WO2015032803A1

WO2015032803A1 - Wind turbine

Info

Publication number: WO2015032803A1
Application number: PCT/EP2014/068705
Authority: WO
Inventors: Joe DALTON
Original assignee: Mainstream Renewable Power Limited
Priority date: 2013-09-05
Filing date: 2014-09-03
Publication date: 2015-03-12
Also published as: GB201315775D0; GB2517935A

Abstract

Provided is an extender for a wind turbine blade, the extender being attachable to a wind turbine blade and operably coupling the blade to a hub of a wind turbine so as to increase the swept area of the blade. The extender includes a first and a second end, wherein the first end is configured to be attached to the hub, and the second end is configured to be attached to the wind turbine blade. Also provided is a wind turbine blade extension kit including a wind turbine blade and an extender for the blade. Further provided is a wind turbine including a hub, one or more wind turbine blades and an extender for each of the blades.

Description

Title

Wind Turbine

Field of the Invention The present teaching relates to wind turbines and in one aspect to a wind turbine blade extender for extending the effective length of a wind turbine blade. In another aspect the present teaching relates to a wind turbine comprising an integrated winch configured so that the one or more of the wind turbine blades can be removed from the hub of the turbine

Background

The use of wind turbines to harness wind energy in order to generate electrical power represents an essential part of incorporating renewable energies in the energy-mix.

Types of wind turbines include horizontal-axis wind turbines (HAWT) and vertical- axis wind turbines (VAWT), with HAWT turbines being the most common. For example, in HAWT wind turbines, the blades of the turbine rotate around a horizontally extending axis.

Figure 1 illustrates a typical horizontal-axis wind turbine 100. The turbine 100 comprises a tower 2 disposed on a base 1 , and a nacelle 3 pivotally attached to the tower. A rotor shaft 4 disposed in the nacelle 3 is connected to a generator 5, and a wind rotor hub 6 is rotatably attached to the rotor shaft 4. A plurality of blades 7 are attached to the rotatable hub 6. The hub 6 is connected to a drive train (not shown) within the nacelle 3. Each of the blades 7 defines a longitudinal axis (L) generally aligned along the radius of a circular area swept out by the blades 7 when they rotate. Each of the blades 7 may comprise a proximal end 8 and a distal end 9. The proximal end 8 is typically attached to the hub 6. The distal end 9 may be the end of the blade farthest from the hub 6. Between the proximal end 8 and the distal end 9, each of the blades 7 may have an aerodynamically-shaped profile that reduces in size as it approaches the distal end 9. The blades 7 may be formed of a material which has relatively low rotational inertia to enable the turbine to accelerate quickly if the wind increases. The blades 7 may be formed, by way of non-limiting example, of composite epoxy laminated materials (including fibreglass, carbon, aramid, basalt etc., fibres that are woven, stranded, matted, chopped, etc.), plastics (including filled and unfilled thermoplastics), laminated wood, metals, and/or any combination thereof.

Conventionally, wind turbine blades are made from composite materials in a single piece. In other words, the blades are single integrated components of the wind turbine and are directly attached to the hub. It is also known to provide an arrangement whereby the blade is transported in parts. Wind turbines have also been developed using retractable blade technology. Such solutions include the concept of a variable length blade including a root portion and a tip portion, wherein the tip portion is configured to be slidably received within the root portion. The tip portion may be extended or retracted relative to the root portion depending on the strength of the wind. In other designs, wind turbine blades have a first blade element, a second blade element and a transition element. In most configurations, the blades, or at least a portion or element of the blades, are attached to the hub. Conventionally, however, the length of wind turbine blades is fixed. Thus the swept area which is defined by the area swept by the blades, is also fixed. The power output of a wind turbine is directly related to the swept area. The greater the diameter of the circle defined by the swept area, the more power the turbine is capable of extracting from the wind. Normally, the swept area of wind turbine blades may be defined by the equation nD²/4 - nd² /4, where D represents the overall diameter of the rotating structure, and d represents the diameter of the hub to which the blades are attached. There is a linear relationship between the energy captured by the blades and the swept area of the blades. In this regard, if the swept area of the blades is increased, the amount of energy captured is proportionately increased.

In one conventional arrangement the blades are directly fixed to the hub and hence the blades cannot pivot about their longitudinal axis. Other configurations provide that the blades are attached to a pitch mechanism in the hub, which adjusts the angle of attack of the blades according to the wind speed to control their rotational speed. The hub is attached to the rotor shaft which drives the generator through a gearbox. Optionally a gearbox may not be included. For example, in a direct drive wind turbine, which does not include a gearbox, the rotor shaft is attached directly to the generator, which spins at the same speed as the blades.

The wind turbine's generator, gears, bearings, and support structure are typically designed around the expected wind load and power production at the site where the turbine is to be located. In geographical areas where low wind speeds are the norm, very long blades are desirable in order to generate as much power as possible from the available wind. Correspondingly in geographical areas where high wind speeds are typical, a wind turbine must control the power production and the mechanical loads developed, with the result that the blade length is typically lower.

Eventually, if the wind speed becomes high enough and the length of the individual blades is too long to cope with the conditions, the turbine must shut down to avoid damaging the components of the wind turbine. Thus short blades are desirable to keep the turbine producing power in high winds.

In view of the foregoing, there is a need for a turbine that can both optimise energy generation at low wind speeds and control the mechanical load of the blades at higher wind speeds. For these reasons and others, there is a need for an improved wind turbine structure which can overcome these problems. Summary

These and other problems are addressed by a wind turbine provided in accordance with the present teaching. Accordingly the present teaching provides a wind turbine according to claim 1 . Also provided is a wind turbine blade extension kit including a wind turbine blade and an extender for the blade as detailed in claim 28. Further provided are wind turbines including the extension kit in accordance with claims 49 and 50. Advantageous features are provided in the dependent claims.

These and other features of the present teaching will be better understood with reference to the following drawings.

Brief Description Of The Drawings The present teaching will now be described with reference to the accompanying drawings in which:

Figure 1 illustrates a typical horizontal-axis wind turbine (HAWT);

Figure 2 illustrates a comparison between a conventional design in which the blades are directly attached to the hub, and the design of the present teaching in which the extender is fitted between the blades and the hub;

Figure 3 illustrates an extender for a wind turbine according to an embodiment of the present teaching;

Figure 4 illustrates a bearing and pitch arrangement at the blade-extender interface according to an embodiment of the present teaching;

Figure 5 illustrates a cross-sectional view of the connecting means between bearing rings of the bearing and pitch arrangement;

Detailed Description Of The Drawings

Exemplary arrangements of a wind turbine provided in accordance with the present teaching will be described hereinafter to assist with an understanding of the benefits of the present teaching. In one aspect the present teaching provides a wind turbine blade extender. Such an extender for a wind turbine will be understood as being exemplary of the type of extender that could be provided and is not intended to limit the present teaching to any one specific arrangement as modifications could be made to that described herein without departing from the scope of the present teaching.

As described above, varying the length of a wind turbine blade changes the swept area of the blade, thereby allowing the amount of power generated from the wind to be regulated. However in contrast to conventional approaches whereby a specific blade was developed for a specific geographical location, the present inventor has realised that a single "standard" length blade can be used for all sites. In order to vary the swept area that is defined by a specific turbine, the present teaching provides an extender that can be coupled to or otherwise used with a standard length blade to vary the actual distance of the blade tip from the turbine hub.

In one configuration the extender is provided in the form of a customised extension structure. The extender is configured to allow it to be fitted between the conventional hub of a wind turbine and the blade, thereby extending the "effective length" of the wind turbine blade. The extender may be provided in one or more geometrical configurations. In one arrangement it is provided having a cylindrical form. The extender may have a solid structure or a hollow structure. A hollow extender structure enables the housing of other elements in the extender, which will be described later. By providing an extender, the swept area of the standard blade is increased. As was discussed above the swept area of a turbine is conventionally defined by nD²/4 - nd² /4, where D represents the overall diameter of the rotating structure, and d represents the diameter of the hub. By adding an extender per the present teaching however, the swept area increases significantly without having to increase the actual length of the blade. For example, if an extender of length L is added, the swept area of the blade now becomes (TT(D+L)²/4 - n(d+L)²/4. In the case of a 50 metre length blade, on a 3 metre diameter hub, the swept area is, per the above equation approximately 8325 m². With the addition of a 3 metre extender, the swept area increases to 9267 m². This represents an available swept area increase of 1 1 %, for the addition of a 3 metre extension. It will be appreciated that if the extender is configured to have a cylindrical form, it does not have to match the geometrical shape of the actual blade which is optimised for its aerodynamic profile. The benefit of the extender is the increase in length of the distance of the tip of the standard blade from the hub. Figure 1 illustrates a wind turbine 100 on a base 1 with a tower 2 supporting a nacelle 3. A plurality of blades 7 are attached to a rotating hub 6. The hub 12 may be connected to a drive train (not shown) within the nacelle 3. Each of the blades 7 defines a longitudinal axis (L) generally aligned along the radius of a circular area swept out by the blade 10 when it rotates.

By using an extender it is possible to increase the effective length of the wind turbine blade and thereby also increase the mechanical load which may be generated through that specific turbine. Figure 2 illustrates a comparison between a conventional design in which the blades 7 are directly attached to the hub 6, and the design of the present teaching in which an extender 10 is fitted between the blades 7 and the hub 6.

Figure 3 illustrates an extender 10 for a wind turbine blade according to an embodiment of the present teaching. The extender 10 may comprise a first end 1 1 that is configured to be attached to the hub of a wind turbine; and a second end 12 that is configured to be attached to the proximal end of a wind turbine blade. The extender 10 may be cylindrically shaped with a hollow interior, thus being in the form of tubing. In this arrangement, it will thus be appreciated that the first and second ends 1 1 and 12 are open, with the attachment effected through the circumferential portions of the first and second ends 1 1 and 12. Other components of the extender 10 to be described later, such as a pitch control mechanism for varying the angle of attack of the blades, a pitch drive, winches for removing or replacing the blades, and reinforcement structures, may thus be housed in the extender 10. As mentioned above however, the extender 10 may be provided in one or more other geometrical configurations. The extender 10 may comprise a single integrated piece, or in preferred configurations be comprised of multiple pieces which are configured to enable the extender 10 to be itself extendible. As discussed above, by using an extender it is possible to increase the effective length of the wind turbine blade and thereby also increase the mechanical load which may be generated through that specific turbine. In this way varying the length of the extender may be usefully employed to vary the effective load generated. This variance in the length can be provided through effecting an extension or a reduction in the effective length of the blade. In one such arrangement, the extender 10 may comprise multiple extendible elements in a telescopic arrangement. Thus the extender 10 may be configured to vary in length according to requirements and/or wind conditions. For example, an extender installed in an underperforming wind farm may be configured to be relatively long in comparison to other situations. In one aspect of the present teaching the length of the blade may be dynamically modified without requiring a removal of the blade from the turbine through use of an extendible extender. In this way the response characteristics of the turbine can be modified in response to changes in prevailing conditions. This can be done in combination with active monitoring of expected conditions for a forecast period. For example, wind conditions in winter months are typically different to those of summer months. A turbine incorporating an extender provided in accordance with the present teaching can have the effective blade length change in situ to provide an optimal response for the different conditions. In one configuration this may require manual intervention by an operator to change the length of the blade extender. In another configuration the turbine may include a mechanical or other motorised actuator that may, in response to a signal provided to the turbine, initiate a change in the effective length of the extender. This could be achieved through either a retraction or extension of the extender as appropriate. The extender 10 may be configured to be attached to the blade 7 via a fixed, detachable, pivotal or geared arrangement. Similarly, the extender 7 may be configured to be attached to the hub 6 via a fixed, detachable, pivotal or geared arrangement. It will be appreciated that any of the above connecting means may be provided at the interfaces between the hub 6 and blade 7 respectively. For example, the connection between the hub 6 and extender 10 may be a fixed arrangement and the connection between the extender 10 and the blade 7 may be a pivotal arrangement. However, the present teaching is not limited to this configuration and for example a pivotal connection may be provided at both the extender-hub interface and the extender-blade interface.

In one embodiment, the extender 10 may be configured to be fixedly connected to the blade 7. That is, the blade 7 may be fixed to the extender 10 during installation of the blade 7. In another embodiment, the second end 12 of the extender 10 may be configured to be detachably connected to the blade 7. Such an arrangement allows the blade 7 to be removed for repair or replacement in case of damage, wear or otherwise. The second end 12 of the extender 10 may also be configured to be pivotally connected to the blade 7, and optionally may be pivotally connected to the blade 7 via a geared arrangement.

In another embodiment, the extender 10 may be configured to be fixedly connected to the hub 6. That is, the extender 10 may be fixed to the hub 6 during installation of the extender 10. In one embodiment, the first end 1 1 may be fastened to the hub 6 using a bolt flange or studs attached to the first end 1 1 . In another embodiment the first end 1 1 of the extender 10 may be integrally formed with the hub 6. It will be appreciated that in this configuration the bearing loading would be even lower than if the extender 10 was a separate element attached to the hub 6. In both these embodiments, it will thus be appreciated that the extender 10 is fixed in relation to the hub 6 to which it is attached.

Alternatively, the first end 1 1 of the extender 10 may be configured to be detachably connected to the hub 6. The first end 1 1 of the extender 10 may be configured to be pivotally connected to the hub 6, and in an optional arrangement, the first end 1 1 of the extender may be configured to be pivotally connected to the hub 6 via a geared arrangement. It will be appreciated by the skilled person that such a pivotal connection to either or both of the hub and extender allows the blade to pivot about its longitudinal axis.

It will be appreciated that a fixed, detachable, pivotal or geared arrangement as described above may be provided at either or both of the first and second ends of the extender at the interfaces between the hub and blade respectively. By providing an arrangement whereby the blade is pivotal with respect to the extender and/or the hub, the blades can rotate about the longitudinal axis of the blades. This will be described in more detail later.

The extender 10 may comprise metal or a composite material. As mentioned above, the extender 10 may be constructed to have one or more reinforcement spars 13 as necessary. It will be appreciated that such spars may be optimally provided when the extender has a hollow configuration, as a reinforcement aid. These spars 13 may be provided on an internal surface 14 or external surface 24 of the extender 10, and may extend in a longitudinal direction from the first end 1 1 to the second end 12 along a substantial portion of the longitudinal length of the extender 10. In one embodiment, a plurality of spars 13 may be formed on the internal surface 14 of the extender 10 and may be disposed at equal intervals around the internal circumference of the extender 10. The spars 13 may also be provided on the external surface 24 of the extender. In any of the arrangements described above, the spars 13 may be integrally formed with the extender 10 or attached thereto as separate components.

High loads may occur in some instances from the dynamic load of the wind turbine blades. As mentioned above, a bearing arrangement may be provided at one or both ends of the extender at the connection interface between the blade and hub respectively. As mentioned above in relation to conventional designs, the blades may be fixedly attached, for example bolted, to the hub. In this case, the blades cannot pivot about their longitudinal axis. Other arrangements provide that the blades are detachable. Bearing loads and rotating speeds vary considerably due to constantly changing winds. These loads are typically transferred securely via the raceways and connections of the blade bearings into the rotor hub. Thus irrespective of whether a pitch control mechanism is incorporated, the bearing arrangement is an important component of the wind turbine in that it has to be able to transfer the moment produced by the wind load to the hub and at the same time enable that the blades can rotate freely and accurately.

In one embodiment of the present teaching, a bearing arrangement may be provided at the blade-extender interface in order to support the mechanical load of the blade, or more specifically, at the interface between the second end 12 of the extender 10 and the proximal end of the blade 7. In this case, the bearing loading may be lower than that for a blade which is directly connected to the hub, providing an equivalent swept area. Accordingly, according to the present teaching, the load on blade bearings may be reduced. Thus, the bearing arrangement at the blade-extender interface may only require a single bearing arrangement. Such a single bearing arrangement is illustrated in Figure 5.

Figure 4 illustrates a bearing arrangement at the blade-extender interface according to an embodiment of the present teaching. Figure 5 illustrates a cross-sectional view of the bearing rings of the bearing arrangement. Referring to Figures 4 and 5, the bearing arrangement may comprise an annularly shaped inner bearing ring 20 operably connected with the blade, and including a first raceway groove 25, and an annularly shaped outer bearing ring 30 operably connected with the extender 10, configured for mating engagement with the inner bearing ring 20, and including a second raceway groove 35 aligned with the first raceway groove 25. A plurality of rolling elements 40 may be disposed in the first and second raceway grooves 25 and 35 to rotatably interconnect the inner and outer bearing rings 20 and 30. The rolling elements may be spherical in shape, for example, ball bearings, or cylindrical. Thus, the blade and extender 10 may be pivotally connected. While the bearing arrangement illustrated in Figure 5 is a single bearing arrangement, it will be appreciated that a double bearing arrangement may also be utilised. A double bearing arrangement comprises two grooves instead of one as in a single bearing arrangement. Figure 3 illustrates a double bearing arrangement 60 at the second end 12 of the extender 10, comprising two parallel bearing rows.

Each of the inner and outer bearing rings 20 and 30 may be annularly shaped and defined by forward and rearward surfaces which are generally flat and oriented mutually parallel to each other, along with radially oriented inner and outer surfaces which are cylindrical in shape and disposed concentrically in relation to the longitudinal axis of the extender 10. The outer bearing ring 30 may comprise fastening means such as apertures 36 for detachably connecting the outer bearing ring 30 to the second end 12 of the extender 10. The outer bearing ring 30 may be connected to an inner circumferential surface 14 of the second end 12 of the extender 10. Alternatively, the outer bearing ring 30 may be manufactured as an integral component of the extender 10. The inner bearing ring 20 may be detachably mounted to an associated blade by at least a fastening means inserted through apertures 26 extending through the inner bearing ring 20. Alternatively, the inner bearing ring 20 may be manufactured as an integral component of the blade. The blade may be further supported by a portion of the proximal end of the blade extending over an outer circumferential portion of the second end 12 of the extender 10.

It will be appreciated by those skilled in the art that such a single or double bearing arrangement may also be provided at the extender-hub interface.

As mentioned above in relation to conventional designs, the blades may be fixedly attached, for example bolted, to the hub. In this case, the blades cannot pivot about their longitudinal axis. It will be appreciated however that in order to optimise operating efficiency, wind turbines need to be aligned to wind conditions. Accordingly, the blades may be adjusted so that they make optimal use of the wind, while it must also be ensured that the blades are not subjected to excessive loads that could cause damage. Thus, it is preferable that the blades are configured to be pivoted about the longitudinal axis of the blades. The rotor shaft bearing supports the blades and rotor and transmits torque to the gearbox.

In order to control the output of the wind turbine, the angle of the blades may be continuously adjusted to the wind speed via rotary movement of the blades through a pitch control mechanism. In one embodiment, the first end 1 1 of the extender 10 may be fixed to the hub 6 and thus the extender 10 is not moveable in relation to the hub 7. In this arrangement, a mechanism may be provided at the second end 12 of the extender 10 at the blade-extender interface so that the blade is pivotal about its longitudinal axis rotatable in and thus moveable in relation to the extender 10 and the hub 6 to which the extender 10 is coupled. Pivoting the blades controls the rotation speed of the rotor shaft, which ensures relatively uniform power generation. The wind turbine blade angle may be adjusted by an electric or hydraulic pitch drive. The extender 10 may thus comprise a pitch control mechanism in order to adjust the angle of attack of the blades. The angle of attack is the angle at which the wind strikes the blades. A fully stalled turbine blade, when stopped, has the flat side of the blade facing directly into the wind. A fully furled turbine blade, when stopped, has the edge of the blade facing into the wind. The pitch control mechanism may enable each blade to be rotated approximately 90° around their longitudinal axis. In one arrangement, the pitch control mechanism may comprise a slewing drive, which enables the blade to be rotated while withstanding high torque loads.

In one embodiment of the pitch control mechanism, a gear segment 50 may be formed on or associated with one of the inner and outer bearing rings 20 and 30, and engages an associated pitch drive to pivot the attached blade to the desired angle. The gear segment 50 may be formed on at least a portion of the inner surface 22 of the inner bearing ring 20, and may be defined by a plurality of teeth 52 shaped to mesh with an associated pinion gear 54 on a drive shaft 55 of the pitch drive. In operation, the drive shaft 55 of the pitch drive may be configured to rotate a predetermined amount which causes the pinion gear 54 on the drive shaft 55 to rotate the inner bearing ring 20, thereby pivoting the attached blade to the desired angle. The pitch control mechanism including the bearing rings may be manufactured as an integrated component of the extender 10, rather than a "bolt-on" to the hub casting in conventional arrangements. This may result in improvements in alignment accuracy for electrically actuated pitch mechanisms, and also frees up space in the hub for additional components. However, the present teaching is not limited to such a configuration, and the pitch control mechanism may also be housed in the hub as in conventional designs. It will also be understood by the person of skill in the art that the pitch bearing arrangement is not limited to the configuration described above. Various other structures may be employed that facilitate pitch control and load bearing.

It will be appreciated that the pitch control mechanism, while necessitating additional components, increases the response rate of blades to changes in wind speed. Also, as a pitch control mechanism may be installed in each of the extenders for a respective blade, "double redundancy" may be provided in relation to individual blade pitching options.

The blades 7 may be replaced, for example, in the case of blade damage or wear. In one embodiment, a winch or winches may be provided proximal to the blade as an integral component of the turbine. In one configuration the winches are mounted on the reinforcement spars 13 of the extender 10. The winches may be used for winching the blades 7 out of their operational configuration, using for example ropes. In the event that a blade needs to be removed for repair and/or replaced, the wind turbine may be put into a "lazy Y" configuration. That is, the blades 7 may be pivoted about the extender 10 from their operational configuration to be in substantial alignment with the longitudinal axis of the rotor shaft. A blade 7 may then be lowered to the ground using the winches. In certain circumstances the winch by itself may suffice to effect the movement of the blade. Other circumstances may require use of a small crane to support the distal end of the blade 7. In either scenario, blade removal necessitated by blade damage or wear may be performed relatively easily.

An element of dynamic balancing may also be undertaken within the wind turbine blade extender 10. This may facilitate adjustments to ensure the minimisation of vibration, without the complication of having to add/remove mass within the blade itself.

Additionally, lightning protection cables may be introduced into the extender through the centreline of the blade, and clamped directly to the hub/extender. This has the potential to reduce current flow through pitch bearings in the event of a lightning strike. Thus, potential damage to pitch bearings due to lightning strikes may be reduced. The present teaching may further provide a wind turbine blade extension kit comprising a standard wind turbine blade and the extender as described above. The extender may be coupled to the blade through a fixed, detachable, pivotal or geared arrangement. The blade-extender combination may be coupled to the hub as described above.

Further, the present teaching provides a wind turbine including a hub, a plurality of wind turbine blades, and an extender as described above coupled to each of the blades. The extender may be coupled at one end to the hub and at the other end to a respective blade. As mentioned above, it will be appreciated that a pivoting or pitch control mechanism may be provided at the hub-extender interface. Thus, in an additional or alternative arrangement to the pivoting or pitch control mechanism provided at the hub-blade interface, a pivoting or pitch control mechanism may be provided at the hub-extender interface. Thus, the extender may be pivotal in relation to the hub as well as the blade being pivotal in relation to the extender. In another arrangement, the extender may be formed as an integral component of the hub. It will be appreciated that an extender provided according to the present teaching enables a single "standard blade" to be used on multiple sites, but the extender may be optimised for site conditions. Such optimisation may comprise varying the length of the extender or using different pitch and bearing arrangements within the extender. Modifications may be made that allow underperforming wind farms to be upgraded, by varying the extender length through an extendible or telescopic arrangement, and modifying controls.

It will be understood that as the present teaching allows for the use of existing standard blades, transportation of such blades may also be standardised, such that they may be provided having a uniform length for transportation.

As the pitch and load bearing mechanisms may be provided in the extender, additional space may be freed up in the hub. For example, a lifting device may be installed in the hub for component replacement.

It will be appreciated that what has been described herein are exemplary arrangements of a wind turbine blade extender that is configured to be fitted at the interface between a wind turbine hub and a standard wind turbine blade, thereby increasing the effective swept area of the blade.

The words comprises/comprising when used in this specification are to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

While the present teaching has been described with reference to some exemplary arrangements it will be understood that it is not intended to limit the present teaching to such arrangements as modifications can be made without departing from the spirit and scope of the present teaching. In this way it will be understood that the present teaching is to be limited only insofar as is deemed necessary in the light of the appended claims.

Claims

1 . A wind turbine blade extender, the extender being attachable to a wind turbine blade and operably coupling the blade to a hub of a wind turbine so as to increase the swept area of the blade, the extender itself being extendible to operably allow modification of the swept area of the blade.

2. The wind turbine blade extender of claim 1 , comprising:

a first end configured to be attached to the hub of the wind turbine; and a second end configured to be attached to the wind turbine blade.

3. The extender of claim 1 or 2, comprising a motor coupled to the extender to effect an extension or retraction of the extender.

4. The extender of any preceding claim, wherein the extender comprises multiple elements, moveable relative to one another to provide an extension or retraction of the extender.

5. The wind turbine blade extender of claim 4, wherein the multiple elements are moveable in response to a signal provided to the extender.

6. The wind turbine blade extender of claim 4 or 5, wherein the multiple elements comprise a telescopic arrangement.

7. The wind turbine blade extender of any preceding claim, having a cylindrical structure.

8. The wind turbine blade extender of any preceding claim, comprising a hollow interior.

9. The wind turbine blade extender of any of claims 2 to 8, wherein the second end of the extender is configured to be fixedly connected to the blade.

10. The wind turbine blade extender of any of claims 1 to 6, wherein the second end of the extender is configured to be detachably connected to the blade.

1 1 . The wind turbine blade extender of any preceding claim, wherein the second end of the extender is configured to be pivotally connected to the blade.

12. The extender of claim 1 1 , wherein the second end of the extender is configured to be pivotally connected to the blade via a geared arrangement.

13. The extender of claim 1 1 or 12, wherein the second end of the extender is configured to be pivotally connected to the blade about a longitudinal axis of the blade.

14. The wind turbine blade extender of any of claims 2 to 13, wherein the first end of the extender is configured to be fixedly connected to the hub.

15. The wind turbine blade extender of any of claims 2 to 13, wherein the first end of the extender is configured to be detachably connected to the hub.

16. The wind turbine blade extender of claim 14 or 15, wherein the first end of the extender is configured to be pivotally connected to the hub.

17. The wind turbine blade extender of claim 16, wherein the first end of the extender is configured to be pivotally connected to the hub via a geared arrangement.

18. The extender of claim 16 or 17, wherein the first end of the extender is configured to be pivotally connected to the hub about a longitudinal axis of the extender.

19. The wind turbine blade extender of any preceding claim, comprising one or more reinforcement spars.

20. The wind turbine blade extender of claim 19, wherein the one or more spars extend longitudinally along a surface of the extender.

21 . The wind turbine blade extender of claim 19 or 20, wherein the one or more spars are formed on an internal surface of the extender.

22. The wind turbine blade extender of any of claims 19 to 21 , wherein the one or more spars are formed on an external surface of the extender.

23. The wind turbine blade extender of any of claims 19 to 22, wherein the one or more spars are integrally formed with the extender.

24. The wind turbine blade extender of any preceding claim, comprising a metal or a composite material.

25. The wind turbine blade extender of any preceding claim, further comprising a lightning protection cable.

26. The wind turbine blade extender of claim 25, wherein the lightning protection cable extends through the extender.

27. The wind turbine blade extender of claim 25 or 26, wherein the lightning protection cable is clamped to the extender.

28. A wind turbine blade extension kit comprising a wind turbine blade and a wind turbine blade extender of any of claims 1 to 27 configured to be attached to the blade.

29. The kit of claim 28, wherein the second end of the extender is configured to be connected to the blade through a bearing arrangement.

30. The kit of claim 28 or 29, wherein the extender and blade are configured to be pivotally connected to each other.

31 . The kit of claim 30, wherein the extender and blade are configured to be pivotally connected through an inner bearing ring and an outer bearing ring.

32. The kit of claim 31 , wherein the inner and outer bearing rings are configured to be rotatably interconnected via corresponding raceway grooves formed in a circumferential surface of each of the inner and outer bearing rings and rolling elements disposed in the raceway grooves.

33. The kit of claim 31 or 32, wherein the outer bearing ring is connected to the extender.

34. The kit of any of claims 31 to 33, wherein the outer bearing ring extends circumferentially around the internal circumferential surface of the extender.

35. The kit of claim 33 or 34, wherein the outer bearing ring is connected with the extender via fastening means and apertures formed in the outer bearing ring.

36. The kit of claim 33 or 34, wherein the outer bearing ring is an integral component of the extender.

37. The kit of any of claims 31 to 36, wherein the inner bearing ring is connected to the blade.

38. The kit of any of claims 31 to 37, wherein the inner bearing ring is an integral component of the blade.

39. The kit of any of claims 29 to 38, wherein the bearing arrangement comprises a single bearing arrangement.

40. The kit of any of claims 29 to 38, wherein the bearing arrangement comprises a double bearing arrangement.

41 . The kit of any of claims 30 to 40 wherein the pivotal connection between the extender and the blade comprises a geared arrangement.

42. The kit of claim 41 , wherein the geared arrangement comprises a pitch control mechanism for varying the angle of attack of the blade.

43. The kit of claim 42, wherein the pitch control mechanism comprises a gear segment formed on one of the inner and outer bearing rings.

44. The kit of claim 43, wherein the gear segment is formed on at least a portion of one of the inner and outer bearing rings.

45. The kit of claim 43 or 44, wherein the gear segment is formed on at least a portion of the inner surface of the inner bearing ring.

46. The kit of any of claims 43 to 45, wherein the gear segment is defined by a plurality of teeth shaped to mesh with an associated pinion gear on a pitch drive shaft.

47. The kit of any of claims 28 to 46 when dependent on any of claims 19 to 27, further comprising a winch configured to be mounted on at least one of the spars for replacing the blade by winching the blade out of its operational configuration.

48. The kit of any of claims 28 to 47, wherein the extender has a different geometrical shape to that of the blade.

49. The kit of any one of claims 28 to 48 comprising a motor, the motor being operably coupled to the extender to effect a change in length of the extender.

50. A wind turbine comprising:

a hub;

one or more wind turbine blades;

a wind turbine blade extender of any of claims 1 to 27 coupled to each of the blades and coupling the blade to the hub.

51 .A wind turbine comprising:

a hub;

one or more kits of any of claims 28 to 49 coupled to the hub.

52. The wind turbine of claim 50 or 51 , wherein the first end of each of the extenders is configured to be connected to the hub through a bearing arrangement.

53. The wind turbine of claim 52, wherein the first end of each of the extenders is configured to be pivotally connected to the hub through a geared arrangement.

54. The wind turbine of claim 53, wherein the geared arrangement is housed in the hub.

55. The wind turbine of any of claims 50 to 54, wherein the bearing arrangement comprises a single bearing arrangement.

56. The wind turbine of any of claims 50 to 54, wherein the bearing arrangement comprises a double bearing arrangement.

57. The wind turbine of claims 50 or 51 , wherein the extenders comprise the same material as that of the hub.

58. The wind turbine of claim 57, wherein the extenders are integrally formed with the hub.

59. The wind turbine of any of claims 51 to 58 when dependent on claim 47, being configured so that the one or more of the wind turbine blades can be removed and/or replaced by winching the blade out of its operational configuration.

60. A wind turbine comprising:

a. A hub

b. at least one wind turbine blade coupled to the hub;

c. a winch provided proximal to the blade, the winch being configured so that the at least one wind turbine blade can be removed and/or replaced by winching the blade out of an operational configuration.

61 .The turbine of claim 60 wherein the winch is operable to move the blade away from the hub and towards the ground on which the turbine is located.