Field of the invention

The present invention relates to sectional blade for a wind turbine, the blade comprising at least a first blade portion and a second blade portion extending in opposite directions from a joint.

Background of the invention

Modern wind turbines comprise a plurality of wind turbine rotor blades, typically three blades, each blade having a weight of up to 15 tons and a length of up to 55 meters, or even more.

Traditionally, a blade comprises two shell parts, one defining a windward side shell part and the other one defining a leeward side shell part. Each of the shell parts are traditionally made in one piece. To reinforce such a blade, a beam- or box-shaped, longitudinal and tubular element, i.e. a spar, can act as a reinforcing beam running lengthways, i.e. in the

longitudinal direction of the blade. The spar is located in the cavity between the two wind turbine shell parts and extends substantially throughout the shell cavity in order to increase the strength and stiffness of the wind turbine blade. A blade may further be reinforced by two or more spars placed lengthways side by side.

As the size of wind turbines and thus wind turbine blades are still growing, the production facilities and the transport means must be increased to handle blades of the required size. This also increases the demand on logistics and increases the associated costs. Summary of the invention

It is an object of the present invention to provide an improved wind turbine blade comprising at least two blade portions, to provide an improved method of manufacturing such a blade, and to provide a wind turbine comprising a sectional blade.

Thus, in a first aspect, the invention provides a sectional blade for a wind turbine, the blade comprising at least a first blade portion and a second blade portion extending in opposite directions from a joint, where at least one insert of a first group of inserts is embedded in the first blade portion and at least one insert of a second group of inserts is embedded in the second blade portion, and where the blade portions are joined by an assembly structure connecting an insert of the first group of inserts to a corresponding insert of the second group of inserts, the corresponding inserts having a common centre line, and where the assembly structure comprises at least two assembly elements being positioned offset from said common centre line. In a preferred embodiment, the joint between the two blade portions may be transverse to the length of the blade, thus allowing for a blade comprising smaller sections compared to a traditional blade being manufactured of shell parts of full-size.

The blade portions may be shorter than normal blade shells, and the blade portions may be easier to transport from a manufacturing site to an assembly site for assembling hereof, compared to blades in one piece. Furthermore, the assembly site can be situated close to the place where the turbine blade it to be used.

By manufacturing the blade of different parts, these parts may be transported unassembled, thereby facilitating transport with the possibility of reducing the associated costs.

The joint may be approximately at the middle part of the blade providing blade portions of approximately the same length. However, the blade portions may also be of different length. As an example, the first blade portion may define a main blade portion, whereas the second blade portion may define a root end portion or a tip end portion.

In an embodiment of the invention, the second blade portion may form a winglet. Winglets can attain different shapes such as e.g. a sharply bent tip in an angle from a few degrees to 90° relative to the lengthwise direction of the blade, or such as a gradually bent tip. Hereby is obtained that the blade may be transported in parts which may e.g. be relatively flat compared to a traditional blade with winglet, thereby facilitating transport with the possibility of reducing the associated costs.

The blade tip influences the performance of the wind turbine blade as well as the noise emission. By detachable mounting of the winglet to the rest of the blade, is obtained e.g. that the blade tip may by exchanged on existing wind turbines to thereby adjust the wind turbine performance or noise emission by attaching tips extending in different angles relative to the lengthwise direction of the blade or tips of different size and/or shape. Also, as the blade tip is often vulnerable to damage during transport, handling, or operation, a detachable blade tip or winglet according to the above may furthermore be advantageous in facilitating the exchange of a damaged blade tip. As longer blades may be preferred at sites with a typically lower wind velocity, a certain type of wind turbine may be manufactured in e.g. two different designs, one with longer blades for a low wind velocity site and another with shorter blades for a high wind velocity site. As the root section may be the same for the two designs, only one of the blade portions may be exchanged depending on whether the wind turbine is to be erected at a site with a lower or higher wind velocity.

Furthermore, the blade may comprise more than one joint and thus comprise more than two blade portions and may comprise two groups of inserts for each joint.

Each blade portion may comprise two shell parts, one defining a windward side shell part and the other one defining a leeward side shell part. These shell parts may be assembled before joining the first and second blade portions.

The inserts which may be metal inserts, may be embedded in the blade portion during manufacturing of the blade portions or alternatively be embedded in the blade portions after manufacturing hereof. When embedding the inserts, they may be adhered to the blade portions, screwed to the blade portions as they may comprise a threaded portion, or by other means attached to the blade portions. By embedded is understood, that at least a part of the insert is surrounded by a part of the blade portion.

At least one insert of the first group of inserts is embedded in the first blade portion.

However, in one embodiment all inserts of the first group of inserts are embedded in the first blade portion. Likewise, all the inserts of the second group of inserts may be embedded in the second blade portion.

The inserts of the first group and the inserts of the second group may each form pairs, so that an insert of the first group corresponds to an insert of the second group, i.e. the number of inserts of the two groups may be equal. An assembly structure connects an insert of the first group of inserts to a corresponding insert of the second group of inserts. The assembly structure may be a common structure connecting each of the insert of the first group of inserts to a corresponding insert of the second group of insert. Alternatively, the assembly structure may comprise a plurality of assembly parts, e.g. one assembly part for each pair of inserts or one assembly parts connecting a number of inserts of the first group of inserts to a corresponding number of inserts of the second group of inserts. The insert of the first group of inserts and the corresponding insert of the second group of inserts have a common centre line. As the inserts are positioned so that they have a common centre line, the risk of damage of the joint caused by torsion can be reduced.

As the assembly structure comprises at least two assembly elements being positioned offset from said common centre line, the assembly elements become capable of counteracting forces which tend to twist or rotate one of the blade portions relative to the other. If the at least two assembly elements are positioned symmetrically the capability of counteracting forces is increased further.

The assembly structure may be able to exert a compressive force allowing the blade portions to be forced towards each other by the assembly structure. This may be possible e.g. by use of an elastic force, by tightening one or more bolts, by tightening a turnbuckle-like structure or by other means.

By applying a mechanical form of fastening on-site assembling of the sectional blade is facilitated, as e.g. no curing of a bonding material is needed. The blade portions may each form a cavity for accommodation of a shaft of an insert. I.e. at least some of the inserts of the first group of inserts and of the second group of inserts may be provided with a shaft which may be an oblong part of these inserts. The blade portions may form a cavity for each shaft thus allowing each of these inserts to be embedded separately. The cavity may have a size and a shape corresponding to the size and shape of the shaft. As the inserts may have shafts of different size and/or shape, the cavities of the blade portions may likewise have different size and/or shapes. It should be understood, that not the total length of the shaft may be embedded, as a part of the shaft may be positioned outside the cavity in some embodiments.

By embedding the inserts in the blade portions, a safe joint between the inserts and the blade portions may be achieved, as the embedded portion of the insert is in contact with the composite material of the blade portion along its length. If the cavities are conical, the joint may be even safer, as the embedded portion of the insert is in contact with more layers of the composite material at the same time.

The shafts may be embedded in the blade portions during manufacturing of the blade portions or alternatively be embedded in the blade portions after manufacturing hereof. When embedding the shafts, the outer surface of the shafts may be adhered to the inner surface of the cavities by use of a bonding material. Alternatively, the shafts may e.g. comprise an outer threading corresponding to an inner threading of the cavities. However, other means may also be used when attaching the shafts to the blade portions.

Each insert may comprise a head forming a flange in contact with a head of a corresponding insert, when the two blade portions have been joined. An insert of the first group of inserts may be connected to an insert of the second group of inserts by positioning these inserts head to head, as an end face of the heads may be adapted for flanged joint.

The head may also be embedded in one of the blade portions, so that the end face of the head is in plane with the end face of the blade portion. Alternatively, at least a part of the head may protrude from the end face of the blade portion. In a further alternative, only a part of the shaft is embedded in the blade portion so that no part of the head is embedded in the blade portion.

The flanges of each group of inserts may extend in a plane, i.e. the end faces of the heads of the first group of inserts may form a flat surface. The end faces of the heads of the second group of inserts may correspondingly form a flat surface. As the flanges may be adapted for flanged joint, the flanges of each group of inserts may extend in one common plane defining a flat surface, thereby facilitating joining of the blade portions.

Two inserts of one of the groups of inserts may have heads of different size and/or shape. The size and/or shape of the inserts may depend on where the inserts are positioned in the joint. As an example, the size and/or shape of an insert depends on the curvature of blade portions at which the insert is positioned. Typically, the heads may be smaller at areas of the joint with relatively large curvature, and correspondingly larger, at least in one direction, at areas of the joint with relatively small curvature or at straight areas of the joint. The number of inserts having different size and/or shape may be more than two, e.g. depending on the size of the wind turbine blade and/or the shape of the blade at the position of the joint. The number of inserts may further depend on the size of the blade. As an example, a number of 40-50 inserts may be needed for a joint near the root end of a wind turbine blade.

However, more or less inserts may also be applicable.

The assembly elements may extend through a head of an insert of the first group of inserts and a head of a corresponding insert of the second group of inserts. It should be understood, that the assembly element may extend through a head of more inserts of the first group of inserts and a head of a corresponding inserts of the second group of inserts, so that more inserts are connected by one assembly structure at the same time. In on embodiment, the assembly element is a bolt. Consequently, the assembly structure may comprise a plurality of assembly elements. Each insert of the first group of inserts may be connected to a corresponding insert of the second group of inserts by assembly elements in the form of bolts. However, more than two bolts may be used for each pair of inserts, such as 3, 4, 5 or even more. The blade portions may be joined by tightening a nut attached to each of the bolts. The bolts may be positioned symmetrically about the insert.

The inserts may be positioned so that at least one assembly element is accessible from outside the blade. This may e.g. be possible by positioning the inserts so that the head extends across the outer surface of the blade portions and forms projections extending upwardly from the outer blade surface. The projections of adjacent heads of the two blade portions may be joined and afterwards, all the projections can be covered by a fairing.

However, the inserts may also be of a size that allows the assembly elements to be accessible from the outside without the heads extending across the outer surface. This may improve the aerodynamic properties of the blade compared to an embodiment in which the heads extend across the outer surface.

Alternatively, the inserts may be positioned so that at least one assembly element is accessible from inside the blade. This may be possible by positioning the inserts so that the head extends across the inner surface of the blade portions and forms projections extending upwardly from the inner blade surface. However, the inserts may be positioned so that at least one assembly element is accessible from outside and at least one assembly element is accessible from inside the blade. Thereby the blade portions may be joined by connecting assembly elements both along the outer surface and the inner surface of the blade portions.

To be able to ensure a more even compressing force along the outer and the inner surfaces of the blade portions, it may be an advantage if the number of assembly elements being accessible from the outside is equal to the number of assembly element being accessible from the inside. Consequently, e.g. 2, 4, 6, and 8 or more assembly elements may be preferred. Thereby there will be substantially no bending or twisting along the length of the insert or in the surrounding composite material of the blade portions. As it may be difficult e.g. to tighten a bolt inside the blade, an assembly element accessible from inside the blade may be different from an assembly element accessible from outside the blade. Consequently, bolts may be used as assembly elements when connecting the inserts from outside the blade, whereas e.g. snap lock members may be used as assembly elements when connecting the inserts from inside the blade. Herein "snap lock" denotes locking features adapted to be locked and unlocked without use of tools or at least only by very simple manipulation of a locking feature, e.g. by turning a lock member etc.

When the blade portions are assembled, a fairing may cover the joint to protect it. The fairing may further improve the aerodynamic properties of the blade in relation to wind. The blade may comprise a first spar element forming part of the first blade portion and a second spar element forming part of the second blade portion. The spar may act as a reinforcing beam in the longitudinal direction of the blade. In this case, at least one insert of the first group of inserts may be attached to the first spar element and at least one insert of the second group of inserts may be attached to the second spar element. In a second aspect, the invention provides a method of assembling a blade for a wind turbine, the method comprising the steps of: providing at least a first blade portion and a second blade portion, embedding at least one insert of a first group of inserts in the first blade portion, embedding at least one insert of a second group of inserts in the second blade portion, and connecting an insert of the first group of inserts to a corresponding insert of the second group of inserts by an assembly structure comprising at least two assembly elements thereby joining the blade portions so that they extend in opposite directions from a joint, the assembly elements being positioned offset from a common centre line of the corresponding inserts.

It should be understood, that the above-mentioned features of the first aspect of the invention may also be applicable in relation to the method of assembling a blade for a wind turbine according to the second aspect of the invention. Thus, the second aspect may comprise any combination of features and elements of the first aspect of the invention. In a third aspect, the invention provides a wind turbine comprising a sectional blade according to the first aspect of the invention It should be understood, that the above- mentioned features of the previously described aspects may also be applicable to the third aspect of the invention. Brief description of the drawings

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

Fig. 1 illustrates a first and a second blade portion being joined by two inserts and an assembly structure,

Fig. 2 illustrates an embodiment of an insert,

Fig. 3 illustrates a blade portion and a group of inserts,

Fig. 4 illustrates a first and a second blade portion being joined by a first and second group of inserts and an assembly structure, and Figs. 5a-5c illustrate different views of another embodiment of a joint.

Detailed description of the drawings

It should be understood that the detailed description and specific examples, while indicating embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

Fig. 1 illustrates a part of a sectional blade 1 for a wind turbine (not shown). The blade 1 comprises at least a first blade portion 2 and a second blade portion 3 which extend in opposite directions from a joint 4. An insert 5 of a first group of inserts is embedded in the first blade portion 2 and an insert 6 of a second group of inserts is embedded the second blade portion 3. The blade portions 2, 3 are joined by an assembly structure 7 which connects the insert 5 of the first group of inserts to the corresponding insert 6 of the second group of inserts. The corresponding inserts 5, 6 have a common centre line (not shown).

The joint 4 between the two blade portions 2, 3 is transverse to the length of the blade 1, thus allowing for a blade 1 which comprises smaller sections compared to a traditional blade being manufactured of shell parts of full-size.

The first and second blade portions 2, 3 are positioned so that they abut each other with substantially no distance here between. The inserts 5, 6 are made of metal and may either have been embedded in the blade portions 2, 3 during manufacturing hereof or alternatively have been embedded in the blade portions after manufacturing hereof. As illustrated, embedded means, that at least a part of the inserts 5, 6 is surrounded by a part of the blade portions 2, 3. The assembly structure 7 connects each insert 5 of the first group of inserts to a

corresponding insert 6 of the second group of inserts. The assembly structure 7 is able to exert a compressive force allowing the blade portions 2, 3 to be forced towards each other by the assembly structure 7.

The blade portions 2, 3 form a cavity for accommodation of a shaft 8 of an insert 5, 6. The cavity has a size and a shape corresponding to the size and shape of the shaft 8.

Fig. 2 illustrates an insert 5, 6 of the first or second group of insert. Each insert 5, 6 comprises a shaft 8 and a head 9. The head 9 forms a flange for contact with a head of a corresponding insert, when the two blade portions 2, 3 are joined. An insert 5 of the first group of inserts can be connected to an insert 6 of the second group of inserts by positioning these inserts 5, 6 head to head.

As illustrated in Fig. 1, a part of the head 9 protrudes from the end face of the blade portions 2, 3.

As also illustrated in Fig. 1, the assembly structure 7 comprises two assembly elements 10 to be positioned so that they extend through the head 9 of the insert 5 of the first group of inserts and the head 9 of the corresponding insert 6 of the second group of inserts. In the illustrated embodiment, the assembly element 10 is a bolt. The assembly elements 10 are positioned offset from the common centre line (not shown) of the inserts 5, 6.

As illustrated in Fig. 2, the assembly structure (not shown) may comprise a plurality of assembly elements (not shown), as the head 9 comprises four holes 11 each being adapted for an assembly element in the form of a bolt 10. The blade portions may be joined by tightening a nut 12 (see Fig. 1) attached to each of the bolts 10 (see Fig. 1).

Fig. 3 illustrates a blade portion 2 and a group of inserts 5 of which only the head 9 is shown. In the illustrated embodiment, the inserts 5 should be used to connect the first spar element 12 to a second spar element (not shown). The first spar element 12 forms part of the first blade portion 2 and a second spar element (not shown) forms part of the second blade portion (not shown). The spar acts as a reinforcing beam in the longitudinal direction of the blade 1. As illustrated in Fig. 1 and 3, the inserts 5, 6 are positioned so that at least one assembly element 10 is accessible from outside and at least one assembly element 10 is accessible from inside the blade. Thereby the blade portions 2, 3 can be joined by connecting assembly elements 10 both along the outer surface and the inner surface of the blade portions 2, 3. It should be understood, that the number of inserts 5 in Fig. 3 is for illustration only. The number of inserts 5, 6 depends on the size of the blade 1. As an example, a number of 40-50 inserts 5 of the first group and an equal number of inserts 6 of the second group of inserts could be a suitable number of inserts for a joint 4 near the root end of a wind turbine blade 1. However, more or less inserts 5, 6 may also be applicable. Fig. 4 illustrates a first blade section 2 and second blade section 3 which have been joined. The inserts 5, 6 are positioned so that at least one assembly element 10 is accessible from outside the blade 1. This is made possible by positioning the inserts 5, 6 so that the head 9 extends across the outer surface of the blade portions 2, 3 and forms projections extending upwardly from the outer blade surface. The projections of adjacent heads 9 of the two blade portions 2, 3 have been joined and afterwards, all the projections have been covered by a fairing 13.

The fairing 13 covers the joint 4 to protect it. And the fairing 13 are further used to improve the aerodynamic properties of the blade 1 in relation to wind.

Figs. 5a-5c illustrate different views of another embodiment of a joint 4. In Fig. 5a, two sets of inserts 5, 6 are illustrated. The shafts 8 are only partly embedded in cavities of the first and second blade portions 2, 3 which blade portions do not abut each other but are positioned with a distance to each other. The heads 9 of the first and second inserts 5, 6 are connected by an assembly structure 7 in the form of two bolts 10 and two nuts 12 for each set of inserts 5, 6. In this embodiment, the heads 9 do not extend across the outer surface of the blade.

Fig. 5b, being a cross sectional view of the joint, illustrates a fairing 13 being used to cover the joint 4 to protects it and to improve the aerodynamic properties of the blade in relation to wind. The distance dl and d2 is of a size sufficient to get the bolts 10 in place and sufficient to tighten the nuts 12. After having tightened the nuts 12, a foam material or similar (not shown) can be positioned in the area between the blade portions 2, 3 before covering the joint with the fairing 13. This foam material can be used as a support for the fairing 13.

Fig. 5c is a plan view of the joint 4 illustrated in Figs. 5a and 5b.