Machines designed to abstract energy from waves utilise the movement and kinetic energy inherent in waves in a fluid. 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 include power harnessing means known as an absorber, which is the part of the device that actually abstracts energy; a particularly favoured absorber type is rotative. Such rotary absorbers can be used to abstract energy from waves either directly from the oscillatory flow within a body of water or from air motivated by an oscillating water column.

The efficiency of turbines and the like rotary machines is dependent upon the relationship between the velocity of fluid travelling through it and the speed of rotation of the rotor. A particular problem is presented to optimising rotary absorbers in wave devices because of the variability of the fluid velocity flowing

through them due to the variability of the amplitude and frequency of natural incident waves.

The present invention seeks to improve the efficiency of wave energy devices incorporating rotative absorbers resulting in increased energy production from machines designed to absorb the kinetic energy present in natural waves.

According to a first aspect of the present invention there is provided a control system for a device which abstracts energy inherent in the waves in a fluid and comprises a rotary absorber which is caused to rotate by fluid flow generated by the waves, the control system comprising means for adjusting the speed of the absorber rotor based on the velocity of the fluid flow to optimise the efficiency of the rotor.

The present invention may be based on adjusting the speed of the rotor in relation both to the general magnitude of the wave motion and also in response to the oscillating flow generated by each individual wave in such a way that the efficiency is enhanced.

In order to achieve the necessary speed control the control system may have the ability to put energy into the turbine and associated components as well as to extract energy. For example, the control system may be such that at any instant in time the flow of power could be into or from terminals of the device. That is to ;

say that the device can be either a source or a sink of energy. This does not preclude the case where there is no power at the tenninals.

The efficiency of any turbine-based device depends crucially on the flow through the device as compared with the flow at its design point for a particular speed of rotation. Accordingly, the control system may be operable to match the drive train characteristics and control system to the sea state by the use of electrical generators and control equipment where the speed of the rotating machinery is matched in an optimum way to the sea state, to extract maximum energy available.

The speed of the rotary absorber may be dynamically modified in line with the sea state, maintaining the optimum relationship between the flow through the absorber and its rotational speed, improving overall efficiency.

The control system may include means for predicting the fluid flow velocity.

Successive waves from which energy is to be abstracted will be different in amplitude and frequency. Instantaneous adjustment of the rotor speed to take account of fluid flow on a real-time basis may not be possible and therefore a prediction of the fluid flow velocity may be used. In practice several parameters, such as wind conditions, seasonal conditions, the velocity of fluid currently flowing through the rotor and monitoring equipment upstream of or on the device, may be used to predict the velocity of fluid likely to travel through the rotor.

The predicted value, or a likely range of values can be used to select a speed or a range of rotor speeds within which the rotor can work efficiently. The adjustment means can then act to keep the rotor speed within the selected range. Accordingly instantaneous adjustments in response to individual waves may not be effected, but rather periodic adjustments may be made to maintain rotor speed within a set range to gain the best average efficiency.

The control system may include means for calculating the fluid velocity. For example, the actual fluid velocity of a current wave may be used in conjunction with the means for predicting fluid flow to arrive at a likely range within which the velocity of the next wave will fall.

The calculating means may comprise an accelerometer on the device itself.

The control system may comprise a programmable logic controller (PLC). The PLC may be pre-programmed with required working parameters.

The means for adjusting the speed of the rotor may comprise means for varying the energy output of the absorber. The more energy output the less energy is retained for rotation and therefore the speed can be reduced.

The means for adjusting the speed of the rotor may comprise means for inputting energy into the absorber.

The control system may therefore have the ability to put energy into the device as well as to extract it.

The speed of the rotor can be changed by varying energy output and input to accelerate or brake the rotor.

In the case of a rotor linked to a generator the generator can be used to vary the speed of the rotor. For example, if the predicted fluid velocity dictates a reduction in rotor speed to match it to the fluid velocity then the loading on the generator can be increased. The generator can be overloaded by taking out more energy than the spinning member has in reserve. The generator can thereby cause deceleration of the rotor. Alternatively a friction braking system may be used.

Conversely, energy can be input into the generator to accelerate the rotor by reducing the loading and/or by applying energy to the rotor.

According to a second aspect of the present invention there is provided a device for abstracting energy inherent in the waves in a fluid, comprising a rotary absorber rotatable by fluid flow caused by waves, the device having a control system for adjusting the speed of the absorber rotor to optimise the relationship between its rotational speed and the velocity of the fluid flow.

The device may therefore have a control system of the type described above.

The absorber may be adapted to rotate in a single direction when acted upon by a fluid flow which alternates in direction. The absorber can then take advantage of both peaks and troughs of waves.

The absorber may comprise a turbine, such as an impulse turbine.

The absorber may comprise a turbine rectifier which, in use, rotates in one direction when acted upon by an alternating fluid flow. Accordingly the rotor rotates in the same direction regardless of the through flow of fluid.

The absorber may be driven, in use, by air flow forced through the absorber by the action of waves. Alternatively, the absorber may be driven, in use, directly by the oscillating fluid flow of the waves themselves.

The absorber rotor may have fixed geometry blades. The use of fixed geometry blades keeps the number of moving parts of the device to a minimum. This is particularly useful because devices are expected to operate maintenance-free for extended periods of time. Of course variable geometry blades could be used to increase rotor efficiency further and movable guide vanes may be provided. The geometry of such variable blades may also be under the control of the control system.

The device may include an oscillating water column linked to the absorber for causing fluid to flow through the rotor. Oscillating water columns, such as those

described in WO 95/27850 and WO 98/55764, provide an efficient method of causing air to be motivated through turbines under the action of waves to allow the energy therein to be harnessed.

In order to add power into and extract energy from the absorber, reluctance switching or solid state switching may be used. In very active sea conditions the device must remain reliable and therefore solid state power systems with no moving parts are advantageous.

According to a third aspect of the present invention there is provided a wave energy array comprising a plurality of wave energy devices described herein.

The control systems of the devices may be linked and energy transfer may be possible between devices in the array. Accordingly power can be moved from one part of the array to another to take account of localised wave conditions and to optimise the efficiency of each of the devices.

According to a fourth aspect of the present invention there is provided a method for optimising the operation of a rotary absorber forming part of a wave energy device, the method comprising the steps of : predicting the velocity of fluid caused to flow through the absorber by waves; and adjusting the rotational speed of the rotor based on the velocity of the fluid flow to optimise the efficiency of the rotor.

The method may comprise the steps of selecting an optimum rotor speed range based on the predicted fluid flow velocity and adjusting the rotor speed so as to be maintained with the selected range.

It is possible to employ a similar associated device coupled to a flywheel with inherently high inertia to provide a source or sink of energy to the system comprising one or more sets of devices. Alternatively the flow of electrical energy could be provided from associated turbine and electrical machine systems in an array of similar units where the input wave energy is varying in an unsynchronised manner. As a further alternative the general electrical supply system could provide such a source or sink.

A particularly effective method or achieving the necessary motor, generator and control technologies is by the use of switched reluctance electrical machine technology.

A switched reluctance electrical rotor and generator arrangement may be preferred as the rotary absorber.

Switched reluctance generators can operate over very wide speed range while maintaining efficiency at close to maximum, which allows for maximum capture efficiency.

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 diagrammatic section of a device for abstracting energy from waves formed according to the present invention; Figure 2 is a schematic representation of a control system for controlling the device of Figure 1; Figure 3 is a flow diagram illustrating the operation of a control system formed according to the present invention; and Figure 4 is a diagrammatic illustration of a wave energy device array formed according to the present invention.

Referring first to Figure 1 there is shown a wave energy device generally indicated 10. The device 10 comprising a cylindrical tube 15 having a float 20 at an upper end thereof and ballast 25 at the other end thereof. The float 20 and the ballast 15 are spaced apart along the length of the tube 15 to allow the tube 15 to float in a generally upright position in a body of water 30.

At the upper end of the tube out of the water 30 a rotor chamber 35 is attached in fluid connection therewith. The rotor chamber 35 houses a turbine arrangement 40 which in this embodiment is a turbine rectifier with a switched reluctance generator. The rotor chamber 35 opens into a plenum chamber 45.

In use of the device a column of water 50 moves up and down inside the tube 15 under the action of waves in the water 30, as described in detail in patent document WO 95/27850 (incorporated herein by reference). The oscillating movement of the column of water 50 inside the tube 15 motivates air to flow axially through the turbine 40 to spin the turbine rotor 41. Because the turbine 40 is a rectifier turbine the rotor 41 rotates continuously in one direction even though it is acted upon by an alternating fluid flow.

The velocity at which the air flows through the rotor 41 is designated V. The speed with which the rotor 41 rotates is designated S. Dependent on the specific form of the rotor it will operate at maximum efficiency only at a fixed relationship between the velocity V and the speed S.

In order to maximise the efficiency of operation of the rotor 41 the device 10 includes a control system 55 which can adjust the speed of the rotor 41 in line with changes in the velocity V due to variations in wave frequency and magnitude.

Referring now also to Figure 2 the control system 55 is linked to the turbine 40 in such a way that it can adjust rotor speed S. The control system 55 adjusts the speed S by varying the amount of energy output from the turbine generator 42 and furthermore has the ability to input energy into generator 42. Changes in the loading on the generator 42 can be used to cause acceleration or braking of the rotor 41. Therefore the turbine 40 may either be a source or a sink of energy. The

control system 55 can dynamically adjust the level of power flowing into or away from the turbine 40.

Referring now to Figure 3 the steps involved in determining the control exerted by the control system 5 5 are shown.

At box 60 the velocity V of fluid which will travel through a rotor is predicted.

The velocity V could be predicted by the use of a number of statistics including: wind speed and direction; historical data for wave patterns at that time of year; real-time fluid velocity values calculated on the device itself; and monitoring equipment positioned upstream of the device. The information required for the prediction calculation and the calculation itself may be performed by a processor on the device itself or at a remote location and then supplied, for example, by telemetry to the device.

The predicted velocity value or a range of likely velocity is designated A at box 65.

At box 70 the value or range of values A is used to select an optimum speed or range of speeds over which the rotor, in use, will function efficiently. For example, a range may be specified of +/-10% away from maximum rotor efficiency based on A.

The optimum rotor speed or range of speeds is designated B at box 75.

At box 80 the current rotor speed S is determined and is adjusted as required so as to be within the range B. Periodic adjustments can be made in order to adjust and maintain the rotor speeds as determined at any moment in time by B.

The breadth of the predicted range of velocity and consequently the speed range selected will be determined partly on the basis of the variability of wave conditions so that a good average efficiency can be achieved. It is not therefore necessary to guarantee maximum efficiency for each wave, but only to achieve efficiency within an acceptable range for as many waves as possible.

Referring now to Figure 4 there is shown an array of devices 110a to 110d of the same general type as the device 10 shown in Figure 1 except that the rotor chamber vents to the atmosphere rather than to a plenum chamber. The devices 110a to 110d are spaced apart over an expanse of water and accordingly may each be subject to varying wave conditions. Accordingly at any one time one or more of the rotors 140a to 140d may need to be accelerated or braked in order to operate at the speed determined by their respective control systems 155a to 155d as required for maximum possible efficiency. The control systems 155a to 155d are linked in terms of the information they supply; and the devices 110a to 110d are able to transfer electrical energy to other devices in the array. Accordingly if the rotor 140a requires accelerating and the rotor 140b requires braking, energy could be transferred from the device 11 Ob to the device 110a. Surplus energy from the array can be transferred away.