Patent Description:
Modern utility-scale wind turbine blades typically comprise a substantially hollow shell supported at least in part by a reinforcing spar structure. In some known wind turbine blades the spar structure comprises a shear web and spar caps, the shear being bonded between the spar caps. The spar structure is designed to take up bending and shear loads experienced by the wind turbine blade during use. The quality of the joint between the shear web and the blade shell is therefore a significant factor affecting the durability of the blade.

<CIT> describes a spar for a wind turbine blade which includes a shear web extending between opposing sides of the blade. A joint is arranged substantially midway between ends of the shear web for sizing the shear web. <CIT> describes a wind turbine where a shear web is connected to a blade component with a wetted composite material.

Typically, the shear web is bonded to the blade shell at the same time as the bonding together of first and second half shells which form the hollow shell of the wind turbine blade. A bead of adhesive is typically deposited in one half shell and further adhesive is deposited along a top surface of the shear web. The shear web is then arranged in the first half shell before the second half shell is arranged on top of the first. The adhesive in the bondlines is then compressed and cured to bond the shells together and to bond the shear web between the half shells.

The present method of bonding shear webs to the blade shells introduces a number of challenges. For example, there is a risk of the shear web sinking too far into the uncured adhesive deposited on the first half shell. There is also a risk of the second half shell lifting up slightly as a result of thermal expansion of the blade shells during the adhesive curing process, when heat is applied to the mould assembly. Both scenarios may lead to the shear web becoming detached from the second half shell. Careful monitoring of the bonding process is therefore required to avoid these problems and thorough analysis of the bond lines is required to detect any problems should they arise.

Against this background it is an object of the present invention to provide an improved process for bonding shear webs to blade shells.

In a first aspect of the invention there is provided a bondline structure according to claim <NUM> for bonding a shear web to a wind turbine blade shell.

The term "inner core" refers to the central component of the bondline structure, i.e. the inner core is the middle part of the bondline structure.

The adhesive of the inner core is preferably an adhesive which expands upon application of heat thereto, such that the inner core of the bondline structure undergoes thermal expansion during heating thereof, for example in a curing process. The one or more outer layers may comprise dry reinforcing fibres which become saturated by the adhesive of the inner core upon compression of the bondline structure.

The adhesive may comprise epoxy or polyester. However, any suitable adhesive may be used. The adhesive may be thixotropic, semi thixotropic, non-thixotropic or combinations thereof, to optimize the flow parameters of the adhesive. Where the one or more outer layers comprise an adhesive, the adhesive in the outer layers is preferably a resin.

The one or more outer layers preferably comprise a prepreg material. Where the one or more outer layers comprise a prepreg material, the reinforcing fibres of the outer layers are saturated with adhesive in the form of a resin matrix.

The one or more outer layers may comprise multiaxial fibres. Preferably the multiaxial fibres comprise biaxial fibres. Alternatively the multiaxial fibres may comprise triaxial or quadriaxial fibres. The fibres are preferably glass fibres. The fibres may alternatively be carbon fibres or any other suitable fibres. Preferably there is a plurality of outer layers.

In the case of a plurality of outer layers, an inside layer (or inside layers) nearest to the core may be formed from a different material to an outside layer (or outside layers) further from the core. The inside layers may be chopped strand mats or needle mats. The outside layers may be continuous strand mats. This allows better compliancy of the outside layers as the inside layers can adapt to the changing shape of the core. In addition the outside layer(s) may have out of plane filaments (i.e. extending transverse to a longitudinal axis of the bondline). This improves the fibre contact between the shear web and the blade shell.

Preferably a low percentage of the reinforcing fibres of the outer layers of the bondline structure are oriented parallel to a longitudinal axis of the bondline structure. Most preferably the outer layers of the bondline structure comprise substantially no fibres oriented parallel to the longitudinal axis of the bondline structure.

The inner core is preferably substantially cylindrical. Most preferably it is substantially circular in cross-section. Alternatively, it may have any other suitably shaped cross section. For example, the inner core may be elliptical, rectangular, hexagonal etc. in cross section. Whilst preferably there are no edges extending in a longitudinal direction of the inner core, if there are any edges, they are preferably rounded.

The one or more outer layers are wrapped around a full circumference of the inner core. In other words, the entire circumference of the inner core is covered by the one or more outer layers. This provides for efficient transfer of loads between the shear web and the blade shell.

Preferably the inner core does not contain any reinforcing fibres. In other words, in the bondline structure, all of the reinforcing fibres are located in the one or more outer layers. In this way, loads between the shear web and the blade shell are transferred efficiently in the one or more outer layers.

The blade shell preferably comprises spar caps. The spar caps may be mutually opposed, located on opposite sides of the blade shell. The spar caps may be embedded within a laminate structure of the blade shell. Alternatively the blade shell may comprise mutually opposed spar caps bonded to an inner surface of an aerodynamic fairing.

In a second aspect of the invention there is provided a method of making a wind turbine blade according to claim <NUM>.

The method may comprise arranging the bondline structure in a bondline region of the first half shell. Preferably the bondline region is defined by the spar caps of the blade shell such that a reinforcing spar structure may be formed by the spar caps and shear web when the shear web is bonded to the blade shell.

The method may additionally comprise arranging a further bondline structure as described above between the second mounting flange and the inner surface of the second half shell. The further bondline structure could be arranged on an upper surface of the shear web, or on the surface of the second half shell. The shear web and the second half shell may be pressed together such that the further bondline structure becomes compressed between the second mounting flange and the inner surface of the second half shell. The method may further comprise curing the adhesive in the further bondline structure such that the further bondline structure bonds the second mounting flange to the second half shell.

The inner core is made from an adhesive, and the step of curing the adhesive may comprise applying heat to the bondline structure, and said application of heat may cause the adhesive of the inner core to expand.

The step of providing the one or more bondline structures may comprise winding fibrous material around an elongate inner core.

The method may further comprise providing a winding machine configured to wind fibrous material around inner core material to form a bondline structure. The winding machine may be arranged at one end of the first half shell. One or more lengths of inner core material may be inserted through the winding machine such that the outer fibrous material may be wound around the inner core material to form a continuous bondline structure. The continuous bondline structure may be pulled out of the winding machine and onto the inner surface of the first half shell.

The winding machine is preferably arranged at the root of the blade shell and the adhesive structure is pulled in a spanwise direction towards the tip end of the blade shell.

Optional and advantageous features described above in relation to any one aspect of the invention are equally applicable to the other aspects of the invention. Repetition of such features is avoided purely for reasons of conciseness.

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

<FIG> is a schematic exploded view of a wind turbine blade <NUM>. The blade <NUM> comprises an outer shell <NUM> of a composite construction and formed of a first (leeward) half shell 12a and a second (windward) half shell 12b. The blade <NUM> extends in a spanwise direction (S) from a root end <NUM> to a tip end <NUM> of the blade, and in a chordwise direction (C) between a leading edge <NUM> and a trailing edge <NUM>. A shear web <NUM> forming part of a reinforcing spar structure is located inside the blade <NUM>. The shear web <NUM> is bonded along lower and upper surfaces <NUM>, <NUM> to respective inner surfaces <NUM>, <NUM> of the blade shells 12a, 12b, as described in further detail later.

In this example, the shear web <NUM> comprises first and second mounting flanges <NUM>, <NUM> arranged along longitudinal edges of an elongate panel <NUM>. The flanges <NUM>, <NUM> extend transversely from the panel <NUM>. In this example, the shear web <NUM> comprises a substantially I-shaped cross section wherein the mounting flanges <NUM>, <NUM> extend transverse to the shear web panel <NUM> on both a first and an opposing second side <NUM>, <NUM> of the shear web <NUM>. In other examples the flanges <NUM>, <NUM> may each extend transverse to the panel <NUM> on the same side thereof, the shear web <NUM> thereby comprising a substantially C-shaped cross section. The present invention is not limited to any particular shape of shear web <NUM>.

The reinforcing spar structure further comprises spar caps (not shown) which, together with the shear web <NUM>, provide structural and torsional rigidity to the wind turbine blade <NUM>. The spar caps are formed of a reinforcing fibrous material such as carbon fibre reinforced plastic (CFRP), and extend longitudinally in the spanwise direction (S) to absorb bending loads along the blade <NUM>. In preferred examples the spar caps are embedded in the laminate structures of the half shells 12a, 12b. In other examples, the blade shell <NUM> may comprise mutually opposed spar caps bonded to an inner surface of an aerodynamic fairing. It will be appreciated that the present invention is equally applicable to wind turbine blades <NUM> comprising any type of blade shell <NUM>.

<FIG> is a schematic perspective view of a bondline structure <NUM> for bonding the shear web <NUM> to the blade shells 12a, 12b. The bondline structure <NUM> comprises one or more outer layers <NUM> comprising reinforcing fibres surrounding an inner core <NUM>. The inner core <NUM> is made from a deformable material. In a particular example, the outer layers <NUM> may comprise reinforcing fibrous fabric that is pre-impregnated with uncured resin (so-called 'prepreg' material). The bondline structure <NUM> may be configured differently and/or may be made from other materials in other examples. Various possibilities will be described later.

<FIG> is a schematic cross sectional view of the bondline structure <NUM>. The outer layers <NUM> may comprise a single layer <NUM> of reinforcing fibres wrapped around the inner core <NUM> a plurality of times such that the bondline structure <NUM> comprises a plurality of outer layers <NUM>. As shown, the outer layers <NUM> may form a continuous spiral in cross section. In other examples, one or more separate sheets of fibrous material may be wrapped around the core <NUM>. In such an example the outer layers <NUM> may form a discontinuous spiral in cross section.

The process of bonding a shear web <NUM> between first and second half shells 12a, 12b using the bondline structure <NUM> will now be described with reference to the remaining figures.

Referring to <FIG>, this shows a first half shell 12a supported in a corresponding first half mould 50a. A first bondline structure <NUM> as described above is arranged on an inner surface <NUM> of the first half shell 12a. In this example the first bondline structure <NUM> is formed of a plurality of discrete lengths which are arranged end to end in the first half shell 12a. The first bondline structure <NUM> is arranged on top of a spar cap <NUM>, which is embedded in the laminate structure of the blade shell 12a.

<FIG> is a schematic cross-sectional view showing a shear web <NUM> supported above the first bondline structure <NUM> on the first half shell 12a. For the purpose of illustration, only the first mounting flange <NUM> of the shear web <NUM> is shown. The shear web <NUM> is aligned with the first bondline structure <NUM> such that the first bondline structure is located between the first mounting flange <NUM> of the shear web <NUM> and the inner surface <NUM> of the first half shell 12a. The shear web <NUM> is then lowered onto the first bondline structure <NUM> in the direction of the arrow A.

Referring now to <FIG>, this is a schematic cross-sectional view of part of a wind turbine blade mould assembly <NUM> in a closed configuration. In this configuration, a second half shell 12b is positioned on top of the first half shell 12a. Prior to closing the mould assembly <NUM>, a second bondline structure <NUM> may be arranged on the upper surface <NUM> of the shear web <NUM> between the second mounting flange <NUM> of the shear web <NUM> and the inner surface <NUM> of the second half shell 12b. Alternatively, the second bondline structure <NUM> may be affixed to the inner surface <NUM> of the second half shell 12b prior to closing the mould.

When the mould assembly <NUM> is closed, the first bondline structure <NUM> is compressed between the first mounting flange <NUM> and the first half shell 12a, and the second bondline structure <NUM> is compressed between the second mounting flange <NUM> and the second half shell 12b. When compressed, the deformable material of the inner core <NUM> in each bondline structure <NUM> serves to resiliently bias against the movement of the shear web mounting flanges <NUM> and <NUM> towards the respective inner surfaces <NUM>, <NUM> of the half shells 12a, 12b. For example in relation to the first mounting flange <NUM> and the inner surface <NUM> of the first half shell 12a, the elasticity of the inner core <NUM> serves to push upwards against the first mounting flange <NUM>. The shear web <NUM> therefore cannot sink to a point such that it would come into contact with the first half shell 12a. Also, in the event of thermal expansion causing the second half shell 12b to lift up and away from the shear web <NUM>, the compressed bondline structures <NUM> will expand slightly, pushing the shear web <NUM> upwards, and ensuring that the shear web <NUM> remains attached to the second half shell 12b via the second bondline structure <NUM>.

The expansion and compression of the bondline structures <NUM> is directly related to the movement of the half shells 12a, 12b and shear web <NUM> relative to one another. As such, the elasticity of the deformable inner core <NUM> serves to ensure that the bondline structure <NUM> follows any movement of the blade shell <NUM> to maintain a connection between the shear web <NUM> and blade shell <NUM> throughout curing of the adhesive in the outer layers <NUM> of reinforcing fibres.

The adhesive in the outer layers of the bondline structures <NUM> (i.e. the resin matrix of the prepreg outer layers) is cured to respectively bond the first and second mounting flanges <NUM>, <NUM> of the shear web <NUM> to the inner surfaces <NUM>, <NUM> of the first and second half shells 12a, 12b. The fibrous material in the outer layers <NUM> of the bondline structures <NUM> results in a strong fibre connection being made between the blade shell <NUM> and the shear web <NUM>. The reinforcing fibres of the bondline structures <NUM> enable more effective transfer of loads between the blade shell <NUM> and shear web <NUM> than was previously achievable using pure adhesive to bond a shear web <NUM> in place.

<FIG> shows an optional stage in the blade manufacturing process, which involves forming the bondline structure <NUM> at the same time as arranging the bondline structure <NUM> in the first half shell 12a. Specifically, a winding machine <NUM> is arranged at the root end <NUM> of the first half shell 12a. The winding machine <NUM> is configured to wind reinforcing fibrous material around elongate core material <NUM> to form the bondline structure <NUM>. Elongate core material <NUM> is fed through the winding machine <NUM> in a spanwise direction (S) towards the tip end <NUM> of the half shell 12a, as indicated by the arrow B1. The winding machine <NUM> wraps the fibrous outer layers <NUM> around the core <NUM>. The completed bondline structure <NUM> is pulled out of the winding machine <NUM> and pulled directly into the first half shell 12a in the direction of arrow B2. The winding machine <NUM> may similarly be used to form the second bondline structure <NUM>.

In other examples the winding machine <NUM> could be arranged at the tip end <NUM> of the first half shell 12a, in which case the bondline structure <NUM> may be pulled out of the winding machine <NUM> in a spanwise direction (S) towards the blade root <NUM>.

A continuous length of core material <NUM> may be fed through the winding machine <NUM>. Alternatively, a plurality of discrete lengths of core material <NUM> may be fed through the winding machine <NUM>. In both cases the winding of fibrous outer layers <NUM> around the core material <NUM> may be continuous. Therefore, the winding process enables a continuous bondline structure <NUM> to be formed from a plurality of discrete lengths of core material <NUM>. As a further alternative, the winding machine <NUM> can be used to produce a plurality of discrete bondline structures <NUM> that are then arranged end to end.

Use of a winding machine <NUM> is advantageous because it enables the bondline structures <NUM> to be produced in a semi-automated process with high repeatability. The bondline structures <NUM> produced have a high degree of uniformity. It is also particularly advantageous to produce the bondline structures <NUM> immediately before they are used, because this avoids the need to transport or store the bondline structures <NUM>.

Whilst a winding machine <NUM> has been described with reference to <FIG> as a possible means for manufacturing a bondline structure <NUM>, in other examples the bondline structure <NUM> may be formed in a number of different processes. For example, the bondline structure <NUM> may be formed in a pultrusion process wherein the elongate core material <NUM> and outer layers <NUM> of reinforcing fibres are pulled through a die. In such a process, the outer layers <NUM> of reinforcing fibres may be coated in an adhesive (e.g. resin) prior to being pulled through the die. Alternatively the outer layers <NUM> of the bondline structure <NUM> may be coated in adhesive after having been pulled through the die with the inner core material <NUM>.

Further, with reference to <FIG>, it is also anticipated that in some examples the bondline structures <NUM> may be arranged in multiple rows. As shown in <FIG>, two bondline structures <NUM> are arranged side by side in the chordwise direction (C) between the first mounting flange <NUM> and the first half shell 12a. A similar arrangement of bondline structures <NUM> may be provided between the second mounting flange <NUM> (not shown) and the second half shell 12b (not shown). More than two rows of bondline structures <NUM> may be used in yet further examples.

In the above examples, the outer layers <NUM> of the bondline structures <NUM> comprise a prepreg material, wherein the reinforcing fibres are pre-impregnated with an adhesive. Any suitable adhesive may be used, including resins such as epoxy resin or polyester resin. Using pre-impregnated fibrous material can aid in achieving a complete and consistent distribution of adhesive when bonding the shear web <NUM> to the blade shell <NUM>. In other examples the outer layers <NUM> may comprise dry fibrous material which later becomes saturated with adhesive when bonding the shear web <NUM> to the blade shell <NUM>. In this case, for example, adhesive may be separately applied to the dry outer layers <NUM>, for example using a brush or roller.

In some examples, a bondline structure comprises an inner core formed of adhesive which is surrounded by outer layers <NUM> which may comprise dry reinforcing fibres. The dry reinforcing fibre of the outer layers may then become saturated by the adhesive of the inner core during compression of the bondline structures in manufacturing a wind turbine blade as described with reference to <FIG>.

In yet further examples, a bondline structure <NUM> comprises an inner core <NUM> made from an adhesive which is surrounded by outer layers <NUM> which may comprise a prepreg material as described above.

Any suitable reinforcing fibres may be used in the outer layers <NUM> of the bondline structures <NUM>. However, glass or carbon fibres are preferred. In preferred examples, the outer layers <NUM> comprise multiaxial reinforcing fibres in order to most effectively transfer loads between the shear web <NUM> and the blade shell <NUM>. The outer layers <NUM> may therefore comprise biaxial, triaxial or even quadriaxial fibrous material in which the fibres are each respectively oriented at two, three or four different angles relative to a longitudinal axis Y of the bondline structure <NUM> (indicated in <FIG>).

Preferably, the bondline structures <NUM> comprises no reinforcing fibres oriented parallel to the longitudinal axis Y of the bondline structure <NUM>, or only a low percentage of fibres oriented parallel to the longitudinal axis Y, such as less than <NUM>%, more preferably less than <NUM>%. The durability of the joint between the shear web <NUM> and the blade shell <NUM> is increased in such examples because the lack of longitudinally-oriented fibres means substantially no shear loads are absorbed in the bondline structure <NUM>. The shear loads are instead substantially wholly absorbed by the shear web <NUM> which is specifically designed to withstand such loading, and the bondline structures <NUM> merely serve to transfer loads between the blade shell <NUM> and shear web <NUM>.

Preferably the elastic modulus of the first bondline structure <NUM> is substantially the same as the elastic modulus of the second bondline structure <NUM>. This may be achieved by forming the inner core <NUM> of the first and second bondline structures <NUM> from the same material. This results in the shear web <NUM> being suspended at a substantially equal distance from each of the first and second half shells 12a, 12b, on account of each of the bondline structures <NUM> being compressed substantially equally. The shear web <NUM> is therefore supported substantially centrally between the two half shells 12a, 12b.

The bondline structure <NUM> comprises an inner core <NUM> made from an adhesive, and the adhesive is preferably an adhesive which expands upon application of heat thereto. For example, during the curing of adhesive in the outer layers <NUM> of a bondline structure <NUM> to bond the shear web <NUM> to the blade shell <NUM>, heat may be applied to the bondline structures <NUM>. Such application of heat in the present example causes the adhesive inner core <NUM> to undergo thermal expansion. It will be understood that the benefits of such a thermally expanding inner core <NUM> are the same as those described throughout with reference to an elastically deformable inner core <NUM>. Primarily, the thermally expanding inner core <NUM> serves to fill any gaps between the mounting flanges <NUM>, <NUM> of the shear web <NUM> and the inner surfaces <NUM>, <NUM> of the blade shell <NUM> to ensure a consistent and thorough bond between the shear web <NUM> and the blade shell <NUM>.

The inner core <NUM> may be any of a number of suitable shapes in cross section. For example the inner core <NUM> may be elliptical, hexagonal or octagonal in cross section. In preferred examples, any edges of the inner core <NUM> extending in the longitudinal direction are rounded. In the most preferred examples, for example as shown in <FIG>, the inner core <NUM> comprises a substantially circular cross section, the inner core <NUM> thereby having no longitudinally-extending edges.

An inner core <NUM> having a substantially circular cross section is particularly advantageous in minimising the occurrence of wrinkles in the outer layers <NUM> of reinforcing fibres when the bondline structure <NUM> is compressed during bonding of the shear web <NUM> to the blade shell <NUM>. The lack of longitudinally extending edges means that an outer surface <NUM> of the inner core <NUM> (indicated on <FIG>) remains smooth even when the bondline structure <NUM> is compressed. The outer layers <NUM> of reinforcing fibres surrounding the inner core <NUM> are therefore also substantially smooth as they conform to the profile of the inner core <NUM> under compression of the bondline structure <NUM>. This serves to reduce any potential stress concentrations in the outer layers <NUM> of reinforcing fibres in a finished blade <NUM> as the fibres remain substantially straight and do not become kinked or bent around edges of the inner core <NUM> when the bondline structure <NUM> is compressed.

To minimise the risk of the outer layers <NUM> wrinkling during compression of the bondline structure <NUM>, the inner core <NUM> preferably comprises both a material having a high Poisson's ratio and a substantially circular cross section.

The bondline structures <NUM> may have any suitable length L. In some examples the bondline structures <NUM> may have a length L substantially the same as that of the shear web <NUM> to be bonded to the blade shell <NUM>. Equally however, the bondline structure <NUM> may be formed of a plurality of discrete lengths to facilitate transport of said structure <NUM> and/or to ease handling during manufacture of the wind turbine blade <NUM>.

Manufacturing a wind turbine blade <NUM> in accordance with the methods described above, and using a bondline structure <NUM> as described above to bond the shear web <NUM> to the blade shell <NUM>, provides a number of advantages over previous techniques for bonding a shear web <NUM> in a wind turbine blade <NUM>. Primarily, a bondline structure <NUM> as described herein provides a more consistent and durable bond between a blade shell <NUM> and a shear web <NUM>, which is less susceptible to defects than previous methods of pure adhesive bonding. The reinforcing fibres of the bondline structure <NUM> provide a stronger connection between the blade shell <NUM> and shear web <NUM> which is capable of effectively transferring loads between the shell <NUM> and shear web <NUM> in a more robust manner.

The inner core <NUM> ensures that any movement of the shells 12a, 12b is mirrored by the bondline structures <NUM> to ensure the shear web <NUM> remains connected to the blade shells 12a, 12b during the blade join-up process and during the curing of the adhesive. The bondline structures <NUM> are able to accommodate thermal expansion of the blade shells 12a, 12b during adhesive curing, and ensure that the shear web <NUM> remains attached to the blade shells 12a, 12b despite some movement of the blade shells.

The bondline structures <NUM> prevent the possibility of the shear webs <NUM> coming into direct contact with the blade shells 12a, 12b and ensure there is always adhesive between the shear web mounting flanges <NUM>, <NUM> and the blade shells 12a, 12b along the entire length of the shear web <NUM>. The problem of a shear web <NUM> sinking too far into adhesive is therefore avoided.

Tooling and manufacturing costs involved in manufacturing a wind turbine blade <NUM> can be greatly reduced by using a bondline structure <NUM> to bond the shear web <NUM> to the shell <NUM> instead of pure adhesive. The deformable and compressible bondline structures <NUM> accommodate variations in the geometry of the shear web <NUM> and blade shell <NUM>. Both the shell <NUM> and shear web <NUM> may therefore be manufactured with a greater dimensional tolerance. Similarly, various wind turbine blade designs having different shell thicknesses or shear web geometry may be formed in a single mould assembly on account of the bondline structures <NUM> accommodating such dimensional variations.

Whilst in the above examples bondline structures <NUM> are provided on both sides of the shear web <NUM>, in other examples a bondline structure <NUM> may be provided only on one side of the shear web <NUM>. In such cases, a simple adhesive bondline may be formed on the other side of the shear web <NUM>.

Claim 1:
A bondline structure (<NUM>) for bonding a shear web (<NUM>) to a wind turbine blade shell (<NUM>) , the bondline structure comprising:
an elongate inner core (<NUM>) made from a deformable material;
one or more outer layers (<NUM>),
the bondline structure being characterized in that:
the one or more outer layers (<NUM>) comprise reinforcing fibres surrounding the inner core,
the inner core is made from an adhesive and the one or more outer layers (<NUM>) are wrapped around a full circumference of the inner core (<NUM>).