WO2024213212A1

WO2024213212A1 - Wind turbine blade root extender

Info

Publication number: WO2024213212A1
Application number: PCT/DK2024/050075
Authority: WO
Inventors: Jens Bredal Nielsen; Anton Bech
Original assignee: Vestas Wind Systems A/S
Priority date: 2023-04-14
Filing date: 2024-03-27
Publication date: 2024-10-17

Abstract

Wind turbine blade root extender A tubular root extender (24) for extending the length of a wind turbine blade (16) has been described. The root extender (24) is positioned between the root end (20) of a blade (16) and a mounting structure (26) of a hub (18), such as a pitch bearing (26) or hub flange. The root extender (24) comprises a plurality of elongate members (38) extending in a longitudinal direction (L) and arranged circumferentially to form the tubular shape of the root extender (24). The elongate members (38) collectively form a segmented ring when the root extender (24) is viewed in transverse cross-section perpendicular to the longitudinal direction (L). The elongate members (38) are made of fibre-reinforced polymeric material, and each elongate member (38) defines a through bore (32) extending through an entire length of the elongate member (38). A plurality of elongate tension elements (36) connect the root end (20) of the blade (16) to the mounting structure (68). Each tension element (36) extends through a respective through bore (32) in an elongate member (38). A first end (78) of each tension element (36) is connected to the mounting structure (68) of the hub (18) and a second end (80) of each tension element (36) is connected to the root end (20) of the blade (16).

Description

Wind turbine blade root extender

Technical field

The present invention relates generally to wind turbine blades, and more specifically to root extenders for extending the length of an existing wind turbine blade.

Background

Modern utility-scale wind turbines typically comprise a rotor mounted at the top of a tower. The rotor comprises one or more blades (typically three) mounted to a hub.

There is a continuous desire to maximise the productivity of wind turbines. For example, it is known to retrofit aerodynamic add-on devices such as flaps and vortex generators to existing wind turbine blades to improve their performance and increase the output of the wind turbine.

The output power of a wind turbine is directly related to the swept area of the rotor, i.e. the area of the circle created by the blade(s) as they sweep through the air. Accordingly, one way of increasing the output power from an existing wind turbine would be to replace the blades with longer blades. Whilst longer blades will increase the swept area, and hence increase the output power of the turbine, this solution is very expensive as the blades are one of the most expensive components of a wind turbine. Furthermore, unless the existing blades can be repurposed, replacing the blades creates significant waste.

An alternative way to increase the output power of an existing wind turbine is to arrange a root extender between the hub and the root of an existing blade. Root extenders are known in the art, and tend to be made from thick sections of steel or casted from concrete. These existing root extenders are therefore very heavy structures and present a number of other disadvantages. For example, they may be difficult and/or expensive to produce. They may also be difficult to install, and may necessitate modifications to the blades or the hub.

Against this background, it is an object of the present invention to provide an improved root extender. Summary of the invention

According to the present invention, there is provided a rotor assembly for a wind turbine, the rotor assembly comprising: a blade extending in a longitudinal direction from a root end to a tip end; a hub comprising a mounting structure; a tubular root extender positioned between the root end of the blade and the mounting structure, the tubular root extender comprising a plurality of elongate members extending in the longitudinal direction and arranged circumferentially to form the tubular shape of the root extender, the elongate members collectively forming a segmented ring when the root extender is viewed in transverse cross-section perpendicular to the longitudinal direction, the elongate members being made of fibre-reinforced polymeric material, and each elongate member defining a through bore extending through an entire length of the elongate member; and a plurality of elongate tension elements connecting the root end of the blade to the mounting structure, each tension element extending through a respective through bore in an elongate member, wherein a first end of each tension element is connected to the mounting structure of the hub and a second end of each tension element is connected to the root end of the blade.

The root extender serves to increase the effective length of the wind turbine blade, enabling the rotor to capture more energy from the wind and increasing the power output of the wind turbine. The root extender may be retrofitted to an existing rotor or included during the initial installation, for example to adapt the rotor diameter to the site conditions or customer requirements.

The root extender has a length extending in the longitudinal direction and a cross-section substantially perpendicular to the longitudinal direction. The root extender is preferably substantially circular in cross-section. The root extender is preferably substantially cylindrical. The root extender preferably has a substantially uniform cross-sectional profile along its length. The cross-sectional profile of the root extender preferably substantially corresponds to the cross-sectional profile of the blade root, which is typically round. However, the blade root and/or the root extender may have a non-circular cross-section, for example oval or elliptical, in some embodiments.

The elongate members are arranged adjacent to one another (i.e. the elongate members are mutually adjacent) around the circumference of the tubular root extender. There are preferably gaps between adjacent elongate members. The gaps are preferably small gaps. In a preferred embodiment the gaps are approximately one millimetre in width. The elongate members are connected together to form a tubular unit. Preferably the elongate members are connected together by adhesive and/or cured polymeric material, which may be supplied to the assembly of elongate members during a resin-infusion process. The gaps between adjacent elongate members are filled with adhesive and/or resin to connect the elongate members together.

Each elongate member has a length extending in the longitudinal direction and a crosssection substantially perpendicular to the longitudinal direction. The elongate members are preferably arranged in a substantially circular configuration when the root extender is viewed in cross-section. However, in the case of a tubular root extender having a noncircular cross-section, the elongate members may be arranged in a non-circular configuration, for example an oval or elliptical configuration.

The elongate members, and hence the root extender, may have a length of about 0.5 to 6 metres, and approximately 2.5 metres in a preferred embodiment. The root extender may have a cross-sectional diameter of about 1 to 4 metres, and approximately 1.8 metres in a preferred embodiment. However, the root extender may have any suitable dimensions. The diameter of the root extender is preferably selected to match the diameter of the root end of the blade. The length of the root extender is preferably selected according to the desired rotor diameter to be achieved.

The elongate members may be substantially rectangular in cross-section. Preferably the elongate members are substantially square in cross-section. The elongate members may each have mutually-opposed inner and outer surfaces and mutually-opposed side surfaces. Each side surface of an elongate member may be arranged adjacent (i.e. facing) a side surface of an adjacent elongate member around the circumference of the tubular root extender. The inner surface of each elongate member may face an interior of the tubular root extender. The outer surface of each elongate member may face an exterior of the root extender.

The side surfaces of the elongate members are preferably substantially planar. The inner and/or outer surfaces of the elongate members may be slightly curved. Accordingly, the curved inner and/or curved outer surfaces of the elongate members may collectively define a curved (e.g. circular) inner and/or outer profile of the tubular root extender. In a preferred embodiment, the inner and outer surfaces are curved to a radius of about 900 millimetres. For the avoidance of doubt, the elongate members are considered to have a substantially square cross-section even when they have slightly curved inner and/or outer surfaces.

An elongate member may have a width defined as the distance between its two opposed side surfaces, measured through the middle of the side faces (when the elongate member is viewed in cross-section). In a particular embodiment, the elongate members have a width of approximately 62 mm, but in other examples the width may be larger or smaller.

An elongate member may have a thickness defined as the distance between its inner and outer surfaces, measured through the middle of the inner and outer surfaces (when the elongate member is viewed in cross-section). In a particular embodiment, the elongate members have a thickness of approximately 70 mm, but in other examples the thickness may be larger or smaller.

Preferably each elongate member comprises a single through bore. However, in other embodiments, the elongate members may comprise multiple through bores. Preferably, each elongate members comprises a maximum of five through bores, more preferably a maximum of three through bores, and most preferably a single through bore. Accordingly, the number of elongate members may correspond to the number of tension elements. Alternatively, if the elongate members comprise multiple through bores, then there may be fewer elongate members than tension elements. In other words, the elongate members may comprise a segment accommodating a single tension element or a segment accommodating multiple tension elements. In further examples, the root extender may include multiple elongate members butted up together in the longitudinal direction.

In a particular example, the root extender when viewed in transverse cross-section comprises a segmented ring formed from ninety elongate members (i.e. ninety segments). The root extender is substantially circular in transverse cross section. Accordingly, each elongate member subtends an angle of approximately four degrees (i.e. 360/90) when seen from a central longitudinal axis of the root extender. In this case, each elongate member is near-square in cross section and defines a single through bore. In an alternative example, each elongate member may define three through bores. In this case, there would only be thirty elongate members defining the segmented ring, and each elongate member would subtend an angle of approximately twelve degrees. In this case, the elongate members may be substantially rectangular in transverse cross section. Preferably, each elongate member subtends an angle of 1 .5 to 6 degrees. Preferably, the elongate members each comprise a relatively small segment of the circumference of the root extender. Preferably, each elongate member subtends an angle of 1-15 degrees, more preferably 2-8 degrees, even more preferably 2-5 degrees, when seen from a central longitudinal axis of the root extender.

The through bores extend in the longitudinal direction through the elongate members. The through bores have a cross-section substantially perpendicular to the longitudinal direction. The through bores are preferably substantially circular in cross-section, although the through bores could have a non-circular cross-section. In a particular embodiment, the through bores have a cross-sectional diameter of approximately 33 mm, but in other examples the diameter may be larger or smaller. The cross-sectional diameter of the through bores must be sufficient to accommodate a tension element. At the same time, it may be desirable to minimise the cross-sectional diameter of the through bores to maximise the volume of fibre-reinforced composite material in the elongate members, and thereby maximising the stiffness of the root extender.

The elongate members are preferably all substantially identical. In particular, the elongate members are preferably all made of the same materials and have the same dimensions and profile as one another. The elongate members may be made from any suitable method. However, in preferred embodiments the elongate members are formed in a pultrusion process, i.e. they are pultrusions. This is particularly advantageous since the elongate members can be made in a continuous pultrusion process and cut to the desired length. The through bore can also advantageously be provided during the pultrusion process by using a suitably-shaped pultrusion die. The pultrusion process enables the elongate members to be formed relatively rapidly and inexpensively and minimises any material wastage.

The elongate members are preferably of substantially uniform cross-section along their length. The pultrusion process is ideal for creating parts of uniform cross-section. The elongate members may be formed by cutting sections of a continuous pultruded part to a desired length. The cut is preferably made perpendicular to the length of the pultruded part.

The elongate members are made from fibre-reinforced polymeric material. The reinforcing fibres may be any suitable fibres, including glass, carbon and aramid fibres. Glass fibres are particularly preferred as they are relatively inexpensive and readily available, yet sufficiently lightweight and capable of providing the requisite stiffness to the root extender. High modulus glass fibres are particularly advantageous due to their high stiffness. Preferably the glass fibres have a Young’s modulus of 80-100 GPa. In a particular example the glass fibres have a Young’s modulus of 86 GPa. Consequently the fibres are stiffer than standard E-glass fibres, which are commonly used to form wind turbine blades, and which have a Young’s modulus of around 70 GPa.

It is particularly preferred that the fibres are unidirectional fibres, which extend substantially in the longitudinal direction, i.e. along the length of the elongate members. Therefore, the unidirectional fibres extend along the length of the root extender. In a preferred embodiment, the elongate members are pultrusions formed from high modulus, unidirectional glass fibres.

The polymeric material of the elongate members is preferably epoxy resin. However, other suitable polymeric materials may include vinyl ester, polyurethane. Epoxy is most preferred as it has been proven to resist delamination and cracking.

The root extender preferably further comprises an outer skin defining an outer surface of the root extender and an inner skin defining an inner surface of the root extender. The plurality of elongate members are preferably arranged between the inner and outer skins. The inner and/or outer skins are preferably made from fibre-reinforced composite material. The inner and/or outer skins preferably comprise multi-directional fibres, for example biaxial fibres. The fibres preferably extend transversely to the fibres of the elongate members. For example, the fibres may advantageously be arranged at approximately +/- 45 degrees to the longitudinal direction. Preferably, the reinforcing fibres of the skins are glass fibres. Preferably these glass fibres are of lower Young’s modulus than the fibres of the elongate members, i.e. they may have a Young’s modulus below 80 GPa and be standard E-glass fibres. For example, these fibres may be E-glass with a Young’s modulus of about 70 GPa. Glass fibres are particularly preferred for the same reasons as already discussed. In a preferred embodiment, the inner and outer skins comprise biaxial glass fibres.

The elongate members of the root extender are held in compression between the root end of the blade and the mounting structure when the tension elements are tensioned. The tension elements are in tension when connecting the blade to the mounting structure. Tensioning the tension elements, for example by applying torque, causes the elongate members to be held in compression between the root end of the blade and the mounting structure. As the elongate members are held in compression, they will transmit a majority of the loads from the blade to the hub in use, e.g. when the rotor is used in a wind turbine.

The tension elements are preferably threaded elements such as studs or bolts. Stud bolts are preferred, which comprise threaded first and second ends, with a non-threaded shank between the two ends. The threads are preferably provided in a rolling process, which results in higher strength threads than cut threads. The tension elements are preferably made of metal, preferably steel.

The first end of each tension element is connected to the mounting structure of the hub and the second end of each tension element is connected to the root end of the blade. The connection may be made by nuts that are screwed on to threaded ends of the tension elements.

The tension elements are suitably longer than the elongate members. Accordingly the tension elements are longer than the tubular root extender. The tension elements extend continuously through the entire length of the through bores, and the ends of the tension elements project longitudinally beyond each ends of the elongate members.

The tension elements are relatively long in comparison to standard tension elements that are conventionally used to connect a blade to a mounting structure. Preferably the tension elements are approximately 0.5 m longer than the root extender. Preferably the tension elements have a length of more than two metres.

The hollow tubular blade extender made of fibre-reinforced composite, e.g. glass-fibre reinforced composite, may have a relatively low axial stiffness in comparison to a root extender made from stiffer materials such as steel, for example. Consequently, the tension elements may experience the blade loads with a higher fraction on the tension elements in comparison to a more rigid extender. If the fraction of loads experienced by the tension elements is too high, it can lead to fatigue of the threads. To reduce the fraction of the blade loads reaching the tension elements, the tension elements may be made softer or more reinforcing fibres may be provided in the elongate members. It is preferable for the root extender to be made as light as possible within reasonable cost. Accordingly, as explained above, the elongate members are preferably made of high modulus unidirectional glass fibres, which results in maximum stiffness in the axial direction. Preferably, the fibre volume fraction of the elongate members is between 60% and 70%, most preferably approximately 65%. The fibre volume fraction is the percentage of the entire volume of an elongate member constituted by the fibres. The remaining volume comprises the polymeric material, e.g. the cured resin. High fibre volume fraction leads to higher stiffness of the elongate elements. Forming the elongate members as pultrusions is particularly advantageous because it allows a higher volume fraction to be achieved than is possible in other moulding processes. Accordingly, in a preferred embodiment the elongate members may comprise unidirectional pultrusions with a fibre volume fraction of 65%.

To further reduce the overall mass of the root extender to reduce the fraction of loads experienced by the tension elements, the tension elements may comprise stud bolts with a reduced shank diameter, i.e. a shank diameter that is less than the diameter of the threaded ends of the tension element. A reduced shank diameter will decrease the mass of the tension elements thus minimising the increase in weight attributable to the root extender. Furthermore, the reduced shank diameter will reduce the stiffness of the tension elements resulting in a greater proportion of the blade loads being experienced by the elongate members and reducing the fraction of loads experienced by the tension elements.

The tension elements may advantageously include a shank or portion of the shank having a non-circular cross-section. For example, the shank or a portion of the shank may have a flattened profile such as an oval or elliptical cross-section. Appropriate orientation of the shank may increase the bending resistance of the tension element and thereby reduce thread fatigue.

A further option for reducing the fraction of blade loads experienced by the tension elements is to use tension elements with a non-straight shank. In particular, the shank may have a wavy or helical profile. A wavy or helical profile may be achieved by processing a straight rod after rolling the screw threads. For example, the straight rod may be arranged in a press tool, having a wavy profile. The press tool will then form the shank of the rod into a wavy undulating shape. Alternatively, the rod could be rolled into a screw-like shape to form a gentle helix. Both options will reduce the axial stiffness of the tension elements. Using these techniques, the axial stiffness of the tension elements can be reduced by 50% and thereby the loads experienced by the threads of the tension elements may be reduced from approximately 40% of the external blade load to about 25%, which is more acceptable and significantly reduces the risk of thread fatigue.

The mounting structure may be a flange of the hub or a pitch bearing. The mounting structure may include a plurality of through holes. The plurality of through holes may be arranged in a circle when viewed in a plane perpendicular to the longitudinal direction. The first end of each respective tension element may extend through a respective through hole in the mounting structure.

The root end of the blade may comprise a plurality of connection elements. The connection elements may be arranged in a circle when viewed in a plane perpendicular to the longitudinal direction. The second end of each respective tension element may engage a respective connection element in the blade root. The connection elements may be embedded within the root end of the blade. The connection elements are preferably threaded bushings. The second ends of the tension elements are preferably threaded ends and are received within the threaded bushings in mating engagement. Alternatively, the connection elements may be studs that project longitudinally from the root end of the blade, or T-bolt connectors.

A plate such as a ring plate may be provided between the elongate members and the blade root and/or between the elongate members and the mounting structure. The plate(s) may serve to distribute loads evenly around the perimeter of the blade root or mounting structure and minimise load concentrations. When positioned between the blade root and the elongate members, the plate may advantageously serve to spread the load from the connection elements over the full area of the end faces of the elongate members. Alternatively, a plurality of washers may be used instead of a plate to spread loads evenly over the end faces of the elongate members. Preferably each washer has a transverse cross-sectional area equal to or greater than the transverse cross-sectional area of the end face of an elongate member. The hub side of the extender may stand directly on the hub structure if this is a fully flat surface.

The present invention also provides a tubular root extender for extending the length of a wind turbine blade. The root extender is configured to be positioned between a root end of a wind turbine blade and a mounting structure of a hub. The root extender comprises a plurality of elongate members extending in a longitudinal direction and arranged circumferentially to form the tubular shape of the root extender. The elongate members are made of fibre-reinforced polymeric material, and each elongate member defines a through bore extending through an entire length of the elongate member. The elongate members collectively form a segmented ring when the root extender is viewed in transverse cross-section perpendicular to the longitudinal direction. The elongate members are arranged in a ring when viewed in cross-section. The through bores are each sized to receive a respective tension element that extends through an entire length of the through bore. A first end of each tension element is connected to the mounting structure of the hub and a second end of each tension element is connected to the root end of the blade.

The present invention also provides a method of making a tubular root extender for extending the length of a wind turbine blade. The method comprises providing a plurality of elongate members extending in a longitudinal direction. The elongate members are made of fibre-reinforced polymeric material. Each elongate member defines a through bore extending through an entire length of the elongate member. The method further comprises arranging the elongate members circumferentially to form a tubular assembly such that the elongate members collectively form a segmented ring when the tubular assembly is viewed in transverse cross-section perpendicular to the longitudinal direction. Thereafter the method comprises connecting the elongate members together.

The elongate members have already been described in detail above, and these details will not be repeated. Accordingly, in a preferred embodiment, the elongate members comprise pultrusions of unidirectional glass-fibre reinforced composite material.

The method may comprise supporting the tubular assembly of elongate members using a jig or other suitable tool.

The method preferably comprises wrapping inner and/or outer surfaces of the tubular assembly of elongate members with fibrous material. Preferably the fibrous material is a fibrous fabric. The fibrous material preferably comprises multi-axial fibres, most preferably biaxial fibres. In a preferred example, biax fabric is used.

The method may further comprise infusing the tubular assembly of elongate members with resin during a resin infusion process. A vacuum bagging process may be used in which the tubular assembly is sealed within a vacuum bag. Air may then be extracted from the vacuum bag and liquid resin may be admitted into the bag. The resin infiltrates between the elongate members and infiltrates through the optional inner and/or outer fabric layers. Once cured, the resin integrates the elongate members together and integrates the optional inner and/or outer fabric layers to form a single tubular unit. In a preferred embodiment, the finished root extender comprises elongate pultrusions disposed between inner and outer skins.

The method preferably comprises arranging the elongate members circumferentially with small gaps between adjacent elongate members. The method may further comprise providing an infusion medium in the gaps between adjacent elongate members. The infusion medium may be a fleece material, for example a fibrous fleece. Preferably the infusion medium is a glass fibre fleece. The infusion medium functions to slow the passage of resin in the spaces between elongate members during the infusion process. Without the infusion medium in the gaps, the resin may race within the gaps and form undesirable seam lines over the elongate members.

The method may comprise bonding the infusion medium to the elongate members. For example, the infusion medium may be bonded to the side surfaces of the elongate members. Dots of adhesive may be applied to the infusion medium or to the surfaces of the elongate members to secure the infusion medium to the elongate members. The adhesive ensures that the infusion medium is kept in the correct position. This adhesive may also serve to connect adjacent elongate members to one another to stabilise the tubular arrangement of elongate members and ensure that elongate members maintain their correct positions when they are wrapped with the inner and/or outer skin layers and during the infusion process.

The elongate members, which are preferably pultrusions, may advantageously be surface activated prior to the infusion process. Surface activation may involve brushing the outer surfaces of the elongate members with an abrasive brush. Alternatively, other suitable techniques may be provided to suitably roughen the surfaces of the elongate members. Activating the surfaces improves the adhesion of resin during the infusion process, and/or improves the spot bonding process described above.

Preferably the root extender assembly is arranged in a vertical orientation for the infusion process. The elongate members may initially be loaded onto a jig in a horizontal orientation and wrapped with inner and/or skin layer in a horizontal position. After the vacuum bag has been arranged, the jig may rotate the assembly to a vertical orientation for the infusion. The vertical orientation may advantageously avoid resin lock offs that cause dry spots in the infused assembly. The assembly is preferably infused from the bottom to the top. Accordingly, a resin inlet port may be provided near the bottom of the bagged assembly, and a vacuum outlet port may be provided near a top of the bagged assembly.

Once the infusion process is complete, the resin is cured over a period of time. An elevated temperature may be used to enhance or shorten the curing process. After curing is complete, the end surfaces of the tubular root assembly may be flattened, for example by end milling. The outer surface of the root extender may finally be painted white or another colour, for example to match the appearance of an existing wind turbine blade.

The present invention also provides a method of increasing the swept area of a wind turbine rotor. The method comprises: providing a wind turbine rotor having a hub and a rotor blade connected to the hub, the rotor blade extending in a longitudinal direction from a root end to a tip end, and the root end being connected to a mounting structure of the hub; disconnecting the rotor blade from the mounting structure; positioning a tubular root extender between the root end of the rotor blade and the mounting structure, the root extender comprising a plurality of elongate members extending in the longitudinal direction and arranged circumferentially to form the tubular shape of the root extender, the elongate members being made of fibre-reinforced polymeric material, and each elongate member defining a through bore extending through an entire length of the elongate member; providing a plurality of elongate tension elements of greater length than the elongate members; providing a tension element in each through bore such that a first end of each tension element projects beyond a first end of the respective elongate member and a second end of each tension element projects beyond a second end of the respective elongate member; connecting the first end of each tension element with the mounting structure of the hub; and connecting the second end of each tension element with the root end of the blade.

Optional features described above in relation to the invention when expressed in terms of a rotor assembly and/or a tubular root extender apply equally to the invention when expressed in terms of a method of making a tubular root extender and/or a method of increasing the swept area of a wind turbine rotor using a root extender. Repetition of these features is avoided purely for reasons of conciseness. Accordingly, it will be appreciated that the features recited in the dependent claims are expressly intended to be combinable with the features of any of the independent claims.

Brief description of the drawings

Embodiments of the invention will now be described, by way of non-limiting example, with reference to the following figures, in which:

Figure 1 is a schematic view of a wind turbine having a rotor comprising a plurality of blades fitted with root extenders in accordance with an example of the present invention;

Figure 2 is a schematic perspective view of a root extender according to an example of the present invention;

Figure 3 is a schematic transverse cross-sectional view of part of the root extender of Figure 2;

Figure 4 is a schematic transverse cross-sectional view of an elongate member forming part of the root extender of Figure 2;

Figure 5 is a schematic longitudinal cross-sectional view of the root extender of Figure 2 positioned between a blade root and a pitch bearing according to an example of the present invention;

Figure 6 schematically shows a press tool forming a tension element with a wavy profile; and

Figure 7 schematically shows part of a tension element with a wavy profile inside a through bore of an elongate member of the root extender of Figure 2.

Detailed description

Figure 1 shows a wind turbine 10 comprising a rotor 12 mounted at the top of a tower 14. The rotor 12 comprises a plurality of wind turbine blades 16, three in this example, connected to a central hub 18. Each blade 16 extends lengthwise in a longitudinal direction L from a root end 20 to a tip end 22. The blades 16 in this example are variable pitch blades and may be turned about a pitch axis P.

A root extender 24 is positioned between the root end 20 of each blade 16 and a respective mounting structure 26 of the hub 18. The mounting structures 26 in this example are pitch bearings (shown schematically in Figure 5). In other examples, the mounting structure 26 could be a hub flange, for example in the case of fixed pitch blades.

The rotor 12 rotates about a generally horizontal axis, extending through the hub 18, substantially perpendicular to the plane of Figure 1. The tips 22 of the blades 16 describe a circle 28 as the rotor turns. The area of this circle 28 is the swept area of the rotor 12.

It will be appreciated that the root extenders 24 (which may also be referred to as blade extenders) extend the effective length of the blades 16, and therefore increase the diameter of the circle 28 described by the blade tips 22. Accordingly, the root extenders 24 increase the swept area of the rotor 12. This results in increased energy capture from the wind, and increased power output from the wind turbine 10 in comparison to a rotor with the same length blades 16 without the root extenders 24.

A root extender 24 according to an example of the present invention is illustrated schematically in Figure 2. The root extender 24 is a tubular structure and extends lengthwise in a longitudinal direction L. The root extender 24 has a length of approximately 2.5 metres, but the root extender 24 could be longer or shorter in other examples. The root extender 24 is substantially circular in cross-section, i.e. in transverse cross-section perpendicular to the longitudinal direction L. In this example, the root extender 24 is substantially cylindrical. The root extender 24 has a diameter D of approximately 1.8 metres. The shape and diameter is selected to correspond to the shape and diameter of the root end of the wind turbine blade to which the root extender 24 will be fitted.

The root extender 24 has a central longitudinal axis 30 extending in the longitudinal direction L through the centre of the cross-section. The central longitudinal axis 30 coincides with the pitch axis P of the blade (shown in Figure 1) when the root extender is installed.

The root extender 24 is a hollow structure defined by a cylindrical wall 31. A plurality of through bores 32 are provided in the wall 31 of the root extender 24. The through bores 32 extend in the longitudinal direction L through the entire length of the root extender 24. The through bores 32 define holes 33 at end faces 34 of the root extender 24. The holes

33 are mutually spaced in a circular arrangement at the end faces 34 of the root extender 24. The through bores 32 are sized to accommodate tension elements 36, e.g. stud bolts (shown in Figure 5), which extend through the entire length of the through bores 32 and connect the blade 16 to the hub 18, as will be described in further detail later.

The root extender 24 is made of composite material, in particular fibre-reinforced polymeric material. Accordingly, the root extender 24 is advantageously lightweight in comparison to known root extenders 24 made predominantly from metal, such as steel, or concrete. In this example, the root extender 24 is made from glass-fibre reinforced polymeric material. The polymeric material may be any suitable resin, for example epoxy.

Figure 3 is a schematic transverse cross-section showing a portion of the wall 31 of the root extender 24. The root extender 24 comprises a plurality of mutually adjacent elongate members 38 arranged circumferentially to form the tubular shape of the root extender 24. The elongate members 38 collectively form a segmented ring when viewed in crosssection. Only a portion comprising four segments of the segmented ring is shown in Figure 3.

The elongate members 38 are arranged close together with small gaps 40 of approximately 1 mm between adjacent elongate members 38. The elongate members 38 are formed of glass-fibre reinforced epoxy. The elongate members 38 are arranged between an inner skin 42 and an outer skin 44, which respectively define an interior surface 46 and an exterior surface 48 of the root extender 24. The skins 42, 44 comprise fibrous material, in particular biaxial glass fabric. The elongate members 38 and skins 42, 44 are integrated together in a resin infusion process to form a single unit, as discussed in further detail later.

The elongate members 38 extend lengthwise in the longitudinal direction L, which is perpendicular to the plane of Figure 3. The elongate members 38 may each have a length of approximately 2.5 metres. The elongate members 38 may have a substantially constant cross-section along their length. The elongate members 38 may have a substantially square, i.e. a near-square cross section (as shown in Figure 3) or a near-rectangular cross section. In this example, each elongate member 38 includes a single through bore 32. The through bores 32 extend through the entire length of the elongate members 38. The through bores 32 in this example are of substantially circular cross section.

Referring also to Figure 4, which shows a single elongate member 38 in cross-section, the elongate member 38 has mutually-opposed inner and outer surfaces 50, 52 and mutually- opposed side surfaces 54, 56. The side surfaces 54, 56 are substantially planar, whereas the inner and outer surfaces 50, 52 are slightly curved. In this example, the inner and outer surfaces 50, 52 are curved to a radius of about 900 millimetres, consistent with the 1.8 m diameter of the root extender 24. The elongate member 38 may also have slightly rounded edges.

The elongate member 38 may subtend an angle 0 of approximately four degrees, when seen from the central longitudinal axis 30 of the root extender 24 (indicated in Figure 2). This angle 0 is the angle between the dashed lines 58 in Figure 4, which converge at the central longitudinal axis 30, although not shown in Figure 4. Accordingly, the side surfaces 54, 56 of the elongate member 38 may be slightly inclined. This geometry enables the elongate members 38 to collectively adopt a substantially circular arrangement when positioned side-by-side.

When the elongate members 38 are all substantially identical and the cross-sectional shape of the root extender 24 is substantially circular, the subtended angle 0 is calculated by dividing the 360 degrees by the number of elongate members 38. In this example, the root extender 24 comprises ninety elongate members 38, each defining a respective through bore 32. Accordingly, each elongate member 38 subtends an angle of approximately four degrees in this example.

The number of elongate members 38 corresponds to the number of tension elements 36 required to connect the blade 16 to the hub 18. In other examples, the elongate members 38 could have more than one through bore 32, in which case fewer elongate members 38 would be required, and each elongate member 38 may subtend a larger angle.

In this example, the elongate member 38 has a width w of approximately 62 mm and a thickness (or height) t of approximately 70 mm. The width w is the distance between the two opposed side surfaces 54, 56, measured through the middle of the side faces as indicated in Figure 4. The thickness (or height) t is the distance between the inner and outer surfaces 50, 52, measured through the middle of the inner and outer surfaces 50, 52, as indicated in Figure 4.

The through bore 32 in this example has a diameter of approximately 33 millimetres, to accommodate an M30 stud bolt.

Returning now to Figure 3, the elongate members 38 forming the root extender 24 are all substantially identical. The elongate members 38 are arranged side-by-side such that each side surface 54, 56 of an elongate member 38 faces a side surface 54, 56 of an adjacent elongate member 38. The inner surfaces 50 of the elongate members 38 face an interior 60 of the tubular root extender 24, and the outer surfaces 52 of the elongate members 38 face an exterior 62 of the root extender 24. When arranged side-by-side, the elongate members 38 collectively form a segmented ring (in this example a substantially circular ring) (in cross-section) in view of the slightly curved and slightly angled geometry of the various surfaces of the elongate members 38 previously described with reference to Figure 4.

The elongate members 38 in this example are formed in a pultrusion process, i.e. the elongate members 38 are pultrusions. For maximum stiffness, the elongate members 38 are made from high modulus glass fibres, preferably having a Young’s modulus of approximately 86 GPa. To further maximise the stiffness of the elongate members 38, the glass fibres are unidirectional glass fibres, with the fibres orientated along the length of the elongate members 38.

The pultrusion process produces a continuous pultruded component of cured glass-fibre composite having the cross-sectional profile shown in Figure 4. The through bore 32 is advantageously formed during the pultrusion process. The elongate members 38 are formed by dividing, e.g. cutting, the continuous pultruded component into the desired lengths, in this case approximately 2.5 m long sections. However, if desired, for example to facilitate handling, the continuous pultruded component could be divided into sections that are shorter than the desired length of the root extender. In this case, the shorter sections could be butted up against each other in the longitudinal direction to form the root extender 24. However, it is presently preferred for the elongate members 38 each to be continuous along the length of the root extender 24, as this reduces the number of components needed to form the root extender 24. The root extender 24 is formed by supporting the elongate members 38 in a circular arrangement using a suitable jig or tool. For example, the tool may comprise a pair of plates each having a circular arrangement of pegs that are inserted partially into the respective through bores 32 in the elongate members 38. The assembly of elongate members 38 is wrapped with biaxial fabric on the inside and outside to form the inner and outer skins 42, 44 shown in Figure 3. The wrapped assembly is then covered with a vacuum bag. Air is removed from the vacuum bag and resin is admitted into the evacuated vacuum bag. The resin infuses through the gaps 40 between the elongate members 38 and infuses throughout the biaxial fabric and is then hardened during a curing process. The cured resin integrates the components together into a single unit. After the resin has cured, the root extender 24 may be painted white or another colour to match or complement the appearance of the blade 16. The end faces 34 of the root extender 24 may be milled to provide a flat surface.

In order to prevent resin from racing too fast within the small gaps 40 between elongate members 38, an infusion medium 64, in this case a fibrous fleece, is provided in each gap 40, as shown in Figure 3. The fibrous fleece 64 is bonded to the side surfaces 54, 56 of the elongate members 38 using small dots of adhesive. The adhesive may also serve to hold the adjacent elongate members 38 together so that they remain in their correct relative positions during the resin infusion process. Prior to assembling the elongate members 38, the surfaces 50, 52, 54, 56 of the elongate members 38 may be activated, for example by brushing with an abrasive brush. This provides a rough surface which promotes adhesion of resin during the infusion process.

Referring now to Figure 5, this shows a cross-section through a root extender 24 positioned between a pitch bearing 26 and a blade root 20 in a rotor assembly 66 according to an example of the present invention. The root extender 24 may be the root extender 24 shown in Figure 2 or Figure 3 or another variant within the scope of the present invention.

The pitch bearing 26 in this example comprises an inner bearing ring 68 and an outer bearing ring 70. A plurality of bearing elements 72, in this case ball bearings, are provided between the inner and outer bearings 68, 70. The inner bearing ring 68 includes a plurality of through holes 74. The through holes 74 are arranged in a circle when viewed in a plane perpendicular to the plane of Figure 5. The root end 20 of the wind turbine blade 16 includes a plurality of connecting elements 76. The connecting elements 76 are arranged in a circle when viewed in a plane perpendicular to the plane of Figure 5. The connecting elements 76 are embedded within the composite structure of the blade root 20. In this example, the connecting elements 76 are bushings, e.g. steel bushings. The bushings 76 are provided with an internal screw thread.

A plurality of elongate tension elements 36 are shown in Figure 5. The tension elements 36 connect the root end 20 of the blade 16 to the inner ring 68 of the pitch bearing 26 in this example. The tension elements 36 extend longitudinally from a first end 78 to a second end 80. In this example, the tension elements 36 are stud bolts. The stud bolts 36 are preferably made of steel. The stud bolts 36 have a threaded first end 78 and a threaded second end 80, and a non-threaded shank 82 between the first and second ends 78, 80.

Each tension element 36 extends through a respective through hole 74 in the inner ring 68 of the pitch bearing 26. Thereafter, the tension elements 36 each extend through an entire length of a respective through bore 32 in the root extender 24. The second end 80 of each tension element 36 engages with a connecting element 76 in the root end 20 of the blade 16.

Accordingly, it will be appreciated that each through bore 32 of the root extender 24 is coaxial with a corresponding through hole 74 in the bearing 26 and coaxial with a corresponding connection element 76 in the blade root 20.

Specifically, in this example, the threaded second ends 80 of the tension elements 36 mate with the internal screw threads of the bushings 76. After screwing the second ends 80 of the tension elements 36 into the blade root 20, the tension elements 36 are each pretensioned by stretching, and the first ends 78 are then secured to the inner ring 68 of the pitch bearing 26 by nuts 84.

Pre-tensioning the tension elements 36 causes the blade 16 to be pulled axially towards the pitch bearing 26. Consequently, the root extender 24 becomes pre-loaded in compression between the blade root 20 and the pitch bearing 26. Accordingly, the tension elements 36 are held in tension, and the root extender 24 is held in compression in the rotor assembly 66. The tension elements 36 are longer than the root extender 24 such that they extend through an entire length of the root extender 24 and still have sufficient length to engage with the pitch bearing 26 at the first end 78 and to engage with the connecting elements 76 of the blade root 20 at the second end 80. The tension elements 36 are therefore longer than the elongate members 38.

In this example, the tension elements 36 may have a length of approximately 3 metres, whereas the elongate members 38 may have a length of approximately 2.5 metres. However, the elongate members 38 may have any suitable length according to the required extension of the blade length. The length of the tension elements 36 may then be selected to be suitably longer than the length selected for the root extender 24.

The tension elements 36 have a diameter that is smaller than an internal diameter of the through bores 32 in the root extender 24. This allows the tension elements 36 to move freely with respect to the through bores 32 when the tension elements 36 are initially inserted through the through bores 32 and when the tension elements 36 are subsequently tensioned. The through bores 32 preferably have a smooth non-threaded internal surface. Accordingly, there is no mating engagement between the tension elements 36 and the through bores 32. It will be appreciated that the through bores 32 act as bushings for the tension elements 36.

Referring still to Figure 5, a plate 86 is provided between the blade root 20 and the root extender 24. The plate 86 ensures that there is an even load distribution on the end face 34 of the root extender 24 and avoids stress concentrations that may otherwise arise due to the connecting elements 76 embedded in the blade root 20, which may protrude slightly from the blade root 20.

The plate 86 in this example comprises a ring plate having a plurality of holes, which are coaxial with the through bores 32 of the root extender 24 and coaxial with the connecting elements 76 in the blade root 20. The plate 86 may be made from steel, and preferably has a thickness of approximately 20 mm. In other examples, a plurality of washers, for example 20 mm thick square washers, may be used instead of a ring plate 86 to distribute the loads evenly over the end faces of the elongate members 38.

Although not shown in Figure 5, a further plate or washers may be provided between the root extender 24 and the inner bearing ring 68. However, this may not be required if the inner bearing ring 68 or other mounting structure to which the root extender 24 abuts presents a flat, even surface.

In order to avoid fatigue of the threads of the tension elements 36 it is important to ensure that the strain on the tension elements 36 is maintained at an acceptable level. It is therefore necessary to design the root extender 24 and the tension elements 36 such that loads are balanced correctly between the root extender 24 and the tension elements 36.

The majority of the blade loads should be handled by the root extender 24 and not by the tension elements 36. Accordingly, the stiffness of the root extender 24 should be maximised. This is achieved in the above examples by using high modulus unidirectional glass fibres in the elongate members 38. Furthermore, the volume fraction of the fibres in the elongate members 38 should be relatively high, for example approximately 65%.

The stiffness of the root extender 24 may be further increased by increasing the quantity of reinforcing fibres in the root extender 24, for example by using thicker elongate members 38. However, this will of course increase the cost and weight of the root extender 24, and so it is desirable to minimise the quantity of material in the root extender 24 as far as possible, within load constraints. Another option for minimising the bending loads experienced by the tension elements 36 is to reduce the axial stiffness of the tension elements 36. Various ways of achieving this are presented below.

In order to reduce the fraction of blade loads reaching the tension elements 36, and thereby to increase the proportion of loads taken by the root extender 24, the tension elements 36 can be designed with reduced axial stiffness. To this end, the shank 82 of the tension elements 36, or a portion of the shank 82, may have a reduced diameter in comparison to the diameter of the threaded ends 78, 80. For example, the threaded ends 78, 80 may have a diameter of approximately 30 mm (M30), whilst the shanks 82 may have a diameter of 28 mm. This can be achieved in a rolling process, where the screw threads are created by rolling a 28 mm steel rod.

A further option for reducing the stiffness of the tension elements 36 is to form the shanks 82 with a wavy or helical profile. Referring to Figure 6, this shows a press tool 88 that can be used to impart an undulating (wavy) profile to a previously straight shank 82. The wavy shank 82 will impart spring-like qualities to the tension element 36. Accordingly, the tension element 36 with a wavy shank 82 has a reduced axial stiffness in comparison to an otherwise similar tension element 36 with a straight shank. In order to accommodate a tension element 36 with a wavy shank 82, the through bores 32 of the elongate members 38 need to have a sufficient clearance (e.g. a sufficient diameter, width or height) to accommodate the amplitude of the undulations, as shown in Figure 7. An alternative technique for reducing the axial stiffness of the tension elements 36 is to form the shanks 82 into a gentle helix, for example by rolling a rod into a screw-like shape.

The blade root 20 and the bearing ring 68 may deform under load, which creates bending in the tension element 36. It is possible to reduce the bending stresses on the threads of the tension elements 36 by forming the shanks 82 of the tension elements 36 with a noncircular cross-sectional profile. For example, the shanks 82 or at least a portion of the shanks 82, may have a flattened profile. By appropriately orienting the shanks 82 the tension elements 36 may be more resistant to bending loads.

Using any of these measures, the loads in the tension elements 36 may be reduced and/or the fatigue resistance of the tension elements 36 may be increased whilst minimising the mass and cost of the root extender 24.

In summary, a tubular root extender 24 for extending the length of a wind turbine blade 16 has been described. The root extender 24 is positioned between the root end 20 of a blade 16 and a mounting structure 26 of a hub 18, such as a pitch bearing 26 or hub flange. The root extender 24 comprises a plurality of elongate members 38 extending in a longitudinal direction L and arranged circumferentially to form the tubular shape of the root extender 24. The elongate members 38 collectively form a segmented ring when the root extender 24 is viewed in transverse cross-section perpendicular to the longitudinal direction L. The elongate members 38 are made of fibre-reinforced polymeric material, and each elongate member 38 defines a through bore 32 extending through an entire length of the elongate member 38. A plurality of elongate tension elements 36 connect the root end 20 of the blade 16 to the mounting structure 68. Each tension element 36 extends through a respective through bore 32 in a respective elongate member 38. A first end 78 of each tension element 36 is connected to the mounting structure 68 of the hub 18 and a second end 80 of each tension element 36 is connected to the root end 20 of the blade 16.

The present invention presents numerous advantages. Instead of replacing one blade 16 with a longer blade to achiever a greater swept area, the root extender 24 will increase production whilst still using the existing blades 16. The root extenders 24 of the present invention therefore reduce scrap, transport and material, and will increase the useful lifetime of current turbines.

The root extenders 24 of the present invention are advantageously lightweight in comparison to prior art root extenders 24 made from steel or concrete. The root extender 24 can be manufactured relatively inexpensively and from readily available materials. The root extender 24 is relatively easy to build. As it is formed of composite materials, no welding is required, and hence there are no joints that are sensitive to fatigue. Composite material is advantageously not susceptible to corrosion, unlike metal. The design is highly flexible and enables root extenders 24 to be built to any dimensions.

The root extender 24 can be installed without any modifications being required to the existing blades 16 or mounting structure 26. The existing tension elements 36 connecting an existing blade 16 to a mounting structure 26 are removed. The root extender 24 is arranged in place between the mounting structure 26 and the existing blade 16. Longer tension elements 36 are then fitted to hold the assembly together. Accordingly, it is not necessary to create any new joints.

Many modifications may be made to the specific examples described above without departing from the scope of the present invention as defined in the following claims. Whilst particular dimensions are discussed in the above examples, these are provided primarily to facilitate understanding of the present invention, but are not intended to be limiting. It will be appreciated that the root extenders can be built to any suitable dimensions, and the number and dimensions of the elongate members, through bores, holes, tension elements etc may vary according to the design requirements and the dimensions and configuration of the particular blade that is to be extended.

Claims

1. A rotor assembly for a wind turbine, the rotor assembly comprising: a blade extending in a longitudinal direction from a root end to a tip end; a hub comprising a mounting structure; a tubular root extender positioned between the root end of the blade and the mounting structure, the tubular root extender comprising a plurality of elongate members extending in the longitudinal direction and arranged circumferentially to form the tubular shape of the root extender, the elongate members collectively forming a segmented ring when the root extender is viewed in transverse cross-section perpendicular to the longitudinal direction, the elongate members being made of fibre-reinforced polymeric material, and each elongate member defining a through bore extending through an entire length of the elongate member; and a plurality of elongate tension elements connecting the root end of the blade to the mounting structure, each tension element extending through a respective through bore in an elongate member, wherein a first end of each tension element is connected to the mounting structure of the hub and a second end of each tension element is connected to the root end of the blade.

2. The rotor assembly of Claim 1, wherein the elongate members are substantially rectangular, preferably substantially square, in cross-section.

3. The rotor assembly of Claim 1 or Claim 2, wherein the elongate members have curved inner and/or outer surfaces.

4. The rotor assembly of any preceding claim, wherein the elongate members are pultrusions.

5. The rotor assembly of any preceding claim, wherein the elongate members comprise unidirectional fibres extending in the longitudinal direction.

6. The rotor assembly of any preceding claim, wherein the elongate members comprise glass fibres having a Young’s modulus of 80 to 100 GPa.

7. The rotor assembly of any preceding claim, wherein the elongate members have a fibre volume fraction of at least 60%, preferably between 60% and 70%, preferably approximately 65%.

8. The rotor assembly of any preceding claim, wherein the root extender further comprises an outer skin defining an outer surface of the root extender and an inner skin defining an inner surface of the root extender, and wherein the plurality of elongate members are arranged between the inner and outer skins.

9. The rotor assembly of Claim 8, wherein the inner and/or outer skin comprise multi- axial reinforcing fibres, preferably biaxial fibres.

10. The rotor assembly of any preceding claim, wherein the elongate members are held in compression between the root end of the blade and the mounting structure and the tension elements are held in tension.

11. The rotor assembly of any preceding claim, wherein each elongate member comprises a single through bore.

12. The rotor assembly of any of Claims 1 to 10, wherein each elongate member comprises a plurality of through bores.

13. The rotor assembly of any preceding claim, wherein each elongate member subtends an angle of approximately 2-5 degrees, when seen from a central longitudinal axis of the root extender.

14. The rotor assembly of any preceding claim, wherein the tension elements are longer than the elongate members.

15. The rotor assembly of any preceding claim, wherein the tension elements comprise stud bolts having a shank or a portion of a shank of non-circular cross-section.

16. The rotor assembly of any preceding claim, wherein the tension elements comprise a shank having a non-straight profile, preferably a wavy or helical profile.

17. The rotor assembly of any preceding claim, wherein the tension elements comprise a shank having a smaller diameter than a diameter of the first and second ends.

18. A tubular root extender for extending the length of a wind turbine blade, the tubular root extender being configured to be positioned between a root end of a wind turbine blade and a mounting structure of a hub, wherein the tubular root extender comprises a plurality of elongate members extending in a longitudinal direction and arranged circumferentially to form the tubular shape of the root extender, the elongate members collectively forming a segmented ring when the root extender is viewed in transverse cross-section perpendicular to the longitudinal direction, the elongate members being made of fibre-reinforced polymeric material, and each elongate member defining a through bore extending through an entire length of the elongate member.

19. A method of making a tubular root extender for extending the length of a wind turbine blade, the method comprising: providing a plurality of elongate members extending in a longitudinal direction, the elongate members being made of fibre-reinforced polymeric material, each elongate member defining a through bore extending through an entire length of the elongate member; arranging the elongate members circumferentially to form a tubular assembly such that the elongate members collectively form a segmented ring when the tubular assembly is viewed in transverse cross-section perpendicular to the longitudinal direction; and connecting the elongate members together.

20. The method of Claim 19, further comprising wrapping inner and/or outer surfaces of the tubular assembly of elongate members with fibrous material.

21 . The method of Claim 19 or Claim 20, further comprising infusing the tubular assembly of elongate members with resin during a resin infusion process.

22. A method of increasing the swept area of a wind turbine rotor, the method comprising: providing a wind turbine rotor having a hub and a rotor blade connected to the hub, the rotor blade extending in a longitudinal direction from a root end to a tip end, and the root end being connected to a mounting structure of the hub; disconnecting the rotor blade from the mounting structure; positioning a tubular root extender between the root end of the rotor blade and the mounting structure, the root extender comprising a plurality of elongate members extending in the longitudinal direction and arranged circumferentially to form the tubular shape of the root extender, the elongate members being made of fibre-reinforced polymeric material, and each elongate member defining a through bore extending through an entire length of the elongate member; providing a plurality of elongate tension elements of greater length than the elongate members; providing a tension element in each through bore such that a first end of each tension element projects beyond a first end of the respective elongate member and a second end of each tension element projects beyond a second end of the respective elongate member; connecting the first end of each tension element with the mounting structure of the hub; and connecting the second end of each tension element with the root end of the blade.