GB2434408A

GB2434408A - Wave energy conversion apparatus and coastal protection method

Info

Publication number: GB2434408A
Application number: GB0601307A
Authority: GB
Inventors: Jack Hardisty
Original assignee: NEPTUNE RENEWABLE ENERGY Ltd
Current assignee: NEPTUNE RENEWABLE ENERGY Ltd
Priority date: 2006-01-24
Filing date: 2006-01-24
Publication date: 2007-07-25
Also published as: GB0601307D0

Abstract

A wave energy conversion device for coastal use comprises a supporting structure (1) resting on or anchored to the sea bed, a float (3) mounted on an arm (4) pivotally connected to the structure to permit the float to move in a substantially vertical plane, and electricity generating means (6, 10. 11) responsive to the movement of the waves to generate electricity. The electricity generating means may comprises a hydraulic pump charging an accumulator which in turn supplies a turbine driving a generator, but in other embodiments, a flywheel and a linear electric generator are employed.

Description

WAVE ENERGY CONVERSION APPARATUS AND COASTAL PROTECTION

METHOD USING SUCH APPARATUS

Field of the Invention

This invention relates to a wave energy conversion apparatus and to a method of maximising energy conversion.

Background to the Invention

With fossil fuel sources rapidly becoming depleted, and nuclear power being the subject of public disquiet on long-term safety issues, there is currently much interest in the extraction of energy from naturally-available and renewable sources such as wind and waves. Many proposals for the conversion of wave energy have been put forward. For example, W099/22137 discloses a point-absorber in the form of a buoy whose vertical movement operates a piston, while W02005/038244 discloses a float driving a generator and flywheel ar-rangement.

Many systems envisage operation in relatively deep water in order to be exposed to the waves with the greatest energy. However, there are problems with this: 1. Deep water locations tend to be a long way from where the elec- trical energy is needed, which means that the transmission infra-structure is costly; 2. The wave direction can vary with time, requiring the structure of the conversion device to be omni-directional or capable of adapt-ing to the direction of the wave movement; 3. Anchoring the conversion device at a deep water location can be difficult and costly; and 4. Making the device robust enough to survive deep-water storm conditions is also costly.

Coastal locations have the advantages that the wave direction tends to be approximately constant, the shallower water makes it easier to provide satis-factory anchorage for the device, storm conditions are less extreme than in the open ocean, and the device is nearer to the location where the electrical energy is required. However, the average amount of energy available in the waves is

Specification NEPTRENE-P1360.GBA2006-O1-24.dOC

smaller than in the deep water locations, and so it is important to make the de-vice as efficient as possible.

In any system involving a float following the wave motion, it is desirable, in order to maximise energy conversion, to arrange for the motion of the float to approach as closely as possible resonance with the wave oscillations. In W02005/038244 it is suggested that the mass of the float might be changed with changing wave conditions, for example by introducing water into the float or removing water from the float, by means of hatches.

Summary of the Invention

According to one aspect of the invention, there is provided a wave en-ergy conversion device for coastal use, comprising a supporting structure rest- ing on or anchored to the sea bed, a float mounted on an arm pivotally con-nected to the structure to permit the float to move in a substantially vertical plane, and electricity generating means responsive to the movement of the waves to generate electricity.

Preferably, the electricity generating means comprise a reciprocating hy-draulic pump connected between the arm and the structure whereby movement of the float in response to the passage of waves causes reciprocation of the pump, a hydraulic accumulator connected to the output of the pump and a hy-draulic reservoir connected to the input of the pump, and a turbine connected between the accumulator and the reservoir, the turbine driving an electric gen-erator.

Preferably, the device comprises a pressure transducer located in front of the float relative to the direction of motion of the waves, in use, and control means for controlling the motion of the float in response to the variations in pressure detected by the transducer. The control means may comprise hydrau- tic control means for controlling the operation of the pump. The hydraulic con-trol means may comprise solenoid- controlled valves operable to vary the stroke of the pump.

The efficiency of the float is maximised by the generation of a reflected wave in the up wave direction which is locked it radians out of phase with the incident waves and by minimising the transmitted wave in the down wave direc-Specmcaon NEPTRENE-P1360.GBA--2006-O1-24.doc tion. This is achieved by the axi-asymmetric shape of the float. The down wave face is substantially a curve of radius equal to the distance from the pivot point whilst the side faces are substantially vertical. The up wave face is curved so as to generate a reflected crest at the lowest part of the oscillation corresponding to the trough and a reflected trough at the highest part of the oscillation corre-sponding to the crest.

It has been recognised that it is desirable to phase lock the motion of the float in resonance with the wave motion to maximise the energy extraction by the float. The present invention provides two mechanisms by which this may be achieved. Firstly, by controlling the motion of the float, through controlled damping of the hydraulic pump cylinder, in response to wave pressure meas-urement on the seabed in front of the device (relative to the wave direction), the float can be closely approximated to resonance with the wave frequency. Sec-ondly, by longer term monitoring of the wave energy spectrum using the same wave pressure measurements to build up a time domain, the mass of the float may be adjusted, by pumping water into or out of it, gradually to take account of the longer-term changes in sea conditions. This also serves to maintain reso-nance.

The inventor's laboratory experiments have demonstrated that the opera-tion of the device can be described as simple harmonic motion: M(t) + B (t) + kz(t) = F(t) Where M = mass of the float and added mass, MA, of the water in wake z = vertical displacement of the float B = torque on the float by the support arm as a linear function of velocity k = buoyancy component of the restoring force as a function of the displacement The natural frequency of oscillation of the float, a, is given by: 1k co=i Spedfication NEPTRENE-P1 360GBA--2006-O1-24.doc And the damped frequency, coo, is given by (DIW2_1 B D 2(M+M4)) So that resonance is achieved when w is equated to the incident wave frequency by varying the float's mass, M, and torque, B. The float displacement is then: A= F MJ(coO2_wy+(Bww/M) 2 Thus, in a preferred embodiment of the invention, a pressure transducer is provided connected to control means for monitoring the amplitude and fre-quency of the waves. The control means can then control a pump to adjust the mass of the float to maintain the resonant frequency of the float at the average wave frequency over a period. The control means may be programmed to ac-cumulate a set of wave pressure measurements with time over a predetermined period, calculate from the set of measurements a wave power spectrum for the set, and determine from the spectrum an optimum frequency at which the wave energy is a maximum, calculate the required mass of the float for a resonant frequency substantially equal to the optimum frequency, and operate the pump to adjust the float mass to the required mass. For example, the control means may be programmed to perform a discrete Fourier transform on a set of data to create the wave power spectrum.

It has been found that, by the combination of these techniques, the mo-tion of the float can be substantially phase locked with the wave motion, and that, as a result, the motion of the float can be increased to up to 5 times the wave height, thereby maximising energy extraction from the waves.

One benefit from efficient extraction of energy from the waves is that, to the shore side of the device, there is substantial calming of the waves. It is be- lieved that, with a plurality of the devices deployed at a sea depth of approxi- mately lOm (say at 200m from the low tide mark on the shore), the devices be-ing spaced apart by about lOOm, erosion of the shore behind the devices can be substantially reduced, halted, or even reversed.

Specification NEPTRENE-P1360.G8A--2006-O1-24.doc

The hydraulic system by which the energy is extracted suitably operates with water so that, if any leakages occur, there is no risk of damaging contami-nation of the sea water. The use of conventional hydraulic fluids would carry the risk of harm to the environment, should any losses occur.

In an alternative embodiment, the electricity generating means comprises a generator linked to a flywheel, and a hydraulic piston linkage between the arm and the flywheel.

In yet another embodiment, the electricity generating means comprises a linear electric generator mounted between the arm and the supporting structure.

An advantage of locating a wave energy conversion device adjacent to the coast is that the waves tend to move in the same general direction, i.e. to-wards the shore. This means that the mounting of the float does not have to allow for adaptation to differing wave directions, simplifying construction and therefore cost. Since coastal waters -typically within 200m of the low water point -are relatively shallow, location of the device in this region also simplifies construction, and connection to the electricity distribution grid is also easier.

The mounting of the pump cylinder to the supporting structure can readily be adapted to make best use of the tidal range at any given location. The mounting of the arm is such as to permit the float to be raised clear of the water in extreme storm conditions to reduce the risk of damage.

Brief Description of the Drawings

In the drawings, which illustrate diagrammatically an exemplary embodi-ment of the invention: Figure 1 is a side elevation of the device according to a first embodiment; Figure 2 is a diagram illustrating the energy conversion components of the device of Figure 1; Figures 3 and 4 are views corresponding to that of Figure 1, showing the movement of the float in response to waves; Figure 5 is a side elevation of the device according to a second embodi-ment; Figure 6 is a side elevation of the device according to a third embodi-ment; and

Specification NEPIRENE-Pi 360.GBA---2006-O1-24.doc

Figure 7 is a graph showing experimental data demonstrating resonance displacement for an axi-asymmetric float of the type used in the device.

Detailed Description of the Illustrated Embodiment

Referring first to Figures 1 to 4, the device has a supporting structure consisting of two pairs of legs 1, each pair set in a generally A-frame configura-tion with cross-members between the pairs supporting the hydraulic, power generating and control modules 2. A hollow float 3 is mounted between a pair of arms 4 which are in turn pivotally mounted at one end thereof on a cross-member extending between the legs 1.

A shaft 5 extends between the arms 4 and is coupled to one end of a hydraulic cylinder 6, the other end of which is pivotally mounted on a cross-member 7 of the supporting structure such that, as the float moves up and down under the influence of the waves, as hereinafter described, the rocking motion of the arms 4 is translated into reciprocation of the piston in the cylinder 6, caus-ing a pumping action.

As can be seen from Figure 2, the cylinder 6 is connected via control valves 8 and 9 between a water reservoir 10 and a water pressure accumulator 11. The pumping action draws water from the reservoir 10 and delivers it under pressure to the accumulator 11, which contains a compressible gas, for exam-pie air, which is compressed as water is pumped into the accumulator, thereby holding the water under high pressure. Water is led from the accumulator 11 through a hydraulic turbine 12, and from there back to the reservoir, which is at atmospheric pressure. The pressure differential gives rise to potential energy in the accumulator converted to kinetic energy in the turbine 12, which drives an electric generator 13. The generator 13 delivers electrical energy to the shore via cables 14, which will run on or under the sea bed to link the device to the electricity supply grid. The pump 6 operates to maintain the pressure in the ac-cumulator so that the flow of water through the turbine 12 can be regulated to maintain a constant generator speed. It will be appreciated that in periods of calm it may not be possible to keep the turbine running, and so the output of the device will not necessarily be available continuously, although residual pressure

Specification NEPTRENE-P1 360.GBA--2006-O1 -24.doc

in the accumulator may enable the turbine to continue operation for short peri-ods of low wave action, for example.

The control system of the device operates to monitor the available wave energy and mean frequency and to modulate the movement of the float to main- tam as closely as possible a state of resonance. A pressure transducer is posi-tioned on the sea bed at a distance of about 50m in front of the device (i.e. on the seaward side of the device), the pressure readings being used to measure the timing and magnitude of the waves. These measurements are used in two ways. Firstly, the movement of the float will tend to lag the wave motion, be-cause of the inertia in the system. This means that the range of movement of the float is smaller than optimum, and therefore the proportion of the energy in the wave which is extracted is also less. If the motion of the float is phase- locked to the wave motion to maintain resonance, the energy which can be ex- tracted is increased substantially. Figures 3 and 4 illustrate the effect of phase-locking. Figure 3 shows the float 6 moving upwards as the peak of the wave passes beneath it, ensuring that the maximum force is exerted on the float.

Figure 4 shows the position as the trough arrives. If the downward motion of the float coincides with the trough accurately, the upthrust on the float is mini-mised, and so the weight of the float maximises the force on the pump. The phase-locking is achieved by control of the valves 8 and 9 controlling the flow of water into and out of the cylinder 6 to counteract the tendency of the float to lag the wave motion. The resistance to motion offered by the cylinder 6 can be controlled by throttling, opening and closing the valves 8 and 9 to ensure that the float is delayed slightly at the trough before being allowed to rise with the wave so that when the peak reaches the float, the maximum upward force is exerted. Similarly, the float is prevented from immediately following the descent from the peak, but is again delayed so as to fall into the trough.

The shaping of the float is important to maximising the energy conver-sion. The rear face of the float is substantially an arc of a circle whose centre is the pivot axis for the arms. This minimises the disturbance as the float de-scends into the wave trough, in turn minimising wave formation to the landward side of the device, since this would represent energy lost. By contrast, the in-SpeciflcaUon NEPTRENE-P1 360.GBA--2006-O1-24.doc curved shape of the front face tends to reflect the oncoming wave and maxi-mises lift. The result of these various factors is that the height of the waves to the rear of the device is substantially smaller than that of the oncoming waves -a significant proportion of the energy in the waves have been extracted. This has the benefit of reducing the eroding effect of the waves on the shore, and by placing a number of the devices along a shore which is prone to wave erosion, for example at lOOm spacing, the result of the calmer water to landward of the devices is the deposit of suspended material, giving rise to deposited spits at intervals defining small bays between them, limiting or preventing further ero- sion. Thus, a double benefit is achieved from the use of the devices: the pro-duction of electricity without the consumption of fossil fuels; and the protection of the coastline.

The other operation of the control system is to monitor the longer-term changes in wave height and frequency due to changes in weather conditions, and to change the mass of the float so that its resonant frequency matches the changing average wave frequency. Sets of data representing wave frequency and amplitude are collected, and the sets are then processed using a discrete Fourier Transform to produce a wave power spectrum. This in turn enables the frequency at which the energy is a maximum to be identified, and from this the necessary mass of the float to be calculated to enable it to resonate with the frequency identified. The processor then causes a pump to operate to pump sea water into or out of the float to achieve the desired mass. It will be appreci-ated that the timescale over which this monitoring and the resultant adjustments are carried out is substantially greater than that for which the control of the float motion is carried out; in the latter case the control is responsive to each wave, while the adjustment of float mass may involve a matter of hours to gradually follow a changing trend in the waves.

Figure 5 shows an alternative arrangement, in which the hydraulic sys-tem is replaced with a flywheel 50 rotated through a hydraulic piston linkage 51 from the float arm 4. The flywheel 50 drives an electric generator (not shown) mounted on the supporting structure. The hydraulic piston linkage 51 ensures that, while the amplitude of oscillation of the float 3 may vary according to wave

Specification NEPTRENE-P1360.GBA--2006-O1-24.doC

height, the rotation of the flywheel will continue, provided that the wave height exceeds a predetermined value. It also permits the motion of the float to be regulated according to the wave motion to phase lock the reflected wave to the oncoming wave, as hereinbefore described, to maximise energy extraction.

Yet another embodiment is illustrated in Figure 6. In place of the hydrau- lic pump in the first embodiment, a linear electric generator 60 is connected be-tween the float arm 4 and the cross-member 7 of the supporting structure.

Since the generator 60 outputs electricity according to the frequency and ampli-tude of the float's motion, it would be necessary to provide some form of electric accumulator 61 to ensure that the electricity supplied to the grid is at a suitably regulated voltage and frequency. The accumulator 61 may involve storage bat- teries and inverter means to output alternating current, and while it might be lo- cated on the supporting structure as indicated in the Figure, it might alterna-tively be appropriate to locate the accumulator on the land, perhaps serving a group of the conversion devices.

Figure 7 is a graph showing the effect on displacement of varying the mass of an axi-asymmetric float. In an experiment with a model float at a scale of 1:100 in incident waves of 0.025m height, resonance displacement of 0.150m was achieved when the mass of the float was about 1.35kg in mass.

Specification NEPTRENE-P1 360.GBA--2006-O1 -24.doc -10-

Claims

CLAIMS

1. A wave energy conversion device for coastal use, comprising a supporting structure resting on or anchored to the sea bed, a float mounted on an arm pivotally connected to the structure to permit the float to move in a sub-stantially vertical plane, and electricity generating means responsive to the movement of the waves to generate electricity.

2. A wave energy conversion device according to Claim 1, wherein the electricity generating means comprises a reciprocating hydraulic pump con-nected between the arm and the structure whereby movement of the float in re-sponse to the passage of waves causes reciprocation of the pump, a hydraulic accumulator connected to the output of the pump and a hydraulic reservoir con- nected to the input of the pump, and a turbine connected between the accumu-lator and the reservoir, the turbine driving an electric generator.

3. A wave energy conversion device according to Claim I or 2, corn-prising a pressure transducer located in front of the float relative to the direction of motion of the waves, in use, and control means for controlling the motion of the float in response to the variations in pressure detected by the transducer.

4. A wave energy conversion device according to Claim 3, wherein the control means comprises hydraulic control means for controlling the opera-tion of the pump.

5. A wave energy conversion device according to Claim 4, wherein the hydraulic control means comprise solenoid-controlled valves operable to vary the stroke of the pump.

6. A wave energy conversion device according to any preceding claim, wherein the float has, in profile an incurved front face, and a convex rear face whose curvature is substantially an arc of a circle with a centre at the pivot point for the arm, where the front face is the face which is directed towards the oncoming waves, in use, whereby the float generates a reflected crest at the lowest part of the oscillation corresponding to the trough of the wave and a re-flected trough at the highest part of the oscillation corresponding to the crest.

Specification NEPTRENE-P1 360.GBA---2006-O1-24.doc -11 -

7. A wave energy conversion device according to any preceding claim, wherein the float is a hollow body and the device comprises a pump to pump water into and out of the float to change the mass thereof.

8. A wave energy conversion device according to Claim 7, compris-ing a pressure transducer located in front of the float relative to the direction of motion of the waves, in use, and control means for monitoring the amplitude and frequency of the waves and for controlling the pump to adjust the mass of the float to maintain the resonant frequency of the float at the average wave frequency.

9. A wave energy conversion device according to Claim 8, wherein the control means is programmed to accumulate a set of wave pressure meas-urements with time over a predetermined period, calculate from the set of measurements a wave power spectrum for the set, and determine from the spectrum an optimum frequency at which the wave energy is a maximum, cal-culate the required mass of the float for a resonant frequency substantially equal to the optimum frequency, and operate the pump to adjust the float mass to the required mass.

10. A wave energy conversion device according to any preceding claim, wherein the hydraulic pump pumps water.

11. A wave energy conversion device according to Claim 1, wherein the electricity generating means comprises a generator linked to a flywheel, and a hydraulic piston linkage between the arm and the flywheel.

12. A wave energy conversion device according to Claim 1, wherein the electricity generating means comprises a linear electric generator mounted between the arm and the supporting structure.

13. A wave energy conversion device, substantially as described with reference to, and/or as shown in, Figures 1 to 4, Figure 5 or Figure 6 of the drawings.

14. A method of protecting a coastal shore from wave erosion, corn-prising locating along a portion of the shore a plurality of wave energy conver-sion devices according to any of the preceding claims, spaced apart at intervals Specffication NEPTRENE-P1360GBA---2006-O1 -24.doc -12- such that the resultant reduction in wave energy causes deposition of solid ma-terial at at least the same rate at which it is removed.

Specfflcatrnn NEPTRENE-P1360.GBA--2006-O1-24.doc