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Title:
A COVERING FOR A WIND TURBINE BLADE
Document Type and Number:
WIPO Patent Application WO/2019/197474
Kind Code:
A1
Abstract:
A covering for a wind turbine blade (20). The covering may have a film (30) having at least one aerodynamic device (42, 43) pre-attached to it. In the manufacture of a wind turbine blade, the film having the aerodynamic device (42, 43) pre-attached is attached to a shell of a wind turbine blade (20).

Inventors:
STEWART, Ian (24 Southcliff Road, Southampton Hampshire SO14 6GE, SO14 6GE, GB)
Application Number:
EP2019/059097
Publication Date:
October 17, 2019
Filing Date:
April 10, 2019
Export Citation:
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Assignee:
VESTAS WIND SYSTEMS A/S (Hedeager 42, 8200 Aarhus N, 8200, DK)
International Classes:
B29C63/02; B29C63/00; B29C63/06; B29C65/00; B29C65/08; F03D1/06; B29L31/08
Foreign References:
CA3019517A12017-10-26
EP2484897A12012-08-08
US20090087314A12009-04-02
Other References:
None
Attorney, Agent or Firm:
VESTAS PATENTS DEPARTMENT (Hedeager 42, 8200 Aarhus N, 8200, DK)
Download PDF:
Claims:
CLAIMS

1 . A method of manufacturing a wind turbine blade, comprising the steps of:

providing a film having at least one aerodynamic device pre-attached thereto, and attaching the film having the aerodynamic device to a shell of a wind turbine blade.

2. A method according to claim 1 , wherein the film and/or the aerodynamic device comprises a plastics material or a thermoplastics material.

3. A method according to claim 2 or claim 3, wherein the film is relatively flexible and the aerodynamic device is relatively stiffer than the film.

4. A method according to any preceding claim, wherein the film has a plurality of the aerodynamic devices pre-attached.

5. A method according to any preceding claim, wherein the film has at least one erosion protection device pre-attached thereto, preferably wherein the erosion protection device is a leading edge erosion protection device.

6. A method according to any preceding claim, further comprising positioning the aerodynamic device on the film at a predetermined location and attaching the aerodynamic device to the film.

7. A method according to claim 5 or claim 6, wherein the aerodynamic device and/or the erosion protection device is attached to the film by welding, preferably by ultrasonic welding.

8. A method according to claim 7, further comprising positioning an ultrasonic welding tool on a side of the film opposite and in the vicinity of the aerodynamic device or erosion protection device to be attached, and activating ultrasonic welding equipment to weld the aerodynamic device or erosion protection device to the film.

9. A method according to claim 8, wherein the aerodynamic device and/or the erosion protection device has an attachment surface which contacts the film and the ultrasonic welding tool has a shape which substantially mates with the shape of the attachment surface.

10. A method according to any preceding claim, wherein the aerodynamic device is one or more of a flap, a Gurney flap, a serrated trailing edge device, a trailing edge device, a roughness strip, or a vortex generator.

1 1 . A method according to any preceding claim, wherein the film has a first edge and a second edge, and the method further comprises bringing the first edge adjacent the second edge when attaching the film to the shell such that the film envelops at least a portion of the wind turbine blade.

12. A method according to any preceding claim, wherein the film has an adhesive backing, and attaching the film to the shell comprises bringing the adhesive backing into contact with the shell.

13. A method according to any preceding claim, wherein the film and the shell of the blade each define one or more datum, and the method further comprises aligning the film datum with the shell datum.

14. A method according to any preceding claim, further comprising applying heat to the film when the film is attached to the shell of the wind turbine blade.

15. A method according to any preceding claim, wherein the film has a thickness of 0.15mm or less, such as 0.1 mm or less, or 0.09mm or less.

16. A method according to any preceding claim, wherein the shell has an airfoil profile in cross section extending from a leading edge to a trailing edge and comprising a suction side and a pressure side;

wherein the film covers at least 50% of the suction side in a given cross section and/or at least 50% of the pressure side in a given cross section.

17. A method according to claim 16, wherein the film covers at least 90% of the suction side in a given cross section and/or at least 90% of the pressure side in a given cross section.

18. A method according to claim 16, wherein the film covers 100% of the suction side in a given cross section and/or 100% of the pressure side in a given cross section.

19. A covering for a wind turbine blade, comprising:

a film having at least one aerodynamic device pre-attached thereto.

20. A covering according to claim 19, wherein the film and/or the aerodynamic device comprises a plastics material or a thermoplastics material.

21 . A covering according to any of claim 19 or claim 20, wherein the aerodynamic device is one or more of a flap, a Gurney flap, a serrated trailing edge device, a trailing edge device, a roughness strip or a vortex generator.

22. A covering according to any of claims 19 to 21 , wherein the film has an adhesive layer on an opposite side of the film to the aerodynamic device.

23. A covering according to any of claims 19 to 22, wherein the film has a thickness of 0.15mm or less, such as 0.1 mm or less, or 0.09mm or less.

24. A wind turbine blade comprising at least one blade shell and a covering according to any of claims 19 to 23, wherein the covering is applied over the blade shell.

25. A wind turbine blade according to claim 24, wherein the shell has an airfoil profile in cross section extending from a leading edge to a trailing edge and comprising a suction side and a pressure side;

wherein the film covers at least 50% of the suction side in a given cross section and/or at least 50% of the pressure side in a given cross section.

26. A wind turbine blade according to claim 25, wherein the film covers at least 90% of the suction side in a given cross section and/or at least 90% of the pressure side in a given cross section.

27. A wind turbine blade according to claim 25, wherein the film covers 100% of the suction side in a given cross section and/or 100% of the pressure side in a given cross section.

28. A wind turbine blade according to any of claims 24 to 27, wherein the covering is applied over the blade shell as a plurality of strips of the film, and wherein the at least one aerodynamic device of adjacent strips of the film are aligned in the longitudinal direction of the blade.

Description:
A COVERING FOR A WIND TURBINE BLADE

FIELD OF THE INVENTION

The present invention relates to a covering for a wind turbine blade, a wind turbine blade having the covering, and a method of manufacturing a wind turbine blade.

BACKGROUND OF THE INVENTION

Wind turbine blades are often provided with additional aerodynamic devices, so-called 'add-ons', such as vortex generators, flaps, etc. during manufacture in order to achieve the required flow characteristics of the blade, or to improve the flow characteristics. The ‘add-ons’ can also be retrofitted to existing wind turbine blades. Particular difficulties arise in positioning and installing these aerodynamic 'add-ons' and other features due to the large size of the blade. Accurate positioning of these devices is required to provide the required aerodynamic characteristics.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a method of manufacturing a wind turbine blade, comprising the steps of providing a film having at least one aerodynamic device pre attached thereto, and attaching the film having the aerodynamic device to a shell of a wind turbine blade.

A second aspect of the invention provides a covering for a wind turbine blade, comprising a film having at least one aerodynamic device pre-attached thereto.

A further aspect of the invention provides a wind turbine blade comprising at least one blade shell and a covering according to the second aspect of the invention, wherein the covering is applied over the blade shell.

The invention is advantageous in that the film with pre-installed aerodynamic device(s) can improve the ease of installation of the aerodynamic device(s) to the blade. Pre-attaching the aerodynamic device to the film may speed up and simplify the process of bonding or otherwise attaching the aerodynamic device to the blade shell. Rather than attaching each aerodynamic device or component individually to the outer surface of the blade shell, a plurality of the aerodynamic devices or components may be pre-attached to the film which may then be applied to the blade shell as one. Attaching the aerodynamic device or component to the film rather than directly to the outer surface of the blade may be easier if the film is more accessible before the film is attached to the blade shell. This may also allow better access for tooling used to bond or otherwise attach the aerodynamic devices to the film. Furthermore, positioning of the aerodynamic device or component on the blade may be simplified as only a single datum may be required to position the film on the blade, rather than a separate datum to position each aerodynamic device or component on the blade.

The film and/or the aerodynamic device may comprise a plastics material or a thermoplastics material.

The film may be relatively flexible and the aerodynamic device may be relatively stiffer than the film.

The film may have a plurality of the aerodynamic devices pre-attached.

The film may have at least one erosion protection device pre-attached thereto, wherein the erosion protection device may be a leading edge erosion protection device.

The method may further comprise positioning the aerodynamic device on the film at a predetermined location and attaching the aerodynamic device to the film.

The aerodynamic device and/or the erosion protection device may be attached to the film by welding, e.g. by ultrasonic welding.

The method may further comprise positioning an ultrasonic welding tool on a side of the film opposite and in the vicinity of the aerodynamic device or erosion protection device to be attached, and activating ultrasonic welding equipment to weld the aerodynamic device or erosion protection device to the film. The aerodynamic device and/or the erosion protection device may have an attachment surface which contacts the film and the ultrasonic welding tool may have a shape which substantially mates with the shape of the attachment surface.

The aerodynamic device may be one or more of a flap, a Gurney flap, a serrated trailing edge device, a trailing edge device, a roughness strip, or a vortex generator, for example.

The film may have a first edge and a second edge, and the method may further comprise bringing the first edge adjacent the second edge when attaching the film to the shell such that the film envelops at least a portion of the wind turbine blade.

The film may have an adhesive backing, and the method may further comprise attaching the film to the shell by bringing the adhesive backing into contact with the shell.

The film and the shell of the blade may each define one or more datum, and the method may further comprise aligning the film datum with the shell datum.

The method may further comprise applying heat to the film when the film is attached to the shell of the wind turbine blade.

The film may have an adhesive layer on an opposite side of the film to the aerodynamic device.

The film may have a thickness of 0.15mm or less, such as 0.1 mm or less, or 0.09mm or less.

The shell of the blade has an airfoil profile in cross section extending from a leading edge to a trailing edge and comprising a suction side and a pressure side; and the film may cover at least 50% of the suction side in a given cross section and/or at least 50% of the pressure side in a given cross section. Or, the film may cover at least 90% of the suction side in a given cross section and/or at least 90% of the pressure side in a given cross section. Or the film may cover 100% of the suction side in a given cross section and/or 100% of the pressure side in a given cross section. The covering may be applied over the blade shell as a plurality of strips of the film, and the at least one aerodynamic device of adjacent strips of the film may be aligned in the longitudinal direction of the blade.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 shows a wind turbine;

Figure 2 shows a wind turbine blade shell;

Figure 3a shows a cross-section of a film for covering the blade shell;

Figure 3b shows a cross-section of the film of figure 3a attached to the blade shell; Figure 3c shows a cross-section of a film having an outer coating and attached to the blade shell;

Figure 4 shows a film with pre-attached erosion protection device and aerodynamic devices;

Figure 5a shows attachment of an erosion protection device to a film using a shaped welding tool;

Figure 5b shows attachment of a serrated trailing edge device to a film using a shaped welding tool;

Figure 6 shows the film of figure 4 enveloping the blade shell;

Figure 7 shows the film of figure 4 attached to the blade shell; Figure 8 shows a portion of the wind turbine blade with the film and devices attached; Figure 9a shows a film before devices are attached;

Figure 9b shows a film with a serrated trailing edge device pre-attached;

Figure 9c shows a film with vortex generators pre-attached;

Figure 9d shows a film with vortex generators and a serrated trailing edge device pre attached;

Figure 10a shows a vortex generator attached at its base to a film;

Figure 10b shows a film laid over a vortex generator to conform to the aerodynamic outer surface of the vortex generator;

Figure 1 1 a shows a film with a Gurney flap pre-attached;

Figure 1 1 b shows attachment of the Gurney flap to the film using a welding tool;

Figure 12a shows a film with a roughness strip pre-attached;

Figure 12b shows attachment of the roughness strip to the film using a welding tool;

Figure 13 shows a portion of a wind turbine blade with a plurality of films having pre attached devices covering a portion of the blade shell;

Figure 14 shows a cross-section view of the chordwise overlap between adjacent films of the blade shown in figure 13; and

Figure 15 shows a wind turbine blade including a plurality of films covering the blade, some of which films have pre-attached aerodynamic devices.

DETAILED DESCRIPTION OF EMBODIMENT(S) In this specification, terms such as leading edge, trailing edge, pressure surface, suction surface, thickness and chord are used. While these terms are well known and understood to a person skilled in the art, definitions are given below for the avoidance of doubt.

The term leading edge is used to refer to an edge of the blade which will be at the front of the blade as the blade rotates in the normal rotation direction of the wind turbine rotor.

The term trailing edge is used to refer to an edge of a wind turbine blade which will be at the back of the blade as the blade rotates in the normal rotation direction of the wind turbine rotor.

The chord of a blade is the straight line distance from the leading edge to the trailing edge in a given cross section perpendicular to the blade spanwise direction.

A pressure surface (or windward surface) of a wind turbine blade is a surface between the leading edge and the trailing edge, which, when in use, has a higher pressure than a suction surface of the blade.

A suction surface (or leeward surface) of a wind turbine blade is a surface between the leading edge and the trailing edge, which will have a lower pressure acting upon it than that of a pressure surface, when in use.

The thickness of a wind turbine blade is measured perpendicularly to the chord of the blade and is the greatest distance between the pressure surface and the suction surface in a given cross section perpendicular to the blade spanwise direction.

The term spanwise is used to refer to a direction from a root end of a wind turbine blade to a tip end of the blade, or vice versa. When a wind turbine blade is mounted on a wind turbine hub, the spanwise and radial directions will be substantially the same. Figure 1 shows a wind turbine 10 including a nacelle 12 supported on a tower 13 that is mounted on a foundation 14. The wind turbine 10 depicted here is an onshore wind turbine such that the foundation 14 is embedded in the ground, but the wind turbine 10 could be an offshore installation in which case the foundation 14 would be provided by a suitable marine platform, such as a monopile or jacket.

The nacelle 12 supports a rotor 15 comprising a hub 16 to which three blades 20 are attached. It will be noted that the wind turbine 10 is the common type of horizontal axis wind turbine (HAWT) such that the rotor 15 is mounted at the nacelle 12 to rotate about a substantially horizontal axis defined at the centre at the hub 6. As is known, the blades 20 are acted on by the wind which causes the rotor 15 to rotate about its axis thereby operating generating equipment through a gearbox (not shown) that is housed in the nacelle 12. The generating equipment and gearbox are not shown in figure 1 since it is not central to the examples of the invention.

Each of the blades 20 has a root end 21 proximal to the hub 16 and a tip end 22 distal from the hub 16. A leading edge 23 and a trailing edge 24 extend between the root end 21 and tip end 22, and each of the blades 20 has a respective aerodynamic high pressure surface (i.e. the pressure surface) and an aerodynamic low pressure surface (i.e. the suction surface) surface extending between the leading 23 and trailing edges 24 of the blade 20.

Each blade has a cross section which is substantially circular near the root end 21 , because the blade near the root must have sufficient structural strength to support the blade outboard of that section and to transfer loads into the hub 16. The blade 20 transitions from a circular profile to an aerofoil profile moving from the root end 21 of the blade towards a "shoulder" of the blade, which is the widest part of the blade where the blade has its maximum chord. The blade 20 has an aerofoil profile of progressively decreasing thickness in an outboard portion of the blade, which extends from the shoulder to the tip end 22.

The blade 20 has an outer shell 25. Such a wind turbine blade may conveniently be formed from two opposing half-shells. Figure 2 shows a schematic cross-section of one example of the shell 25. The shell 25 may be manufactured as a set of corresponding half-shells 25a, b. Each of the half-shells 25a, b are separately moulded before being joined together (at a leading edge and a trailing edge) to form the blade.

It will be appreciated that the blade shell 25 need not be formed as two half shells which are subsequently joined together but may be formed as a unitary shell structure, together with the shear web, in a "one shot" single shell process. It will further be appreciated that the blade 20 may have a split blade construction which comprises a plurality of blade sections which are joined together to form the complete blade. The precise construction of the blade and blade shell is not central to the invention and so further detailed description is omitted for clarity.

The shell 25 is covered with a thin film 30. The film may be a thermoplastic film. The film 30 may include a self-adhesive layer 32 on its underside as shown in Figure 3a. The thermoplastic film material has the ability to reversibly stretch and shrink with the application of force and heat, and so local shape changes can be accommodated for by using, for example, a hot air gun to prevent bubbles and wrinkles being formed. The film 30 may be secured to the outer surface of the shell 25 using the adhesive layer 32 (Figure 3b). Alternatively, an adhesive layer or coating may be applied to the shell 25 before application of a film without an adhesive backing layer.

The film 30 may provide an outer aerodynamic surface for at least a portion of the wind turbine blade 20. The film 30 may be applied to the shell 25 more quickly, and possibly more cheaply, than a conventional gel coat and/or paint coat typically applied to the outer surface of the blade shell. Alternatively, a coating 33 such as a paint coat may then subsequently be applied on top of the outer surface 31 of the film 30 if required, as shown in Figure 3c.

The film 30 may be supplied in rolled sheet form. Depending on the type of film, the film 30 may be stretched and shrunk using heat and force so that changes in the curvature of the wind turbine blade 20 are adhered to and the film 30 closely consolidates to the surface of the shell 25. Bubbles and wrinkles may also be removed by applying heat to the film.

Figure 4 shows an example wherein the film 30 includes aerodynamic devices 42,43 and an erosion protection strip 41 pre-attached to the film 30. The aerodynamic devices 42,43 and erosion protection strip 41 , collectively called add-ons, are attached to the film at respective locations that correspond to their intended locations on the wind turbine blade 20. The erosion protection strip 41 is a leading edge protection strip that extends approximately 40mm either side of the leading edge and protects the leading edge of the wind turbine blade 20 from erosion due to droplet (e.g. rain, hailstone, etc.) or particle (e.g. ice, sand, dust, insect debris, etc.) impingement. The aerodynamic devices in this example are a serrated trailing edge device 42 and a set of vortex generators 43. The vortex generators are provided on the suction side of the blade 20 to energise the boundary layer of the aerodynamic surface by producing a turbulent flow that remains attached to the surface of the blade 20, although could be placed at any location on the blade in order to alter the flow characteristics.

Attaching the aerodynamic devices 42,43 and erosion protection strip 41 to the film 30 prior to attaching to the shell 25 may provide better access for attaching the add-ons 41 -43 to the film, as compared with the conventional approach of attaching the add ons directly to the blade shell after any surface coatings have been applied.

The add-ons 41 -43 may be attached to the film by welding, adhesive bonding, or any other suitable means for attaching or affixing the add-ons. The attachment technique or process may be dependent on the film material used. In the case of a thermoplastic film, ultrasonic welding may be a suitable process for attaching the add-ons. Ultrasonic welding may provide a strong bond between the add-ons 41 -43 and the film 30. Ultrasonic welding may be performed using a welding tool that is shaped to the geometry of the add-on to be attached to the film. The geometry of the add-on may be selected to conform to the geometry of the portion of the blade shell where the add-on is to be located when the film is subsequently attached to the blade shell. In this way the film can conform to the shape of the add-on and since the shape of the add-on conforms to the shape of the blade shell geometry, a good conformity of the film to the blade shell can be achieved.

Figures 5a and 5b show examples of ultrasonic welding tools 51 ,52 used to attach the film 30 to the leading edge protection strip 41 and serrated trailing edge device 42, respectively. The ultrasonic welding tools 51 ,52 are positioned on the film 30 beneath the add-ons 41 -43 so that good contact can be made at the bonding surface. The welding tools 51 ,52 are shaped to substantially match the portion of the shell 25 surface to which the add-ons 41 -43 attach.

For example, Figure 5a shows the ultrasonic welding tool 51 of the leading edge protection strip 41 that is shaped to contour to the inside surface of the leading edge protection strip 41 , which is itself shaped to be accepted onto the leading edge of the shell 25. Figure 5b shows the ultrasonic welding tool 52 of the serrated trailing edge device 42 shaped so that the serrated trailing edge device 42 fits onto the trailing edge of the shell 25. In this example the serrated trailing edge device 42 has been modified so that the device 42 sits over the trailing edge of the shell, rather than being bonded only to the pressure side of the wind turbine blade 20, as is more conventional. Serrated trailing edge devices can reduce noise, particularly at the blade tip end, during operation of the wind turbine.

Figure 6 shows the film 30, with pre-attached erosion device 41 and aerodynamic devices 42,43, being attached to the shell 25. The film 30 is applied to the shell 25 using the leading edge centreline LC (Shown in Figure 8) as a datum. The film 30 is wrapped around the shell so that two spanwise edges 34,35 of the film are brought together and at least a portion of the wind turbine blade 20 is enveloped by the film 30, with the edges 34,35 extending along the blade 20 between the root end 22 and the tip end 24. The adhesive layer 32 of the film 30, if present, may then attach to the shell 25, with force and heat optionally being applied to conform the film 30 to the geometric shape of the shell 25. The film 30 may also include an inboard edge 37 and an outboard edge 38 that are adjacent and extend between the two opposing spanwise edges 34,35, the importance of which will be described later with reference to figure 15.

Figure 7 shows the film 30 attached and conforming to the surface of the shell 25 to create the aerodynamic surface. The two spanwise edges 34,35 of the film 30 join to form a spanwise overlap region 36 aft of the vortex generators 43, so that the spanwise overlap region 36 is positioned in the turbulent flow created by the vortex generators 43. If the spanwise overlap 36, also called a seam or join, is located aft of the vortex generators 43, the impact on auditory evoked potential (AEP)/noise is minimised as the boundary layer is already tripped due to the vortex generators 43. The first edge 34 is shown to overlap the second edge 35 so that a smoother aerodynamic profile is created. Any wrinkles and bubbles on the film 30 can optionally be removed by applying heat to the film 30 so that the aerodynamic surface of the film 30 is as conformal as possible.

Figure 8 shows the film 30 attached to the blade 20, so that the erosion protection device 41 is positioned to protect the leading edge 23, the serrated trailing edge device 42 is positioned at the trailing edge 24, and a series of vortex generators 43 are positioned between the leading edge 23 and trailing edge 24, and aft of the maximum blade thickness position. The blade 20 is shown to include a cut-out 38 in the film 30 that exposes a lightning conducting receptor 48 that is intended to protect the blade 20 by attracting lightning strikes to protected parts of the blade. The cut-out 38 may be formed after the film 30 is attached to the shell 25, or could be formed prior to attaching the film to the shell 25.

Figures 9a-9d show various films before attachment to the blade. Figure 9a shows the film 30 before it is attached to the shell 25, and before any aerodynamic devices or erosion protection devices are pre-attached. The film may be a thermoplastic polyurethane thin sheet. The film may be sized to fit over a portion of the shell 25 outer surface. Figures 9b-d show examples of the film that do not include an erosion protection device 41 . Figure 9b shows an example of the film 30 with a serrated trailing edge device 42 pre-attached to the film 30. Figure 9c shows the film 30 with vortex generators 43 pre-attached to the film 30. Figure 9d shows the film 30 with a serrated trailing edge device 42 and vortex generators 43 pre-attached to the film 30 before it is attached to the shell 25.

As shown in Figure 10a, the base 44 of the vortex generators 43 may be attached to the outer surface 31 of the film 30, so that the film 30 conforms to the shape of the shell 25 outer surface, and the vortex generators 43 are impacted directly by the air flow around the blade 20. An alternative shown in Figure 10b is to cover the aerodynamic outer surface of the vortex generators 43 with the film 30 so that the vortex generators are attached to the opposite side of the film 30. In this case the film 30 is contoured to the aerodynamic shape of the vortex generator 43, so that the air flow directly impacts the film 30. This can be achieved by thermoforming of the film 30 onto the vortex generator 43. The aerodynamic devices that attach to the films 30 shown in Figures 1 to 10 include a serrated trailing edge device 42 and vortex generators 43, however the aerodynamic devices could be a different aerodynamic device such as a flap, a Gurney flap or a roughness strip. It will be appreciated that the above described illustrated examples of the films are not exhaustive and other add-ons and add-on combinations may be conceived by those skilled in the art.

Figure 1 1 a shows a Gurney flap 45 pre-attached to a film in a location so that it projects from the trailing edge or near to the trailing edge region. As with previously described aerodynamic devices 41 -43, the Gurney flap 45 is attached to the film 30, e.g. by ultrasonic welding using a welding tool 55 as shown in Figure 1 1 b. Alternatively, or in addition, the film 30 may include a roughness strip 46 pre-attached to the film, as shown in Figure 12a. The roughness strip 46 is attached to the film 30, e.g. by ultrasonic welding using a welding tool 56, as shown in Figure 12b. The welding tools 55,56 may have a shape that substantially matches the portion of the shell 25 surface to which the aerodynamic device 45,46 attaches.

The covering for the blade 20 may comprise of a plurality of films 30. The films may be provided as strips. Each film strip may have respective add-ons/devices pre-attached. For instance, Figure 13 shows three films 30 that each envelop a portion of the blade 20. Having a plurality of films 30 that each correspond to a spanwise portion of the blade may provide the advantage of making it easier and simpler to position the add ons, as each add-on can be associated to a respective film strip with a single datum, or at least fewer datums, as compared with aligning and affixing each add-on individually to a respective datum on the blade. It may also have the advantage that damaged films 30 can be isolated, repaired and replaced individually in sections, rather than stripping away an undetermined section of gel/paint coat and subsequently trying to reapply an equal coating thickness layer to the shell 25.

The films 30 may envelop portions of the surface of the blade 20 and form spanwise overlapping regions 36 between opposing spanwise edges 34,35 of the film 30, as shown in previous examples. However, there may also be chordwise overlapping regions 39 between adjacent films 30, such that, for example, the inboard edge 37 of a first film 30 overlaps an outboard edge 38 of a second film 30. A cross-section of these chordwise overlapping regions 39 between adjacent films 30 is shown in Figure 14. The seam of the overlapping regions 39 is oriented so that it is aligned with the flow direction on the blade 20.

Figure 15 shows a wind turbine blade with a covering. The covering comprises a plurality of films, or strips of film, 30 that substantially cover the outer surface of the blade 20. The erosion protection device 41 may extend approximately the final third of the leading edge of the blade 20 span, whilst the serrated trailing edge device 42 may extend approximately the final third of the leading edge of the blade 20 span except for approximately the final 3 metres of the blade 20 span due to lightning strike protection. As a result, not all of the films 30 that make the covering for the wind turbine blade 20 comprise erosion protection strips 41 and/or an aerodynamic device 42. Additionally, some of the films 30 comprise add-ons 41 ,42 spanning only a portion of the film 30. In an alternative example, a portion of the blade 20 may be coated with a layer of gel/paint coating, whilst another portion of the blade 20 may be covered with a plurality of films 30 with pre-attached aerodynamic devices.

Whilst the examples shown in Figures 4-15 have a limited combination of add ons/devices, such as erosion protection strips 41 and aerodynamic devices 42-46, it will be understood that any combination of add-ons/devices may be envisaged in order to fulfil the requirements of the aerodynamic surface of the blade 20. Furthermore, during the lifetime of the wind turbine 10 the erosion protection strip 41 or aerodynamic devices 42-46 on the film 30 may be periodically replaced or upgraded, e.g. by removing and replacing one or more of the films/strips or by removing and replacing individual add-ons/devices whilst the film 30 is attached to the blade shell due to the existing devices becoming damaged or being superseded by improved technology.

In an example, the erosion protection strip 41 and aerodynamic devices 42-46 may not extend beyond the edges of the film 30 to which they are pre-attached. Instead, the devices may be provided in sections so that if a device needs to extend beyond the edges of one film, a portion of the device will be attached to a first film 30, and a further portion of the device will be attached to an adjacent film 30. This has the advantage of making it simpler and easier to manufacture the films 30 with pre-attached aerodynamic devices 42-46, and easier to apply the films 30 to the wind turbine blade shell 25. The film 30 may be applied to the shell 25 using the leading edge centre line LC as a datum. Alternatively, different datums can be used when applying a film. For example, the film 30 can be applied using the trailing edge centre line TC as a datum for applying the film 30. The film 30 may be applied using a plurality of datums. For example, the leading edge centreline LC can be used as a first datum, and the trailing edge centre line can be used as a secondary centre line.

The spanwise overlap regions 36 are shown to occur aft of the vortex generators 42 where the boundary layer has already been tripped, however the spanwise overlap region 36 may be located at any chordwise location. For instance, the spanwise overlap 36 can be positioned forward of the vortex generators, or on the pressure side of the blade 20, or could be positioned underneath either an erosion protection strip 41 or an aerodynamic device 42-46.

In an alternative embodiment, the opposing edges 34,35 of a film 30 may not have an overlap region 36 and/or there may be no overlapping regions 39 between adjacent films 30. Instead the edges may abut so that no increase in thickness of the covering is produced.

The film 30 may be a thermoplastic film, e.g. made from polyurethane, although other materials may be used such as, Polyvinylidene fluoride (PVDF) or Poly(methyl methacrylate) (PMMA), or a combination of materials.

The blade 20 may require a particular pattern or colour scheme. The pattern or colour scheme may be selected to depend on the location on the blade 20 or the location of the wind turbine 10 site. The film 30 material may contain colouring/pigments to give the film 30 a desired colour, or a transfer layer, or a gel/paint coat 33 over an entire blade or at discrete locations to achieve a desired patterning or colour scheme. The film 30 may also contain further additives, such as UV protective materials for example.

As noted, the film 30 is a“thin” film. The film needs to be thin so that it can stretch and conform to the outer surface of the wind turbine blade. In an example, the film has a thickness of 0.15mm or less. In other examples, the film may have a thickness of 0.1 mm or less or even 0.09mm or less. The use of a thin film allows it to be relatively elastic such that it can be applied to the blade without any wrinkles or bubbles being formed.

The film 30 effectively wraps around the wind turbine blade. In other words, the film may cover a relatively large area of the wind turbine blade so that the blade is enveloped. The shell of wind turbine blade has an airfoil profile in cross section extending from the leading edge to the trailing edge and comprising the suction side and the pressure side. In a given cross section (i.e. in a single plane) the film covers 100% of the airfoil profile, such that the film is wrapped around the suction side and the pressure from the leading edge to the trailing edge.“In a given cross section” means that the film is wrapped around the blade at that particular spanwise location, and that at other spanwise locations there may be no film or a different percentage covering of film. For example, the film may cover at least 50% of the suction side in a given cross section and/or at least 50% of the pressure side in a given cross section. Alternatively, the film may cover at least 90% of the suction side in a given cross section and/or at least 90% of the pressure side in a given cross section Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.