WO2010049708A2

WO2010049708A2 - Improved apparatus for generating power from wave energy

Info

Publication number: WO2010049708A2
Application number: PCT/GB2009/002594
Authority: WO
Inventors: Trevor John Whittaker
Original assignee: Aquamarine Power Limited
Priority date: 2008-10-31
Filing date: 2009-10-30
Publication date: 2010-05-06
Also published as: GB201107934D0; WO2010049708A3; GB2476778A; GB0820021D0

Abstract

A wave energy conversion device (1), for use in relatively shallow water, includes a base portion (2) for anchoring to the bed of a body of water. A flap portion (6) is pivotally connected to the base portion (2) and is biased to the vertical. The flap portion (6) oscillates backwards and forwards about the vertical in response to wave motion. The flap portion (6) includes an upper portion (8) upstanding above the pivot axis (7) for extracting energy from the wave motion and a lower portion (10) that has a biasing mass located below the pivot axis. The biasing mass provides a restoring force acting to bias the flap to the vertical when the flap portion (6) oscillates. Power extraction means is provided for extracting energy from the movement of the flap portion.

Description

IMPROVED APPARATUS FOR GENERATING POWER FROM WAVE ENERGY

The present invention relates to an apparatus for generating power by extracting energy from waves. A control system for use therewith is also described.

Concerns about global warming and environmental pollution caused by the use of fossil fuels in energy generation has resulted in a move towards so-called ^λgreen' energy sources, or renewable energy sources such as tidal movement , wave power and wind power .

It has long been recognised that the waves in the sea and other bodies of water provide a vast and substantially untapped quantity of energy and many inventions have been made with the goal of achieving the aim of extracting power from the sea. One type of device for recovering wave energy is a downwards hanging flap or a pendulum which is reciprocally swung or rocked by waves in a caisson and the reciprocal movement of the pendulum is converted to electric power. Such an arrangement is described in US Patent No. 4,580,400. An alternative arrangement is a seabed mounted or supported structure having a hinged lever attached to a panel for reciprocation motion and such an arrangement is described in International Publication No. WO 2004/007953 Al. This arrangement is used in relatively deep water, at a preferred depth of about L/2, where L is the wavelength of the waves expected at the location of use. A somewhat similar arrangement is disclosed in WO 03/036081 where a reciprocating body is situated entirely underwater in a water basin of intermediate depth. In contrast an alternative device described in WO 98/17911 is for use in shallow waters. It makes use of the "translation" waves formed where deep-water waves break or are broken as they run up the seashore. The device has a flap, which is pushed backwards by the translation waves and returns to the upright between each wave impulse using springs.

There are numerous other examples of other wave power generating apparatus. Whilst such devices have been previously proposed they have fundamental failings for various reasons including lack of robustness in what is a very hostile environment; the need to ^Λover engineer' devices so as to make them suitable for use in hostile environments with consequent cost and maintenance implications; the need to utilise substantial anchorage devices for holding such apparatus in a secure manner on the seabed; and relatively substantial maintenance and repair costs for such devices .

In particular, previously proposed devices have generally been inefficient. The quantity of power captured from the incident waves has tended to be low and the subsequent conversion of the captured power into electricity poor.

The devices have tended to produce power unevenly with large ^λspikes' in the output, making it difficult to provide a smooth power output suitable for delivery into an electrical grid system.

In our previous application (WO2006/100436) we disclosed a wave energy conversion- device for use in relatively shallow water. The device includes a base portion formed and arranged for anchoring to the bed of a body of water in use of the device and an upstanding flap portion pivotally connected to the base portion. The flap portion is biased to the vertical in use and formed and arranged to oscillate backwards and forwards about the vertical in response to wave motion acting on faces of the flap portion. Power extraction means for extracting energy from the movement of the flap portion is also described in that application. The device of W02006/100436 is intended for efficient extraction of energy from waves in relatively shallow water, say up to 20m deep. In use the base portion of the device is anchored to the bed of a body of water with the flap portion facing the wave motion and the base portion and the flap portion extend vertically through at least the entire depth of the water, to present a substantially continuous surface to the wave motion throughout the full depth of water from the wave crest to the sea bed.

Whilst the device of WO2006/100436 does provide a practical means of extracting energy from waves, there is a continuing need to improve wave powered energy devices to enhance their ability to efficiently extract energy and to provide devices that are economic in manufacture and placement in a suitable location. At the same time practical wave powered devices must be sufficiently robust in the demanding environment of an ocean or other water body, where substantial wave forces act under normal weather conditions on the device and its associated foundations . Under storm conditions exceptionally high loadings can be found requiring employment of a large safety margin and/or methods for stowing the energy capture device to protect it from the worst conditions anticipated. It is an object of the present invention to provide a wave energy conversion device that addresses at least one of the aforementioned problems .

The present invention provides a wave energy conversion device, for use in relatively shallow water, comprising: a base portion formed and arranged for anchoring to the bed of a body of water in use of the device; a flap portion pivotally connected to said base portion, said flap portion being biased to the vertical in use and formed and arranged to oscillate, in use, backwards and forwards about the vertical in response to wave motion acting on faces of the flap portion; said flap portion comprising an upper portion upstanding above the pivot axis for extracting energy from the wave motion and a lower portion, said lower portion having a biasing mass located below the pivot axis and providing a restoring force acting to bias the flap to the vertical when the flap portion oscillates; and power extraction means for extracting energy from the movement of the flap portion.

Providing an oscillating flap portion having a biasing mass (i.e. a counterweight), which provides a restoring force, below the pivot axis, allows construction of a flap type wave energy conversion device which can be more readily tuned to the wave periods that exist in relatively shallow waters, up to say 20m deep. Providing a biasing mass below the pivot reduces the period of oscillation of the flap and, when the period of oscillation is tuned to the period of the wave motion, the flap portion will exhibit larger angular movement and velocities, allowing the possibility of a higher rate of power take off (energy extraction) . Preferably the device is formed and arranged so that when the base portion is anchored to the bed of a body of water with the flap portion facing the wave motion, the base portion and the flap portion extend vertically through at least the entire depth of the water, to present a substantially continuous surface to the wave motion throughout the full depth of water from the wave crest to the sea bed.

It will be understood that although the flap portion is biased to the vertical, in some (weak) sea states, or where the wave motion is not regular, the flap portion may from time to time not oscillate through the vertical on every wave motion.

By presenting a substantially continuous surface to the wave motion throughout the depth of the water (the Vater column' ) , the flap portion of the invention can efficiently capture the maximum amount of energy from the wave motion prevailing at a given location.

Relatively small gaps above, below or in the flap portion, can have a deleterious effect on the power capture factor of a device of the invention. The power capture is defined as the ratio of the power captured by a device to the power available from the waves incident on the device width.

For example, a gap between the base portion and the flap portion or between the flap portion and the seabed, through which wave motion can pass, can cause significant power capture losses. The inventors have identified that a loss of at least 30% or more in power capture can occur by having a gap between the base portion and the flap portion.

Accordingly it is preferred that the base portion and the flap portion are formed and arranged to operate substantially without a gap between them. Similarly the inventors have identified that if the flap portion does not extend up to the water surface in the wave crest then losses occur over the top of the flap. Relatively small holes or passages through the flap portion have a similar effect. Thus preferably the flap portion is formed and arranged to extend up through the surface of the water i.e. the flap pierces the water surface under normal calm conditions .

Preferably the flap portion is formed and arranged with the base portion to account for changes in the depth of the water at a given location caused by tidal change and also to account for the expected variations in wave height i.e. the flap portion and the base are sized so that the flap will pierce the water surface at all expected tide levels and sea states. This allows capture of wave energy throughout the full depth of the water ie the water column, including at the surface in all but the most exceptional (high) sea states. Providing some ^freeboard' to the flap, a portion projecting above the water surface, makes allowance for tidal and wave variation.

Preferably the device is formed and arranged for location at a mean water depth of between 6 to 20 metres, desirably between 8 and 16 metres. At these shallower depths, the available surge wave energy, in typical sea locations at least, is substantially greater than in the deeper waters often used by other wave energy conversion devices. At these depths the wave period of typical sea states can vary from typically 4 up to 15 seconds (inland sea or Atlantic ocean) to up to 20 seconds in the Pacific where longer swells tend to be found.

Tuning a flap portion that is built with sufficient strength to survive in ocean conditions (and which can be tuned to match reasonably closely to the sea state periods expected) can be difficult. A typical flap of about 12m in height and 18m in width may have a mass of 200 tonnes or more. Furthermore the oscillation of such a large flap portion, if built without a biasing mass below the pivot point, results in very substantial uplift or heave forces acting on the base or foundation of the device. By providing a biasing mass on a lower flap portion, below the pivot axis, the present invention allows a greater range of tuning to be accomplished, without resorting to the use of springs or other complex damping devices in many cases. The under-slung positioning of the biasing mass also means that buoyancy can be reduced without diminishing the restoring force. At the same time the uplift or heave forces, tending to drag or uproot the base

(foundation) of the device and caused by the oscillating mass of the upper flap portion are mitigated or even substantially removed by the provision of the biasing mass below the pivot axis .

The resulting restoring force provided by the biasing mass in the lower flap portion is dependent on the size of the mass and its distance from the pivot axis. As the power extracted from the oscillating flap depends on the surface area of the upper flap portion presented to the wave motion, the lower flap portion is preferably as short as possible commensurate with achieving the desired period for the flap portion as a whole. i.e. preferably the lower flap portion has its biasing mass located close to the pivot connection to allow the upper flap portion to be as tall as possible for interaction with the water column.

Preferably the upper flap portion is provided with substantial buoyancy. A buoyant upper flap portion provides a restoring force to the vertical when the flap portion in located in a body of water. The combination of an upper flap portion provided with substantial buoyancy and a lower flap portion having a mass below the pivot is preferred as it can allow the flap to be tuned to a wide range of sea states of different periods and in particular to be tuned to typical sea states as found in relatively shallow waters at up to 20 metres in the Atlantic or pacific oceans or in large inland seas. Advantageously the flap portion is formed and arranged to have a natural period (oscillating frequency) of less than 15 seconds. A wave period of 15 seconds or less is typical of that found in Pacific Ocean swells in relatively shallow water. More preferably the flap portion is formed and arranged to have a natural period (oscillating frequency) of between 5 and 13 seconds. Most preferably the flap portion is formed and arranged to have a natural period of between 6 and 10 seconds. A period of between 6 and 10 seconds is typical of the sea states found in the Atlantic Ocean and large inland seas .

Preferably the weight (downwards force) provided by the lower flap portion, below the pivot axis, plus the weight provided by the upper flap portion, above the pivot axis, is equal to or greater than the buoyancy force (if any) provided by the upper flap portion, when the device is located in water for use .

Additionally the weight provided by the biasing mass of the lower flap portion in addition to the mass of the remainder of the flap and supporting structure can be greater than the total vertical forces provided by the buoyancy of the upper flap portion. In this way unnecessary vertical foundation forces can be avoided.

Additional weight (downwards force) acting on the base helps to secure the base to the bed of the body of water where the device is located, reducing the need for extensive foundations (for example piles) to secure the device to the bed of the body of water.

The additional weight also has the beneficial effect of cancelling at least some of the heave forces acting on the foundation through the motion of the flap.

For example the weight provided by the mass below the pivot axis may be from 1 to 4 times the weight provided by the resultant weight of the flap above the pivot axis and the buoyancy force ^' (if any) provided by the upper flap portion, when the device is located in water for use.

Advantageously the moment provided by the mass below the pivot axis in the flap portion is greater than the moment of the flap portion above the pivot axis in a device of the invention. Conveniently the buoyancy of the flap portion is adjustable. This permits adjustment of the restoring force for the flap portion. The buoyancy can be provided in a flap portion by having chambers in the structure of the flap, which can be filled with air or other gas, or may be filled with a foam material. For example, where the flap portion comprises tubing sections the tube sections can be air filled, at least to some extent. Conveniently the buoyancy of the flap portion is adjusted by flooding or partial flooding of one or more air filled chambers.

Desirably the flap portion has a high centre of buoyancy and a low centre of mass. The upper part of the upper flap portion undergoes the greatest motion in use, as it is furthest from the pivot, and so it has the greatest forces acting on it. By having a flap with a reduced mass

(higher buoyancy) in its uppermost parts the bending forces acting on it are reduced. The desired properties may, for example, be achieved by providing a flap portion comprising horizontally stacked tubing sections with the diameter of the tubing used increasing towards the top of the flap. Flooding or partially flooding tubing near the pivot point of the flap provides a low centre of mass whilst the larger diameter tubing near the top of the flap gives a large air volume to provide buoyancy centred towards the top of the upper portion of the flap. The lower portion of the flap may also be designed for flooding, to provide the mass giving the restoring force. However, it is desirable to reduce the size of the lower portion relative to the upper portion. Therefore mass provided in the lower portion, below the pivot, is advantageously of a denser material than water, for example concrete, lead or steel, iron ore, rock or heavy gravel .

As the lower portion of the flap containing the mass is below the pivot axis the lower portion of the flap moves in the opposite direction to the upper portion of the flap i.e. the lower portion of the flap moves in the opposite direction to the direction of the wave forces striking the upper portion of the flap and powering the motion of the flap. Thus the motion of the lower portion of the flap may tend to slow the flap down, against the desired direction of motion. This effect can be mitigated in a number of ways. The lower flap portion may be made as compact as possible, with the biasing mass being formed from a high density material and located close to the pivot. The lower flap portion may also be shaped to minimise the drag forces as it moves through the water. For example the lower flap portion may comprise one or more "keel shaped" (i.e. hydrodynamically efficient) masses shaped to move through the water with reduced drag, in comparison with simple shapes such as flat plates.

Alternatively or additionally the base portion may include at least one deflector plate, formed to deflect the wave motion away from the lower flap portion and onto the upper portion of the flap, above the pivot axis.

In a particularly preferred alternative the lower flap portion comprises a substantially cylinder shape concentric with the pivot axis. The benefit of a lower flap portion of this form is that unlike other possible shapes it does not radiate waves, as the flap rotates about the pivot axis . To provide the restoring force the cylinder is not homogeneous in mass around the pivot axis but has a concentration of mass below the pivot axis to provide the desired restoring force.

Alternative or additional independent biasing means may be provided. For example, springs or torsion bars formed and arranged to urge said flap portion to a generally vertical orientation with respect to said base portion. The independent biasing means can be adjustable if required.

It will be appreciated that the height of waves at any given point is not consistent throughout the year and at any one given time or season of the year the incident wave period will differ. Thus to maximise the efficiency and performance of the device, the flap portion may be formed and arranged to change its natural period. For example in a process referred to as "slow tuning" which may change the natural period of the flap on a seasonal basis. Thus preferably there is provided means for altering the centre of buoyancy of said flap portion; altering the buoyancy force; moving the centre of mass of the flap portion with respect to said base portion; altering the centre of mass of the flap portion; and/or altering the characteristics of said biasing means .

Advantageously the upper flap portion has a generally rectangular form. Other flap shapes are possible. The rectangular form may be of a generally stiffened flat plate, however, depending on the construction method of the flap portion other generally rectangular bodies can be made. If the upper flap portion is composed of a flat plate or flat plates, it is preferred that they are made of a composite, reinforced structure. This improves the ability of the flap portion to withstand the forces imposed by the wave motion. For example, the upper flap portion may be constructed of plates comprising two outer skins of steel plate with steel reinforcing bars placed at regular intervals between them, and welded to the inner surface of each plate. In use for a flap portion the spacing between the reinforcing bars and the outer skins can be filled with a material such as concrete to provide added strength and adjust buoyancy. The flap portion may be constructed of modular components. For example the flap may comprise sections of generally circular in section piping or tubing arranged in a plane, by stacking horizontally or vertically parallel and adjacent each other, to give a generally rectangular form to the flap. Advantageously where the tubing is stacked horizontally to form the flap portion the sections of piping or tubing may be of different diameters . An upper flap portion with smaller sections of tubing near the pivot and larger sections of tubing near towards the top edge has some advantages with regard to the control of biasing and the robustness of the flap portion as discussed hereafter.

A flap portion constructed of pipe sections in this manner has a number of advantages . The ^Λinodular' construction of the flap portion allows for easy transport to a construction site where the flap is assembled. Tubes have an inherent strength able to withstand considerable forces such as those from strong wave motion, particularly impact, torsion and buckling forces. However, the forces of the wave surge acting on a face of the upper flap portion tend to be increased, at the lines where the tubing sections abut, by a ^λfunnelling' effect of the curves of the tubing. Advantageously, where the tubes abut, a packing material is provided to reduce local wave impact forces. Preferably at least the upper part of the upper flap portion is provided with a resilient or compliant surface. The surface serves to absorb the energy of transient impacts, avoiding damage to the flap portion. For example, where the upper flap portion is comprised of large tubing sections, the tubing section may have a smaller diameter tubing, of a resilient material, wound spirally round it or slid on as a sleeve. This provides a compliant layer on the surface of the large tubes. A flap portion constructed of tubing sections also presents the possibility of ready adjustment of the buoyancy of the flap and thus of the biasing effects.

The height of the whole device, base portion and flap portion, is sized to suit the depth where the device is located, with the flap portion piercing the water, at least under calm conditions.

Preferably the upper flap portion has a width at least equal to its height. Power capture has been found to be dependent on the width of a flap portion, as described hereafter with reference to specific embodiments. More preferably the width of the flap portion is between 1 and 3 times the height of the upper flap portion. For the preferred water depth of 8 to 16m and the expected wave patterns in seas at these depths a width range of 10 to 30m gives relatively efficient energy capture, up to 80% for some wave periods and/or embodiments .

Preferably the flap portion has rounded or contoured top edge and/or side edges radiused in the range of from 0.5 to 2m, preferably 1 to 1.5m. As described hereafter with reference to specific embodiments, providing rounded side edges to the flap portion increases the power capture, by reducing the loss of power due to vortex shedding as waves move round the edges of a flap portion. Suitable contouring or curvature of the side edges of a given flap portion can readily be determined by suitable experimentation .

Existing known designs which utilise a seabed mounted base and a pivoting flap have focussed on a flap which remains substantially below the sea surface and the present invention leads in quite a different direction insofar as the upper flap portion is formed and arranged to pierce the water surface.

The flap is positioned in the sea so that one of the faces of the plate (of the upper flap portion) faces directly into the prevailing direction of the waves at the chosen location. The wave pressure on the face of the upper flap portion causes a differential pressure and thereby causes it to oscillate back and forth about its pivots.

As discussed above it is preferred that the upper flap portion pierces the water surface with some freeboard available. As the upper flap portion is tilted by wave action from the vertical, the amount of the upper flap portion piercing the water surface (the freeboard) reduces. This can lead, depending on the size of the wave, to power being lost as part of the wave passes over the upper flap portion.

This effect can be mitigated by the provision of an additional structure at the top of the upper flap portion, which interacts more positively with waves at the surface even when the flap portion is tilted and freeboard reduced. For example the upper flap portion may have an additional substantially flat plate attached along its top edge, at right angles to the plane of the flap, to form a λT" , a closed ^λY' or an inverted ^λL' shaped structure. In all cases it is preferred that these additional structures have rounded edges, for smooth flow of water over and around them.

Alternatively, the top of the upper flap portion may have an alternative shape, for example, the top edge of the flap may have a generally cylindrical form, of a diameter substantially greater than the general thickness of the flap portion. This arrangement is particularly preferred where the flap portion is of a modular form, constructed of a series of horizontally laid tubing sections. The top edge of the flap portion is simply constructed by adding a tubing section of a greater diameter to the top of the λ stack' of ^λ standard' tubing sections.

Other shapes may be envisaged, with the profile of the part of the upper flap portion that pierces the water being made to improve power capture when the upper flap portion is near the expected maximum tilt angle, in normal sea conditions .

As used herein the term Relatively shallow waters' is intended to cover waters having a depth in the range of from 6 to 20 metres and thus it will be appreciated that for such an arrangement the device, that is the base portion and said flap portion may have a height slightly greater than the mean depth of the water in which the device is in use. Mean depth refers to the average depth between high and low tides where the device is in use in tidal waters .

Preferably to minimise loads on the device during extreme weather/wave events, and to facilitate maintenance, said flap portion is formed and arranged so that it may be laid more or less horizontal on the seabed (or the like) . Preferably this functionality is achieved by flooding the flap with water so that it sinks to the seabed or driving the flap portion to the seabed and latching it into a fixed position.

Preferably to minimise potentially damaging loads during extreme weather/wave events the surface area of the flap portion can be reduced to minimise its coupling effect with an incident wave. This may be achieved by one of the following means : - the upper flap portion is inflatable and it can be deflated so as to reduce its size; a large portion of the upper flap' s surface detaches in extreme events i.e. the flap portion is frangible or is designed to break, at a defined position, under extreme loading leaving the rest of the device undamaged; the upper part of the upper portion of the flap, preferably the upper most portion which pierces the surface of the water in use of the device, is formed and arranged to be retractable into the rest of the upper flap portion during extreme weather/wave events. This arrangement prevents damage to said top portion. There may be provided a plurality of devices according to the present invention, thus in another aspect the present invention provides an energy generating system comprising a plurality of wave energy conversion devices of the type described above and interconnected with each other.

To provide a smooth energy output from an array of wave energy conversion devices according to the present invention the flap portion of adjacent devices may be cascaded at an angle to the predominant wave direction so that the distance between the first and last flap is between one quarter and one half of a wavelength in the direction of wave propagation.

One significant problem though of existing designs is that maintenance costs are generally high due to the requirement to utilise heavy lifting gear for maintenance purposes. The present invention avoids or minimises such disadvantages by utilising components, in particular the upper flap portion, which are neutrally buoyant, thereby making them easy to handle. This may be achieved by utilising foam or other low density materials attached to the components of the device or introducing voids or chambers into the components which may be filled with air to increase buoyancy or filled with ballast (typically water) as required.

Advantageously to compensate for tide levels, both daily and throughout the year, the axis of rotation of the flap portion (pivot axis) may be moved up and down with respect to the base portion. Thus the pivot axis may be raised or lowered with respect to the sea bed when in use. Preferably the flap portion may be mounted on a support shaft which is itself held between two support portions for pivoting, that allow the flap portion and support shaft to move up and down (due to the flap portion' s buoyancy) in response to variations in tide level.

Alternatively the flap portion may be mounted on the support shaft which is mounted on actuators or other means which may be formed and arranged with control means to move the flap portion up or down according to tidal conditions. In all cases, where the flap portion can be moved up and down, the base portion and the flap portion continue to present a substantially continuous surface to the wave motion throughout the depth of the water. This can be arranged, for example by providing moveable deflector plates on the base portion, which rise as the flap portion is raised, to present a continuous surface of base portion deflector plate and flap portion to the wave motion.

Preferably said power extraction means utilises high pressure hydraulic fluid to drive a hydraulic motor, desirably a variable flow and speed hydraulic motor. The fluid is pressurised by the oscillation of the flap portion, preferably by means of a piston and cylinder driven by the flap portion, which pressurises the hydraulic fluid. The benefit of the variable flow and speed motor is that the flow can be continuously adjusted, preferably by computer control, to make the most efficient use of the power output of the flap portion. The computer control matches the operating parameters of the variable speed motor to the flow of hydraulic fluid, generated by the action of the flap portion. More preferably the power extraction means comprises a hydraulic motor, which is connected via a flywheel energy store to a variable speed electrical generator system. The variable speed electrical generator system may, for example, comprise a variable speed motor/induction generator, which is connected to an electrical grid system by a motor inverter and line rectifier. In use the output from the hydraulic motor is used to power the flywheel from which energy is extracted via the variable speed electrical generator system to supply electricity to the grid system. The flywheel is kept spinning in its optimum operating range by the controlled rate of power extraction. Preferably the control of the variable speed electrical generator system is via a computer control system.

Preferably the control of operation of the wave power generating device and its power extraction means is by a linked computer control system. The control system adjusts the operating parameters of the flap portion, the hydraulic motor, and the variable speed electrical generator system, to optimise the output of electrical power from the device in real time.

The computer control system monitors the operation of the flap portion, the hydraulic circuit that contains the hydraulic motor, the flywheel and the variable speed electrical generator system and adjusts parameters according to an appropriate algorithm.

Preferably the wave energy conversion device further comprises sensors, which determine the pattern and strength of waves before they strike the flap portion. These sensors allow adjustment of the parameters of the wave power generating device and power extraction means in a predictive fashion by said computer control system. The sensors may, for example, be positioned ahead of the flap portion.

The present invention also provides a method for extracting energy from waves comprising the steps of: a) providing a wave energy conversion device according to the invention; b) locating said device on the bed of a body of water with a depth of between 6 to 20m, with its flap portion facing the direction of waves; c) extracting wave energy from the waves in a said body of water.

Further preferred features and advantages of the present invention will now be described with reference to the accompanying drawings in which: - Fig. 1 is a schematic perspective view of a wave energy conversion device of the invention;

Fig. 2a is a schematic front elevation view of a flap portion for use in a wave energy conversion device of the invention; Figs. 2b and 2c are respectively plan and side elevations of parts of the lower flap portion of figure 2a;

Fig. 3a is a schematic perspective view of another flap portion of the invention;

Fig. 3b is a partial cross sectional view of the flap portion of figure 3a;

Figure 4 is a schematic end elevation of parts of a wave energy conversion device of the same general form as that of figure 1; Fig. 5 is a schematic layout of a power takeoff system for use with the invention;

Figs. 6 (a to d) show three embodiments of a device of the invention constructed from tubing sections; Fig. 7 shows a further embodiment of a device of the invention constructed from tubing sections;

Fig. 8 illustrates graphically test results from a device of the invention; and

Fig.9 illustrates graphically further test results from a device of the invention.

A wave energy conversion device, generally indicated by reference no. 1, is shown in schematic form in Fig. 1 and comprises a base portion 2 of two foundation legs 4 which are anchored to the bed 5 of a body of water in use of the device. An upstanding flap portion 6, of generally rectangular form, is mounted for rotation about a pivot axis 7 to the base 2 and in use is placed to face the direction of wave motion, indicated by the arrow W. The flap portion 6 has an upper portion 8 which extends in use up through the depth of a body of water to pierce the surface (see figure 4) . A lower flap portion 10 extends downwards below the pivot axis 7 close to the bed 5 of the body of water .

The upper portion 8 of the flap 6 is rendered buoyant, by means of being partially filled with air, which acts to provide a restoring force to the flap when it oscillates away from the vertical. The lower portion 10 of the flap 6 is filled with a material denser than water, for example concrete. This biasing mass located in the lower portion 10 of the flap provides a restoring force against the motion of the upper portion 8 of the flap portion 6 when it is driven to oscillate by the motion of waves acting on it. The combination of the buoyancy provided in the upper portion 8 of the flap and the mass provided below the pivot axis 7 in the lower portion 10 of the flap results in a flap which is tuned to prevailing wave periods at a location where the device 1 is placed.

In this example the lower portion 10 of the flap portion 6 moves against direction of the wave driven upper portion 8, thereby reducing the potential power take off. Furthermore the height of flap portion being driven by the wave motion is reduced by the need to provide room for the mass below the pivot axis 7. Nevertheless a tuned flap portion 6 utilising a biasing mass below the pivot axis 7 will tend to have a higher angular movement (larger oscillations and velocity) whilst at the same time developing lower torque. The reduced torque results in reduced uplift or heave forces, tending to drag or uproot the base 2 (foundation) of the device. The device 1 is also provided with a suitable power extraction unit (not shown - see Fig. 5) for extracting the power generated by the movement of the flap portion 6.

Figure 2a shows in schematic front elevation an alternative flap portion to that of figure 1. The flap portion 6 includes an upper flap portion of the same form as that of figure 1. However, in this example the lower flap portion 10 includes three biasing masses 12 attached to the upper flap portion 8 and hanging below the pivot axis 7. The masses 12 may be made from solid steel, lead or concrete for example. Each mass 12 is shaped

(streamlined) to present reduced resistance to the water as the flap portion 6 oscillates in response to wave motion.

Each mass 12 has an upper narrow connecting web 14 to the rest of the flap portion 6 and a larger main mass 16 suspended below. (See the side elevation view of one of the masses 12 in figure 2b) . Figure 2c is a plan view of one of the masses showing the streamlined shape, akin to that of a yacht keel, which is presented by the mass 12 to the direction of wave motion W and the direction of oscillation of the flap portion 6.

The arrangement of figures 2 has the benefit that the main part (main mass 16) of the masses 12 is displaced further from the pivot connection 7 by virtue of the connecting webs 14, resulting in a greater restoring force being provided for a given amount of mass. At the same time the streamlined keel shapes present reduced resistance and drag through the water as the lower flap portion 10 moves in the direction opposite that of the upper flap portion 6.

Figure 3a shows in perspective another flap portion 6 with a further alternative lower flap portion 10 arrangement. In this example the lower flap portion 10 is generally cylindrical in form, centred around the pivot axis 7. The cylindrical lower flap portion 10 includes a mass 18 located inside the power flap portion and below the pivot connection as indicated by the dashed lines in figure 3a. The mass 18 may be of steel lead or concrete for example. The partial cross section of figure 3b shows the mass 18 in cross-hatching. The lower flap portion also includes a void 20 which, depending on the desired tuning of the flap portion 6 may be air or water filled. The provision of a cylindrical lower flap portion 10 has the benefit that as it oscillates as indicated by the curved arrows in figure 3b substantially no waves are radiated, which would act to reduce the power absorbed by the upper flap portion 6.

Figure 4 is a schematic end elevation of a device generally similar to that of figure 1, with the base unit (parts 2 in figure 1) not shown for clarity. In this example the device includes two deflector plates 22 which are angled upwards towards the flap portion 6 on either side. These deflector plates act to direct wave motion from near the bed 5 of the water up onto the upper portion of the flap 8 thereby increasing the energy absorbed. At the same time the deflectors plates 22 mitigate losses of energy uptake caused by the motion of the lower flap portion 10, which is against that of the upper flap portion 8. Furthermore losses of energy caused by wave motion at the gap 24 between the bottom of the flap portion 6 and the bed 5 of the water body are also prevented.

Deflector plates 22 may be fitted to devices having any design of flap portion 6. The can be sized to provide the benefits described above even when the pivot axis 7 is adjustable in height above the bed 5 of the water body. Alternatively they may themselves be adjustable to provide the optimum deflection of wave motion depending on adjustments made to the operation of the flap portion.

Fig. 5, is a schematic illustration of a power takeoff system for conversion of the oscillating motion of a wave energy conversion device of the invention to electricity. The oscillating motion of the flap portion of a device of the invention (not shown in this figure but generally as shown in Fig. 1) is coupled by a suitable linkage (not shown) and a driving rod 26 to a hydraulic ram (piston) 28 which reciprocates in a cylinder 30 and is double acting. The cylinder 30 forms part of a hydraulic circuit 32 to which it is connected by an outlet point 34 at a discharge end 36 of the cylinder and an inlet port 38 at the opposite (inlet) end 40 of the cylinder 30.

A fluid flow passage 42 fitted with a non-return valve 44 allows hydraulic fluid 46, in the circuit 32, to flow through the ram 28 (piston) from the inlet end 40 of the cylinder to the discharge end 36.

In use as the ram 28 oscillates back and forth in the cylinder 30, hydraulic fluid is forced through the fluid flow passage 42 into the discharge end 36 of the cylinder during the closing stroke of the ram 28. On the opening stroke of the ram 28 the fluid cannot flow back through the fluid flow passage 42 because of the operation of the non-return valve 44 and so is pumped out of the outlet port 34 of the cylinder 30. The driving rod 26 has a cross sectional area that is half of the cross-sectional area of the cylinder 30. This means that the cross sectional area of the ram (piston) 28 facing the inlet end of the cylinder 30 is twice that facing the outlet end of the cylinder 30. Consequently the ram 28 is double acting and pumps the same volume of hydraulic fluid on both its opening and closing strokes. This pumping action pressurises the hydraulic fluid in the circuit 32. The pressure in the hydraulic circuit 32, caused by the action of the ram in the cylinder is used to drive a variable displacement hydraulic motor 48 through which the fluid passes. Fluid used to drive the hydraulic motor then passes into a reservoir 50 where it is held available to be drawn back into the cylinder, via a second nonreturn valve 52 and the inlet port 38.

An accumulator 53, which is a pressure cylinder containing air 54, is connected to the pressure circuit between the cylinder 30 and the hydraulic motor 48. As fluid is pumped out of the cylinder into the hydraulic circuit the air 54 is compressed to store some of the pressure produced by the pumping action of the ram 28. This has the effect of smoothing variations in the pressure of the fluid entering the hydraulic motor 48, allowing more efficient operation.

The hydraulic motor 48 drives a flywheel 55 which stores energy from the hydraulic motor 48 until it is converted into electricity by an induction generator/motor 56 which connects to the flywheel. The output from the induction generator 56 is converted via a motor inverter 57 and line rectifier 58 into an electrical output 56 suitable for connection to an electricity grid (not shown) . The induction generator/motor and its associated inverter and rectifier form a variable speed electrical generator system which is used to keep the flywheel 48 spinning within its optimum range by extracting power from the flywheel in a controlled manner. The generator/motor is computer controlled to vary the extraction of energy from the flywheel in response to surges in the flywheel speed. To optimise the output from this system the hydraulic motor 48 is controlled by a computer control system 59. (Connections from the computer control system to the various elements of the generating system are not shown for reasons of clarity in the figure.) The computer control system 59 monitors inter alia ram velocity, hydraulic pressure and the rotational speed of the hydraulic motor in order to determine the optimal displacement for the motor at any given moment. The computer control system 59 also serves to tune the device to the prevailing wave period such that the force and angular velocity are in phase, depending on the sea characteristics as required.

Figure 6a shows a device of the invention 1, which has a 12m by 12m flap portion 6 attached by pivots 7 to a base portion 2, which is approximately 2m high. The upper flap portion 8 consists of a horizontally stacked array of tubing sections 60 with diameters of 1.8m. The tubing sections 60 have 50mm spacings 62 between them, which are filled with a packing material 64. A driving rod 66 is pivotally attached to each side of the upper flap portion 8. These connect to pistons inside hydraulic cylinders 68 which are pivotally attached to the base portion 2. A deflector plate 22 fills the spacing between the bottom tubing section 72 of the upper flap portion 8 and the seabed 74 and conceals the lower flap portion 10 (not shown) which can be of the same general form of that of figure 2a. In use, when the upper flap portion 8 oscillates in response to wave action the driving rods 66 are driven to cause hydraulic fluid in the cylinders 68 to be pressurised by the action of the pistons (see fig.5) . The pressurised fluid then flows out and returns via pipework connections 76 into the hydraulic circuit of rest of the power take off system (not shown, see Fig. 5 for example) .

Figure 6b shows another embodiment of a device 1 of similar configuration to that of Fig. 6a except that curved end sections 78 ( ^Λend effectors' ) are located at each side edge 80 of the flap portion. In tests these end effectors 78 have been shown to improve power capture significantly.

Figure 6c shows a yet further embodiment, which has the same configuration of that of Fig. 6b, but with the provision of additional tubing sections 82 located at the top of the flap portion. In use these provide additional buoyancy and the additional structure also gives more positive interaction with waves at the water surface when the upper flap portion 8 is tilted.

Figure 6d shows an embodiment of the same form as that of figure 6c except that a single driving rod 66 and piston 68 is provided for power take off, mounted centrally. The driving rod 66 is pivotally attached to the flap 6 and the piston 68 is mounted on a cross member 83 of the base 2.

All of the devices of figures 6 can have the buoyancy of the upper flap portion adjusted, for example by flooding the tubing sections 60 with water. In general to provide a high centre of buoyancy the lower tubing sections in the upper flap portion 8 will be flooded with water, with upper tubing sections kept filled with air. Figures 7 show a device of the invention 1 similar to that of Fig. 6a but with rounded side edges 18 and top portion 20.

Figure 7a shows the device 1 in perspective view, with the power take off or extraction means not shown apart from the driving rods 66 and hydraulic cylinders 68. Figures 7b to 7d show the same device in elevation, side elevation and cross section (along X-X of 7a) respectively.

In this embodiment the flap portion 6 is about 18m wide and the device 1 is of the order of 12m high to give particularly effective power capture at a water depth of up to 12m. The upper flap portion 8 is constructed of four horizontally disposed tubing sections 60, each of 1.8m diameter. The spacings 62 between each tubing section 60 are larger than those of the upper flap portion 8 of Fig. 6a, about Im and are filled by curved plates 84. The required substantially continuous surface to be presented to the wave motion is completed by the curved deflector plates 22 fitted to the base portion 2 which conceal the lower flap portion 10 which in this example has the cylindrical form of that shown in figure 3a.

Test results

Test results carried out on model flap devices, with and without a biasing mass in a lower flap portion acting as a restoring force have shown that tuning to a shorter period can be achieved. When scaled up to a realistic device for typical relatively shallow water sea states the tuning achieved matches more closely expected sea state periods . The graph of figure 8 shows results obtained, when model flap portions representing 12 m high and 18m wide overall dimensions were studied. Curve A shows a flap without a biasing mass below the pivot axis. Curve B shows the results when a mass was added below the pivot axis . The provision of an added mass below the pivot axis resulted in a decrease in oscillation period from about 18s to about 14s together with an increased angular rotation. These results indicate the benefit of utilising a restoring force mass below the pivot point for tuning to or towards the periods anticipated for real sea states .

Further more detailed tests have been carried out to investigate the benefits obtainable by making use of a biasing mass below the pivot axis, especially for flap type wave energy conversion devices such as those described herein.

Increasing the height of the pivot in a given depth of water decreases the height of the flap for the same water depth thereby reducing the natural period of the flap.

However, placing a counterweight below the pivot tunes the device to even shorter periods and allows tuning to or at least towards the optimum for efficient energy production in relatively shallow water (20m or less as described before) whilst using an appropriately sized and robust flap construction.

Tuning the natural period (T_n) of a wave energy converter towards the dominant period of the incoming waves is important because this increases the motion of the device, thereby also increasing the velocity and the power that can be extracted (power = torque x velocity) . The dominant period of waves in the north Atlantic lies in the range 1-9 seconds. The natural period of a generally- rectangular flap of width 18m, height 12m and thickness 1.8m and having a density of 250kg/m³ is approximately 16 seconds. Because the natural period of such a device is a lot longer than the dominant period of the incoming waves, the power production could benefit significantly from lowering the natural period. The natural frequency ω_n (T_n = 2π I ω_n) of such a flap is defined as

where k_p is the pitch stiffness, which is calculated as

k_p = O.S(p_w - pf)W_fT_fd*g + m_cp_cg

(The parameters defined by the symbols in these equations are defined below in Table 1)

I_a is the added moment of inertia, which is calculated with a wave interaction analysis software programme (WAMIT (2006) - from WAMIT, Inc. Chestnut Hill, MA 027467-2504, USA) and J is the flap moment of inertia, which is calculated as

Using these equations for the flap device of the dimensions discussed above [k_p = 9.6 MNm/rad; I = 2 -10⁶ kg m²; T_a = 60 -10^s) gives a natural period of 16 seconds. If the dimensions for a flap with counterweight are now chosen as in Table 1 below:

Table 1

Then the resulting pitch stiffness is calculated as 10.6MNm/rad and I₁ the moment of inertia of the flap is 2.4 -10⁶ kg m².

This pitch stiffness can be split into contributions due to the buoyancy of the flap and the downward force on the counterweight. Their respective values are 5.8 and 4.8 MNm/rad.

The added moment of inertia for a flap of these dimensions, calculated with WAMIT, is shown in figure 9.

The natural pitching period for this design is calculated to be 10.6 seconds (J_a = 27.5-10⁶ kg m²) . This is significantly lower than for the flap without a counterweight described above, bringing the natural period closer to the dominant wave period. Because the flap moment of inertia is practically- negligible compared to the added moment of inertia the natural frequency of the flap depends mainly on the ratio k_p/I_a. Changes in flap height influence the added moment of inertia greatly as it is proportional to d³. This means that added moment of inertia plays a big role together with pitch stiffness when altering the natural period of a flap by changing the height, whereas pitch stiffness is the only influencing factor when keeping the geometry the same .

This means that when operating in relatively shallow waters it is desirable for the upper portion of the flap, above the pivot axis to be as big as possible for a given width, to allow good energy extraction. Therefore the effect of changing the biasing mass (counterweight) becomes important The reduction in height of the upper portion of the flap above the pivot axis necessitated by providing the biasing mass below it should be kept to a minimum to allow maximum energy extraction for a given arrangement .

As an example, to show the beneficial effect of increasing the mass of the counterweight while keeping the geometry constant the mass is doubled for the flap with the counterweight defined in table 1 above. Doubling the mass to 326 Tonnes increases the flap' s moment of inertia to 3.9-10⁶ kg m² and it doubles the part of the pitch stiffness that is due to the counterweight (this relates linearly to gravitational force and therefore also to mass) . This results in a total pitch stiffness of 15.4 MNm/rad. The added moment of inertia remains constant because the geometry of the flap doesn' t change (any small change in geometry that might be necessary to accommodate the increased size of the weight will be relatively close to the hinge which means that the influence on added moment of inertia will be small) .

These changes would bring the natural period of the oscillator (flap) down to 9.2 seconds (I_a = 28.9-10⁶ kg m²) , which is again closer to the dominant wave period in the north Atlantic.

It should be noted that increasing the moment arm of the biasing mass (moving the centre of mass of the counterweight further away from the pivot axis) would have a similar effect of shortening the natural period.

Doubling this parameter decreases the natural period by a slightly smaller amount than doubling the mass does because the flap' s moment of inertia increases by the square of this distance and increasing the moment arm will also increase the added moment of inertia as more of the body is in contact with the water at a greater distance from the pivot axis.

Claims

1. A wave energy conversion device, for use in relatively shallow water, comprising: a base portion formed and arranged for anchoring to the bed of a body of water in use of the device; a flap portion pivotally connected to said base portion, said flap portion being biased to the vertical in use and formed and arranged to oscillate, in use, backwards and forwards about the vertical in response to wave motion acting on faces of the flap portion; said flap portion comprising an upper portion upstanding above the pivot axis for extracting energy from the wave motion and a lower portion, said lower portion having a biasing mass located below the pivot axis and providing a restoring force acting to bias the flap to the vertical when the flap portion oscillates; and power extraction means for extracting energy from the movement of the flap portion.

2. A device as claimed in claim 1 wherein the weight provided by the mass below the pivot axis in the flap portion is equal to or greater than the resultant weight of the flap portion above the pivot axis.

3. A device as claimed in claim 2 wherein the weight provided by the mass below the pivot axis in the flap portion is greater than the resultant weight of the flap portion above the pivot axis .

4. A device as claimed in claim 3 wherein the weight provided by the mass below the pivot axis is from 1 to 4 times more than the weight provided by the resultant weight of the flap portion above the pivot axis.

5. A device as claimed in claim 1 wherein the moment provided by the mass below the pivot axis in the flap portion is greater than the moment of the flap portion above the pivot axis .

6. A device as claimed in any preceding claim wherein the buoyancy of the flap portion is adjustable.

7. A device as claimed in any preceding claim wherein the flap portion has a high centre of buoyancy and a low centre of mass .

8. A device as claimed in any preceding claim wherein the lower flap portion is shaped to minimise the drag forces as it moves through the water.

9. A device as claimed in claim 8 wherein the flap portion below the pivot axis comprises one or more keel shaped masses.

10. A device as claimed in claim 8 wherein the lower flap portion comprises a substantially cylinder shape concentric with the pivot axis and the cylinder shape has a concentration of mass below the pivot axis.

11. A device as claimed in any preceding claim wherein the base portion includes at least one deflector plate, formed to deflect wave motion away from the lower flap portion and onto the upper portion of the flap, above the pivot axis .

12. A device as claimed in any preceding claim further comprising additional biasing means for the flap portion.

13. A device as claimed in any preceding claim formed and arranged to have a flap portion natural period of less than 15 seconds .

14. A device as claimed in any one of claims 1 to 12 formed and arranged to have a flap portion natural period of between 5 and 13 seconds.

15. A device as claimed in any one of claims 1 to 12 formed and arranged to have a flap portion natural period of between 6 and 10 seconds.

16. A device as claimed in any preceding claim formed and arranged so that when the base portion is anchored to the bed of a body of water with the flap portion facing the wave motion, the base portion and the flap portion extend vertically through at least the entire depth of the water, to present a substantially continuous surface to the wave motion throughout the full depth of water from the wave crest to the sea bed.

17. A device as claimed in any preceding claim wherein the base portion and the flap portion are formed and arranged to operate substantially without a gap between them.

18. A device as claimed in any preceding claim wherein the flap portion and the base are sized so that the flap will pierce the water surface at all expected tide levels and sea states .