Reinforcing structure for a wind turbine blade

Abstract

A reinforcing structure for a wind turbine blade A reinforcing structure for a wind turbine blade (12) is described. The reinforcing structure comprises one or more pultruded strips (42C) having spanwise grooves (54). The grooves (54) impart transverse flexibility to the strips (42C), allowing the strips (42C) to conform to the curvature of a wind turbine blade mould (44). An associated method of making a reinforcing structure for a wind turbine blade (12) is described. The method comprises providing an elongate mould (44) extending in a longitudinal direction and defining a mould surface at least part of which is concave-curved in transverse cross section. One or more pultruded strips (42C) with spanwise grooves (54) are arranged in the mould (44) to form the reinforcing structure. The pultruded strip(s) are bent along the grooves (54) so that they substantially conform to the transverse curvature of the mould surface. In preferred embodiments the reinforcing structure is a spar cap (36).

Claims

1. A method of making a reinforcing structure for a wind turbine blade through a moulding process, the method comprising: providing an elongate mould extending in a longitudinal direction and defining a mould surface at least part of which is concave-curved in transverse cross section; arranging one or more pultruded strips in the mould to form the reinforcing structure, the one or more pultruded strips having one or more longitudinally-extending grooves; and bending and/or cracking the one or more pultruded strips along the one or more grooves such that the one or more pultruded strips substantially conform to the transverse curvature of the mould surface to define a plurality of planar strip portions, each of the plurality of planar strip portions having a lower surface that is closest to the mould surface, wherein the lower surface remains flat relative to the transverse curvature of the mould surface upon completion of the moulding process.

2. The method of claim 1, wherein the reinforcing structure is a spar cap.

3. The method of claim 1, comprising stacking a plurality of the pultruded strips in the mould to form a stack in which each strip is flat in transverse cross section; and bending and/or cracking the stack of strips in the mould along at least one groove of at least one strip such that the stack of strips conforms to the transverse curvature of the mould surface.

4. The method of claim 1, further comprising: at least partially covering the one or more pultruded strips with a vacuum bagging film to form a sealed region in the mould enclosing the one or more pultruded strips; and removing air from the sealed region such that the vacuum bagging film exerts a force on the one or more pultruded strips and causes the one or more pultruded strips to bend and/or crack along the one or more grooves.

5. The method of claim 1, wherein the mould is a wind turbine blade mould, and the method comprises forming the reinforcing structure in the mould integrally with a shell of the wind turbine blade.

6. The method of claim 1, wherein the one or more pultruded strips are made from fibre-reinforced polymeric material.

7. A method of making a reinforcing structure for a wind turbine blade through a moulding process, the method comprising: providing an elongate mould extending in a longitudinal direction and defining a mould surface at least part of which is concave-curved in transverse cross section; arranging one or more pultruded strips in the mould to form the reinforcing structure, the one or more pultruded strips having one or more longitudinally-extending grooves; and cracking the one or more pultruded strips along the one or more grooves such that the one or more pultruded strips substantially conform to the transverse curvature of the mould surface to define a plurality of planar strip portions upon completion of the moulding process.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a wind turbine comprising a plurality of wind turbine blades according to an embodiment of the present invention;

(2) FIG. 2 is a cross-section through one of the wind turbine blades of FIG. 1;

(3) FIG. 3a schematically illustrates a relatively wide pultruded strip in transverse cross-section;

(4) FIG. 3b schematically illustrates the relatively wide pultruded strip of FIG. 3a arranged in a curved mould, where the pultruded strip forms a bridge in the mould and does not conform to the curvature of the mould;

(5) FIG. 4a schematically illustrates two relatively narrow pultruded strips in transverse cross-section;

(6) FIG. 4b schematically illustrates the two relatively narrow pultruded strips of FIG. 4a arranged side-by-side in a curved mould, the strips together conforming to the curvature of the mould;

(7) FIG. 5a schematically illustrates a pultruded strip according to an embodiment of the invention in transverse cross-section, the strip having a plurality of spanwise grooves;

(8) FIG. 5b schematically illustrates the pultruded strip of FIG. 5a arranged in a curved mould, where the strip is bent along the spanwise grooves and conforms to the curvature of the mould;

(9) FIG. 5c schematically illustrates the pultruded strip of FIG. 5b where the curved strip is cracked along the spanwise grooves into multiple sections;

(10) FIG. 5d schematically illustrates a pultruded strip according to an embodiment of the invention in transverse cross-section, the strip having a plurality of spanwise grooves formed in the lower surface of the strip;

(11) FIG. 6a schematically illustrates a stack of pultruded strips according to an embodiment of the invention arranged in a mould to form a reinforcing structure of a wind turbine blade;

(12) FIG. 6b schematically illustrates part of a wind turbine blade shell comprising a reinforcing structure according to an embodiment of the invention embedded in the shell structure;

(13) FIG. 7a illustrates a pultrusion process according to an embodiment of the invention for making pultruded strips according to embodiments of the present invention;

(14) FIG. 7b is a schematic cross-section through a pultrusion die for use in the pultrusion process; and

(15) FIG. 7c is a schematic perspective view of a pultruded strip according to an embodiment of the invention.

DETAILED DESCRIPTION

(16) FIG. 1 shows a wind turbine 10 comprising a plurality of wind turbine blades 12. The blades 12 are connected at their respective root ends 14 to a wind turbine blade hub 16. Each blade 12 extends in a longitudinal or ‘spanwise’ direction S from its root end 14 to a tip end 18. Typically the blades 12 are substantially circular at their root ends 14, where they connect to the hub 16. The blades 12 then develop an airfoil profile in transverse cross-section moving in the spanwise direction S towards the tip 18.

(17) FIG. 2 is a schematic transverse cross-sectional view of one of the wind turbine blades 12 of FIG. 1, taken along the line 2-2 in FIG. 1. It can be seen from FIG. 2 that the blade 12 has an airfoil profile in this region. The blade 12 extends in a ‘chordwise’ direction C between a leading edge 20 and a trailing edge 22. The blade 12 comprises an outer shell 24, which in this example is formed from a windward half-shell 26 and a leeward shell 28 that are bonded together along their leading and trailing edges 20, 22. The windward and leeward shells 26, 28 are both curved between their leading and trailing edges 20, 22 to define the airfoil profile of the blade 12. The windward and leeward shells 26, 28 may be made separately in respective half-moulds. However, in other embodiments, the outer shell 24 of the blade 12 may be formed as a single piece.

(18) The outer shell 24 may be of composite construction, and typically comprises inner and outer skin layers 30, 32 made primarily of glass-fibre reinforced plastic (GFRP). For increased strength, core material 34 such as foam or balsa may be incorporated in the shell structure, for example between the inner and outer skin layers 30, 32, to form a sandwich structure.

(19) The wind turbine blade 12 also comprises reinforcing structures. In this embodiment, the reinforcing structures include spar caps 36 and a shear web 38. In this embodiment, the blade 12 comprises two spar caps 36, which are associated respectively with the windward and leeward shells 26, 28 of the blade 12. The two spar caps 36 are arranged opposite one another in the region of maximum thickness of the airfoil profile (i.e. where the windward and leeward shells 26, 28 are furthest apart).

(20) The shear web 38 is bonded between the spar caps 36. The spar caps 36 are integrated with the outer shell 24 of the blade 12. In this example the spar caps 36 are embedded within the structure of the outer shell 24. In other embodiments, the spar caps 36 may be bonded to inner surfaces 40 of the windward and leeward half-shells 26, 28.

(21) The spar caps 36 in this example are of generally rectangular cross-section. The shear web 38 in this example is generally I-shaped in cross-section. These components may have other suitable shapes in other embodiments. The spar caps 36 and the shear web 38 are longitudinal elements, which extend longitudinally in the spanwise direction of the blade 12, i.e. transverse to the plane of the page in FIG. 2. Typically they extend along the whole or a majority of the length of the blade 12. Accordingly, for modern utility-scale wind turbines, these components may have lengths in excess of eighty metres.

(22) In this embodiment, each spar cap 36 is formed from one or more pultruded strips of carbon-fibre reinforced plastic (CFRP). Preferably the spar caps 36 are each formed from a stack of pultruded strips 42, as will be described in further detail later.

(23) It will be appreciated from FIG. 2 that the spar caps 36 are integrated with curved portions of the outer shell 24 of the wind turbine blade 12. In order for the spar caps 36 to integrate effectively with the blade shell 24, it is important that the spar caps 36 can conform to this curvature. As will now be described with reference to FIGS. 3a-4b, this presents challenges where relatively wide spar caps 36 are required, such as is the case for single-web blades such as the blade 12 shown in FIG. 2.

(24) Referring to FIG. 3a, this shows a pultruded strip 42a of CFRP in transverse cross-section. The strip has a width W of approximately 330 millimetres, and a thickness T of approximately 5 mm. The strip 42a has a width W appropriate for a wide spar cap for a single-web blade, and is relatively wide in comparison to the width of pultrusions used to form existing narrower spar caps, which tend to be approximately half this width. Such existing spar caps may be found in dual-web blades, which include two pairs of spar caps spaced in the chordwise direction and bridged by a pair of shear webs. An example of a dual-web blade can be seen in FIG. 1 of WO2014079456.

(25) Referring to FIG. 3b, this schematically illustrates the relatively wide pultrusion 42a of FIG. 3a when arranged in a wind turbine blade mould 44. In order to define the curved profile of the blade, the mould surface 46 is concave-curved in transverse cross section. Due to its large width, the pultrusion 42a is not able to lie flush against the curved mould surface 46. Instead, the pultrusion 42a effectively forms a ‘bridge’ in the mould 44 and results in a large gap 48 beneath the pultrusion 42a between the pultrusion 42a and the mould surface 46. Such large gaps 48 should be avoided when laying up blade materials since they ultimately result in resin-rich pockets forming weaknesses in the blade structure. Accordingly, the wide pultruded strip 42a is not able to conform effectively to the curvature of the mould 44.

(26) FIG. 4a shows a pair of pultruded strips 42b of CFRP in transverse cross-section and arranged side-by-side. Each strip 42b has a width W of approximately 165 mm and a thickness T of approximately 5 mm. The combined width of the two pultrusions 42b is approximately 330 mm, i.e. approximately equal to the width of the relatively wide pultrusion 42a described above with reference to FIG. 3a.

(27) Referring to FIG. 4b, this illustrates the two pultruded strips 42b of FIG. 4a when arranged side-by-side in a wind turbine blade mould 44. It can be seen that each pultrusion 42b is able to lie substantially flush or flat against the mould surface 46. Accordingly, the pultrusions 42b conform well to the curvature of the mould 44.

(28) Whilst it is possible to form a wide spar cap 36 from multiple side-by-side pultrusions 42b, this is a relatively expensive solution. For example, the cost of producing two ‘half width’ pultrusions 42b is greater than the cost of producing a single ‘full width’ pultrusion 42a. The cost of handling, storing and laying-up multiple narrow pultrusions 42b is also greater than the cost of handling, storing and laying-up fewer wider pultrusions 42a.

(29) The above discussion illustrates the problem that wide pultrusions 42a do not conform well to curved moulds and may form ‘bridges’ in the mould. Whilst narrower pultrusions can conform to the curvature of the mould, and can be used to form a wide spar cap 36, they are a relatively expensive solution. The present invention provides a cost-effective solution, which will now be described in further detail with reference to the remaining figures.

(30) FIG. 5a is a schematic transverse cross-sectional view of a pultruded strip 42c according to an embodiment of the present invention. The strip is made from CFRP. In this embodiment, the strip has a width W of approximately 330 mm and a thickness T of approximately 5 mm. As such, the strip 42c is relatively wide and has the same dimensions as the wide strip 42a described above with reference to FIG. 3a. It will be appreciated that these dimensions are not intended to be limiting and strips according to the invention having other suitable widths and thicknesses may be formed depending upon the particular requirements, for example the size, shape and structural requirements of a particular wind turbine blade or spar cap.

(31) The pultruded strip 42c in FIG. 5a is substantially flat in cross-section and comprises upper and lower surfaces 50, 52 in the orientation of the strip 42c shown in FIG. 5a. The strip 42c comprises a plurality of longitudinally-extending grooves 54, which are also referred to as ‘spanwise grooves’. The grooves 54 are defined in the upper surface 50 of the strip 42c. In this example, the strip 42c has two grooves 54, but in other examples the strip 42c may have a single groove or more than two grooves.

(32) In this example, the grooves 54 serve to divide the cross-section of the strip 42c into three portions 56a-c of substantially equal width. However, in other embodiments, the grooves 54 may be located in other positions, such that the strip 42c may be divided into portions of unequal width.

(33) The grooves 54 may have any suitable dimensions. In this example each groove 54 has a width ‘w’ of approximately 2 mm and a depth ‘d’ of approximately 3 mm. The depth d of a groove 54 therefore corresponds to approximately 60% of the thickness T of the strip 42c in this example. In other examples the grooves 54 may have a different depth d. Preferably, however, the depth d of a groove 54 is at least 50% of the thickness T of the strip 42c, and more preferably the groove 54 extends through a majority of the thickness T of the strip 42c. This advantageously imparts flexibility to the strip 42c allowing the strip 42c to be able to bend or flex along the grooves 54, as will now be described with reference to FIG. 5b.

(34) Referring to FIG. 5b, this schematically shows the pultruded strip 42c of FIG. 5a arranged in a curved mould 44, e.g. a wind turbine blade mould. In other embodiments the mould 44 may be a dedicated spar cap mould, for example. The strip 42c is bent along each groove 54, which allows the strip 42c to conform to the curvature of the mould 44. Specifically, in this example, the lower surface 52 of each of the three portions 56a-c of the strip 42c lies substantially flush with or substantially flat against the portion of the curved mould surface 46 beneath it. The spanwise grooves 54 impart chordwise flexibility to the pultruded strip 42c and allow the strip 42c to bend or flex along the grooves 54 to conform to the curvature of the mould 44.

(35) In some embodiments, bending the strip 42c along the grooves 54 may cause the strip to crack along the grooves 54, i.e. crack about the lines 55 indicated in FIG. 5b. As shown in FIG. 5c, the resulting cracked strip 42c may be divided into multiple separate sections 56a, 56b, 56c such that the cracked strip 42c conforms to the transverse curvature of the mould 44. The gaps between the sections 56a, 56b, 56c may advantageously fill with resin, for example during a subsequent resin infusion process, causing the cracks to ‘heal’.

(36) Referring to FIG. 5d, in another embodiment the grooves 54 may be formed in the lower surface 52 of the strip 42c. It will be appreciated that this is equivalent to arranging the strip 42c in the mould 44 upside down, in comparison to orientation of the strip 42c shown in FIG. 5b. It can be seen that in this orientation, the strip 42c is still able to conform to the curvature of the mould 44 by bending (or cracking) along the grooves 54.

(37) In this orientation of the strip 42c, the grooves 54 widen slightly at the mouths of the grooves when the strip flexes to conform to the transverse curvature of the mould 44.

(38) FIG. 6a is a schematic partial transverse cross-sectional view of a wind turbine blade mould 44 and a layup 58 for a blade shell 24 (indicated in FIG. 2). The layup 58 comprises an outer skin layer 32, which may be formed, for example, from one or more layers of fabric material such as woven or non-woven glass-fibre fabric. A plurality of pultruded strips 42c with spanwise grooves 54 are stacked on top of the outer skin layer 32 to form a stack 60. The stack 60 forms a reinforcing structure of the blade, in this case a spar cap 36. Foam core material 34 may be arranged alongside one or both sides of the stack 60 of pultrusions 42c. An inner skin layer 30, e.g. comprising one or more glass-fibre fabric layers, is arranged on top of the stack 60 and the core material 34 to complete the layup.

(39) There are three pultruded strips 42c in this example, but the stack 60 of pultrusions 42c may include any number of strips 42c in other embodiments. Alternatively a single strip 42c of suitable thickness may be used in other embodiments. In this example, each strip 42c is substantially identical to the strip 42c described above with reference to FIG. 5a. However, in other embodiments, the strips 42c may be different to the strip 42c in FIG. 5a. For example, they may have different dimensions to the strip 42c in FIG. 5a, and/or they may have a different number or configuration of grooves 54. The strips 42c in the stack 60 could also be different to one another according to the particular requirements.

(40) It can be seen form FIG. 6a, that each strip 42c is substantially flat (i.e. not bent) when it is arranged in the mould 44. Accordingly, the strips 42c may initially form a bridge in the mould 44 similar to that described above with reference to FIG. 3b. The method of assembling the spar cap 36 in the mould 44 may therefore comprise stacking the strips 42c when flat to form a stack 60 of flat strips 42c in the mould 44. The strips 42c are easier to handle and stack when flat, so this provides an advantage during the layup process.

(41) Once the constituent components of the blade shell have been laid up in the mould 44, the layup 58 is then then covered with a layer of vacuum bagging film 62. The vacuum bagging film 62 is sealed against suitable surfaces, for example against leading- and trailing-edge mould flanges (not shown) to form a sealed region 64 encapsulating the layup 58. A vacuum pump (not shown) is connected to the sealed region 64 and is used to remove air from this region. This causes the vacuum bagging film 62 to bear down against the layup 58.

(42) The vacuum bag 62 contracts around the layup 58 and exerts a force on the layup 58, which causes the stacked pultruded strips 42c to bend along their spanwise grooves 54 (see FIG. 6b). The strips 42c therefore deform (i.e. bend) during the vacuum process to conform to the curvature of the blade mould 44. This is particularly advantageous since no manual intervention is required to bend the strips. However, in other embodiments the strips 42c could be bent using manual intervention if required.

(43) With the vacuum maintained, resin (not shown) is supplied to the sealed region 64. The resin infuses throughout and in-between the various structural components and layers of the blade shell. Once cured, the resin forms a hard matrix that integrates the various components together. Heat may be applied to the layup 58 and/or mould 44 to accelerate the curing process.

(44) Once the resin has cured, the vacuum bag 62 is removed and the wind turbine blade shell 24 is removed from the mould. The resulting blade shell 24, part of which is illustrated in FIG. 6b, comprises a spar cap 36 embedded within the shell structure. The spar cap 36 comprises a stack of pultruded strips 42c, each strip 42c having spanwise grooves 54, and each strip 42c being bent along the grooves 54 such that the strips 42c and the stack 60 conforms to the chordwise curvature of the blade shell 24.

(45) In addition to allowing the pultruded strips 42c to conform to the curvature of the blade mould 44 (FIG. 6a), the grooves 54 advantageously enhance the resin-infusion process since they provide longitudinal channels or pathways for resin to flow between stacked strips 42c. This facilitates and enhances the permeation of resin between adjacent strips 42c in the stack 60 and hence improves bonding between the strips 42c.

(46) In other embodiments, one or more of the stacked strips 42c may be orientated as shown in FIG. 5d, i.e. such that the grooves 54 are on the underside of the strip 42c.

(47) This orientation of the strip(s) 42c provides a particular advantage, since the grooves 54 widen when the strip 42c flexes thereby maximising the space for resin to flow between stacked strips.

(48) A method of making the pultruded strip 42c with spanwise grooves according to the invention will now be described with reference to FIGS. 7a-7c.

(49) Referring to FIG. 7a, this shows a pultrusion apparatus 70 and process for forming pultruded strips according to embodiments of the invention. The pultrusion apparatus 70 comprises a heated pultrusion die 72 and a resin bath 74 containing liquid resin 76 such as epoxy resin. In this example, carbon fibres 78 are pulled through the resin bath 74 and coated in resin 76 before being pulled through the pultrusion die 72. The pultrusion die 72 defines an aperture 80 shaped to define the cross-sectional shape of the pultruded strips 42c.

(50) Referring additionally to FIG. 7b, which is a cross-sectional view of the pultrusion die 72, it can be seen that the aperture 80 is substantially rectangular in shape. The pultrusion die 72 additionally includes groove-forming features 82. These features 82 protrude into the rectangular aperture 80. The formations 82 are shaped to define the cross-sectional shape of the grooves 54 in the pultruded strips 42c formed by the die 72.

(51) The formations 82 are preferably dimensioned to form grooves 54 in the pultrusions 42c that have a depth of at least 50% of the thickness of the pultrusions 42c.

(52) Referring again to FIG. 7a, the resin-coated carbon fibres 78 are pulled through the heated die 72 in the direction of the arrow 84. The die 72 shapes the fibres into a strip 42c with longitudinal grooves 54 whilst simultaneously curing the resin. Accordingly, the cured strip 42c emerges from the other side of the die.

(53) The nature of the pultrusion process results in a strip 42c of uniform cross-section along its length with grooves 54 that extend along the full length of the strip. The resulting cured strip 42c with spanwise grooves 54 is shown in perspective view in FIG. 7c.

(54) Aside from the initial tooling costs associated with the pultrusion die 72, the cost of producing pultruded strips with grooves 54 is substantially the same as producing strips of equivalent dimensions without grooves (such as the strip 42a described above with reference to FIG. 3a). Accordingly, the present invention allows relatively wide strips to be formed that are able to conform to a curved mould, thus representing an advantageously reduced-cost solution in comparison to the alternative of using multiple, narrower strips arranged side-by-side, as discussed above in relation to FIGS. 4a and 4b.

(55) The above examples are provided for illustrative purposes only and are not intended to limit the scope of the invention as defined in the accompanying claims. Accordingly, many modifications may be made to the examples described above without departing from the scope of the invention defined in the claims. For example, strips of other dimensions and/or with other numbers or dimensions of grooves may be formed.