GB1572086A

GB1572086A - System for extracting energy from waves

Info

Publication number: GB1572086A
Application number: GB38490/76A
Authority: GB
Original assignee: Vickers Ltd
Current assignee: Vickers Ltd
Priority date: 1977-09-16
Filing date: 1977-09-16
Publication date: 1980-07-23

Description

(54) SYSTEM FOR EXTRACTING ENERGY FROM WAVES (71) We, VICKERS LIMITED, a British company of P.O. Box 177, Vickers House, Millbank Tower, Millbank, London, SWI P 4RA., do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to the extraction of energy from waves and is concerned with the provision of an improved system there or.

According to the invention there is provided a system for the extraction of energy from waves on the surface of a body of liquid, comprising a duct, having a bend section therein, in which a column of liquid may be trapped and also having an immovable inlet directly exposed to the action of the waves concerned, the duct being positioned so that, in use, the liquid column is caused to oscillate by the wave action, and an energy extraction device positioned to be operated directly or indirectly by the oscillating liquid, the system being sited at a particular loction at a fixed height relative to the bed of the body of liquid and the system, as regards all parts thereof except any as may be sited onland, being arranged in a submerged condition so that no part of the system projects upwardly to the surface of the body of liquid.

The waves may act directly or indirectly on the liquid column.

Preferably, a resonant condition will be set up for the liquid column so as to extract as much energy as possible for any given wave conditions.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by wa of example, to the accompanying drawings, in which: Figures la to Id and Figure 3 illustrate several embodiments of the invention; Figure 2 shows a curve illustrating the frequency response of a linear system as a means of explaining the effect obtained when introducing damping; Figure 4 illustrates the principle on which a development of the invention is based; Figure 5 shows one possible further means of extracting energy from an oscillating body of water; Figure 6 to 8 show diagrammatically different embodiments; and Figure 9 and 10 illustrates two further practical embodiments.

All the energy extraction systems to be described are either fixed installations or (Figure 10) tethered to the sea bed, so as in all cases to be sited at a fixed location relative to the sea bed.

Referring to Figure la, the system shown consists of a fully submerged U-shaped duct or tunnel 1, in the form of a trap, at an inshore location and an energy extraction device or power unit 2 incorporated in the duct. In use, the U-tube will of course fill with sea water and the mass of water thus formed will be caused to oscillate with the motion of the waves thereby driving the power unit.

If the duct is "turned" to a particular wave frequency, the system will provide a head amplification which greatly enhances the performance of the system. A tuned water duct of constant cross-sectional area is not, however, suitable for maximum wave loading. This is because for dull tuning the period of the mass of water in the duct given by

(where I is the length of the column of water) must equal the period of the waves given (in deep water) by

which requires that I equals 7r On the other hand (as illustrated in Figure la) maximum loading occurs when the distance l, between the open ends of the tunnel is related to the wave length A by the expression

where n= 1, 2, 3 etc.Thus the lowest value of l, corresponds to n=l and is equal to half the wave length and it will readily be seen that I for the tuned duct is smaller than l, for the duct with maximum loading.

The system of Figure 1 h, however, overcomes this difficulty be using a duct 1 composed of successive lengths 11, la and l of respective cross-sectional areas Al, A2 and A3. The period of oscillation for this duct is given by the expression::

and both full tuning and maximum peak differential pressure may be obtained by choosing the parameters l" 12 and 13 and Al, A2 and A3 such that the period of the mass of water in the duct is equal to that of the waves, and also such that the tunnel length between its open ends is equal to

It is important to note that whilst full tuning of a lightly damped system suffers from the known disadvantages of significant degradation in system response for small changes in forcing frequency and of inability to control the parameters of the system, the system disclosed herein has the advantage that energy extraction device 2 itself introduces damping.This, as illustrated in Figure 2 were elJ, is the undamped natural frequency of the duct and 5 is the damping ratio, significantly increases the frequency band width of the system.

The energy extraction device 2 conveniently comprises a low-head turbine of the propeller or Kaplin type. Besides frequency band width consideration, the turbine blades will require to have variable pitch in order to control the efficiency of the turbine when there occur variations in forcing load. Unless the turbine can be operated in both directions of oscillation, it will be necessary to have two turbines in respective trap branches with appropriate valving in each duct if energy is to be continuously extracted from the system.

Alternatively, if the turbine blade pitch angle can be reversed at each half cycle, continuous rotation can be obtained.

In order not to have to locate a large installation underwater at an inshore location, either of the systems shown in Figures Ic and Id may be adopted, in which a part of the system is located onland. It should be noted that in the Figure Ic embodiment where one end of the duct is exposed to the wave action and the other end is open to the atmosphere, the peak wave loading is half that of the Figure la system.

However, and with reference to Figure Id -full loading with an inshore installation can be obtained, starting from the Figure lc arrangement by modifying the central section of the trap to include two bends so as to position the turbine onland below ground level and to expose both ends of the duct 1 to the wave action with a separation between the duct ends in the direction of propogation of the waves equal to half the wavelength.

Turning now to Figure 3, here the trap 1, incorporating the energy absorption or extraction device 2, is located onland but still contains a mass of water and is connected to the sea water by an inverted U-tube 3, of which the part located inshore is submerged and which contains air so that the mass of water in the trap 1 is loaded by the waves indirectly via an air column 4.

Only one end of the trap in Figure 3 is connected to the water, the other end being vented to the atmosphere.

Installations (not forming part of this invention) which are sited at the sea surface, especially in deep water, have the disadvantage that they have to be specially designed to withstand extreme storm and wave conditions which occur from time to time. On the other hand, environmental conditions below the surface of the sea are less severe than those at the surface, whatever the surface conditions. Thus, the described systems, which are submerged as regards all parts thereof except any as may be located onland, will accordingly have a greater chance of survival against adverse environmental conditions than a similar system which is disposed entirely or even in part at the surface of the area.

Some totally submerged systems, intended to be sited in deep water, will now be described.

Referring to Figure 4, a sea water "U" trap 1 or an "L" shaped duct bend section open at one end to the sea with a bell mouth 5 is connected by its other end to a large fluid-tight air reservoir 6 so as to be in communication with the air volume of the reservoir. The assembly comprising the reservoir and trap is installed entirely below sea level with the level of water in the trap at a depth 'Zo' below mean sea level and the air in the reservoir at ambient sea pressure.

Provided the air volume of the reservoir is relatively large (e.g. approximately 100 times the volume of the water column in the trap) such that the pneumatic stiffness of the air in the reservoir is small compared with the hydrostatic stiffness of the water in the trap resulting from the displacement Z of the water column from its position at rest, the water column will be caused by the action of the waves to oscillate at a frequency corresponding with the passage of waves passing over the open end of the trap. When the length of the water column I= 27t.

or thereabouts where A is the wave length, a resonant condition will be established in the trap with an amplification of the water motion and pressure head of the sea water in the duct with respect to the amplitude and pressure .fluctuation of the waves themselves, the amplification resulting in a water column amplitude of above five to ten times the wave height. The length of the water column may be adjusted by providing the reservoir with means for adjusting the air pressure or volume of the reservoir 6, so that the trap may be 'tuned' to resonate over the range of wave forcing frequencies which will occur in practice.Since the driving force is provided by the fluctuating sea pressure and wave particle motion at the open end of the duct, the provision of the bell mouth 5 of greater diameter than the duct proper will increase the amount of wave energy captured by the system. Since, also, the system is intended for operation in a fully submerged condition, it will not be exposed to the action of wind and surface waves and so it will not be necessary to restrict or limit operation of the system under storm conditions. The system is capable of operation in all depths of water (except extreme depths), including at the sea bed.

In a development of this embodiment, a plurality of water-filled traps are arranged around the periphery of the air reservoir at intervals and are connected to be in communication with its internal space. This system enables the energy output to be increased without significantly increasing the bulk of the system. This modification can be similarly incorporated in other embodiments too.

Referring to Figure 5, there is shown a further means of extracting energy from an oscillating body of water in a trap. The energy extraction device 2 shown in Figure 5 is intended to be used in the Figure 4 system although it could equally be incorporated in the right-hand limb of the traps 1 of the Figures lc, and 3 systems or in other systems.

The energy extraction device comprises a float 7 which oscillates up-and-down in the left-hand limb of the trap I as the water oscillates. This up-and-down motion of the float is converted into rotary motion by a rack and pinion arrangement 8 to drive an electrical generator 9.

Figures 6 to 8 illustrate different embodiments for converting the potential and kinetic energy of the oscillating sea water into a more useable form of energy.

Referring to Figure 6, a one way valve A is mounted in the trap adjacent the bell mouth 5 to allow sea water to enter the trap.

An outlet duct 10 is in communication with the trap and incorporates a one-way valve B, adjacent the trap for allowing sea water to pass from the trap to a turbine. When the valve A is open, valve B is closed, and conversely when valve A is closed, valve B is open. By this means the oscillating flow within the trap is "rectified", that is converted into a unidirectional flow in the duct 10 which may be used to drive a low pressure turbine of the 'Kaplin'/'Strathflo' or similar type connected to the duct 10 immediately down-stream of valve 'B'.

In the modification of Figure 7, the valve B allows the "rectified" flow to enter a further reservoir 11 containing a quantity of sea water with an air cushion above the sea water and provided with a one-way valve C allowing sea water to leave the reservoir 11 by way of an outlet duct 12. In a modification, the reservoir 11 may be raised relative to the reservoir 6 so that the duct 10 opens into the reservoir 11 below the level of the sea water in that reservoir, the reservoirs 6 and 11 and valves B and C then conveniently being arranged in linear disposition.

As sea water is discharged into the second reservoir the air pressure within it is caused to increase above ambient hydrostatic pressure owing to the accumulation of sea water in the reservoir. By adjustment of the opening and closing pressures of valves B and C, short term energy storage and smoothing of the sea water is obtained and a continuous discharge from valve C is created, which may be used to drive a lowpressure turbine, downstream of the valve C, or fed ashore by a pipeline to a shore based storage reservoir and hydro-electric generating station.

Another modification is shown in Figure 8 in which the bottom portion of the air reservoir 6 is provided with an outlet duct 13, including a one-way valve D which allows sea water in the reservoir 6 to leave to flow along the duct 13 to a low-pressure turbine or to a shore based storage reservoir and hydro-electric generating plant.

The datum level 14 of the water column at rest is chosen such that in operation the amplitude of oscillation Z is greater than the height of the inlet to the air reservoir 6 above the datum level 14 of the water column, whereby water is caused to spill into the air reservoir from the trap. As sea water accumulates in the reservoir while the valve D remains closed, the pressure is caused to rise, thereby reducing the length of the water column and hence its natural frequency of oscillating becomes 'detuned' from the wave forcing frequency. This effect can be minimised by making the air reservoir sufficiently large so that its pneumatic stiffness remains small compared with the hydrostatic stiffness and by appropriate choice of system damping.

Again, by adjustment of the opening pressure for valve D a form of short-term energy storage and smoothing is provided and continuous discharge provided for driving the low-pressure turbine or for piping ashore to a land based generating station. This particular system as compared with that of Figure 7 eliminates two one-way valves and the associated duct losses.

In the described systems, the air reservoir 6 provides a large margin of excess buoyancy and consequently it may be tethered to the sea bed by mooring lines held in tension, to provide a high degree of stiffness and rigidity of the system against forces acting upon it owing to wave motion and other effects. Further, the submerged depth of the system may be controlled by adjustment of the length of mooring lines, to provide a means of controlling the wave power extracted by the system in severe storm conditions.

Figure 9 illustrates a system in which the trap comprises an inner tube 15, provided with the bell mouth 5, and an outer, concentric tube 16. The bottom of the tube 16 is closed by a horizontal partition wall 17 which subdivides the interior of a tank 18 so as to provide an upper, air reservoir 6 and a lower reservoir 11 containing a quantity of sea water with an air cushion above. A valve B allows sea water from the trap to enter the reservoir 11 and this reservoir is provided with an outlet duct 12 incorporating a oneway valve C.

It will be appreciated that this system operates in an identical fashion to the Figure 7 embodiment. It has the advantage that it is very compact in construction.

Mooring lines 19 tether the system to the sea bed as shown.

Another practical system is shown in Figure 10 which operates identically to the Figure 8 embodiment but in which the trap is of annular construction extending around the periphery of the reservoir 6. This trap, in having an annular inlet or mouth exposed to the action of the waves, is able to absorb a much larger quantity of energy than the trap of the Figure 8 system in which the bell mouth 5 of the trap has a relatively small area.

The described systems can each function as an omni-directional resonant absorber of wave energy over an area of sea surface substantially greater than that of the trap inlet opening.

It will be appreciated that in all the systems described, any duct end leading into the water should be positioned so that the .water discharged into the sea does not damp the incident waves by generating two opposed currents. Furthermore, the open end of the water duct subjected to leading by the waves should be located at a site with a depth greater than the critical depth at which the waves break.

WHAT WE CLAIM IS: 1. A system for the extraction of energy from waves on the surface of a body of liquid, comprising a duct, having a bend section therein, in which a column of liquid may be trapped and also having an immovable inlet directly exposed to the action of the waves concerned, the duct being positioned so that, in use, the column is caused to oscillate by the wave action, and an energy extraction device positioned to be operated directly or indirectly by the oscillating liquid, the system being sited at a particular location at a fixed height relative to the bed of the body of liquid and the system, as regards all parts thereof except any as may be sited onland, being arranged in a submerged condition so that no part of the system projects upwardly to the surface of the body of liquid.

2. A system according to claim 1, wherein the bend section is in the form of a "U" trap.

3. A system according to claim 1 or 2, and installed at a site, wherein the duct is open at both ends to the action of the waves.

4. A system according to claim 3, wherein the ends of the duct are spaced-apart in the general direction of propagation of waves at the site and the spacing between the duct ends closely equals half the wavelength at that site for maximum wave loading of the system.

5. A system according to claim 4, wherein the duct cross-sectional area changes along its length in such manner that the system in operation is not only subjected to maximum wave loading but it is also "tuned" by

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Claims

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reservoir 6 is provided with an outlet duct 13, including a one-way valve D which allows sea water in the reservoir 6 to leave to flow along the duct 13 to a low-pressure turbine or to a shore based storage reservoir and hydro-electric generating plant.

The datum level 14 of the water column at rest is chosen such that in operation the amplitude of oscillation Z is greater than the height of the inlet to the air reservoir 6 above the datum level 14 of the water column, whereby water is caused to spill into the air reservoir from the trap. As sea water accumulates in the reservoir while the valve D remains closed, the pressure is caused to rise, thereby reducing the length of the water column and hence its natural frequency of oscillating becomes 'detuned' from the wave forcing frequency. This effect can be minimised by making the air reservoir sufficiently large so that its pneumatic stiffness remains small compared with the hydrostatic stiffness and by appropriate choice of system damping.

Again, by adjustment of the opening pressure for valve D a form of short-term energy storage and smoothing is provided and continuous discharge provided for driving the low-pressure turbine or for piping ashore to a land based generating station. This particular system as compared with that of Figure 7 eliminates two one-way valves and the associated duct losses.

In the described systems, the air reservoir 6 provides a large margin of excess buoyancy and consequently it may be tethered to the sea bed by mooring lines held in tension, to provide a high degree of stiffness and rigidity of the system against forces acting upon it owing to wave motion and other effects. Further, the submerged depth of the system may be controlled by adjustment of the length of mooring lines, to provide a means of controlling the wave power extracted by the system in severe storm conditions.

Figure 9 illustrates a system in which the trap comprises an inner tube 15, provided with the bell mouth 5, and an outer, concentric tube 16. The bottom of the tube 16 is closed by a horizontal partition wall 17 which subdivides the interior of a tank 18 so as to provide an upper, air reservoir 6 and a lower reservoir 11 containing a quantity of sea water with an air cushion above. A valve B allows sea water from the trap to enter the reservoir 11 and this reservoir is provided with an outlet duct 12 incorporating a oneway valve C.

It will be appreciated that this system operates in an identical fashion to the Figure 7 embodiment. It has the advantage that it is very compact in construction.

Mooring lines 19 tether the system to the sea bed as shown.

Another practical system is shown in Figure 10 which operates identically to the Figure 8 embodiment but in which the trap is of annular construction extending around the periphery of the reservoir 6. This trap, in having an annular inlet or mouth exposed to the action of the waves, is able to absorb a much larger quantity of energy than the trap of the Figure 8 system in which the bell mouth 5 of the trap has a relatively small area.

The described systems can each function as an omni-directional resonant absorber of wave energy over an area of sea surface substantially greater than that of the trap inlet opening.

It will be appreciated that in all the systems described, any duct end leading into the water should be positioned so that the .water discharged into the sea does not damp the incident waves by generating two opposed currents. Furthermore, the open end of the water duct subjected to leading by the waves should be located at a site with a depth greater than the critical depth at which the waves break.

WHAT WE CLAIM IS:

1. A system for the extraction of energy from waves on the surface of a body of liquid, comprising a duct, having a bend section therein, in which a column of liquid may be trapped and also having an immovable inlet directly exposed to the action of the waves concerned, the duct being positioned so that, in use, the column is caused to oscillate by the wave action, and an energy extraction device positioned to be operated directly or indirectly by the oscillating liquid, the system being sited at a particular location at a fixed height relative to the bed of the body of liquid and the system, as regards all parts thereof except any as may be sited onland, being arranged in a submerged condition so that no part of the system projects upwardly to the surface of the body of liquid.

2. A system according to claim 1, wherein the bend section is in the form of a "U" trap.

3. A system according to claim 1 or 2, and installed at a site, wherein the duct is open at both ends to the action of the waves.

4. A system according to claim 3, wherein the ends of the duct are spaced-apart in the general direction of propagation of waves at the site and the spacing between the duct ends closely equals half the wavelength at that site for maximum wave loading of the system.

5. A system according to claim 4, wherein the duct cross-sectional area changes along its length in such manner that the system in operation is not only subjected to maximum wave loading but it is also "tuned" by

arranging for the period of oscillation of the liquid column to match closely the period of the waves concerned.

6. A system according to claim 3, 4 or 5, wherein the energy extraction device is located at an onland location.

7. A system according to claim 1 or 2, wherein the duct is open at one end to the action of the waves and at the other end to the atmosphere at an onshore location.

8. A system according to claim 2, wherein the duct includes an inverted U-tube connected to one end of the trap, whereby, in use, liquid in the trap may be exposed to the action of the waves concerned via an air column in the inverted U-tube.

9. A system according to claim 7 or 8, wherein the length of the liquid column is so chosen as to match the period of oscillation of the liquid column closely to the period of the waves concerned.

10. A system according to any preceding claim, wherein the energy extraction device is installed in the bend section.

11. A system according to claim 10, wherein the energy extraction device comprises a low-head turbine.

12. A system according to claim 11, wherein the turbine blades have variable pitch.

13. A system according to claim 11 or 12, wherein the turbine blade pitch angles are reversible.

14. A system according to any one of claims 10 to 12, wherein the bend section and a further bend section each include an energy extraction device and are incorporated in respective parallel-arranged branches with appropriate valving so arranged that, in use, the turbines operate alternately for the different directions of fluid oscillation in the duct.

15. A system according to claim 1, wherein the bend section is in communication at one end with the interior of a reservoir for gas.

16. A system according to any one of claims 7 to 9 or to claim 15, wherein the energy extraction device comprises a float positioned in the duct to float, in use, in liquid at an interface between gas in the duct and the liquid column, an electrical generator and means arranged to convert oscillatory linear motion of the float, brought about in use by the action of the waves concerned, into rotary motion to drive the generator.

17. A system according to claim 15, wherein the energy extraction device comprises a low-head turbine.

18. A system according to claim 15 or to claim 16 when appended to claim 15 or claim 17, wherein the said immovable inlet is a bell mouth.

19. A system according to claim 15, or to claim 16 when appended to claim 15, or to claim 17 or 18, and installed at a site, wherein the length of the liquid column is closely equal to W2n where A is the wavelength of the waves.

20. A system according to claim 15, or to claim 16 when appended to claim 15, or to claim 17, 18 or 19, wherein the entire system is fully submerged.

21. A system according to claim 15, or to claim 16 when appended to claim 15, or to any one of claims 17 to 20, comprising a plurality of such ducts of which the bend sections are arranged around the periphery of a single reservoir for gas at intervals and are connected to be in communication with the interior of the reservoir.

22. A system according to claim 17 or to any one of claims 18 to 21 when appended to claim 17, wherein an outlet duct is in communication with the bend section and is connected to supply liquid to the turbine, and wherein there are provided a first oneway valve in the duct having the bend section therein fdr admitting liquid into the bend section and a second one-way valve in said outlet duct for permitting the supply of liquid to the turbine.

23. A system according to claim 22, wherein the outlet duct is connected to a further reservoir for gas in which a quantity of liquid is stored in use with a gas cushion above and which is provided with a further outlet duct, this latter duct being connected to supply liquid to the turbine and incorporating a third one-way valve to permit such supply.

24. A system according to claim 22, wherein the bend section is in the form of a concentric tube arrangement and the first and second mentioned reservoirs are provided by a tank sub-divided by a partition wall to provide the first-mentioned reservoir above the second-mentioned reservoir.

25. A system according to claim 17 or any one of claims 18 to 21 when appended to claim 17, wherein the system is so sited and arranged that, in operation, liquid in the bend section overspills, as the liquid column oscillates, into the gas reservoir, and wherein an outlet duct, provided with a oneway valve, is connected to supply liquid in the gas reservoir to the turbine.

26. A system according to claim 25, wherein the bend section is of annular construction, extending around the periphery of the gas reservoir.

27. A system according to claim 1, wherein the bend section is so arranged that as the liquid column oscillates in use, part of the liquid overspills out of the bend section, and wherein means are provided for conveying the overspilt liquid to the energy extraction device.

27. A system according to claim 15, or to claim 16 when appended to claim 15, or to any one of claims 17 ot 27, wherein the entire system is sited in a submerged state and tethered to the bed of the liquid by mooring lines.

29. A system for the extraction of energy from waves, substantially as hereinbefore described with reference to any one of Figures la, lb, Ic, ld, and 3 of the accompanying drawings.

30. A system for the extraction of energy from waves, substantially as hereinbefore described with reference to Figures 4 and 5, to Figures 4 and 6, to Figures 4 and 7, to Figures 4 and 8, to Figures 4 and 7 as modified by Figure 9, or to Figures 4 and 8 as modified by Figure 10 of the accompanying drawings.

31. A method of extracting energy from waves, substantially as hereinbefore described with reference to any one of Figures la, lb, Ic, ld, and 3 of the accompanying drawings.

32. A method of extracting energy from waves, substantially as hereinbefore described with reference to Figures 4 and 5, to Figures 4 and 6, to Figures 4 and 7, to Figures 4 and 8, to Figures 4 and 7 as modified by Figure 9, or to Figures 4 and 8 as modified by Figure 10 of the accompanying drawings.