GB2547194A

GB2547194A - Vertical axis wind turbine

Info

Publication number: GB2547194A
Application number: GB1602271.7A
Authority: GB
Inventors: Mccarty David; Pappas Victor
Original assignee: Wind Spirit Energy Co LLC
Current assignee: Wind Spirit Energy Co LLC
Priority date: 2016-02-09
Filing date: 2016-02-09
Publication date: 2017-08-16
Also published as: GB201602271D0

Abstract

A vertical axis wind turbine 10 comprises a stationary mast 130 having a longitudinal axis 131, and upper and lower rotational members 120, 110 rotatable about upper and lower portions of the mast respectively. Vanes 160 extend between the upper and lower rotational members, each vane comprising a flexible elongate member 140 and a flexible plate 150 attached to, and extending along, one side of the elongate member. The elongate member has a length greater than the distance between the upper and lower rotational members, so that the elongate member and the attached plate bend outwardly from the mast when a first end and a second end of the elongate member are attached to the upper and lower rotational members respectively. The turbine may include a first engaging means to engage the lower rotational member to drive an electrical generator. The turbine may include a tachometer for measuring the speed of rotation of the turbine, a second engaging means for engaging the lower rotational member, and an automated controller for connecting the lower rotational member to a second electrical generator means via the second engaging means when the tachometer measures the speed above a threshold value.

Description

VERTICAL AXIS WIND TURBINE

The present invention relates to a vertical axis wind turbine.

Generating electricity from wind power is one of the most successful renewable energy technologies, and its use is expected to continue to grow rapidly. The UK, for example, plans to expand Britain’s wind generating capacity by at least three-fold by 2020. There is a legally-mandated goal for Ontario, Canada, to obtain 15% of its electrical power from wind by 2025, which will require the equivalent of 26,500 additional 1.5 Mw wind turbines to be built. Denmark intends to double its wind power capacity by 2030, even though 25% of its currently operating wind turbines are expected to have reached the end of their operational lives by then. Germany faces a similar situation where it is set to increase its wind power capacity while replacing many of its 25,000 active wind turbines which are now approaching the end of their 20 year life span. In addition, the World Energy Council predicts that the market for wind turbines in Asia, Russia, Latin America, and Africa, will surpass the combined demand of the USA and Europe by 2020.

Commercial wind turbines in operation today are almost exclusively of the horizontal axis type. The most commonly used size has a capacity of 1.5 Mw using blades 50 metres (164 ft.) in length. On average these cost USD 1.37 million per Mw to construct. These horizontal axis wind turbines (HAWT) also have relatively high annual maintenance expense. This is mainly due to the nacelle containing all the heavy equipment (gear box, inverter, generator, etc.) which is necessarily located at the top of their support towers and many nacelles are located over 100 metres above the surface.

Vertical axis wind turbines (VAWT), in a variety of configurations, are not extensively used. The majority of those in operation are small units which are not connected to an electrical grid. VAWTs have an advantage in being very quiet in operation, and they rotate at very low wind velocities, and regardless of wind direction, without the need for expensive wind sensing and blade orientation equipment. The California Institute of Technology (“Cal Tech”) has carried out exhaustive studies and field trials which shows that a VAWT requires as little as one-tenth of an area of land to yield the same amount of electricity as a similar-size HAWT. However, until now, VAWTs have been expensive to build in relation to their output (USD per Kw capacity). The largest VAWT ever built was in Quebec, Canada. It had 110 metre (361 ft.) long blades, and a 3.8 Mw capacity. However, it cost approximately USD 110 million to construct, or about USD 29 million per Mw. This expense has overshadowed the benefits of a VAWT over a HAWT.

It is an object of the present invention to provide a less costly, more efficient VAWT.

According to the present invention there is provided a vertical axis wind turbine, comprising: a stationary mast having a longitudinal axis; an upper rotational member or crown ring rotatable about an upper portion of the mast; a lower rotational member or base ring rotatable about a lower portion of the mast; and a plurality of vanes wherein each vane extends between the upper and lower rotational members and each vane comprises a flexible elongate member or a long supporting bar, called a stringer, and a flexible plate attached to the elongate member and extending along one side of the elongate member, the elongate member having a length greater than the distance between the upper and lower rotational members and parallel to the longitudinal axis so that the elongate member and the attached plate bend outwardly from the mast when a first end of the elongate member is attached to the upper rotational member and a second end of the elongate member is attached to the lower rotational member.

The vertical axis wind turbine retains the generic advantages that vertical axis wind turbines have over the far more commonly used horizontal axis wind turbines. The vertical axis wind turbine overcomes the inherent high costs of construction of both types by replacing the expensive labour intensive components with simple designs using run-of-the-mill materials and components significantly lowering the manufacture cost, and by incorporating an improved vane design with a simple airfoil configuration to enhance power generating capacity. The vanes are built from two components: a flexible elongate member and a flexible plate. Each of these may be made from a flat piece of aluminum cut to the correct size and shape at a mill. These vanes replace the more expensive, precision-engineered, laboriously manufactured airfoils of other designs. This is possible because the fastened elongate member is automatically and easily curved during construction of the turbine to impart an airfoil shape to the vane which increases aerodynamic efficiency in capturing wind energy. The flexible elongate member also acts as the supporting member for the entire vane, reducing the necessary strength and weight of the plate. Thus, the flexible elongate member component makes the assembly both lighter and more efficient, and enables it to harness wind energy at lower wind velocities.

Furthermore, the simplified components allow the vanes to be installed during construction in such a manner, and at no additional cost, as to enable either clockwise or counterclockwise rotation of the turbine by changing the direction that the plate extends from the flexible elongate member.

In addition, the vertical axis wind turbine is safe for the environment. It is quiet, free from any type of pollution, does not represent a hazard to birds, bats, or other wildlife. At the end of its useful life, the parts of the turbine, except perhaps for the mast and any foundation constructed for the turbine, can be recycled.

The vertical axis wind turbine can be constructed in a wide range of diameters and mast lengths to accommodate the power requirements of onshore and offshore grid-connected wind farms. Its light weight and quiet operation also make it suitable for off-grid connections for industrial parks, office complexes, shopping malls, and even for individual buildings such as hospitals, college dormitories, and various commercial buildings.

The upper and lower rotational members may have bearings.

The opposite ends of the elongate member may extend beyond the respective ends of the plates attached thereto.

The first end of the elongate member of each vane may be attached to the upper rotational member so that the elongate member initially extends from the upper rotational member in a direction substantially perpendicular to the longitudinal axis. The second end of the elongate member of each vane may be attached to the lower rotational member so that the elongate member initially extends from the lower rotational member in a direction substantially parallel to the longitudinal axis. Thus, the overall shape of the vertical axis wind turbine would be similar to a conventional light bulb.

The vertical axis wind turbine may include first engaging means for engaging the lower rotational member wherein an electrical generator means is arranged to be driven by rotation of the lower rotational member via the first engaging means. As the wind blows against the vertical axis wind turbine, it causes the vanes to revolve about the mast on the lower rotational member and the upper rotational member. The lower rotational member may be affixed to one or more rotation assemblies to transform the kinetic energy of the rotating turbine into electrical energy. The rotation assembly may comprise the first engaging means. The rotation assembly may comprise appropriate gearing to a generator and associated equipment to generate electricity. The rotation assembly will normally comprise a brake and rotation lock, a gear box, the generator, an AC inverter, and a connection to an electrical grid. Alternatively, a doubly-fed induction generator may be directly connected to an electrical grid. The size, design, and components of the rotation assembly are engineered to optimize performance of a wind turbine of specific capacity operating under known or projected environmental conditions.

The vertical axis wind turbine may include a tachometer for measuring the speed of rotation of the wind turbine, second engaging means for engaging the lower rotational member, and an automated controller for connecting the lower rotational member to a second electrical generator means via the second engaging means when the tachometer measures the speed above a threshold value or exceeds a pre-set number. This permits the wind turbine to operate at higher wind velocities and generate more electricity than its rated capacity. Such an arrangement is not aerodynamically, structurally or economically feasible on horizontal axis wind turbines. A second rotation assembly may be provided which includes the tachometer and the controller for engaging/ disengaging. The second rotation assembly may comprise the second engaging means. The second engaging means may be substantially diametrically opposed to the first engaging means with respect to the mast. The second rotation assembly may be placed at or near ground level, but level with and diametrically opposed (180 degrees) to the first or primary rotation assembly.

The plate of each vane may extend substantially at a tangent to a radius of the wind turbine about the longitudinal axis.

The flexible plate of each vane may have an edge distal to the elongate member which is convex. The flexible plate may have an edge proximal to the elongate member and which is straight. The flexible plate may have a pair of end edges extending between the proximal edge and the distal edge. The end edges may be of substantially equal length, and the maximum width of the flexible plate between the straight proximal edge and the convex distal edge is substantially four times the length of each of the end edges.

The vertical axis wind turbine can be set to rotate in either a clockwise or counterclockwise direction by the simple, cost-neutral expedient of installing its vanes with the convex edge angled to the right of radial, when viewed from above for clockwise rotation or to the left of radial for anti-clockwise rotation.

The convex edge of the vane may be known as the “leading edge”, and the straight edge of the vane may be known as the “trailing edge”. The term “diameter” refers to the maximum diameter of the internal structure of the wind turbine created by the flexible elongate member when they are attached to the lower and upper rotational members.

In an embodiment, the flexible elongate members are attached, preferably by welding, to trailing edge of plates to form the vanes. Unattached ends of the flexible elongate members which extend beyond the plate serve as a bottom tail and a top tail. The bottom tail of the vane is attached to the lower rotational member and the top tail is attached to the upper rotational member. This bends the vane into an airfoil shape with the leading edge extending outward from the mast to catch the wind. The top tail may be fixed to the upper rotational member in a nearly horizontal or horizontal orientation. The bottom tail may be fixed to the lower rotational member in a nearly vertical or vertical orientation.

The second end of the elongate member of each vane may be attached to the lower rotational member so that the elongate member initially extends from the lower rotational member in a direction substantially perpendicular to the longitudinal axis. Thus, the overall shape of the vertical axis wind turbine would be effectively spherical. This shape is suitable for offshore installations, or in wind farm areas potentially subject to flooding as a spherical configuration raises the position of the base ring on the mast. This permits the rotation assembly to be placed on a foundation or platform raised higher (above sea or ground level) than in the light bulb configuration without increasing the length of the mast.

The plate of each vane may have an edge distal from the elongate member of the vane and the edge overlaps the elongate member of an adjacent vane. This may provide the vertical axis wind turbine with the appearance of a solid or near solid object, particularly when the turbine is rotating.

The elongate member of each vane may comprise a flat strip. The plane of the strip may be substantially perpendicular to a plane extending through the longitudinal axis when the strip is attached to the upper and lower rotational members.

The plate may have a length along the elongate member greater than the distance between the upper and lower rotational members.

The flexible plate of each vane may comprise aluminium.

The flexible plate of each vane may be a single member. This is suitable for smaller configurations of the wind turbine. Alternatively, the flexible plate of each vane may comprise a plurality of members. This is suitable for larger configurations of the wind turbine. These members may be fixed together, preferably by butt welding.

In a wind farm configuration a VAWT will generate more electricity per unit of land area than a HAWT. Thus, VAWTs can be placed closer together to reduce the amount of expensive land required. Efficiency can be significantly enhanced if adjacent wind turbines rotate in opposite directions.

The vertical axis wind turbine increases the power generating capacity of wind turbines. The most popular model HAWT has a 1.5 Mw nameplate capacity with three blades, each 50 metres (164 ft.) long, and a 100 metre (328 ft.) diameter. Using the industry accepted formula (tit2 where r = length of a single blade) to determine a turbine’s “swept area” and its direct, albeit crude, relationship to power generation, the HAWT has a swept area of 7,854 square metres. In comparison, the vertical axis wind turbine in a comparable size would have a diameter of only 40 metres (130 ft.), but each of its multiple vanes, preferably approximately 40 to 44 in number for a wind turbine of this diameter, would be 61 metres (200 ft.) long due to its shape. This provides it with a swept area of 11,690 square metres indicating a nameplate capacity of 2.2 Mw. This is a 47% greater potential power generating capacity than the HAWT.

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

Figure 1 is a side view of the vertical axis wind turbine in accordance with one embodiment of the invention installed on the ground with an attached rotation assembly;

Figure 2 is a top view of the vertical axis wind turbine;

Figure 3 is a side view of a vane of the vertical axis wind turbine laid flat;

Figure 4 is a side view of the vertical axis wind turbine;

Figure 5 is a perspective view of the vertical axis wind turbine;

Figure 6 is a detail showing the attachment of the vane to a crown ring of the vertical axis wind turbine;

Figure 7 is a detail showing the attachment of the vane to a base ring of the vertical axis wind turbine, and the rotation assembly;

Figure 8 is a detail similar to Figure 7 and shows an additional rotation assembly;

Figure 9 is a side view of a vertical axis wind turbine in accordance with another embodiment of the invention with an attached rotation assembly; and

Figure 10 is a detail showing the attachment of the vane to a base ring of the vertical axis wind turbine of Figure 9.

Referring to Figures 1 to 7 of the accompanying drawings, a vertical axis wind turbine 10 utilizes thin, flat plates 150 fixed to flexible elongate members or long supporting bars called stringers 140, the attached assembly forming a vane 160. Several such vanes 160 are assembled vertically presenting a curved airfoil surface to catch the wind and rotate the entire wind turbine 10. The vanes 160 are fixed to a cylindrical lower rotational member or ring called a base ring 110 and to a cylindrical upper rotational member or ring called a crown ring 120 that revolve about a stationary vertical mast 130. The vanes 160 are attached such that in the final assembly, the vanes 160 will partially enclose a space about the mast 130 in a shape generally resembling a light bulb. The vanes 160 will collect wind from all directions to drive one or more rotation assemblies 190 even at low wind velocities. In the preferred embodiment, the rotation assembly is placed at or near ground level on top of foundation 180.

The mast 130 is the main support structure of the vertical wind turbine 10 and has a longitudinal axis 131 which extends in a vertical direction. The mast 130 is engineered for a specific wind turbine diameter and projected environmental factors including lightning protection. For turbines intended to be placed at or near ground level, the total length of the mast 130 is generally less than two times longer than the widest diameter of the wind turbine 10. For units placed higher to obtain the benefits of greater wind velocity at higher altitudes, the mast 130 would be appropriately engineered for greater length and sturdiness. The mast 130 may be constructed of concrete, fiberglass, carbon fiber, steel, or aluminum. The base of the mast 130 is anchored in a foundation 180 preferably comprising concrete or reinforced concrete.

The base ring 110 and crown ring 120 are solid, circular metal components, preferably composed of aluminum, which are attached near the bottom and top of the mast 130, respectively. The base ring 110 and crown ring 120 are stepped and are configured to revolve about the fixed mast 130. In the preferred embodiment, the base ring 110 and crown ring 120 revolve on double row tapered roller bearings (not shown), but either component may also rotate on other types of bearings known in the art.

The stringers 140 when fastened to the base ring 110 and the crown ring 120 determine the shape and diameter of the wind turbine 10 and the curve of the vanes 160. The stringers 140 form the stiffening, supporting element of the vanes 160. Stringers 140 are preferably made of rectangular/flat aluminum bars, Grade 6061-T6511, 1.9 cm (0.75 inch) thick and 15.24 cm (6 inches) wide, with a preferred length 1.57 times that of the intended widest diameter of the wind turbine 10, plus an additional preferred length of 1.22 metres (4.0 ft.) to provide top and bottom tails 142, 141 of the stringer 140 to be affixed to the crown ring 120 and the base ring 110, respectively. The top and bottom tails 142, 141 are the parts of the stringer 140 that extend beyond the plate 150 and each tail 142, 141 may be 61 cm (24 inches) long. In alternative embodiments, the stringers 140 may be of different sizes and composed of materials other than aluminum such as steel, carbon fiber, or fiberglass. With the preferred aluminum construction, the stringers 140 should be attached to the plates 150, crown ring 120, and base ring 110 by welding. The top tails 142 of the stringers 140 may be welded to an upper face 121 of a larger diameter portion 122 of the crown ring 120 wherein the larger diameter portion 122 is below a smaller diameter portion 123 of the crown ring 120 (see Fig. 6). The bottom tails 141 of the stringers 140 may be welded to an outer edge face 111 of a larger diameter portion 112 of the base ring 110 wherein the larger diameter portion 112 is above a smaller diameter portion 113 of the base ring 110 (see Fig. 7). Other means of attachment such as rivets, bolts, or glue may be utilized as appropriate if materials non-receptive to welding are used for any of the components.

The plates 150 are the part of the wind turbine 10 that will catch the wind in order to rotate the wind turbine 10 and drive a generator 194 of the rotation assembly 190. Each plate comprises a straight edge 151, a convex edge 152, a bottom end 153, and a top end 154 (see Fig. 3). In the preferred embodiment, the plates 150 are made of aluminum. Grade 3003 H14 aluminum plate is suitable for smaller configurations of the invention. Grade 5052 H32 aluminum plate in thicker sizes is preferred for larger versions because of its strength, amenability to forming and welding, and for offshore installations because of its high salt and corrosion resistance.

In a preferred embodiment, the bottom end 153 and top end 154 are both sized to 30 cm (12 inches). The convex edge 152 smoothly arcs outward until the plate 150 reaches a maximum width. In the preferred embodiment, the maximum width of the plate 150 is 122 cm (48 inches), four times the width of a bottom end 153 or a top end 154. Instead of aluminum, each plate 150 may alternatively be constructed of plastic, carbon fiber, composite materials, or steel, but aluminum is preferred due to its combination of strength, low weight, and flexibility. Regardless of material, the plates 150 may either be composed of a single unitary piece or of several pieces joined together along the same plane. When the plates comprise aluminum as in the preferred embodiment, if not in a single piece, the individual sections are preferably attached by buttwelding.

Each vane 160 is formed from one plate 150 and one stringer 140 fixed together. In the preferred embodiment, the stringers 140 have a length greater than that of a plate 150 by a distance at least equal to the radius of the crown ring 120 and the height of the base ring 110. The stringer 150 is fixed to the straight edge 151 such that some of the length of the stringer 140 remains free on each end of the vane to form the bottom tail 141 and the top tail 142. The bottom tail 141 is attached to the base ring 110 and the top tail 142 is attached to the crown ring 120, preferably by welding. Because the vanes 160 have a length greater than the distance between the base ring 110 and crown ring 120, each vane 160 must be bent outward from the crown ring 120 and inward to the base ring 110.

It is preferable to connect the top tail 142 of the vane 160 to the crown ring 120 in a horizontal or nearly horizontal orientation. To achieve this orientation it is preferred that the crown ring 120 has a horizontal or nearly horizontal face so that the top tail 142 of the vane 160 can simply be welded to the face of the crown ring 120. Gravity will then assist bending the entire vane 160 so that the bottom tail 141 may be fastened to the base ring 110.

In the preferred embodiment, the bottom tail 141 of the vane 160 is connected to the base ring 110 in a vertical or nearly vertical orientation. It is preferred that the base ring 110 has a vertical or nearly vertical face so that the bottom tail 141 of the vane 160 can simply be welded to the face of the base ring 110. This configuration results in the preferred shape of the wind turbine 10 resembling a light bulb.

In this light bulb shape, the upper portion of the wind turbine 10 captures more energy from the wind than the lower portion due to its increased leverage and altitude.

The above described components represent the inventive steps of the present invention. The remaining components necessary for wind power generation, including the upper and lower bearings and the rotation assembly, are primarily off-the-shelf equipment of the appropriate size for the intended turbine, all of which are well known to the art.

The rotation assembly 190 (see Fig. 7) typically comprises engaging means 191 for engaging a lower part 113 of the base ring 110, a brake and rotation lock 192, a gear box 193, the generator 194, an AC inverter 195, a deicing system 196, and a connection 197 to an electrical grid, all of which are well known to the art.

Referring to Figure 8, the vertical axis wind turbine 10 may also include a second rotation assembly 170 comprising a tachometer 171, and an engage/disengage controller 172, in addition to engaging means 173 for engaging the base ring 110, a gear box 174, a generator 175, an AC inverter 176, and a connection 177 to the electrical grid. While these components are well known to the art, the installation of the second rotation assembly 170, to enhance the power generating capacity of the wind turbine 10 during periods of high wind velocity, is a feature unique to the larger versions of the present invention. Installation of a second rotation assembly is not a feasible option for HAWTs for obvious aerodynamic, structural, and economic reasons. When an optional second rotation assembly 170 is employed in the present invention it is preferably installed at the same height as the primary rotation assembly 190, usually at or near ground level, and directly (180 degrees) opposite the primary rotation assembly 190.

Referring to Figures 9 and 10, another embodiment of the vertical axis wind turbine 200 has a spherical configuration which is obtained by the bottom tail 241 and top tail 242 both being simply attached or affixed to the base ring 210 and crown ring 220, respectively, horizontally or nearly horizontally. The crown ring 220 may be the same as the crown ring 120 of the vertical axis wind turbine 100 having a light bulb configuration. The base ring 210 is similar to the crown ring 220 and the bottom tails 241 of the stringers 240 may be welded to a lower face 211 or preferably an upper face 214 of a larger diameter portion 212 of the base ring 210 wherein the larger diameter portion 212 is above a smaller diameter portion 213 of the base ring 210. For the present invention in a 39.62 metre (130 ft.) diameter size the rotation assembly 290 could thus be raised approximately 10 metres (33 ft.) higher than the rotation assembly 190 of the vertical axis wind turbine 100 having a light bulb configuration without increasing the length of the mast 130. In this instance, the rotation assembly 290 is shown placed on a platform 281 above the foundation 190.

Other shapes are also possible.

Whilst particular embodiments have been described, it will be understood that various modifications may be made without departing from the scope of the claimed invention.

Claims

CLAIMS:

1. A vertical axis wind turbine, comprising: a stationary mast having a longitudinal axis; an upper rotational member rotatable about an upper portion of the mast; a lower rotational member rotatable about a lower portion of the mast; and a plurality of vanes wherein each vane extends between the upper and lower rotational members and each vane comprises a flexible elongate member and a flexible plate attached to the elongate member and extending along one side of the elongate member, the elongate member having a length greater than the distance between the upper and lower rotational members and parallel to the longitudinal axis so that the elongate member and the attached plate bend outwardly from the mast when a first end of the elongate member is attached to the upper rotational member and a second end of the elongate member is attached to the lower rotational member.

2. The vertical axis wind turbine as claimed in claim 1, wherein the first end of the elongate member of each vane is attached to the upper rotational member so that the elongate member initially extends from the upper rotational member in a direction substantially perpendicular to the longitudinal axis.

3. The vertical axis wind turbine as claimed in claim 2, wherein the second end of the elongate member of each vane is attached to the lower rotational member so that the elongate member initially extends from the lower rotational member in a direction substantially parallel to the longitudinal axis.

4. The vertical axis wind turbine as claimed in claim 2, wherein the second end of the elongate member of each vane is attached to the lower rotational member so that the elongate member initially extends from the lower rotational member in a direction substantially perpendicular to the longitudinal axis.

5. The vertical axis wind turbine as claimed in any preceding claim, wherein the plate of each vane extends substantially at a tangent to a radius of the wind turbine about the longitudinal axis.

6. The vertical axis wind turbine as claimed in any preceding claim, wherein the plate of each vane has an edge distal from the elongate member of the vane and the edge overlaps the elongate member of an adjacent vane.

7. The vertical axis wind turbine as claimed in any preceding claim, wherein the flexible plate of each vane has an edge distal to the elongate member which is convex.

8. The vertical axis wind turbine as claimed in claim 7, wherein the flexible plate has an edge proximal to the elongate member which is straight.

9. The vertical axis wind turbine as claimed in claim 8, wherein the flexible plate has a pair of end edges extending between the proximal edge and the distal edge.

10. The vertical axis wind turbine as claimed in claim 9, wherein the end edges are of substantially equal length, and the maximum width of the flexible plate between the straight proximal edge and the convex distal edge is substantially four times the length of each of the end edges.

11. The vertical axis wind turbine as claimed in any preceding claim, wherein the elongate member of each vane comprises a flat strip.

12. The vertical axis wind turbine as claimed in claim 11, wherein the plane of the strip is substantially perpendicular to a plane extending through the longitudinal axis when the strip is attached to the upper and lower rotational members.

13. The vertical axis wind turbine as claimed in any preceding claim, including first engaging means for engaging the lower rotational member wherein an electrical generator means is arranged to be driven by rotation of the lower rotational member via the first engaging means.

14. The vertical axis wind turbine as claimed in claim 13, including a tachometer for measuring the speed of rotation of the wind turbine, second engaging means for engaging the lower rotational member, and an automated controller for connecting the lower rotational member to a second electrical generator means via the second engaging means when the tachometer measures the speed above a threshold value.

15. The vertical axis wind turbine as claimed in claim 14, wherein the second engaging means is substantially diametrically opposed to the first engaging means with respect to the mast.

16. The vertical axis wind turbine as claimed in any preceding claim, wherein the plate has a length along the elongate member greater than the distance between the upper and lower rotational members.

17. The vertical axis wind turbine as claimed in any preceding claim, wherein the flexible plate of each vane comprises aluminium.

18. The vertical axis wind turbine as claimed in any preceding claim, wherein the flexible plate of each vane is a single member.

19. The vertical axis wind turbine as claimed in any one of claims 1 to 17, wherein the flexible plate of each vane comprises a plurality of members.

20. The vertical axis wind turbine as claimed in any preceding claim, wherein opposite ends of the elongate member extend beyond the respective ends of the plates attached thereto.