WAVE-ENERGY EXTRACTION

Abstract

A wave energy extractor such as a wave energy converter (WEC) or wave energy dissipater has a floating body with a channel at the fore or aft end, the channel having a sloping base, side walls, in some embodiments an end wall, and an opening for receiving waves. Resonance effects enhance the energy extracted from the waves and transferred to the floating body. Embodiments include WECs such as hinged raft WECs and heaving buoy WECs. The channels may also be fitted to a floating breakwater.

Claims

1-43. (canceled)

44. A wave energy extractor comprising a floating body having a wave receiving channel, the wave receiving channel having a sloping base and a wave receiving opening, both the sloping base and the wave receiving opening being provided between opposing first and second side walls, wherein at least part of the sloping base extends beneath a still water surface in a still water rest position.

45. A wave energy extractor according to claim 44 wherein the said wave receiving channel has a channel resonant frequency at least partly defined by the sloping base and/or the first and second side walls, and wherein the said channel is configured to cause resonance of waves received by the wave receiving channel through the wave receiving opening and having the said channel resonant frequency.

46. A wave energy extractor according to claim 45 wherein the said wave receiving channel is configured to cause resonance of the waves, within a wavelength range, that are received through the wave receiving opening.

47. A wave energy extractor according to claim 44, wherein the ratio of the length of the sloping base, relative to the width of the channel, to the average distance between the opposing first and second side walls, is in the range 1 to 4 and/or wherein the ratio of the depth of the second end of the sloping base, relative to the still water rest position, to the average distance between the opposing first and second side walls, is in the ratio of 0.2 to 1.

48. A wave energy extractor according to claim 44, wherein the distance between the side walls is in the range of 0.8-16 m and/or wherein the sloping base extends upwards at a mean angle of 10° to 60° relative to the horizontal, when the wave energy extractor is at rest.

49. A wave energy extractor according to claim 44, installed in a body of water at a location receiving waves which have an annually averaged mean period, wherein the ratio of distance between the side walls to the wavelength corresponding to said mean period is in the range 1:10 to 1:50.

50. A wave energy extractor according to claim 44 wherein the said sloping base is a first sloping base and the wave receiving channel is a first wave receiving channel, wherein the wave energy extractor further comprises a second wave receiving channel comprising a second sloping base provided between the first said side wall and a third side wall, the said first and third side walls opposing each other, wherein at least part of the second sloping base extends beneath a still water surface in a still water rest position.

51. A wave energy extractor according to claim 50, wherein the second wave receiving channel has a second channel resonant frequency at least partly defined by the second sloping base and/or the first and third side walls, and wherein the said second wave receiving channel is configured to cause resonance of waves received by the second channel and having the said second channel resonant frequency.

52. A wave energy extractor according to claim 44 wherein the wave energy extractor has only a single floating body and/or the said floating body is not connected to another floating body by a joint.

53. A wave energy extractor according to claim 44 wherein the sloping base and the first and second side walls have longitudinal axes which extend in a first direction, wherein the said floating body has a further wave receiving channel comprising a further sloping base and further wave receiving opening, both the further sloping base and the further wave receiving opening being provided between a further pair of opposing side walls, at least a portion of the said further sloping base extending beneath the still water surface in the still water rest position, wherein the further sloping base and the further pair of side walls have longitudinal axes which extend in a second direction different from the first direction.

54. A wave energy extractor according to claim 53 wherein the said further channel has a further channel resonant frequency at least partly defined by the further sloping base and/or the said further pair of walls, and wherein the said further channel is configured to cause resonance of waves received by the said further channel through a wave receiving opening thereof and having the said further channel resonant frequency.

55. A floating breakwater comprising the wave energy extractor of claim 44, wherein the said wave energy extractor is a wave energy dissipater.

56. A wave energy extractor according to claim 44 comprising the said (first) floating body and a further (second) floating body to which it is connected by a joint and the WEC generates energy from the relative movement of the first and second floating bodies.

57. A wave energy extractor according to claim 44, wherein the sloping base extends upwards at a mean angle of 10° to 70° relative to the horizontal, when the wave energy extractor is at rest and/or wherein the sloping base is substantially parallel to the horizontal when the wave energy extractor is at rest.

58. A method of extracting wave energy, the method comprising: providing a floating body having a wave receiving channel, the wave receiving channel having a sloping base and a wave receiving opening, both the sloping base and wave receiving opening being provided between opposing first and second side walls, wherein at least part of the sloping base extends beneath a still water surface in a still water rest position; and the channel receiving waves through the wave receiving opening, causing resonance of the received waves to thereby extract wave energy.

59. A method according to claim 58, comprising the initial step of measuring properties of the waves at a location, averaged over a period of time, and selecting the floating body and/or the wave channel from amongst a plurality of possible floating bodies and/or wave channels having different configuration taking into account the measured properties.

60. A method according to claim 59, comprising measuring properties of the waves at a first location and a second location, the second location having a wavelength corresponding to the mean period between waves, at a greater wavelength than at the first location, and installing a wave energy extractor comprising a said floating body at each of the first location and the second location, wherein the distance between the opposing first and second side walls of the channel of the floating body installed at the first location is greater than the distance between the opposing first and second side walls of the channel of the floating body installed at the second location.

61. A method according to claim 59, comprising fitting a wave channel attachment, which comprises the said wave receiving channel, side walls and sloping base, to a floating body.

62. Apparatus comprising a buoyant hull tethered to an anchor, the buoyant hull having a wave receiving channel comprising a sloping base and a wave receiving opening, at least part of the sloping base extending beneath a still water surface in a still water rest position, the wave receiving channel being configured such that incoming waves received by the channel through the wave receiving opening cause the body to move relative to the anchor to thereby extract wave energy.

63. Apparatus according to claim 62 wherein the anchor is suspended in a body of water under the buoyant hull and/or wherein the anchor comprises the floor of a body of water containing the buoyant hull and/or wherein the anchor is a structure which is adjacent the buoyant hull.

64. Apparatus according to claim 62 wherein the wave receiving channel has a resonant frequency such that waves received by the wave receiving channel through the wave receiving opening and having the said resonant frequency resonate in the said wave receiving channel to thereby cause the buoy to move relative to the anchor to thereby drive the said energy extraction.

65. A method of generating or storing power, the method comprising: providing a buoyant hull tethered to an anchor provided beneath the buoyant hull, the buoyant hull having a wave receiving channel comprising a sloping base and a wave receiving opening, at least part of the sloping base extending beneath a still water surface in a still water rest position; and incoming waves being received by the wave receiving channel through the wave receiving opening, thereby causing the buoyant hull to move relative to the anchor to thereby extract wave energy from the waves to thereby generate or store power.

Description

DESCRIPTION OF THE DRAWINGS

[0124] An example embodiment of the present invention will now be illustrated with reference to the following Figures in which:

[0125] FIG. 1A is a perspective view of a channel, FIG. 1B is a plan view and 1C is a cross-section through the channel located in water;

[0126] FIG. 2A is a perspective view of a buoyant hull having the channel of FIG. 1A, FIG. 2B is a plan view and 2C is a cross-section through the buoyant hull located in water;

[0127] FIG. 3 illustrates key dimensions of a channel;

[0128] FIG. 4A is a graph of the simulated peak water elevation of incident waves within the channel, as a function of l, b, α and d; and FIG. 4B is a photograph of peak water elevation of incident waves in an example embodiment;

[0129] FIG. 5 is a graph of the variation with λ/b in surge, heave and pitches forces exerted on a channel such as that of FIG. 1, with α=20°, d/b=0.5 and l/b=2, compared to a channel with a non-sloping (horizontal in the neutral position) surface and a hull having a vertical fore end wall with no channel;

[0130] FIGS. 6A through 6F are cross-sections through alternative channels;

[0131] FIGS. 7A through 7E are cross-sections through further alternative channels;

[0132] FIG. 8A is an example of a floating hinge wave energy converter (WEC) with a channel according to the invention and FIG. 8B is a cross-section therethrough;

[0133] FIG. 9 is a schematic diagram of a heaving buoy WEC;

[0134] FIG. 10 is a schematic diagram of another heaving buoy WEC;

[0135] FIG. 11 is a schematic diagram of a heaving buoy wave energy converted attached to a sea wall;

[0136] FIG. 12A is a perspective view, 12B is a plan view and 12C is a cross-section through a floating hull having a plurality of adjacent channels;

[0137] FIG. 13A is a perspective view, 13B is a plan view and 13C is a cross-section through a floating hull having channels at the fore and aft ends;

[0138] FIG. 14 is a perspective view of a hinged WEC having channels at the fore and aft ends;

[0139] FIG. 15 is a perspective view of a WEC or breakwater installation;

[0140] FIG. 16 is a perspective view of a floating breakwater;

[0141] FIG. 17A is a perspective view of a further embodiment of a hinged WEC, FIG. 17B is a cross-section through the hinged WEC of FIG. 17A and FIG. 17C is a plan view of the WEC of FIGS. 17A and 17B;

[0142] FIG. 18 is a plot of the power response curve of the embodiment of FIGS. 17A-17B compared to that of a standard raft;

[0143] FIG. 19 is a plot of wave channel excitation torque at various wave channel angles;

[0144] FIG. 20 illustrate the simulated effect of a (individual) channel with a sloping base on the surrounding wave field in comparison with the field around horizontal and vertical plates, and FIG. 21 is a close-up of region A around the channel;

[0145] FIG. 22 is a diagram of an example embodiment of a WEC including damping plates;

[0146] FIG. 23 is an example embodiment of a damping plate; and

[0147] FIG. 24 is an array of plots indicating wave fields of an example wave channel compared to those of a horizontal plate and a vertical plate;

DETAILED DESCRIPTION OF AN EXAMPLE EMBODIMENT

[0148] With reference to FIGS. 1A-1C, a channel 1 is provided as a component of a wave energy extraction device such as a wave energy converter (WEC) or dissipater. FIG. 1C shows the position of the channel relative to the still water surface 2 when a wave energy extraction device having the channel is at rest in still water (the still water rest position, referred to herein as a neutral position). In this description, directions and orientations such as horizontal and vertical refer to the directions when the channel is oriented in this neutral position. Label W indicates the direction of incident waves during normal operation.

[0149] The channel has a longitudinal axis 7, a sloping base 4 which extends from a first end 6 to a second end 8 from where an end wall 10 extends vertically. Side walls 12 and 14 are provided at opposite lateral sides of the sloping base. In this example, the side walls extend above the water line in the neutral position. Given the direction of incident waves, the first end 6 is at the fore end of the channel and the second end 8 is at the aft end. The area above the fore end functions as the opening 9 through which waves enter the channel.

[0150] The channel may be provided as an individual component for attachment to, or integration into, a wave energy extractor and may have fixtures 16 (such as bolt holes) for such purpose. However, FIGS. 2A through 2C show the channel 1 provides at the fore of a buoyant hull 100 having a buoyant component 102 and being a component of a wave energy extractor. Although the buoyant hull may be made of a material, such as a plastic material or fibreglass, which has sufficiently low density to be buoyant, it typically comprises a relatively dense resilient material (e.g. steel) with one or more internal components filled with a low-density material, such as air, or vacuum.

[0151] We have found that, surprisingly, a channel such as that shown in FIGS. 1 and 2, is especially useful for wave energy extraction. Without wanting to be bound by theory we propose that the channel creates resonance effects leading to this enhanced wave energy extraction.

[0152] With reference to FIG. 3, characteristic dimensions of the basic channel 1 are the length, l, of the sloping base; the depth, d, of the second end of the channel, relative to the water surface in the neutral position; b, the beam (width) of the channel, between the side walls; and α, the angle at which the sloping base extends below the horizontal (in the neutral position). Practical embodiments may include additional characteristic dimensions, for example, the bases may have a variable slope, the side walls need not be parallel, the end wall need not be vertical.

[0153] FIG. 4A illustrates the peak wave elevation within the channel as a function of l, d, b and α as simulated using linear wave theory. In linear wave theory, the wavelength, λ, is a function of radial frequency, ω, and water depth, h, via the dispersion relation:

[00001]${\omega }^2=\frac{2.Math.\pi .Math.g}{\lambda }.Math.\tan .Math.\frac{2.Math.\pi .Math.h}{\lambda }$

for finite depth water, or

[00002]${\omega }^2=\frac{2.Math.\pi .Math.g}{\lambda }$

when the water can be assumed to be of infinite depth. Here, g is the gravitational constant. Rather than ω, we here use λ. Dimensions are non-dimensionalised by the beam of the channel, i.e. l/b, d/b, λ/b.

[0154] The choice of dimensions of the channel are important as they effect the resonant modes of the WEC and, as a result, the energy which it may generate. Of particular importance are the angle of slope of the sloping base of the wave receiving channel, and the length-to-width ratio (i.e. the ratio of the length of the base to the width of the base (distance between side walls) of the wave receiving channel. For example, a longer and narrower base will excite a stronger resonance at a longer wavelength than will a shorter and wider base.

[0155] FIG. 4B shows a practical implementation. Waves travel right to left in the picture into channel 1 which generally corresponds to the channel of FIG. 1 with corresponding labelled features. The height of the water surface 3 is substantially raised above the still water level 2 at the moment shown in the picture.

[0156] FIG. 5 shows the variation with λ/b in the surge, heave and pitch forces on a hinged floating raft WEC having a channel according to FIGS. 1 and 2. It can be seen that there is a resonance effect, with substantially increased applied forces, at λ/b of between about 15 and 20, compared to corresponding devices with a horizontal base or vertical end wall. These additional forces are indicative of a surprising amount of energy extraction from the waves and it is useful that the peak wavelength is substantially greater than the beam (and so in practical embodiments typically the length) of the wave energy extraction device, thereby enabling a relatively compact device to be provided to efficiently extract energy from the relatively long wavelengths components of typical ocean waves.

[0157] It is not required that the channel has the shape of FIG. 1 or 2. Some alternative cross sections are shown in FIGS. 6A through 6F. In 6A, the top edge 18 of the side walls are generally horizontal in the neutral position. In FIG. 6B, the end wall is not vertical but is sloped (in this example at 45° to the vertical). In FIG. 6C, there is no discrete end wall as such. In FIG. 6D, the sloping base has a first sloping section 4A and a second sloping section 4B which extends from the first sloping section, towards the second end of the sloping base, at a greater angle relative to the horizontal, than the first sloping section. Alternatively, second sloping section 4B can be seen a portion of the end wall 10. In FIG. 6E, the tops of the side walls are also sloped, extending at a different angle (in this example at a greater angle relative to the horizontal) to the sloping base. In FIG. 6F, the side walls do not extend the whole way to the first end of the sloping base and so the sloping base extends forewards of the channel.

[0158] In FIGS. 1, 2 and 6A through 6F, the sloping base is formed by a plate, and the distance between the sloping base of the channel 4 and the underside 20 of the channel is constant. In FIG. 7A, the sloping base 4 has a different slope to the underside of the channel—indeed in this example the underside of the channel slopes downward towards the second end. The underside of the channel 22 below the end wall slopes at a different angle to the end wall. In FIG. 7B, the sloping base has a first region, 4A and a second region 4B, which is towards the second end and slopes at a greater angle relative to the horizontal than the first region and there is not a distinct end wall. In the embodiment of FIG. 7C, the sloping base curves (and is concave, with a slope which increases towards the second end), and blends into the end wall 10. In FIG. 7D, the upper edge 18 of the side walls and the sloping base slope at different angles relative to the horizontal in the still water position. The side walls are trapezoidal in this example (in some other examples they are rectangular, or scalene trapezoids, for example). The floating body is attached to a second floating body 200 by a hinged joint 202. The embodiment of FIG. 7E corresponds except that the first end of the channel has a lip 24 in the form of a hydrodynamic fairing.

[0159] FIG. 8A shows a floating hinge WEC 300 having a first floating body 102 having a channel 1 at a fore end thereof, and a second floating body 200, connected to the first floating body by a hinged joint 202. An electricity generator, or other energy conversion apparatus, such as a pump (e.g. a hydraulic pump or water pump) 204 may be located in the hinge region to convert received wave energy into another form (e.g. electricity, fluid power etc.)

[0160] With reference to FIG. 8B, which is a cross-section through the first floating body, the floating body has a chassis 111, formed for example from metal, fiberglass, concrete or a plastics material, and in some embodiments including a buoyant core 112 (e.g. a sealed compartment containing air, or another gas, or a vacuum, or a low-density plastics material, and optionally one or more ballast chambers) has an antic-corrosion coating 114 formed on the external surface of the chassis. The surface is also treated with an anti-fouling coating (e.g. paint) on the external surface to restrict the growth of organisms.

[0161] However, the invention is also useful with for example a heaving buoy WEC, such as that shown in FIG. 9 in which a floating body 100 comprises buoyant hull 102 with a channel 1 as described in FIG. 2 at a fore end thereof. The floating hull is tethered through a connector 50 to an anchor (in this case, the sea bed 52) through an electricity generator 54 (or pump) which generated electricity (or pressurised hydraulic fluid or water) from the heaving motion of the floating body.

[0162] FIG. 10 shows another heaving buoy WEC in which the floating body 100 has two channels 1, fore and aft, the connector is a pole 50 up and down which the WEC moves with wave motion and a generator or pump is provided within the body of the WEC. In the embodiment of FIG. 11, the floating body is connected through an arm 56 to an anchor in the form of a sea wall 58. The arm drives a generator or pump 54 to thereby provide human-usable energy extracted from the waves. As well as generating energy from heaving motion, such WECs may generate energy from surging motion and rotation (pitch), for example.

[0163] Thus, a WEC may be referenced to the sea floor, a wall or other anchor, or self-referenced (e.g. to a submerged but not fixed anchor connected to the floating body).

[0164] With reference to FIGS. 12A through 12C, a plurality of adjacent channels 1A, 1B, may be provided. As well as a first channel 1A having sloping base 4A and side walls 12, 14, there is a second adjacent channel with a further sloping base 4B, which shares side wall 14 with the first channel and has a further side wall 15. In this example, the channels have end walls 10. The channels may have different dimensions, for example in order to have different resonant wavelengths. They may for example have different beams, and or bases which slope at different angles.

[0165] FIGS. 13A through 13C, a plurality of channels 1A, 1B, are formed at opposite (fore, aft) ends of the buoyant component 102. Each has a respective sloping base 4A, 4B, side walls 12A, 12B opposite 14A, 14C respectively, and an end wall 10A, 10B. FIG. 14 shows a WEC based on this principle, with first and second buoyant hulls 102, 200 having channels at their fore and aft ends respectively, and a hinge 202 therebetween. As with the embodiment of FIG. 8, the hinge has a power take-off such as an electricity generator or pump.

[0166] In a practical implementation a plurality of WECs may be located adjacent to each other and typically parallel to extract energy from a broad front of waves. FIG. 15 shows an example of a floating WEC 400 formed from a plurality of floating hulls having channels according to the invention at fore and aft ends, and which are connected by rods 60, with power take-offs generating electricity or pumping fluid using the rotational motion of the floating hulls relative to each other.

[0167] As well as being useful for a WEC, such as a wave energy generator, channels according to the invention are useful as wave energy dissipaters, for example as floating (or sea-floor based) breakwaters, or in other context where it is advantageous to dissipate energy, such as floating booms, or around the hull of a ship or periphery of an oil platform, for example. Thus, the structure of FIG. 15 is also useful as a floating breakwater, in which the adjacent hulls are coupled with either fixed rods (without rotating joints) or with brakes (e.g. disc brakes) or other energy dissipating connections therebetween.

[0168] In a further example of a floating breakwater, an elongate, and potentially jointed, floating hull 102 has a plurality of channels 1 according to the invention, on either (or just one) side. The resonance effects in the channels extract energy efficiently from waves. Energy dissipation can be enhanced by the presence of joints which dissipate heat due to friction, drag plates, vanes or orifices, which dissipate energy hydrodynamically.

[0169] The wave energy dissipater causes the reflection of wave energy in respect of the waves received into the wave receiving channel or channels. The or each channel acts to create resonance, increasing the wave height within the channel with the result that the wave energy is reflected out of the channel in a different direction from the direction in which it entered the channel. This causes a decrease in the height of waves in the lee of the wave energy dissipater (i.e. the wave energy dissipater acts as a breakwater). One skilled in the art will appreciate that in this embodiment, energy is not dissipated as the wave energy is reflected, not absorbed.

[0170] Turning now to FIGS. 17A-17C, a further example embodiment of the invention is a floating WEC having a first floating body (here in the form of a fore hull) 104 and a second floating body (here in the form of an aft hull) 106 joined by a hinge 108 (cross-sectional and plan views of this example embodiment can be seen in FIGS. 19 and 20, respectively). In this example, both the first floating body 104 and the second floating body 106 are buoyant. The first floating body 104 has two adjacent fore wave receiving channels 1A, 1B, the channels having a sloping base, a sloping end wall, and three side walls (including a common wall which functions as a side wall of each of the two adjacent channels) which thereby form the two adjacent channels 1A, 1B, into which (and out of which) water may flow in use. In this example, the second floating body 106 has an aft wave receiving channel 110, the channel having a sloping base and three side walls (however no distinct end wall) which forms two adjacent channels into which (and out of which) water may flow in use. The fore wave receiving channel 1 and the aft wave receiving channel 110 are not buoyant in this example, however in other examples they may be (e.g. at least partially) buoyant. The principal wave direction is indicated with arrow W.

[0171] One skilled in the art will appreciate that the choice of the angle of slope of both the base of both the aft wave receiving channel and the fore wave receiving channel will have an effect on the volume (and hence the mass) of fluid that may be received and therefor constrained by the channels, and therefore also on the resonant behaviour of the system. Accordingly, the geometry of the respective wave receiving channels will be influenced by the expected environment that the WEC is intended for use in, and the dynamics of the waves (e.g. frequency distribution, wave heights and water depth) expected in that environment.

[0172] As can be seen most clearly from FIG. 17B, this example embodiment wave receiving channel front stiffeners 114 and rear stiffeners 120 and a mooring attachment 116 on the first wave receiving channel (one skilled in the art will recognise that these are optional features which nevertheless may be included, for example, to improve the structural strength of the first wave receiving channel). The first floating body has a first length 132 and the second floating body has a second length 134 which make up the majority of the full length 130 of the WEC (the remaining length being made up from the hinge. The hinge is defined by a first floating body hinge axis 122 and a second floating body hinge axis 124, such that the first floating body can move rotationally relative to the second floating body and so that both floating bodies may be below the waterline 2, to a greater or lesser extent. The centre of the hinge is typically a first depth 128 below the waterline 2 and the second floating body typically extends a second depth 126 below the waterline (i.e. in use).

[0173] As can be seen most clearly from FIG. 17C, further optional features include a first Y-stiffener 136 of the first floating body and a second Y-stiffener 142 of the second floating body. The WEC further has an access hatch 146. The hinge has a first floating body nacelle 138 and a second floating body nacelle 144 as well as torque transfer plates 140. Line 146 indicates a cross section through the aft wave receiving channel, the view through that cross section indicated by feature 150, from which it can be seen that the aft wave receiving channel also has a channel lip.

[0174] In the example embodiments of FIGS. 17A-17C, waves may enter either the fore wave receiving channels 1A, 1B or the aft wave receiving channel 110. The force of waves on each channel causes the first floating body 104 to move relative to the second floating body 110. A power response curve for the embodiment of FIG. 17 can be seen in FIG. 18, compared to that of a standard raft. Table 1 below compares the predicted energy production of the embodiments of FIGS. 17A-17C, compared to that of a standard raft (The data of Table 1 relates to an example site in the North Sea). It has been found surprisingly that a WEC such as that shown in FIGS. 17A-17C may produce almost three times as much energy than a standard raft each year, despite being both smaller and lighter.

TABLE-US-00001 TABLE 1 Length Mass Annual Energy Production m tonne MWh/year Embodiment of 37 218 566 FIG. 17 Standard Raft 49 245 189

[0175] In particular, in the embodiment of 17A-17C, the full length 130 of the WEC is 37 m (the first floating body has a first length 132 of 25 m and the second floating body has a second length 134 of 11 m, the remaining length being made up by the hinge). In comparison, a standard raft is typically 49 m in length. The angle of the slope of the sloping base of the wave receiving channel of the first floating body of the WEC is an important factor here, and modelling and experimentation have led to the choice of angle of slope of between 20° and 30°. The angle of the slope of the sloping base of the wave receiving channel of the second floating body is 70°.

[0176] In general, we have found that the resonance effects described herein pull wave energy from adjacent regions into the wave channels (this has the effect that the floating body has an incident wave cross-section greater than its physical cross-section perpendicular to the direction of wave travel). The wave height at the channel is increased and the resonances effect increases the forces on the floating bodies/hulls typically thereby increasing movement of the floating body.

[0177] FIGS. 20 and 21 illustrate the simulated effect of a (individual) channel with a sloping base on the surrounding wave field in the presence of parallel waves travelling from left to right. The scale of the right hand column is |η| (normalised wave elevation) and of the left hand column is the real part of that. The first row shows the wave field around a wave channel according to the present invention. The second row shows the wave field around a non-wave channel geometry in the form of a horizontal plate with no depth. The third row shows the wave field around a non-wave channel geometry in the form of a vertical plate (orthogonal to the still water surface). As can be seen the wave channel creates a much larger disturbance in the wave field, due to wave resonance within the channel. In the first column this can be seen from the modulation of otherwise straight wave crests. The second column show the modulation with |η| of 1 being uniform modulation. FIG. 21 is a close-up of region A around the channel. Clear resonance within the channel can be seen.

[0178] FIGS. 22 and 23 shows computer simulations of example embodiments of a WEC wherein the simulations have been provided with vertical damping plates 220 positioned above the or each wave channel 1. FIG. 22 shows an example embodiment of a WEC having a damping plate and FIG. 23 is a close up of a wave channel with a damping plate.

[0179] Such computer simulations are helpful for calculating the response of a WEC (including a damping plate) in the presence of water waves. In particular, it has been found that the provision of the damping plates is helpful in solving linear wave equations which otherwise predict resonant responses which are unrealistically large. In practice, such resonant responses have not been found. The addition of the damping plates to the simulations has the effect of providing solutions that are more realistic. In such simulations, the damping plate is treated as being substantially massless but as applying damping forces to water. In such simulations, they act to create a pressure on the water surface that thereby alters the flow of the water, damping the motion of the water. In some respects the damping plates may therefore be considered to be virtual plate.

[0180] FIG. 24 is a series of plots indicating the predicted behaviour (i.e. behaviour predicted in a computer simulation) of a WEC both with and without a damping plate.

[0181] In WEC embodiments, the extracted energy is converted to another human-usable form using a power take-off such as an electricity generator, pump etc.

[0182] In energy dissipating embodiments, this extracted energy is primarily or entirely not used for a useful purpose but is instead dissipated, with the purpose of reducing wave intensity.

[0183] The dimensions of the channel, or channels, particularly the spacing between the side walls (the beam) may be selected depending on the location where the wave energy dissipater is to be installed. The properties of waves at the location, notably the average period between waves and/or the power spectral density of waves, are measured over a long period of time (e.g. a day, week, month or year). A channel with dimensions which will prove optimal at that location given the measured wave properties is then designed or selected from amongst a plurality of available configurations. In particular, the beam of the channel is typically determined as a fraction of the wavelength associated with the measured mean period between waves, taking into account measurements or simulations such as those shown in FIG. 5. Thus, wave energy extractors (such as WECs or dissipaters) may be provided with channels having different dimensions at different locations which have waves with different properties. For example, the beam of the channels at different locations may be a function of (e.g. proportional to) the wavelength associated with the mean period of waves at the various locations.