EP2057374A2

EP2057374A2 - Apparatus for converting wave energy into electricity

Info

Publication number: EP2057374A2
Application number: EP07789210A
Authority: EP
Inventors: Dennis James Carey
Original assignee: Neptune Energy Ltd
Current assignee: Neptune Energy Ltd
Priority date: 2006-08-15
Filing date: 2007-08-14
Publication date: 2009-05-13
Also published as: WO2008020190A2; GB2440945B; WO2008020190A3; GB2440945A; GB0616210D0

Abstract

An apparatus for converting wave energy into electricity comprises an inlet, an inlet chamber coupled to the inlet and arranged to allow water to flow from the inlet to the inlet chamber. The inlet and inlet chamber form an oscillating water column. An outlet for connection to electricity generating means and an outlet chamber in the form of a reservoir coupled to the outlet are arranged to allow water to flow to the outlet. Between the oscillating water column and reservoir, a gas chamber is arranged such that that a gas in the gas chamber may be trapped by water in the inlet chamber and the outlet chamber. A weir is arranged in the gas chamber such that water may flow over the weir. Periodic variation in pressure of water at the inlet causes water to periodically oscillate in the oscillating water column transmitting pressure to the reservoir by compression of gas in the gas chamber forcing water to pass through the outlet. The outlet chamber is arranged between the inlet and inlet chamber, thereby allowing a larger than normal radius of curvature of the flow path and avoiding detrimental effects such as vortex shedding.

Description

APPARATUS FOR CONVERTING WAVE ENERGY INTO ELECTRICITY

FIELD OF THE INVENTION

The invention relates to apparatus for generating electricity by harnessing wave energy.

BACKGROUND OF THE INVENTION

Large bodies of water present a vast source of renewable energy in the form of winds, waves and tides. Devices designed to extract energy from waves harness the kinetic and potential energy of a wave and convert a portion of it into a useful form such as electrical energy. Such devices can be used in any environment where natural water waves are present such as oceans, lakes, estuaries etc.

Several apparatus for generating electricity from wave power, henceforth referred to as wave generators, are known, the most common being the oscillating water column (OWC). An OWC can consist of a partially submerged hollow structure open to the sea below the water line, the structure essentially enclosing a volume of air above a column of water. OWC systems use the motion of the waves to compress air within a container. Incident waves cause the water column to rise and fall, which alternately compresses and decompresses the air column. The air flows to and from the atmosphere via a turbine through which energy is extracted from the system and used to generate electricity. OWCs are the most widely used design for wave generators, the most common being positioned on or near the shoreline, or on breakwaters, etc.

A "tapchan" or "tapered channel" device consists of a reservoir positioned a few metres above sea level and a tapered channel that is wider at the mouth (which is open to the sea) and becomes narrower as it penetrates the reservoir. Incoming waves increase in height as they move up the channel, eventually overflowing the lip of the channel and pouring into the reservoir. Tapchan devices convert the kinetic energy of the wave into potential energy, which is subsequently converted into electrical energy by a generator as the water is fed back to the sea through a pipe.

Existing devices designed to generate electricity from wave power experience several problems. Devices located at or near the water surface experience large wave impact loads in extreme sea states and weather conditions. They are therefore more prone to wear and damage. Fully submerged devices have been proposed but have contained too many moving parts to be reliable and regular servicing is more difficult for such devices.

The inventors have appreciated that it would be desirable for an oscillating water column wave generator to be fully submerged in order to reduce the effects from extreme sea states and so that it may be placed on or near the seabed. The inventors have further appreciated that a simplified wave generator with a minimal number of moving parts would reduce the need for servicing and provide a far more robust device than previous designs, and should function over a range of water depths.

The inventors have also appreciated that the main source of energy loss, particularly in an oscillating flow, is large scale vortex shedding from surfaces where there is a rapid change in flow direction such as around sharp corners and that it would be desirable to reduce the effect of vortex shedding by introducing more streamline flow in critical areas.

SUMMARY OF THE INVENTION

The invention is defined in the claims to which reference is now directed.

An embodiment of the invention provides an apparatus for converting wave energy into electricity in the form of an omni-directional, fully submerged, resonating oscillating water column, in which the arrangement of input and output ducts, and the arrangement of the reservoirs and weir, achieve vastly improved hydrodynamic flows, and the effective elimination of vortex shedding, all achieved without the inclusion of moving parts. Resonant oscillations are caused in an oscillating water column open to the sea via an inlet due to periodic pressure variations from the change in sea level as waves pass overhead. The oscillating water column contains an inlet chamber coupled to the inlet and arranged to allow water to flow from the inlet to the inlet chamber. A reservoir in the form of an outlet chamber is separated from the inlet chamber by a volume of gas in a gas chamber and a weir is positioned in the gas chamber so that water can flow from the inlet chamber to the reservoir when oscillations in the OWC are large enough. The periodic pressure variations are transmitted to the reservoir by compression of the volume of gas and flow of water over the weir and results in water flowing through an outlet connected to the outlet chamber and through electricity generating means such as a turbine, which may be located between the reservoir and the outlet. The phenomenon known as vortex shedding is reduced within the apparatus by arranging the outlet chamber between the inlet and inlet chamber.

Water flowing in the inlet and through to the inlet chamber undergoes a substantial change in flow direction. Positioning the outlet chamber between the inlet and inlet chamber allows the radius of curvature within the flow path of the water to be increased. By providing a flow path with large curvature, as opposed to sharp edges, vortex shedding can be dramatically reduced and improve the efficiency of the apparatus.

As a preferred feature, the outlet chamber is arranged between the inlet and the inlet chamber by defining a wall of the inlet chamber by the outer wall of the outlet chamber. Additionally a portion of the inlet chamber may be curved around and/or underneath the outlet chamber.

Preferably, a means to direct flow in the form of a centre body cone is provided between the inlet and the inlet chamber to introduce greater radial curvature in the flow path of the water from the inlet to the inlet chamber and further reduce vorticity and turbulence within the apparatus. This centre body could, for example, have a hyperbolic or parabolic profile. As a further preferred feature the dimensions of the inlet chamber and the volume of the gas chamber are such that the natural frequency of the periodic oscillations of water within the inlet chamber is substantially close to that of the periodic variation in pressure of water at the inlet.

BRIEF DESCRIPTION OF THE FIGURES

An embodiment of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 shows an example of a known wave generator that uses an oscillating water column; Figure 2 shows a simplified diagram of a "U tube" to illustrate the principles of operation of a fully submerged oscillating water column; Figure 3 shows a known arrangement of a fully submerged wave generator; and Figure 4 shows a schematic diagram of a device embodying the invention.

DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

An embodiment of the invention uses a fully submerged oscillating water column. An example of a known oscillating water column is shown in figure 1. A structure 101 encloses a volume of air 102 above a column of water 104 which is open to the sea. Incoming waves 103 cause a change in the height of the water level 105 inside the structure 101. Air in the volume 102 is alternately compressed and decompressed as the water level rises and falls respectively. If a path 106 is provided to the atmosphere, air will flow back and forth through turbine generator 107 and generate electricity. Whilst figure 1 shows a shoreline device that can be located on cliff-sides and coastal regions, this design can be adapted for deepwater however the design requires exposure to the atmosphere to function.

OWC wave energy devices can be tuned to any predominant frequency of the incoming waves by altering the dimensions of the device. The natural period of oscillation of an OWC open to atmospheric pressure is exactly the same as the period of a deep water wave of length λ when the length of the water column L = λ/2π, or approximately 20m for an average Atlantic swell, where λ = 120 m.

When a wave system impinges on the inlet to an OWC of appropriate length, the fluctuating sea pressure forces the water column to oscillate at its natural frequency, with a substantial increase in amplitude compared with the incident wave. This condition is known as resonance and the wave energy captured by the OWC is greatly enhanced.

A completely submerged OWC must be connected to an enclosed air space, the purpose of which is to provide the pneumatic stiffness necessary to maintain the water column in an oscillating condition. The volume of the air space, in the static situation, is ideally approximately equal to the volume of the OWC.

The principles of operation of a fully submerged oscillating water column will first be discussed and can be best understood with reference to a simplified "U tube" device as shown in figure 2.

Figure 2 shows a simplified known device with an inlet 201 , an outlet 202, an oscillating water column in the form of a U tube 203, a reservoir 204, a volume of enclosed air 205 and a weir 206 separating the U tube 203 from the reservoir 204.

The combination of a U-tube 203, air chamber 205, and reservoir 204, provide a dynamic system in which a continual interchange takes place between the kinetic and potential energy of the wave-train with fluctuating hydrostatic, hydrodynamic and pneumatic pressures within the wave generator, which create accelerating and decelerating fluid particles which result in oscillating velocities and liquid levels in regular phased relationships to each other.

Waves passing above the inlet increase the water pressure in the column causing the water in the U tube to oscillate on one side of the weir with an amplitude that varies with both the wave frequency and height of the approaching waves. The amplitude of oscillation of a passing wave is amplified within the U tube such that water moves a further distance up the tube than the height of the passing wave. At some stage water in the U tube will spill over the weir 206 into the reservoir 204, where it becomes trapped resulting in a rise of the internal air pressure.

As water spills into the reservoir the air space is compressed and pressure is raised above the surrounding hydrostatic pressure so that hydraulic power is available to drive a water turbine. Once the device accumulates a sufficient quantity of water in the reservoir, a pressure differential is produced capable of driving electricity generating means 209. A steady state oscillatory operation is achieved when the outflow through the turbine is matched to the inflow over the weir. For a device such as that shown in figure 2 to function correctly the diameter of the outlet needs to be adjusted to the sea conditions in which the device is operating.

When the OWC is subjected to waves close to its natural frequency, resonance will occur. At resonance the amplitude of the passing wave can be magnified by

3 to 4 times providing the greatest possible variation in the water level 207. The natural frequency of the OWC depends upon the dimensions of the device, particularly the length of the water column and the volume of air enclosed, which would ideally be tuned for the particular environment in which it is intended to be used.

The air within the air volume 205 performs several functions. Since air resists compression it behaves as a "mechanical spring", without which the U tube would not oscillate. As the reservoir fills up due to overtopping over the weir, the volume available for the air to occupy decreases. The resulting increase in the air pressure adds to the restoring force provided by gravity, which is necessary for oscillation of the water within the L) tube. It also stores energy to create hydraulic power since the increase in pressure that results provides an extra pneumatic effect that forces water through the turbine. The air absorbs energy over several wave cycles as more water spills over the weir. This creates a smoother power output since output power is, to a certain extent, decoupled from the input wave power. Furthermore, in extreme sea states, the air prevents possible overloads by de-tuning the OWC from waves of large amplitude. The air detunes the device since under increased pressure it forces water in the U tube 203 below the weir height, effectively shortening the OWC, changing the dimensions and raising its natural frequency above the frequency of the waves.

Allowing water to flow over the weir from the oscillating U tube into the reservoir converts an oscillating input flow into a continuous outflow and prevents water from flowing back and forth through the turbine. This is analogous to a diode, in electrical terms, to rectify an alternating current into a direct current. The mass of air contained in the air volume 205 is fixed; consequently when the volume is reduced (compressed) by water level 207 rising up the OVVC, the pressure must rise and push water out of the reservoir, which is replaced as more water spills over the weir. The water always flows in one direction through the turbine because when the inlet and outlet flows are matched to the prevailing sea conditions the internal air pressure is always greater than the external hydrostatic/sea pressure at the turbine/outlet 209/202. For the air pressure to be greater than the pressure at the outlet it is necessary for the outlet to be located at a greater depth within the sea/water than the water level in the reservoir.

Figure 3 shows a known configuration of a fully submerged wave generator featuring an omni directional oscillating water column.

An inlet 301 is provided that is open to the body of water in which the device is placed. A 3-dimensional annular "U tube" OWC, similar in functionality to the U tube of figure 2, is formed from an inlet chamber, in the form of an inner annulus 302, and inlet 301. An outlet chamber is provided in the form of a reservoir 303 into which water spills when oscillations in the OWC, formed from inlet 301 and inner annulus 302, exceed the height of the weir 304, the weir comprising the separating lip between the inner annulus and the reservoir. An outlet is provided in the form of outlet pipe 310, which comprises of a cylindrical tube connected to the reservoir 303 at one end and open to the body of water in which the device is submerged at the other. A means of generating electricity 311 is provided within the outlet pipe.

The apparatus is fully submerged with air in gas chamber 305. Water in the inner annulus 302 and reservoir 303 traps the air in the gas chamber. The water pressure in the inner annulus is dependent upon the height of water above the inlet 301 , which varies as waves pass over. As a wave passes over above the inlet, the pressure in the water column increases, causing the water level in the inner annulus to rise. Air trapped in the gas chamber 305 is compressed as the reservoir height increases and water spills over the weir. The pressure differential due to the compressed air forces water in the reservoir through a turbine and generates electricity. The reservoir has a larger width (the perpendicular distance between weir 304 and the outer wall) than that of the inner annulus. Oscillations within the inner annulus, as a result of waves passing overhead, will have a greater amplitude than oscillations within the reservoir and water will not be able to flow back over the weir from the reservoir to the inner annulus.

An embodiment of the invention provides an omni-directional, fully submerged, oscillating water column. Figure 4 shows a cross section of such a device.

The apparatus comprises of three hollow cylinders arranged concentrically about an inlet in the form of inlet 401 that is open to the body of water in which the device is placed. A 3-dimensional annular "U tube" OWC, similar in functionality to the U tube of figure 2, is formed from an inlet chamber in the form of an outer annulus 402 (defined by the annular spacing between cylinders 407 and 408) and inlet 401 (as defined by cylinder 409).

An outlet chamber (defined by the annular region between cylinders 408 and 409) is provided in the form of a reservoir 403 into which water spills when oscillations in the OWC, formed from inlet 401 and outer annulus 402, exceed the height of weir 404, the weir comprising the separating lip on cylinder 408 between the outer annulus and the reservoir. The outer annulus connects to the inlet by curving underneath the reservoir.

An outlet is provided in the form of outlet pipe 410, which comprises of a cylindrical tube connected to the reservoir 403 at one end and open to the body of water in which the device is submerged at the other. The outlet does not need to be aligned with the passing waves and there can be more than one outlet connected to the reservoir. A means of generating electricity 411 is provided within the outlet pipe(s) and is preferably a turbine. The apparatus is fully submerged with air in gas chamber 405. Water in the outer annulus 402 and reservoir 403 traps the air in the gas chamber. The water pressure in the outer annulus is dependent upon the height of water above the inlet 401 , which varies as waves pass over. As a wave passes over above the inlet, the pressure in the water column increases, causing the water level in the outer annulus to rise. Air trapped in the gas chamber 405 is compressed as the reservoir height increases and water spills over the weir. The pressure differential due to the compressed air forces water in the reservoir through a turbine and generates electricity. As with the device of figure 3, the reservoir has a larger width (the perpendicular distance between weir 404 and the inner wall 409) than that of the outer annulus, although they may still have the same cross sectional area. Oscillations within the outer annulus, as a result of waves passing overhead, will have a greater amplitude than oscillations within the reservoir and water will not be able to flow back over the weir from the reservoir to the inner annulus. Both the outer annulus and the inlet are arranged concentrically with rounded inlet geometry and duct curvature to minimise the possibility of vortex shedding and energy loss within the internal duct flow.

Vortex shedding creates eddies and turbulence, which destroys streamline flow, where fluid particles slide smoothly over each other. It occurs when any fluid, liquid or gaseous, is forced suddenly to change its direction of flow along or adjacent to a solid surface. For example, when the wind blows around the corner of a building, it often produces a local whirlwind that sucks up leaves and debris. It is the reason why cars, trains and aeroplanes are streamlined in shape for the purpose of reducing resistance to forward motion. When an aeroplane stalls, the airflow separates from the upper surface of the wing and a region of stationary or stagnant air is created above the wing with a loss of lift and increase in drag. Vortex shedding affects the integrity and behaviour of a wide range of engineering structures and systems, including tall chimneys, bridges, ship's hulls, marine structures and wave generators.

Research shows that over 90% of the kinetic energy flux within and around an oscillating water column, can be dissipated in eddies and turbulence, with similar detrimental effects on the energy captured, unless potentially critical areas of flow are identified and eliminated by careful design.

Predicting the onset of vortex shedding and associated turbulence in critical areas, in advance of experimental verification, particularly in periodic and unsteady flows, is a highly specialised branch of engineering and an outstanding problem in fluid mechanics that remains in the field of experimental and theoretical research and expert knowledge.

In the known embodiment shown in fig 3, a sudden reversal in flow direction of

180° is required by the OWC around the sharp corner at 313, the most critical region of the design. Vortex shedding is highly probable in this region where large-scale turbulence will be carried down stream on either side of the duct wall as the flow oscillates back and forth, with an associated loss of kinetic energy. A similar negative effect would apply at corners 312 and 314. All these areas would be critical to energy loss.

By reversing the position of the OWC and reservoir, as shown in the embodiment of figure 4, the required reversal in flow direction by the OWC is achieved more gradually by increasing the radius of curvature of the duct at 412 and 413, so that the possibility of vortex shedding in this region is greatly reduced. The narrowing of the flow volume between the inlet and the outer annulus occurs far more gradually than, for example, in the design of figure 3. Consequently a smoother streamlined duct flow can be anticipated with substantial improvement in the energy captured.

Additionally, vortex shedding and turbulent flow in the inlet is greatly reduced by including a centre body in the form of a hyperbolic cone 414 within the inlet 401 , which serves to introduce curvature in the path of the water as it passes from the inlet into the outer annulus.

The embodiment shown in figure 4 has, in fact, been subject to large-scale experimental verification and is known to be an acceptable design. The effect of including the length of the bend in the OWC also has the beneficial effect of reducing the overall height of the device. For optimum power output the apparatus is required to remain close to resonance for as broad a spectrum of wave frequencies as possible. The natural frequency of the apparatus is dependant on the length of the OWC, and the volume and pressure of the air in region 405. Each apparatus will therefore be tuned, through these parameters, to optimise efficiency in the light of prevailing conditions of depth and wave characteristics. For example, the inlet of a device such as that of figure 4 should have a diameter of approximately λ/2π, or approximately 20m for an average Atlantic swell, where λ ~ 120 m.

By altering the curvature within the OWC it is possible to alter the range of frequencies over which the device is responsive. The device shown in figure 4 remains at or near resonance over a large range of incoming wave frequencies and can be said to have a broad bandwidth response. A device with a broad bandwidth response will be near resonance over a broader range of incoming wave frequencies but will exhibit a smaller maximum oscillation amplitude to a device with a narrow bandwidth response being tuned to a narrower range of frequencies. A broad bandwidth response is more practical for average sea conditions. The embodiment of the invention shown in figure 4 combines a low cost, easy to build design with a broad bandwidth response.

Figure 4 shows the outer annulus 402 as forming part of the OWC in conjunction with the inlet 401. In the known embodiment of figure 3 the OWC may be formed from the inlet 301 and inner annulus 302 with the outer annulus being the reservoir 303. Due to the arrangement of the reservoir between the inlet and the outer annulus, the embodiment of figure 4 incorporates a larger bend radius in the geometry of the OWC than that of figure 3, achieving the appropriate length of water column within a more compact configuration. The result is a virtual elimination of large scale vortex shedding due to the increase in the radius of curvature of the OWC, particularly curving regions 412 and 413, within the structure. Smoother oscillatory flow within the structure is achieved in comparison to devices such as the one shown in figure 3 and overall energy capture is dramatically improved. While the apparatus may be formed from hollow cylinders to provide the annular regions that form the inlet, the outer annulus and reservoir, it should be appreciated that other shapes could be used.

A duct may be provided for the purposes of injecting gas into the gas chamber

405. Properties of the gas such as pressure and composition may then be controlled to adjust the resonance effect.

The weir height can be designed to optimise the power output of the device for the sea conditions typical of its proposed location. The cross sectional area of the

U tube may be constant, convergent or divergent along its length, and annular in form, for the purpose of maximising the energy capture capability of the device and matching its performance to the to the local wave energy spectrum.

The electricity generating means is preferably a water turbine however it will be appreciated that any suitable generating means could be used. Although the electricity generating means can be located in the outlet pipe 410, this is not essential. Two or more devices could be connected together from their outlets and the electricity generating means could be located at or near the connecting junction.

By placing several devices such as the embodiment of figure 4 in a regular array, constructive hydrodynamic interference can result in an increase in the overall performance and efficiency of each individual device and a reduction in the number of wave generators required. The optimum spacing at which individual devices are positioned depends upon the sea conditions and the dimensions of the devices themselves.

The principle features of a device embodying the invention are that it is stationary, completely submerged, positionable on the sea bed, removed from wave impact damage in extreme seas, contains no moving parts, and that the water in the OWC U tube is tuned to resonate over a range of wave frequencies, so that the amplitude of movement inside the U tube is substantially increased in relation to the incident wave amplitude outside. A device embodying the invention may be constructed with a cellular structure consisting of a double skin of steel reinforced laminated concrete, and could incorporate sand/gravel as ballast. The walls would be stiffened and stabilised with appropriate webs, suitably contoured to minimise interference with the internal water flow. These webs may be radially disposed vertical webs of solid section. The device could instead be constructed from a buoyant steel structure and allowed to float at a particular depth.

Advantageously, sprayed and laminated construction would be used as it produces a smoother and denser surface finish compared with conventional shuttered concrete methods, with greater resistance to marine growth and a corresponding benefit to the hydrodynamic performance and maintenance requirements of the structure.

Units may be constructed in sheltered water near the chosen installation site, from the base upwards, using a floating mould with the aid of floating slipforms and pre-cast cellular elements. These elements could be pre-cast on site as construction proceeds, the outer walls being fabricated in steel reinforced laminated concrete, and interlocked together by a proprietary process to form a homogeneous cellular structure of great strength and rigidity.

The spray and laminate process produces a very high quality and highly reinforced concrete at the outer surface of the structure, with much higher strength/weight ratios when compared with conventional vibrated concrete. The process places the steel content to the outside of the structural element away from the neutral axis, and where the reinforcing steel is more structurally effective.

The complete unit could then be floated to the open water installation site and flooded to sink it in the chosen position. Suitable ballast materials could be gathered from local sources and pumped into interconnecting cellular void spaces to provide negative buoyancy as required. The unit could then be re- inflated with air via an air duct until the volume of water within the OWC is sufficient for resonance and oscillation of the water column occurs. For maintenance purposes the whole unit may be re-floated and removed by pumping with air and dumping the sand ballast, or the turbine/generators can be detached and raised separately to the surface.

It will be appreciated by the skilled person that examples of construction methods are for illustration only and that other construction methods and materials could be used.

Claims

1. Apparatus for converting wave energy into electricity comprising: an inlet; an inlet chamber coupled to the inlet and arranged to allow water to flow from the inlet to the inlet chamber; an outlet for connection to electricity generating means; an outlet chamber coupled to the outlet arranged to allow water to flow to the outlet; a gas chamber arranged between the inlet chamber and outlet chamber such that that a gas in the gas chamber may be trapped by water in the inlet chamber and the outlet chamber; a weir arranged in the gas chamber such that water may flow over the weir from the inlet chamber to the outlet chamber; the inlet, inlet chamber, outlet chamber and outlet being arranged with respect to one another such that periodic variation in pressure of water at the inlet causes water to periodically oscillate in the inlet chamber, wherein pressure is transmitted from the inlet chamber to the outlet chamber by compression of gas in the gas chamber; the dimensions of the apparatus being such that the resulting variation in gas pressure causes water from the outlet chamber to pass through the outlet; wherein the outlet chamber is arranged between the inlet and inlet chamber.

2. Apparatus for converting wave energy into electricity according to claim 1 wherein a wall of the inlet chamber is defined by the outer wall of the outlet chamber.

3. Apparatus for converting wave energy into electricity according to claim 1 or 2 wherein a portion of the inlet chamber is curved around the outlet chamber.

4. Apparatus for converting wave energy into electricity according to claim 3 wherein the curved portion of the inlet chamber curves under the outlet chamber.

5. Apparatus for converting wave energy into electricity according to any proceeding claim wherein the outlet chamber is arranged concentrically between the inlet and the inlet chamber.

6. Apparatus for converting wave energy into electricity according to any proceeding claim wherein a centre body is provided between the inlet and the inlet chamber to introduce curvature in the flow path of the water from the inlet to the inlet chamber.

7. Apparatus for converting wave energy into electricity according to claim 6 wherein the centre body is a hyperbolic or parabolic cone.

8. Apparatus for converting wave energy into electricity according to any proceeding claim in which the dimensions of the inlet chamber and the volume of the gas chamber are such that the natural frequency of the periodic oscillations of water within the inlet chamber is substantially close to that of the periodic variation in pressure of water at the inlet.

9. Apparatus for converting wave energy into electricity according to any proceeding claim wherein a gas duct is connected to the enclosed volume of gas allowing the volume and pressure of the gas to be controlled.

10. Apparatus for converting wave energy into electricity according to any proceeding claim wherein the weir height is adjustable.

11. Apparatus for converting wave energy into electricity according to any proceeding claim wherein the inlet, the inlet chamber and the outlet chamber are formed from the annular regions defined by a plurality of concentric hollow cylinders.

12. Apparatus for converting wave energy into electricity according to any proceeding claim wherein the electricity generating means is located between the outlet chamber and the outlet.