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Title:
"WAVE ENERGY CONVERTER"
Document Type and Number:
WIPO Patent Application WO/2021/068030
Kind Code:
A1
Abstract:
A method and apparatus of harnessing wave energy is described. A wave energy harnessing reservoir has an inlet side and an outlet side, the inlet side being arranged to open or close and the outlet side including a diffuser in which a turbine is located. The reservoir extends from close to the seabed to well above the mean sea level, enabling it to capture nearly the entire energy flux of an ocean wave. When a wave enters the reservoir the inlet side closes, resulting in the formation of a raised water level within the reservoir. The raised water level creates a pressure head used to drive the turbine. The reservoir has retractable feet which allow it to be secured in use on the sea floor in one position, and then easily floated to another position when required.

Inventors:
DE GEETER PIETER JAN (AU)
Application Number:
AU2020/051074
Publication Date:
April 15, 2021
Filing Date:
October 06, 2020
Export Citation:
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Assignee:
DE GEETER PIETER JAN (AU)
International Classes:
F03B13/14; E02B9/08; F03B13/22
Attorney, Agent or Firm:
ARMOUR IP PTY LTD et al. (Broadway Nedlands, Western Australia 6009, AU)
Download PDF:
Claims:
Claims

1. A wave energy converter for harnessing wave energy from a body of water, the wave energy converter including: a reservoir having an inlet side and an outlet side, the inlet side including a screen arranged to permit one-way water flow, and the outlet side including a rear wall, the screen extending vertically between a lowest level and a highest level, the height of the screen being at least 80% of the height of the reservoir; an outlet shaft extending to at least a rear wall, the outlet shaft having a turbine located therein; and a plurality of retractable feet arranged to engage with a seabed.

2. A wave energy converter as claimed in claim 1 , wherein the retractable feet are moveable between an engaged position in which the feet engage with the seabed and elevate the reservoir to a position of negative buoyancy, and a retracted position in which the reservoir is permitted to float freely.

3. A wave energy converter as claimed in claim 2, wherein the retractable feet are further moveable into an extended position in which the reservoir is lifted above the mean sea level.

4. A wave energy converter as claimed in any preceding claim, wherein the height of the screen is greater than 90% of the height of the reservoir.

5. A wave energy converter as claimed in any preceding claim, wherein the screen has two portions, a first portion arranged to locate generally below mean sea level and a second portion arranged to locate about mean sea level.

6. A wave energy converter as claimed in claim 5, wherein the second portion is horizontally spaced relative to the first portion.

7. A wave energy converter as claimed in any preceding claim, wherein the screen is moveable between an open configuration and a closed configuration, and is caused to close or to open based on predetermined water levels within the reservoir.

8. A wave energy converter as claimed in claim 7, wherein the opening and closing of the screen is a result of pressure differential between the static water pressure inside the reservoir and the water pressure outside the reservoir.

9. A wave energy converter as claimed in claim 8, wherein the screen is caused to open when the forward kinetic energy of a wave exceeds the potential energy of the water within the reservoir.

10. A wave energy converter as claimed in any preceding claim, wherein the screen comprises a plurality of blades, each blade being rotatable about a pivot axis generally transverse to the direction of wave propagation.

11. A wave energy converter as claimed in claim 10, wherein each blade is rotatable about a horizontal pivot axis.

12. A wave energy converter as claimed in claim 11 , wherein each blade is shaped as a vane, with the pivot axis being off-centre.

13. A wave energy converter as claimed in any one of claims 10 to 12, wherein each blade has a density greater than that of the body of water.

14. A wave energy converter as claimed in claim 13, wherein the density of each blade is between 101 % and 120% of that of the body of water.

15. A wave energy converter as claimed in any preceding claim, wherein the outlet shaft is a diffuser.

16. A wave energy converter as claimed in claim 15, wherein the diffuser has an outlet diameter at least twice the diameter of the turbine.

17. A wave energy converter as claimed in any preceding claim, wherein the rear wall has a vertical wave-capturing portion arranged to locate generally above mean sea level.

18. A wave energy converter as claimed in claim 17, wherein the rear wall includes an inclined portion arranged to locate generally below mean sea level.

19. A wave energy converter as claimed in any preceding claim, wherein the reservoir includes a ceiling which extends at least partially across an upper surface of the wave energy converter.

20. A wave energy converter as claimed in claim 19, wherein the ceiling is formed by a resiliently deformable sheet or membrane.

21. A wave energy converter as claimed in claim 19 or claim 20, wherein the ceiling includes a forward portion having a horizontal screen similar in operation to the vertically extending screen.

22. A wave energy converter as claimed in any preceding claim, wherein the reservoir includes a selectively operable volume diminishing means.

23. A wave energy converter as claimed in claim 22, wherein the volume diminishing means is an inflatable bag.

24. A wave energy converter as claimed in any preceding claim, wherein the wave energy converter includes a control means arranged to determine at least one of wave direction and wave amplitude.

25. A wave energy converter as claimed in claim 24, wherein the control means is arranged to retract the retractable feet in response to a measured wave direction which is more than a predetermined angle away from normal to the screen.

26. A wave energy converter as claimed in any preceding claim, wherein movement of the reservoir is preferably constrained by at least one cable.

27. A wave energy converter as claimed in claim 26, wherein the wave energy converter includes a low-stretch cable extending between a primary clump weight and the reservoir.

28. A wave energy converter as claimed in claim 27, wherein a secondary clump weight is located along the low-stretch cable.

29. A wave energy converter as claimed in any one of claims 26 to 28, wherein the wave energy converter has a second cable extending rearwardly of the reservoir, to prevent seaward movement of the reservoir.

30. A wave energy converter as claimed in claim 29, wherein the second cable includes electrical cabling for transporting electricity from a generator associated with the turbine.

31. A wave energy converter as claimed in claim 29 or claim 30, wherein the wave energy converter includes means for maintaining a relatively low tension on the second cable.

32. A wave energy converter as claimed in any preceding claim, wherein the reservoir has a height extending at least 80% of the distance from the water surface to the sea floor.

33. A wave energy converter as claimed in claim 32, wherein the reservoir has a height extending at least 90% of the distance from the water surface to the sea floor.

34. A wave energy converter as claimed in any preceding claim, wherein the reservoir has a wave guiding means hinged to a lower edge of the inlet side, the wave guiding means extending to the sea floor.

35. A wave energy converter as claimed in any preceding claim, wherein the reservoir has side walls extending between the screen and the rear wall, the side walls tapering inwardly towards the rear wall.

36. A wave energy conversion installation including a plurality of side-by- side coupled wave energy converters as claimed in any preceding claim.

Description:
“WAVE ENERGY CONVERTER”

Field of the Invention

[0001 ] The present invention relates to the harnessing of wave energy, such as for the production of electricity.

Background to the Invention

[0002] In recent decades there has been an increasing desire to harness renewable energy sources in order to supply energy (generally as electricity), rather than relying on non-renewable sources which are commonly fossil-fuel based. Commercial scale installations for harnessing of solar power, wind power and tidal power, amongst others, are continuing to be established.

[0003] The harnessing of wave energy has long been considered a potential source of renewable energy. Many different approaches have been used in attempts to efficiently convert wave energy into electricity. Broadly speaking, these systems fall into four categories.

[0004] Floating systems use buoys or other floating members connected to an energy conversion unit located on the seabed, below the floating member. The energy conversion unit is arranged to convert reciprocal motion of the floating member into electricity.

[0005] Floating flap systems used a hinged flap which rotates in response to passing waves. Typically, the rotation is used to drive a pump which supplies water under pressure to a hydroelectric turbine at a low efficiency.

[0006] Compressible devices use the vertical motion of a floating member to pressurise a gas, such as air. This may be done by use of an elastic membrane arranged to move up or down in response to the wave motion, thus varying the volume of an enclosed air chamber and causing pressure which may be used to drive a generator. In other systems, the floating member may be in the nature of a plunger plate which operates as a piston, driving compressed air directly through an outlet located above the water. [0007] All of these systems harness wave energy in the vertical direction; that is, transverse to the direction of wave propagation. They are not capable of harnessing wave energy in the horizontal direction.

[0008] The fourth category of wave energy harnessing systems is that encompassing ‘overtopping’ devices. These devices encourage waves to travel up a ramp and over an edge into a reservoir. The flow of water into the reservoir provides a head of pressure which can be used to power a hydraulic turbine. Such devices are relatively inefficient, as only a relatively small part of the wave amplitude can be captured. The wave energy associated with the remainder of the wave is lost to reflection. In addition, the freefall of water into the reservoir can generate turbulence, which results in further efficiency losses.

[0009] The present invention seeks to provide a method of harnessing wave energy which captures energy in both the direction of propagation of the wave and in the transverse direction (horizontal and vertical).

[0010] The present invention seeks to improve upon the method of harnessing wave energy disclosed in international patent publication number WO 2018/191779, the contents of which are herein incorporated by reference.

[0011] In particular, the present invention seeks to more efficiently capture wave energy which would otherwise pass beneath a wave energy converter.

Summary of the Invention

[0012] According to one aspect of the present invention there is provided a wave energy converter for harnessing wave energy from a body of water, the wave energy converter including: a reservoir having an inlet side and an outlet side, the inlet side including a screen arranged to permit one-way water flow, and the outlet side including a rear wall, the screen extending vertically between a lowest level and a highest level, the height of the screen being at least 80% of the height of the reservoir; an outlet shaft extending to at least a rear wall, the outlet shaft having a turbine located therein; and a plurality of retractable feet arranged to engage with a seabed.

[0013] Advantageously, the wave energy converter of the present invention is able to harness kinetic energy in the body of water at a depth below the wave amplitude.

[0014] It is anticipated that the action of the wave against the rear wall will result in the creation of a standing wave which may have a height up to twice that of the incipient wave.

[0015] The retractable feet are preferably part of ‘spuds’ as used in the offshore oil and gas industry. It is preferred that the retractable feet are moveable between an engaged position in which the feet engage with the seabed and elevate the reservoir to a position of negative buoyancy, and a retracted position in which the reservoir is permitted to float freely.

[0016] The retractable feet may be further moveable into an extended position in which the reservoir is lifted above the mean sea level, such as for inspection and/or maintenance purposes.

[0017] It is preferred that the lowest level of the screen is close to a base of the reservoir. Accordingly, the height of the screen is preferably greater than 90% of the height of the reservoir. It is considered that the smaller the ‘lip’ between the base of the reservoir and the lowest level of the screen, the more efficient the capture of wave energy flux.

[0018] The screen may have two portions, a first portion arranged to locate generally below mean sea level and a second portion arranged to locate about mean sea level. It is preferred that the second portion is horizontally spaced relative to the first portion. [0019] The screen is preferably moveable between an open configuration and a closed configuration. The screen may be caused to close or to open based on predetermined water levels within the reservoir. In a preferred embodiment, the opening and closing of the screen is a result of pressure differential between the static water pressure inside the reservoir and the water pressure outside the reservoir. In one arrangement, the screen is caused to open when the forward kinetic energy of a wave exceeds the potential energy of the water within the reservoir.

[0020] The screen may comprise a plurality of blades, each blade being rotatable about a pivot axis generally transverse to the direction of wave propagation. It is preferred that each blade is rotatable about a horizontal pivot axis.

[0021 ] The blades may each be shaped as a vane, with the pivot axis being off-centre. It is preferred that each blade has a density greater than that of the body of water. It is most preferred that the density of each blade is relatively close to that of the body of water, for instance, between 101% and 120%.

[0022] The outlet shaft may be a diffuser. It is preferred that the diffuser has an outlet diameter at least twice the diameter of the turbine.

[0023] The rear wall preferably has a vertical wave-capturing portion arranged to locate generally above mean sea level.

[0024] The distance between the screen and the wave-capturing portion of the rear wall may be based on the expected wavelength of the waves being harnessed. It is preferred that this distance is less than one third of the expected wavelength. In a preferred embodiment, the distance is about one fifth of the expected wavelength.

[0025] The rear wall may include an inclined portion arranged to locate generally below mean sea level, and is arranged to provide additional support for the outlet shaft. [0026] The reservoir may include a ceiling which extends at least partially across an upper surface of the wave energy converter. The ceiling may be formed by a resiliently deformable sheet or membrane. It is anticipated that such a device could increase the capacity of the wave energy harnessing reservoir in appropriate conditions, and provide a means for storing excess potential energy.

[0027] The ceiling may include a forward portion having a horizontal screen similar in operation to the vertically extending screen.

[0028] The reservoir may include a selectively operable volume diminishing means. The volume diminishing means may be an inflatable bag or similar. The inflatable bag may be associated with a compressed air pump which is arranged to operate under predetermined conditions.

[0029] The wave energy converter may include a control means arranged to determine at least one of wave direction and wave amplitude. In a preferred embodiment, the control means is arranged to retract the retractable feet in response to a measured wave direction which is more than a predetermined angle away from normal to the screen. It will be appreciated that retraction of the retractable feet permits the wave energy converter to float, and allows it to skew into the path of oncoming waves. The arrangement is such that the retractable feet are extended to reengage with the seabed once the wave energy converter has moved to a new position.

[0030] Movement of the reservoir is preferably constrained by at least one cable. The wave energy converter may include a low-stretch cable extending between a primary clump weight and the reservoir. In a preferred embodiment, a secondary clump weight may be located along the low-stretch cable. It will be appreciated that during normal use the primary clump weight is sufficiently heavy that it cannot be dislodged. In an unusual event (for instance, unusually strong waves), the primary clump weight is able to ‘give’ by sliding along the seabed, thus reducing the risk of damage to the wave energy converter. Similarly, unusually high impact wave energy may be partly absorbed by the secondary clump weight lifting off the seabed.

[0031] The wave energy converter may have a second cable extending rearwardly of the reservoir, to prevent seaward movement of the reservoir. The second cable may include electrical cabling for transporting electricity from a generator associated with the turbine. The wave energy converter may include means for maintaining a relatively low tension on the second cable, such as a constant tension winch.

[0032] It is anticipated that the reservoir will have a height sufficient to extend at least 80% of the distance from the water surface to the sea floor, with greater than 90% being preferred. A wave guiding means may be provided between a lower edge of the inlet side and the sea floor in order to ensure all available energy is captured within the reservoir. Preferably the wave guiding means is hinged to the lower edge of the inlet side.

[0033] The reservoir has side walls extending between the screen and the rear wall. In a preferred embodiment the side walls taper inwardly towards the rear wall.

[0034] It will be appreciated that the wave energy converter of the present invention can be used in a modular fashion; that is, a plurality of wave energy converters can be coupled side-to-side to increase the total energy output.

Brief Description of the Drawings

[0035] It will be convenient to further describe the invention with reference to preferred embodiments of the present invention. Other embodiments are possible, and consequently the particularity of the following discussion is not to be understood as superseding the generality of the preceding description of the invention. In the drawings:

[0036] Figure 1 is a side view of a wave energy converter according to the present invention; [0037] Figure 2 is a plan view of the wave energy converter of Figure 1 ;

[0038] Figure 3 is a rear view of the wave energy converter of Figure 1 ;

[0039] Figure 4 is a plan view of the wave energy converter of Figure 1 , shown in a tethered position;

[0040] Figure 5 is a side view of the wave energy converter of Figure 1 , shown in the tethered position of Figure 4;

[0041 ] Figure 6 is a plan view of four wave energy converters such as those of Figure 1, assembled in a modular fashion;

[0042] Figure 7a is a side view of a portion of a screen from within the wave energy converter of Figure 1 , shown in an open position;

[0043] Figure 7b is a side view of the screen portion of Figure 7a, shown in a closed position;

[0044] Figure 7c is a front view of the screen portion of Figure 7a, shown in the open position;

[0045] Figure 8 is a rear view of the wave energy converter of Figure 1 , shown in a maintenance position;

[0046] Figure 9 is a plan view of a wave energy converter in accordance with a second embodiment of the present invention;

[0047] Figure 10 is a plan view of four wave energy converters such as those of Figure 9, assembled in a modular fashion.

[0048] Figure 11 is a side view of a wave energy converter in accordance with a third embodiment of the present invention;

[0049] Figure 12 is a plan view of the wave energy converter of Figure 11 ; [0050] Figure 13 is a rear view of the wave energy converter of Figure 11; and

[0051] Figure 14 is a rear view of the wave energy converter of Figure 11, shown in a maintenance position.

Detailed Description of Preferred Embodiments

[0052] The general operation of the present invention can be described with reference to Figures 1 to 3.

[0053] Figures 1 to 3 show a wave harnessing reservoir 10 located above the seabed 12. The wave harnessing reservoir has a base 14 located relatively near the seabed 12, and an uppermost portion 16 extending well above mean sea level 18. In an example installation in 6m to 7m of water, it is anticipated that the base 14 may locate between 1m and 2m above the seabed 12, and the uppermost portion 16 may locate about 4m above the mean sea level 18.

[0054] The reservoir 10 has an inlet side 20 facing towards approaching waves, and an outlet side 22 at its lee side.

[0055] The inlet side 20 includes a screen formed by a series of blades 24. The blades 24 extend along the inlet side 20, generally parallel to the shoreline. The blades 24 are moveable between an open configuration in which each blade is generally horizontal and where a relatively large gap is formed between adjacent blades 24, and a closed configuration in which each blade is generally closed to the seabed 12 and there is little or no gap between adjacent blades 24. The operation of the blades 24 will be described in further detail below.

[0056] The screen formed by the blades 24 has two portions: a first portion 26 extending from the base 14 to about the mean sea level 18, and a second portion 28 extending above the mean sea level 18. The first portion 26 and the second portion 28 are both vertical. The second portion 28 is rearwardly spaced relative to the first portion 26 by a horizontal step 30. In the embodiment of the drawings the first portion 26 has a height of 4.8m, the second portion 28 has a height of 4.2m, and the horizontal step 30 has a width of 2m.

[0057] The screen is moveable between an open configuration in which water can readily flow into the reservoir 10, and a closed configuration in which water is restricted from flowing into the reservoir 10 from the inlet side 20.

[0058] A protective barrier 48, for instance formed from round steel bars or strips spaced at about 0.3m, is employed on the outside of the blades 24 to prevent the ingress of large fish, marine mammals, or flotsam into the reservoir 10.

[0059] The outlet side 22 includes a rear wall 32. The rear wall 32 has a vertical wave-capturing portion 34 extending above the mean sea level 18, an inclined central portion 36 extending away from the wave-capturing portion 34 in a downward and rearward direction, and a vertical lower portion 38 extending between the central portion 36 and the base 14. In the embodiment of the drawings the lower portion 38 is spaced from the first portion 26 of the screen by about 15m, and the wave-capturing portion 34 is spaced from the second portion 28 of the screen by about 9m. The central portion 36 is inclined at about 35° relative to the horizontal.

[0060] A diffuser 40 is located with an opening 42 positioned generally centrally of the reservoir 10. The diffuser 40 passes through the rear wall 32 to an exit 44 located on the shore side of the rear wall 32. A hydraulic turbine 46 is located at a throat of the diffuser 40. In the embodiment of the drawings the exit 44 is spaced about 3m rearwardly of the lower portion 38 of the rear wall 32.

[0061] As an ocean wave approaches the inlet side 20 of the reservoir 10, the blades 24 are open to allow it to move into the inside of the reservoir 10. This is shown as arrows 50 in Figure 7a. As the wave impacts on the wave- capturing portion 34 of the rear wall 32, much of the wave energy is reflected back. As this occurs, the blades 24 are quickly rotated into their closed configuration as shown in Figure 7b, effectively trapping the wave energy within the reservoir 10.

[0062] The result of this is to create a standing wave with a height against the wave-capturing portion 34 up to twice the amplitude of the incipient wave.

[0063] This standing wave creates a pressure head in the reservoir 10, which can be then used to force water through the diffuser 40, and to drive the turbine 46.

[0064] The horizontal step 30 allows air to be easily expelled from the reservoir 10 as the water level rises within.

[0065] In the embodiment of Figures 1 to 8, the reservoir 10 has straight sides perpendicular to the screen and the rear wall 32. In a preferred alternative embodiment as shown in Figures 9 and 10, the sides taper towards the rear wall 32 in order to increase the water fill level within the reservoir.

[0066] An articulated ramp 58 extends from the base 14 of the screen to the sea floor 12, preventing wave energy flux from passing beneath the reservoir 10. The articulated ramp 58 is hinged about a horizontal axis.

[0067] The blades 24 are shown in greater detail in Figures 7a, 7b and 7c.

[0068] Each blade 24 is generally aerofoil shaped, and located on a pivot axis 52. The pivot axis 52 is off-centre relative to the blade 24. Each blade 24 has a density slightly greater than that of water, meaning that its weight force is slightly greater than its buoyancy force. A small pressure differential between one side of the blade and the other will thus be sufficient to cause rotation.

[0069] When pressure within the reservoir 10 becomes higher than the outside, a pressure force is applied to the inside of the blades 24. Due to the off-centre position of the pivot axis 52, a larger degree of force acts on a lower portion of each blade 24 than on an upper portion. This generates a moment which causes the blade to rotate into the closed configuration of Figure 7b. When pressure drops inside the reservoir (after being used to drive the turbine 46) the water pressure on the outside of the blades 24 will act against them, moving them back to the open configuration of Figure 7a.

[0070] In order for the wave harnessing reservoir 10 to efficiently convert wave energy, it must be correctly oriented relative to incoming waves, and energy loss due to sway and heave motions must be minimised.

[0071] The mechanism employed for achieving this is the use of retractable feet. The reservoir has four hydraulically deployable “spuds” 60, located near four corners of the base 14. The spuds 60 have a deployed position as shown in Figures 1 to 3, whereby the spuds 60 extend from the base 14 to the sea floor 12. The arrangement is that the spuds 60 extend to a point where the reservoir 10 is actively lifted upwards such that the second portion 28 of the screen is above the mean sea level 18. In this position the reservoir has negative buoyancy, and thus weight of the reservoir 10 is borne by the spuds 60 against the sea floor 12. This weight bearing provides a frictional force sufficient to prevent horizontal movement of the reservoir 10 during normal operations.

[0072] The reservoir 10 includes sensors (not shown), which may be vanes or similar, to determine whether or not the screen is appropriately facing the direction of incoming waves. If the wave direction should change beyond a pre-determ ined margin, the wave energy converter is arranged to realign itself by means of the following process.

[0073] The spuds 60 retract, lowering the reservoir 10 to a position of neutral buoyancy whereby it floats freely in the water. The general position of the reservoir 10 is maintained by a low stretch cable 62 (such as a steel cable) which extends from a primary clump weight 64 to the inlet side 20 of the reservoir 10. When freely floating, the reservoir 10 will naturally skew to a position wherein the screen is normal to the incoming wave front. Once in this position, the spuds 60 are extended to return the reservoir to a secure position. Figure 4 shows an example of the reservoir 10 moving between two different angular orientations based on different wave directions.

[0074] During the floating operation the reservoir 10 is prevented from surging seaward by the provision of a rear support cable 66, extending from the outlet side 22 of the reservoir 10 to a rearwardly positioned clump weight or anchor 68. The rear support cable 66 is maintained in tension by use of a constant tension winch 69 mounted to the reservoir 10.

[0075] The reservoir 10 has further features to accommodate wave conditions outside design operating conditions. In the event of seas being unusually low, the incoming wave height will be reduced, substantially reducing the pressure head used to drive the turbine 46. In order to mitigate this problem, the reservoir 10 includes a selectively operable volume diminishing means, being an inflatable bag 70 fixed to the wave-capturing portion 34 of the rear wall 32. Under pre-determ ined conditions (for instance, a period of below-threshold mean wave height), the inflatable bag 70 expands to reduce the volume of the wave-capturing space behind the second portion 28 of the screen. This has the effect of raising the height of the standing wave within the reservoir, and thus increasing the head on the turbine 46.

[0076] In the event of seas being unusually high, wave energy could be lost by overflow of the reservoir 10. The wave energy converter is able to capture this energy using a two-part ceiling 72, the ceiling 72 having a forward part 74 and a rear part 76. The forward part 74 consists of blades 24, and extends horizontally about one third of the distance from the second portion 28 of the screen to the wave-capturing portion 34 of the rear wall 32. The rear part 76 is formed of a resilient membrane which is able to stretch into a domed configuration 78, and extends from the forward part 74 to the rear wall 32. Unusually high waves have the effect of stretching the rear part 76 both to expand the volume of the reservoir 10 (and the head on the turbine 46) and also to act as a potential energy store. [0077] The possible effect of excessive wave impact is mitigated by use of a secondary clump weight 80 located along the low stretch cable 62. The secondary clump weight 80 is significantly lighter than the primary clump weight 64. Excessive wave energy will, in the first instance, be absorbed by lifting of the secondary clump weight 80 off the sea floor 12 as the wave’s impact forces the reservoir to move in the direction of wave propagation. Further excessive wave energy will be accommodated by dragging of the primary clump weight 64. Together, these mechanisms are arranged to absorb excess energy (for instance in a high storm) and prevent damage to the reservoir 10.

[0078] The turbine 46 drives a shaft 82 connected to a generator 84. The electrical power thus generated is transferred along an umbilical cable 86, which extends along the rear support cable 66 to the rearwardly positioned clump weight 68 and thence to the shore.

[0079] The arrangement of the reservoir 10 is such that a plurality of reservoirs 10 can be fixed together side-by-side in a modular fashion in order to increase the total output. This is shown schematically in Figure 6. In this arrangement the primary clump weight 64 is formed in a modular fashion, with a number of individual clump weights 64a fixed together. Each of the reservoirs 10 is connected by a low stretch cable 62a to a common secondary clump weight 80.

[0080] Figure 6 shows the plurality of reservoirs 10 being laterally aligned such that they present a generally planar screen. It will be appreciated that the reservoirs 10 may be angled with respect to each other to present a slightly concave inlet.

[0081] Figure 8 shows the reservoir 10 in condition for inspection and/or maintenance. This is achieved by extension of the spuds 60 over such length that the turbine 46 is above the mean sea level 18. [0082] Figure 9 shows an alternative reservoir 110, similar in most respects to the reservoir 10 of Figures 1 to 8. The significant difference is that the alternative reservoir 110 has side walls 112 which taper inwardly from the screen 24 to the rear wall 32. In the embodiment of Figure 9 the degree of inclination is about 20° relative to the direction of wave propagation.

[0083] The effect of the tapered side walls 112 is to increase the water fill level within the reservoir.

[0084] Alternative reservoirs 110 can be hinged together in a similar fashion to that of Figure 6. Such an arrangement of alternative reservoirs 110 can be seen in Figure 10. It will be seen that in this embodiment the alternative reservoirs 110 are arranged to present a generally concave inlet.

[0085] Figures 11 to 14 show a further alternative reservoir 210, similar in most respects to the reservoir 10 of Figures 1 to 8 and the reservoir 110 of Figure 9, with like numbers referring to like features. Significant differences include the removal of the horizontal step 30, the removal of the inclined portion 36 of the rear wall 32, the position of the inflatable bag 70, and a narrowing of the triangular shape of the reservoir 110 and associated change in positioning of the spuds 60. Significantly, the reservoir 210 employs two side-by-side turbines 46 with diffusers 40 including a common wall 241.

[0086] It will be appreciated that the operation of the vanes and skewing system described above, as well as the operation of the inflatable bag 70, are preferably managed by an electronic control system. It is envisaged that this will be a fully autonomous process controlled by Artificial Intelligence.

[0087] Calculations suggest that the wave energy converter 10 is capable of converting wave energy flux at a net output efficiency of at least 75%, as opposed to about 33% for known prior art systems. At a relatively low cost of fabrication, towage, installation and operation the price per delivered kWh is anticipated to be substantially lower than for other forms of large-scale energy generation. [0088] Additionally, the wave energy converter is envisaged to be:

• environmentally friendly, without polluting the sea or atmosphere;

• able to act as an artificial reef (if fish pass through the inlet screen they will be able to safely exit through the large openings between blades of a slowly rotating turbine or impeller);

• harmless to all forms of life; and

• unlikely to cause horizon pollution because of low freeboard.

[0089] Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention.