Patent Description:
Modern wind turbine blades typically comprise a shell defining the aerodynamic contour of the blade and one or more longitudinally-extending spars which act as the main load-bearing structures of the blade. A spar typically comprises a shear web connected between opposed spar caps provided respectively on windward and leeward sides of the blade. Spar caps are configured to absorb bending loads experienced by the blade in use and therefore typically comprise a material with a high tensile strength, such as carbon fibre reinforced plastic (CFRP). In some wind turbine blades, the spar caps are formed of a plurality of strips of reinforcing material arranged in a plurality of side-by-side stacks to allow the spar caps to conform to the chordwise curvature of the blade.

The aerodynamic profile of a modern wind turbine blade typically twists along the length of the blade to capture energy from the wind most effectively. A mould configured to form the blade shell therefore typically comprises a mould surface with an inclination that varies along its length. However, the varying inclination of the mould surface introduces challenges when manufacturing a blade having a spar cap formed of stacked strips. In particular, the stacked strips have a tendency to move or slide on steeply inclined sections of the mould due to gravity, and maintaining the strips in their intended positions can be difficult. <CIT> describes a known method to manufacture a blade having a spar cap formed of stacked strips.

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

In a first aspect of the invention there is provided a wind turbine blade comprising a blade shell that extends in a spanwise direction from a root end to a tip end, and in a chordwise direction from a leading edge to a trailing edge. The blade shell comprises a spar cap formed from a plurality of substantially planar strips of reinforcing material, the strips being arranged in a plurality of stacks extending longitudinally in the spanwise direction and arranged side-by-side in the chordwise direction. In each stack an uppermost strip defines an upper surface of the stack, a lowermost strip defines a lower surface of the stack, and longitudinal edges of the stacked strips define side surfaces of the stack. The blade further comprises a retaining clip comprising a plurality of side-by-side substantially U-shaped sections. The U-shaped sections each comprise a pair of mutually-spaced side portions defining a stack-receiving region therebetween, and the side portions are joined by a bridging portion. At least some of the stacks are located in the stack-receiving regions of the retaining clip, such that the side portions of the U-shaped sections abut side surfaces of the stacks. Each U-shaped section of the retaining clip is inverted with respect to its neighbouring U-shaped section(s) such that the bridging portions of the respective U-shaped sections extend alternately across the upper and lower surfaces of the stacks in the chordwise direction.

During manufacture of the blade, the retaining clip maintains the positions of the strips in their respective stacks and prevents the strips from sliding or otherwise moving with respect to each other.

The inverted arrangement of neighbouring U-shaped sections means that a U-shaped section of the retaining clip having a bridging portion that extends across the upper surface of a stack will be immediately adjacent to at least one U-shaped section having a bridging portion that extends across the lower surface of a stack. Similarly, a U-shaped section with a bridging portion that extends across the lower surface of a stack will be immediately adjacent to at least one U-shaped section having a bridging portion that extends across the upper surface of a stack. The retaining clip may resemble a square waveform.

The 'uppermost' and 'lowermost' strips of the stack refer to the uppermost and lowermost strips at the spanwise location of the retaining clip. It will be appreciated that the number of strips in the stack may vary in different spanwise locations of the blade, for example the number of strips may drop off towards the ends of the stacks such that the spar cap tapers in height towards its ends.

The retaining clip may have a first U-shaped section at a first end and a second U-shaped section at a second end. These U-shaped sections are also referred to as "endmost" U-shaped sections. The retaining clip may have any number of intermediate U-shaped sections between the endmost sections. The number of U-shaped sections in total preferably corresponds to the number of side-by-side stacks. In such cases, each U-shaped section of the retaining clip may accommodate a single stack. In other examples, each U-shaped section may accommodate multiple stacks.

At least one U-shaped section of the retaining clip, preferably an endmost section, may comprise a return flange extending from a side portion of the U-shaped section. The return flange may extend substantially parallel to and spaced apart from the bridging portion of that U-shaped section. A stack may be located between the bridging portion and the return flange such that the bridging portion extends across one of the upper or lower surfaces of a stack and the return flange extends at least partially across the other of the upper or lower surfaces of the stack. During manufacture of the blade, the stack may be gripped between the return flange and the bridging portion.

An end of the retaining clip may comprise a locating flange. The locating flange may extend in the chordwise direction beneath a blade shell component located adjacent the spar cap. The blade shell component is preferably a panel of core material. The core material may comprise a material such as structural foam, polystyrene, or balsa wood for example.

The locating flange may extend from a side portion of an endmost U-shaped section. The locating flange may be substantially co-planar with the return flange. The two flanges may extend chordwise in opposite directions from the side portion of an endmost U-shaped section.

The side-by-side stacks may be spaced apart slightly in the chordwise direction by the side portions of the U-shaped sections of the retaining clip.

The wind turbine blade may comprise one or more pairs of retaining clips arranged side-by-side in the spanwise direction. The U-shaped sections of one of the retaining clips in each pair may be inverted relative to the U-shaped sections of the other retaining clip in the pair.

Preferably, each stack is located in a U-shaped section of one of the retaining clips in the pair and in a corresponding U-shaped section of the other retaining clip in the pair. Accordingly, a bridging portion of a U-shaped section of one of the retaining clips in the pair may extend across one of the upper or lower surfaces of a stack, whilst a bridging portion of a U-shaped section of the other retaining clip in the pair may extend across the other of the upper and lower surfaces of the stack.

The wind turbine blade may comprise a plurality of retaining clips, or a plurality of pairs of retaining clips. The plurality of retaining clips, or the plurality of pairs of retaining clips may be mutually spaced in the spanwise direction.

One or more of the stacks may comprise a tapered end region. The or each retaining clip may be positioned outside of the tapered end region.

A chordwise cross-sectional profile of the stack-receiving regions of the retaining clip preferably substantially corresponds in shape to a chordwise cross-sectional profile of the stacks. Preferably the stacks and the stack-receiving regions of the retaining clip are substantially rectangular in chordwise cross-section.

Side portions of the U-shaped sections may have a height that substantially corresponds to a height of the stacks. The stacks comprise a plurality of planar strips of reinforcing material which are arranged in layers. The stacks may also comprise layers of material (such as glass or carbon fabric) interleaved between the planar strips. The height of the stacks is the distance between the uppermost layer of the stack and the lowermost layer of the stack.

The bridging portions of the U-shaped sections may have a length that substantially corresponds to a chordwise width of the stacks.

The or each retaining clip may be formed of a fibre-reinforced polymer material. The or each retaining clip may be a pre-cured component. The retaining clip may therefore comprise resin that is already cured before the strip is positioned in relation to the stacks. The retaining clip could also be formed from a high strength plastic moulding.

In a second aspect of the invention there is provided a method of manufacturing a wind turbine blade. The method comprises providing a blade shell mould that extends in a spanwise direction from a root end to a tip end and in a chordwise direction from a leading edge to a trailing edge, and providing a plurality of substantially planar strips of reinforcing material. The method further comprises arranging the strips in the mould in a plurality of stacks to form at least part of a spar cap, the stacks extending in the spanwise direction and being arranged side-by-side in the chordwise direction. In each stack an uppermost strip defines an upper surface of the stack, a lowermost strip defines a lower surface of the stack, and longitudinal edges of the stacked strips define side surfaces of the stack. The method further comprises providing a retaining clip comprising a plurality of side-by-side substantially U-shaped sections. Each U-shaped section comprises a pair of mutually-spaced side portions defining a stack-receiving region therebetween, the side portions being joined by a bridging portion. The method further comprises arranging at least some of the stacks in the stack-receiving regions of the retaining clip, such that the side portions of the U-shaped sections abut the side surfaces of the stacks. Each U-shaped section of the retaining clip is inverted with respect to its neighbouring U-shaped section(s) such that the bridging portions of the respective U-shaped sections extend alternately across the upper and lower surfaces of the stacks in the chordwise direction.

In a further aspect of the invention there is provided a retaining clip for a wind turbine blade spar cap configured to retain strips of reinforcing material in a stacked formation and restrict movement of a plurality of stacks of strips relative to one another. The clip comprises a series of substantially U-shaped sections each having a pair of mutually-spaced side portions joined by a bridging portion. Each U-shaped section is inverted with respect to its neighbouring U-shaped section(s).

Optional features described in relation to the first aspect of the invention are equally applicable to any other aspect of the invention, repetition of these 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> shows a schematic exploded view of a wind turbine blade <NUM>. The blade <NUM> comprises a first half shell 12a and a second half shell 12b which each extend in a spanwise direction (S) from a root end <NUM> of the blade <NUM> to a tip end <NUM>, and in a chordwise direction (C) between a leading edge <NUM> and a trailing edge <NUM>. The first and second half shells 12a, 12b are joined together to form a blade shell <NUM>. The blade shell <NUM> defines an aerodynamic contour and is configured to capture energy from wind incident on the blade <NUM>. The blade shell <NUM> may twist along its spanwise length to capture wind energy most effectively.

The blade <NUM> further comprises longitudinally-extending spar caps <NUM> to take up bending loads experienced by the blade <NUM> in use. The half shells <NUM> comprise spar caps <NUM> which may be embedded in laminate layers of the half shell <NUM>, forming a so-called structural shell. The spar caps <NUM> are formed of a plurality of substantially planar strips of reinforcing material <NUM> arranged side-by-side in stacks <NUM> which extend longitudinally in the spanwise direction (S). The blade <NUM> may further comprise shear webs <NUM> bonded between the mutually-opposed spar caps <NUM> of the first and second half shells 12a, 12b to form spar structures which provide structural support for the blade <NUM> in use.

As described by way of background above, the strips of reinforcing material <NUM> may comprise a relatively stiff material such as carbon fibre reinforced plastic (CFRP). As such, the strips <NUM> may comprise an inherent elastic resilience, due to the tensile nature of the reinforcing material, which causes the strips <NUM> to be somewhat resistant to twisting along their longitudinal length. This can present issues during manufacture of the blade <NUM>, as will now be described with reference to <FIG>.

<FIG> shows a plurality of strips of reinforcing material <NUM> arranged on a mould surface <NUM> of a blade shell mould <NUM> in a plurality of stacks <NUM> to form a spar cap <NUM>. Longitudinal edges <NUM> of the strips <NUM> in a stack <NUM> define side surfaces <NUM> of that stack, and an uppermost strip <NUM> and lowermost strip <NUM> in each stack <NUM> define upper and lower surfaces <NUM>, <NUM> of that stack. The stacks <NUM> shown in <FIG> should be substantially rectangular in chordwise cross section. However, the strips <NUM> in the stacks <NUM> are not restrained and may therefore move relative to one another in the mould <NUM>.

To form a blade shell <NUM> which twists along its spanwise length, the mould <NUM> may comprise a mould surface <NUM> having an inclination that varies between a root end of the mould <NUM> and a tip end of the mould <NUM>. In some cases, the elastic resilience of the reinforcing material can constrain the twist of the strips <NUM> such that the strips <NUM> do not conform to the variations in the inclination of the mould surface <NUM> unaided. The transition between a relatively flat outboard portion of the mould surface <NUM> and a relatively steeply inclined surface <NUM> in a root portion can be particularly problematic.

Internal stresses resulting from twisting the strips <NUM> act to pull the strips <NUM> back to their flat, neutral state. These torsional stresses cause a first longitudinal edge 36a of each strip <NUM> to be pressed down towards the mould surface <NUM> whilst a second longitudinal edge 36b of each strip <NUM> is lifted from the mould surface <NUM> as the strip <NUM> attempts to untwist itself. As shown in <FIG>, this may result in the formation of a step between adjacent stacks <NUM> where the upper surfaces <NUM> of neighbouring stacks <NUM> are not flush, i.e. are not co-planar.

Steps between adjacent stacks <NUM> can cause material that is arranged on top of the stacks <NUM> in the mould <NUM>, such as glass fibre mats, to become wrinkled. Such wrinkling can cause stress concentrations in the laminate structure of the shell <NUM>, and also presents an uneven surface for the shear webs <NUM> to bond to, decreasing the strength of the connection between the webs <NUM> and the half shells <NUM>.

A portion of the mould surface <NUM> configured to form a root portion of the blade shell <NUM> with a substantially circular cross-sectional profile may be relatively steeply inclined. The steep inclination of the mould surface <NUM> in this region can cause the strips <NUM> in the stacks <NUM> to slide relative to one another. This undesired sliding may result in a so-called "book effect", whereby the stack <NUM> resembles a spine cocked book. This can result in misalignment of the strips <NUM>, and may also result in other shell components arranged adjacent to the stacks <NUM> being incorrectly positioned in the mould <NUM>.

In order to overcome at least some of the above described difficulties involved in manufacturing a wind turbine blade <NUM>, the strips <NUM> may be held in place by a retaining clip <NUM> such as those shown in the remaining figures.

With reference now to <FIG>, a retaining clip <NUM> according to an example of the present invention comprises a plurality of substantially U-shaped sections <NUM> that are configured to receive the stacks <NUM> of strips <NUM> that form a spar cap <NUM>. The retaining clip <NUM> shown in <FIG> comprises three side-by-side U-shaped sections <NUM>. The U-shaped sections <NUM> are formed of mutually-spaced side portions <NUM> that are joined by bridging portions <NUM> extending transversely between the side portions <NUM>. Each U-shaped section <NUM> is inverted with respect to its neighbouring U-shaped section or sections. As such, the bridging portions <NUM> of adjacent U-shaped sections <NUM> extend between opposite ends of the side portions <NUM>.

A first U-shaped section 50a defines a first end <NUM> of the retaining clip <NUM>, and a second U-shaped section 50b defines a second end <NUM> of the retaining clip <NUM>. The first and second U-shaped sections 50a, 50b may therefore be referred to as endmost U-shaped sections. The retaining clip <NUM> in <FIG> additionally comprises an intermediate U-shaped section 50c between the endmost U-shaped sections 50a, 50b. The retaining clip <NUM> may comprise any number of intermediate U-shaped sections 50c between the endmost U-shaped sections 50a, 50b, including no intermediate U-shaped sections 50c. For example, the retaining clips <NUM> shown in <FIG> comprise first and second U-shaped sections 50a, 50b and no intermediate U-shaped sections 50c.

The U-shaped sections <NUM> define stack-receiving regions <NUM> in which the stacks <NUM> of strips <NUM> are arranged as shown in <FIG>. Each stack-receiving region <NUM> may be configured to receive a single stack <NUM> of strips <NUM>. When viewed in chordwise cross section, the shape of the stack-receiving regions <NUM> preferably corresponds to a cross-sectional profile of the stacks <NUM> as shown in <FIG>. For example, where the stacks <NUM> of strips <NUM> are substantially rectangular in cross section, the corresponding stack-receiving regions <NUM> in which the stacks <NUM> are arranged are preferably also substantially rectangular in cross-sectional shape.

The retaining clip <NUM> is configured to form a close fit around the stacks <NUM>. As such, the side portions <NUM> of the U-shaped sections <NUM> preferably have a height that substantially corresponds to a height of the stack <NUM> arranged in a given receiving region <NUM>. Similarly, the length of each bridging portion <NUM> preferably corresponds to a chordwise width of the stack <NUM> arranged in the respective receiving region <NUM>. The bridging portion <NUM> fixes the distance between the side portions <NUM>. Substantially matching the length of the bridging portion <NUM> to the width of a stack <NUM> therefore means that the stack <NUM> is gripped in the stack-receiving region <NUM> between the side portions <NUM> by an interference fit.

When the stacks <NUM> are arranged in receiving regions <NUM> of the retaining clip <NUM>, the side surfaces <NUM> of the stacks <NUM> abut the side portions <NUM> of the U-shaped sections <NUM>. The side portions <NUM> abutting the side surfaces <NUM> of the stacks <NUM> prevent the strips <NUM> from sliding, thereby avoiding the previously described "book-effect", and maintaining the position of the strips <NUM> relative to one another in the chordwise direction (C).

The side-by-side stacks <NUM> are spaced apart slightly in the chordwise direction (C) by the side portions <NUM> of the U-shaped sections <NUM> of the retaining clip <NUM>. As such, the retaining clip <NUM> ensures that a gap is maintained between adjacent stacks <NUM>. During manufacture of the half shell <NUM>, the gap between stacks <NUM> may help to promote a thorough infusion of resin throughout the materials arranged in the mould <NUM>.

With the stacks <NUM> arranged in the stack-receiving regions <NUM> of the retaining clip <NUM>, the inverted orientation of neighbouring U-shaped sections <NUM> relative to one another results in the bridging portions <NUM> of the U-shaped sections <NUM> extending alternately across the upper and lower surfaces <NUM>, <NUM> of the stacks <NUM> in the chordwise direction (C). As shown in <FIG>, alternating which surface <NUM>, <NUM> the bridging portions <NUM> extends across means that, for example, the bridging portion 54a of the first U-shaped section 50a extends across the upper surface <NUM> of a stack <NUM>, the bridging portion 54c of the neighbouring intermediate U-shaped section 50c extends across the lower surface <NUM> of a stack <NUM>, and the bridging portion 54b of the neighbouring second U-shaped section 50b again extends across the upper surface <NUM> of a stack <NUM>. When viewed in chordwise cross section, the retaining clip <NUM> may therefore substantially resemble a square wave. The retaining clip <NUM> may be considered to weave between the side-by-side stacks <NUM> in the chordwise direction (C).

A stack <NUM> arranged in a U-shaped section <NUM> having a bridging portion <NUM> that extends across the lower surface <NUM> of the stack <NUM>, exerts a downwards pressure on that bridging portion <NUM> of the retaining clip <NUM>. The retaining clip <NUM> utilises this downwards pressure to counteract an upwards pressure, caused by the internal torsional stresses described previously, exerted by a neighbouring stack <NUM> on a bridging portion <NUM> that extends across the upper surface <NUM> of that stack. The alternating bridging portions <NUM> therefore ensure that the strips <NUM> of a stack <NUM> cannot untwist because the bridging portion <NUM> bears against the upper surface <NUM> and presses the stack <NUM> against the mould surface <NUM>.

The retaining clip <NUM> may further comprise a return flange <NUM> to further secure the strips <NUM> in their respective stacks <NUM>, and to secure a stack <NUM> in position relative to the other stack(s) <NUM>. The return flange <NUM> may extend in the chordwise direction (C) from a side portion <NUM> of a U-shaped section <NUM>, preferably an endmost U-shaped section 50a, 50b, and at least partially across an upper or lower surface <NUM>, <NUM> of a stack <NUM> arranged in a receiving region <NUM> as shown in <FIG>. As shown in <FIG>, the retaining clip <NUM> may comprise two return flanges <NUM>, i.e. each of the endmost U-shaped sections 50a, 50b of the retaining clip <NUM> may comprise a return flange <NUM> extending from a respective side portion <NUM>.

The return flanges <NUM> extend from the opposite end of a respective side portion <NUM> to that from which the bridging portion <NUM> extends. The return flanges <NUM> are therefore spaced apart from the bridging portion <NUM> of a given U-shaped section <NUM>. One or more of the return flanges <NUM> may be substantially parallel to the bridging portion <NUM> of a given U-shaped section <NUM>. In <FIG> the bridging portions 54a, 54b of the first and second U-shaped sections 50a, 50b extend across the upper surfaces <NUM> of their respective stacks <NUM>, and the return flanges <NUM> therefore extend partially across the lower surfaces <NUM> of the stacks <NUM>. Stacks <NUM> of strips <NUM> located between a bridging portion <NUM> and a return flange <NUM> may be gripped between said flange <NUM> and bridging portion <NUM> to restrict movement of the strips <NUM> in the stack <NUM> relative to one another. The return flanges <NUM> additionally help to fix the retaining clip <NUM> in position relative to the stacks <NUM>. By clipping around one or more of the endmost stacks <NUM>, the retaining clip <NUM> is better equipped to withstand the elastic forces in the strips <NUM> that act to spring the strips <NUM> apart in the stacks <NUM>.

The retaining clip <NUM> may also comprise a locating flange <NUM> extending in the chordwise direction (C) from a side portion <NUM> of an endmost U-shaped section 50a, 50b. Such a locating flange <NUM> may be configured for arrangement beneath blade shell components <NUM> arranged adjacent to the stacks <NUM> during manufacture of the blade shell <NUM>, as will be described later in more detail. The locating flange <NUM> extends in the chordwise direction (C) in the opposite direction to the return flange <NUM>, i.e. away from the stack-receiving region <NUM>. Where the return flange <NUM> is configured to interface against a lower surface <NUM> of a stack <NUM>, the locating flange <NUM> may be substantially co-planar with the return flange <NUM> so that the retaining clip <NUM> sits flush against the mould surface <NUM>.

With reference still to <FIG>, the strips <NUM> may be individually arranged in stacks <NUM> and with the retaining clip <NUM> in the mould <NUM> during manufacture of the blade shell <NUM>. Alternatively, the strips of reinforcing material <NUM> may be arranged in side-by-side stacks <NUM> offline (i.e. outside of the blade shell mould <NUM>) as a sub-assembly to reduce in-mould time during manufacture of the blade shell <NUM>. Regardless of where the strips <NUM> are arranged, the process for arranging the strips <NUM> together with a retaining clip <NUM> to form a spar cap <NUM> is the same.

The strips of reinforcing material <NUM> are arranged in stacks <NUM> which, when arranged in the mould <NUM>, extend longitudinally in the spanwise direction (S). The stacks <NUM> are arranged in the stack-receiving regions <NUM> of a retaining clip <NUM> configured with U-shaped sections <NUM> that are adjacent one another in the chordwise direction (C). The stacks <NUM> arranged in the U-shaped sections <NUM> are therefore arranged side-by-side in the chordwise direction (C). Where the retaining clip <NUM> comprises a return flange <NUM>, the stack <NUM> is clipped into the receiving region <NUM> and held in place by the bridging portion <NUM> of that U-shaped section <NUM> and the return flange <NUM> interfacing against opposing surfaces <NUM>, <NUM> of the stack <NUM>.

Manufacturing the blade shell <NUM> may also include arranging other blade shell components <NUM>, such as core material panels of structural foam, adjacent to the stacks <NUM> in the mould <NUM>. If the retaining clip <NUM> comprises a locating flange <NUM> extending chordwise from a side portion <NUM> of a U-shaped section <NUM>, the method may also include arranging the retaining clip <NUM> and/or stacks <NUM> in the mould <NUM> such that the locating flange <NUM> is positioned between the blade shell components <NUM> and the mould surface <NUM>. For example, this may include sliding the retaining clip <NUM> and/or stacks <NUM> in the chordwise direction (C) to arrange the locating flange <NUM> beneath the shell components <NUM>. Alternatively, the retaining clip <NUM> and/or stacks <NUM> may first be arranged in the mould <NUM>, with the shell components <NUM> then being arranged on top of the locating flange <NUM> and adjacent to the stacks <NUM> in the mould <NUM>.

The retaining clip <NUM> maintains the position of each strip <NUM> in its respective stack <NUM> during manufacture of the blade shell <NUM>. The side portions <NUM> of the retaining clip <NUM> prevent the strips <NUM> from sliding or otherwise moving with respect to each other, and the bridging portions <NUM> extending over the upper surfaces <NUM> of the stacks <NUM> prevent the strips <NUM> from untwisting or pulling away from the mould surface <NUM>. The retaining clip <NUM> therefore ensures that the strips of reinforcing material <NUM> are accurately aligned in the blade shell mould <NUM>, and that other blade shell components <NUM> arranged adjacent to the stacks <NUM> are accurately positioned.

The blade materials arranged in the mould <NUM> may be infused with resin to form the half shell <NUM>. In another example, the blade materials may comprise pre-impregnated fibres. The retaining clip <NUM> holds the strips <NUM> in the stacks <NUM> throughout manufacture of the blade <NUM> and remains embedded in the blade shell <NUM> even after manufacture of the blade <NUM> is complete, e.g. after the resin cures. The retaining clip <NUM> preferably has a spanwise width that is very small in comparison to the total length of the spar cap <NUM>. Inclusion of the clip <NUM> in the laminate structure of a half shell <NUM> therefore has no detrimental effect on the load-bearing capacity of the spar cap <NUM>. Further the retaining clip <NUM> is preferably lightweight so as not to unduly increase the mass of the blade <NUM>. The retaining clip <NUM> preferably has a thickness that is as small as possible (such as <NUM> thick or less) in order to avoid deviations to the fabric layers that are subsequently placed on the clips.

<FIG> shows a pair of retaining clips <NUM> arranged with the stacks <NUM> of the spar cap <NUM>. The retaining clips <NUM> are arranged side-by-side in the spanwise direction (S), and the stacks <NUM> of strips <NUM> are arranged in U-shaped sections <NUM> of both retaining clips <NUM>. The retaining clips <NUM> may be arranged such that U-shaped sections <NUM> of a first clip 48a of the pair are inverted relative to corresponding U-shaped sections <NUM> of a second retaining clip 48b in the pair. The retaining clips 48a and 48b clips may be configured in substantially the same manner, with each clip <NUM> simply being inverted in orientation relative to the other. The upper surface <NUM> of each stack <NUM> interfaces with a bridging portion <NUM> of a retaining clip <NUM>, ensuring that each of the stacks <NUM> is pressed down onto the mould surface <NUM>.

A bridging portion <NUM> of the first or second retaining clip 48a, 48b extends across each of the upper and lower surfaces <NUM>, <NUM> of a stack <NUM>. For example, as shown in <FIG>, a bridging portion 54a of the first retaining clip 48a extends across the upper surface <NUM> of the first stack <NUM>, and a bridging portion (not shown) of the second retaining clip 48b extends across the lower surface <NUM> of that stack <NUM>. A pair of retaining clips <NUM> arranged in this manner effectively serves to lock the strips <NUM> in position in each stack <NUM> by sandwiching the stack <NUM> between the respective bridging portions <NUM> of the clips <NUM>. Where both the first and second retaining clips 48a, 48b comprise a locating flange <NUM>, the locating flanges <NUM> may extend above and below a portion of adjacent blade shell components <NUM>, thereby also locking such adjacent blade shell components in place.

The wind turbine blade <NUM> may comprise a plurality of retaining clips <NUM> arranged along the spanwise length of the spar cap <NUM>. Similarly, the blade <NUM> may comprise a plurality of pairs of retaining clips <NUM> arranged along the length of the spar cap <NUM>. The retaining clips <NUM>, or pairs of clips <NUM> may be mutually spaced in the spanwise direction (S) of the blade <NUM>. For example, the clips <NUM> or pairs of clips <NUM> may be spaced at intervals of between <NUM> to <NUM> along the length of the spar cap <NUM>. Spacing the clips <NUM> or pairs of clips <NUM> apart in the spanwise direction (S) minimises any adverse effect of including a bridging portion <NUM> of a clip <NUM> in the bondline between a shear web <NUM> and the spar cap <NUM>.

The retaining clips <NUM> are preferably arranged along the spar cap <NUM> in spanwise regions where the inclination of the mould surface <NUM> changes and thereby introduces torsion into the strips of reinforcing material <NUM>. As explained above, the retaining clips <NUM> bear against upper surfaces <NUM> of the stacks <NUM> to ensure the strips <NUM> and stacks <NUM> are held in position despite the varying inclination of the mould surface <NUM>.

Spar caps <NUM> in some wind turbine blades <NUM> may comprise one or more stacks <NUM> having a tapered end region <NUM> at a tip end or root end of the spar cap <NUM> (as shown in <FIG>). The tapered end region <NUM> of a stack <NUM> may be formed by varying the number of strips <NUM> stacked on top of one another in different spanwise regions along the spar cap <NUM>. For example, the number of strips <NUM> in a stack <NUM> may decrease near a root end and/or near a tip end of the stack <NUM> such that the spar cap <NUM> tapers in height near its root end and/or tip end. The tapered end region <NUM> may be provided to transfer load from the spar cap <NUM> to the laminate of the blade shell at the root and tip ends of the blade <NUM>.

Where a blade <NUM> comprises stacks <NUM> with one or more tapered end regions <NUM>, the retaining clip or clips <NUM> are preferably positioned outside of the tapered end region <NUM> to avoid interfering with the shear load transfer from the spar cap <NUM> to the laminate of the shell. For example, the strips <NUM> may be formed of carbon fibre and the blade shell laminate from glass fibre and by placing the clips <NUM> outside of the tapered regions, they will not interfere with the shear load transfer from the carbon to the glass.

<FIG> are provided merely to illustrate other examples of retaining clips <NUM> in accordance with the invention. For example, as noted above, a retaining clip <NUM> may comprise a first and second U-shaped section 50a, 50b and no intermediate sections 50c therebetween, as shown in <FIG> shows a retaining clip <NUM> comprising inverted U-shaped sections 50a, 50b, and <FIG> shows a retaining clip <NUM> that additionally comprises return flanges <NUM> as previously described. It will be appreciated that the description provided previously with regards to features such as the inverted neighbouring U-shaped sections <NUM>, their side portions <NUM> and bridging portions <NUM>, and the return and locating flanges <NUM>, <NUM> for example is also applicable to the retaining clips <NUM> shown in <FIG>. As such, further description of such features will not be repeated here in the interest of conciseness.

A retaining clip <NUM> as described with reference to any of the accompanying figures may be formed of any suitable material. Preferably, the retaining clip <NUM> may be formed of fibre reinforced composite material such as glass fibre reinforced plastic (GFRP). For example, the retaining clip <NUM> may be formed of one or more layers of biaxial glass fibre plies laminated with a polymer resin. The resin is preferably cured, i.e. set before the stacks <NUM> of strips <NUM> are arranged with the retaining clip <NUM>. As such, the retaining clip <NUM> may be a pre-cured component, i.e. cured before arrangement in the mould <NUM>. The material of the retaining clip <NUM> preferably has a high tensile strength to resist the elastic forces exerted by the twisted strips of reinforcing material <NUM>.

Preferably, the surface of the retaining clip <NUM> is activated (such as via abrasion or removal of a peel ply layer) so that the clip properly adheres to the surrounding blade materials during the manufacture of the blade <NUM>.

A retaining clip <NUM> may be configured such that multiple stacks <NUM> may be arranged in a single receiving region <NUM> defined by a U-shaped section <NUM> of the clip <NUM>. Alternatively or additionally, a retaining clip <NUM> may be configured to retain strips <NUM> in only some of the stacks <NUM> forming a spar cap <NUM>. For example, a spar cap <NUM> may have three side-by-side stacks <NUM> and a retaining clip <NUM> as described herein may only have two side-by-side U-shaped sections <NUM> that are each configured to receive a single stack <NUM>. As such, the invention is not limited to the number of U-shaped sections <NUM> matching the number of side-by-side stacks <NUM> that form the spar cap <NUM> of the wind turbine blade <NUM>.

It will be appreciated that terms such as "interfacing", "abut" and "bear against" include both direct and indirect "interfacing", "abutment" and "bearing against". That is to say such terms are intended to cover examples wherein one or more intermediate components, such as a spacer, may be arranged between interfacing abutting surfaces.

Claim 1:
A wind turbine blade (<NUM>) comprising:
a blade shell (<NUM>) that extends in a spanwise direction from a root end (<NUM>) to a tip end (<NUM>), and in a chordwise direction from a leading edge (<NUM>) to a trailing edge (<NUM>), the blade shell comprising a spar cap (<NUM>) formed from a plurality of substantially planar strips of reinforcing material (<NUM>), the strips being arranged in a plurality of stacks (<NUM>) extending longitudinally in the spanwise direction and arranged side-by-side in the chordwise direction, whereby in each stack an uppermost strip (<NUM>) defines an upper surface of the stack, a lowermost strip (<NUM>) defines a lower surface of the stack, and longitudinal edges (<NUM>) of the stacked strips define side surfaces (<NUM>) of the stack;
a retaining clip (<NUM>) comprising a plurality of side-by-side substantially U-shaped sections (<NUM>) each comprising a pair of mutually-spaced side portions (<NUM>) defining a stack-receiving region (<NUM>) therebetween, the side portions being joined by a bridging portion (<NUM>);
wherein at least some of the stacks (<NUM>) are located in the stack-receiving regions (<NUM>) of the retaining clip (<NUM>), such that the side portions of the U-shaped sections (<NUM>) abut side surfaces (<NUM>) of the stacks, and each U-shaped section of the retaining clip is inverted with respect to its neighbouring U-shaped section(s) such that the bridging portions (<NUM>) of the respective U-shaped sections extend alternately across the upper and lower surfaces of the stacks in the chordwise direction.