Wind turbine blades

A method of making a wind turbine blade comprises stacking a plurality of strips of fibre-reinforced polymeric material one on top of another to form a stack of strips (40); strapping the stack of strips together by means of at least one strap (41) made from a fibrous material, and thereby forming a strapped stack; infusing the strapped stack with resin; and curing the resin to form an elongate spar structure in which the at least one strap (41) is integrated with the stack of strips.

FIELD OF THE INVENTION

The present invention relates to wind turbine blades and to methods of manufacturing wind turbine blades. More specifically, the present invention relates to wind turbine blades that include a stack of load-bearing reinforcing strips integrated within the structure of the shell.

BACKGROUND TO THE INVENTION

FIG. 1ais a cross-sectional view of a wind turbine rotor blade10. The blade has an outer shell, which is fabricated from two half shells: a windward shell11aand a leeward shell11b. The shells11aand11bare moulded from glass-fibre reinforced plastic (GRP). Parts of the outer shell11are of sandwich panel construction and comprise a core12of lightweight foam (e.g. polyurethane), which is sandwiched between inner13and outer14GRP layers or ‘skins’.

The blade10comprises a first pair of spar caps15aand15band a second pair of spar caps16a,16b. The respective pairs of spar caps15aand15b,16aand16bare arranged between sandwich panel regions of the shells11aand11b. One spar cap15a,16aof each pair is integrated with the windward shell11aand the other spar cap15b,16bof each pair is integrated with the leeward shell11b. The spar caps of the respective pairs are mutually opposed and extend longitudinally along the length of the blade10.

A first longitudinally-extending shear web17abridges the first pair of spar caps15aand15band a second longitudinally-extending shear web17bbridges the second pair of spar caps16aand16b. The shear webs17aand17bin combination with the spar caps15aand15band16aand16bform a pair of I-beam structures, which transfer loads effectively from the rotating blade10to the hub of the wind turbine. The spar caps15aand15band16aand16bin particular transfer tensile and compressive bending loads, whilst the shear webs17aand17btransfer shear stresses in the blade10.

Each spar cap15aand15band16aand16bhas a substantially rectangular cross section and is made up of a stack of pre-fabricated reinforcing strips18. The strips18are pultruded strips of carbon-fibre reinforced plastic (CFRP), and are substantially flat and of rectangular cross section. The number of strips18in the stack depends upon the thickness of the strips18and the required thickness of the shells11aand11b, but typically the strips18each have a thickness of a few millimetres and there may be between three and twelve strips in the stack. The strips18have a high tensile strength, and hence have a high load bearing capacity.

The blade10is made using a resin-infusion process as will now be described by way of example with reference toFIGS. 1band 1c. Referring toFIG. 1b, this shows a mould20for a half shell of a wind turbine blade in cross-section. A glass-fibre layer22is arranged in the mould20to form the outer skin14of the blade10. Three elongate panels24of polyurethane foam are arranged on top of the glass-fibre layer22to form the sandwich panel cores12referred to above. The foam panels24are spaced apart relative to one another to define a pair of channels26in between. A plurality of pultruded strips18of CFRP, as described above with reference toFIG. 1a, are stacked in the respective channels26. Three strips18are shown in each stack in this example, but there may be any number of strips18in a stack.

Referring toFIG. 1c, once the strips18have been stacked, a second glass-fibre layer28is arranged on top of the foam panels24and the stacks of pultruded strips18. The second glass-fibre layer28forms the inner skin13of the blade10. Next, vacuum bagging film30is placed over the mould20to cover the layup. Sealing tape32is used to seal the vacuum bagging film30to a flange34of the mould20. A vacuum pump36is used to withdraw air from the sealed region between the mould20and the vacuum bagging film30, and resin38is supplied to the sealed region. The resin38infuses between the various laminate layers and fills any gaps in the laminate layup. Once sufficient resin38has been supplied to the mould20, the mould20is heated whilst the vacuum is maintained to cure the resin38and bond the various layers together to form the half shell of the blade. The other half shell is made according to an identical process. Adhesive is then applied along the leading and trailing edges of the shells and the shells are bonded together to form the complete blade.

Other examples of rotor blades having spar caps integral with the shell are described in EP 1 520 983, WO 2006/082479 and UK Patent Application GB 2497578.

The CFRP pultruded strips18extend along the majority of the length of the wind turbine blade10. Modern wind turbine blades may be in excess of eighty metres long, and so it will be appreciated that these strips are very long and heavy. The length and weight of the strips presents challenges relating to the manufacture of the blades, and relating to the handling and transportation of the strips. The present invention aims to address these challenges by providing a convenient method of manufacturing this type of wind turbine blade, and by providing apparatus for use in the method.

SUMMARY OF THE INVENTION

Against this background, and from a first aspect, the invention resides in a method of making a wind turbine blade, the method comprising: stacking a plurality of strips of fibre-reinforced polymeric material one on top of another to form a stack of strips; strapping the stack of strips together by means of at least one strap made from a fibrous material, and thereby forming a strapped stack; infusing the strapped stack with resin; and curing the resin to form an elongate spar structure in which the at least one strap is integrated with the stack of strips.

When making a wind turbine blade according to a method of the invention, the straps therefore remain in place on the stack as the stack is infused with resin and as the resin is cured. The straps serve to hold the strips together and in alignment with one another during the resin infusion process and during curing of the resin. This results in a high degree of alignment of the strips in the finished elongate spar structure of the wind turbine blade, which improves the performance of the wind turbine blade. Furthermore, because the straps remain in place during the infusion process, there is no need to remove the straps from the stack before infusion, which speeds up the manufacturing process and reduces the amount of manual input required.

The method may comprise arranging the strapped stack in a mould and infusing the strapped stack with resin in the mould.

Preferably, the method comprises comprising stacking and strapping the strips outside the mould and transferring the strapped stack into the mould.

The mould may be a blade shell mould, and the method may comprise arranging the strapped stack in the blade shell mould together with other structural components of the blade, and infusing the strapped stack and the other structural components with resin in the blade shell mould. In this way, the strapped stack and the other structural components may be infused in a single infusion process, thereby reducing the number of process steps required.

Alternatively, the method may further comprise transferring the cured elongate spar structure from the mould to a blade shell mould and integrating the cured spar structure with other structural components of the blade in the blade shell mould. Integrating the cured spar structure with other structural components of the blade may comprise infusing the cured spar structure and the other structural components with resin.

In other embodiments, the strips may be stacked outside the mould, and then transferred to the mould to be strapped and infused. The strips may also be stacked directly into the blade mould, then strapped together inside the blade mould. In still other embodiments, the strips may be stacked and strapped outside the mould to form the strapped stack, and the strapped stack may then be fully or partially cured outside the mould. The fully- or partially-cured strapped stack may then be transferred to the mould for subsequent resin infusion with the other structural components of the wind turbine blade.

Transferring the strapped stack may comprise lifting the strapped stack and lowering the strapped stack into the blade mould. In this way, the stack can be transferred to the mould with relatively little movement. Alternatively, the strapped stack could be transferred to the mould by sliding the strapped stack into the mould from one end.

The method may comprise strapping the stack of strips together by wrapping the or each strap around the stack of strips.

To wrap the strap around the stack particularly securely, the method may comprise fixing a first end of the strap to the stack and wrapping the remainder of the strap around the stack such that a second end of the strap overlaps the first end, and fixing the overlapping first and second ends together.

So that the first end of the strip may be fixed to the stack whilst avoiding damage to the strap or the stack, the method may comprise fixing the first end of the strap to the stack by clamping the first end of the strap to the stack using a removable clamp, and may comprise removing the removable clamp from the strap after fixing the overlapping first and second ends together.

The method may comprise bonding the overlapping first and second ends of the strap together by means of an adhesive layer. The adhesive layer may be a layer of thermoplastic adhesive material. In this way, the overlapping ends can be bonded by application of heat. Using a thermoplastic adhesive material also permits flexibility of the bond between the overlapping ends.

The adhesive layer may have a web, mesh or grid structure. This allows resin to infuse through and into the adhesive layer, so that the adhesive layer does not interfere with the infusion process.

To hold the strips in place particularly securely, the method may comprise tensioning the strap around the stack of strips. A particular advantage of tensioning the strap is that as the strap is tensioned around the stack, any that are initially out of alignment are pulled into alignment by the tension in the strap.

To guard against wrinkling of fibrous layers forming part of the blade structure, the method may comprise arranging a layer of pre-cured material over the stack and arranging one or more fibrous layers over the pre-cured layer before infusing the components with resin.

The invention also extends to a wind turbine blade made according to the method described above.

From another aspect, the invention resides in an elongate spar structure for a wind turbine blade, the spar structure comprising a stack of strips of fibre-reinforced polymeric material strapped together with at least one strap made of a fibrous material, wherein the strap is integrated with the stack by cured resin.

The at least one strap may be made from a fibrous material having a density between 50 and 500 grams per square metre. In this way, the fibre density may be low enough to permit unhindered infusion of resin through the strap. The at least one strap may be made of a glass fibre material.

The at least one strap may be wrapped around the stack such that first and second ends of the strap overlap. To fix the strap in place, a layer of adhesive material may be provided between the overlapping first and second ends of the strap.

The adhesive material may be a thermoplastic adhesive material, such that the overlapping ends can be bonded together by application of heat.

To permit infusion of resin into and through the layer of adhesive material, such that the bond between overlapping end regions does not affect the infusion process, the layer of adhesive material may be formed as a web, mesh or grid structure.

The elongate spar structure may comprise a plurality of straps.

The invention also extends to a wind turbine blade comprising one or more elongate spar structures described above.

From another aspect, the invention resides in an infusible strap configured to be secured around a stack of fibre reinforced strips of polymeric material, the stack of strips forming a spar structure of a wind turbine blade, and the strap being configured to maintain the relative alignment between the strips when the stack is lifted and transferred into a wind turbine blade mould, and to maintain the relative alignment of the strips during a resin-infusion stage, wherein the infusible strap is compatible with the resin used in the infusion process and is further configured to permit resin to infuse therethrough so that the strap can be integrated with the spar structure in a finished wind turbine blade.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 2illustrates a wind turbine blade mould20for moulding a wind turbine blade of the type described above by way of introduction to the present invention. More specifically, the blade mould20is used for making wind turbine blades comprising spar caps that extend along a longitudinal axis L, and that comprise a plurality of strips18of fibre reinforced polymeric material arranged one on top of another in a stack.

An alignment zone, generally indicated at60, is defined on the factory floor adjacent the mould20. Pultruded strips18made from carbon-fibre reinforced plastic (CFRP) are fed to the alignment zone60from a down-stream strip manufacturing station or strip feed station (not shown) in a feed direction F. At the alignment zone60, the strips18are stacked up to form two stacks40that will later form spar caps15a,15b,16a,16b, (seeFIG. 1a) and the stacks are aligned relative to one another as they will be aligned in the mould20.

As seen inFIGS. 2 and 3, the stacks40are strapped together with a plurality of straps41that keep the strips18aligned during handling of the stacks40, and during subsequent process stages.

Once the stacks40have been strapped together, the stacks40are lifted from the alignment zone60, with their relative positions maintained by a support structure (not shown) and then lowered into the mould20. Other structural components of the blade are then laid in place around the stacks40. The structural components are covered with a vacuum bag to form a sealed region, air is evacuated from the sealed region, and the sealed region is infused with resin. The resin infuses between the structural components of the blade, including around the stacks40and the straps41. The resin is then cured, forming a blade shell having integrated spar caps15a,15b,16a,16b, with each spar cap15a,15b,16a,16bhaving a plurality of integrated infusible strap41.

Referring now toFIG. 3, the straps41that hold the stacks40together are infusible straps. Each strap41is made of a lightweight glass fibre cloth or another suitable fibrous material, having a density of approximately 200 grams per square metre (gsm). The fibrous nature of the straps41allows resin to infiltrate into the straps41during the resin infusion process, as will be described in more detail later. Each strap has a thickness of approximately 0.05 to 0.4 mm, and a width of approximately 50 mm to 150 mm.

The straps41fit tightly around the stack40such that the straps41are under tension. This prevents the strips18moving out of alignment with one another during handling of the stack40, and once the stack40is arranged in the blade mould20. The straps41are pulled tightly around the stack40.

As best seen inFIG. 4a, end portions44of the strap41overlap one another at an upper surface of the stack40and are bonded to one another by an adhesive layer45to form a joint48.

Referring toFIG. 4b, the adhesive layer45is a layer of thermoplastic adhesive material formed into an open web structure. The thermoplastic adhesive material adheres the end portions44of the strap41together, whilst permitting flexibility of the joint48, so that the strap41can be placed under tension without adversely affecting the joint48. The open web structure allows infusion resin to infiltrate through the joint48, so that the joint48does not interfere with the later infusion process.

The thermoplastic adhesive material is, for example, a copolyester, an aliphatic polyurethane or any other suitable thermoplastic adhesive material. The material is selected so as not to hinder the later infusion process; in particular, an adhesive is selected that does not react chemically with the infusion resin.

A method of making a wind turbine will now be described, with particular reference toFIGS. 5 to 20.

First, a plurality of strips18are stacked one on top of another to form a stack40, as shown inFIG. 5.

Next, the stack40is strapped together by wrapping a strap41around the stack40. To wrap the strap41sufficiently tightly, a first end portion44aof the strap41is clamped in place against the stack40using a flat, removable clamping piece46that extends across the strap41and clamps47arranged either side of the strap41, as illustrated inFIG. 6. The adhesive layer45is arranged over the first end portion44aas shown inFIG. 7, and the strap41is pulled tightly around the stack40, until the strap41is under the required tension. As shown inFIGS. 8 and 9, a second end portion44bof the strap41is pulled over the first end portion44aof the strap41, such that the end portions44of the strap41overlap with the adhesive layer45sandwiched between the overlapping end portions44. Heat is applied to the end portions by a heating iron, so as to activate the thermoplastic adhesive material and bond the end portions44of the strap41together. The strap41is trimmed if required, and the clamps47and removable clamping piece46are removed leaving the strap41in place around the stack40, as shown inFIG. 10.

Once the first strap41has been fitted as described, further straps41are fitted to the stack40at regular intervals along the stack40, as shown inFIG. 11.

The stack40is then arranged in the alignment zone60defined on the factory floor, with other stacks40, as shown inFIG. 12. The stacks40are aligned relative to one another in the alignment zone60in a configuration that matches the eventual configuration of the spar caps15a,15b,16a,16bin the finished blade10.

The stacks40are then supported in this configuration by a support structure70, as shown inFIG. 13. Once supported, the stacks40are transferred to the mould20in the sequence shown inFIGS. 13 to 16. The stacks are lifted upwardly in the direction100, then moved transversely in the direction200, and finally lowered in the direction300to be laid on the mould surface21with a pair of stacks40arranged on each side of a foam panel24. The support structure70is then removed leaving the stacks40in place in the mould20, as shown inFIG. 17.

During the transfer process, the straps41maintain the alignment of the strips18in the stacks40. In this way, little, if any, subsequent adjustment of the strips18is required once the stacks40have been arranged in the mould20. The support structure70also maintains the alignment of the stacks40during transfer, so that there is no need to rearrange the stacks40after the transfer process.

Once the stacks40have been arranged in place in the mould, other structural components of the blade are laid on top of the stacks40, as shown inFIGS. 18 to 20. First, as shown inFIG. 19, a layer of pre-cured mesh material50is laid directly on top of the stack40, such that the straps41are sandwiched between the stack40and the pre-cured mesh50. Layers of dry glass fibre fabric52are then laid over the pre-cured mesh50, as shown inFIG. 20. The layer of pre-cured mesh50is relatively stiff so that it acts as a wrinkle-preventing layer to prevent the straps41from creating wrinkles in the layers of glass fibre fabric52. The structural components are then covered with a vacuum bag to form a sealed region, air is evacuated from the sealed region using a vacuum pump, and resin is introduced to the sealed region, where it infuses between the components.

As the resin infuses between the structural components of the blade, it infuses around the stacks40and into the fibrous material of the straps41. The fibrous material of the straps41is of a low density, thereby ensuring that the strap41does not interfere with the infusion process. The open structure of the adhesive layer45means that resin can also infuse into and through the adhesive layer45. The inert nature of the thermoplastic adhesive material means that the layer of adhesive material does not react chemically with the resin. In this way, the strap41is infused with resin in the same way as the other components of the blade. When the resin is cured, the strap41is therefore integrated with the stack40, by the cured resin such that the strap41forms a structural component of the spar cap15a,15b,16a,16b.

Accordingly, a particular advantage of the present invention is that it is not necessary to remove the straps41once the stacks40are in place in the mould20. This removes a step from the manufacturing process, increasing the speed of the process, and reducing the amount of manual labour required. Furthermore, as the straps41remain in place during the infusion process, the straps41also serve to maintain the alignment of the strips18in the stack40as the components are infused with resin, and as the resin is cured.

Although in the embodiment described the straps act primarily to hold the strips of the stack together and in alignment with one another, in other embodiments the straps may also be used to lift the stack into position in the mould.

In some embodiments, a single spar cap may be formed from more than one stack of strips. In this case, the stacks that form the spar cap are arranged side-by-side, and are strapped together by a single strap. This arrangement is useful for creating wider spar caps.

In the embodiment described the joint of the strap41is arranged on an upper surface of the stack40. However, in other embodiments, the joint may be arranged on a side of the stack, particularly if the stack includes a large number of strips and is of a relatively large height. In such embodiments, the joint sits next to the foam panel when the stack is arranged in the mould, and hence the joint is shielded by the foam panel. This further mitigates against the joint causing wrinkles in the glass fibre layers of the wind turbine blade.

The present invention is therefore not limited to the exemplary embodiments described above and many other variations or modifications will be apparent to the skilled person without departing from the scope of the present invention as defined in the following claims.