WO2018191779A1

WO2018191779A1 - "wave energy converter"

Info

Publication number: WO2018191779A1
Application number: PCT/AU2018/050348
Authority: WO
Inventors: Pieter Jan De Geeter
Original assignee: Pieter Jan De Geeter
Priority date: 2017-04-18
Filing date: 2018-04-18
Publication date: 2018-10-25
Also published as: AU2018202718A1

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 tunnel 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.

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. A number of 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 sea bed, 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.

[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 system 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] A search of prior art has revealed German patent number

DE3017257 which seeks to harness both kinetic and potential energy of at least a top part of a wave form.

Summary of the Invention

[001 1 ] According to one aspect of the present invention there is provided a method of harnessing wave energy from a body of water, the method including the steps of:

providing a wave energy harnessing reservoir having an inlet side and an outlet side, the inlet side including a screen moveable between an open configuration and a closed configuration, and the outlet side including a rear wall arranged to extend above a waterline of the body of water, 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 wave energy harnessing reservoir;

orienting the wave energy harnessing reservoir so that it is generally aligned in a direction of wave propagation;

allowing the screen to be open to receive a wave travelling through the body of water;

causing the screen to close when the water level within the wave energy harnessing reservoir reaches an elevated level;

using stored energy within the water in the wave energy harnessing reservoir to drive a turbine; and

causing the screen to open when the water level within the wave energy harnessing reservoir reaches a lowered level.

[0012] Advantageously, the method of the present invention is able to harness kinetic energy in the body of water at a depth below the wave amplitude.

[0013] 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.

[0014] 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.

[0015] 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. [0016] 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.

[0017] 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%.

[0018] The turbine may be located within a tunnel, the tunnel extending between an interior of the wave energy harnessing reservoir and an exterior of the reservoir. The tunnel outlet is preferably located outside the rear wall; that is, the tunnel preferably extends through the rear wall.

[0019] The tunnel may be a converging-diverging tunnel. It is preferred that the tunnel has an outlet diameter at least twice the diameter of the turbine.

[0020] The inlet side may be generally planar. Alternatively, the inlet side may consist of a number of sets of screen, covering an arc of up to about 210°.

[0021 ] The distance between the screen and the rear wall may be based on the expected wave length of the waves being harnessed. It is preferred that this distance is less than one third of the expected wave length. In a preferred embodiment, the distance is about one fifth of the expected wave length.

[0022] An optimal distance between the screen and the rear wall may be calculated as a function of the design wave height. The ratio of the distance between the screen and the rear wall to the 'submerged' height of the screen (that is, the height between the lowest level of the screen and mean sea level) is preferably close to 1 .9, although ratios within the range 1 .0 to 3.5 are considered to be acceptable without excessive loss of efficiency.

[0023] The inlet side may be wider than the outlet side. The arrangement may be generally trapezoidal when viewed in plan view.

[0024] The reservoir may have side walls which taper towards each other above the water line. This may assist in increasing the ultimate height of the captured wave.

[0025] The reservoir may include an expandable portion, which may be bounded 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.

[0026] The reservoir may be anchored to the seabed by means of anchoring cables. In one embodiment, at least one of these cables may be associated with an energy converter, such as hydraulic or pneumatic energy converter, at the anchor point. In this embodiment, the reservoir may be suitably ballasted to enable it to move with the ocean current, with energy from this movement being captured by the energy converter.

[0027] 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.

[0028] In an alternative embodiment, the reservoir may be fixed to the sea floor, for instance by means of piles or by provision of appropriate ballast.

[0029] According to a second aspect of the present invention there is provided a wave energy harnessing reservoir having an inlet side and an outlet side, the inlet side including a screen moveable between an open configuration and a closed configuration, and the outlet side including a rear wall arranged to extend above a waterline of a body of water, 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 wave energy harnessing reservoir;

the screen having a submerged height representing the height between the lowest level of the screen and the waterline;

the reservoir having an effective length equal to the distance between the screen and the rear wall;

wherein the ratio of submerged height : effective length is in the range of 1 .0 to 3.5.

[0030] Preferably, the ratio of submerged height : effective length is in the range of 1 .6 to 2.5.

[0031 ] Most preferably, the ratio of submerged height : effective length is about 1 .9.

Brief Description of the Drawings

[0032] 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:

[0033] Figure 1 is a perspective of a wave harnessing reservoir according to the present invention;

[0034] Figure 2 is a partially cutaway view of the wave harnessing reservoir of Figure 1 ;

[0035] Figure 3 is a partial front view of the wave harnessing reservoir of Figure 1 ;

[0036] Figure 4 is a side view of the wave harnessing reservoir of Figure 1 ; [0037] Figure 5 is a partial plan view of the wave harnessing reservoir of Figure 2;

[0038] Figure 6a is a side view of a screen from within the wave harnessing reservoir of Figure 2, shown in an open position;

[0039] Figure 6b is a side view of the screen of Figure 6a, shown in a closed position;

[0040] Figure 7 is a schematic cross sectional view of a wave harnessing reservoir according to the present invention;

[0041 ] Figure 8a is a graphical analysis of the operation of a wave on the wave harnessing reservoir of the present invention;

[0042] Figure 8b is a graphical analysis of the operation of a wave on the wave harnessing reservoir of the prior art;

[0043] Figure 9 is a schematic plan view of an alternative wave harnessing reservoir in accordance with the present invention;

[0044] Figure 10 is a side view of a further alternative wave harnessing reservoir in accordance with the present invention; and

[0045] Figures 1 1 to 15 show alternative tethering means for wave harnessing reservoirs in accordance with the present invention.

Detailed Description of Preferred Embodiments

[0046] The general operation of the present invention can be described with reference to Figures 1 to 7.

[0047] Figures 1 to 7 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 25m of water, it is anticipated that the base 14 may locate between 2m and 5m above the seabed 12, and the uppermost portion 16 may locate about 5m above the mean sea level 18.

[0048] The reservoir 10 has an inlet side 20 facing towards the open sea, and an outlet side 22 facing towards the shoreline.

[0049] 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 parallel to the seabed 12 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.

[0050] The blades 24 extend from the base 14 to the uppermost portion 16, that is, through the full height of the reservoir 10. In an alternative

embodiment, the reservoir may include a small lip or fixed barrier extending upwardly from the base 14 before a lowest blade 24. It is considered that the higher the barrier, the less effective the wave harnessing operation. As such, a barrier of more than 20% of the height of the reservoir is considered to render the invention impractical. A barrier height of less than 10% is considered more efficient, with as little barrier as possible being preferred.

[0051 ] The screen is therefore generally 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.

[0052] A protective barrier 25, for instance formed from round steel bars spaced at about 0.3m, is employed on the outside of the blades 24 to prevent the ingress of large fish or flotsam into the reservoir 10. [0053] The outlet side 22 includes a rear wall 26. The rear wall 26 extends well above the mean sea level 18.

[0054] A tunnel 28 is located with an opening 30 positioned generally centrally of the reservoir 10. The tunnel 28 passes through the rear wall 26 to an exit 32 located on the shore side of the rear wall 26. The tunnel 28 is formed generally as a converging-diverging tunnel. A hydraulic turbine 34 is located at the narrow point of the tunnel 28.

[0055] Referring to Figure 7, as an ocean wave 40 approaches the inlet side 20 of the reservoir, the blades 24 are open to allow it to move into the inside of the reservoir 10. This is shown as arrows 42. As the wave 40 impacts on the rear wall 26, much of the wave energy is reflected back as shown by arrows 44. As this occurs, the blades 24 are quickly rotated into their closed configuration, effectively trapping the wave energy within the reservoir 10.

[0056] The result of this is to create a standing wave 46, with a height against the rear wall 26 up to twice the amplitude of the wave 40.

[0057] This standing wave 46 creates a pressure head in the reservoir 10, which can be then used to force water along the path of arrows 48, through the tunnel 28, and to drive the turbine 34.

[0058] It is anticipated that the distance between the inlet side 20 and the rear wall 26 will be as close as possible to one fifth of the wave length of the ocean wave 40. This distance may be defined as the effective length of the reservoir 10.

[0059] It is calculated that the effective length of the reservoir 10 may be based on the submerged height of the screen formed by blades 24; that is, the distance between the lowest blade 24 and mean sea level 18. [0060] Calculations suggest that the optimum ratio of effective length : submerged height is about 1 .9. It is considered that a ratio of between 1 .6 and 2.5 will be reasonably close to optimum, and a ratio between 1 .0 and 3.5 will be within about 25% of optimum.

[0061 ] The reservoir 10 has tapered side walls 52, which assist in concentrating wave energy towards the rear wall 26. The inlet side 20 is generally vertical within the ocean, but has a tapered, recessed upper portion 54 located above mean sea level 18. The rear wall 26 has an inwardly tapering upper section 56. The combination of the tapered upper portion 54 of the inlet side 20 and upper section 56 of the rear wall 26 forces the standing wave 46 to a higher level, increasing the head operating the turbine 34.

[0062] The blades 24 are shown in greater detail in Figures 6a and 6b.

[0063] Each blade 24 is generally aerofoil shaped, and located on a pivot axis 60. The pivot axis 60 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 62 is slightly greater than its buoyancy force 64. A small pressure differential between one side of the blade and the other will thus be sufficient to cause rotation.

[0064] When pressure within the reservoir 10 becomes higher than the outside, a pressure force 66 is applied to the inside of the blades 24. Due to the off-centre position of the pivot axis 60, 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 6b. When pressure drops inside the reservoir (after being used to drive the turbine 34) the water pressure on the outside of the blades 24 will act against them, moving them back to the open configuration of Figure 6a. [0065] The efficiency of the present invention may be seen with references to Figure 8a and 8b, with 8a representing the present invention and 8b representing a style of reservoir found in the prior art.

[0066] In both cases, the combined kinetic and potential energy of an incident wave is shown as a series of arrows. The energy lost underneath the reservoir 10 in Figure 8a is small (not shown in Figure 8b).. The energy lost due to reflection by a large 'lip' in Figure 8b is significant.

[0067] The ratio of effective length (Lo) to submerged height (SP) in Figure 8a is an optimal 1 .9. In Figure 8b, it can be seen that this ratio is close to 5.0.

[0068] It is also important to consider the distance between the seabed 12 and the base 14 of the reservoir 10. It is preferred that this distance be less than 20% of the submerged height of the screen. In fact, it is considered that a distance of about 10% is considered preferable.

[0069] The reservoir shown in Figures 1 to 6 is suitable for installation in environments where the direction of wave propagation is reasonably consistent, perpendicular to the shore line. Where the direction of wave propagation is not consistent, it may be necessary to have inlets and associated screens along several sides of the reservoir, a schematic plan view of one such reservoir 70 is shown in Figure 9, with the coastline marked as 72 and different possible directions of wave propagation marked with arrows 74.

[0070] A further style of reservoir 80 is shown in Figure 10. The reservoir 80 of Figure 8 includes an elastic membrane 82 forming part of a base of the rear wall 26. This provides additional capacity for the reservoir 80, with the membrane 82 providing storage of potential energy from excessive waves (for instance, in high seas) which can then be discharged via the turbine 34. [0071 ] It is anticipated that the reservoir 10 will be anchored to the seabed 12 using anchor cables 84. In one embodiment, shown in Figure 1 1 , at least one anchor cable 84 is associated with a hydraulic or pneumatic energy convertor 86 located at the anchoring point. In this embodiment the cable 84 is a low-stretch cable, able to transmit the back-and-forth sway of the reservoir 10 into energy activating the energy converter 86 to generate electrical energy.

[0072] In an alternative arrangement, the reservoir 10 may be fixed to the seabed 12 using piles 90 as shown in Figure 12, or using increased weight (ballast) 94 as shown in Figure 13.

[0073] Fixing the reservoir 10 to the seabed 12 means that, in theory, all available wave flux energy is able to be captured and used for driving the turbine 34. Where the reservoir 10 is anchored via cables 84 to the seabed 12, there will necessarily be a gap between the base 14 of the reservoir 10 and the seabed 12.

[0074] In order to increase efficiency of energy capture, it is considered that this gap should be as small as possible. It is also envisaged that in an alternative embodiment, shown in Figure 14, a hinged ramp 96 may extend from the base 14 of the reservoir 10 to the seabed 12 in order to close the gap.

[0075] Figure 15 shows a proposed anchoring system for the reservoir 10, with a single anchoring point 98 located on the seabed 12, and two low- stretch cables 84 extending from the anchoring point 98 to the reservoir 10. With such an arrangement, the reservoir 10 is able to slew so as to remain square to incoming waves. A stretchable tether 100 may be included on the shore side of the reservoir 10.

[0076] Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention. For instance, it is envisaged that a plurality of tunnels 28 and/or turbines 34 may be employed within a single reservoir 10 to provide a more rapid outflow of water.

Claims

1 . A method of harnessing wave energy from a body of water, the method including the steps of:

2. A method of harnessing wave energy as claimed in claim 1 , wherein the height of the screen is preferably greater than 90% of the height of the reservoir.

3. A method of harnessing wave energy as claimed in claim 1 or claim 2, wherein the screen may be caused to close or to open based on predetermined water levels within the reservoir.

4. A method of harnessing wave energy as claimed in claim 1 or claim 2, 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.

5. A method of harnessing wave energy 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.

6. A method of harnessing wave energy as claimed in claim 5, wherein each blade is rotatable about a horizontal pivot axis.

7. A method of harnessing wave energy as claimed in claim 5 or claim 6, wherein each blade is shaped as a vane, with the pivot axis being off- centre.

8. A method of harnessing wave energy as claimed in any one of claim 5 to 7, wherein each blade has a density greater than that of the body of water.

9. A method of harnessing wave energy as claimed in claim 8, wherein the density of each blade is between 101 % and 120% of that of the body of water.

10. A method of harnessing wave energy as claimed in any preceding claim, wherein the turbine is located within a tunnel, the tunnel extending between an interior of the wave energy harnessing reservoir and an exterior of the reservoir.

1 1 . A method of harnessing wave energy as claimed in claim 10, wherein the tunnel outlet is located outside the rear wall.

12. A method of harnessing wave energy as claimed in claim 10 or claim 1 1 , wherein the tunnel is a converging-diverging tunnel.

13. A method of harnessing wave energy as claimed in claim 12, wherein the tunnel has an outlet diameter at least twice the diameter of the turbine.

14. A method of harnessing wave energy as claimed in any preceding claim, wherein the inlet side is generally planar.

15. A method of harnessing wave energy as claimed in any one of claims 1 to 13, wherein the inlet side may consist of a number of sets of screen, covering an arc of up to about 210°.

16. A method of harnessing wave energy as claimed in any preceding claim, wherein the inlet side is wider than the outlet side.

17. A method of harnessing wave energy as claimed in any preceding claim, wherein the distance between the screen and the rear wall is less than half of the expected wave length.

18. A method of harnessing wave energy as claimed in claim 17, wherein the distance is about one fifth of the expected wave length.

19. A method of harnessing wave energy as claimed in any preceding claim, wherein the ratio of the distance between the screen and the rear wall to the submerged height of the screen is within the range 1 .0 to 3.5.

20. A method of harnessing wave energy as claimed in claim 19, wherein the ratio is within the range 1 .6 to 2.5.

21 . A method of harnessing wave energy as claimed in claim 20, wherein the ratio is about 1 .9.

22. A method of harnessing wave energy as claimed in any preceding claim, wherein the reservoir has side walls which taper towards each other above the water line.

23. A method of harnessing wave energy as claimed in any preceding claim, wherein the reservoir includes an expandable portion, which is bounded by a resiliently deformable sheet or membrane.

24. A method of harnessing wave energy as claimed in any preceding claim, wherein the reservoir is anchored to the seabed by means of anchoring cables.

25. A method of harnessing wave energy as claimed in claim 24, wherein at least one of these cables is associated with an energy converter at the anchor point.

26. A method of harnessing wave energy as claimed in any preceding claim, wherein the reservoir will have a height sufficient to extend at least 80% of the distance from the water surface to the sea floor.

27. A method of harnessing wave energy as claimed in claim 26, wherein the reservoir has a height sufficient to extend at least 90% of the distance from the water surface to the sea floor.

28. A method of harnessing wave energy as claimed in any preceding claim, wherein a wave guiding means is provided between a lower edge of the inlet side and the sea floor.

29. A method of harnessing wave energy as claimed in any one of claims 1 to 25, wherein the reservoir is fixed to the sea floor, for instance by means of piles or by provision of appropriate ballast.

30. A wave energy harnessing reservoir having an inlet side and an outlet side, the inlet side including a screen moveable between an open

configuration and a closed configuration, and the outlet side including a rear wall arranged to extend above a waterline of a the body of water, 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 wave energy harnessing reservoir;

wherein the ratio of submerged height : effective length is in the range of 1 .1 to 3.0.

31 . A wave energy harnessing reservoir as claimed in claim 30, wherein the ratio of submerged height : effective length is in the range of 1 .6 to 2.5.

32. A wave energy harnessing reservoir as claimed in claim 31 , wherein the ratio of submerged height : effective length is about 1 .9.