GB1593983A

GB1593983A - Devices for extracting energy from wave power

Info

Publication number: GB1593983A
Application number: GB19199/77A
Authority: GB
Original assignee: ENERGY SECRETARY OF STATE FOR
Current assignee: ENERGY SECRETARY OF STATE FOR
Priority date: 1978-05-08
Filing date: 1978-05-08
Publication date: 1981-07-22

Description

(54) IMPROVEMENTS IN OR RELATING TO DEVICES FOR EXTRACTING ENERGY FROM WAVE POWER (71) I, SECRETARY OF STATE FOR ENERGY in Her Britannic Majesty's Government of the United Kingdom of Great Britain and Northern Ireland, London, a Body Corporate do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to devices for extracting energy from waves.

One example of a device adapted to generate electricity from energy extracted from waves is provided by the buoy device invented by Yoshio Masuda and described in British Patent Specification No.

1,014,196 in which the oscillation of a column of water in a chamber is arranged to drive air through an air turbine; another example is in co-opending Application No.

34378/76. Serial No. 1580901.

A breakwater is an example of a device intended to extract energy from waves to provide calmer regions on its leeward side.

According to one aspect of the invention there is provided a device for extracting energy from waves on a liquid on which the device is adapted to float, the device having a chamber with an opening for the flow of the liquid into and out of the chamber to provide in the chamber a quantity of the liquid which is arranged to oscillate in operation of the device to extract energy from said oscillations, wherein the resonant frequency of oscillation of said quantity of the liquid is arranged to be controlled from the incoming waves in such a manner that said resonant frequency is arranged substantially to match the predominant frequency of the incoming waves.

According to another aspect of the present invention, there is provied a device for extracting energy from waves on a body of liquid on which the device is adapted to float, the device having a chamber with an opening for the flow of the liquid into and out of the chamber to provide in the chamber a quantity of the liquid which is arranged to oscillate in operations of the device to extract energy from said oscillations, wherein there are provided first means arranged to be responsive to the frequency of the incoming waves; second means arranged to be responsive to the resonant frequency of oscillation of the liquid in the chamber, and third means capable of effecting a change of said resonant frequency and adapted to cooperated with said first and second means to arrange that said resonant frequency substantially matches said predominant frequency of the incoming waves.

It will be understood that although there is usually a range of frequencies of incoming waves, there will generally be a predominant frequency within this range of frequencies.

Preferably, the liquid in the chamber is arranged to oscillate with a 90 phase lag with respect to the phase of the incoming waves.

The resonant frequency may be changed by changing the mean height of the quantity of the liquid in the chamber, and said mean height may be changed by changing the depth of immersion of the device.

Alternatively, the resonant frequency may be changed by providing one or more movable sides to the chamber to change the ratio of the average free surface area to volume of the liquid in the chamber or to change the average area of the free surface of this liquid.

Resonance of the oscillating liquid in the chamber is caused by the mass of the oscillating liquid, together with the action of the air compressed above this liquid and the effect of gravitational force on this liquid providing restoring forces on the oscillating liquid.

The present invention will now be particularly described by way of example only with reference to the accompanying drawings, in which: Figure I shows a diagrammatic sectional representation of a device for generating electrical energy from sea waves; Figure la shows a plan view in the direction of arrow X of Figure 1; Figure lb shows to an enlarged scale a sectional representation of the directional valve used in the device shown in Figure l; and Figure 2 shows a diagrammatic sectional representation of the device similar to that of Figure 1.

In the above Figures like parts have like numerals.

Referring now to Figures I and la the device shown is of rectangular form in plan as shown in Figure la, and comprises a forward buoyancy tank 2 and an aft buoyancy tank 3 with respect to the direction of the incoming wave indicated by the arrow 'A', both buoyancy tanks 2 and 3 having rounded lower edges. The tanks 2 and 3 define opposing walls of a chamber 4 having a roof 5 and containing air as a working gas. The chamber 4 at its base is open to the sea to provide a quantity of sea water inside the chamber 4 in communication with the sea. An inlet tube 12 from the rear of the device, with a nonreturn valve 13, is provided for the indraught of air into the chamber 4 as the surface 6 of the sea water in the chamber 4 falls, a hood 14 protecting the inlet end of the inlet tube 12 from spray.A non-return outlet valve 7 in the roof 5 allows air to be expelled from the chamber 4 as the surface 6 rises. The buoyancy tanks 2 and 3 contain sea water, the amount of which is arranged to be changed to alter the depth of immersion of the device and thereby the height of the quantity of sea water in the chamber 4.

A casing 17 extends about the roof 5 and downwardly to below the surface at the rear of the aft buoyancy tank 3 to define an airspace 10 which holds the air expelled from the chamber 4 at a substantially uniform pressure under the restoring force provided by the depressed sea level. An air turbine 18 in an upstanding duct 19 fed from an orifice 20 in the casing 17 is arranged to drive an electric generator (not shown) from the outflow of air from the space 10 through the orifice 20.

The device is shown with the sea at its mean lowest level and about to rise in the direction of arrow 'B', and the sea water inside the chamber 4 at its mean level and falling in the direction of arrow 'C', the surface 6 of the sea water inside the chamber 4 is therefore shown at a 900 phase lag behind that of the motion of the surface outside the device.

A float 25 outside the forward buoyancy tank 2 is rigidly joined by a connecting rod 26 to a piston member 27 in a cylinder 28 arranged with its longitudinal axis in a vertical orientation with respect to the sea level. The piston member 27 is provided with a rubber sealing ring 29 located in an annular groove 30 in the piston member 27.

A directional valve 35 arranges the flow to and from the cylinder 28 and reference is now made to Figure lb for further details.

In Figure lb, the directional valve 35 shown in median section has a cylindrical valve chamber 34 in which is disposed a spool-shaped valve member 43 having three cylindrical disc portions 44, 45 and 46 joined in equispaced relationship by two stem portions 47. An elongated stem 48 extending downwardly from the lower disc portion 46 extends through the end of the valve chamber 34 where it is sealed by a gland 49.

A port 31 at the upper end of the cylinder 28 and adjacent to the directional valve 35 connects to two valve ports 32 and 33 at the valve chamber 34. Two tube ports 32a and 33a are disposed diametrically opposite the valve ports 32 and 33 and connect to a tanks dip tube 37 and sea dip tube 38 respectively.

The tanks dip tube 37 as shown in Figure 1 to which reference is again made extends through the space 10 and downwardly into and to the bottom of the aft buoyancy tank 3, and has a branch dip tube 37a which extends downwardly into and to the bottom of the forward buoyancy tank 2. The sea dip tube 38 extends downwardly below (not shown) the lowest sea level in the chamber 4.

A float 50 in the chamber 4 is pivotally connected by a pin 51 to one end of an arcuate-shaped lever 52. The lever 52 extends through a slot 54 above the forward buoyancy tank 2 and is pivotally mounted on a pivot pin 55 located in a post member 56 upstanding from the top of the buoyancy tank 2. The lever 52 at its other end is disposed about the valve stem 48 and has forks 57 disposed about a peg 58 projecting radially from the valve stem 48 so that the position of the float 50 determines the position of the valve member 43 within the valve chamber 34.

Downward movement of the piston member 27 in the cylinder 28 creates a suction which can withdraw water into the cylinder 28 from both the buoyancy tanks 2 and 3 through tanks dip tube 37, and from the sea through sea dip tube 38, and upward movement of the piston member 27 causes the piston member to discharge seawater from the cylinder 28. The source from which the seawater is sucked into the cylinder 28 and the place to which it is discharged is controlled by the directional valve 35.

Referring now to Figure lh, the spacing of the ports 32 and 33, and the valve disc portions 44, 45 and 46 are arranged relative to each other and with the motion of the float 50 as follows:- When the sea level outside is at its lowest level and the seawater in the chamber 4 is at its mean level, the valve disc portion 46 is arranged to close valve port 33 whilst the other valve port 32 is allowed to remain open. At an intermediate position in the upward travel of the valve member 43, as the seawater in the chamber 4 falls, both valve ports 32 and 33 are opened. At the end of the upward travel of the valve member 43 and the lowest seawater level in the chamber 4, valve port 32 is closed by disc portion 45 and valve port 33 open.The reverse sequence occurs during the upward travel of the seawater surface 6 to mean seawater level in the chamber 4. On the seawater surface 6 rising above mean seawater level in the chamber 4 the valve member 43 is moved downwardly by the lever 52 and at an intermediate position in the downward travel of the valve member 43 both valve ports 32 and 33 are open. As the valve member 43 moves towards the lower end of its downward travel, valve port 33 is allowed to remain open but valve port 32 is closed by disc portion 44. The reverse sequence occurs as the valve 43 return to a position corresponding to mean seawater level in the chamber 4.

This valve 35 sequence may be expressed as: Valve Position Port Sequence At mean seawater level in chamber 4 (32) open, (33) closed At intermediate upward travel of valve member 43 both (32) and (33) open At end of upward travel (32) closed, (33) open At intermediate position in return stroke both (32) and (33) open At mean seawater level (32) open, (33) closed At intermediate position in downward travel of member 43 both (32) and (33) open At end of downward travel (32) closed, (33) open At intermediate position in return stoke both (32) and (33) open At mean seawater level (32) open, (33) closed It can be seen that when the phase of oscillation of the seawater in the chamber 4 lags 90 behind the phase of the incoming waves in the direction of arrow 'A' no net pumping occurs.If a phase change occurs between the motions of the seawater in the chamber 4 and the incoming waves, the sequence of opening and closing the ports 32 and 33 and the pumping sequence of the piston member 27 move out of phase and seawater is sucked from or discharged into the buoyancy tanks 2 and 3 to change the depth of immersion of the device until the opening and closing sequence of the ports 32 and 33 and the pumping sequence of the piston member 27 are in phase again and no net pumping occurs. The relative dwell time at each of the aforesaid valve positions may be arranged to suit particular applications.

The device being an oscillatory machine has an efficiency of power absorption which is frequency dependent. The maximum efficiency of power absorption is determined by matching the resonant frequency of the oscillations of the quantity of water in the chamber 4 to the predominant frequency of the incoming waves. The resonant or natural frequency is mainly determined by the depth of immersion of the foremost side of the device, to the incoming wave. As this immersion increases the natural frequency of the quantity of liquid decreases.

The resonant frequency and the maximum efficiency are uniquely identified by the phase difference of 90O between the oscillation of the quantity of water in the chamber 4 and the motion of the incoming wave just in front of the forward buoyancy tank 2. The device of Figure 1, by arranging for its depth of immersion to be adjusted in relation to the frequency of the incoming wave, is maintained substantially at its maximum power absorption position.

Against very high seas, the ability to sink a device for extracting energy from sea waves may be essential in many applications. In this respect, use of the invention is beneficial since the biggest waves are associated with the largest wavelengths, and under these conditions a device utilising variable immersion to optimise the efficiency of power absorption will automatically be at its greatest immersed depth. The proportion of the device exposed to the incoming waves will then be relatively small so that the largest waves will tend to pass over the device rather than into it.

As an alternative to changing the mean height of the quantity of water in the chamber 4, the resonant frequency may be changed as shown in Figure 2 to which reference may be made.

In Figure 2, the device shown is similar in many respects to that shown in Figure 1, but the forward wall of the chamber 4 is now provided by a plate 60 joined by a hinge 61 at its lower edge 63 to the lower part of the forward wall of the chamber 4.

The plate 60 has flexible sealing members 62 of synthetic rubber or of a plastics material at its lower edge 63 and side edges (not shown) joined both to the plate 60 and to the walls of the chamber 4 by sealing strips 64 secured to the plate 60 by screws (not shown).

A small orifice 65 in the plate 60 allows liquid to flow behind the plate 60 to ensure that there is no substantial change in the buoyancy of the device from a change in inclination of the plate 60. The restricted flow through the orifice 65 is such that the height of the liquid behind the plate 60 does not change to a substantial extent during one oscillation of the liquid, and it is for this reason that the flexible sealing members 62 are provided since the gaps between the plate 60 and the walls of the chamber 4 are too large to serve the function of the orifice 65.

The angle of the plate 60 is controlled by a double-acting hydraulic actuator comprising a cylinder 68 and a piston member 69, the cylinder 68 being pivotally connected to the forward wall of the chamber 4 and the piston member 69 pivotally connected to the top of the plate 60. Unlike the arrangement of Figure 1, the tubes 37 and 38 are connected to either end of the double-acting actuator 68 so that the actuator 68 in a mean position is notionally unchanged when the column of water in the chamber 4 oscillates with a 900 phase lag behind that of the incoming waves (i.e. no net pumping).This mean position is changed at other phase differences since unequal pumping then occurs through the tubes 37 and 38, and this changes the inclination of the plate 60 and thereby the volume of water in motion in the chamber 4 and the average area of its free surface, and produces a change in the frequency of the oscillations of the water in the chamber 4.

The positions and widths of the disc portions 44, 45 and 46 in the directional valve 35 should be adjusted in relation to the positions of the ports 32a and 33a (see Figure lb) so that ports 32a and 33a are never both open together unless a pressure relief valve (not shown) is provided.

Alternative valves 35 may be used, and additional inclinable plates 60 may be provided, for example, at the aft wall of the chamber 4.

The features of variable depth of immersion and an inclinable plate 60 may both be provided in a device for a particular application, and may be used severally or in combination in a breakwater device similar to the devices shown in Figures I or 2, but without the turbine 18, energy of the incoming waves being dissipated by the flow of air through the orifice 20.

The feature of Figure 2 may also be used to change the amplitude of the oscillations of the quantity of water.

A plurality of devices incorporating the invention may be joined together, for example, to form an elongate barrier or breakwater, and may be made from materials such as metals, or of a reinforced concrete construction, e.g. ferro-concrete or glass fibre reinforced concrete.

WHAT WE CLAIM IS:- 1. A device for extracting energy from waves on a liquid on which the device is adapted to float, the device having a chamber with an opening for the flow of the liquid into and out of the chamber to provide in the chamber a quantity of the liquid which is arranged to oscillate in operation of the device to extract energy from said oscillations, wherein the resonant frequency of oscillation of said quantity of the liquid is arranged to be controlled from the incoming waves in such a manner that said resonant frequency is arranged substantially to match the predominant frequency of the incoming waves.

2. A device as claim in Claim 1, wherein the resonant frequency of the liquid in the chamber is arranged to be changed by changing the depth of immersion of the device and thereby the mean height of the liquid in the chamber.

3. A device as claimed in Claim 1, including a movable side of the chamber for changing the resonant frequency of the liquid in the chamber by changing the ratio of average free surface area to volume of the liquid in the chamber or by changing the average area of the free surface of the liquid in the chamber.

4. A device for extracting energy from waves on a body of liquid on which the device is adapted to float, the device having a chamber with an opening for the flow of the liquid into and out of the chamber to provide in the chamber a quantity of the liquid which is arranged to oscillate in operation of the device to extract energy from said oscillations, wherein there are provided first means arranged to be responsive to the frequency of the incoming waves, second means arranged to be responsive to the resonant frequency of oscillation of the liquid in the chamber, and third means capable of effecting a change of said resonant frequency and adapted to co

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. chamber 4, the resonant frequency may be changed as shown in Figure 2 to which reference may be made. In Figure 2, the device shown is similar in many respects to that shown in Figure 1, but the forward wall of the chamber 4 is now provided by a plate 60 joined by a hinge 61 at its lower edge 63 to the lower part of the forward wall of the chamber 4. The plate 60 has flexible sealing members 62 of synthetic rubber or of a plastics material at its lower edge 63 and side edges (not shown) joined both to the plate 60 and to the walls of the chamber 4 by sealing strips 64 secured to the plate 60 by screws (not shown). A small orifice 65 in the plate 60 allows liquid to flow behind the plate 60 to ensure that there is no substantial change in the buoyancy of the device from a change in inclination of the plate 60. The restricted flow through the orifice 65 is such that the height of the liquid behind the plate 60 does not change to a substantial extent during one oscillation of the liquid, and it is for this reason that the flexible sealing members 62 are provided since the gaps between the plate 60 and the walls of the chamber 4 are too large to serve the function of the orifice 65. The angle of the plate 60 is controlled by a double-acting hydraulic actuator comprising a cylinder 68 and a piston member 69, the cylinder 68 being pivotally connected to the forward wall of the chamber 4 and the piston member 69 pivotally connected to the top of the plate 60. Unlike the arrangement of Figure 1, the tubes 37 and 38 are connected to either end of the double-acting actuator 68 so that the actuator 68 in a mean position is notionally unchanged when the column of water in the chamber 4 oscillates with a 900 phase lag behind that of the incoming waves (i.e. no net pumping).This mean position is changed at other phase differences since unequal pumping then occurs through the tubes 37 and 38, and this changes the inclination of the plate 60 and thereby the volume of water in motion in the chamber 4 and the average area of its free surface, and produces a change in the frequency of the oscillations of the water in the chamber 4. The positions and widths of the disc portions 44, 45 and 46 in the directional valve 35 should be adjusted in relation to the positions of the ports 32a and 33a (see Figure lb) so that ports 32a and 33a are never both open together unless a pressure relief valve (not shown) is provided. Alternative valves 35 may be used, and additional inclinable plates 60 may be provided, for example, at the aft wall of the chamber 4. The features of variable depth of immersion and an inclinable plate 60 may both be provided in a device for a particular application, and may be used severally or in combination in a breakwater device similar to the devices shown in Figures I or 2, but without the turbine 18, energy of the incoming waves being dissipated by the flow of air through the orifice 20. The feature of Figure 2 may also be used to change the amplitude of the oscillations of the quantity of water. A plurality of devices incorporating the invention may be joined together, for example, to form an elongate barrier or breakwater, and may be made from materials such as metals, or of a reinforced concrete construction, e.g. ferro-concrete or glass fibre reinforced concrete. WHAT WE CLAIM IS:-

1. A device for extracting energy from waves on a liquid on which the device is adapted to float, the device having a chamber with an opening for the flow of the liquid into and out of the chamber to provide in the chamber a quantity of the liquid which is arranged to oscillate in operation of the device to extract energy from said oscillations, wherein the resonant frequency of oscillation of said quantity of the liquid is arranged to be controlled from the incoming waves in such a manner that said resonant frequency is arranged substantially to match the predominant frequency of the incoming waves.

operate with said first and second means to arrange that said resonant frequency substantially matches the predominant frequency of the incoming waves.

5. A device as claimed in Claim 4, wherein the first means comprises, a first float means, connected to a pump means for displacing the liquid, and adapted to float on the incoming waves so as to operate the pump means from displacement of the first float means, the second means comprises, a second float means connected to a flow diverting means so as to operate the flow diverting means from displacement of the second float means, the flow diverting means being connected in series flow arrangement with the pump means, and the third means comprises, a buoyancy tank means to which the pump means is arranged to discharge liquid through the flow diverting means or suck liquid from through the flow diverting means so as to change the depth of immersion of the device and thereby the mean height of the liquid in the chamber to effect a change in the resonant frequency of oscillation of the liquid in the chamber, the flow diverting means being adapted to control the quantity of liquid discharged to or sucked from the buoyancy tank means by the pump means so as to control the depth of immersion of the device.

6. A device as claimed in Claim 4, wherein the first means comprises, a first float means, connected to a pump means for pumping the liquid, and adapted to float on the incoming waves so as to operate the pump means from displacement of the first float means, the second means comprises, a second float means connected to a flow diverting means so as to operate the flow diverting means from displacement of the second float means, the flow diverting means being connected in series flow arrangement with the pump means, and the third means comprises, a movable wall of the chamber, and a hydraulic actuator means connected in series flow arrangement with the flow diverting means, and arranged to move the movable wall in response to flow of liquid from the flow diverting means so as to change the ratio of average free surface area to volume of the liquid in the chamber or to change the average area of the free surface of the liquid in the chamber, thereby to effect a change in the resonant frequency of oscillation of the liquid in the chamber.

7. A device as claimed in Claim 5 or Claim 6, wherein means are provided for downwardly biasing the first float means onto the liquid.

8. A device as claimed in Claim 5 wherein the buoyancy tank means comprises a forward buoyancy tank and an aft buoyancy tank, and said liquid is arranged to be simultaneously discharged to or sucked from the forward buoyancy tank and the aft buoyancy tank by the pump means.

9. A device as claimed in Claim 3 or Claim 6, wherein the movable side or wall is pivotally connected to the device.

10. A device as claimed in any one of the preceding claims, wherein the resonant frequency of the liquid in the chamber is arranged to oscillate with about a 90" phase lag with respect to the phase of the incoming waves.

II. A device as claimed in any one of the preceding claims, wherein the oscillation of the liquid in the chamber is arranged to drive air through an air turbine, thereby to extract energy from said waves.

12. A device as claimed in any one of Claims I to 10, wherein the oscillation of the liquid in the chamber is arranged to drive air through an orifice, thereby to dissipate energy from said waves.

13. A device for extracting energy from waves on a liquid on which the device is adapted to float, substantially as hereinbefore described with reference to Figures 1, la and lb of the accompanying drawings.

14. A device for extracting energy from waves on a liquid on which the device is adapted to float, substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.