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Title:
VERTICAL AXIS WIND TURBINE
Document Type and Number:
WIPO Patent Application WO/2019/073189
Kind Code:
A1
Abstract:
Apparatus (1) for harvesting wind energy on a large scale consists of an annular support structure (4), floating on a body of water (2), which can rotate about its hub (3). Multiple vertical aerofoil units (6, 76, 206), mounted at equal intervals around the support structure (4), can pivot independently about a vertical axis (17, 217). The aerofoil units (6, 76, 206) are each held at a suitable angle of attack relative to the ambient wind (7) by vertical canard aerofoils (13, 213) mounted to a boom (12, 212) extending forwards from a leading edge (10) of the main aerofoil (8, 208) of each aerofoil unit (6, 76, 206). A cam follower mechanism (21) in the pivot mounting of each aerofoil unit (6, 76, 206) directs the angle of attack of the canard aerofoils (13, 213), so that the angle of attack of the respective main aerofoil (8, 208) is optimised for the current position of the aerofoil unit (6, 76, 206) around the rotating support structure (4). The main aerofoils (8, 208) thus generate a torque causing the apparatus (1) to rotate. Electrical power is generated from the rotation of the apparatus (1), for example by generators at the hub (3).

Inventors:
MERCER, Guy Nigel (8 Geldock Road, Little Billing, Northampton NN3 9PH, GB)
Application Number:
GB2018/000134
Publication Date:
April 18, 2019
Filing Date:
October 15, 2018
Export Citation:
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Assignee:
MERCER, Guy Nigel (8 Geldock Road, Little Billing, Northampton NN3 9PH, GB)
International Classes:
F03D7/06; F03D3/06
Domestic Patent References:
WO2014080030A12014-05-30
WO1991009225A11991-06-27
Foreign References:
US9546643B22017-01-17
CA2886279A12016-09-24
US5676524A1997-10-14
Other References:
None
Attorney, Agent or Firm:
BANFORD, Jonathan (Franks & Co Limited, Carlton House26 Billing Road, Northampton NN1 5AT, GB)
Download PDF:
Claims:
CLAIMS:

1. Wind energy gathering apparatus comprising a horizontally-extending centrosymmetric support structure, rotatable about a vertical axis extending through central hub means of the apparatus, and a plurality of aerofoil means, each said aerofoil means extending substantially vertically upwardly from the support structure adjacent its circumference, and being spaced equiangularly around said circumference, wherein each said aerofoil means is individually controllably rotatable about a vertical axis and is provided with control means adapted to align the aerofoil means at an optimum angle of attack relative to an incident airflow, said optimum angle of attack being such as to optimise a torque about the central hub means produced by the incident airflow flowing over the respective aerofoil means.

2. Wind energy gathering apparatus as claimed in Claim 1, wherein the optimum angle of attack is such as to produce a controllable level of torque about the central hub means from the respective aerofoil means.

3. Wind energy gathering apparatus as claimed in either Claim 1 or Claim 2, wherein the optimum angle of attack of each of the plurality of aerofoil means is controllable so as to produce from each aerofoil means a torque directed in the same rotational sense about the central hub means.

4. Wind energy gathering apparatus as claimed in any of the preceding claims, wherein the support structure of the apparatus is supported on a body of water.

5. Wind energy gathering apparatus as claimed in Claim 4, wherein the central hub means is mounted on a bed of the body of water, for example on pillar means extending upwardly from the bed of the body of water.

6. Wind energy gathering apparatus as claimed in Claim 4, wherein the central hub means is also supported on the body of water and is anchored in position, for example by a plurality of cables extending to anchor means in the bed of the body of water.

7. Wind energy gathering apparatus as claimed in any one of the preceding claims, wherein the support structure and aerofoil means of the apparatus are so supported that a minimum of the weight thereof is borne by the central hub means.

8. Wind energy gathering apparatus as claimed in any one of the preceding claims, wherein each aerofoil means of the apparatus is provided with control means to align said aerofoil means controllably at an optimum angle of attack independently of each other aerofoil means.

9. Wind energy gathering apparatus as claimed in Claim 8, wherein said control means adapted to align the aerofoil means is wholly mechanical and comprises no electrical or electronic control elements.

10. Wind energy gathering apparatus as claimed in Claim 9, wherein the control means adapted to align the aerofoil means comprises vane means mounted pivotably to the aerofoil means.

11. Wind energy gathering apparatus as claimed in Claim 10, wherein said vane means is so operatively connected to the aerofoil means that a change in an alignment of the vane means causes a change in the angle of attack of the aerofoil means.

12. Wind energy gathering apparatus as claimed in either Claim 10 or Claim 11, wherein each said vane means is disposed upwind of the respective aerofoil means.

13. Wind energy gathering apparatus as claimed in any one of Claims 10 to 12, wherein said vane means form a canard arrangement with the respective aerofoil means.

14. Wind energy gathering apparatus as claimed in any one of Claims 10 to 13, wherein the vane means is mounted to an elongate body extending irom a leading edge of the aerofoil means.

15. Wind energy gathering apparatus as claimed in any one of the preceding claims, wherein each said aerofoil means is rotatable about a vertical axis located between a leading edge and a centre of pressure of the aerofoil means.

16. Wind energy gathering apparatus as claimed in any one of the preceding claims, wherein each aerofoil means has a profile symmetrical about its chord.

17. Wind energy gathering apparatus as claimed in any one of Claims 10 to 14, wherein the control means of each aerofoil means comprises means to change a pitch of the vane means relative to a remainder of the aerofoil means.

18. Wind energy gathering apparatus as claimed in Claim 17, wherein the means to change the pitch of the vane means comprises cam means and cam follower means co-operatively connected to extend between the support structure and the rotatable aerofoil means.

19. Wind energy gathering apparatus as claimed in Claim 18, wherein the means to change the pitch of the vane means further comprises means to convert linear motion of the cam follower means to rotational motion.

20. Wind energy gathering apparatus as claimed in Claim 19, wherein the means to change the pitch of the vane means further comprises means to transmit said rotational motion to the vane means.

21. Wind energy gathering apparatus as claimed in any one of the preceding claims wherein the vane means is provided with limiter means adapted to restrict the pitch of the vane means to a particular range of angles.

22. Wind energy gathering apparatus as claimed in any one of the preceding claims, comprising electrical energy generation means.

23. Wind energy gathering apparatus as claimed in Claim 22, wherein said electrical energy generation means is operatively connected to the central hub means of the apparatus.

24. Wind energy gathering apparatus as claimed in Claim 23, wherein the electrical energy generation means is driven by the rotation of the hub means.

25. Wind energy gathering apparatus as claimed in Claim 4, wherein the support structure is surrounded by dam means adapted to retain water flows induced by the rotation of the support structure through the body of water.

26. Wind energy gathering apparatus as claimed in Claim 25, wherein at least one electrical energy generation means is mounted to the dam means, and is adapted to be driven by said water flows.

27. Wind energy gathering apparatus as claimed in Claim 26, wherein means are provided to pump air beneath the support structure to create an air cushion and/or a sheath of bubbles adjacent the support structure, in order to reduce hydrodynamic drag on the support structure as it rotates through the body of water.

Description:
VERTICAL AXIS WIND TURBINE

The present invention relates to apparatus and installations for collecting energy from the wind. More particularly but not exclusively, it relates to installations for harvesting wind energy which rotate about a vertical axis.

Simple vertical axis wind turbines have been used for many years. They can in general be classified as being based on drag, like an anemometer (e.g. the Savonius type wind turbine), or it being based on lift generated by aerofoils (e.g. the Darrieus type wind turbine, or the Giromill wind turbine, which operates on the same principles).

A more recent development of the Giromill is the Cycloturbine. This has vertical aerofoil blades, mounted so that they can pivot about their axes, with a weathervane linked to the aerofoils to control the angle of the attack of each aerofoil relative to the ambient airflow. This provides better self-starting and smoother power output from the Cycloturbine design compared to the Giromill. A disadvantage of such vertical axis wind turbines (VAWTs) is that the entire mass of the rotating structure must be supported on a central rotational bearing, and in the case of the Giromill/Cycloturbine types, the mass is concentrated along the diameter. Their sizes and weights are hence restricted. In order to limit centrifugal loads, and to avoid overheating the bearings, the speeds of existing VAWTs have to be limited. The potential power outputs of VAWTs are hence proportionately restricted.

Horizontal axis wind turbines are currently more efficient and more effective than existing VAWTs. However, while they are widely used, both on land and in offshore "wind farms", the bending loads on the blades of the wind turbine "propellers" are close to the maximum that can be achieved, using current materials. The horizontal axis wind turbines are hence approaching their maximum dimensions. Such wind turbines also have to be shut down to prevent overload and propeller/mounting damage in high winds.

The present invention revisits VAWT technology, addressing the drawbacks of existing horizontal axis wind turbine (HAWT) systems and permitting the power output per unit to be scaled up by a factor of 50 to 100. In particular, the problems currently associated with large diameter systems are addressed.

An additional issue for existing wind turbines, whether vertical axis or horizontal axis, is that they have no means of storing energy. The electricity that they generate must immediately be distributed, with separate energy storage facilities being required if the supply to be distributed exceeds the immediate demand. The apparatus of the present invention seeks to address the issue of energy storage as well as energy collection.

It is hence an object of the present invention to provide a vertical axis wind turbine installation that obviates some or all of the above shortcomings of existing wind turbine systems, and provides large scale, efficient and versatile wind energy harvesting, storage and supply.

According to the present invention, there is provided wind energy gathering apparatus comprising a horizontally-extending centrosymmetric support structure, rotatable about a vertical axis extending through central hub means of the apparatus, and a plurality of aerofoil means, each extending substantially vertically upwardly from the support structure adjacent its circumference, said aerofoil means being spaced equiangularly around said circumference, wherein each said aerofoil means is individually controllably rotatable about a vertical axis and comprises control means adapted to align the aerofoil means at an optimum angle of attack relative to an incident airflow, said optimum angle of attack being such as to optimise a torque about the central hub means produced by the incident airflow flowing over the respective aerofoil means.

Preferably, the optimum angle of attack is such as to produce a controllable level of torque about the central hub means from the respective aerofoil means. Advantageously, the optimum angle of attack of each of the plurality of aerofoil means is controllable so as to produce from each aerofoil means a torque directed in the same rotational sense.

In a preferred embodiment, the support structure of the apparatus is supported on a body of water.

Advantageously, the central hub means is then mounted on a bed of the body of water, for example on pillar means extending upwardly from the bed of the body of water.

Alternatively, the central hub means is also supported on the body of water and is anchored in position, for example by a plurality of cables extending to anchor means in the bed of the body of water.

Preferably, the support structure and aerofoil means of the apparatus are so supported that little or none of their weight is borne by the central hub means.

Preferably, each aerofoil means of the apparatus is provided with means controllably to align it at an optimum angle of attack independently of each other aerofoil means.

Advantageously, said control means adapted to align the aerofoil means comprises vane means mounted pivotably to the aerofoil means. Said vane means may be so operatively connected to the aerofoil means that a change in an alignment of the vane means causes a change in the angle of attack of the aerofoil means.

Each said vane means may be disposed upwind of the respective aerofoil means.

Said vane means may form a canard arrangement with the respective aerofoil means.

The vane means may be mounted to an elongate body extending from a leading edge of the aerofoil means.

Preferably, each said aerofoil means is rotatable about a vertical axis located between a leading edge and a centre of pressure of the aerofoil means.

Each aerofoil means preferably has a profile symmetrical about its chord.

Preferably, the control means of each aerofoil means comprises means to change a pitch of the vane means relative to a remainder of the aerofoil means.

Advantageously, the means to change the pitch of the vane means comprises cam means and cam follower means co-operatively connected between the support structure and the rotatable aerofoil means.

The means to change the pitch of the vane means may further comprise means to convert linear motion of the cam follower means to rotational motion. The means to change the pitch of the vane means may further comprise means to transmit said rotational motion to the vane means.

The vane means may be provided with limiter means adapted to restrict the pitch of the vane means to a particular range of angles.

Preferably, the apparatus comprises electrical energy generation means.

Said electrical energy generation means may be operatively connected to the central hub means of the apparatus.

The electrical energy generation means may then be driven by the rotation of the hub means.

In embodiments in which the support structure of the apparatus is supported on a body of water, the support structure is preferably surrounded by dam means adapted to retain water flows induced by the rotation of the support structure.

One or more electrical energy generation means may then be mounted to the dam means, adapted to be driven by said water flows.

Means may be provided to pump air beneath the support structure to create an air cushion and/or a sheath of bubbles adjacent the support structure, in order to reduce hydrodynamic drag on the support structure as it rotates. Embodiments of the present invention will now be more particularly described, by way of example and with reference to the Figures of the accompanying drawings, in which:

Figure 1 is a plan view from above of a first wind energy harvesting apparatus embodying the present invention;

Figure 2 is a side elevation of an aerofoil unit isolated from the wind energy harvesting apparatus of Figure 1 ;

Figure 3 is a frontal elevation of the aerofoil unit of Figure 2;

Figure 4 is a plan view from above of the aerofoil unit of Figure 2;

Figure 5 is an exploded side elevation of the aerofoil unit of Figure 2 complete with its base element;

Figure 6 is a cross-sectional side elevation of the base element of the aerofoil unit of Figure 2;

Figure 7 is an exploded cross-sectional side elevation of a first part of a vane control mechanism of the aerofoil unit of Figure 2;

Figure 8 is a plan view from above of a cam follower unit isolated from the vane control mechanism of Figure 7;

Figure 9 is a plan view from above of the upper and lower cams isolated from the vane control mechanism of Figure 7;

Figure 10 is a schematic plan view from above of the aerofoil unit of Figure 2, showing a second part of the vane control mechanism;

Figure 11 is a schematic plan view from above of how wind passing over the aerofoil units of the apparatus of Figure 1 interacts with a tangential velocity of the rotating apparatus to produce an apparent wind relative to the aerofoil unit, at a series of positions around the apparatus;

Figure 12 is a side elevation of an alternative aerofoil unit, isolated from a wind energy harvesting apparatus embodying the present invention;

Figure 13 is a plan view from above of the alternative aerofoil unit of Figure 12; Figure 14 is an exploded side elevation of the alternative aerofoil unit of Figure 12, complete with an alternative base arrangement;

Figure 15 is a plan view from above of an alternative cam follower unit isolated from the vane control mechanism of the alternative aerofoil unit of Figures 12 and 14; Figure 16 is a schematic plan view from above of the aerofoil unit of Figure 12, showing a second part of its vane control mechanism; and

Figure 17 is a schematic plan view from above of a third aerofoil unit embodying asymmetric aerofoil sections.

Referring now to the Figures, and to Figure 1 in particular, there is shown a wind energy harvesting apparatus 1 in the form of a vertical axis wind turbine. This particular embodiment of the present invention is a full-scale version, 2400m in diameter, most of which is supported by floating on a body of water 2, such as a sea or even an ocean. There is a central hub unit 3, surrounded by a circular support structure 4, to which it is connected by a series of high-tensile steel cables 5.

The support structure 4 is here shown as a continuous annular structure, for example comprising hollow concrete castings, although in other embodiments the support structure 4 may comprise a series of individual modules linked in a circle by suitable connecting members. Smaller apparatus might be made with support structures closer to discs, rather than as a braced ring

In this embodiment, the central hub unit 3 is mounted on an underwater pillar (not visible) built on the bed of the body of water 2, with the central hub unit 3 above water, substantially in the plane of the floating support structure 4. As a result, little or none of the weight of the support structure 4 is supported on the central hub unit 3. In other embodiments, the central hub unit 3 also floats, but is anchored in position by cables anchoring it to the sea bed.

The central hub unit 3 is so constructed that the support structure 4 is freely rotatable about a vertical axis through the central hub unit 3. It can rotate clockwise or anticlockwise, as is shown in Figure 1.

In other versions, adapted to operate on land, the support structure 4 would be supported by rollers or wheels, running along a circular track.

A number of aerofoil units 6 (in this example, eight) are mounted to an upper surface of the support structure 4, spaced equally. Each aerofoil unit 6 (shown in more detail in Figures 2-4 below) comprises a symmetrical aerofoil extending vertically, which is so mounted to the support structure 4 that it can be rotated through 360° about a vertical axis. In this embodiment, each aerofoil unit 6 is approximately 300 metres high. Usually, more than eight aerofoil units 6 would be used, but Figure 1 is limited to those shown for clarity. The aerofoil can thus be aligned at a desired angle of attack (AO A), relative to a wind direction 7, so that the force exerted on the aerofoil unit 6 produces a torque about the central hub unit 3 that encourages rotation of the apparatus 1. As can be seen, this involves a different alignment of the aerofoil unit 6 at different points around the support structure 4. The aerofoils switch from being aligned to port of the wind direction 7, to being aligned to starboard of the wind direction 7, and back, at two points located roughly at right angles to the wind direction 7. [Note: the actual alignments of the aerofoils and apparent wind directions experienced are a little more complex than is shown in Figure 1, for example needing to take into account the motion of the support structure 4 and the aerofoil units 6 mounted thereto, into the wind 7, down- wind 7 and crosswind; this is explained in more detail below].

Figure 2 shows an individual aerofoil unit 6 separated from the support structure 4. The aerofoil unit 6 comprises a main aerofoil 8, extending vertically from a horizontal aerofoil base 9. The main aerofoil 8 in this example has a constant profile throughout its height, with a leading edge 10 intended to be directed generally to windward and a trailing edge 11 intended to be aligned generally downwind. The main aerofoil 8 in this example is symmetrical about a vertical plane (see also Figure 4). A rigid boom 12 extends from the leading edge 10 of the main aerofoil 8. Two vertically-extending canard aerofoils 13 extend upwardly and downwardly, respectively, from a distal end of the boom 12. The canard aerofoils 13 are pivotably mounted about a vertical axis, and are coupled so that both will be at the same pitch angle relative to the boom 12 and main aerofoil 8. The control and effects of the canard aerofoils 13 are described below. To stabilise the main aerofoil 8, a forestay 14 is provided, comprising a cable extending from a point on the leading edge 10 of the main aerofoil 8 to a point on the windward rim of the aerofoil base 9.

Figure 2 also shows a pivot axle 15 of the aerofoil unit 6, which extends downwardly from a centre of the aerofoil base 9. The whole aerofoil unit 6 is rotatable about this pivot axle 15. It is notable that the main aerofoil 8 is located such that the vertical axis 17 through the pivot axle 15 is much closer to the leading edge 10 than to the trailing edge 11. As a result of this arrangement, with the aerofoil 8 extending across the aerofoil base 9 so far that the trailing edge 11 is close to or extends beyond the rim of the aerofoil base 9, the inherent structural strength of the aerofoil 8 is sufficient to stabilise it in a downwind direction, and so there is no need for a backstay to correspond to the forestay 14. In any case, there is no room for a conventional backstay, which would instead have to be within the aerofoil 8.

Figure 3 shows the profile of the main aerofoil 8, viewed from in front with the boom 12 and the canard aerofoils 13 in the vertical plane of symmetry through the main aerofoil 8. For lateral stability, a lateral stay 16 extends diagonally from a point on each side face of the main aerofoil 8 generally level with the boom 12 to a corresponding point on the rim of the aerofoil base 9. For maximum efficiency, the drag produced by the forestay 14 and the lateral stays 16 can be minimised by enclosing them in a shroud having a low-drag aerofoil profile.

Figure 4 shows the profiles of the main aerofoil 8 and of the canard aerofoils 13 most clearly. The boom 12 is relatively broad, here having the same width as the main aerofoil 8, for stability, but since it only overlaps with a small proportion of the leading edge 10 (see Figures 2 and 3), this has little effect on the efficiency of the main aerofoil 8. (The boom 12 is hollow, not just supporting the canard aerofoils 13 but also containing their control arrangements, as described below).

Figure 4 also shows most clearly the vertical axis 17 through the pivot axle 15, about which the main aerofoil 8 rotates. As noted above, the vertical axis 17 is close to the leading edge 10 of the main aerofoil 8. This means that the centre of pressure 18 of the main aerofoil 8 (the point through which the net wind pressure on the main aerofoil 8 will act), which is located at approximately one-quarter of the chord (the line from the leading edge 10 to the trailing edge 11), is behind the vertical axis 17. Meanwhile, the wind pressure on the canard aerofoils 13 is exerted at a point forward of the vertical axis 17. The wind pressures on the canard aerofoils 13 and on the main aerofoil 8 are hence exerted on opposite sides of the vertical axis 17, and while the wind pressure on the canard aerofoils 13 will be much lower than the wind pressure on the main aerofoil 8, because of their lower area, the wind pressure on the canard aerofoils 13 is exerted much further from the vertical axis 17 of rotation, and so has proportionately higher leverage.

This arrangement is vital for the control of the alignment of the main aerofoil 8, which will be described below. (Note: the canard aerofoils 13 are shown extending in line with the main aerofoil 8 and the rigid boom 12, but in practice they will be controllably turned to port or to starboard as part of the control arrangements of the aerofoil unit 6). Figure 5 shows how the main aerofoil 8 and the aerofoil base 9 fit on to the support structure 4. The support structure 4 is shown as a solid body for simplicity, but in practice would contain sizeable voids so that it floats on the body of water 2, braced to the central hub unit 3 and a far side of the support structure 4 by the cables 5.

The pivot axle 15 at the centre of the underside of the aerofoil base 9 is received into a bearing housing 19, fixed to the support structure 4. The pivot axle 15 is free to rotate in the bearing housing 19. Since the aerofoil unit 6 rotates to track changes of a local apparent wind direction 7, conventional bearing technology will be sufficient to cope with changes of angle measured in degrees per minute, unlike horizontal axis wind turbines and existing vertical axis wind turbines, which need bearings capable of carrying significant weights at hundreds of revolutions per minute, maybe thousands.

These bearing arrangements are shown in more detail in Figure 6. It should be noted, incidentally, that the particular bearing type illustrated here, within the bearing housing 19, is a representative bearing type only. In the ultimate scaled-up versions of this apparatus 1 , more complex bearings are likely to be required to support both radial and axial loadings. The selection of the exact bearing type and size for a given size and weight of aerofoil unit 6 should be a routine engineering choice. Since the exact bearing type used is unlikely to be relevant to the essential subject matter of the present invention, a schematic example of a bearing is illustrated here.

Figure 5 also shows, in general terms, a roller guide arrangement 20, mounted to the support structure 4, which retains and guides the run of the aerofoil base 9 as it rotates. The details of this arrangement are shown more clearly in Figure 7. Also shown in general in Figure 5 are a cam follower arrangement 21, mainly mounted to the bearing housing 19 in the assembled aerofoil unit 6, but here shown exploded for clarity, and also a link arm arrangement 22, mounted to the aerofoil base 9 adjacent the pivot axle 15, but which is also operatively connected to the cam follower arrangement 21 in the assembled aerofoil unit 6. These are both shown in more detail in Figure 7, and the link arm arrangement 22 also in Figure 8. Their function as part of the control arrangements for the canard vanes 13 is described below; in the context of these Figures.

Figure 6 shows at a larger scale how the bearing housing 19 is mounted to the support structure 4, being received into a cooperating socket 23. A radial flange 24 extends outwardly from the generally cylindrical bearing housing 19, just above its midpoint. This flange 24 rests on the support structure 4, and in turn supports the cam follower arrangement 21, which encircles a upper portion of the bearing housing 19 extending above the flange 24 (see Figure 7 and description below).

Figure 7 shows the cam follower arrangement 21, portions of the link arm arrangement 22, and the roller guide arrangement 20 in more detail.

The cam follower arrangement 21 comprises a bottom cam 25, of generally annular form, which rests on the flange 24 and fits around the upper portion of the bearing housing 1 , a lower portion of the bearing housing 19 being held in the socket 23 as described above. (The outer profile of the bottom cam 25 is shown in Figure 10). A cam spacer 26, also of annular form, rests on top of the bottom cam 25 and fits around the upper portion of the bearing housing 19. In turn, a top cam 27, of generally annular form, rests on top of the cam spacer 26, and also fits around the upper portion of the bearing housing 19. (The outer profile of the top cam 25 is also shown in Figure 10). A bearing cover 28, again annular in form, is located on top of the top cam 27. The bearing cover 28 also acts as a clamp to hold the cam follower arrangement 21 together, for example by bolts extending through the bearing cover 28, top cam 27, cam space 26 and bottom cam 25 into the flange 24, and possibly through the flange 24 into the support structure 4 to help secure the bearing housing 19 in place. An inner profile of the bearing cover 28 is sized to allow the pivot axle 15 into the bearing housing 19.

A final portion of the cam follower arrangement 21 is the cam follower itself 29, which also effectively forms part of the link arm arrangement 22 (see below and Figure 8). The cam follower 29 is conveniently a half-annular or C-shape, sized to fit around the cam spacer 26 and between the bottom 25 and top 27 cams. The only parts of the cam follower 29 which should touch any part of the rest of the cam follower arrangement 21 are a bottom cam follower roller 31 extending downwardly at a first end of the cam follower remote 29 and a top cam follower roller 32 extending upwardly at a second end of the cam follower from the first. The bottom cam follower roller 31 follows an outer profiled run of the bottom cam 25 and the top cam follower roller 32 follows an outer profiled rim of the top cam 27. (A single cam and follower roller could be used, but the arrangement shown, with the two opposed cam follower rollers 31, 32, is believed to be more accurate and reliable. It also does not rely on using a spring or other biasing structure to hold the cam follower roller 31 or 32 firmly in contact with the outer profiled run of the cam 25 or 27).

The cam follower 29 is fixed to a remainder of the link arm arrangement 22 on the underside of the aerofoil base 9 via two spacer columns 30, shown in dotted lines in Figure 7 as they would be located above the plane of the image in Figure 7. (See Figure 8 and associated description below for details of the remainder of the link arm arrangement).

As Figure 7 also shows, the roller guide arrangement 20 comprises an circular wall 33 with an in-turned upper edge, mounted to the support structure 4 in its assembled form, and a circumferential roller element 34 extending radially outwardly from a circumference of the aerofoil base 9. The roller element 34 contacts an underside of the in-turned upper edge of the wall 33, along which it runs as the aerofoil unit 6 rotates relative to the support structure 4, guiding the aerofoil base 9 to help ensure smooth and controlled rotation. This arrangement also to a degree acts as a seal to prevent debris or even seawater reaching the mechanisms described above.

Figure 8 shows a plan view of the link arm arrangement 22. The semi-annular shape of the cam follower 29 is evident, with the bottom and top cam follower rollers 31, 32 located at respective remote ends. The spacer columns 30 extend between an upper surface of the cam follower 29 and an under surface of a horizontal elongate link beam 35. Two link arms 36, 37 are pivotably mounted to the link beam 35. The link arms 36, 37 extend in parallel, and the link beam 35 extends in parallel to the chord of the aerofoil 8, shown by dotted line 38. A first 36 of the link arms is pivotably linked at its distal end to the aerofoil base 9, at a point on the chord, and the second 37 of the link arms is fixed adjacent its distal to a rotatably-mounted vertical drive shaft 39, which passes through an aperture in the aerofoil base 9, also at a point on the chord. (Refer back to Figure 7 for full details). The link beam 35 and the link arms 36, 37 thus form a parallel motion linkage, mounted to the aerofoil unit 6 and connected to drive shaft 39 (the function of which will be set out below in the context of Figure 10).

Figure 9 shows the bottom cam 25 and the top cam 27 and their respective profiled outer rims. The bottom 25 and top 27 cams are mirror-images of each other. When the bottom and top cam follower rollers 31, 32 are following the outer profile of their respective cams 25, 27, for most of the circumference of the cams 25, 27, the cam follower rollers 31, 32 each trace a portion of a circle and the cam follower 29 follows.

However, each cam 25, 27 has two opposed zones 40, 41 where there is a step change in the radius of the outer profile. As a result, when the bottom cam follower roller 31 is following the outer profile of the bottom cam 25, and is urged outwardly at zone 40, for example, the top cam follower roller 32, following the outer profile of the top cam 27 is at the opposite side of the cam follower arrangement 21, is passing through zone 40 of the top cam 27, where it would be able to move inwardly. The reverse occurs half a revolution later when each cam follower roller 31 , 32 passes simultaneously through the respective zone 41 on the respective cam 25, 27. Thus, as the cam follower rollers 31, 32 traverse the zones 40, 41, there is a lateral movement of the cam follower 29 as a whole. Referring back to Figure 8, a lateral movement of the cam follower rollers 31, 32 along the chord 38 moves the cam follower 29 parallel to the chord 38, and so the link beam 35 (to which the cam follower 29 is mounted via the spacer columns 30) also moves, parallel to the chord 38. The link arms 36, 37 are constrained at their distal ends and so turn by a small angle relative to the link beam 35. The end result of this sequence is that the distal end of the second link arm 37 pivots, and the vertical drive shaft 39 turns with it. The drive shaft 39 is thus turned back and forth through a small angle, but only when the cam follower 29 (and hence the entire rotatable aerofoil unit 6) is passing through either of two substantially opposed rotational positions, relative to the support structure 4.

The effect of this turning of the vertical drive shaft 39 is shown in Figure 10. This drive shaft 39 runs vertically up within the main aerofoil 8, until it reaches the level of the boom 12. (Note: this course of the drive shaft 39 is shown by a dashed line in Figures 2 and 6, not labelled therein for simplicity).

At this point, the drive shaft 39 is coaxially connected to a first pulley wheel 42. A belt or chain 43 extends around the first pulley wheel 41 and along an interior of the boom 12 to its distal end, at which the belt or chain 43 extends around a second pulley wheel 44. The second pulley wheel 44 is fixedly mounted to a pivot axle for the canard aerofoils 13, such that rotation of the second pulley wheel 44 changes the pitch of the canard aerofoils 13 relative to the boom 12 and the main aerofoil 8. Thus, the small backwards and forwards turns of the drive shaft 39 produced by the cam follower 21 and link arm 22 arrangements are transmitted, via the pulley wheels 42, 44 and the belt or chain 43 running between them, to become back-and-forth switches of the pitch of the canard aerofoils 13.

The belt or chain 43 in this embodiment contains two spring/resilient sections 45. Thus, a sudden or extreme rotation of the drive shaft 39 will not be transmitted immediately or in full to the canard aerofoils 13. (Additionally, if the canard aerofoils 13 are displaced suddenly by gusts of wind or the like, the spring sections 45 will absorb and oppose more extreme motions of the canard aerofoils 13).

There is a further restriction imposed on the pivoting motion of the canard aerofoils 13. A pitch limiter 46 is fitted, linking the boom 12 and the canard aerofoils 13, which prevents the pitch of the canard aerofoils 13 going outside a preselected range on either side of neutral.

All the control elements necessary to govern the angle of attack of the main aerofoil 8 of each aerofoil unit 6, relative to the wind direction 7, have now been described. They function as follows.

As noted above with reference to Figure 1, a symmetrical aerofoil, such as the main aerofoil 8 of each aerofoil unit 6, provides no lift (i.e. no lateral force to either side) when it is completely aligned with the incident airflow. (NB: the apparent direction and speed of the incident airflow will not be the same as the wind velocity, due to the superimposed motion of the support structure 4, but the exact effect that this has on the aerofoil units 6 will be discussed below, in the context of Figure 11). When a symmetrical aerofoil 8 is turned so that its leading edge 10 is pointing to port of the incident airflow, a net force will be generated, passing through the centre of pressure and directed generally to port. Conversely, if the leading edge 10 is pointing to starboard of the incident airflow, a net force will be generated, directed through the centre of pressure and generally to starboard. Aligning the main aerofoil 8 to port or starboard, depending on the position of the particular aerofoil unit 6 around the circular support structure 4, will create a torque about the central hub unit 3 that is in the desired direction of rotation of the support structure 4. It will thus be necessary to switch the alignment of the aerofoil 8 over, in effect from the port tack to the starboard tack and vice versa, as it moves from position to position with the rotation of the support structure.

In addition to ensuring the correct overall alignment of the aerofoil 8, it is necessary to control the exact angle of attack of the aerofoil 8, so as to obtain a torque within design limits from the airflow available. (Note: in principle, the drag forces due to passage of air over the aerofoil 8 will also have an effect, but for aerofoil profiles such as those shown, at normal angles of attack, drag can be neglected without too great inaccuracy).

Additionally, as the support structure 4 rotates, the aerofoil units 6 must rotate relative to it, to maintain a desired alignment to the incident airflow. The aerofoil unit 6 must therefore be able to "follow the wind", whichever tack it is on. In principle, one could measure local airflows adjacent each aerofoil unit 6 and use computer controls and servo motors to align the aerofoil unit 6 at the calculated ideal angle at all times. However, such electronic systems would add to cost and complication, and are likely to require constant monitoring and maintenance. In the preferred versions of the present invention, therefore, straightforward aerodynamic/mechanical arrangements are chosen, such as those described above.

As well as the canard aerofoils 13 shown, other aerodynamic arrangements are possible for guiding the main aerofoil 8 to produce an optimum angle of attack, such as steerable aerofoils located behind the trailing edge 11, analogous to the elevators of an aircraft with a conventional tail structure. These trailing aerofoils could also be combined with canard aerofoils 13, and a variety of additional control surfaces and similar features from aircraft design, such as flaps and variable incidence (swept) wings, could also be employed. In each variation, the relative positions of the pivot axis 15 of the aerofoil 8, the centre of pressure of the particular aerofoil profile, and the size and location of the chosen steering vanes must be carefully balanced to optimise the control response. It is also believed that the system would be viable with the steering vanes mounted on top of the main aerosol 8, either above the leading edge 10 or above the trailing edge 11, although this would greatly increase the length of the control runs to turn the steering vanes. The most preferred option at present, however, is the canard arrangement described in detail herein.

The ideal disposition for the aerofoil units 6 of the present invention is where the main aerofoil 8 is aligned into the incident airflow 7, at an optimum angle of attack, AO Amain- The canard aerofoils 13 on the end of the boom 12 are meanwhile aligned at an angle to the boom 12 and the main aerofoil 8, the pitch angle (Pitchcanard) > with both said angles being in the same sense, to port or to starboard. Thus, the angle of attack of the canard aerofoils 13 is always greater than that of the main aerofoil 8, i.e. AOAcanard - AOA ma i„ + Pitch ca mrd- This provides a balanced aerofoil unit 6 with the force exerted by the incident airflow on the canard aerofoils 13 at a first distance in front of the vertical pivot axis 17 balancing the (greater) force exerted on the main aerofoil 8, which acts at a (shorter) second distance behind the pivot axis 17.

If the incident airflow direction moves to increase the angles of attack, the force exerted on the canard aerofoils 13 increases by less than the force exerted on the main aerofoil 8, and so the main aerofoil 8 turns back into the wind, reducing the angles of attack, until a balance is again achieved. Conversely, if the incident airflow direction moves to lower the angle of attack, the force exerted on the canard aerofoils 13 is reduced by less than the force exerted on the main aerofoil 8, and so the canard aerofoils 13 pull the main aerofoil 8 further round, to increase the angles of attack. Thus, the main aerofoil 8 is maintained at the desired angle of attack to produce the designed level of torque. This occurs both for minor variations of wind direction and for long-term changes, and also when the incident airflow direction change is due to the rotation of the support structure 4 and hence the aerofoil unit 6 to a different angle relative to the wind.

Excessive angles of attack are avoided, because the canard aerofoils 13 are always at a greater angle of attack than the main aerofoil 8. Thus, if there is a gust of wind and the angle of attack of the canard aerofoils 13 becomes so high that the aerofoils 13 stall, the balancing force from the canard aerofoils 13 is lost, and the unbalanced force on the main aerofoil 8 turns it promptly back into the wind. When it has turned back far enough, the canard aerofoils 13 will no longer be stalled, and the balance can be re-established.

This control arrangement also copes with increases in the speed of the incident airflow. If the canard aerofoils 13 are pivoted at a point in front of their centre of pressure (like the main aerofoil 8), then increased airflow will tend to reduce their pitch relative to the boom 12 and main aerofoil 8. This change of pitch is accommodated by the spring sections 45 in the belt/chain 43 (see Figure 10). The force on the canard aerofoils 13 will fall while that on the main aerofoil 8 stays constant, unbalancing the system which turns back into the wind until a new balance is reached, at a lower angle of attack for the main aerofoil 8. The torque produced by the airflow over the main aerofoil 8 is lower at the lowered angles of attack, which balances the increase in torque due to the increased magnitude of the airflow, and so the apparatus 1 as a whole can continue to operate at increased wind speeds.

Thus, the apparatus of the present invention has an inbuilt opposition to overspeeding at high ambient wind speeds, unlike HAWTs, which have to be braked under positive control to avoid overspeed, or even have their blades feathered to take them out of service entirely in high winds, for safety's sake.

As mentioned above, for the net torque to be in the desired direction of rotation, wherever the aerofoil unit 6 is around the support structure 4, the angle of attack must be to port of the incident airflow around half of the circumference of the support structure and to starboard around the other half. The vital switching between the two opposite alignments is driven by the cam and cam follower arrangement described above (see Figures 7 to 10).

The cams 25, 27 are fixed to the support structure 4. Thus, as the support structure 4 rotates, the alignment of the cams 25, 27 will rotate with it. Meanwhile, the cam follower 29 is fixed to the aerofoil base 9, and rotates with the aerofoil unit 6, which the canard arrangement is holding at an angle of attack close to the incident airflow. However, at two opposed positions on the circumference of the support structure 4, the wind direction is parallel to the tangential motion of the support structure (see more on this point below with reference to Figure 11) and the incident airflow over the aerofoil 8 is thus in the same direction as the wind. The cams 25, 27 are oriented such that in these two positions, the cam follower 29 meets one or other of the stepped zones 40, 41. As a result, the cam follower 29 then moves laterally, and via the parallel motion in the link arm arrangement 22, the vertical drive shaft 39 is turned. This drives the mechanism of belt/chains 43 and pulleys 42, 44 within the boom 12 to turn the pitch of the canard aerofoils 13 from port to starboard or vice versa.

Thus, the aerofoil unit 6 is carried round half a rotation of the support structure with the canard aerofoils 13 pitched to port; at one of the above positions, the canard aerofoils 13 are switched over to a starboard pitch, as described above; the aerofoil unit 6 continues around for another half rotation with the canard aerofoils 13 pitched to starboard until it reaches the second of the above positions; and the canard aerofoils 13 are switched back to a port pitch. Since the pitch direction of the main aerofoil 8 follows that of the canard aerofoils 13, the main aerofoils 8 of each aerofoil unit 6 are hence pointing in the correct direction to contribute a torque in the direction of rotation of the support structure 4, and wind energy is thus converted into rotational energy of the support structure 4.

It should be noted in the above description that the incident airflow over the aerofoils 8, 13 - i.e. the apparent wind that they are experiencing - is not the same as the true wind passing over the apparatus, in either speed/strength or direction. Once rotation of the support structure 4 has been established, the wind velocity over the ground or water must be combined with the tangential velocity of the rotating support structure 4 relative to the ground/water, to give the resultant velocity of the aerofoil unit 6 carried on the support structure relative to the air. The tangential velocity of the support structure in the apparatus 1 described is calculated as reaching up to 40mph/60km.h "1 . Wind speeds are likely to be of similar magnitude, so the resultant or apparent wind velocity over the aerofoils 8, 13 will be affected greatly by the angle between the wind and tangential velocities.

This effect is shown in Figure 11, which shows the vector triangles for combining wind and tangential velocities at eight aerofoil unit 6 locations (marked as 61-68) around the support structure 4. Figure 11 is effectively split into two, to prevent the vector triangles overlapping (note that the central hub unit 3 appears in both halves). In all locations, the external wind direction is from right to left. Three vector triangles are superimposed for each location: one for a low wind speed L lower than the magnitude of the tangential velocity, one for a wind speed E equal to the magnitude of the tangential velocity, and one for a high wind speed H significantly greater than the magnitude of the tangential velocity (isolated vectors to represent these three speeds are also shown between the two halves of the Figure). The tangential velocity is marked as T in all cases (shown as the effective headwind, so in the opposite direction to the rotation of the apparatus 1, shown adjacent the central hub unit 3). The dashed lines are radii of the support structure, for reference. The apparent or resultant winds experienced by each aerofoil unit for each wind speed are marked with hollow arrow heads Δ, except for locations 61 and 65.

As can be seen, there is a significant variation in the magnitude and direction of the resultant/apparent wind experienced at different locations, and with different wind speeds. Nevertheless, the canard control of the angle of attack of the main aerofoil and the cam-based switching of the canard pitch both involve the incident airflow - i.e. the apparent wind - and so the control systems of the present invention are capable of steering each of the main aerofoils to the correct angles for optimum torque directed to speed up the rotation of the apparatus as a whole, even with this wide variation of apparent wind speeds and directions..

At locations 61 and 65, the aerofoil unit 6 would be heading straight into the wind and running directly downwind, respectively, so all vectors are collinear, including those for the resultant/apparent wind (omitted for clarity). As can be seen at location 65, when the apparatus 1 is rotating at a tangential speed T lower than the wind ground speed H, the apparent/resultant wind is still in a useful direction (i.e. the wind can still catch up with the aerofoil unit 6). It is only when T matches wind speed E (or worse, exceeds wind speed L) that this side of the apparatus may start to lose effectiveness. In practice, this means that the arrangements are at their maximum effectiveness at start-up, and the effectiveness dies off as highest rotational speeds are approached. Since this means that the approach to top speed is likely to be asymptotic, the likelihood of runaway operation is significantly lowered, and it may be possible to run the apparatus in stronger winds than for HAWTs which have to be switched off outside a relatively narrow wind speed range.

The apparatus described is thus an effective wind energy collection apparatus. However, it does more. As it collects wind energy, the support structure 4 rotates faster and faster, and if the wind drops off, it will continue to rotate - it is effectively a giant flywheel, and acts as an energy storage system. When power is generated (see below), the energy is taken from the rotational kinetic energy of the support structure 4, irrespective of whether any wind energy is being collected by the aerofoil units 6 at that time. Existing HAWTs may need to be turned off, or expensive additional energy storage arrangements employed, if the energy collected much exceeds that for which there is an immediate need. The apparatus thus helps to even out supply/demand variation, e.g. through the day and night.

Energy collection from the apparatus can be carried out in more than one way, too. A straightforward electrical generator can be mounted at the central hub 3, converting rotational energy to electricity. With the 2400m diameter support structure envisaged rotating with a peripheral speed of about 60km.h _1 , the rotational velocity is only one revolution every 7 or 8 minutes. Thus, gearing up will be necessary to produce higher rotational velocities to drive conventional electrical generators producing AC electricity at 50 or 60Hz, rather than the gearing down required for generators driven by steam turbines. Incidentally, this very low rotational velocity means that the bearings required at the central hub can be substantially conventional, and the large versions of the apparatus may be easier to construct than smaller versions having to rotate at higher angular velocities to give the same peripheral speeds compatible with normal wind speeds.

A further energy generation possibility results from the preferred apparatus that floats on a body of water. In this case, it is planned that a circular dam, possibly 3000m in diameter, would surround the support structure 4, for example for protection from excessive waves. This could either be a solid wall built on the sea bed, or a tethered floating collar with a membrane wall extending downwards to or toward the sea bed. As the support structure 4 rotates within this dam, it is likely that hydrodynamic forces will cause water adjacent the support structure to flow along with it. As a first benefit, the effective mass, momentum and rotational kinetic energy of the support structure 4 will be that much greater, making it a "bigger" energy storage apparatus at little extra cost.

Additionally, this creates a strong flow of water circling within the (stationary) dam. It should be possible to locate hydroelectric generators, mounted to an inner wall of the dam, within this flow, and to extract energy from the apparatus by this means, as well as or instead of the generator arrangements at the central hub described above. These generators would not even have to be particularly efficient at collecting energy from the water flow, since the energy not collected would remain in the circulating stream within the dam. Initial calculations indicate that the apparatus could produce 750MW in electrical energy taken off at the central hub, compared with 5MW for a single HAWT, such as those used in offshore "wind farms". 750MW is more than could be extracted from a wind farm of similar area to the apparatus. With the hydroelectric generation system added, this power output could be raised further. Wave energy could also be collected and transferred to the water streams circulating within the dam, and it is believed that the total power produced could then reach 1.3GW.

One problem with wind farms using an array of individual HAWTs is that a "fence" of turbines can effectively cause a "wind shadow" of relatively low wind speeds trailing some distance downwind, so the HAWTs cannot be too closely packed, or energy collection efficiency will suffer. For the apparatus of the present invention, as long as the diameter of the support structure 4 is about eight times the height of the aerofoils 8, any wind shadow effect from the wind meeting the upwind half of the circle of aerofoils would have died away, leaving the downwind half of the circle of aerofoils to operate in very much the same wind speeds and with similar efficiency. Thus, if say 40% of the possible wind energy is extracted by the upwind aerofoils, and the downwind aerofoils collect 40% of the remaining 60%, the apparatus as a whole would collect a total of 64% of the possible wind energy. This compares well with the maximum theoretical effectiveness of 59% for a conventional HAWT and the limit of maybe 50% that is found in practice.

A possible issue with the rotating support structure floating in water is the hydrodynamic drag from driving it through the water at what would be fairly high speeds for (non-planing) water vessels. In order to overcome this, it is envisaged that air would be pumped out below the support structure, constrained by membrane walls and even a floor, to form an air bearing around and beneath the support structure 4, greatly reducing the frictional resistance experienced. (There will be plenty of energy available from the apparatus to power such an arrangement, after all.)

As well as the basic structures shown above, further research and development work has led to alternative structures, which show signs of even better performance, particularly on scale-up. These are shown in Figures 12 to 16; essentially Figure 12 shows a variation on what is shown in Figure 2, Figure 13 shows a variation on Figure 4, Figure 14 shows a variation on Figure 5, Figure 15 shows a variation on the arrangements of Figure 8, and Figure 16 shows a variation on the arrangements of Figure 10.

A further alternative form of the invention is also described below with reference to Figure 17, which can usefully be compared with Figures 4 and 13.

Referring now to Figure 12, a second aerofoil unit 76 is shown, which can be substituted for the aerofoil unit 6 shown in Figure 2. The second aerofoil unit 76 has the same main aerofoil 8 as aerofoil unit 6, mounted on essentially the same aerofoil base 9, and extending vertically upwardly with a constant symmetrical profile, having a leading edge 10 intended to be aligned generally to windward and a trailing edge 11 intended to be aligned generally downwind. A rigid boom 12 extends from the leading edge 10, and two canard aerofoils 13 extend upwardly and downwardly, respectively, from the distal end of the boom 12. Although the canard aerofoils 13 in Figure 12 are shown as the same size as in Figure 2, it is believed that in this second aerofoil unit 76 it may be possible for them to be relatively smaller, while still producing sufficient steering effect.

The aerofoil base 9 of the second aerofoil unit 76 is again rotatably supported on a pivot axle 15 and a roller guide assembly 20 that engages with a rim of the aerofoil base 9.

However, the second aerofoil unit 76 differs in that it has improved control arrangements for the canard aerofoils 13. The second aerofoil unit 76 is provided with a second link arm arrangement 72, which has a rearranged geometry, relative to the link arm arrangement 22 shown for the aerofoil unit 6. Details of this are shown in Figure 15, but in the context of Figure 12, the important difference is that the vertical drive shaft 39 is now located between the leading edge 10 and the pivot axle 15, rather than to the downwind side of the pivot axle 15, coincident with the centre of pressure 18 of the aerofoil 8 (compare Figures 4 and 13). Locating the vertical drive shaft 39 close to the leading edge 10, and hence closer to the canard aerofoils 13 themselves, has several advantages, but is only possible for larger examples of the aerofoil units, while in smaller models, only the arrangement of the aerofoil unit 6 can conveniently fit.

Figure 14 shows the second aerofoil unit 76 and its mounting to a second support structure 84, which is a development of the simple support structure 4 shown in Figures 5 and 6. The pivot axle 15 fits rotatably into an identical bearing housing 19, and a cam follower arrangement 21 identical to that described above surrounds the bearing housing 9 and interacts with the second link arm arrangement 72 on the second aerofoil unit 76 in substantially the same manner as it does with the link arm arrangement 22 on the aerofoil unit 6 (differences are described below).

In this embodiment, however, rather than the support structure 4 comprising a simple annular body of concrete (i.e. concrete containing voids and low-density fillers so that the support structure 4 floats), the second support structure 84 comprises a large annular channel, tank or trough defined by a circular outer wall 81, a concentric circular inner wall 82 and a base 83; the annular support structure 4 is mounted roughly midway between the outer 81 and inner walls 82 on an annular concentric circular rib 85 extending upwardly from the base 83. The volume within the second support structure 84 is filled with water 86. As the apparatus 1 as a whole rotates, the water 86 will also rotate, as a result of drag where it contacts the walls 81, 82 and base 83. Thus, the rotating mass for the second support structure 84 will be much greater than for the simple support structure 4, the angular momentum will be proportionately higher for any given rotational speed, and so the second support structure 84 behaves as a significantly superior "flywheel" or energy store - it will both retain more energy for subsequent use and even out the effects on speeds of rotation of variation in ambient wind speeds. The added angular momentum also helps to overcome hydrodynamic drag on an outer surface of the second support structure 84.

This embodiment is shown equipped with an air bearing arrangement as referred to above. Air is pumped out beneath the base 83 of the second support structure 84, forming an aerated volume 87 of water full of bubbles, contained between the base 83 and downward extensions 88, 89 of the outer wall 81 and inner wall 82 respectively. The downward extension 88 of the outer wall 81 extends further downwardly than the downward extension 89 of the inner wall 82, to help retain the aerated volume 87 as the second support structure 84 rotates. The viscosity of air is around fifty times less than the viscosity of water, which should result in much lower drag forces. When the total surface area of the base 83 of the whole second support structure 84 is considered, this drop in drag will greatly improve the rate at which the apparatus can be brought up to designed rotational speeds, and should retain much more stored energy by maintaining the speed of rotation than would be expended to pump the air into the aerated volume 87.

Figure 15 shows in detail the rearranged geometry of the second link arm arrangement 72, compared to the link arm arrangement 22 shown in Figure 8. The semi-annular cam follower 29 is the same in each, as are its bottom 31 and top 32 cam follower rollers (the entire cam follower arrangement 21 is the same for the aerofoil unit 6 and the second aerofoil unit 76). In each instance, two spacer columns 30 extend vertically between the cam follower 29 and the link arm arrangement 22, 72, thus transmitting motions of the cam follower 29 to the link arm arrangement 22, 72.

However, the straight link beam 35 of Figure 8 is in this version replaced by a second link beam 95, from which extend first and second lateral projections 93, 94. The simple link arms 36, 37 are replaced by a longer first link arm 96 and a much shorter second link arm 97. The first link arm 96 is pivotably mounted to a distal end of the first lateral projection 93 and the second link arm 97 is pivotably mounted to a distal end of the second lateral projection 94. The second link arm 97 is in turn pivotably mounted to a fixed point on the underside of the aerofoil base 9 and located on the chord 38 of the aerofoil 8), while the first link arm 96 is fixedly mounted to a lower end of the vertically-extending rotatable drive shaft 39 (see also Figure 12), also at a point lying on the chord 38 of the aerofoil 8.

The second link beam 95 extends parallelly to the chord 38, as does a line joining the respective distal ends of its lateral projections 93, 94. The second link arm arrangement 72 thus forms a parallel motion linkage, as in the case of the link arm arrangement 22, but the separation of the two pivot points on each of the first and second link arms 96, 97 is much less than the separation of the two pivot points on each of the link arms 36, 37 of the link arm arrangement 22 of Figure 8. Thus, linear (or near-linear) motions of the link beam 95, transmitted from the cam follower 29, lead to much greater angular displacements of the first 96 and second 97 link arms than for the link arms 36, 37 of the link arm arrangement 22. The vertical drive shaft 39, being coupled to the first link arm 96, is therefore twisted through a much greater angle, for a given movement of the cam follower 29. The benefits will be described below.

The other change in geometry between the link arm arrangements 22, 72 is that the first link arm 96 is now located between the leading edge 10 of the aerofoil 8 and the pivot axle 15, within the arc of the cam follower 29, while the second link arm 97 is now located outside said arc, maintaining the separation of the first 96 and second 97 link arms. This allows the vertical drive shaft 39, now mounted to the first link arm 96, to be located between the leading edge 10 and the pivot axle 15/vertical axis 17.

One further feature shown in Figure 15 is the elongate extension 98 of the first link arm 96. A distal end 99 of this extension can be connected to a spring or other resilient arrangement, to damp down sudden or jerky movements of the cam follower 29, second link beam 95 and first link arm 96. This smoothes out the consequent rotation of the vertical drive shaft 39, reducing stresses both in this part of the mechanism and in the next part of the mechanism that controls the alignment of the canard aerofoils 13.

This part of the mechanism is shown in Figure 16. As comparison with Figure 10 will show, most of this mechanism is unchanged. The aerofoil 8, the boom 12 extending from its leading edge 10, and the canard aerofoils 13 are the same, and the aerofoil 8 is pivotable about the vertical axis 17 (i.e. a vertical axis passing through the pivot axle 15). However, as described above, in this variant of the invention the vertical drive shaft 39, which runs vertically upwards through the aerofoil 8 from the link arm arrangement 72, is located between the leading edge 10 and the vertical axis 17. One immediate advantage of this is simply that the control runs leading to the canard aerofoils 13 are that much shorter, cheaper, responsive and reliable.

The vertical drive shaft 39 is again coaxially connected to a pulley wheel - here a third pulley wheel 102. A belt or chain 43 extends around the third pulley wheel 102, through the interior of the boom 12, and around a fourth pulley wheel 104 adjacent the distal end of the boom 12. The fourth pulley wheel 104 is fixedly mounted to a pivot axle for the two canard aerofoils 13, such that rotation of the fourth pulley wheel 104 changes the pitch of the canard aerofoils 13 relative to the boom and the main aerofoil 8. Thus, the vertical drive shaft 39 turning backwards and forwards turns the third pulley wheel 102, which moves the belt 43, which turns the fourth pulley wheel 104, to produce backwards and forwards changes in the pitch angle of the canard aerofoils 13.

As for the mechanism of Figure 10, the variant mechanism of Figure 16 has two spring/resilient sections 45 in the belt or chain 43, to help to absorb sudden or extreme rotations of the vertical drive shaft 39 and third pulley wheel 102 and obviate extreme or jerky rotation of the canard aerofoils 13. Additionally, if local gusts of wind briefly displace the canard aerofoils 13, or if there is a constant high wind, this arrangement can absorb and oppose this motion without significant feedback to the remainder of the control mechanism (and as noted above, this would lead to a lower effect from the canard aerofoils 13, the main aerofoil 8 would turn slightly into the wind, and so the whole aerofoil unit 6, 76 would automatically feather itself slightly, taking proportionately less energy from the higher wind and avoiding overspeeding in a positive feedback loop). A further constraint on extreme movements of the canard aerofoils 13 is again provided by the presence of a pitch limiter 46 to prevent the canard aerofoils 13 going outside a preselected pitch range relative to the boom 12 and hence the main aerofoil 8.

In the variant of Figure 16, however, due to the alterations in the link arm arrangement 72 described above, the vertical drive shaft 39 turns through a significantly greater angle for a given movement of the cam follower 29. To produce the same desired changes in pitch angle for the canard aerofoils 13, the third pulley wheel 102 hence has a significantly smaller diameter than the first pulley wheel 42, and still generates the same linear movements of the belt or chain 43. Thus, a finer and more responsive control over the pitch angle of the canard aerofoils 13 can be achieved.

If it for any reason becomes necessary to halt the apparatus 1, the control arrangements shown in Figures 10 and 16 make it straightforward - all that is needed is to disengage the drive/control runs to the canard aerofoils 13. For example, a simple dog clutch could be included in the drive shaft to each canard aerofoil 13. Disengaging the drive shaft would release the canard aerofoil 13 to swing into line with the local airflow, and so it would no longer deflect the main aerofoil 8 to catch the wind and the main aerofoil 8 in turn would swing into line with the wind and stop producing "lift".

The apparatus 1 described above has a symmetrical profile for both its main aerofoils 8 and its canard aerofoils 13. While this is preferred in most situations, asymmetric aerofoil profiles, as used in aircraft wings, can also be used, as shown in Figure 17.

A third aerofoil unit 206 can be used in place of either of the aerofoil units 6, 76 described above. This comprises an asymmetric main aerofoil 208 mounted to pivot about a vertical axis 217, and having a centre of pressure 218. A boom 212 extends to windward from the main aerofoil 208, at its distal end are mounted two controllably pivotable asymmetric canard aerofoils 213. Each of these pivots about a vertical axis 214, which is well to windward of a respective centre of pressure 219. Ideally, the same control arrangements are present as in Figure 16, or failing that Figure 10. Although it is not essential, the embodiment shown has the same asymmetric profile for the main 208 and canard 213 aerofoils, so they have the same AO A/performance characteristics.

When an asymmetric aerofoil is at a positive angle of attack (AOA), it will produce greater lift to drag ratios that a symmetrical aerofoil. However, when it is at a negative angle of attack, as is going to be the case for half of a rotation of the apparatus 1, it has lower coefficients of lift and lower lift to drag ratios.

Normally, this would be undesirable, but as mentioned above, wind shadow can be a problem unless the apparatus 1 is very large in diameter. If apparatus 1 embodying the present invention has to be built at a size where the downwind aerofoil units do experience a wind shadow from the upwind aerofoil units, then the asymmetric third aerofoil units 206 would have benefits. Arranged to have a positive AOA while on the upwind side of the apparatus 1 and a negative AOA on the downwind side, the apparatus 1 would be more effective at absorbing wind energy on the upwind side, and less efficient on the downwind side, but this would be less important as there would be less energy to absorb in the wind shadow anyway.

One further detail of the third aerofoil 206 is that rather than being on the respective true chords 238, 239 (shown by dashed lines), the support boom 212 and all pivot points of the asymmetric main aerofoil 208 and the asymmetric canard aerofoil 238 should lie on a line 248 extending through the centres of pressure 218, 219 of the aerofoils 208, 213 - this gives an angle of attack corresponding to the zero life condition of the aerofoil profile employed. The line 248 can then be considered as the neutral axis of this aerofoil system. Normally, the AOA is considered to be the angle of the air flow to the physical chord of an aerofoil; however, when controlling such a system with asymmetric aerofoils 208, 213, it is better to equate the AOA to the angle between the air flow and this neutral axis 248.

As can be seen, while it is possible to operate the invention with asymmetrical aerofoils, in most cases it will be more straightforward to rely on the less complex symmetrical aerofoils shown in the majority of the Figures above.