GB2110763A - Method and apparatus for extracting energy from water waves - Google Patents

Abstract

An elongate rotor is secured semi-submerged in a body of water with its longitudinal axis stable with respect to the mean water level and aligned with the direction of the wave movement. The rotor has compartments 2 on its surface which collect and retain water on one side and release it on the other side, the compartments being spaced along the length of the rotor. As the waves rise and fall the compartments fill alternately with water and trapped air, so as to force the rotor to rotate. Groups of rotors are preferably connected in oppositely rotating pairs to cancel out the overall torque, an electrical generator being driven by the rotors. The rotors are self-buoyant and tethered in position. <IMAGE>

Description

SPECIFICATION Method and apparatus for extracting energy from water waves The present invention relates to a method and apparatus for extracting energy from water waves using a rotor drivable by the waves.

Various mechanical devices have been proposed for extracting energy from waves in the sea and elsewhere. The energy available is considerable, particularly in oceanic waves such as those arriving at the west coast of Europe from the North Altantic. However, the energy must be extracted over large areas of sea and this together with the forces involved has imposed limitations on the devices so far proposed.

According to one aspect of the present invention I provide a method of extracting energy from water waves using a rotor rotatable by the waves, wherein the rotor is of elongate cylindrical configuration having positioned along it a plurality of longitudinally extending compartments uniformly spaced around its axis, and is secured partially submerged in the water with its longitudinal axis substantially stable with respect to the mean water level, each compartment being shaped to collect and retain water when on one side of the rotor and discharge it when on the other side.

According to another aspect of the present invention I provide apparatus for extracting energy from water waves comprising a rotor of elongate cylindrical configuration having positioned along it a plurality of longitudinally extending compartments uniformly spaced around its axis, and means for securing the rotor partially submerged in water with its longitudinal axis substantially stable with respect to the mean water level, each compartment being shaped to collect and retain water when on one side of the rotor and discharge it when on the other side.

A method and apparatus for extracting energy from water waves according to the present invention will now be described by way of example and with reference to the accompanying drawings in which: Figures 1 and 2 are diagrammatic views from opposite sides of part of a rotor in use in the apparatus; Figures 3 to7 are sectional views taken across the rotor at positions A to E respectively on Figs. 1 and 2; Figure 8 is a partial plan view of an apparatus incorporating the rotor of Figs. 1 to 7.

Figure 9 is an end view of part of the rotor showing a modified construction; Figure 10 is a diagrammatic sectional view of part of the rotor showing a further modified construction; and Figure ii is a diagrammatic sectional view of part of the rotor showing a yet further modified construction.

In the present method, travelling waves on the surface of a body of water are used to rotate a rotor which is positioned with its axis substantially stable with respect to the mean water level, so that the waves rise and fall relative to the rotor. A part of the rotor is shown in Figs. 1 to 7. This comprises a central hub 1 and a plurality of peripherally arranged compartments 2. The compartments are uniformly arranged around the hub axis and each have an inner wall 3 formed by the periphery of the hub, side walls 4 formed by longitudinally extending curved blades secured to the hub and end walls 5 formed by radially extending divider plates secured to the hub. The divider plates are spaced along the rotor at intervals substantially equal to the hub diameter.

The rotor is itself buoyant or is supported by a buoyant framework so that when placed in a body of water it floats partly submerged with the rotor axis substantially parallel to the mean water level. In Figs. 1 and 2 the rotor is positioned with its axis aligned to the direction of travel of the main wave fronts so that each wave passes along the rotor, the water level 6 varying along the length of the rotor. The wave movement considered at each point on the rotor causes the water level to rise as each peak approaches and then fall as each trough approaches. In the drawings the wave height is arbitrarily shown as being equal to the outer rotor diameter but it may be greater or less than this.

When the water level rises at a given point on the rotor the compartments having their openings facing generally upwards successively fill with water while air is trapped under the curved side walls 4 of the compartments having their openings facing generally downwards. This trapped air cannot escape sideways due to the end walls 5 and so produces an upward thrust on one side of the rotor and a consequent coupled about the rotor axis tending to rotate it.

When the water level then falls again water will be caught and held in the compartments having their openings facing generally upwards whereas it will drain out of the compartments having their openings facing generally downwards. The trapped water applies a downward load on the side of the rotor opposite to that in which the air was previously trapped and so produces a couple tending to rotate it in the same direction.

When freely rotating the motor will rotate in synchronism with the wave movement so that it rotates half a revolution (180 ) in half the wave period. When rotating at a slower speed than this there will be a torque developed on the rotor which is at a maximum when the rotor is stalled.

However, the maximum power output will be obtained at an intermediate speed and the rotor in Figs. 1 to 8 is shown rotating at a speed of approximately 1 /3rd of the synchro nous speed in order to illustrate the operation of the rotor in detail.

In Figs. 1 and 2 the diameter of the rotor has been greatly exaggerated in comparison to the wave length of the waves so as to more clearly show the rotor operation. This means that the end walls 5 of the compartments would be extremely close together and they have therefore not been shown in Figs. 1 and 2. Fig. 8 however shows the rotor diameter and compartments in their true relative pro portions. Also on Figs. 1 and 2 the level 7 of the water in the compartments is shown as well as the water level 6 outside the rotor. In Fig. 1 the waves are passing from right to left whereas in Fig. 2 they are passing from left to right.

At the top of the wave peak (position E on Figs. 1 and 2) water covers the rotor and air is trapped in the compartments having their openings facing generally downwards (see Fig. 7). The trapped air applies an inward thrust on one side of the rotor which turns the rotor as seen in Fig. 7 in a counter clockwise direction. As the water level 6 falls at the approach of a wave trough water is successively caught and held in the compartments having their openings facing generally upwards, producing an increasing downward thrust on the opposite side of the rotor as the upward thrust from the trapped air decreases due to the falling water level.

At position A (Fig. 3) the water level 6 is at its mean value, the combination of downward thrust from the water containing compartments above the water level and upward thrust from the air containing compartments below the water level on the opposite side maintaining the overall counter clockwise torque on the rotor. As the water level continues to fall and the rotor rotates, eventually all the air containing compartments are above the water level 6 and water containing compartments which have been carried round by the rotation of the rotor are each successively lifted above the water level until air can enter at the lower open end of the compartment and the water be vented. This venting action takes place over the zone between the intersection of lines 6 and 7 on Fig. 2 and the bottom of the wave trough and is shown in Fig. 4.Since each compartment must be lifted above the water level in order to be vented there is a small downward force produced which opposes the rotation of the rotor but this is overcome by the much larger downward thrust of the water containing compartments on the other side of the rotor, particularly as the venting occurs near the bottom of the rotor where the effective moment of the force is small.

At the bottom of the wave trough in position C venting is completed and as shown in Fig. 5 the rotor has water caught and held in the compartments having their openings facing generally upwards, so as to generate a torque in the counter-clockwise direction equivalent to that generated by the trapped air when at the wave peak shown in Fig. 7.

As the wave rises again air is trapped in successive compartments having their openings facing generally downwards, producing an increasing upthrust as the downward thrust from the water containing compartments above the water level decreases. Eventually all the water containing compartments are below water level and air filled compartments which have been carried round by the rotation of the rotor are each successively pushed below the water level 6 until water can enter the upper open end of the compartment and flood it.

This flooding action takes place over the zone between the intersection of lines 6 and 7 on Fig. 1 and the top of the wave peak and is shown in Fig. 6. Since each compartment must be pushed below the water level to allow the water to enter it there is a small upward force produced which opposes the rotation of the rotor but this is overcome by the much larger upthrust from the air trapped on the other side of the rotor, particularly as the flooding occurs near the top of the rotor where the moment of the force is small. This corresponds to the venting action which takes place just prior to the wave trough. The extent of the flooding and venting zones depends on the speed of the rotor relative to the wave.

In Figs. 1 and 2 the wave is shown as having a height equal to the rotor diameter.

Waves of smaller height will also operate the rotor but at a lower output torque and speed.

With larger waves the efficiency of energy extraction from the waves will be reduced.

The wave length is not critical except that it must be long in comparison to the spacing of the compartments and walls otherwise due to the shape of the wave air can escape from the compartments as the water level rises, reducing the amount of air trapped. Decreasing the spacing in order to trap more air can however restrict the speed of venting and flooding of the compartments thus reducing the output torque. Reducing the size of the opening into each compartment in order to increase the amount of air or water trapped in the rotor will also tend to restrict the speed of venting and flooding.

In the rotor shown in Figs. 4 to 8 the side walls 4 curve so that their outer ends are approximately 20 behind the inner ends secured to the hub, taken in the direction of rotation of the rotor.

A typical rotor for use in the North Atlantic ocean off the coast of Europe could be of several hundred metres or more long with an outer diameter of approximately 5m, a hub diameter of approximately 3m and compartments of length approximately 3m. The length of the rotor gives stability with respect to the mean water level (resistance to pitch and heave) and the rotor must have sufficient beam stiffness to resist undue flexing. Sea waves are of complex shape but apart from July, August, September, waves in the North Atlantic have a mean height above 2m for about 85% of the time and above 7m for about 10% of the time, and a 5m wave is fairly tyical. A 5m wave would typically have a period of approximately 1 Os and a length of approximately 1 50m.This is the sea condition shown in the drawings, although as explained above the scale along the rotor in Figs. 1 and 2 has been changed to enable the whole wave to be shown. The compartments in this rotor are spaced to give approximately 50 compartments in each wave length of the chosen 5m wave, although as explained ear lier the exact spacing is not critical. The various dimensions can be varied for different sea conditions.

If the rotor is allowed to rotate freely at zero torque it will rotate at a theoretical maximum rate of 180 in 5 seconds (6rpm) i.e. the time taken for a particle of water in a compartment to travel from the crest of a wave to the bottom of the following trough. Maximum torque is developed when the rotor is station ary but the optimum speed for maximum output power depends on the exact compart ment shape as the compartments above the rotor axis carry more water than those below and vice versa for the air carrying compart ments. It is however quite possible that the optimum speed will be approximately 1 /3rd of the maximum speed and this has therefore been used in the drawings.Since sea conditions and in particular average wave height and period vary it may be advantageous to continually adjust the rotor speed by means of the load driven by the rotor so as to always have the optimum speed for maximum output power, using some form of feedback arrange ment.

The rotor may be linked to an electrical generator or generators either mounted within the hub of the rotor or in a casing at one or both ends, preferably the end facing generally away from the waves so that any disruption of the waves by the casing is at a minimum.

However, the torque reaction on the rotor would tend to disturb its trim. This can be reduced by linking widely spaced half sub merged floats on either side of the rotor torque reaction arm from the generator but this would induce pitching of the apparatus as the waves traversed the floats. A vertical torque reaction arm in conjunction with horizontal mooring cables could be used but would be a fairly complex construction.

A simpler and preferable apparatus is shown in Fig. 8. This apparatus has two rotors 8, 9 arranged side by side and joined at their ends. The rotors are separated by a sufficient distance to ensure they do not complete unduly for the energy in the wave fronts and are linked via a power transmission at the end facing generally away from the waves to a central electrical generator 1 0. The power transmission is housed in a casing 11 carrying the generator 10 and may be a mechanical transmission using gears and drive chains or some other means such as a hydraulic or pneumatic transmission. The rotors are identical except that one is arranged to rotate clockwise and the other anticlockwise, the power transmission keeping them in synchronism.This balances the large torques generated and also eliminates any tendency for the apparatus to crab slowly sideways when operating. The rotors are linked at the end facing generally into the waves by an open framework 1 2 so as to minimise the disturbance to oncoming waves and are buoyant, the degree of buoyancy being adjustable by flooding or emptying compartments inside the hubs of the rotors so as to set the optimum position of the rotor axes with respect to the mean water level. This is determined by experiment and may vary along the length of the rotors.

Diagonal bracing 1 3 limits lozenging of the unit.

In an alternative arrangement a generator or generators is provided for each rotor, the generators being linked by torque reaction arms rather than the power transmission casing 11.

In a modified construction several rotors may be mounted in parallel in a rigid frame.

The rotors may be substantially shorter than in the earlier described arrangements and rely on the frame to hold them stable with respect to the mean water level. The torques can again be balanced if necessary by arranging the rotors in counter rotating pairs.

An array of these double or multiple rotor arrangements may be positioned at sea, tethered together by mooring lines and/or rigid lateral links and detachable from one another for repairs or maintenance. Electric power from the generators would be taken ashore by cable.

In any of the above arrangements each rotor may be additionally supported intermediate its ends by one or more steady bearings.

The apparatus is tethered either to a conventional mooring line or a buoy. A conventional mooring line will tend to disturb the trim of the rotors which may however be compensated by using the buoyancy compartments in the hubs of the rotors. This trim compensation could be made automatic by operating pumps to control the buoyancy compartments in response to tension in the mooring line. A preferred arrangement is to tether the apparatus to a buoy submerged below the water level and itself moored to the sea bed by the taut line technique. If only one end of the apparatus were moored to the buoy, or the other end was allowed some freedom of movement by its mooring lines, then the apparatus would tend to align itself with the local direction of travel of the main wave front.

Although the apparatus is preferably used with the rotor axes generally aligned with the expected direction of travel of the main wave front, so that the waves pass along the rotors, the rotors will also operate when the waves pass diagonally or transversely across them.

This is due to the irregular nature of ocean seas and the relatively short length of individual wave crests so that the rotor will lie across several crests and troughs. The rotor will also operate with waves from the opposite end. Thus the direction of the waves is not critical and the apparatus may be used at sites where this direction is more than usually variable. The apparatus may also be used in sea conditions where significant wave energy arrives from more than one direction simultaneously.

However, where the apparatus is to be used in a position where the main wave fronts come predominantly from one direction then the rotors may be aligned with this direction and to reduce turbulence at the ends facing the waves the rotors my be streamlined and may gradually taper in diameter to accommodate the reduction in height of the waves as they pass along the rotors.

As mentioned previously it is important that the compartments are vented and flooded rapidly in order to reduce the resulting loss of torque. The venting and flooding may be made more rapid by incorporating vanes or valves in the compartments or by a modification to the shape of the end walls 5 as seen in Fig. 9. In Fig. 9 the end wall of each compartment is cut away at 1 4 to give a straight outer edge rather than an arc. This allows the water to flow over substantially the whole edge of the end wall when the compartments start to flood (see Fig. 6) rather than over the lowest part of the wall only, so that the water enters the compartments faster.

In Fig. 10 there is shown a vane 1 5 which extends along the mouth of a compartment between the end walls and enables air to escape from the compartment easier during flooding. This speeds up the flooding. The vane can be any suitable shape and can extend between the side walls 4 rather than the end walls.

In Fig. 11, there is shown a flap valve 16 fitted into each side wall 4. The flap valve opens when the water level reaches the lower end of the opening covered by the valve and allows the compartment to fill from below.

This speeds up the flooding of the compartment and a similar action speeds up the venting by allowing air to enter through the flap valve. A light spring tends to hold the valve flap shut. There will however be some loss of water/air through the valve after the rotor has been flooded/vented which could reduce the torque unless a means for locking it shut when nearing the top and bottom of the rotor is incorporated.

When using a valve such as in Fig. 11 the opening of the compartment can be reduced or even closed and the shape changed to trap more air/water. A valve could also be put in the opening of the compartment so as to hold air/water for a greater proportion of that part of the cycle where positive work is done or even in the end wall 5 or inner wall 3 if suitably controlled.

Whereas Figs. 3 to 7 show a presently preferred form of the compartments 2, they may take different forms. In particular, they may be either fixed or moveable with respect to the rotor hub, and the compartment walls themselves may be flexible or movable, so as to maximise the amount of working fluid in the compartments during rotation of the rotor.

The number of compartments and their size and design may vary along the rotor to accommodate the change in wave height as energy is extracted. The compartments may be in line or staggered, both longitudinally and circumferentially, and the side walls may be parallel to the rotor axis or on a slight spiral. The number of compartments around the rotor may also vary.

The rotor body must be of adequate beam stiffness to resist undue flexing in the sea and this can be provided by the rotor itself or its associated support structure.

Claims (12)

1. A method of extracting energy from water waves using a rotor rotatable by the waves, wherein the rotor is of elongate cylindrical configuration having positioned along it a plurality of longitudinally extending compartments uniformly spaced around its axis, and is secured partially submerged in the water with its longitudinal axis substantially stable with respect to the mean water level, each compartment being shaped to collect and retain water when on one side of the rotor and discharge it when on the other side.

2. A method according to claim 1 wherein each compartment is of a length considerably less than that of the rotor.

3. A method according to claim 1 or 2, wherein the rotor's longitudinal axis is substantially parallel to the expected direction of travel of the main wave front.

4. A method according to any preceding claim, wherein two or more rotors are used, the rotors being connected so that each rotor is linked to a similar rotor rotating in the opposite direction.

5. Apparatus for extracting energy from water waves comprising a rotor of elongate cylindrical configuration having positioned along it a plurality of longitudinally extending compartments uniformly spaced around its axis, and means for securing the rotor partially submerged in water with its longitudinal axis substantially stable with respect to the mean water level, each compartment being shaped to collect and retain water when on one side of the rotor and discharge it when on the other side.

6. Apparatus according to claim 5, wherein each compartment is of a length considerably less than that of the rotor.

7. Apparatus according to claim 5 or 6, wherein the rotor comprises a central hub, a plurality of parallel longitudinally extending compartment side walls protruding outwardly from the hub and a plurality of compartment end walls extending between the compartment side walls at positions spaced along the rotor.

8. Apparatus according to claim 7, wherein the outer free end of each compartment side wall is displaced about the axis with respect to the inner end in the direction opposite that of rotation of the rotor.

9. Apparatus according to claim 8, wherein the compartment side walls are curved in a plane perpendicular to the rotor axis.

1 0. Apparatus according to any one of claims 5 to 9, wherein the rotor is supported for rotation at each end by support means which also support a second similar parallel rotor having its compartments shaped so that the rotors rotate in opposite directions.

11. Apparatus according to claim 10, wherein the two rotors are linked at one end by a power transmission which couples both rotors to an electrical generator carried by the support means.

12. Apparatus according to claim 10 or 11 wherein more than one such pair of rotors are supported by the support means.

1 3. Apparatus according to any of claims 5 to 12, wherein the rotor is self-buoyant.

1 4. A method of extracting energy from water waves substantially as herein described with reference to and as illustrated by the accompanying drawings.

1 5. Apparatus for extracting energy from water waves substantially as herein described with reference to and as illustrated by the accompanying drawings.