rexresearch.com

Rubber 'Snake' Could Help Wave Power Get A Bite Of The Energy Market

A device consisting of a giant rubber tube may hold the key to producing affordable electricity from the energy in sea waves. Invented in the UK, the 'Anaconda' is a totally innovative wave energy concept. Its ultra-simple design means it would be cheap to manufacture and maintain, enabling it to produce clean electricity at lower cost than other types of wave energy converter. Cost has been a key barrier to deployment of such converters to date.

Named after the snake of the same name because of its long thin shape, the Anaconda is closed at both ends and filled completely with water. It is designed to be anchored just below the sea's surface, with one end facing the oncoming waves.

A wave hitting the end squeezes it and causes a 'bulge wave'* to form inside the tube. As the bulge wave runs through the tube, the initial sea wave that caused it runs along the outside of the tube at the same speed, squeezing the tube more and more and causing the bulge wave to get bigger and bigger. The bulge wave then turns a turbine fitted at the far end of the device and the power produced is fed to shore via a cable.

Because it is made of rubber, the Anaconda is much lighter than other wave energy devices (which are primarily made of metal) and dispenses with the need for hydraulic rams, hinges and articulated joints. This reduces capital and maintenance costs and scope for breakdowns.

The Anaconda is, however, still at an early stage of development. The concept has only been proven at very small laboratory-scale, so important questions about its potential performance still need to be answered. Funded by the Engineering and Physical Sciences Research Council (EPSRC), and in collaboration with the Anaconda's inventors and with its developer (Checkmate SeaEnergy), engineers at the University of Southampton are now embarking on a programme of larger-scale laboratory experiments and novel mathematical studies designed to do just that.

Using tubes with diameters of 0.25 and 0.5 metres, the experiments will assess the Anaconda's behaviour in regular, irregular and extreme waves. Parameters measured will include internal pressures, changes in tube shape and the forces that mooring cables would be subjected to. As well as providing insights into the device's hydrodynamic behaviour, the data will form the basis of a mathematical model that can estimate exactly how much power a full-scale Anaconda would produce.

When built, each full-scale Anaconda device would be 200 metres long and 7 metres in diameter, and deployed in water depths of between 40 and 100 metres. Initial assessments indicate that the Anaconda would be rated at a power output of 1MW (roughly the electricity consumption of 2000 houses) and might be able to generate power at a cost of 6p per kWh or less. Although around twice as much as the cost of electricity generated from traditional coal-fired power stations, this compares very favourably with generation costs for other leading wave energy concepts.

"The Anaconda could make a valuable contribution to environmental protection by encouraging the use of wave power," says Professor John Chaplin, who is leading the EPSRC-funded project. "A one-third scale model of the Anaconda could be built next year for sea testing and we could see the first full-size device deployed off the UK coast in around five years' time."

The Anaconda was invented by Francis Farley (an experimental physicist) and Rod Rainey (of Atkins Oil and Gas).  There may be advantages in making part of the tube inelastic, but this is still under assessment.

Wave-generated electricity is carbon-free and so can help the fight against global warming. Together with tidal energy, it is estimated that wave power could supply up to 20% of the UK's current electricity demand.

The two-year project 'The Hydrodynamics of a Distensible Wave Energy Converter' is receiving EPSRC funding of just over £430,000.

*A bulge wave is a wave of pressure produced when a fluid oscillates forwards and backwards inside a tube.

Inventor(s):     MACDONALD FARLEY FRANCIS JAMES; TASMAN RAINEY RODERICK CHARLES
Classification: - international:     F03B- European:     F03B13/14C; F03B13/18F; F04B17/00; Y02E10/38
Also published as:    GB2434840 /    US2009007557 / WO2007088325 / EP1979609 / CA2640583

Abstract -- A generally horizontal distensible (elastically flexible) tube 1 in the sea containing water and oriented in the direction of wave travel with distensibility such that the propagation velocity of pressure waves inside the tube (referred to as bulge waves) is the same as the velocity of propagation of the waves in the sea outside. Energy is then transferred from the ocean waves to the bulge wave. Energy extraction means at the stem and/or bow deliver useful energy. Said energy extraction means may comprise pistons actuating pumps or linear or rotating generators, or may comprise one-way valves with the effect that water is pumped into a pressure system or into the distensible tube. In a preferred embodiment one-way valves at or near the stern admit water to the tube which flows out at the bow through a water turbine driving an electric generator. The tube may be suspended below the surface from floats 4, or may be supported on the sea bed (figure 2).

The invention relates to an apparatus for extracting useful energy from the waves of the sea.

James Lighthill in reference [1] shows how pressure waves can propagate along a distensible tube. The pressure causes the tube to dilate locally and this reduces the velocity of propagation.

The more distensible the tube, the slower is the wave velocity. It is convenient to refer to these waves in the tube as "bulge waves". Lighthili applies his analysis to blood flow in arteries.

This invention, on a much larger scale, applies the same principle to extract energy from ocean waves. A long distensible tube full of water is oriented in the direction of wave propagation and the velocity of the bulge wave inside the tube is more or less equal to the velocity of the ocean waves outside. In this case energy is transferred from the ocean to the bulge wave which grows along the length of the tube. At the end of the tube a piston or other means is used to capture the energy of the bulge wave and generate useful power.

Many prior wave energy inventions use flexible membranes and/or tubes oriented in the direction of wave travel, but none appear to rely on the distensibility of a tube made (or partly made) of an elastic material, as a means of storing wave energy prior to conversion. The novelty of this invention is the use of a tube with elastic walls carrying bulge waves matched to the velocity of the ocean waves.

Definitions Elastic: A substance, material or object is elastic if it can be deformed by an applied force and return to its original shape when the force is removed. An elastic object obeys Hooke's law that the strain produced is substantially proportional to the applied stress. All solid materials are more or less elastic up to some limiting strain. For example the limiting strain for steel is about 0.1% while for rubber the limiting strain may be around 50%. By highly elastic we mean a substance, material or object for which the limiting strain is greater than 5%. The elasticity of an object depends upon its shape as well as the material from which it is made. Thus a helical spring made of steel can be highly elastic in the direction of its principal axis, although the' steel itself is not.

Distensible: A tube is distensible if it responds to changes of internal pressure with a proportional change of its cross-sectional area from its undisturbed value. Distensible tubes have highly elastic walls, either because they are made of elastic material or because they are in some way folded or corrugated. For a tube of cross-sectional area S with internal pressure p, the distensibility is defined as D = (uS) dS/dp (1).

It is important for this invention to distinguish between distensibility and flexibility: some examples may make this clear. A motor car tyre is flexible but not distensible: when inflated it is elastic for small deformations. The inner tube of the motor car tyre is distensible. An inflatable boat is flexible but not distensible: its size does not vary with the inflation pressure.

This is because inflatable boats are made of reinforced elastomeric sheet which is flexible but not highly elastic.

Bulge wave: As described by Lighthill in reference [1], in a distensible tube a longitudinal pressure wave, associated with a change of cross-section and a longitudinal fluid velocity, can propagate along the tube. This wave is called a bulge wave. The velocity of propagation of the bulge wave is c where c2 = 1/(pD), p is the density of the fluid inside and D the distensibility as defined above in equation (1).

Bow and stern: For a long object in the sea oriented generally in the direction of wave propagation, the end facing into the waves will be referred to as the bow: the other end facing in the direction of propagation will be referred to as the stem.

The invention According to this invention in its first characteristic the wave energy converter comprises a long distensible tube, generally horizontal, immersed or partially immersed in the sea and oriented generally in the direction of wave propagation, said tube being open or closed at the bow and furnished with energy extraction means at one or both ends, the distensibility of the tube being chosen so that the velocity of the bulge wave along the tube is generally equal to or close to the velocity of the waves in the surrounding sea.

The tube is filled with water or other liquid of similar density which may with advantage be at a pressure higher than that in the surrounding sea.

According to the invention the cross-section of the distensible tube may be of any shape and the elasticity of the walls may vary around the circumference, part of the circumference in some embodiments being substantially inelastic. Furthermore the shape, size and elasticity of the cross-section, and consequentially the distensibility, may with advantage vary along the length of the tube.

According to the invention in its second characteristic the walls of said tube may be comprised of any highly elastic material such as natural or synthetic rubber with or without fibre reinforcement or a highly elastic arrangement of less elastic substances such as helical springs, corrugated metal or a reticulated structure of flexible membranes inflated with compressed air or other fluid.

According to the invention in its third characteristic the energy extraction means at the ends of the tube may compnse any machinery or process which is driven by the oscillating pressure and oscillating longitudinal velocity inside the tube, for example without limitation one or more turbines or pistons operating at any angle to the horizontal actuated by the water pressure inside said tube and driving hydraulic pumps or linear or rotating generators, or overtopping means allowing water inside the tube to be driven over a weir or through one or more non-return valves into a reservoir at elevated pressure, a separate non-return valve allowing water to enter the tube from the sea when the pressure inside is low, or any combination of the above.

In an alternative embodiment the energy extraction means comprises a vertical tube containing water with means for adjusting the height of the water surface and with a piston moving more or less vertically. In a further alternative the vertical tube is closed at the top except for a hole furnished with a float valve which allows air to escape but not water and is further furnished with a non-return valve leading to a hydraulic accumulator, with the effect that when the water inside the tube reaches the top of the tube the float valve closes and water is driven at high pressure into said hydraulic accumulator.

According to the invention in its fourth characteristic the distensible tube may be located on the sea bed, fixed in position by conventional attachments according to the art or ballasted with liquid or solid ballast so as to sink to the sea bed. Alternatively the tube may be fixed at some distance below the sea surface by attachment to a supporting frame attached to the sea bed. In another embodiment the distensible tube may be furnished with buoyancy means the whole being ballasted to float with said tube partly or wholly submerged. In this case the tube is held in position with moorings according to the art.

Some specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 shows in side elevation and in lateral cross-section a distensible tube furnished with buoyancy chambers floating close to the water surface;

Figure 2 shows in side elevation a distensible tube ballasted to rest on the sea floor;

Figure 3 shows a vanety of possible cross-sections of the distensible tube;

Figure 4 shows the cross-section of a distensible tube with inflated reticulated walls;

Figure 5 shows in lateral longitudinal section extraction means comprising a piston moving horizontally and driving a hydraulic pump;

Figure 6 shows in lateral longitudinal section energy extraction means comprising a piston moving vertically and driving a hydraulic pump;

Figure 7 shows in lateral longitudinal section energy extraction means comprising a hydraulic ram pump driving water at high pressure into a hydraulic accumulator;

Figure 8 shows in lateral longitudinal section energy extraction means comprising a transition to a narrow rigid pipe which carries high pressure water ashore;

Figure 9 shows in lateral longitudinal section energy extraction means comprising a one-way valve at the stern of the tube and a turbine at the bow driving an electric generator;

Figure 9 shows in lateral longitudinal section energy extraction means comprising a one-way valve at the stern of the tube and a turbine at the bow driving an electric generator;

Figure 10 shows in lateral longitudinal section an improved one-way valve system which may be used with the turbine and electric generator illustrated in Figure 9;

Figure 11 shows in section an energy converter comprising two distensible vessels connected by a substantially rigid pipe.

Particular embodiments of the invention will now be described by way of example with reference to the figures. Figure 1 illustrates by way of example in side elevation a long distensible tube 1 with rigid bow 2 furnished with a multiplicity of hollow buoyant chambers 4 with the effect that the apparatus floats with the tube 1 more or less horizontal and slightly below the sea surface 3. The device is held in position by moorings 7 according to the art. The walls of the tube 1 are highly elastic and made for example of natural or synthetic rubber as illustrated in the cross-section view AA in Figure 1. The high elasticity of the walls has the effect of making the tube 1 distensible, the said elasticity being chosen so that the velocity of the bulge wave propagating inside the tube is close to the velocity of the waves in the sea outside.

At the stern and/or bow the tube is furnished with energy extraction means 5 of which there are many alternative embodiments which will be described in detail below.

The operation of the device is as follows. The oscillating pressure and pressure gradient outside the Lube wall due to the ocean waves excites a bulge wave near the bow which propagates along the tube at the bulge wave velocity. As the bulge wave moves along the tube, the ocean wave is moving along the tube at the same speed and at each point contributes a further increase in pressure. The result is a cumulative more or less linear increase in the amplitude of the bulge wave, which in effect progressively sucks energy in from the wave. Depending on its length, the oscillating internal pressure amplitude at the end of the tube can be 3-5 times the amplitude of the oscillating pressure in the ocean wave. Useful energy is then extracted from the oscillating pressure at the end of the tube, as explained in detail below. In a typical case the amplitude of the bulge wave at the stern of the tube 1 is such that the tube must expand and contract by about 50% in cross-sectional area from its undisturbed value.

In an alternative embodiment, illustrated in side elevation in Figure 2, the distensible tube 1 is furnished on its lower surface with a multiplicity of flexible bags 8 filled with ballast means, for example without limitation sand, gravel or liquid mud, with the effect that the tube 1 is held firmly on the sea bed 9. It may be further located by means of moorings 7. In a preferred embodiment the flexible bags 8 may be joined together to comprise one long bag with the same effect. This embodiment is useful in shallow water such that the ocean waves on the sea surface 3 produce a significant pressure oscillation at the depth of the distensible tube 1, exciting a bulge wave as explained above. The tube is furnished at the stern with energy extraction means 5 of which there are many alternative embodiments which will be described in detail below. The operation of the device is similar to that described above.

Figure 3 illustrates in cross-section by way of example a variety of constructions which may be adopted for the distensible tube. The cross-section may be of any shape. To achieve the large changes in cross-sectional area mentioned above, all or part of the circumference of the tube must be highly elastic. Figure 3a illustrates an embodiment in which the walls of the tube 10 are made of natural or synthetic rubber, the elasticity of the walls being chosen to achieve the correct distensibility as specified above. The elasticity of the walls need not be the same at all points of the circumference. Figure 3b illustrates by way of example an embodiment in which the lower side of the tube is a substantially inelastic plate 11, while the rest of the circumference lOis highly elastic. Figure 3c illustrates a construction in which the sides of the tube 10 are elastic but the top and bottom 11 comprise inelastic plates. In a further alternative, illustrated in Figure 3d, the top and bottom 11 of the tube are inelastic but the sides of the tube 12 are corrugated; in this case the tube can expand and contract vertically like a conventional metal bellows, and the distensibility is controlled by the vertical spring constant of the corrugated walls. Figure 3e illustrates an embodiment in which the whole circumference of the cross-section is corrugated, the distensibility being controlled by the circumferential spring constant of the walls. There can be any number of corrugations. Figure 3f illustrates an embodiment in which the cross-section is normally elliptical, but can expand out to a more circular shape with greater cross-sectional area by the bending of the walls, which are effectively corrugated as in Figure 3e, but with only two corrugations.

In a preferred embodiment illustrated in transverse cross-section in Figure 4a the walls of the tube compnse a reticulated structure of flexible membranes, inflated by compressed air or other fluid, according to the art of inflated structures. Said membranes may be themselves highly elastic or alternatively fibre-reinforced elastomenc sheets with limited elasticity. Although the flexible membranes comprising a structure may be themselves substantially inelastic, an inflated structure can be highly elastic: well known examples are a motor car tyre and a football. The principles are illustrated in Figures 4b and 4c which show part of an inflated structure comprising a multiplicity of similar cells joined together in a linear array. When the cells are inflated with compressed air the upper and lower membranes adopt the shape that maximizes the volume of the cell; this is achieved when the upper and lower membranes lie on the circle circumscribing the corner points 16, 17, 18 and 19. This circle is shown by dotted lines in Figure 4b. It will be seen that in Figure 4b the upper and lower membranes become rather flat, with the result that in this case the structure is not significantly elastic in the horizontal direction.

In Figure 4c however the internal vertical membranes are shorter, with the effect that the upper and lower membranes, which again follow the shape of the circumscribing circle, are substantially curved. It results that the structure is highly elastic in the horizontal direction. The effective modulus of elasticity of the structure can be varied by changing the pressure inside the cells.

In the embodiment of the distensible tube illustrated in transverse cross-section in Figure 4a, the dimensions of the cells are so chosen that the inner and outer membranes are highly curved with the effect that the wall of the enclosed hollow tube is highly elastic in the circumferential direction and the distensibility of the tube is large. The distensibility of the tube can be varied by changing the inflation pressure of the wall with the effect that the velocity of the bulge wave inside the tube can easily be adjusted from time to time to match the prevailing wave conditions.

This is a major advantage of this embodiment for wave energy conversion. In some embodiments the walls of the distensible tube may be made of a highly elastic material such as natural or synthetic rubber said walls further comprising internal spaces which may be inflated with air or other fluid with the effect that the distensibility of the tube may be adjusted from time to time.

In all the embodiments illustrated in Figures 3 and 4, the cross-section of the tube may be the same at all longitudinal positions along the tube. Or with advantage the dimensions or the circumferential elasticity of the cross-section may vary along the tube.

Particular embodiments of the energy extraction means mounted at the stem of the distensible tube will now be described with reference to Figures 5 to 10. In the embodiment illustrated by way of example in Figure 5 a rigid cylindrical tube 20 is attached to the distensible tube 1 at its stern end. The piston 21 slides inside the rigid tube 20 and via a connecting rod 23 drives a conventional hydraulic pump 25 which delivers hydraulic fluid which may be oil, air or water to a useful output via the connecting pipes 26. The space behind the piston 21 is with advantage filled with air and vented to the atmosphere via tube 22. In operation the bulge wave propagating along the distensible tube 1 builds up to a large amplitude as it reaches the stem.

The oscillating pressure in the bulge wave drives the piston to and fro with the effect that the said hydraulic fluid is pumped under pressure to a useful output.

Another embodiment of the energy extraction means is illustrated by way of example in Figure 6. In this embodiment the distensible tube 1 is tenninated at the stern with a bent tube 29 connected to a rigid cylinder 30 with piston 31 A hydraulic pump 33 is supported for example on an open framework 34 and connected to the piston by means of the connecting rod 32. A reservoir 35 contains water 36 and the water level is maintain above the mean level of the sea by auxiliary pumps (not illustrated). This reservoir is connected to the energy extraction means by a nan-ow pipe with the effect that the mean water level in the cylinder 30 is maintained substantially above the level of the sea but the pipe is too narrow to pass the bulge wave pressure oscillations to the reservoir. The piston 31 is buoyant and on average floats on the water in the cylinder 30. The rising and falling of the water level in the cylinder 30 under the action of the bulge wave drives the piston to and fro vertically with the effect that useful hydraulic energy is generated by the hydraulic pump 33 and passed to a useful output via connecting pipes (not illustrated) according to the art. In this embodiment, if the wave energy is very high so that the bulge wave is exceptionally large, the piston 31 will rise above the end of the cylinder 30 and water will spill out of the cylinder into the surrounding sea with the effect of relieving the excess pressure in the system and protecting it from damage. The piston will fall back into the cylinder and the lost water will be replaced by water from the reservoir via the pipe 37. If the bulge wave oscillation is of large amplitude the pressure inside the distensible tube may fall below the sea water pressure outside with the effect that the walls of the distensible tube could collapse inwards. To avoid this, the tube may with advantage be furnished with a one-way valve 38 which allows sea water to enter the tube if the pressure inside is lower than outside.

Another embodiment of the energy extraction means is illustrated by way of example in Figure 7. In this embodiment the distensible tube 1 is terminated at the stern by a bend connected to a vertical tube 42. Which is closed at the top close to sea level by a bulkhead 43. Said bulkhead is furnished with a hole 44 fitted with a float valve 45 with the effect that air can flow freely in and out of the tube 42 but water cannot escape. The bulkhead is further furnished with a one-way valve 46 leading to a hydraulic accumulator 47 which contains water under pressure according to the art. In this embodiment the water surface 41 inside the vertical tube 42 is on average more or less the same as in the sea outside but is driven up and down through a large amplitude by the bulge wave inside the tube. As the water surface 42 rises the air above it is vented to the atmosphere via the hole 44; but when the water reaches the bulkhead the float valve closes and a high pressure shock is generated. This forces some water through the one-way valve 46 into the hydraulic accumulator 47 with the effect that energy is captured to the hydraulic accumulator. From the accumulator sea water under pressure may be led off through the pipe 48 to do useful work according to the art. The water thus lost from the tube 42 is replaced from the sea when the pressure in the bulge wave goes negative via the one-way valve 38 substantially as described above. The overall effect of this embodiment is that the bulge waves cause sea water to be pumped at high pressure to a useful output with no moving parts (apart from the float valve and one-way valves). In this embodiment the distensible tube 1 may optionally be open to the sea at the bow.

Another embodiment of the energy extraction means is illustrated by way of example in Figure 8. In this embodiment the distensible tube 1 is connected to a long rigid output pipe 51 by means of an intermediate transition and matching section 50. Said transition and matching Section 50 may comprise several stages (not illustrated) with changes of cross-section and of distensibility with the effect that the bulge wave with comparatively small pressure oscillations and with large longitudinal velocity oscillations is converted without substantial loss or reflection into a wave with high pressure oscillations and low velocities, the latter being more suitable for transmitting energy to shore through a comparatively narrow pipe.

In the embodiment illustrated by way of example in longitudinal side elevation in Figure 9 the distensible tube 1 is furnished at the stern with a partially distensible end pipe 60 furnished with one or more one-way valves 61 which allow sea water to enter the pipe but not to leave.

The tube is further furnished at the bow with a rigid pipe 62 open to the sea and fitted with an internal frame 63 which supports an electric generator 65 and water turbine 64. The operation of the system is as follows. The pressure inside the distensible tube 1 is generally higher than the pressure outside. But during the low pressure phase of the bulge wave arriving at the stern, the pressure in the end pipe 60 is lower than the pressure in the sea with the effect that water is sucked into said end pipe through the one-way valve or valves 61. During the high pressure phase of the bulge wave the one-way valve is closed and water cannot leave the tube. The result is a net intake of water at the stern which maintains the high average pressure in the distensible tube 1 and the result is a more or less uniform flow of water along the tube from stern to bow.

Said water flows out into the sea at the bow through the turbine 64 generating electricity according to the art. A mooring 7 serves to hold the device in position. With advantage the end pipe 60 may be furnished with one or more side chambers 66 which may be distensible or rigid and may contain air. Said side chambers have the effect of improving the matching of the bulge wave and smoothing the flow of water to the turbine. In this embodiment the distensible tube 1 may in addition be furnished with a multiplicity of one-way valves 68 dispersed along the length of the tube with the effect of allowing more water to enter the tube at these positions and enhancing the flow through the turbine 64.

Figure 10 shows by way of example in longitudinal lateral cross-section another embodiment of the one-way valve system which may be attached to the stern of the distensible tube or at some position along the length of the tube and used in combination with a turbine at the bow as described with reference to Figure 9. The purpose of this embodiment is to increase the pressure inside the distensible tube 1 with a view to avoiding any tendency of the tube to collapse inwards. Referring to Figure 10, in this embodiment the distensible tube 1 is furnished at its stern with a rigid tube 70 with sliding piston 71 which compresses and expands the corrugated bellows 73. The high pressure inside the tube 1 is balanced by the helical spring 74. The space 78 inside the bellows communicates with the interior of the distensible tube 1 through the one-way valve 72 and with the sea via one-way valve 75, while the space between the bellows and the rigid tube 70 may with advantage be vented to the atmosphere via the tube 76. In operation the oscillating pressure in the bulge wave inside the distensible tube 1 drives the piston 71 to and fro. When moving to the left in Figure 10 it sucks water from the sea via one-way valve 75 into the space 78. When moving to the right it drives the water from space 78 through the one-way valve 72 into the distensible tube 1. In this embodiment the distensible tube 1 is furnished at the bow with a turbine and electric generator substantially as already illustrated and described in relation to Figure 9 with the effect that the energy of the waves causes water to be pumped through the turbine generating useful electricity. Many other arrangements of pistons may be adopted according to the art with the effect of pumping water at elevated pressure into the distensible tube 1. In particular, there may with advantage be a number of chambers 78, which may be brought into operation in varying numbers, by locking the one-way valves. In this way the minimum bulge wave pressure required to move the piston may be varied, to suit the wave conditions, and other more complex control strategies adopted.

In an alternative embodiment similar to Figure 9 but not illustrated, the one-way valve 61 at the stern may be reversed, allowing water to leave the end pipe 60 but not to enter and a turbine can be located in this stream of water at the stern. In another alternative instead of passing through a turbine, the flow of water can be directed into an elevated reservoir either in the sea or on land with the effect that water is pumped from the sea to the reservoir. This may be used for example for flushing out estuaries or supplying fish farms.

In another embodiment illustrated by way of example in longitudinal cross-section in Figure 11 two distensible vessels of any shape 80 containing water or other fluid are connected by a more or less ngid pipe 81 with the effect that under the action of the waves water flows to and fro along the pipe between the said vessels. Any energy extraction means described above may be connected to the vessels with the effects substantially as already described. In particular either one or both the vessels may be furnished with one or more one-way valves 82 or more elaborate one-way valve systems as illustrated in Figure 10 with the effect that water is pumped from the sea into the vessels. The vessels may be further furnished with one or more turbines 83 generating electric power substantially as already described.

It will be apparent that some particular features of one of the alternative embodiments described by way of example above may be combined with particular features of another embodiment, all within the scope of the invention.

References [1] James Lighthill, Waves in Fluids, Cambridge University Press (1978), p. 96 ff

Inventor(s):     FARLEY FRANCIS JAMES MACDONALD
Classification:- international:     F03B13/14; F03B13/00- European:     F03B13/14; F03B13/14C; F03B13/22; Y02E10/38

Abstract -- A wave energy converter makes use of pressure waves (bulge waves) that travel along a generally horizontal distensible tube 1 in the sea 7. The tube 1 contains water under pressure and is oriented in the direction of wave travel. Its flexibility / expandability is such that the velocity of pressure waves inside the tube is the same as the velocity of the waves in the sea. The tube 1 has highly elastic walls incorporating or surrounded by helical reinforcement members 2. The velocity of the pressure waves can be tuned to match the wave velocity by regulating the pressure of the water inside the tube. Energy is then transferred from the ocean waves to the bulge wave. Energy extraction means 6 at the end of the tube 1 delivers useful energy as electricity or high pressure sea water.


The invention is an apparatus for extracting useful energy from the waves of the sea, and is an improved embodiment of the invention described in reference [IJ.

James Lighthill in reference (21 shows how pressure waves can propagate along a distensible tube. The more distensible the tube, the slower is the wave velocity. It is convenient to refer to these waves in the tube as "bulge waves". Lighthill applies his analysis to blood flow in arteries. This invention, on a much larger scale, applies the same principle to extract energy from ocean waves. A long distensible tube full of water is oriented in the direction of wave propagation and the velocity of the bulge wave inside the tube is more or less equal to the velocity of the ocean waves outside. In this case energy is transferred from the ocean to the bulge wave which grows along the length of the tube. At the end of the tube a piston or other means is used to capture the energy of the bulge wave and generate useful power.

Many prior wave energy inventions use flexible membranes andlor tubes oriented in the direction of wave travel, but none appear to rely on the distensibility of a tube made (or partly made) of an elastic material. The novelty of this invention is the use of a tube with elastic walls carrying bulge waves matched to the velocity of the ocean waves. By adding helical reinforcement to the elastic walls the distensibility of the tube can readily be controlled.

Elastic tubes reinforced by helical windings are known to enclose a smaller volume when stretched by an axial force and have therefore been proposed as pumping means in wave power converters using the vertical heaving of floating bodies. In contrast according to this invention the tube is used at maximum volume, no axial force is applied to the tube which is immersed in the water and responds to the lateral pressure exerted directly upon it by the waves. The propagation of bulge waves in an elastic tube with helical reinforcement and the application to wave power conversion are both novel concepts.

Definitions Elastic: A substance, material or object is elastic if it can be deformed by an applied force and return to its original shape when the force is removed. An elastic object obeys Hooke's law that the strain produced is substantially proportional to the applied stress. All solid materials are more or less elastic up to some limiting strain. For example the limiting strain for steel is about 0.1% while for rubber the limiting strain may be around 50%. By highly elastic we mean a substance, material or object for which the limiting strain is greater than 5%. The elasticity of an object depends upon its shape as well as the material from which it is made. Thus a helical spring made of steel can be highly elastic in the direction of its principal axis, although the steel itself is not.

Distensible: A tube is distensible if it responds to changes of internal pressure with a more or less proportional change of its cross-sectional area from its undisturbed value. Distensible tubes have highly elastic walls, either because they are made of elastic material or because they are in some way folded or corrugated. For a tube of cross-sectional area S with internal pressure p. the distensibility is defined as D = (1/5) dS/dp (I).

It is important for this invention to distinguish between distensibility and flexibility: some examples may make this clear. A motor car tyre is flexible but not distensible: when inflated it is elastic for small deformations. The inner tube of the motor car tyre is distensible. An inflatable boat is flexible but not distensible: its size does not vary with the inflation pressure.

This is because inflatable boats are made of reinforced elastomeric sheet which is flexible but not highly elastic. A garden hose is flexible but not distensible. A toy balloon is distensible.

Bulge wave: As described by Lighthill in reference 12), in a distensible tube a longitudinal pressure wave, associated with a change of cross-section and a longitudinal fluid velocity, can propagate along the tube. This wave is called a bulge wave. The velocity of propagation of the bulge wave is c where c2 = 1/(pD), p is the density of the fluid inside and D the distensibility as defined above in equation (1).

Bow and stern: For a long object in the sea oriented generally in the direction of wave propagation, the end facing into the waves will be referred to as the bow: the other end facing in the direction of propagation will be referred to as the stern.

Pitch: The pitch of a helix is the angle between the lines of the helix and the transverse plane perpendicular to the axis of the helix.

The invention According to this invention in its first characteristic the wave energy converter comprises a long distensible tube, generally horizontal, immersed or partially immersed in the sea and oriented generally in the direction of wave propagation, said tube being closed and containing water under pressure and furnished with energy extraction means at one or both ends, the distensibility of the tube being adjusted so that the velocity of the bulge wave along the tube is generally equal to or close to the velocity of the waves in the surrounding sea. The tube is of circular cross-section and the diameter may with advantage vary along the length of the tube.

According to the invention in its second characteristic the impermeable wall of the tube is comprised of a highly elastic material such as natural or synthetic rubber and furnished with a multiplicity of helical reinforcement means made of any material. Said reinforcement means may comprise without limitation wires, cables, tapes, ropes, strings or cords made of substantially inextensible material such as metal, natural or synthetic fibre or partially extensible synthetic material such as high modulus polyethylene, polyester or keviar. Each element of said reinforcement means is in the form of a helix circumscribing the tube or embedded in the wall of the tube extending from one end of the tube to the other and fixed to the tube at each end. With advantage the helical reinforcement means are parallel to each other and equally spaced around the circumference of the tube. Said helices may be right-handed or left-handed or both and of constant and identical pitch. Right-handed and left-handed helices may with advantage be woven together to comprise a reticulate structure or they may be of different diameters so that one fits inside the other.

According to the invention in its third characteristic the distensible tube with helical reinforcement is filled with water connected to pressure means. Said pressure means ensures that the pressure inside the tube is maintained and controlled according to the art. It may comprise for example without limitation a hydraulic accumulator with elastic walls or a hydraulic accumulator containing air under pressure or a hydraulic accumulator in which the contained water is raised vertically or a tube supplying water under pressure from the shore or from an auxiliary vessel or structure.

The effect of the pressure inside the tube is twofold. On the one hand it tends to expand the tube laterally but, because of the helical reinforcement, this can only occur if the tube contracts axially. On the other hand the pressure acting on the ends of the tube tends to lengthen it axially. If the pitch angle of the helical reinforcement is close to 35 degrees, these two tendencies cancel each other and the tube is maintained in stable equilibrium. If this equilibrium is disturbed, for example by squeezing the tube in one place, a bulge wave will be generated and will propagate along the tube. The velocity of propagation of the bulge wave along the tube is proportional to the square root of the pressure inside. If the velocity of the bulge wave is equal to the velocity of propagation of the waves in the sea, then energy is transferred from the sea to the bulge wave and a bulge wave of large amplitude arrives at the stern.

According to the invention in its fourth characteristic the energy extraction means at the ends of the tube may comprise any machinery or process which is driven by the oscillating pressure and oscillating longitudinal velocity inside the tube and produces useful hydraulic or electrical energy, for example without limitation one or more turbines or pistons operating at any angle to the horizontal actuated by the water pressure inside said tube and driving hydraulic pumps or linear or rotating electric generators, or overtopping means allowing water inside the tube to be driven over a weir or through one or more non-return valves into a reservoir at elevated pressure.

In an alternative embodiment the energy extraction means comprises a vertical tube containing water closed at the top except for a hole furnished with a float valve which allows air to escape but not water and is further furnished with a non-return valve leading to a hydraulic accumulator, with the effect that when the water inside the tube reaches the top of the tube the float valve closes and according to the art of the hydraulic ram pump some water is driven at high pressure into said hydraulic accumulator.

According to the invention in its fifth characteristic the distensible tube is furnished with buoyancy means and ballasted to float with the tube partly or wholly submerged. The tube is moored and held in position with moorings according to the art. In another embodiment the distensible tube may be located on the sea bed, fixed in position by conventional attachments according to the art or ballasted with liquid or solid ballast so as to sink to the sea bed or may be fixed at some distance below the sea surface by attachment to a supporting frame attached to the sea bed.

Some specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows in side elevation a distensible tube with a multiplicity of helical reinforcement means; Figure 2 illustrates in side elevation and partly in cross Section an enlarged view of the distensible tube showing for clarity only one element of the helical reinforcement means; Figure 3 shows in cross sectional side elevation further detail of the stern section comprising a particular embodiment of the pressure means and energy extraction means; Figure 4 shows in cross sectional side elevation an alternative embodiment of the stern section with energy extraction means using the principle of the hydraulic ram pump to deliver water at high pressure to useful output A particular embodiment of the invention will now be described by way of example with reference to the figures. Figure 1 illustrates by way of example in side elevation a long distensible tube 1 made of highly elastic material, filled with water and circumscribed by a multiplicity of reinforcement means 2 each in the form of a helix made of substantially inelastic material for example without limitation steel wire, steel tape, steel cable, steel rope or siring, tape, or cord made of natural or synthetic fibre or any combination of these. With advantage the reinforcement means may comprise steel cable covered with PVC or nylon. For clarity, only one helix 2 is shown by way of example in the enlarged view of a part of the distensible tube illustrated in side elevation and partly in section in Figure 2. With advantage the helices 2 are equally spaced around the circumference of the tube I. With advantage the tube may be furnished with an equal number of left-handed and right-handed helices. Said left-handed and right-handed helices cross in many places and may with advantage be of slightly different diameters; or they may be woven together to comprise a reticulate structure. With advantage the pitch angle of the helices is close to 35 degrees. With advantage the helices 2 may be embedded in the wall of the distensible tube 1. With advantage the tube 1 may be furnished with a multiplicity of circular collars, not illustrated, furnished with grooves or slots to locate the reinforcement means during assembly, said collars being highly elastic.

Referring again to Figure 1, the tube is furnished at the bow with a bow section 4 which may have rigid walls or flexible walls made of fabric coated with elastomer. The distensible tube I and the reinforcement means 2 are both firmly attached to the bow section. The bow section may be furnished with anchoring means 5 for attaching the whole machine to the sea bed according to the art. Likewise at the stern, the tube and the reinforcement means are both firmly attached to the stern section 6. The stern section 6 may incorporate any of a variety of energy extraction means which convert the oscillating pressure and oscillating flow in the bulge wave into useful hydraulic or electrical power. Some of these have been described in reference 11].

As illustrated in Figure 1, the distensible tube I may be furnished with a multiplicity of buoyancy means 3 with the effect that it floats, partially or fully immersed, near the surface 7 of the sea shown as a broken line in the figure. Alternatively it may be baliasted so that it rests on the sea floor, or it may be fixed to a framework attached to the sea floor.

An embodiment of the energy extraction means is illustrated by way of example without limitation in Figure 3. In this embodiment the energy extraction means is combined with the pressure means used to regulate the pressure inside the distensible tube 1. The distensible tube I and the helical reinforcement means (not illustrated) are finnly attached to the rigid stern section 6. A pressure vessel 10 attached to the stern section 6 is divided into two volumes 11 and 12 by the vertical partition 13 which does not extend completely to the top of the pressure vessel.

Volume 11 serves as a high-pressure accumulator while volume 12 serves as a low-pressure accumulator. Both volumes are partly filled with water 14 while the space 15 above the water surface is filled with air or some other gas at high pressure. The two volumes communicate via the space 16 above the partition 13 with the effect that the gas pressure in the two volumes is always the same. The water surface 17 in the high-pressure accumulator 11 is higher than the water surface 18 in the low-pressure accumulator 12 with the effect that water flows through the turbine 19 generating electricity. These water levels are maintained by flow out of and into the bulge tube via the one-way valves 20 and 21 respectively, said valves being illustrated in Figure 3 by way of example without limitation as duck bill valves. With advantage the system may be furnished with a multiplicity of one-way valves operating in each direction and a multiplicity of turbines.

The operation of the energy extraction means illustrated in Figure 3 is as follows. The average pressure in the distensible tube I is equal to the pressure of the air 15 in the pressure vessel 10 plus the average hydrostatic head arising from the water levels 17 and 18 relative to the mean depth of the tube. The air pressure is adjusted so that the velocity of the bulge wave in the tube is close to the velocity of the waves in the sea with the effect that energy is captured from the sea in the form of oscillating bulge waves in the tube. During the high-pressure phase of the bulge wave arriving at the stern, water is propelled through the one-way valve 20 into the high-pressure accumulator 11 with the effect that the water level 17 in this accumulator rises.

Conversely during the low-pressure phase of the bulge wave, water is sucked out of the low-pressure accumulator 12 via one-way valve 21 into the bulge tube and consequently the water level 18 in this accumulator falls. The overall effect is that water is driven from the distensible tube I via the one-way valve 20 into the high-pressure accumulator, through the turbine 19, into the low-pressure accumulator 12 and back into the distensible tube 1 via one-way valve 21 with the effect that water is circulated through the turbine and the bulge wave energy is converted into electrical power. The two accumulators 11 and 12 have the effect of smoothing the flow through the turbine. Small differences in the total volume of water in the pressure vessel are accommodated by the compressibility of the air above the water surfaces. In very rough seas the water level 17 in the high-pressure accumulator may rise to the top of the partition 13 and overflow into the low pressure accumulator with the beneficial effect of limiting the power output. With advantage the distensible tube 1 is inclined slightly upwards towards the stern, with the effect that any air which may accidentally be sucked into the tube from the low-pressure accumulator via the one-way valve 21 is immediately pumped back into the pressure vessel 10 via one-way valve 20.

An alternative embodiment of the energy extraction means illustrated in Figure 4 converts the wave energy into hydraulic power in the form of high pressure sea water, which may be used for desalination or for generating electricity by means of a Pelton wheel or turbine according to the art. Referring to Figure 4, the distensible tube us connected to the stern section 6 which comprises a rigid tube 40 bent upwards. The tube 40 is closed at the top by a strong horizontal bulkhead 42 which communicates via the hole 43 with a closed vessel 44 containing gas under pressure. The pressure inside said vessel determines the pressure inside the bulge tube I and may be adjusted to regulate the velocity of the bulge wave as described above. The hole 43 is furnished with a float valve 45 which allows gas to flow freely through the hole but closes whenever the surface 41 of the water inside the tube rises to the level of the bulkhead. The bulkhead 42 is further furnished with a closed vessel 47 which serves as high pressure hydraulic accumulator containing water and gas and storing water at high pressure according to the art.

The tube 40 communicates with said hydraulic accumulator 47 through a hole which is normally closed by the one-way valve 46. The operation of the system is as follows. The bulge wave arriving at the stern section 6 causes the water level 41 in tube 40 to rise and fall while the air above the surface vents to the vessel 43. When the rising water reaches the bulkhead 42 the float valve suddenly closes giving rise to a high pressure hydraulic shock in the tube 40 according to the well known principle of the hydraulic ram pump. Said high pressure shock drives water through the one-way valve 46 into the hydraulic accumulator 47 with the effect of increasing the volume of high pressure water stored in the accumulator. This water is led through pipe 48 to a useful output, for example without limitation to drive a Pelton wheel generating electricity or for desalination by reverse osmosis. A small amount of the said high pressure water is used to drive a hydraulic motor or turbine 49 to operate the water pump 50 which pumps water from the sea 7 into the tube 40 to replace the water which passed through the one-way valve 46. To achieve this requires only a few per cent of the captured energy because the pressure in the tube 40 is much lower than the pressure in the accumulator 47. The small arrows in Figure 4 show the direction of water flow inside the pipes. In Figure 4 the float valve 45 and one-way valve 46 are illustrated by way of example without limitation as ball valves, but any appropriate design of valve may be used according to the art.

In all cases the operation of the distensible tube wave energy converter is as follows. The oscillating pressure and pressure gradient outside the tube due to the ocean waves excites a bulge wave near the bow which propagates along the tube at the bulge wave velocity. As the bulge wave moves along the tube, the ocean wave is moving along the tube at the same speed and at each point contributes a further increase in pressure. The result is a cumulative more or less linear increase in the amplitude of the bulge wave, which progressively sucks in energy from the wave. In effect the bulge is surfing in front of the wave picking up energy as it moves.

Depending on the length of the tube, the oscillating pressure amplitude at the stem can be 3 1o5 times the amplitude of the oscillating pressure in the ocean wave. Useful energy is then extracted from the oscillating pressure at the end of the tube, as explained above.

References [I] Francis J.M. Farley, Distensible tube wave energy converter, British patent application GB 0602278.4 filed 4 Feb 2006, PC'FIGB2007/000201 filed 23 Jan 2007 [2] James Lighthill, Waves in Fluids, Cambridge University Press (1978), p. 96ff

Inventor(s):     FARLEY FRANCIS JAMES MACDONALD
Classification:- international:     F03B13/14; F03B13/00 - European:     F03B13/14C; Y02E10/38

Abstract -- The water rises and falls inside a partially submerged hollow chamber, open to the sea at the bottom but closed at the sides and top, except for a hole equipped with a float valve 29. When the water reaches the top of the chamber the float valve 29 closes and some water is driven at high pressure through a non-return valve (7, fig.1) into a hydraulic accumulator. This hydraulic ram may be mounted on an elongated hull or hulls 20 with superstructure 21 supporting elevated ballast 24 which is varied to bring the roll period into resonance with the waves. Auxiliary floats 22 on each side above the waterline contact the water when the roll angle is large and prevent capsize. Water pumped by the rams at high pressure may be used to generate electricity, for desalination or, if the equipment is deployed in a fresh water lake, for irrigation.; The hydraulic ram may be incorporated into a floating buoy (fig. 2).

The invention relates to a floating apparatus for extracting useful energy from the waves of the sea using a hydraulic ram pumping water as the power output mechanism. The invention comprises two components; a new power conversion mechanism based on the well known hydraulic ram and a new floating structure which resonates with the waves in roll Hydraulic rams have been used for many years for pumping water from a low pressure stream into an elevated reservoir. The sudden closing of a valve in a pipe, in which water is flowing, creates a pressure pulse which drives some water through a one-way valve to the output. So far this principle has not been used for wave energy conversion and the concept needs to be adapted to this end According to the present invention the motion of a water surface relative to a structure is used to operate a hydraulic ram generating high pressure water directly. The water is pumped into a pressure vessel hydraulic accumulator or an elevated reservoir, with the result that power is extracted from the waves.

It is well known that efficient conversion with a floating structure of moderate size may be obtained if the structure oscillates in resonance with the waves. This condition obtains if the natural period of the structure oscillating in a calm sea is close to the period of the incoming waves. But for most floating bodies of moderate size the natural period of oscillation in heave, roll or pitch is too short to satisfy this condition. According to this invention the natural period of a floating body in roll or pitch may be lengthened by appropriate design of the hull, by lowering the centre of buoyancy and by raising the centre of gravity with ballast means above the waterline which increases the moment of inertia and reduces the stability Energy may then be extracted from the enhanced rolling or pitching motion that ensues.

According to the invention in its first characteristic the wave powered hydraulic ram comprises a partially submerged hollow chamber of any shape, with a large opening to the sea at one side or at the bottom, the top of the chamber being located more or less level with the sea surface when there are no waves.

According to the invention in its second characteristic the said chamber is furnished at or near the top with one or more holes fitted with float valves. The float valve allows air to flow freely in and out of the chamber but, when the water level inside the chamber reaches the top, the valve closes and the hole is blocked so water cannot leave the chamber through the hole. With advantage the float valve is close to neutrally buoyant in sea water. When the water rising inside the chamber reaches the top, the drag exerted by the water on the valve lifts the valve and it snaps shut But when the water outside the chamber is falling, or equally when the chamber is rising through the water, the pressure of water above the valve causes it to open and water flows downwards through the valve, into the chamber and out at the bottom or the open side as the case may be.

According to the invention in its third characteristic the said chamber is furnished at or near the top with a hole communicating with a pressure vessel, containing water and air under pressure This hole is normally closed by a non-return valve which allows fluid to flow out of the chamber into the pressure vessel but not to return. Said pressure vessel functions as a hydraulic accumulator according to the art. When the water rising inside the chamber under the action of the waves reaches the top of the chamber, the float valve closes suddenly and some water is forced out of the chamber into the pressure vessel. A delivery pipe connected near the bottom of the pressure vessel leads the water under pressure to a useful output. There may be a multiplicity of holes with non-return valves connecting the chamber of the hydraulic ram to the pressure vessel. A multiplicity of hydraulic rams may deliver water under pressure to a single pressure vessel or a multiplicity of pressure vessels According to the invention, the operation of the wave powered hydraulic ram is as follows. The action of the waves causes the water surface to rise and fall inside the chamber or the chamber to rise and fall relative to the water surface When the water surface reaches the top of the chamber the float valve closes generating a high pressure inside the chamber according to the well known principles of the hydraulic ram and water is forced out of the chamber through the non-return valve or valves into the pressure vessel or vessels. From said pressure vessel, water under pressure flows through the delivery pipe to a useful output. The water under pressure may be used for any kind of useful work, for example to generate electricity, for desalination or, if the machine is operated in a fresh water lake, for irrigation.

When a float valve in a wave powered hydraulic ram closes, there will be a sudden pulse of pressure which may generate noise and impulsive forces which can fatigue the structure According to the invention, it is arranged that when the valve closes a small volume of air is trapped in a pocket at the top of the said chamber with the effect that this air acts as a compliant air cushion and the peak pressure inside the chamber is reduced. For the same purpose according to the invention the float valve may be solid or hollow and made of an elastomeric material such as a fibre-reinforced polymer with the effect that the deformation of the material smoothes out the pressure transient. With advantage said float valve may comprise an elastomeric ball seating into a conical socket or an elastomeric cylinder seating into a tapered slot. When the valve is open said ball or cylinder is located near said hole or slot by a cage structure according to the art.

With advantage the said chamber may be wide at the bottom and narrower at the top with the effect that the water rising inside the chamber is accelerated and generates a higher pressure when the said float valve closes. Also with advantage the length and shape of the chamber may be chosen so that the natural oscillation frequency of the water inside it resonates with the prevailing external wave frequency according to the art.

According to the invention one or more wave powered hydraulic rams as described above may be mounted on any structure fixed to the shore or sea bed with the result that the rams are actuated by the rising and falling of the sea surface. One or more wave powered hydraulic rams may also be mounted on any floating structure in which case the rams will be actuated by the heaving, rolling and pitching motion of the structure as well as by the waves outside the structure. The said floating structure may be of any shape or form for example without limitation a buoy, a vertical cylinder, a ship or a structure with multiple hulls or multiple floats.

In all cases a multiplicity of hydraulic rams may feed water under pressure into a single hydraulic accumulator or into a multiplicity of hydraulic accumulators in series or parallel. The water under pressure may be used for any kind of useful work, for example to generate electricity either on the structure or on a neighbouring structure or onshore, for desalination or, if the machine is operated in a fresh water lake, for irrigation.

According to the invention one or more wave powered hydraulic rams may be mounted on a floating resonant oscillator. Said resonant oscillator comprises in the fourth characteristic of the invention one or more hulls which may with advantage be elongated and oriented with their long axes generally parallel to the approaching wave fronts and perpendicular to the direction of wave propagation. If more than one, the hulls are fixed to each other by means of a rigid framework The hull or hulls support above sea level a superstructure which projects laterally on each side of the hull or hulls The invention further comprises in its fifth characteristic one or more ballast means supported on the said superstructure above the level of the sea, with the effect of raising the centre of gravity of the resonant oscillator and reducing its lateral stability with the result that its natural roll period is increased. By correctly choosing the quantity of ballast means and its height above sea level, the roll period of the oscillator can be made equal to the prevailing period of the incoming waves with the result that the structure rolls laterally to and fro in resonance through large angles under the action of the waves In a preferred embodiment the ballast means comprises water in one or more elevated tanks mounted on the said superstructure and the quantity of water is varied from time to time by filling or emptying the tanks, with the effect that the natural period of the oscillator may be adjusted to correspond to the period of the waves prevailing in the sea at any time.

In a preferred option the invention further comprises one or more submerged closed hollow vessels fixed underneath the hulls some distance below the waterline, with the effect that when the structure rolls the upthrust on the said hollow vessels tends to decrease the lateral stability and increase the natural roll period. By moving the said water ballast from the said elevated tanks into said submerged hollow vessels the natural roll period may be further adjusted.

The invention further comprises in its sixth characteristic two or more auxiliary floats mounted on the said superstructure and projecting laterally on each side of the hull or hulls above the level of the sea If the oscillator rolls through a large angle these auxiliary floats come into contact with the water with the effect of limiting the roll angle and preventing the oscillator from capsizing According to the invention one or more wave powered hydraulic rams as described above are mounted on the hull or hulls of the resonant oscillator described above with the effect that the rolling, heaving or pitching of the resonant oscillator, together with the rising and falling of the water surface outside the structure, actuates the rams as described above and causes water to be pumped under pressure into pressure vessels and thence to a useful output.

Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows an embodiment of the wave powered hydraulic ram in vertical cross section.

Figure 2 shows in plan and vertical elevation a floating buoy wave energy converter incorporating a multiplicity of hydraulic rams Figure 3 shows in plan and vertical elevation a resonant oscillator wave energy converter in the form of a catamaran furnished with hydraulic rams.

A particular embodiment of the invention will now be described with reference to Figure 1 which shows the principal components of the wave powered hydraulic ram in vertical cross section In this figure the approximate level of the water surface when there are no waves and the chamber is in its equilibrium position is shown by the dashed line. Referring to this figure, the wave powered hydraulic ram comprises a partially immersed hollow chamber 1, closed on all sides and at the top but open to the sea at the bottom, a hole 2 with float valve 3, and one or more exit holes 6 furnished with non-return valves 7 connecting to the pressure vessel 8 which contains water and air under pressure. The hole 2 is more or less level with the undisturbed water level shown by the dashed line in Figure 1. The action of the waves causes the water surface 9 to rise and fall with respect to the chamber 1, while the air trapped above the water can flow in and out of the chamber through the hole 2 When the water surface in its upward motion approaches the top of the chamber, the float valve 3 rises and suddenly closes off the hole 2.

The result is a sudden rise in pressure at the top of the chamber and following the well-known principles of the hydraulic ram some water is forced through the exit hole or holes 6 via the non- return valve or valves 7 into the pressure vessel 8. From there it flows via pipe 10 which may be rigid or flexible to a distant elevated reservoir or further hydraulic accumulators (not illustrated).

The high pressure water in the accumulator or reservoir can then be used to deliver useful energy according to the usual principles of hydraulic power. With advantage the chamber is furnished above the waterline with a small pocket 5. When the float valve closes as described above, the air trapped inside said pocket 5 acts as a compressible cushion and limits the peak pressure inside the chamber At the top of the chamber 1 of the hydraulic ram, there may be a multiplicity of holes 2 each furnished with a float valve 3, and operating as described above Another embodiment of the wave powered hydraulic ram is illustrated in plan and elevation in Figure 2 which shows a floating buoy destined to extract energy from the waves. The upper part of Figure 2 shows a plan view of the buoy seen from above The lower part of this figure shows the buoy in elevation with a cut-away part shown in section Referring to this figure, the buoy comprises a cylindrical hull 11 closed on all sides, surrounded by a skirt 12 which may be vertical or tapered and inclined. The space between the body of the buoy 11 and the skirt 12 is closed at the top by the bulkhead 15 and divided into a multiplicity of separate spaces 16 by the vertical partitions 13. These spaces serve as chambers for a multiplicity of hydraulic rams which pump water under pressure through the multiplicity of pipes 14 into the pressure vessel 8 located near the centre of the buoy. Water under pressure leaves the pressure vessel via pipe 10 to a useful output as described above. The hull can be of any shape and the chambers surrounding it can be of any shape.

With advantage the buoy can be furnished with a superstructure in the form of a lattice frame (not illustrated) which supports elevated ballast means, preferably water in an elevated tank, and further supports auxiliary floats (not illustrated) located laterally above the waterline. With advantage the buoy may further comprises one or more submerged closed hollow vessels (not illustrated) fixed underneath the buoy some distance below the waterline. The functions of the elevated ballast means, the auxiliary floats and the submerged hollow vessel have been described above.

The operation of each of the multiplicity of hydraulic rams is as follows. The buoy floats on the sea with the undisturbed water surface approximately level with the bulkhead 15 Under the action of the waves the buoy heaves, pitches and rolls with the result that the water surface 9 inside the chamber 16 between the skirt 12 and the buoy 11 rises and falls freely relative to the bulkhead 15 while air flows in and out of the said chamber through the hole 2. When the surface 9 of the water inside the chamber 16 reaches the bulkhead, the float valve 3 closes, blocking the hole 2 and forcing the rising water to flow under pressure through the non-return valve 7 via the pipe 14 into the pressure vessel 8. The flexible pipe 10 is connected to the pressure vessel and leads the water under pressure to the shore or to a neighbouring platform where it is used to do useful work. Alternatively the water under pressure may drive a Pelton wheel or other form of turbine mounted on the buoy, so as to generate electricity which is then conducted ashore or to a neighbouring platform by flexible cables.

It will be appreciated that the vertical partitions 13 serve to separate the multiplicity of hydraulic rams from each other so that each responds independently to the local motion of the water relative to the individual bulkheads 15. This enables the buoy to extract energy from its rolling or pitching motion, as well as from the vertical heaving of the system as a whole The vertical partitions may also serve as attachment points for the mooring cables 17.

The size of the chambers required for the hydraulic rams to capture a given amount of power may be calculated as follows. When the float valve closes, the total kinetic energy of the water in the chamber is transferred as hydraulic energy to the pressure vessel This happens once per wave cycle. In general the amplitude of motion of the buoy relative to the water surface will be Q times the wave amplitude and the cross section of the chamber at the lower end may be q times the cross section at the top. It is well known that the maximum capture width for a small floating body moving in heave is 1/k where k is the wave number (2 times pi divided by the wavelength) of the waves in the sea, see reference 1. If the body moves in pitch the capture width can be 2/k and if it moves in heave and pitch together the capture width can be 3/k. It follows from these considerations that to obtain a capture width of n/k, the total volume V of water in all the chambers should be V- A3 - 8r2qQ2 where X is the wavelength in the sea. For a buoy of diameter 20 m with typical values of the parameters, ?. = 150 m, Q=5, q 3, n = 3, this implies that the volume of water to be trapped between the buoy 10 and the skirt 12 is fairly large but not unreasonable. Power could then be captured over an effective frontage (capture width) of for example 75 m.

A multiplicity of hydraulic rams generally as described above and illustrated by way of example in Figures 1 and 2 may be mounted more or less at sea level on any structure fixed to the shore or sea bed and a multiplicity of hydraulic rams may feed water under pressure into a single hydraulic accumulator. A multiplicity of hydraulic rams with chambers of any shape generally as described above and illustrated by way of example in Figures 1 and 2 may be mounted more or less at sea level at the front, side or back of a floating structure of any shape and may be used to extract energy from the movement of the waves and the movement of the structure.

Another embodiment of the wave powered hydraulic ram will now be described with reference to Figure 3 which shows a resonant oscillator in the form of a catamaran furnished with hydraulic rams destined to extract power from the sea. The upper part of this figure shows the oscillator in plan view as seen from above, with the hydraulic rams shown in section. The lower part of this figure shows the oscillator in elevation as seen from the side, with the hydraulic rams shown in section. Referring to Figure 3, this embodiment comprises a floating structure with two or more long parallel hulls 20 which may be of any shape connected by a superstructure in the form of a lattice frame 21. The said superstructure supports two or more auxiliary floats 22 of any shape, located above the waterline symmetrically on either side of the hulls. The said auxiliary floats may with advantage contain ballast of any kind, in particular water ballast. To locate the ballast the said floats may with advantage be divided by internal vertical transverse partitions (not illustrated). The superstructure further supports centrally one or more elevated containers or reservoirs 24 which may be filled with ballast of any kind, and in particular with water ballast. To locate the ballast the said elevated container 24 may with advantage be divided by internal vertical transverse partitions (not illustrated). This embodiment may with advantage further comprise one or more closed hollow vessels 26 located centrally below the waterline by a lattice frame attached to the said superstructure Depending on the state of the waves, said hollow vessel may be empty or may contain ballast, in particular water ballast, pumped as occasion demands from the auxiliary floats 22 or from the elevated container 24. To locate the ballast the said closed hollow vessel may with advantage be divided longitudinally by internal vertical transverse partitions (not illustrated). This embodiment further comprises a multiplicity of hydraulic rams 28, substantially as described above and illustrated in figures 1 and 2, attached to the hulls 20. Each hydraulic ram is furnished with a float valve 29 comprising for example without limitation a cylinder of elastomeric material which seats into a tapered rectangular slot 30. The chamber 28 of each hydraulic ram is connected via a non-return valve (not illustrated) to a pressure vessel 31 containing air and water under pressure. The pressure vessels may with advantage be connected together by pipes (not illustrated) and may be connected to a further central hydraulic accumulator (not illustrated). A flexible pipe (not illustrated) is connected to the pressure vessels and leads the water under pressure to the shore or to a neighbouring platform where it is used to do useful work. Alternatively the water under pressure may drive a Pelton wheel or other form of turbine mounted on the resonant oscillator, so as to generate electricity which is then conducted ashore or to a neighbouring platform by flexible cables.

The operation of the said resonant oscillator furnished with hydraulic rams is as follows. The structure is oriented so that the waves approach laterally from the left or right in Figure 3 The ballast in the elevated container 24 and the auxiliary floats 22 is adjusted, preferable by pumping water, so that the roll period of the structure is the same as the prevailing wave period in the sea.

The structure then rolls with a large amplitude driving the hydraulic rams 28 up and down in the water. The rising and falling of the water surface engendered by the waves increases the relative motion of the water surface relative to the rams. Substantially as described above, it then results that the rams pump water under pressure into the pressure vessels 31 and this water is led off to do useful work according to the art. In large waves the oscillator may roll with large amplitude with the effect that the auxiliary floats 22 enter the water and prevent the structure from capsizing. In storms it is desirable to move ballast from the elevated container 24 and from the auxiliary floats 22 into the said closed hollow vessels 26, preferably by pumping water from one to the other, with the effect that the structure becomes more stable and does not capsize.

Hydraulic rams operated by waves are particularly well adapted to supplying high pressure sea water for desalination plants. They may also be used in fresh water lakes to pump water for irrigation References [1] Johannes Fames, "Ocean Waves and Oscillating Systems", Cambridge University Press, 2002, pp. 216-217.