GB2451627A

GB2451627A - Tidal power installation, with two reservoirs or basins and a channel containing an energy extractor, used to convert tidal flows to a unidirectional flow

Info

Publication number: GB2451627A
Application number: GB0715181A
Authority: GB
Inventors: Ray Larter
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-08-06
Filing date: 2007-08-06
Publication date: 2009-02-11
Anticipated expiration: 2027-08-06
Also published as: GB2451627B; GB0715181D0

Abstract

A tidal power installation has two reservoirs,70,80, eg in a natural bay 10; a channel 60 with entry 62 and exit 64 portions which are, possibly via sluice gates 22,24,32,34,42,44, connectable to the sea 50 and each reservoir; and turbines 90 between these portions. The installation provides a continuous unidirectional flow through the turbines 90. The turbines may form a barrier or dam and have a gate (91, figure 6), which may contain floatation means to track variations in sea level and may have supports (95, figure 6) and a telescopic guide (93', figure 12), and an impeller (102, figure 9) for driving a compressor or generator. The compressor may be effected by a double piston arrangement (200, figure 14) with an off-centre cam (215, figure 14) and compressed air may be stored in circular or hexagonal tubes (130, figure 13) made of plastic with carbon fibre support. The gate (91, figure 6) has an aperture (92, figure 6) and is typically made from steel coated with an antifouling layer, its supports (95, figure 6) may be tapered and the guide (93, figure 6) may have openings underwater. The reservoirs 70,80 may be separated by triangular or trapezoidal walls (30, figure 4) comprising layers of parallel tubular members (35, figure 4), possibly containing interlocking serrations or grooves (38, figure 5) on the outer surface, which may be held together by retention straps (36, figure 4) and may be clad with sheeting (39, figure 4). At least one of the tubes (35, figure 4) contains a filler material such as aggregate.

Description

INSTALLATION FOR HARNESSING ENERGY FROM TIDAL FLOWS

The present invention relates to energy extraction from tidal flows.

Generation of electricity by harnessing the power represented by tidal movement is known using conventional apparatus such as tidal barrages.

The simplest generating system for tidal plants involves a dam, known as a barrage across an estuary. Sluice gates on the barrage allow the tidal basin to fill on the incoming high tides and to exit through the turbine system on the outgoing tide. Alternatively, flood-generating systems, which generate power from the incoming tide are possible.

The disadvantages relating to such conventional systems lie with the fact that they are only able to generate/extract power for a limited number of hours per day when the tide is flowing in the correct direction at the correct flow rate.

Consumer demand does not typically reflect the timing of this availability.

Furthermore, the apparatus lies dormant for a significant portion of any 24 hour period. It is desirable to provide an installation which can generate power over a greater portion of the day.

According to the present invention there is provided an installation for continuously harnessing energy from tidal flows comprising: a primary reservoir for receiving water from the sea; a secondary reservoir for receiving water from the sea; and a channel comprising:

I

an entry portion, selectably connectable to the sea and to each of the reservoirs; an exit portion, selectably connectable to the sea and to each of the reservoirs; and an energy extractor located in the channel between the entry portion and the exit portion.

By providing a primary and secondary reservoir together with a channel in such a configuration, a constant flow of water through the channel can be achieved and maintained such that the energy extractor can constantly receive water and be driven thereby to enable electricity to be generated constantly if so required. The motion of the tidal water is effectively converted into a circulatory flow to achieve this level of consistency.

Sluice gates may be provided between the channel and each of the sea and the respective reservoirs. The primary reservoir may be a naturally occurring bay or it may be an artificial enclosure.

The energy extractor may comprise a gate for receiving fluid from the entry portion and an impeller for driving a compressor or generator, the impeller being configured to receive water from the gate. The installation may comprise a barrier for separating the entry portion from the exit portion, wherein the gate may be slideably mounted in the barrier and adapted to track variations in sea level and the installation may comprise a conduit for coupling the gate to the impeller and delivering fluid therebetween.

According to a second aspect there is provided an energy extractor for extracting energy from a fluid flow stream comprising: a barrier for separating the fluid flow stream into an upstream portion and a downstream portion, the barrier comprising a gate for receiving fluid from the upstream portion; and an impeller, for driving a compressor or a generator, configured to receive fluid via the gate and to be rotatably driven thereby.

The gate may be slideably mounted in the barrier and may be configured to track a surface of the fluid in the upstream portion of the fluid flow stream.

The gate may comprise floating means to increase buoyancy thereof. The floating means may comprise a sealed chamber which may be located within a body of the gate.

According to a third aspect there is provided a wall for separating reservoirs in an installation as described above, the wall comprising a first layer of tubular members laid upon a second layer of tubular members, the tubular members in each layer being arranged substantially parallel to one another, wherein at least some of the tubular members comprise a filler material.

The outer surface of each tubular member may comprise serrations. The serrations may be provided by splines or otherwise formed by peaks and troughs. The tubular members may be secured together by retention straps.

The wall may be constructed by a method comprising the following steps, assembling and bonding tubular members to one another to create an extended length of tube, dispensing the length of tube into the sea, directing the tube to a location on a sea bed, filling the tube with a filler material. The assembling, bonding and dispensing steps may be performed on/from a vessel floating on the surface of the sea. The method may include capping one or both ends of the extended length of tube. A plurality of tubes may be laid substantially parallel to one another to form a first layer of tubes. A second layer of tubes may be laid upon the first layer to create a taller barrier.

As each tube reaches the seabed the tube may be vibrated to assist in location of the tube in relation to an already laid tube, especially if the tubes are splined or otherwise ridged and the ridges of respective tubes are to interlock with one another. The method may include a step of securing the plurality of tubes to one another, for example by retention straps. The completed wall may subsequently be clad.

According to a fourth aspect there is provided a method for harnessing energy from an installation, the installation comprising a primary reservoir, a secondary reservoir each for receiving water from the sea; and a channel comprising an entry portion, an exit portion and an energy extractor located in the channel between the entry portion and the exit portion, the method comprising the following steps: a) connecting the entry portion of the channel to the sea and connecting the exit portion of the channel to the primary reservoir during the flow tide to permit the primary reservoir to receive sea water; b) isolating the exit portion of the channel from the primary reservoir and connecting the exit portion of the channel to the secondary reservoir to permit the secondary reservoir to receive sea water; C) isolating the exit portion of the channel from the secondary reservoir, isolating the entry portion of the channel from the sea, connecting the entry portion of the channel to the primary reservoir and connecting the exit portion of the channel to the sea to permit the primary reservoir to release sea water; d) isolating the entry portion from the primary reservoir and connecting the entry portion to the secondary reservoir to permit the secondary reservoir to release sea water; and e) isolating the entry portion of the channel from the secondary reservoir and isolating the exit portion of the channel from the sea.

A step of isolating the entry portion of the channel from the sea and connecting the entry portion of the channel to the primary reservoir to permit the secondary reservoir to receive water from the primary reservoir may be carried out after step (b) above.

According to a fifth aspect there is provided a method for continuously harnessing energy from the sea comprising the steps of: converting a tidal flow of water to a continuous flow of water; and channelling the unidirectional flow of water through an energy extractor irrespective of the state of the tide.

The invention will now be disclosed with respect to the accompanying drawings in which: Figure 1 is a schematic plan view of an installation for harnessing power from the sea; Figure 2 represents a sequence of operation of the installation shown in Figure 1; Figure 3 is an isometric view of an alternative configuration of the installation; Figure 4 illustrates an interior wall of the installation; Figure 5 illustrates a further detail of the wall shown in Figure 4; Figure 6 is an isometric view of extractors used in the installation of Figures 1 and 3; Figure 7 an extractor having additional buoyancy; Figure 8 is a side view of the extractors of Figure 6; Figure 9 is a plan view of the extractors of Figure 6; Figure 10 is a downstream view of the extractors of Figure 6; Figure 11 is an isometric view of pontoons supporting the extractors of Figure 6; Figure 12 shows an extractor for use in waters experiencing greater tidal amplitude; Figure 13 illustrates an energy storage device; and Figure 14 illustrates a compressor.

Figure 1 illustrates a coastal bay adapted to enable energy from the tidal flow into and out of the bay to be harnessed to generate electricity. The bay 10 is effectively enclosed by providing an outer wall 20 across the span of a mouth of the bay 10. Sluice gates 22, 24 are provided in openings of the outer wall 20 so that the sea 50 becomes selectably connectable to the bay 10.

Interior walls 30, 40 are provided to define a channel 60 between the two sluice gates 22, 24 in the outer wall 20. A first interior wall 30 extends across the mouth 15 of the bay 10 on the coastal side of the outer wall 20 as shown.

Thus a primary reservoir 70 is defined, having a volume corresponding to a significant proportion of bay 10. Sluice gates 32, 34 are provided in interior wall 30 so that the primary reservoir 70 is selectably connectable to channel and, consequently, to the sea 50. A second interior wall 40 also extends across the mouth 15 of the bay 10 between the first interior wall 30 and the outer wall 20 thereby defining a secondary reservoir 80. Sluice gates 42, 44 are provided in interior wall 40 so that the secondary reservoir 80 becomes selectably connectable to channel 60, and, consequently, selectably connectable to the sea 50.

Extractors 90, for harnessing energy from the flow of water therethrough are located within channel 60 and will be described in more detail later.

Figures 2 illustrate a sequence of operation of the installation illustrated in Figure 1. Figure 2a represents an inflowing tide and shows that sluice gates 24 and 32 have been opened to permit passage of water from the sea 50 along channel 60 passing through extractors 90 and into primary reservoir 70.

As the momentum of the tide begins to reduce and the high tide point is approached, the primary reservoir 70 is isolated from channel 60 by closing sluice gate 32. Sluice gate 42 is opened and water from the sea 50 continues to pass along channel 60 and therefore through extractors 90 and into secondary reservoir 80 as illustrated in Figure 2b. This configuration is maintained until the water level of the sea 50 and the water level of the secondary reservoir 80 reach equilibrium with one another. At this point, the tide will have begun to recede, becoming an outflow tide. The secondary reservoir 80 is then isolated from channel 60 by closing sluice gate 42. Sluice gate 24 is also closed to prevent water in channel 60 flowing back into the sea therethrough.

Sluice gate 22 is now opened together with sluice gate 34 such that the primary reservoir 70 is connected to the sea 50 via channel 60. Water passes along channel 60 through extractors 90 and into the sea 50 as illustrated in Figure 2d. As the tide approaches low tide, the primary reservoir 70 is isolated from channel 60 by closing sluice gate 34. Water flow through channel 60 and hence through extractors 90 is continued by opening sluice gate 44 thereby connecting the secondary bay 80 to the sea 50 as illustrated in Figure 2e. This configuration is maintained until the tide has turned when channel 60 is once again isolated from the sea by closing sluice gate 22 and the secondary reservoir 80 is isolated from the channel 60 by closing sluice gate 44. Sluice gates 24, 32 are then opened to permit water to flow from the sea 50 into primary reservoir 70 as illustrated in Figure 2a and so the cycle continues. The tidal motion of the water flow is effectively converted into a continuous water flow through channel 60.

An optional step is illustrated in Figure 2c whereby the sluice gate 24 connecting the sea 50 to the upstream portion 62 of channel 60 is closed and the sluice gate 34 between the primary reservoir 70 and the upstream portion 62 of channel 60 is opened prior to closing sluice gate 42 between the secondary reservoir 80 and the downstream portion 64 of channel 60. Thus a fluid flow path is created between the primary reservoir 70 and the secondary reservoir 80 so that the latter can be further filled (or topped up) by the former.

The duration of each step may be curtailed so that rather than the water levels on each side of the extractor 90 becoming equal, the sequence may be progressed to the next stage in order to maintain a distinct difference in water levels upstream and downstream of the extractor 90. In this way, efficiency and performance of the extractor can be maintained.

The above mentioned cycle is repeated every 12 hours or so with the tide such that there are four bulk tidal movements in every 24 hour period. As an example, the bay 10 may have an area of approximately 1. 5km2 and may experience a tidal lift of 2m. Approximately 4.5 million cubic tonnes of water passes through channel 60 during each tidal movement resulting in 18 million cubic tonnes of water in any 24 hour period. A significant quantity of electricity can be generated by introducing an energy extractor 90 into the channel 60. The number of extractors 90 introduced into the channel 60 depends on the space available and the demand for the electricity. An increased number of extractors 90 can be accommodated if the extractors 90 are offset from one another when they are installed so that they form a barrier at an angle to the flow as illustrated in Figure 1 rather than the barrier being normal to the bulk flow within the channel 60.

Bulk flow of sea water still enters and leaves the bay 10 carrying with it all the sea flora and fauna associated therewith. Consequently, environmental impact on the habitat in the bay 10 remains low. Alternatively, a filter may be incorporated into the sluice gates 22, 24 to capture detritus or rubbish floating on the surface of the sea or even to prevent passage of larger sea life such as sharks. In so doing, the quality and safety of the water in the bay 10 may be improved for users of the beach in the bay.

A delay in the incoming tide may be experienced, when compared to that in adjacent coastal bays, as the head of water builds upstream of the extractors but the magnitude of the tidal lift will remain approximately the same.

If a suitably sized coastal bay 10 is not available or it is particularly undesirable to modify the coastline by introducing walls 20, 30, 40 across a mouth 15 thereof, an alternative configuration as illustrated in Figure 4 may be implemented. In Figure 3, the primary reservoir 70' and the secondary reservoir 80' are artificially created inland of the beach/coastline. Secondary channels 62', 64' are provided to connect the sea 50 to channel 60'.

Whilst the outer wall 20 experiences the full force of the sea 50 and is, consequently, erected via known, robust sub-sea construction techniques (e.g. the Mulberry system), the interior walls 30, 40 do not experience such harsh conditions and lend themselves to an alternative construction as illustrated in Figures 4 and 5. The tapered wall 30 comprises layers of arrays of tubes 35. The tubes are laid approximately parallel to one another, each vertical layer comprising one less tube than the layer below as shown in Figure 5, to create a wall having a triangular or trapezoidal cross section.

Each tube is filled (not shown) with aggregate or other similar filler material to provide a solid, robust construction. The array of tubes is secured together by retention straps 36 positioned at intervals along the longitudinal extent of the wall 30. The entire construction may be clad with sheeting in addition to the retention straps 36.

The lowest level of tubes may be located directly onto the sea bed 52 or alternatively, as shown in Figure 5, cradles 37 may be provided on the sea bed 52 to securely receive the first layer of tubes 35. The outer circumference of each tube 35 is provided with splines or grooves or has an otherwise serrated profile such that when one tube 35 is placed adjacent another tube these serrations interlock to prevent relative movement therebetween.

The wall 30 may be constructed using a modified pipe laying technique, whereby the tube 35 is positioned by and dispensed from a vessel floating on the surface of the water. The end of the tube 35 may be capped prior to insertion of the empty tube into the water. As part of the tube 35 reaches the sea bed 52, aggregate or other filler material is introduced (possibly under pressure) into the other end of the tube 35 that is still located at the surface of the water on the vessel. The filler material may be delivered directly to the portion of the tube 35 that is already located on the sea bed by inserting a pipe down the tube 35 and gradually withdrawing the pipe as the tube 35 is laid. The other end of the tube 35 is then capped to retain the filler material within the tube 35. If the tube 35 is provided in sections then these sections are assembled and bonded together into appropriate lengths on the vessel as the tube 35 is dispensed into the water.

The number of layers of tube 35 used to assemble the wall is determined by the anticipated depth of the water at high tide together with the diameter of pipe used. Advantages associated with this method of construction include the fact that the empty tube 35 is light and therefore easy to manipulate into place accurately (e.g. by divers located on the sea bed). The assembled lengths of tube can be vibrated from the vessel on the surface of the water as they are laid on the sea bed to encourage the grooves of the tube 35 to engage with one another so that they are securely fitted together.

An array of extractors 90 is installed in channel 60 effectively dividing the channel into an upstream portion 62 and a downstream portion 64. Figures 6 through 12 illustrate extractors 90 in more detail. Figure 6 illustrates an upstream view of an array of three extractors 90, in other words the side presented to upstream portion 62 of channel 60. Each extractor 90 comprises a gate 91, slideably mounted in a guide 93 which in turn is fixedly mounted on the sea bed 52. Each guide 93 is located between two gate supports 95, the combination of guides 93 and supports 95 provides a continuous dividing barrier across channel 60.

Each gate 91 comprises an aperture 92 for receiving water from the upstream portion 62 of channel 60. The gate 911s typically made from steel (coated with an antifouling layer) and as such is particularly heavy. Buoyancy means may be provided to support most of the weight of the structure of the gate 91 to enable the gate 91 to be translated vertically to thereby track the level of the water in the upstream portion 62 of channel 60 and keep the aperture 92 approximately at the surface of the water. The buoyancy means includes one or more of a sealed tank (not shown) located within a body of the gate 91 which is filled with a material lighter than water, for example air or foam. In this example, a float 94 is illustrated, connected to the upper part of the gate 91 above aperture 92 to provide additional buoyancy to the gate 91.

Furthermore the gate 91 may comprise an additional or enlarged sealed tank (as illustrated in Figure 7) to further increase the buoyancy of the gate 91. By providing buoyancy means the weight of the gate 91 can be overcome so that neutral buoyancy of the gate might be approached, thus permitting the gate to float at the surface of the sea and hence track the changes in water level.

The guide 93 is provided with openings in a surface that is presented to the upstream portion 62 of channel 60 to enable water to enter below the gate 91 as it rises and lowers with the tide. Thus interference with the sliding movement of the gate 91 due to suction can be avoided.

Figure 8 illustrates a side view of a single extractor 90. A support 95 is illustrated in the centre of the figure obscuring the gate 91. The upstream portion 62 of channel 60, having a relatively high water level, is represented to the right of the support 95. The downstream portion 64 of channel 60, having a water level lower than that of the upstream portion 62, is represented to the left of the figure. Water is conveyed from the upstream portion 62 to the downstream portion 64 through aperture 92 in gate 91 as described above. A leading edge of each support 95 may be tapered (as illustrated in Figure 9) or otherwise configured to channel the water into the aperture 92. By constricting the water flow in this way the flow rate is increased through the aperture 92.

Impeller 102 (illustrated in Figure 9) is provided within a housing 104. The housing 104 is mounted upon a pontoon 106 or other floating means in the downstream portion 64 of channel 60. Pontoon 106 enables the impeller 102 and its housing 104 to track the fluctuating water level of the downstream portion 64 of channel 60. The housing 104 is connected to gate 91 via chute 108. In use, chute 108 serves to direct water passing through aperture 92 into the housing 104 where the water interacts with the impeller 102. Thus the impeller 102 is rotatably driven about a central axis 110. An outlet 112 is provided in the housing 104 to enable water to exit from the housing and pass into the downstream portion 64 of channel 60.

Chute 108 may diverge from an inlet thereof to an outlet thereof to encourage water to pass therethrough as rapidly as possible and avoid excessive build up of pressure at an inlet to the housing 104. The housing 104 can be tilted by a few degrees so that the central axis 110 is off vertical. Such tilting serves to further align the impeller 102 with the flow of water to reduce changes in direction and therefore momentum of the flow, thus minimising energy losses from the installation.

Figure 10 illustrates a downstream view of the extractors 90 and shows the floating means represented in Figure 8. An alternative pontoon device is illustrated in Figure 11, in this example, an asymmetric pontoon 120 is provided to float the housing 104 on the surface of the water. The asymmetric configuration of pontoon 120 accommodates the significant variation in loading between one portion of the impeller 102 and the other. Preferably, water simply passes from an upstream end of the impeller 102 to a downstream end of the impeller, in so doing the direction of the water is not significantly modified. By minimising the variation in the direction of the bulk flow of the water within the impeller 102, the efficiency and performance of the impeller is rnaximised. The unloaded portion of the impeller 102 carries very little or no water in normal operation and is, therefore, very light when compared to the loaded side. The asymmetric pontoon accommodates this asymmetric loading by providing a large float 122 to support the heavy side of the impeller 102/housing 104 combination and a smaller float 124 to support the empty side of the impeller/housing 104.

The amplitude of the tide can vary significantly from one coastal location to another furthermore at different times of the year the amplitude of tide can vary. For example, spring tides can be particularly exaggerated. In such circumstances, it may be beneficial to enable the gate 91 to be raised or lowered to a greater extent. The guide 93 can be replaced by a telescopic guide 93' as illustrated in Figure 12. The guide 93' is a multi-part component, each slideably nested within one another. Each part of the guide allows a particular amplitude of movement so that the overall telescopic guide 93' permits a greater extension in use, whilst being more compact in its collapsed configuration.

In operation, the configuration of the channel 60 and reservoirs 70, 80 together with sluice gates 22, 24, 32, 34, 42, 44 is such that water always passes along channel 60 from an upstream portion 62 to a downstream portion 64 thereof. Consequently, water always passes from the upstream portion 62 of the channel 60, through respective apertures 92 of each extractor 90, down each chute 108 and into each housing 104. As discussed above, the momentum of this water exerts a force on each impeller 102 thus causing impeller to rotate about a respective axis 110 before exiting the housing 104 through outlet 112 and into downstream potion 64 of channel 60.

Rotation of each impeller 102, in turn, drives generating means, compressing means or both.

If generating means is used then electricity is generated directly and supplied to the national grid for delivery to consumers. Excess electricity could be used locally to the installation for example to supply a water desalination plant.

Alternatively, rotation of the impeller 102 can be used to drive a compressor.

The compressor compresses a gas such as air and this compressed air is retained as a means of storing energy. The compressed air can readily be stored within the support structures 95 as depicted in Figure 13. Figure 13 illustrates an array of substantially parallel tubular vessels 130 arranged in a substantially honeycomb configuration, whereby each vessel is in contact with up to six other vessels along the length to gain additional support. The vessels may have a circular cross section as shown or they may be hexagonal in cross section. The cross section of the vessels may, alternatively, be approximately square and packed tightly so that each vessel is in contact with up to four other vessels.

Each vessel 120 is connected to a high pressure pipe network 135 from the compressors and is configured to receive compressed air therefrom. The tubular vessels 130 may be provided by plastic tubing encapsulated in a carbon fibre support jacket. Storing compressed air in this way enables the generation of electricity to be delayed until such time as there is consumer demand for the electricity. At such time the compressed air is conveyed from the storage vessels 130 and is used to drive a generator for generating electricity.

The compression of air may be effected by a double piston arrangement 200 as illustrated in Figure 14. The piston arrangement 200 comprises a first piston 202 and a second piston 204 located at diametrically opposite sides of the impeller housing 104. Each piston 202, 204 is driveably connected to a respective connecting rod 206, 208 and is slidably mounted within a tubular sheath 210, 212. The two connecting rods 206, 208 of respective pistons 202, 204 are connected to one another and the connection point thereof is directly coupled to an off centre cam 215 located on the impeller 102. As the impeller 102 rotates the connecting rods 206, 208 are alternately driven into each tubular sheath 210, 212 to move the respective piston 202, 204 in and of the sheath (as illustrated in the sequence of Figures 14a-14d), thereby causing any gas contained within the sheath to be compressed. As the piston 202, 204 is withdrawn from the sheath 210, 212, new air is drawn into the sheath through a one-way valve and the subsequent activation of the piston serves to compress the new volume of air.

Claims

1. An installation for continuously harnessing energy from tidal flows comprising: a primary reservoir for receiving water from the sea; a secondary reservoir for receiving water from the sea; and a channel comprising: an entry portion, selectably connectable to the sea and to each of the reservoirs; an exit portion, selectably connectable to the sea and to each of the reservoirs; and an energy extractor located in the channel between the entry portion and the exit portion.

2. An installation according to Claim 1, wherein sluice gates are provided between the channel and each of the sea and the respective reservoirs.

3. An installation according to Claim 1 or Claim 2, wherein the primary reservoir is a naturally occurring bay.

4. An installation according to Claim 1 or Claim 3, wherein the primary reservoir is an artificial enclosure.

5. An installation according to any preceding claim, wherein the energy extractor comprises: a gate, for receiving water from the entry portion; and an impeller for driving a compressor or generator, the impeller being configured to receive water from the gate.

6. An installation according to Claim 5, comprising: a barrier for separating the entry portion from the exit portion, the gate being slideably mounted in the barrier and adapted to track variations in sea level; and a conduit for coupling the gate to the impeller and delivering water therebetween.

7. An energy extractor for extracting energy from a fluid flow stream comprising: a barrier for separating the fluid flow stream into an upstream portion and a downstream portion, the barrier comprising a gate for receiving fluid from the upstream portion; and an impeller, for driving a compressor or a generator, configured to receive fluid via the gate and to be rotatably driven thereby.

8. An energy extractor according to Claim 7, wherein the gate is slideably mounted in the barrier and is configured to track a surface of the fluid in the upstream portion of the fluid flow stream.

9. An energy extractor according to Claim 8, wherein the gate comprises floating means to aid buoyancy thereof.

10. An energy extractor according to Claim 9, wherein the floating means comprises a sealed chamber.

11. An energy extractor according to Claim 10, wherein the sealed chamber is located within a body of the gate.

12. A wall for separating reservoirs in an installation according to any of Claims 1 to 6, the wall comprising a first layer of tubular members laid upon a second layer of tubular members, the tubular members in each layer being arranged substantially parallel to one another, wherein at least some of the tubular members comprise a filler material.

13. A wall according to Claim 12, wherein the outer surface of each tubular member comprises serrations.

14. A wall according to Claim 13, wherein the serrations are provided by splines.

15. A wall according to any of Claims 12 to 14, wherein the tubular members are secured together by retention straps.

16. A method for harnessing energy from the sea using an installation, the installation comprising a primary reservoir, a secondary reservoir each for receiving water from the sea; and a channel comprising an entry portion, an exit portion and an energy extractor located in the channel between the entry portion and the exit portion, the method comprising the following steps: a) connecting the entry portion of the channel to the sea and connecting the exit portion of the channel to the primary reservoir during the flow tide to permit the primary reservoir to receive sea water; b) isolating the exit portion of the channel from the primary reservoir and connecting the exit portion of the channel to the secondary reservoir to permit the secondary reservoir to receive sea water; c) isolating the exit portion of the channel from the secondary reservoir, isolating the entry portion of the channel from the sea, connecting the entry portion of the channel to the primary reservoir and connecting the exit portion of the channel to the sea to permit the primary reservoir to release sea water; d) isolating the entry portion from the primary reservoir and connecting the entry portion to the secondary reservoir to permit the secondary reservoir to release sea water; and e) isolating the entry portion of the channel from the secondary reservoir and isolating the exit portion of the channel from the sea.

17. A method according to Claim 16, comprising a further step, between steps b) and c), of isolating the entry portion of the channel from the sea and connecting the entry portion of the channel to the primary reservoir to permit the secondary reservoir to receive water from the primary reservoir.

18. A method for continuously harnessing energy from the sea comprising the steps of converting a tidal flow of water to a unidirectional flow of water; and channelling the unidirectional flow of water through an energy extractor irrespective of the state of the tide.