 36 which is connected directly to the turbine shaft 18 by a shaft coupling 38, to transfer a rotational drive force from the turbine 12 to the generator l4. The turbine 12 includes a turbine rotor 40 (the pelton wheel) which mounted for rotation within the turbine housing 30 on sealed or shielded bearing assemblies 42. The generator rotor 20 is also mounted on similar sealed or shielded bearing assemblies 44. By providing sealed or shielded bearing assemblies, the requirement to conduct maintenance to re- lubricate the bearings during operational lifetime of the unit 10 is reduced.

The unit 10 also includes a drive fluid inlet port 46, which is provided in the turbine housing portion 30, positioned to direct a stream of drive fluid onto the blades (not shown) of the pelton wheel 40, as indicated by the arrow A in Fig. 1. The unit 10 also includes three fluid exit/exhaust ports or outlets 48, 50 and 52, the outlet 48 provided in the turbine housing portion 30, the outlet 50 in the coupling housing portion 32 and the outlet 52 in the generator casing 34. Also, the turbine housing portion 30 includes a flow port 54 and the generator casing 34 a flow port 56. These ports 54 and 56 provide fluid communication between the turbine 12 and the generator 14, to expose the generator 14 to the drive fluid 16. These ports 54, 56 thus enable the pressure within the unit 10 to equalise in operation.

The inlet 46 and the three outlets 48, 50 and 52 define three flow paths for flow of drive fluid 16 through the unit 10. The first flow path is defined between the inlet 46 and the outlet 48, indicated by the arrows ABi the second between the outlet 46, the flow port 54 and the outlet 50, indicated by the arrows A,B' C; and the third between the inlet 46, the flow ports 54 and 56 and the outlet 52, indicated by the arrows A,B',C'-D.

The first flow path A-B from inlet 46 to outlet 48 provides the most direct passage for the drive fluid 16, and the lowest drop in pressure of the drive fluid 16 across the unit 10. However, it may be desired to direct fluid along the second flow path from inlet 46 to outlet 50, or in particular along the third flow path from inlet 46 to outlet 52, such that the drive fluid 16 flows through the generator 14. In use, drive fluid 16 flowing along the third flow path passes through the generator 14 between the rotor 22 and the stator 24, and thereby cools the generator rotor 20 and windings 24, which produce heat when in operation. In use, flow of drive fluid through the three outlets 48, 50 and 52 (and thus through the three flow paths) is typically adjusted to give optimum unit performance in terms of managing fluid flow; managing flow of any condensate; and cooling parts of the unit.

Fluid is supplied to the inlet 46 along a supply conduit 58, which is shown in the schematic view of Fig. 2, and the unit 10 includes a manifold 60 which connects the three outlets 48, 50 and 52. This allows simultaneous flow of fluid through the unit 10 and out each of the outlets 48, 50 and 52, or the manifold 60 may include appropriate valves or the like for selectively closing one or more of the outlets 48, 50 and 52, to direct the drive fluid along a selected one or more flow paths. The manifold 60 is connected to an outlet conduit 62 from where the drive fluid passes to a desired location, for example, into a pipeline and to an onshore storage/processing facility.

The outlets 48, 50 and 52 also facilitate evacuation of the unit 10 prior to start-up. This is achieved by ensuring the inlet 46 is closed, and then drawing any air (or other fluids) out of the unit through one or more of the outlet 48, 50 and 52, for example, by combining a bleed vent with the outlet 52. Evacuating the unit in this way further reduces the likelihood of ignition of the drive fluid 16, thus preventing any explosive mixture being present within the unit. The generated AC output is taken off through output wires 64 and 66 and used to provide electrical power to the installation control systems.

Turning now to Fig. 3, there is shown an electricity generating assembly comprising a plurality, in the illustrated embodiment, three electricity generating units lOa, lob, lOc, the electricity generating assembly indicated generally by reference numeral 110. Like components of the units lOa, lOb and lOc with the unit 10 of Figs. 1 and 2 share the same reference numerals with the addition of the suffix a, b or c respectively. Each of the electricity generating units lOa, lOb and lOc are of similar structure to the generator unit 10 shown in Figs. 1 and 2. The generator units lOa, lob and lOc are connected in series, fluid supplied into the first unit lOa via supply conduit 58 and inlet 46a, the drive fluid passing through the unit lOa and from there via outlet conduit 62a into the unit lOb through inlet 46b. The unit lOc is connected to the unit lob in a similar fashion. This combination of units lOa, lob and lOc provides an assembly 110 which optimises electricity generation.

It will be understood that, in use, there is a drop in pressure of the drive fluid 16 across each of the units lOa, lob and lOc. Thus the pressure of the drive fluid entering the unit lOb is lower than that entering the unit lOa, and the pressure of fluid entering the unit lOc lower than that entering the unit lob. Accordingly, the operating pressure of the unit lOb is lower than that of the unit lOa, and the operating pressure of the unit lOc is lower than that of the unit lOb. Accordingly, there may be structural changes in the design of the units lOa, lOb, lOc, for example, in dimensions of the respective turbines, to account for the lower operating pressure of the drive fluid and expansion of the fluids through the units.

Turning now to Fig. 4, there is shown an electricity generating unit in accordance with an alternative embodiment of the present invention, the unit indicated generally by reference numeral 210.

The unit 210 is similar to the unit 10 of Figs. 1 and 2 and like components share the same reference numerals incremented by 200. For brevity, only the differences between the unit 210 and the unit 10 will be described herein.

The unit 210 includes three drivers in the form of pelton wheel turbines 212a, 212b, 212c which are connected in series and which are coupled by a common drive shaft 218 to a generator 214. An outlet 248a of the turbine 212a is coupled to an inlet 246a of the turbine 212b. In a similar fashion, an outlet 248b of the turbine 212b is coupled to an inlet 246c of the turbine 212c. An outlet 248c of the turbine 212c is coupled to outlets 250, 252 of the unit 214 in a similar fashion to the unit 10 described above.

Internal shaft seals (not shown) are provided between each of the turbines 212a, 212b and 212c, as the pressure in each turbine is progressively lower.

However, the shaft seals do not compromise integrity of the unit 210. This is because, in the event of wear of the seals, there would never be a leak path to the exterior of the unit 210; wear of the seals would only lead to a pressure variation in the adjoining turbine, which would be detected by appropriate pressure gauges.

The defective seal could then be renewed without a safety issue arising.

Various modifications may be made to the foregoing within the scope of the present invention.

For example, any other hazardous fluid may be used as the drive fluid, such as alternative hydrocarbon containing fluids. The unit may be used in place of conventional electricity generating units

which function on non hazardous drive fluid, for example, where it is desired to reduce maintenance or to achieve generator cooling.

The unit may comprise an inlet corresponding to each outlet, and thus may include a dedicated inlet for each outlet.

The driver may comprise a turbine having a plurality of turbine stages, for example, a plurality of adjacent rotor/stator stages having respective rotor/stator blades.

Alternatively, the driver may comprise a motor such as an air motor, an axial flow turbine, or any other suitable alternative driver.

A continuous drive shaft may extend between the driver (turbine) and the generator.

The electricity generating units of the assembly may be coupled in parallel such that each unit experiences a common pressure of the drive fluid.

The invention offers advantages including low maintenance costs; an inherently safe design; versatility in operation over large pressure drops; adaptability of the design to high or low power output; and easy coupling of one unit to another to maximise power output from a single gas stream.

Claims

1. An electricity generating unit comprising: a fluid actuated driver; and a generator coupled to the driver and adapted to be driven by the driver to generate electricity; wherein, in use, at least part of the generator is adapted to be exposed to a drive fluid used to actuate the driver.

2. An electricity generating unit as claimed in claim 1, wherein the unit comprises a housing in which the driver and the generator are mounted.

3. An electricity generating unit as claimed in either of claims 1 or 2, wherein the generator is directly coupled to the driver.

4. An electricity generating unit as claimed in claim 3, wherein the generator is coupled to a drive shaft of the driver.

5. An electricity generating unit as claimed in any preceding claim, wherein the generator comprises a rotor and a stator and wherein the rotor is directly coupled to the driver.

6. An electricity generating unit as claimed in any preceding claim, wherein the unit is adapted to operate at a pressure above ambient atmospheric pressure.

7. An electricity generating unit as claimed in claim 6, wherein the driver is adapted to be actuated by drive fluid at a pressure above ambient atmospheric pressure.

8. An electricity generating unit as claimed in any preceding claim, wherein the driver is adapted to be actuated by a hazardous fluid.

9. An electricity generating unit as claimed in claim 8, wherein the driver is adapted to be actuated by a flammable and/or explosive fluid.

10. An electricity generating unit as claimed in either of claims 8 or 9, wherein the driver is adapted to be actuated using a natural gas.

11. An electricity generating unit as claimed in any preceding claim, wherein the unit is adapted to be coupled directly to a source of natural gas.

12. An electricity generating unit as claimed in claim 11, wherein the unit is adapted to be coupled to a gas well by production tubing, and is provided at surface.

13. An electricity generating unit as claimed in claim 12, wherein the unit is adapted such that the entire production of gas from the well flows through the unit.

14. An electricity generating unit as claimed in claim 12, wherein the unit is adapted such that only part of the total production from the well flows through the unit.

15. An electricity generating unit as claimed in any one of claims 11 to 14, wherein the unit is adapted to be provided as part of a production facility for recovering the gas.

16. An electricity generating unit as claimed in any one of claims 1 to 10, wherein the unit is adapted to be coupled to a supply of natural gas.

17. An electricity generating unit as claimed in claim 16, wherein the unit is adapted to be provided in a pressure let down facility of a gas supply network.

18. An electricity generating unit as claimed in any preceding claim, wherein the generator is in fluid communication with the driver.

19. An electricity generating unit as claimed in claim wherein the driver and the generator are mounted in a fluid chamber of the unit.

20. An electricity generating unit as claimed in claim 19, wherein the unit comprises a housing in which the driver and the generator are mounted, and wherein the fluid chamber is defined by the housing.

21. An electricity generating unit as claimed in claim 20, wherein the housing is sub-divided into a driver chamber portion and a generator chamber portion, the driver and generator chamber portions in fluid communication.

22. An electricity generating unit as claimed in any preceding claim, wherein the unit comprises an inlet for flow of drive fluid into the unit, and a plurality of outlets.

23. An electricity generating unit as claimed in any preceding claim, wherein the unit defines a plurality of drive fluid flow paths for flow of drive fluid through the unit.

24. An electricity generating unit as claimed in claim 23, wherein the unit defines a first drive fluid flow path for flow of fluid between an inlet and a first outlet, a second drive fluid flow path for flow of drive fluid between an inlet and a second outlet, and a third drive fluid flow path for flow of drive fluid between an inlet and a third outlet.

25. An electricity generating unit as claimed in either of claims 23 or 24, wherein, in use, flow through the flow paths is adapted to be balanced to optimise performance of the unit.

26. An electricity generating unit as claimed in any one of claims 23 to 25, wherein the unit comprises a single inlet common to each flow path.

27. An electricity generating unit as claimed in any one of claims 23 to 25, wherein the unit comprises an inlet corresponding to each flow path.

28. An electricity generating unit as claimed in any preceding claim, wherein the unit comprises at least one outlet arranged to define a driver fluid flow path through at least part of the generator.

29. An electricity generating unit as claimed in any one of claims 23 to 28, wherein a selected one or more of the fluid flow paths is adapted to be closed by closing a respective outlet.

30. An electricity generating unit as claimed in any one of claims 23 to 29, wherein the unit comprises a control assembly associated with the outlets for controlling flow of fluid through the flow paths.

31. An electricity generating unit as claimed in any preceding claim, wherein the unit comprises a plurality of drivers.

32. An electricity generating unit as claimed in any preceding claim, wherein the driver is a turbine.

33. An electricity generating unit as claimed in claim 32, wherein the turbine has a plurality of turbine stages.

34. An electricity generating unit as claimed in either of claims 32 or 33, wherein the turbine is a radial flow turbine.

35. An electricity generating unit as claimed in any one of claims 32 to 34, wherein the turbine is a pelton wheel type turbine.

36. An electricity generating unit as claimed in either of claims 32 or 33, wherein the driver comprises an axial flow turbine.

37. An electricity generating unit as claimed in any one of claims 1 to 31, wherein the driver is an air motor.

38. An electricity generating unit as claimed in any preceding claim, wherein at least part of the generator is isolated from the drive fluid.

39. An electricity generating unit as claimed in claim 35, wherein the generator comprises a stator having stator windings and wherein the stator windings are isolated from the drive fluid.

40. An electricity generating unit as claimed in claim 39, wherein the windings are coated with a coating material.

41. An electricity generating unit as claimed in claim 40, wherein the windings are coated subsequent to assembly of the stator.

42. An electricity generating unit as claimed in claim 40, wherein the material used to form the windings is coated prior to assembly of the stator.

43. An electricity generating unit as claimed in claim 39, comprising an insulating sleeve adapted to be located around the stator windings subsequent to assembly of the stator.

44. An electricity generating unit as claimed in any preceding claim, wherein the unit is adapted to be evacuated prior to use.

45. An electricity generating unit as claimed in any one of claims 1 to 43, wherein the unit is adapted to be filled with an inert fluid prior to use. 3?

46. An electricity generating unit as claimed in any preceding claim, comprising a plurality of fluid actuated drivers coupled to the generator.

47. An electricity generating assembly comprising a plurality of electricity generating units, each electricity generating unit having: a fluid actuated driver; and a generator coupled to the driver and adapted to be driven by the driver to generate electricity; wherein, in use, at least part of the generator is adapted to be exposed to drive fluid used to actuate the driver.

48. An electricity generating assembly as claimed in claim 47, wherein the electricity generating units are units as claimed in any one of claims 1 to 45.

49. An electricity generating assembly as claimed in either of claims 47 or 48, wherein the electricity generating units are connected in series.

50. An electricity generating assembly as claimed in either of claims 47 or 48, wherein the electricity generating units are coupled in parallel.

51. An electricity generating assembly as claimed in any one of claims 47 to 50, wherein at least one of the units comprises a plurality of fluid actuated drivers coupled to the respective generator.