Wind turbine blade manufacture

A method of making a prefabricated root section (26) for a wind turbine blade (10) is described. The method comprises: providing a male mould (28) extending longitudinally in a spanwise direction between an inboard end (30) and an outboard end (32) and extending transversely in a chordwise direction between a leading edge (34) and a trailing edge (36), the male mould (28) defining a male mould surface (38) of convex curvature in the chordwise direction; providing a root plate (48) having one or more root inserts (50) projecting therefrom, the or each root insert (50) being arranged along an arcuate path; arranging one or more inner fibrous layers (40) on the male mould surface (38); arranging the root plate (48) at the inboard end (30), of the male mould (28) such that the or each root insert (50) overlays an inner fibrous layer at the root end of the mould (28). The method further comprises arranging one or more outer fibrous layers on top of the inner fibrous layers (40) and on top of the or each root insert (50), providing resin to the fibrous layers (40) and to the or each root insert (50) and curing the resin to form a prefabricated root section (26) for subsequent use in the manufacture of a wind turbine blade (10). Curing the resin to form the prefabricated root section (26) is conducted before removing the prefabricated root section (26) from the male mould (28).

TECHNICAL FIELD

The present invention relates generally to the manufacture of wind turbine blades.

BACKGROUND

The blades of modern utility-scale wind turbines generally comprise a hollow blade shell formed of composite materials, such as glass-fibre reinforced plastic. The blade shell is typically formed of windward and leeward half shells, which are manufactured in respective female half moulds of a blade mould assembly. The root end of the blade shell is typically significantly thicker than the remainder of the shell in order to support the static and dynamic loads of the blade in use.

In existing blade manufacturing techniques, the increased shell thickness at the root end is achieved by building up a large number of glass-fibre layers at the root ends of the half moulds. However, it is time consuming to lay up such a large number of layers. The process is made more complicated by the steep sides of the mould at the root end, as it can be difficult to secure the fabric layers in place without slippage. Consequently, the laminate layers at the root end are prone to creasing and wrinkling, which may compromise the structure of the blade. Aside from the layup time, a significant amount of time is also required to cure the resin in these thick root laminates. These aspects of the blade production process therefore contribute significantly to the overall time required to make the blade.

The moulds of modern wind turbine blades are expensive to produce, since they are very large structures, typically in excess of fifty metres in length, and usually include integral heating systems and sensor systems, which are utilised during curing of the blade shell laminates. In addition to being expensive, the moulds occupy significant space within the blade manufacturing facility. Therefore, most blade manufacturing facilities may only have one or two moulds.

In order to increase the rate of production of wind turbine blades, it is desirable to minimise the ‘in-mould time’, i.e. the time in which the mould is utilised to produce a blade. It is against this background that the present invention has been developed.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided a method of making a prefabricated root section for a wind turbine blade. The method comprises: providing a male mould extending longitudinally in a spanwise direction between an inboard end and an outboard end and extending transversely in a chordwise direction between a leading edge and a trailing edge, the male mould defining a male mould surface of convex curvature in the chordwise direction; providing a root plate having one or more root inserts projecting therefrom, the or each root insert being arranged along an arcuate path; arranging one or more inner fibrous layers on the male mould surface; arranging the root plate at the inboard end of the male mould such that the or each root insert overlays an inner fibrous layer at the root end of the mould. The method further comprises arranging one or more outer fibrous layers on top of the inner fibrous layers and on top of the or each root insert, providing resin to the fibrous layers and to the or each root insert and curing the resin to form the prefabricated root section for subsequent use in the manufacture of a wind turbine blade, wherein curing the resin to form the prefabricated root section is conducted before removing the prefabricated root section from the male mould.

The one or more inner fibrous layers may be arranged directly on the male mould surface or they may be arranged on top of one or more other layers, for example on top of a release layer or coating layer.

The or each root insert is configured to provide a means of connecting the blade to a wind turbine hub. Preferably the root inserts comprise pre-cured composite structures, for example cured resin reinforced with glass and/or carbon fibre. The root inserts are preferably pultruded components. The root inserts may comprise an aperture or a plurality of apertures configured to receive a respective bolt in a blade-hub connection. The apertures may each be defined by a bushing, for example a sleeve, which is preferably made from metal such as steel. The bushing may be embedded within the pre-cured composite structure of the root insert. The aperture or the bushing preferably comprises an internal screw thread. Alternatively, the root inserts may comprise threaded rods that are partially embedded within the composite structure and project out from one end to receive a nut in a blade-hub connection.

It will be appreciated that the inner layers may be arranged on the mould before or after the root plate is arranged. If the inner layers are arranged first, the root insert(s) may subsequently be arranged on top of the inner layers. If the root plate is arranged first, the inner layers may subsequently be inserted beneath the root insert(s).

Resin may be provided in the method in any number of ways. Preferably, resin is provided in a resin infusion process, for example vacuum-assisted resin transfer moulding (VARTM). A resin infusion process may comprise arranging a vacuum film over the male mould and sealing the vacuum film against a surface, such as a flange of the mould, to define a sealed region encapsulating the layup. A vacuum pump may be employed to remove air from the sealed region. At the same time, resin is admitted into the sealed region. The resin infuses throughout the fibrous layers and between the root insert(s) and the fibrous layers.

Instead of a vacuum film, a silicone sheet or equivalent may be used to cover the lay-up during the infusion process. This may result in the root section having a very smooth outer surface.

Alternatively, or additionally to an infusion process, the fibrous layers may be pre-impregnated with resin, for example they may comprise prepreg materials. Resin may alternatively be provided using any other suitable technique, for example wet hand lay-up, whereby resin is applied to the various components using a brush or roller for example.

Curing the resin to form the prefabricated root section is conducted before removing the prefabricated root section from the male mould. Surprisingly it was found that performing the curing in the male mould lead to significantly better quality of the prefabricated root section as well as the final wind turbine blade. It could be theorized, without being limited thereto, that this is due to better alignment of fibres in the products since the fibrous layers in the prefabricated root section wherein the resin is cured cannot shift or create even small wrinkles during subsequent handling of prefabricated root section, whereas handling of a collection of fibrous material before curing (particularly when this involves rotation from a male mould to a female mould) always will lead to some shifting of the fibrous layers or fibres within the fibrous layers.

Curing the resin may comprise arranging the male mould in an oven. The male mould is preferably a relatively inexpensive structure. For example, the male mould is preferably not a heated mould.

The method may further comprise attaching the root plate to the male mould to fix the position of the root plate and root insert(s) relative to the male mould, for example a face plate of the male mould. The root plate preferably comprises datum features for aligning with corresponding datum features of the male mould, such as on the face plate of the male mould. The root plate may be attached to the male mould by bolts and/or clamps for example.

After curing the resin, the method may further comprise removing the prefabricated root section from the male mould, turning the prefabricated root section through substantially 180 degrees around a horizontal axis to flip the prefabricated root section upside-down, and supporting the prefabricated root section in a separate support frame.

The method may further comprise conducting one or more finishing operations on the prefabricated root section when it is supported in a support frame. These operations may include cutting or trimming the edges of the root section; inspecting and/or conducting non-destructive testing of the structure; sanding and/or painting the surface; removing a peel ply layer if included in the structure.

The number of fibrous layers arranged at the inboard end of the mould may be more than the number of fibrous layers arranged at the outboard end such that the resulting prefabricated root section is thicker at its inboard end than at its outboard end.

In a second aspect of the present invention there is provided a prefabricated root section for use in the manufacture of a wind turbine blade. The prefabricated root section comprises an outer shell formed as a cured composite laminate, the outer shell extending longitudinally in a spanwise direction between an inboard end and an outboard end and extending transversely in a chordwise direction between a leading edge and a trailing edge, a root plate arranged at the inboard end of the outer shell, and one or more root inserts projecting from the root plate, the or each root insert being arranged along an arcuate path and being embedded within the outer shell between inner and outer layers of the laminate.

The prefabricated root section is a large, cured composite member that forms a large spanwise part of the blade root. Preferably the prefabricated root section has a spanwise length constituting between 5% to 20% of the total length of a wind turbine blade in which it is to be incorporated. Alternatively, the pre-fabricated root section may have a spanwise length of between 5 m and 15 m. In a particularly preferred embodiment, the total blade length is approximately 80 m to 120 m, whilst the length of the prefabricated root section is approximately 8 m to 12 m (i.e. approximately 10% of the total blade length). It is preferred that the prefabricated root section forms a part of an outer surface of the blade shell of the as-cured blade shell. In other words, in the part of the outer blade shell that is formed by the prefabricated root section, no fibre layers are arranged closer to the outer surface of the blade than the prefabricated root section.

The root plate of the prefabricated root section may comprise datum features for aligning with corresponding datum features of a face plate of a wind turbine blade mould. If datum features are also used for aligning of the root plate to the male mould during the preparation of the prefabricated root section, it is preferred that the same datum features of the root plate also are used for aligning the prefabricated root section to (the face plate of) the wind turbine blade mould.

The outer shell of the prefabricated root section may taper in thickness moving from the inboard end towards the outboard end. Preferably the inboard end of the root section has a thickness of between 80-150 mm and the outboard end has a thickness between 2 mm to 10 mm.

In a further aspect of the present invention there is provided a method of making a wind turbine blade. The method comprises: providing a female blade mould extending in a spanwise direction between first and second ends; arranging fibrous material in the blade mould; arranging a prefabricated root section as described above adjacent to the blade mould; providing resin to the fibrous material and between the fibrous material and the prefabricated root section; and curing the resin to form an outer shell portion of the blade integrated with the prefabricated root section.

In a further aspect of the present invention there is provided a method of making a wind turbine blade. The method comprises: providing a female blade mould extending in a spanwise direction between first and second ends; arranging fibrous material in the blade mould; arranging a prefabricated root section made according to the method as described above, in or adjacent to the blade mould; providing resin to the fibrous material and between the fibrous material and the prefabricated root section; and curing the resin to form an outer shell portion of the blade integrated with the prefabricated root section.

In a highly preferred embodiment, the prefabricated root section is arranged to form part of an outer surface of the blade after curing the resin. Traditionally, the outer shape of a wind turbine blade shell has been carefully shaped to ensure a perfect airfoil. This required the use of female mould for blade shell. Surprisingly, it was found that for the blade root section a less than perfect airfoil, which is achieved using a male mould, can be more than compensated by improvement in the structural quality of the resulting composite material. Arranging of the prefabricated root section to form part of an outer surface of the blade after curing of the resin may be done by arranging the prefabricated root section inside the female mould or by arranging the prefabricated root section adjacent to the female mould as discussed elsewhere. By forming part of an outer surface of the blade after curing the resin is meant that at least a part of the prefabricated root section will be on the outer surface of the final blade shell after normal blade finish, such as for example gluing, grinding, or polishing, and only be covered by for example paint, vortex generators, or sensors, but not layers of fibrous material.

The blade mould is preferably a half-mould shaped to form one half of a wind turbine blade, e.g. a windward (or pressure) half or a leeward (or suction) half of the blade shell. The blade mould may be a full length blade mould, in which case the first end may be shaped as the root end of the blade and the second end may be shaped as the tip end. In this case, the prefabricated root section is supported in the root end of the mould. Alternatively, the blade mould may be truncated in comparison to a full-length mould. If the mould is truncated at the first end, the prefabricated root section may be supported separately, for example in a separate support frame arranged adjacent to the first end of the mould. The mould may also be truncated at the second end, for example if it is used to form part of a modular wind turbine blade. By the prefabricated root section being arranged adjacent to the blade mould is here meant that most of the prefabricated root section is outside the (female) blade mould and it encompass that a smaller part of the prefabricated root section used for creating a transition between the root section and the rest of the blade shell may be inside the (female) blade mould. For example, a part of the prefabricated root section being tapered for enhancing transfer of forces to a corresponding tapered section of the rest of the blade shell may be arranged inside the blade mould to facilitate for heating via the mould for curing of resin used between parts of the prefabricated root section and the rest of the blade shell. Also, layers of fibrous material may be arranged over at least a part of the prefabricated root section and reaching towards and over a part of the blade shell within the (female) blade mould. Use of a prefabricated root section adjacent to the main blade mould allows for use somewhat smaller blade moulds, which typically will lead to a considerable reduction in cost for the mould, since the cost of a mould is more or less proportional to the surface area of the mould.

The method may comprise arranging the prefabricated root section on top of the fibrous material.

The female mould or the support frame arranged adjacent to the first end of the female mould may comprise a face plate at the first end, the face plate being provided with datum features, and the method may comprise aligning datum features of the root plate of the prefabricated root section with the datum features of the face plate. Such use of datum features is highly advantageous as it allows for very precise alignment of the prefabricated root section to the rest of the blade, and particularly precise alignment of the root inserts of the prefabricated root section to the rest of the blade, which was found to lead to better predictability of blade performance and over time may lead to reduced fatigue degradation of the parts of the wind turbine for example through reduction of vibration.

The prefabricated root section may be arranged in a portion of the blade mould that is substantially unheated during curing of the resin.

The method may further comprise supporting the prefabricated root section in a support frame arranged adjacent to the first end of the blade mould. The blade mould may be located inside a building and at least part of the support frame may be arranged outside the building.

In yet a further aspect of the present invention there is provided a wind turbine blade made according to the method described above, or having a prefabricated root section as described above.

DETAILED DESCRIPTION

FIG. 1is a schematic perspective view of a wind turbine blade10according to an example of the present invention. The blade10extends in a spanwise direction (S) between a root end12and a tip end14, and in a chordwise direction (C) between a leading edge16and a trailing edge18. The blade10comprises an outer shell20formed as a composite laminate, for example from glass-fibre reinforced plastic (GFRP). The outer shell20comprises first and second half shells22,24, e.g. a leeward (suction) half shell22and a windward (pressure) half shell24. The half shells22,24are bonded together by adhesive along their respective leading and trailing edges16,18. The root end12of each half shell22,24comprises a prefabricated root section26.

A method of manufacturing a prefabricated root section26and a method of manufacturing the blade10incorporating the prefabricated root sections26will now be described by way of example with reference to the remaining figures.

FIG. 2is a schematic perspective view of a male mould28shaped to form a prefabricated root section26of the wind turbine blade10. The male mould28extends longitudinally in a spanwise direction (S) between an inboard end30and an outboard end32, and transversely in a chordwise direction (C) between a leading edge34and a trailing edge36. The male mould28defines a male mould surface38of convex curvature in the chordwise direction (C). The inboard end30of the mould surface38has a semi-circular profile in this example, whilst the outboard end32is elongated in the chordwise direction (C) to form part of an airfoil profile. In other embodiments, the mould surface38may have a substantially constant profile (e.g. semi-circular) along its entire spanwise length.

In order to produce the root section26, one or more inner fibrous layers40(also known as ‘plies’) are initially arranged on the male mould surface38. These layers40will form the innermost structural layers of the root laminate. In this example, the fibrous layers40are glass-fibre fabric layers. The fabric42may have fibres extending in any suitable direction, for example it may comprise unidirectional fibres, or it may be biax or triax fabric. Preferably the fabric42is dry fabric, but in other examples it may contain resin, e.g. prepreg or semipreg.

In this example, the fabric42is stored on a roll44before being draped or stretched over the male mould surface38. An edge46of the fabric42may initially be clamped along an edge34,36of the male mould28, in this case along the trailing edge36. The fabric42is then unrolled in the chordwise direction (C) over the convex mould surface38before being clamped along the leading edge34. In other examples, the fabric42could be unrolled in the opposite direction, i.e. from the leading edge34towards the trailing edge36.

A plurality of inner layers40are preferably arranged on the mould28. The layers40may be arranged successively until a required thickness (X) has been built up. Alternatively, the layers40may be pre-assembled as a preform and draped over the male mould surface38in a single step. The layers40may have the same or different dimensions in the spanwise direction (S). In preferred embodiments, layers40with successively shorter spanwise (S) lengths may be used to create a laminate that tapers in thickness (X) moving from the inboard end30towards the outboard end32of the mould28. One or more other layers such as coating layers or release layers may be applied to or arranged on the mould surface38prior to arranging the inner layers40.

Use of a male mould28to form a root section26is particularly advantageous because the fabric layers40do not tend to form wrinkles or creases when arranged on the male mould28. The convex curvature of the mould28means that any creases or wrinkles in the layers40are naturally removed under the influence of gravity. The difficulties and problems associated with arranging plies against the steep sides at the root end of a female blade mould are therefore avoided.

FIG. 3illustrates the next stage of the fabrication process, which involves arranging a root plate48at the inboard end30of the male mould28. The root plate48in this example is a metal plate having a semi-circular profile. A plurality of root inserts50(also known as ‘spears’) project from the root plate48along an arcuate path (in this case along a semi-circular path). Each root insert50is bolted to the root plate48by a respective bolt52.

FIG. 4shows a root insert50by way of example. The root insert50comprises a pre-cured composite structure such as GFRP or CFRP (carbon-fibre reinforced plastic), which is preferably formed in a pultrusion process. The root insert50is wedge shaped and tapers in thickness (Y) moving from a proximal end54(arranged at the root plate48—shown inFIG. 3) towards a distal end56. The proximal end54has a generally rectangular profile. The root insert50comprises a bushing58, preferably made of metal, which is embedded within the composite structure. The bushing58defines an aperture60at the proximal end54of the root insert50. The bushing58has an internal screw thread. Bolts52(shown inFIG. 3) are received in the bushings58in order to connect the root inserts50to the root plate48during the manufacturing process, and subsequently to connect the finished blade10to a wind turbine hub.

The root insert50shown inFIG. 3is merely one example of a root insert and root inserts having different shapes, sizes and configurations may also be used in the present method. In other embodiments, a root insert may be provided that includes a plurality of apertures for receiving a respective plurality of bolts. For example, a single root insert having a profile corresponding to the combined profile of the plurality of root inserts shown inFIG. 3may be used.

Referring again toFIG. 3, the root plate48is arranged at the inboard end30of the male mould28such that the root inserts50overlay the inner fibrous layers40at the inboard end30. It will be appreciated that the inner fibrous layers40could be arranged on the mould28before or after the root plate48is arranged. For example, the inner fibrous layers40could be inserted beneath root inserts50that have been previously arranged on the mould28.

The root plate48in this example comprises datum features62for aligning with corresponding datum features64of the male mould28. The datum features62in this example are bolt holes in the root plate48, which precisely align with bolt holes64at the inboard end30of the male mould28. The root plate48is bolted to the male mould28with a series of bolts66(shown inFIG. 5) that extend through these bolt holes62,64. In other examples, any suitable fastening means or clamps may be used to secure the root plate48to the male mould28. The datum features62,64ensure precise alignment of the root plate48and root inserts50with the male mould28.

FIG. 5is a schematic perspective view of a further stage in the manufacture of the root section26in which one or more outer fibrous layers68(e.g. ‘plies’) are arranged on top of the inner fibrous layers40(shown inFIG. 2) and on top of the root inserts50(shown inFIG. 3). These outer fibrous layers68will form the outermost structural layers of the root laminate. The root inserts50thereby become sandwiched between the inner and outer fibrous layers40,68at the inboard end30of the male mould28.

The outer fibrous layers68are preferably glass-fibre fabric layers. The fabric70may have fibres extending in any suitable direction, for example it may comprise unidirectional fibres, or it may be biax or triax fabric. Preferably the fabric70is dry fabric, but in other examples it may contain resin, e.g. prepreg or semipreg. In this example, the outer layers68are draped or stretched over the male mould surface38in substantially the same way as previously described for the inner layers40. In other examples, a plurality of outer layers68may be preassembled as a preform before being arranged on the male mould28in a single step.

The fibrous layers, i.e. the inner fibrous layers40and the outer fibrous layers68, are preferably arranged such that the total number of fibrous layers40,68arranged at the inboard end30of the mould28is greater than the total number of fibrous layers40,68arranged at the outboard end32. The resulting root laminate is therefore thicker at its inboard end72than at its outboard end74(as shown inFIG. 9).

Referring toFIG. 6, after the various components have been arranged on the male mould28, resin76may be supplied to the components in a vacuum assisted resin transfer moulding (VARTM) process. Specifically, the mould28is covered with a vacuum film78, which is sealed against a surface80such as a flange of the mould28to define a sealed region encapsulating the components. A vacuum pump82is employed to remove air from the sealed region. Concurrently, resin76is admitted into the sealed region. The resin76infuses throughout the fibrous layers40,68and between the root inserts50and the fibrous layers40,68, aided in part by the vacuum pressure within the sealed region.

It will be appreciated that a VARTM process is only one of a number of applicable methods for providing resin76to the fibrous layers40,68and to the root inserts50, and the invention is by no means limited to the use of a VARTM process. In other examples resin76may already be contained in the fibrous layers40,68in the case of prepreg or semipreg materials, or wet hand lay-up may be used, whereby the resin76is applied to the various components using tools such as a brush or roller.

Referring now toFIG. 7, the next stage in the process involves curing the resin76in the root section26. In this example the male mould28is arranged in an oven84to cure the resin76. This is particularly advantageous because it provides very uniform heating and accurate control of the temperature throughout the curing cycle such that thermal stresses and other temperature-related defects are avoided or minimised. Using an oven84to cure the resin76also avoids the need for heating devices and associated sensors built in to the male mould28. Accordingly, a relatively inexpensive and simple male mould28can be used.

Referring toFIG. 8, once the resin76has cured, the prefabricated root section26is removed from the male mould28. The prefabricated root section26is turned through substantially 180 degrees around a horizontal axis to flip the prefabricated root section upside-down and supported in a separate support frame86. The root section26may be stored on the frame86until it is required. In addition, one or more finishing operations may be performed on the root section26whilst it is supported in the frame86. Such operations may include cutting or trimming the edges88,90of the root section26, grinding or re-finishing, inspection and non-destructing testing, and/or the removal of a peel ply layer if one has been included in the laminate shell92of the root section26. It is advantageous to perform such operations with the root section26supported in a separate frame86because the male mould28is then made available for making a further root section26in parallel with these finishing operations being performed.

FIG. 9shows the finished prefabricated root section26made according to the method described above. The prefabricated root section26comprises an outer shell92formed as a cured composite laminate. The outer shell92extends longitudinally in a spanwise direction (S) between an inboard end72and an outboard end74and extends transversely in a chordwise direction (C) between a leading edge88and a trailing edge90. The outer shell92tapers in thickness (Z) moving from the inboard end72towards the outboard end74. The root inserts50described previously are embedded between the inner and outer laminate layers40,68at the thick inboard end72of the shell laminate92.

In this example the outer shell92has a thickness (Z) of between 80 mm to 150 mm at the inboard end72, and a thickness (Z) of between 2 mm to 10 mm at the outboard end74. The prefabricated root section26preferably has a spanwise length (L) constituting between 5% to 20% of the total length of a wind turbine blade10in which it is to be incorporated. In this example the prefabricated root section has a spanwise length (L) of approximately 8 m and is designed to be incorporated in a blade10approximately 80 m in length, the root section26therefore constituting approximately 10% of the total blade length. However, the method described above can be used to make root sections26of any dimensions with any suitable shell thicknesses (Z).

The vacuum-bagging process described previously may result in the prefabricated root section26having an outer surface94that is not completely smooth. It has surprisingly been found that this rough or textured surface94presents some aerodynamic advantages in the finished blade10and can help airflow to remain attached to the surface of the blade10. However, in some cases it may be desirable to form the root section26with a very smooth outer surface94. In this case, a silicone sheet or equivalent may be used to cover components during the infusion process instead of a vacuum film78. This results in the root section26having a very smooth outer surface94.

A method of making a wind turbine blade incorporating the prefabricated root section26will now be described by way of example with reference toFIG. 10.

FIG. 10shows a female wind turbine blade mould96extending in a spanwise direction (S) between first and second ends98,100. In this example, the blade mould96is a half mould shaped to form a half shell22,24of a wind turbine blade10, e.g. a leeward half shell22or a windward half shell24. The blade mould96in this example is a full length blade mould96, wherein the first end98has the shape of the blade root12, and the second end100is shaped to form the blade tip14. A face plate102is secured at the first end98of the blade mould96. The face plate102is provided with datum features104, which are configured to align with datum features62,106of the root plate48of the prefabricated root section26.

Several layers of fibrous material108(e.g. glass fibre and/or carbon fibre layers) are arranged in the female blade mould96to form the laminate layers of the blade shell20. Other blade material or components such as core material and spar caps may be arranged in the mould96according to the particular blade design. The prefabricated root section26is then lifted from its support frame86(shown inFIG. 8) and lowered into the root end98of the female blade mould96. The prefabricated root section26is preferably positioned such that it at least partially overlaps some of the previously arranged fibrous layers108at the first end98of the blade mould96. The datum features62,106of the root plate48are aligned with the datum features104of the face plate102, and the root plate48is bolted or otherwise secured firmly to the face plate102.

Resin is provided to the fibrous material108and between the fibrous material108and the prefabricated root section26. The resin may be provided by any suitable means, and may for example be provided in a VARTM process similar to that described above with reference to the manufacture of the prefabricated root section26. Alternatively, or additionally, prepreg or semipreg materials already containing resin may be used. The resin is then cured to form a finished half shell22,24comprising an integrated prefabricated root section26.

In the above process, the blade mould96preferably includes integral heating means, which are utilised during the curing process. However, as the root section26of the blade10has been prefabricated, there is little or no requirement for heating in this region110of the blade mould96. Accordingly, the root end98of the blade mould96may be substantially unheated during the curing process of the rest of the blade since the prefabricated root section has already been cured. This allows the cost and complexity of the blade mould96to be reduced in comparison to prior art blade moulds which require complex heating devices at the root end to cure the thick laminate in this region. The overall cure time for the blade10is significantly reduced in comparison to conventional blade manufacturing processes, where heating and/or curing of the thick root laminate was the rate-limiting step. The shell thickness outside the root section26is approximately the same thickness or thinner than the outboard end74of the root section26(e.g. between 2-10 mm). Therefore, there are no very thick sections to be cured during the process for the shell outside the root section26.

Although a full-length female blade mould96is shown inFIG. 10, in other examples the blade mould96may be truncated at the first and/or second end98,100in comparison to a full-length blade mould96shown inFIG. 10. If the blade mould96is truncated at the first end98, the prefabricated root section26may be supported outside the blade mould96adjacent to the first end98(for example in a separate support frame). This advantageously allows a shorter and less expensive blade mould96to be used. Furthermore, small variations in dimension of the outer surface94of the prefabricated root section26due to the layup on the male mould28will not lead to misalignment when placed inside the female mould96. The blade mould96may be located inside a blade manufacturing building, whilst part or all of the support frame may be arranged outside the building. This reduces the space inside the building that is required for making the blade10, and may allow more blade moulds96to fit within a building of a given size. In further examples, the mould96may alternatively or additionally be truncated at the second end100, for example if it is used to form an inboard section of a modular wind turbine blade.

A similar process may be used to form the other half shell22,24. Once both half shells22,24have been formed, they are then assembled together to form the complete blade10. This may typically involve providing adhesive along the leading and trailing edges16,18of one or both half shells22,24before clamping the half shells together. Shear webs or a spar may be bonded between the half shells22,24during this process in accordance with the particular blade design.

The present invention presents many advantages over the conventional method of manufacturing wind turbine blades described by way of background. In particular, the present invention provides a faster, more flexible and less expensive process for making blades10and increases the rate of production of blades10.

The use of male moulds28to form the root sections26avoids the problems associated with arranging fabric against the steep sides at the root end of a conventional female blade mould. The root section26can be laid up relatively quickly on a male mould28and a higher quality root laminate92with fewer imperfections can be achieved.

The male mould28is relatively short in comparison to a blade mould96making it possible to fit within a conventional industrial-scale curing oven84, which is not possible with a full length blade mould96. This allows a multiplicity of relatively simple and inexpensive male moulds28to be used since they do not require integral heating apparatus. An oven84also provides uniform heating with high levels of control, resulting in a very high quality root laminate92.

The time in which the main blade moulds96are utilised (the ‘in mould time’) is significantly reduced in comparison to conventional blade production processes. The prefabricated root section26of the present invention can be prepared in advance and relatively quickly positioned in place during production of a blade10. The time-consuming process of laying up a root section in the main (female) blade mould96is therefore avoided. The production of prefabricated root sections26can also take place whilst the main blade moulds96are in use to further increase the output from the blade manufacturing facility.

The time taken to cure the resin in the main blade shell laminate22,24is significantly reduced in comparison to conventional processes as the resin is already cured in the thick prefabricated root section26. Consequently, it may be possible to dispense with heating devices in the root end98of the main blade mould96allowing cheaper blade moulds96to be produced. The cost of the main blade moulds96can be further reduced by truncating the moulds96and supporting the prefabricated root sections26outside the moulds96.

Many modifications may be made to the above examples without departing from the scope of the present invention as defined in the accompanying claims. For example, the shape and dimensions of the male moulds28may be varied according to the particular requirements. The skilled person would also readily appreciate that there are many alternative ways to lay up the fibrous material40,68on the male moulds28.