Technical field

The present invention relates generally to modular wind turbine blades and more specifically to a method of assembling a modular wind turbine blade.

Background

There is a continuing desire to generate increased levels of power from onshore and offshore wind farms. One way to achieve this is to provide modern wind turbines with larger wind turbine blades to increase the swept area of the rotor such that the wind turbine captures more energy from the wind. A wind turbine blade may be designed as a modular assembly formed of two or more blade modules to facilitate transport of the components. The blade modules may then be connected together at the wind farm site to form the blade.

Modern wind turbine blades typically include a reinforcing spar structure to provide structural support to an outer shell of the blade. A spar structure typically comprises longitudinally-extending spar caps that absorb bending loads experienced by the blade in use. Spar caps of adjacent blade modules may be connected to transfer loads across the interface between the blade modules. A joint connecting adjacent spar caps must be configured to withstand and transfer the high bending loads in the spar caps in use.

In some examples, interfacing surfaces of the adjacent spar caps may be connected together using adhesive or resin. However, inconsistencies in the joint, such as a varying bond gap thickness between the interfacing surfaces, can cause weakened areas in the joint. A varying bond gap thickness may be caused by misalignments between the interfacing surfaces when arranging the blade modules for connection. Further, an inconsistent bond gap thickness may result from the manufacture of the spar caps, i.e. the interfacing surfaces may not be manufactured with wholly corresponding surface geometry. Additionally, connecting spar caps of adjacent blade modules at a wind turbine site can be challenging. Voids and dry spots in the joint between spar caps, resulting from an incorrect application of adhesive or an incomplete resin infusion, can interrupt load paths between the spar caps and cause stress concentrations in the joint.

It is against this background that the present invention has been developed. Summary

According to the present invention there is provided a method of assembling a modular wind turbine blade. The method comprises providing a first blade module comprising a first spar cap having a tapered end portion defining a first scarfed surface, and providing a second blade module comprising a second spar cap. The method further comprises providing an elongate connecting element that is either a) separate from the first and second spar caps, or b) an end portion of the second spar cap. The connecting element has a tapered end portion defining a second scarfed surface. The method further comprises arranging the first and second blade modules end-to-end and connecting the first and second spar caps via a scarf joint formed between the first and second scarfed surfaces. Connecting the first and second spar caps comprises arranging pre-preg fibrous material between the first and second scarfed surfaces. The pre-preg fibrous material comprises fibrous material that is pre-impregnated with uncured resin. Connecting the first and second spar caps further comprises curing the resin in the pre-preg fibrous material to thereby connect the tapered end portion of the first spar cap to the tapered end portion of the connecting element.

Pre-preg fibrous material is fibrous material that is pre-impregnated with an uncured resin prior to arrangement between the first and second scarfed surfaces. In some examples, the pre-preg fibrous material may comprise woven fibres, unidirectional fibres, biaxial fibres, multiaxial fibres, or fibres in the form of a chopped strand mat. Uncured resin comprises various separate polymer chains that are not bonded together, i.e. not crosslinked. Accordingly, curing the resin comprises crosslinking the polymer chains in the resin. Crosslinking the polymer chains may occur at ambient temperature, or in some examples, heat may be applied to the scarf joint to promote crosslinking of the polymer chains in the resin.

Manufacturing and assembly tolerances may result in small misalignments between the first and second scarfed surfaces when these are arranged to form the scarf joint. As such, a bond gap defined between the first and second scarfed surfaces may vary in thickness. The arrangement of pre-preg fibrous material in the scarf joint between the first and second scarfed surfaces advantageously alleviates potential risks related to a varying bond gap thickness. The pre-preg fibrous material arranged between the first and second scarfed surfaces advantageously provides a cushioning effect when connecting the first and second spar caps via the scarf joint. For example, the pre-preg fibrous material comprises fibrous material and uncured resin, and is therefore configured to conform to the contours of the first and second scarfed surfaces between which it is arranged. The pre-preg fibrous material cushions variations in the first and second scarfed surfaces, i.e. smoothing out misalignments and minor surface defects, by filling any recesses or discontinuities in the scarfed surfaces.

The pre-preg fibrous material may have a fibre volume fraction which allows the pre-preg fibrous material to adapt to the shape between the first and second scarfed surfaces. Excess resin in the pre-preg fibrous material will allow the thickness of the pre-preg material to slump in thickness when some resin is pressed to other areas with less pressure. Such low-pressure areas can be locations, where the distance between the scarfed surfaces is larger and/or the resin is pressed out of the joint area. The excess resin in the pre-preg fibrous material can hereof be distributed inside the joint area from narrow bond distances to larger bond distances. For a pre-preg material with unidirectional fibre directions, the fibre volume fraction (FVF) for an incompressible fibre layer thickness is 70- 80% FVF. A pre-preg material with a lower FVF (i.e. more resin) can be pressed to a smaller thickness when the resin is redistributed. The FVF for the pre-preg fibrous material of the present invention may be from 30% to 70%, preferably from 45% to 55%, such as 50%. The FVF is the ratio of fibre to resin by volume in the material.

Further, the provision of pre-preg fibrous material between the scarfed surfaces ensures that resin is provided throughout the bond gap such that there are no dry spots or voids in the scarf joint. During the curing process, the viscosity of the resin in the pre-preg fibrous material decreases initially such that the resin mobilises throughout the bond gap to fill any variations or discontinuities in the scarfed surfaces. The thorough provision of resin in the scarf joint ensures that stress concentrations between the spar cap and connecting element are minimised, and that a continuous load path is provided to transfer loads between the spar cap and connecting element in use.

In some examples, the first spar cap and/or the connecting element may comprise a stack of pre-cured layers. Preferably, the pre-cured layers may comprise reinforcing fibres fixed in a cured polymer resin matrix. For example, the pre-cured layers may comprise carbon fibre reinforced polymer (CFRP). The spar cap and the connecting element preferably comprise a stack of pre-cured pultrusions, such as CFRP pultrusions. The use of pultrusions is advantageous because a pultrusion process enables close control of fibre orientation in the spar caps and connecting element. Accordingly, pultrusions may comprise longitudinally-extending reinforcing fibres in a highly uniform arrangement, which improves the strength of the spar caps and connecting element.

Arranging pre-preg fibrous material between the first and second scarfed surfaces is particularly advantageous in examples wherein the first spar cap and/or the connecting element comprise a stack of pre-cured layers. Whilst the use of pre-cured layers may be advantageous for improving the structural performance of the spar caps and connecting element, such pre-cured layers can introduce additional challenges when forming the scarf joint between the first and second scarfed surfaces. For example, pre-cured layers may be substantially rigid and therefore unable to conform to the contours of their opposing scarfed surfaces, potentially causing voids in the bond gap, or a bond gap having a variable thickness. Further, misalignments between pre-cured layers may also result in a non-uniform bond gap thickness.

In some examples, the tapered end portion of the first spar cap may be formed by staggering ends of the pre-cured layers in the longitudinal direction to define the first scarfed surface. Pre-cured layers of the connecting element may be arranged in a complementary manner with ends staggered in the longitudinal direction to form a tapered end portion defining the second scarfed surface. It will be appreciated that staggering the pre-cured layers in the first spar cap and/or connecting element in the longitudinal direction may form a stepped scarfed surface. In such examples, the staggered ends may therefore result in a bond gap between the first and second scarfed surfaces that varies in thickness due to the stepped formation of the or each scarfed surface. Arranging pre-preg fibrous material between the first and second scarfed surfaces in such examples is particularly advantageous because the fibrous material and resin in the pre-preg material fills any misalignments or voids between the first and second scarfed surfaces.

In some examples, the pre-cured layers may each comprise a tapered end, i.e. each of the pre-cured layers may be chamfered. The chamfered ends of the pre-cured layers may define the scarfed surfaces. In such examples, the scarfed surfaces defined by the chamfered ends of the pre-cured layers may be smoother, i.e. may comprise less severe variations, than the stepped scarfed surfaces described above. As such, variations in the bond gap thickness may be less severe. However, misalignments between the pre-cured layers may still cause variations in the bond gap thickness. For example, longitudinal misalignment of the chamfered ends may result in a scarfed surface having flat regions where the chamfered end of a pre-cured layer meets an upper or lower surface of an adjacent layer, instead of aligning seamlessly with the adjacent chamfered end to define a continuous scarfed surface. Again in such an example, the pre-preg fibrous material arranged between the first and second scarfed surfaces alleviates variations in the bond gap thickness because the fibrous material and the resin fill any voids between the first and second scarfed surfaces.

In some examples, the spar caps and/or connecting element may comprise interlayers arranged between the pre-cured layers. The interlayers may comprise fibrous material. The interlayers may facilitate improved resin infusion between the pre-cured layers when forming the spar caps and/or connecting element. In some examples the interlayers may comprise pre-preg fibrous material, i.e. fibrous material that is pre-impregnated with resin to further ensure a thorough provision of resin between the pre-cured layers. However, interlayers between the pre-cured layers separate the pre-cured interlayers and may therefore result in a non-continuous scarfed surface. In some examples the interlayers may extend beyond the ends of the pre-cured layers, e.g. overlapping onto the scarfed surfaces. The presence of such interlayers on the scarfed surfaces may cause variations in the bond gap thickness. The pre-preg fibrous material arranged between the first and second scarfed surfaces alleviates such variations in the bond gap thickness because the fibrous material conforms to the scarfed surfaces and the projecting interlayers to avoid the formation of any voids in the scarf joint.

The method may further comprise arranging a plurality of layers of pre-preg fibrous material between the first and second scarfed surfaces. For example, each layer of prepreg fibrous material may comprise a layer of woven fibres, a layer of unidirectional fibres, a layer of biaxial fibres, a layer of multiaxial fibres, or a layer of fibres in the form of a chopped strand mat. A plurality of layers of pre-preg fibrous material may conform more closely to the contours of the scarfed surfaces. Further, arranging a plurality of layers of pre-preg fibrous material between the first and second scarfed surfaces may increase the amount of fibrous material in the scarf joint, thereby increasing the strength of the joint.

The pre-preg fibrous material may comprise an electrically conductive material. For example, the pre-preg fibrous material preferably comprises carbon fibre. In preferred examples, the first spar cap and the connecting element may each comprise an electrically conductive material. Preferably, the second spar cap may comprise an electrically conductive material. For example, the first spar cap and/or the connecting element and/or the second spar cap may comprise carbon fibre. The pre-preg fibrous material arranged between the first and second scarfed surfaces may therefore facilitate the conduction of electricity between the first and second spar caps. In the event of a lightning strike, the conductive fibrous material in the scarf joint may advantageously ensure that the electrical conductivity between the first and second spar caps is continuous to thereby avoid any electrical arcs, or “flashovers”, at the scarf joint.

The pre-preg fibrous material comprises fibrous material pre-impregnated with resin. The fibrous material may be pre-impregnated with a toughened resin, for example SE75 from Gurit. The use of toughened resin (compared to a non-toughened resin) may improve the joint strength between the first and second scarfed surfaces, thereby providing a stronger scarf joint between the first spar cap and the connecting element.

In some examples, the method may further comprise priming the first scarfed surface by applying adhesive to the first scarfed surface. The adhesive may be an epoxy adhesive. Additionally or alternatively, the method may comprise priming the second scarfed surface by applying adhesive to the second scarfed surface. Applying a primer to the first and/or second scarfed surface may improve the adhesion of the pre-preg fibrous material to the first and/or second scarfed surface. For example, the adhesive may have a higher surface energy than the first and second scarfed surfaces and may therefore provide a higher surface tension (than the scarfed surfaces) for the resin in the pre-preg fibrous material to bond to.

Priming the scarfed surfaces with an adhesive may be particularly advantageous in examples where the spar caps and/or connecting element comprise pre-cured layers such as pultrusions. During manufacture of the pultrusions, an internal mould release agent (IMR) may be included in the polymer resin mixture to facilitate the movement of the pultrusion through a pultrusion die. However, whilst beneficial for the pultrusion process, such an IMR reduces the surface energy of the pultrusions. In some examples, residues of an IMR agent may be present on the scarfed surface of the spar cap and/or connecting element and may therefore reduce the surface energy of the scarfed surface. Priming the scarfed surface with adhesive may provide a surface with higher surface energy, and increased wettability, for bonding the pre-preg fibrous material to the spar cap and connecting element.

Applying a primer to the first and/or second scarfed surfaces may additionally aid in reducing variations in the bond gap thickness. For example, the adhesive may fill recesses or other surface roughness on the first and/or second scarfed surface, thereby further improving the strength of the scarf joint.

The adhesive may be applied to the first and/or second scarfed surfaces in a liquid form. The adhesive then forms a layer of e.g. 0.1 mm to 1 mm on the scarfed surfaces. The adhesive is allowed to dry to a solid state or a semi-solid state before the scarfed surfaces are handled. This prevents the adhesive from being damaged or scraped of the surface.

The method may further comprise arranging one or more electrically conductive elements in the adhesive and in electrical contact with the pre-preg fibrous material and at least one of the first or second scarfed surfaces to form an electrically conductive bridge through the adhesive. The electrically conductive elements may be in direct electrical contact with a scarfed surface or the pre-preg fibrous material, or alternatively the electrically conductive elements may be in indirect electrical contact with a scarfed surface or the pre-preg fibrous material via another conductive element. In either example, the electrically conductive elements advantageously ensure that electrical conductivity is maintained through the scarf joint between the spar cap and connecting element, even if adhesive is used to prime the scarfed surfaces.

The electrically conductive elements may be applied to the adhesive when it is in its liquid form. The electrically conductive elements can then sink through the primer to make contact with the underlying scarfed surface.

The electrically conductive elements may comprise electrically conductive fibres. In some examples, the electrically conductive fibres may be short chopped fibres, for example the fibres may be between 1 mm and 5 mm in length. In other examples, the electrically conductive elements may comprise electrically conductive fibre tows. In preferred examples, the electrically conductive fibre tows may comprise carbon fibre tows.

In some examples, the electrically conductive elements may comprise a first layer of unidirectional electrically conductive fibre tows and a second layer of unidirectional electrically conductive fibre tows in electrical contact with the first layer of fibre tows and oriented transverse to the first layer. As such, the electrically conductive fibre tows may be arranged to resemble a grid, or mesh, of electrically conductive fibre tows. In some examples the electrically conductive elements may comprise a layer of woven electrically conductive fibres. For example, the electrically conductive fibres may be woven to form a plain weave layer, i.e. the electrically conductive fibres may form a fibrous fabric layer.

In some other examples, the electrically conductive elements may be elastically compressible, and the method may further comprise compressing the electrically conductive elements between the first and second scarfed surfaces when assembling the modular wind turbine blade. Such a configuration helps to ensure that the conductive elements in the adhesive are brought into contact with the pre-preg fibrous material and at least one of the first or second scarfed surfaces.

The method may comprise providing the adhesive and the pre-preg fibrous material as a pre-assembled slab. The method may further comprise arranging the slab between the first and second scarfed surfaces. As such, in some examples adhesive may be applied to the first scarfed surface and/or to the second scarfed surface when the slab is arranged between the first and second scarfed surfaces. The method steps of arranging the prepreg fibrous material between the first and second scarfed surfaces and applying adhesive to the first scarfed surface and/or to the second scarfed surface may therefore be completed at the same time in some examples. Configuring the adhesive and pre-preg fibrous material as a pre-assembled slab may facilitate a faster modular blade assembly at a wind turbine site.

In some examples, the elongate connecting element may be separate from the first and second spar caps. The second spar cap may have a tapered end portion defining a third scarfed surface. The connecting element may have a second tapered end portion defining a fourth scarfed surface. In such an example, the method may further comprise connecting the first and second spar caps via a second scarf joint formed between the third and fourth scarfed surfaces. Connecting the first and second spar caps may further comprise arranging pre-preg fibrous material between the third and fourth scarfed surfaces. The prepreg fibrous material may comprise fibrous material that is pre-impregnated with uncured resin. Connecting the first and second spar caps may further comprise curing the resin in the pre-preg fibrous material to thereby connect the tapered end portion of the second spar cap to the second tapered end portion of the connecting element.

It will be appreciated that features and method steps described herein with reference to forming the scarf joint between the first and second scarfed surfaces are equally applicable to the second scarf joint formed between the third and fourth scarfed surfaces. Such features are not repeated here in relation to the second scarf joint purely for reasons of conciseness.

According to the present invention there is provided a modular wind turbine blade assembled in accordance with the method as described herein.

The pre-preg fibrous material in the modular wind turbine blade may comprise an electrically conductive material. That is to say, the fibrous material between the first and second scarfed surfaces of the modular wind turbine blade, initially provided as pre-preg fibrous material before curing in accordance with the above-described method, may be electrically conductive to facilitate continuous electrical conductivity between the spar cap and connecting element.

The modular wind turbine blade may further comprise a layer of adhesive on the first scarfed surface. Additionally or alternatively, the modular wind turbine blade may further comprise a layer of adhesive on the second scarfed surface. As previously described with reference to the method of assembling the modular wind turbine blade, the or each layer of adhesive may help to form a scarf joint of increased strength.

The modular wind turbine blade may further comprise one or more electrically conductive elements in the or each layer of adhesive. The electrically conductive elements are preferably in electrical contact with the fibrous material in the scarf joint and at least one of the first scarfed surface or the second scarfed surface. As previously described above with reference to the method, the provision of an electrically conductive element in the adhesive forms an electrically conductive bridge through the adhesive layer, thereby ensuring continuous electrical conductivity through the scarf joint in examples where the or each scarfed surface is primed with adhesive.

Brief description of the drawings

Examples of the present invention will now be described by way of non-limiting example only, with reference to the accompanying figures, in which:

Figure 1 is a schematic exploded view of a modular wind turbine blade comprising first and second blade modules and a connecting element that is an end portion of a spar cap of the second blade module; Figure 2 is a schematic exploded view of a modular wind turbine blade comprising a connecting element that is separate from the spar caps of the first and second blade modules;

Figure 3 is a schematic cross-sectional view of a scarf joint formed between a spar cap and the connecting element, the scarf joint comprising fibrous material arranged between scarfed surfaces of the spar cap and connecting element;

Figure 4 is a schematic cross sectional view of the scarf joint in an example where the scarfed surfaces are primed with adhesive; and

Figures 5 to 7 are schematic cross-sectional views of the scarf joint in different examples where the spar cap and connecting element comprise pre-cured layers configured to define the scarfed surfaces.

Detailed description

As described above by way of background, assembling a wind turbine blade from a plurality of blade modules may facilitate the provision of larger wind turbine blades whilst still enabling transport of large blade parts to the wind turbine site. The schematic exploded views of Figures 1 and 2 show examples of a modular wind turbine blade 10 comprising a first blade module 12a and a second blade module 12b that are connectable together to form the blade 10. Unless otherwise stated, the following description applies equally to the blade 10 shown in Figure 1 and to the blade 10 shown in Figure 2.

With reference to Figures 1 and 2, the first blade module 12a comprises a first spar cap 14a which may be part of a spar structure that provides structural support to an outer shell 16a of the first blade module 12a. Similarly, the second blade module 12b comprises a second spar cap 14b which may be part of a spar structure that provides structural support to an outer shell 16b of the second blade module 12b.

As will be described later in more detail, the first and second spar caps 14a, 14b are configured to be connected together via a scarf joint 18. As such, the first spar cap 14a has a tapered end portion 20a defining a first scarfed surface 22a. The modular wind turbine blade 10 further comprises a connecting element 24 for connecting the first and second blade modules 12a, 12b. With reference initially to Figure 1 , the connecting element 24 may be defined by an end portion 20b of the second spar cap 14b. For example, the connecting element 24 may be defined by a portion of the second spar cap 14b that extends beyond the outer shell 16b of the second blade module 12b. Referring to Figure 2, in other examples the connecting element 24 may be separate from, and connectable to, the first and second spar caps 14a, 14b. In both examples, the connecting element 24 has a tapered end portion 26a defining a second scarfed surface 22b. The second scarfed surface 22b is configured for connection to the first scarfed surface 22a, i.e. to the first spar cap 14a.

With reference still to Figures 1 and 2, to assemble the modular wind turbine blade 10 the first and second blade modules 12a, 12b are arranged end-to-end. For example, the first and second blade modules 12a, 12b are preferably arranged such that longitudinal axes of the first and second spar caps 14a, 14b are substantially co-axial. The first and second spar caps 14a, 14b are connected via a scarf joint 18 formed between the first and second scarfed surfaces 22a, 22b. The scarf joint 18 helps to transfer loads safely and effectively between the first and second spar caps 14a, 14b. Advantageously, the scarf joint 18 comprises pre-preg fibrous material 28 arranged between the first and second scarfed surfaces 22a, 22b as will now be described in more detail with reference to the remaining figures.

Figure 3 shows a schematic cross-sectional view of the scarf joint 18. The scarf joint 18 may be a scarf joint formed between the first spar cap 14a and the connecting element 24 defined by the end portion 20b of the second spar cap 14b, as shown in Figure 1. Alternatively, the scarf joint 18 may be a scarf joint formed between the first spar cap 14a and the separate connecting element 24 as shown in Figure 2.

Pre-preg fibrous material 28 is arranged between the first and second scarfed surfaces 22a, 22b to provide a cushioning effect between the scarfed surfaces. For example, the pre-preg fibrous material 28 alleviates misalignments and/or minor surface defects in the first and second scarfed surfaces 22a, 22b by conforming to the contour of the first and second scarfed surfaces. As such, the pre-preg fibrous material 28 takes up variations in the thickness of a bond gap 34 defined between the first and second scarfed surfaces 22a, 22b, as shown most clearly in Figures 5 to 7. The pre-preg fibrous material 28 comprises fibrous material that is pre-impregnated with uncured resin, i.e. the fibrous material is impregnated with resin prior to arrangement between the first and second scarfed surfaces 22a, 22b.

In some examples, as shown in Figure 3, a plurality of layers of pre-preg fibrous material 28 may be arranged between the first and second scarfed surfaces 22a, 22b. A plurality of layers may allow the pre-preg fibrous material 28 in the scarf joint 18 to conform more closely to each of the first and second scarfed surfaces 22a, 22b to thereby provide an improved cushioning effect.

After arranging the pre-preg fibrous material 28 between the first and second scarfed surfaces 22a, 22b, the resin in the pre-preg fibrous material 28 is cured to connect the tapered end portion 20a of the first spar cap to the tapered end portion 26a of the connecting element 24. During the curing process, the resin in the pre-preg fibrous material 28 mobilises and fills any voids or recesses in the first and second scarfed surfaces 22a, 22b, thereby providing a thorough resin application between the scarfed surfaces, ensuring that there are no voids or dry spots in the scarf joint 18.

With reference still to Figure 3, the pre-preg fibrous material 28 may comprise an electrically conductive material. Similarly, the spar caps 14a, 14b and connecting element 24 may comprise an electrically conductive material. In such examples, the conductive pre-preg fibrous material 28 provides an electrically conductive bridge across the scarf joint 18 to ensure continuous electrical conductivity between the first and second spar caps 14a, 14b. Advantageously such a configuration minimises the risk of flashovers, i.e. electrical arcs across the scarf joint 18, in the event of a lightning strike, because the conductive pre-preg fibrous material 28 provides a safe path between the spar caps 14 for the electric current.

As shown in Figure 4, in some examples the method may additionally involve priming the first and/or second scarfed surface 22a, 22b by applying adhesive 30 to the scarfed surfaces. The adhesive 30 may have a greater surface energy than the first and second scarfed surfaces 22a, 22b, and may therefore provide a more wettable surface to which the pre-preg fibrous material 28 may be bonded. Accordingly, in some examples priming the scarfed surfaces 22a, 22b with adhesive 30 may help to form a stronger scarf joint 18. In other examples, the pre-preg fibrous material 28 may comprise fibrous material preimpregnated with resin and an adhesive promoter. The adhesive promoter may reduce the surface tension of the resin and thereby increase the wetting of the scarfed surfaces 22a, 22b to help form a strong scarf joint 18.

Referring still to Figure 4, in examples where the first and second scarfed surfaces 22a, 22b are primed with an adhesive 30, one or more electrically conductive elements 32 may be arranged in the adhesive 30. The electrically conductive elements 32 are preferably arranged in electrical contact with the pre-preg fibrous material 28 and at least one of the first or second scarfed surfaces 22a, 22b to form an electrically conductive bridge through the adhesive 30. It will be appreciated that use of such electrically conductive elements 32 is particularly advantageous in examples where the spar caps 14a, 14b, connecting element 24 and pre-preg fibrous material 28 all comprise an electrically conductive material. As such, the electrically conductive elements 32 help to maintain the electrical conductivity through the scarf joint 18, despite the scarfed surfaces 22a, 22b being primed with an adhesive 30. This facilitates the use of any adhesive 30 for priming the scarfed surfaces 22a, 22b, i.e. not limited to electrically conductive adhesive, whilst still maintaining the electrical conductivity through the scarf joint 18. As shown in Figure 4, by way of an example of electrically conductive elements 32, such elements may comprise a plurality of electrically conductive fibre tows.

Whilst not shown in the accompanying figures, in some examples the previously-described priming adhesive 30 and pre-preg fibrous material 28 may be provided as a pre-assembled slab. That is to say, the pre-preg fibrous material 28 and the priming adhesive 30 may be arranged together as a separate slab prior to being arranged between the first and second scarfed surfaces 22a, 22b. It follows that the method of assembling the blade 10 in such an example includes arranging the pre-assembled slab between the first and second scarfed surfaces 22a, 22b. It will be appreciated that by arranging the pre-assembled slab between the first and second scarfed surfaces 22a, 22b, the pre-preg material 28 is arranged between said scarfed surfaces 22a, 22b and the priming adhesive 30 is arranged on the respective scarfed surface 22a, 22b in a combined process step.

Figures 5 to 7 show schematic cross-sectional views of the scarf joint 18 between scarfed surfaces 22a, 22b of different spar caps 14 and connecting elements 24 to illustrate the benefits of arranging pre-preg fibrous material 28 in the scarf joint 18 in various different examples. Figures 5 to 7 show examples wherein a bond gap 34 defined between the scarfed surfaces 22a, 22b has a varying thickness t, and where the variations in bond gap thickness tare accommodated by the pre-preg fibrous material 28. In each of the examples shown in Figures 5 to 7, the spar cap 14 and connecting element 24 comprise a stack of pre-cured layers 36. For example, the pre-cured layers 36 may be pultrusions and preferably comprise fibre reinforced polymer, such as carbon fibre reinforced polymer (CFRP).

With reference initially to Figure 5, in some examples the tapered end portion 20 of the spar cap 14 and/or the tapered end portion 26 of the connecting element 24 may be formed by staggering, i.e. offsetting, ends of the pre-cured layers 36 in the longitudinal direction L. As such, scarfed surfaces 22a, 22b defined by the tapered end portions 20, 26 of the spar cap 14 and connecting element 24 may be stepped. Whilst the respective tapered end portions 20, 26 still define an overall scarfed surface 22a, 22b, as shown in the example of Figure 5, the bond gap thickness t in such an example may vary significantly.

In the example shown in Figure 6, the spar cap 14 and connecting element 24 comprise pre-cured layers 36 having chamfered ends which may help to reduce the severity of the step between adjacent pre-cured layers 36. As seen in Figure 6, chamfering the ends of the pre-cured layers 36 may help to reduce the variation in bond gap thickness t. Chamfering the ends of the pre-cured layers 36 may also help to transfer loads more gradually between the spar cap 14 and connecting element 24, i.e. between opposing precured layers 36 on each side of the scarf joint 18.

The ends of the pre-cured layers 36 may be chamfered prior to arrangement of the layers in the stack. As such, in some examples the chamfered ends of the pre-cured layers 36 may be slightly misaligned in the longitudinal direction L, as shown in Figure 6 for example. Such misalignment may cause discontinuities, i.e. flat regions, in the scarfed surfaces 22a, 22b. It will be appreciated that such misalignments may also result in an inconsistent bond gap 34 of varying thickness t.

Figure 7 shows an example wherein the spar cap 14 and connecting element 24 comprise fibrous interlayers 38 between pre-cured layers 36 in the respective stack of layers. The interlayers 38 may be included for a number of reasons, such as improved resin infusion when manufacturing the spar cap 14 and connecting element 24, or for example to provide electrically conductive material in the spar cap 14 and connecting element 24. However, the inclusion of such interlayers 38 may also result in variations in the bond gap thickness t. For example, where the pre-cured layers 36 are separated by an interlayer 38, the scarfed surface 22 may be discontinuous, i.e. comprising a step or a flat region. Further, in some examples the interlayers 38 may be arranged to extend beyond the end of the pre-cured layers 36, for example to ensure that the interlayers 38 can make electrical contact with another electrically conductive component such as the pre-preg fibrous material 28 in the scarf joint 18. Accordingly, the inclusion of interlayers 38 may result in a bond gap 34 of varying thickness t.

As shown in the examples of Figures 5 to 7, the pre-preg fibrous material 28 arranged between the first and second scarfed surfaces 22a, 22b provides a cushioning effect that alleviates the variations in the scarfed surfaces. In particular, the pre-preg fibrous material 28 conforms to each of the scarfed surfaces 22a, 22b. The fibrous material and resin in the pre-preg fibrous material 28 fills the discontinuities in the scarfed surfaces 22a, 22b to ensure there are no voids or weak spots in the scarf joint 18.

Whilst the method of assembling the modular wind turbine blade 10 has been described above with reference to a first scarf joint 18 formed between the first scarfed surface 22a of the first spar cap 14a and the second scarfed surface 22b of the connecting element 24, it will be appreciated that in some examples the method may include forming a second scarf joint 18. For example, in examples where the connecting element 24 is separate from the first and second spar caps 14a, 14b, as shown in Figure 2, the second spar cap 14b may have a tapered end portion 20b defining a third scarfed surface 22c, and the connecting element 24 may have a second tapered end portion 26b defining a fourth scarfed surface 22d.

It will be appreciated that the method of forming a scarf joint 18 between the first and second scarfed surfaces 22a, 22b is equally applicable to the second scarf joint 18 formed between the third and fourth scarfed surfaces 22c, 22d and will not be repeated herein with reference to the third and fourth scarfed surfaces merely for conciseness. For the avoidance of doubt, forming the second scarf joint 18 between the third and fourth scarfed surfaces 22c, 22d also includes the arrangement of pre-preg fibrous material 28 between the scarfed surfaces 22c, 22d, and the above described advantages of arranging such prepreg fibrous material 28 are equally applicable to the second scarf joint 18.

The description provided herein serves to demonstrate a plurality of possible examples of the present invention. Features described in relation to any of the examples above may be readily combined with any other features described with reference to different examples without departing from the scope of the invention as defined in the appended claims.

Further, it will be appreciated that the above description and accompanying figures are provided merely as an example of the present invention. Many alternatives to the specific examples described above are therefore possible without departing from the scope of the invention as defined in the appended claims.