The present invention relates generally to wave energy devices which abstract energy inherent in waves, and particularly to the natural oscillating frequency of such wave energy devices in a body of water.

Water waves are caused by the interaction of three forces, namely wind, surface tension and gravity. Wind causes air to pass over the surface of the sea, which in turn disturbs the position of the water surface. The wave is formed as a response to this disturbance, by surface tension on the small scale and gravity when dealing with larger disturbances. As the length of time or the length of water (called fetch) over which the wind has been acting increases, so too does the height and length of the resultant wave. The amount of energy present in waves can be considerable.

Machines designed to abstract energy from waves utilise the movement and kinetic energy inherent in waves in fluids. Each wave has a peak and a trough exerting short period cyclic forces upon adjacent bodies. Typically wave energy absorption devices convert, by design, a portion of the energy in waves into a useful form, such as electrical energy. Such devices can be used, for example, in the offshore seaboard, lakes, estuaries and oceans or the like wherever natural water waves are encountered.

Wave energy devices can be classified based on the location of the device with respect to the shoreline, i.e. shoreline devices, nearshore devices and offshore devices.

Shoreline devices have the advantage of relatively easier maintenance and installation and do not require deep water moorings and long underwater electrical cables. The less energetic wave climate at the shoreline can be partly compensated by the concentration of wave energy that occurs naturally at some locations by refraction and/or diffraction. Nearshore devices are situated in shallow waters (typically 10m to 25m water depth). Offshore devices are situated in deeper water, with typical depths of more than 40m.

When considering the real sea state, there are many waves of different lengths and the displacement of the water surface is hence a varying frequency and varying amplitude signal. These variations present a difficulty for wave energy converters, namely the requirement of adaptability to different sea states.

Any buoyant body on the surface of a liquid will oscillate if it is given an initial displacement. An important characteristic of this wave energy device is that its natural frequency, i.e. that frequency at which it oscillates when unconstrained, is the frequency in which it must be excited to have maximum amplitude oscillation, in other words this is its resonant frequency.

If the device is to be used to extract power from its supporting liquid its efficiency will be a maximum when the frequency of the waves within the liquid equals the natural frequency of the floating body.

Normally the dynamic characteristics of floating wave energy devices are set by the design of fixed parameters, primarily the plan area and shape, the buoyancy and the mass of the device. The natural frequency of such a floating 'fixed form' device, (that is one not able to change its character), occurs in a narrow band of incident wave frequencies.

In order to maximise the power absorption from a variety of different frequency waves it is necessary to alter the resonant frequency.

According to a first aspect of the present invention there is provided a control system for a wave energy device, the system comprising means for adjusting the natural frequency of oscillation of the device in use.

The invention therefore relates to improvements to energy capture by controlled changes to wave energy devices to influence and change its dynamic characteristics.

The concept is that the features, (a means of tuning), will facilitate a wide band response from floating point absorber devices providing considerable enhancement to energy capture and production.

Energy capture and output will be enhanced by supplementing the device with tuning features that are capable of varying the dynamic characteristics of the floating device to harmonise with the incident wave frequency. The enhancement of energy absorption and conversion is considerable when the natural frequency of the device is capable of adjustment, (tuning), to match the incident wave frequencies. This tuning facility enables the device to maintain its natural frequency, (resonance), - over a wide band of incident wave frequencies. Such tuning features may also be utilised to 'detune' the device's resonance to assist with maintenance and aid storm survivability.

The adjustment means may comprise means for varying the water plan area of the device. The present invention may therefore be based on the principle of using the water plan area to affect the natural frequency. Further, the present invention also envisages dynamically changing the plan area in response to changes in wave conditions.

The adjustment means may comprise means for varying the buoyancy of the device. This may be achieved by variation to the plan area of the device. This may be accomplished by mechanisms that will enlarge or reduce this plan area.

The plan area variation may be accomplished by arranging an additional outer toroid of buoyancy chambers, open to the sea below normal water level. Each buoyancy chamber is fitted with suitable controllable valves/vents to control the passage of the fluid from the chamber and thus, upon operating, varying the

effective plan area of the device. By opening/closing the vents, progressively, the effective plan area may be progressively reduced/increased as required to effect tuning.

The adjustment means may therefore comprise one or more chambers which are selectively buoyant. For this purpose the or each chamber may comprise an air- side valve for selectively determining the buoyancy thereof.

The adjustment means may comprise means for varying the mass of the device. This may be achieved by the simple expedient of -arranging suitable buoyancy chambers that are capable of being flooded and evacuated with/of sea water under controlled conditions using a suitably designed system to effect tuning.

Changes to one or more parameters, (such as mass and plan area), can be used to affect the dynamic characteristics and the natural frequency response of the device.

Alternatively or additionally a number of passages or vents of suitable dimensions, passing vertically through the existing buoyancy chambers, may be provided such that fluid (water) is free to flow through these passages. These may each be fitted with suitable vents/valves to control the passage of the fluid and thus upon opening/closing effectively vary the plan area of the device. By opening/closing the passages progressively, the effective plane area can be progressively reduced/increased as required to effect tuning.

Other means including the utilisation of a fixed shape of such form that enables an effective change to the plan area when the device is in a dynamic state may also be used.

The system may include means for establishing the wave conditions in which the device is floating and adjusting the natural frequency of the device in response to the wave conditions.

A suitably programmed computer may be used to identify the operating conditions and apply a requisite variations to effect changes (such as the mass and/or the plan area) of the device. This will ensure the floating device's dynamic characteristics are tuned to ensure the natural frequency of the device is in harmony with the wave frequency incident upon the floating device.

In normal operating conditions the mechanisms to achieve tuning may be operated and controlled by a suitable system to ensure the device is 'tuned' to harmonize with waves incident upon the device.

The energy absorption characteristics and thus energy production from a floating- point absorber wave energy device equipped with such a system will be considerably enhanced.

According to a second aspect of the present invention there is provided a device having a control system as described herein.

Devices designed to absorb the energy of sea waves take .many forms. The principle of the present invention is that the performance of floating point absorbers can be enhanced with the application of tuning features to ensure the natural frequency (resonance) of the device may be varied with suitably designed controls, ideally over the full range of natural wave frequencies incident upon the device.

The system may, in principle, be applicable to many types of wave energy converters. For example, the device may comprise an oscillating water column. The oscillating water column type of device comprises a partly submerged structure which has an opening to the sea below the water line, thereby enclosing a column of air above a column of water. As waves impinge on the device they cause the water column to rise and fall, which alternatively compresses and depressurises the air column. This air is allowed to flow to and from the atmosphere through a turbine which drives an electric generator. Both conventional (i.e. unidirectional) and self-rectifying air turbines are known.

The device may comprise a heaving buoy. A heaving buoy consists of a floating body, typically cylindrical or spherical, following the water surface in the vertical plane and reacting either against, for example, the seabed or a submerged drag plate.

According to an alternative aspect there is provided a wave energy device comprising means for adjusting the natural frequency of oscillation of the device in use.

According to an alternative aspect there is provided a control system for a wave energy device, comprising means for varying the excursion of the device in response to waves.

According to an alternative aspect there is provided a wave energy device comprising means for varying the excursion of the device in response to waves.

The system may be adapted to provide for an increase and/or decrease in the excursion magnitude. Accordingly the system can be used not only to "rune" the device to match the incoming wave periods but also to "de-tune" the device to aid storm survivability, for maintenance and ease deployment of buoy.

The present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a section of a wave energy device formed according to a first embodiment of the present invention;

Figure 2 is a section of a wave energy device formed according to an alternative embodiment of the present invention;

Figure 3 is a section of buoyancy chamber formed according to an alternative embodiment of the present invention;

Figure 4 is a plan view of a buoyancy system formed according to an alternative embodiment of the present invention; Figure 5 is a section of the buoyancy chamber taken along line A of Figure

4;

Figure 6 is a section of a selectively buoyant chamber taken along line B of Figure 4;

Figure 7 is a section of an alternative selectively buoyant chamber; and Figure 8 is a section of an alternative selectively buoyant chamber.

Referring first to Figure 1 there is shown a wave energy device generally indicated 10. The device 10 is of the oscillating water column type and is shown floating in a body of water 15.

The operation of oscillating water columns is well known to those in the art and will not be described in detail. However, in brief the device 10 includes a column 1 1 which is partly submerged and is open to the sea below the water line 15 thereby enclosing a column of air 12 above a column of water 13. As waves impinge on the device 10 they cause the water column 13 to rise and fall which alternately compresses and depressurises the air column 12. The air is allowed to flow to and from the atmosphere through a turbine (not shown) which drives an electric generator.

The device 10 further comprises a series of buoyancy chambers 14 positioned around the circumference of the column 1 1. The device further comprises a plurality of selectively buoyant chambers 120 which in this embodiment are positioned radially outwardly of the buoyancy chambers 14.

The buoyancy chambers 14 form permanently buoyant structures which always contribute to keeping the device 10 afloat and contribute to the water plan area. The selectively buoyant chambers 120, on the other hand, are formed so that they can selectively form part of the effective water plan area i.e. the area of the device 10 in contact with the water 15.

The chambers 120 are provided with valves 121 on their air side and unlike the chambers 14 the chambers 20 are open on their water side. As shown on the left hand side of the Figure, with the valve 21 in an open position water 15 can enter the chamber 20 and assume the same level on the interior as on the exterior. On the right hand side of the Figure the valve 21 is shown closed which prevents water from rising into the chamber 20 so that the water level in the enclosed area moves essentially with the device 10 itself and the water plane area of the device is increased. Accordingly, closing one or more of the valves 21 increases the water plane area of the device and reduces the natural period of the device in heave. With the valves open the behaviour of the device is similar to that of a device without such additional buoyancy aids.

It is therefore possible to change the water plan area of the device by opening or closing more or less selectively buoyant chambers 20 and in doing so the natural frequency of oscillation of the device can be adjusted. This allows, for example, the natural frequency of the device 10 to be adjusted so as to match it with incident waves, i.e. to tune it. By matching the resonant frequency of the device 10 to incoming wave periods the maximum amplitude of oscillation of the device can be achieved which can be used to achieve increases in energy capture efficiency due to the movement of the air column 12. However, it is also possible to de-tune the response of the device so that its excursion is, for example, minimised. This could be useful, for example, to aid storm survivability, ease access for maintenance or ease deployment of the buoy.

Referring now to Figure 2 there is shown a device 110 according to an alternative embodiment. The device 110 is very similar to the device 10 shown in Figure 1 except that the selectively buoyant chambers 120 are laterally open to water 1 15 at openings 122. Again, on the left hand side of the Figure the chamber 120 is shown with a valve 121 in an open position thus allowing water 1 15 to enter the chamber 120 through the opening 122; whereas on the right hand side of the Figure the valve 121 is shown in a closed position thus restricting entry of water 115 through the permanently submerged opening 122.

Referring now to Figure 3 there is shown a selectively buoyant chamber 220 formed according to an alternative embodiment.

In this embodiment the chamber 220 includes a valve 221 on the air side of the chamber 220 and also a valve 223 on the water side of the chamber 220.

In this embodiment the valve 221 is a non-return valve and this means that air present in the chamber 220 can be pumped out rather than remaining to be compressed which can lower efficiency of operation. The valve 221 therefore has the function of a diode so that it can be used to reduce the volume of trapped air and reduce the undesirable effect of air compressibility.

Referring now to Figure 4 there is shown a buoyancy system 330 formed according to an alternative embodiment of the present invention.

The system 330 comprises a plurality of chambers arranged in a circular pattern so as to be arrangable around the perimeter of a wave energy device.

The system comprises two types of chambers designated 314 and 320.

As shown in Figure 5, the chambers 314 are completely enclosed and function as buoyancy aids at all times.

As shown in Figure 6 the chambers 320 are open on their water side and include a vent 321 on their air side.

The chambers 314, 320 therefore have the same effect as the chambers 14, 20 shown in Figure 1. However, the arrangement of the chambers 314, 320 in Figure 4 is such that the chambers 320 are positioned on the same circumferential path as the chamber 314 rather than along a radially outwardly displaced path as shown in Figure 1. The basic trim of a device does not therefore have to be altered significantly.

Referring now to Figure 7 a selectively buoyant chamber 420 formed according to an alternative embodiment is shown for use in conjunction with the system 330 shown in Figure 4.

The chamber 420 is the same as the chamber 120 shown in Figure 2 in that it includes a lateral opening 422.

Figure 8 shows a further alternative chamber 520 in which not only a first lateral opening 522 is shown but also a second lateral opening 523 is provided.