GB2466209A

GB2466209A - Wind turbine wake expansion device

Info

Publication number: GB2466209A
Application number: GB0822632A
Authority: GB
Inventors: Carsten Hein Westergaard
Original assignee: Vestas Wind Systems AS
Current assignee: Vestas Wind Systems AS
Priority date: 2008-12-11
Filing date: 2008-12-11
Publication date: 2010-06-16
Also published as: GB0822632D0

Abstract

A wind turbine comprises a nacelle 112 mounted on a tower 114, a rotor 116 mounted for rotation about the nacelle 112, and a wake expansion device for expanding the wake downstream of the rotor 116 in a radial direction. The wake expansion device may be moveable between retracted and deployed positions, and may be deployable behind the rotor 116 to slow the wind speed. The wake expansion device may be a parachute (figures 5 and 6). The wake expansion device may be a fairing 150 around the nacelle 112 which is deployable into a substantially conical shape by actuators 152 in response to measured wind speed. The wake expansion device may be a cone in front of the rotor 116 which extends over a rotor hub 120 and blades 118 (figures 10, 13 and 14).

Description

WAKE EXPANSION DEVICES FOR WIND TURBINES

This invention relates to wind turbines and, in particular, to devices for increasing the energy extracted from the air by a wind turbine.

Wind turbines extract kinetic energy from air and convert that energy into electrical energy. It is well understood that the amount of power that can be generated by a wind turbine P = C, 1/2 pU3.A where p is the air density, U is the air velocity and A is the swept area of the rotor. C, is the coefficient of performance and has a maximum value of 16/27 known as the Betz limit which is the maximum amount of energy that can be extracted by optimum flow of air through a rotor plane with a given area. If all the energy were extracted from the wind the air would be reduced to a standstill and could not leave the wind turbine. The turbine would then effectively block the incident wind which would pass around the rotor plane to fill the void behind the rotor and no energy would be extracted by the rotor. The Betz limit of 16/27 (or 59%) applies to any wind turbine which uses a rotor to extract energy from the air.

It is also well understood that the wake behind a wind turbine rotor expands out beyond the area swept out by the rotor. This is illustrated in Figure 1 which shows a rotor of diameter A0 and a wake of diameter Aw where Aw > A0. This may be understood by considering the airflow through the wind turbine. The turbine extracts kinetic energy from the air and slows it down so that the air downwind of the turbine is moving more slowly than that upwind of the turbine. The amount of air entering the swept rotor area is the same as that leaving the swept area, thus the air behind the rotor must occupy a larger cross section behind the rotor plane.

This area of lower speed air behind the rotor is referred to as the wake.

However, the wake also extends in front of the rotor as the transition between Ao and Aw is smooth as can be seen from Figure 1.

It is well understood that the Betz limit cannot be altered and much of modem wliid turbine design is concerned with trying to get a practical wind turbine as close to that limit as possible over a range of operating conditions. It has been proposed to increase the amount of energy extracted by increasing the collecting area. Figure 2 shows a flared casing around the wind turbine rotor that effectively operates as a fan in reverse.

The Betz limit is not affected but the energy extracted by the rotor is increased as the wind velocity through the rotor is increased. However, given the size of modern wind turbines, the approach illustrated in Figure 2 is impractical.

We have appreciated that the amount of energy that can be extracted by a wind turbine may be increased by expanding the wake of the turbine. If the wake is expanded, a greater volume of air will be drawn through the rotor enabling more energy to be extracted.

Accordingly, the invention broadly provides a wake expansion device for a wind turbine. The wake expansion device is additional to the components of a conventional wind turbine such as the nacelle. More specifically there is provided a wind turbine comprising a nacelle mounted on a tower, a rotor mounted for rotation about the nacelle to extract energy from incident wind, and a wake expanding device for expanding the wake produced downwind of the rotor by wind passing through the rotor, the wake expansion being in the radial direction of the rotor.

As explained above, the wake expansion device has the advantage of increasing the size of the wake A behind the swept rotor which increases airflow through the rotor and so increases the amount of kinetic energy which can be converted into electrical energy. This enables wind turbines to operate at a higher power when wind speeds are below the rated power, and so to exceed the current Cp values which are presently being achieved in practice. Above rated wind speed it is generally not attractive S 3 to extract further energy, although embodiments of the invention offer the possibility, as this may overload the electric generator.

A secondary effect of expanding the wake is to speed up air flow in the inner part of the rotor. It is well known that the inner parts of the rotor are very difficult to make efficient, predominantly due to the high angles of attack of the blades. Speeding up the airflow over this part reduces the angle of attack and assists increasing rotor efficiency.

Preferably, the wake expansion device is movable between a retracted position and a deployed position. This is advantageous as it enables the wake expansion device to be deployed at relatively low wind speeds but to be retracted at higher wind speeds so avoiding excessive wind loading on the wind turbine components, when the benefits of the wake expansion device are generally not needed.

Preferably, a control device controls movement of the wake expansion device between the retracted and deployed positions in response to measured wind speed. Wind speed may be measured by an anemometer on the turbine nacelle and the control device may be part of a larger wind turbine control system. It is advantageous to be able to control deployment of the wake expansion device automatically along with control of other turbine components.

In one preferred embodiment the wake expansion device is deployable behind the nacelle and may comprise a device for reducing the speed of air behind the nacelle. Such a device has the advantage that a reduction in air speed forces the wake to expand outwards thus providing for wake expansion. The wake expansion device may be a parachute or a kite or a similar device and may be housed in a guide tube extending from the rear of the nacelle into the tower when in the retracted position. Preferably, a winch is mounted on the tower for deployment and retraction of the wake expansion device. The winch may be mounted on a rail on a platform within the tower to cope with the nacelle yawing with respect to the tower.

The winch may be under the control of the wind turbine control system to deploy the parachute or other device automatically at a given wind speed.

Where the wake expansion device is a parachute or other similar device such as a kite it is preferred that the parachute has a launch parachute attached which is arranged in an at least partially deployed position when the parachute is retracted within the guide tube.

In a further preferred embodiment of the invention, the wake expansion device may be arranged around the nacelle. Preferably, the wake expansion device is a conical fairing and preferably the fairing is concave.

This arrangement has the advantage that air, having passed through the rotor, passes over the conical fairing and is pushed outwards to expand the wake. This effect may be optimised by positioning the wake device such that the fairing extends from a position on the nacelle near the rotor to a position of maximum width behind the nacelle.

In one preferred embodiment, the wake expansion device arranged around the nacelle comprises a plurality of guide plates each pivotably mounted at an end thereof to a side of the nacelle, whereby a free end of the guide plates can pivot between a retracted and a deployed position.

Preferably an actuator, such as a plurality of motor driven spindles mounted in the nacelle moves the plates between the retracted and deployed positions.

The actuators may respond to the wind turbine control system to deploy the plates into their extended positions at lower wind speeds so expanding the wake behind the rotor.

Preferably, adjacent sides of the guide plates are connected by connecting surfaces. Thus, the wake expansion device, when deployed, is substantially conical. The connecting surfaces may be made of a flexible material such a sail cloth or may comprise a pair of triangular plates arranged between adjacent guide plates, the triangular plates of each pair being hinged to an adjacent plate and to one another.

A substantially conical wake expander is advantageous as it provides for a more efficient wake expansion. The arrangements enable a substantially conical wake expansion device to be provided which is collapsible at lower wind speeds. Preferably, in the expanded position, the angle of the guide plates to their respective nacelle sides is over 200.

In a further preferred embodiment, the wake expansion device comprises a cone arranged in front of the rotor and extending over the rotor hub and a portion of the rotor blades having an aerodynamic cross section. The diameter of the cone may be between 10 -20% of the rotor diameter.

Such an arrangement has the advantage that wake expansion behind the rotor can be provided for by a wake expansion device placed in front, or up stream, of the rotor as air incident on the rotor is deflected by the cone onto faster rotating portions of the rotor blades.

The cone may comprise a plurality of retractable plates and the wake expansion device comprises an actuator for moving the plates into a deployed position to form the cone. This arrangement enables the cone to be deployed only when wind speed falls below a given level. The actuator may be controlled by the turbine controller.

In one preferred embodiment, the wind turbine is a downwind turbine where wind passes over the nacelle before meeting the rotor. Preferably, the wake expansion device comprises a conical fairing arranged around the nacelle and extending from an apex behind the nacelle to a base adjacent to the rotor. This arrangement is also advantageous in that it directs air to faster moving parts of the turbine blades enabling greater energy extraction at lower wind speeds.

Embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 (referred to above) illustrates the expansion of air behind a wind turbine rotor to form a wake; Figure 2 (referred to above) illustrates a known proposal to increase airflow through a rotor; Figure 3 illustrates the principle of wake expansion embodying the present invention; Figure 4 illustrates how wake expansion can be achieved up wind of the wind turbine; Figure 5 illustrates a first embodiment of the invention showing a retracted parachute; Figure 6 shows the embodiment of Figure 5 in the fully deployed position; Figure 7 illustrates a front view of a second embodiment of the invention; Figure 8 is a section on the line A -A in Figure 7; Figure 9 is a rear perspective view of the embodiment of Figures 7 and 8; Figure 10 is a side view of a third embodiment of the invention particularly suited to a downwind turbine; and Figure 11 shows a fourth embodiment of the invention in which a wake expander is mounted on the nacelle housing; Figure 12 is a view on the line A -A in Figure 11; Figure 13 is a side view of a fifth embodiment of the invention in which a wake expander is mounted on the upwind side of the rotor; Figure 14 is an end view of the embodiment of Figure 13; and Figure 15 is a graph of power against wind speed for an operational wind turbine.

In the embodiments to be described, wake expansion devices are used which increase the size of the wake behind the rotor so increasing the flow of air through the rotor enabling energy to be recovered from a greater volume of air. In some cases, a device is mounted in front of the rotor, that is upwind of the rotor, to affect the wake behind the rotor. Horizontal axis wind turbines (HAWTs) may be upstream or downstream turbines.

An upwind turbine has the rotor facing the wind and is the most commonly implemented wind turbine design. The nacelle is behind the rotor and thus does not shade the rotor from the oncoming wind. In a downwind turbine, the rotor is on the lee side of the tower and incident air passes first over the nacelle onto the rotor. Downwind turbines have the advantage that they follow the wind passively and do not need a yaw mechanism to keep the rotor facing the wind. Except where stated, the embodiments below are suitable for use with either upwind or down wind turbines although, for the sake of brevity, they are only described in relation to one or the other of the two types.

The embodiments described below are suitable for fitting to existing wind turbines to increase the amount of energy that can be extracted. Existing commercial wind turbines are designed to operate at a rated power or capacity, for example, 2 -3 MW for a 90 metre diameter rotor. Figure 15 is a graph of power against wind speed and it can be seen that once the wind speed is sufficiently high for the generator to be operating at rated power, the power output is constant. There is, therefore, little point in trying to increase the amount of energy that can be recovered at the rated wind speed Vrated. However, it is advantageous to be able to increase the energy recovered at lower wind speeds so as to approach rated power at wind speeds below Vrated. Thus, in the embodiments described below, the wake expansion devices are retractable so that they may be deployed at lower speeds and then retracted once the wind speed reaches Vrated.

However, the invention is not limited to retractable devices but is also applicable to turbines which are designed to incorporate embodiments of the invention at any wind speed. The advantage of a non-retractable or stationary wake expansion device is one of mechanical simplicity.

However, the disadvantage is increased loading at higher wind speeds, compared to a turbine without such a device, everything else being equal.

Figures 3 and 4 show how the wake of a wind turbine may be expanded by placing a blockage in the air flow. In Figure 3, a smooth cone-shaped device is mounted on a pole or tower behind the wind turbine rotor approximately in line with the rotor axis. Wake air behind the rotor will flow smoothly over the wake cone generating vortices behind the cone and spreading the wake air outwards and so expanding the wake.

For the avoidance of doubt, the term wake expansion is intended to refer to increasing the width of the wake in the radial direction of the rotor rather than expanding the length of the wake behind the rotor. Thus, wake expansion refers to an increase in the dimension Aw of Figure 1 compared to the value of Aw for a conventional wind turbine.

Figure 4 shows the effect of mounting the cone of Figure 3 in front of the turbine rotor. In both the Figure 3 and 4 examples the wind turbine is an upwind turbine so that the cone of Figure 4 affects the airstream before it meets the rotor blade. In this position, the cone will direct the airstream towards the outer parts of the rotor blades. These parts are faster moving and can extract more energy or make the blades more effective due to a reduced angle of attack. Figures 3 and 4 illustrate that the invention will work both when the device is placed upstream and downstream with respect to the rotor. For an idealized flow (potential flow), in principle it does not matter where the device is placed to expand the wake. However, S 9 in practice, one solution may prove more effective than other taking into account many practical considerations and the actual blade design.

Figures 5 and 6 illustrate a first embodiment of the invention. Each figure shows a wind turbine 10 comprising a nacelle 12 rotatably mounted on a tower 14. A rotor 16 is mounted for rotation around a generally horizontal axis to turn the rotor of a generator housed in the nacelle. The rotor has a plurality of rotor blades 18, typically 2 or 3, of which only one is shown in Figures 5 and 6 for simplicity. A cone or spinner 20 is arranged over the front of the rotor to protect the rotor and to deflect incident air onto the rotor blades.

A retractable or deployable wake expansion device is mounted within the tower and nacelle for deployment behind the nacelle. In the embodiment illustrated, the wake expansion device is a parachute which slows down air in the wake to force it radially outwards, to increase Aw. However, any other type of trailing device that slows down the air could be used. For example a kite could be used instead of a parachute.

As shown in Figures 5 and 6, the parachute has a launch parachute 22 and a main parachute 24. The launch parachute does not contribute significantly to wake expansion and is deployed either fully or partially at all times. As can be seen from Figure 5 in particular, the launch parachute is mainly in the lee of the nacelle and so will have minimal effect on the wake. The main parachute is stored in a guide tube 26 arranged in the nacelle and the tower. The guide tube is typically made of GRP and comprises two sections: A first section 28 mounted in the tower 14 and a second section 30 mounted in the nacelle. During assembly of the wind turbine the two parts of the guide tube are fitted together to form the complete tube. At the base of the parachute, and also mounted in the tower 14, is a winch 32 which is used to deploy the parachute.

The winch is preferably mounted on a rail 34 on a platform within the tower. The rail is required to enable the winch to follow the turbine yawing position as the nacelle will rotate about the tower axis in dependence on the wind direction.

It can be seen from Figures 5 and 6 that the position of the parachute with respect to the nacelle and rotor is not symmetrical. Although the parachute could be arranged on the rotor axis, symmetry is not required in either the horizontal or vertical directions.

Thus, in Figure 5 the parachute is shown collapsed within the guide tube 26 with the launch parachute 22 fully or partially in the wind trailing behind the nacelle. The launch parachute 22 provides a first pull when the winch 32 is released to launch the main wake-expansion parachute 24. The winch is required to ensure that the support strings 36 (Figure 6) of the parachute are free of the nacelle. The support strings 36 are connected to a winch cable 38 by a spindle link which, with the winch cable fully extended, extends behind the nacelle as shown in Figure 6. As can also be seen in Figure 6, the launch parachute 22 is attached to the main parachute 24 by a cord 42 which is attached to support strings 40 of the launch parachute.

The embodiment of Figures 5 and 6 may be deployed when the wind speed is below a level required for the turbine to operate at rated power.

As the wind speed rises, the main parachute may be winched in and then winched out again when the wind speed falls. The winch may be driven automatically by a motor which responds to a main wind turbine controller which can drive the winch motor in response to detected wind speed. The wind turbine controller is well known and may control a number of different wind turbine variables, such as blade pitch, in response to wind speed which is typically sensed by an anemometer on the upper surface of the nacelle. The controller is illustrated schematically at 50 in figures 5 and 6. S 11

The diameter of the wake-expansion parachute will depend on the diameter of rotor with which it is used. The thrust that the parachute gives to the tower will depend on the size of the parachute and this increased thrust is balanced with the achievable wake expansion. It is presently considered that a parachute of diameter between I 5-25m, preferably 2Dm should be used for a 90m rotor diameter. Thus, the parachute diameter is approximately 16-28% and preferably 22% of the rotor diameter. A 20m parachute requires a winch range of no less than 25m thus requiring that the winch is arranged around 20m from the top of the tower.

It will be appreciated that the embodiment of Figures 5 and 6 is particularly suited to an upwind turbine. Deployment of a parachute behind the rotor in a downwind turbine would be complicated by the need to deploy the parachute through the rotor and would therefore be much more complex.

Although the device described may appear to load the turbine construction overwhelmingly, the actual additional loading is estimated to be less than 10%, even with a parachute area as large as 20% of the rotor diameter.

The embodiment of Figures 5 and 6 uses a device that is deployable behind the nacelle to expand the wake by slowing the wake air. A second embodiment of the invention illustrated in Figures 7-9 uses a wake expansion device arranged around the nacelle, that is immediately behind the rotor for an upwind turbine or immediately in front of the rotor for a downwind turbine. The embodiment of Figures 7-9 comprises a retractable device which, when deployed, increases the apparent size of the nacelle to expand the wake by forcing air outwards.

In Figures 7-9, components of the wind turbine referred to in Figures 5 and 6 have the same reference numerals incremented by 100. A retractable plate 150 is mounted on each of the four sides of the nacelle. This plate is hinged to the nacelle at the rotor end of the nacelle and may be pushed outwards, rotating about the hinge, by an actuator 152 mounted within the nacelle housing, preferably towards the rear of the nacelle. The actuator 152 as shown in Figures 7-9 comprises a motorised spindle. The actuator 152 shown in the figures has a motor 154 and spindle 156. The plates are rectangular. However, as can be seen from Figures 7 and 8, the plate on the underside of the nacelle includes a cut-away 158 to accommodate the tower. Thus, the lower plate can move between the extended and retracted positions without interfering with the tower.

Preferably, the actuator extends the plates 150 to an angle of between 200 and 300 to the wall of the nacelle to which they are hinged.

In the embodiment illustrated in Figures 7-9, the rectangular guide plates are each interconnected by a pair of triangular plates 160, 162. The triangular plates 160, 162 are hinged to each other along their common side and also each hinged to an adjacent main guide plate 150. As can be seen from the deployed configuration of Figure 9, the triangular connecting plates 160, 162 and the main plates 150 provide an eight-sided cone to be formed around the nacelle which guides air passing through the rotor radially outwards so expanding the wake behind the rotor. In the retracted position, not shown, each of the triangular plates of a given pair will collapse and fold in under one of the main guide plates 150.

In an alternative embodiment, also not shown, the triangular guide plates may be replaced by a flexible material such as a fabric, for example, sail cloth. This construction has the advantage of being simpler than the use of triangular hinged plates. Any suitable fabric may be used bearing in mind the hostile conditions in which the wind turbine is deployed.

The triangular plates or the fabric are each examples of a collapsible connecting surface between the main plates. These connecting surfaces may be omitted entirely. Although such a construction does not deflect air as efficiently as constructions using connecting surfaces, it is simpler and may provide a reasonably practical trade-off. S 13

Similarly to the embodiment of Figures 5 and 6, the actuators 152 may be controlled by a turbine controller and actuated in response to detected changes in wind speed.

The embodiment of Figures 7-9 may be used for both upwind and downwind turbines. For a downwind turbine the plates may be hinged at the end of the nacelle opposite the rotor. Figure 10 shows a variant of this arrangement in which the guide plates are curved and form a cone 170 around the nacelle having an apex behind the nacelle. Although not illustrated in detail in Figure 10, the cone may be made up of retractable plates in the same manner as the embodiment of Figures 7-9 and operable under the control of a turbine controller. As can be seen from Figure 10, the cone 170 deflects incident air smoothly in a radically outwards direction so that it impinges the rotor blades at a location that is further away from the hub than if the cone was not present and where the blades have a higher rotational speed enabling more energy to be extracted.

Figures 11 and 12 show side and end views of 180 a conical wake expander arranged around the nacelle and extending behind the rear end of the nacelle. In this embodiment the wake expander may be fixed, that is not retractable and the conical wall of the expander is concave. The expander may extend from a front position located some way behind the rotor blade.

Figures 13 and 14 show a further embodiment of the invention in which the wake expansion device is mounted on the upwind side of the turbine. The same reference numerals are used for common features with Figures 5 and 6 incremented by 200. This embodiment is particularly suited to an upwind turbine. As can be seen from Figures 5-9, a wind turbine has a nose cone or spinner over the centre of the rotor. The spinner protects the rotor from ingress of dirt and directs air flow onto the blades. The spinner typically fits over the blade roots at a point where they have a circular cross section. In the embodiment of Figures 13 and 14 the diameter of the spinner is increased greatly such that it affects the wake behind the rotor causing it to expand. It is preferred that the diameter of the spinner 220 shown in Figures 13 and 14 is between 10% and 20% of the rotor diameter. Thus, for a 90m rotor diameter the spinner diameter is between 9 and 18m. As can be seen from Figures 13 and 14, the enlarged spinner or front guide cover extends over the rotor blades to a point where the rotor blades have an aerofoil cross section. The precise arrangement will depend on the specific blade design and the blades may be optimised to interact with the enlarged front guide cover. The optimal shape will also depend on the shape of the nacelle 212.

The front guide cover may replace the spinner or may be located over an existing spinner where the device is fitted to an existing wind turbine to increase energy extraction at low wind speeds.

The front guide cover is supported from the main hub 270. A structural cover 280 is provided on the rear side of the front guide cover and attached to the hub. The rear structural cover 280 fits in between the blades 218 and is preferably a truncated cone having a concave surface.

As indicated by the dashed lines 290 in Figure 13, the front guide cover may be retractable allowing the front guide cover to be deployed when the wind speed falls below a given level. Thus, the cover may be made from a number of triangular plates 295 which can fold onto one another when retracted.

The various embodiments described show how the wake of a wind turbine may be expanded in a radial direction by fitting a device behind the turbine, around the nacelle or in front of the rotor. The effect of all these devices is to increase the amount of air that is drawn in through the rotor so increasing the amount of energy that can be extracted.

Various modifications to the embodiments illustrated are possible within the scope of the invention. Although the various embodiments have been illustrated individually, various combinations could be used. For example, the deployable parachute embodiment of Figures 5 and 6 could be used in combination with one or both of the collapsible expander on the nacelle housing of Figures 7-9 and the front guide cover of Figures 13 and 14.

Other combinations are possible. S 16

Claims

CLAIMS1. A wind turbine comprising a nacelle mounted on a tower, a rotor mounted for rotation about the nacelle to extract energy from incident wind, and a wake expanding device for expanding the wake produced downwind of the rotor by wind passing through the rotor, the wake expansion being in the radial direction of the rotor.
2. A wind turbine according to claim 1, wherein the wake expansion device is movable between a retracted position and a deployed position.
3. A wind turbine according to claim 2, comprising a control device, wherein the control device controls movement of the wake expansion device between the retracted and deployed positions in response to measured wind speed.
4. A wind turbine according to claim 2 or 3, wherein the wake expansion device is deployable behind the nacelle.
5. A wind turbine according to any of claims 1 to 4, wherein the wake expansion device comprises a device for reducing the speed of air behind the nacelle.
6. A wind turbine according to claim 4 or 5, comprising a guide tube extending from the rear of the nacelle into the tower for housing the wake expansion device in the retracted position.
7. A wind turbine according to claim 6, comprising a winch mounted on the tower for deployment and retraction of the wake expansion device.
8. A wind turbine according to claim 7, wherein the winch is mounted on a rail on a platform within the tower.
9. A wind turbine according to any of claims 1 to 8, wherein the wake expansion device is a parachute.
10. A wind turbine according to claim 9, wherein the parachute has a launch parachute attached thereto, wherein the launch parachute is arranged in an at least partially deployed position when the parachute is retracted within the guide tube.
11. A wind turbine according to claim 1, 2 or 3, wherein the wake expansion device is arranged around the nacelle.
12. A wind turbine according to claim 11, wherein the wake expansion device comprises a fairing arranged around the nacelle.
13. A wind turbine according to claim 12 wherein the fairing extends from a position on the nacelle near the rotor to a position of maximum width behind the nacelle.
14. A wind turbine according to claim 11, wherein the wake expansion device comprises a plurality of guide plates each pivotably mounted at an end thereof to a side of the nacelle, whereby a free end of the guide plates can pivot between a retracted and a deployed position.
15. A wake expansion device according to claim 14, comprising an actuator for moving the plates between the retracted and deployed positions.
16. A wind turbine according to claim 15, wherein the actuator is mounted in the nacelle and comprises a plurality of motor driven spindles, each moving a respective one of the plurality of plates.
17. A wind turbine according to any of claims 14 to 16, wherein adjacent sides of the guide plates are connected by connecting surfaces S 18 whereby the wake expansion device, when deployed, is substantially conical.
18. A wind turbine according to claim 17, wherein the connecting surfaces are made of a flexible material.
19. A wind turbine according to claim 17, wherein the connecting surfaces each comprise a pair of triangular plates arranged between adjacent guide plates, the triangular plates of each pair each being hinged to an adjacent plate and to each other.
20. A wind turbine according to claim 1, 2 or 3 wherein the wake expansion device comprises a cone arranged in front of the rotor and extending over the rotor hub and a portion of the rotor blades having an aerodynamic cross-section.
21. A wind turbine according to claim 20, wherein the diameter of the cone is between 10-20% of the rotor diameter.
22. A wind turbine according to claim 20 or 21, wherein the cone comprises a plurality of retractable plates, the wake expansion device further comprising an actuator for moving the plates into a deployed position to form the cone.
23. A wind turbine according to claim 1, 2 or 3, wherein the wind turbine is a downwind turbine and the wake expansion device comprises a conical fairing arranged around the nacelle and extending from an apex behind the nacelle to a base adjacent the rotor.
24. A wind turbine, substantially as herein described with reference to Figures 5 and 6, 7-9, 10, 11 and 12 or 13 and 14 of the accompanying drawings.